are constant, while in special processes, the combinations of circumstances are ..... the strength of steel, depending on the treatment process (Figures 9 and 10).
Scientific Monographs
Jozef BÁRTA, Ph.D.
WELDING THIN STEEL SHEETS TREATED BY NITRO-OXIDATION
Author’s acknowledgement to Emília Mironovová for translation. Cover design: Libros, Ltd.
This scientific monograph originated from the author's dissertation thesis defended at the Slovak University of Technology in Bratislava, Faculty of Materials Science and Technology in Trnava. Reviewers: Bernard Benko, Professor, Ph.D. Pavol Sejč, Assoc. professor, Ph.D.
Author’s contact address: Jozef Bárta, Ph.D. Slovak University of Technology in Bratislava Faculty of Materials Science and Technology in Trnava http://www.mtf.stuba.sk
© Köthen, Hochschule Anhalt (FH), 2012 2
Abstract This thesis deals with weldability of nitro-oxidized low carbon steel sheets as well as mecha
nical properties and formability of the materials treated in this way. Specimens
welded by different technologies were analysed by visual inspection, macro-analysis, microanalysis and the method of measuring the mechanical properties. The thesis compares the standard arc welding technologies with those using concentrated energy source and hybrid ones, while focusing on their ability to achieve minimum distortion of surface layer. Most technologies bring about excessive porosity which is due to the decomposition of nitrooxidised layer, irregular bead formation and excessive spatter. Laser welding seems to be the most suitable welding technology for these types of materials, as it produces the narrowest weld joints. The most stable results were achieved by using a disc laser of 1 kW power output, 20 mm/s welding speed and parallel direction of blowing the argon shielding gas with the welding head movement of 10 l/min. The weld joint structure did not show any abnormalities in phase composition, as might have been expected due to the nitrides in the surface layers of the welded materials after nitro-oxidation. Laser welding caused dissolution of the surface layer to 1 mm distance from the fusion boundary.
Key words
nitro-oxidation, micro-structure, macro-structure, mechanical properties, corrosion-resistance, welding technologies.
3
LIST OF ABBREVIATIONS CMT
– Cold Metal Transfer
GMAW
– Gas Metal Arc Welding
MIG
– Metal Inert Gas
MAG
– Metal Active Gas
GTAW
– Gas Tungsten Arc Welding
TIG
– Tungsten Inert Gas
WIG
– Wolfram Inert Gas
T
– Temperature [°C]
Rm
– tensile strength [MPa]
Re
– yield strength [MPa]
A
– Ductility [%]
KU
– impact strength [J]
I
– current [A]
U
– Voltage [V]
P
– Power [W]
HAZ
– heat-affected zone
HT HAZ
– High temperature heat-affected zone
WM
– weld metal
BM
– base metal
HV
– hardness according to Vickers
F
– Force [N]
GDOES
– Glow Discharge Optical Emission Spectroscopy
QDP
– Quantitative Depth Profiling
EDX
– Energy Dispersive X–ray Analysis
STN
– Slovak Technical Standard
EN
– European Standard
ISO
– International Standard Organization
BH
– bake hardening
DP
– dual phase
TRIP
– transformation-induced plasticity
IF
– interstitial-free
HSLA
– high-strength low alloyed 4
UV
– ultraviolet
DZ
– diffusion zone
E
– Corrosion potential
EPR
– electrochemical potentio-kinetic reactivation
5
INTRODUCTION Rapid industrial development brought about the enormous utilization of raw materials in the last decades of the previous century. This has initiated high competitiveness of the manufacturers who are trying to win in the market by pushing down the price. The low prices make customers buy new products rather than have the old ones repaired. To eliminate or at least minimize environmental impacts, the late 20th and the early 21st centuries were dedicated to a better use of raw materials mainly through recycling and efficient use of materials. People have developed the new technologies enabling to produce smaller, lighter and cheaper parts made of cheaper and available raw materials. Simultaneously, the manufacturing of stronger, more precise and powerful devices has called for better quality of material properties. To avoid the increase in size, it was necessary to modify the materials in order to meet the above-mentioned requirements. Materials can be treated by various ways in the process of both manufacturing and further treatment. The most common materials used in the industry are ferrous and non-ferrous metals, and their alloys. The application of alloying materials increases the price especially in the case of large-scale semi-products. One of the possible ways of saving resources and reducing the financial costs is surface treatment of the materials, applied mainly in the case of increased demand for the material’s properties such as corrosion resistance, hardness, strength, etc. Surface treatment of unalloyed materials, for instance, can produce a tough core and hard surface of the material. Nitrogen saturation of materials’ surface has been known for a long time. So has the surface saturation with nitrogen followed by oxidation. This process provides several benefits in terms of significantly improved corrosion resistance, strength, hardness and other properties. The material itself is of no use, however, if it cannot be further processed. In practice, materials must gain the desired shape either by forming or another process, and then be assembled into a new unit. Specific features gained by the material in the process of nitrooxidation should be attained as much as possible by the final product. It is therefore necessary to know the behaviour of such materials in the process of welding, and determine which of the wide range of welding technologies will reflect the specifications of the nitro-oxidation process. The present monograph is devoted to the nitration of DC 01 EN 10130-91 material in a fluid environment with subsequent oxidation, the analysis of its properties and its joining. 6
1.
NITROGEN SATURATION OF MATERIAL SURFACE Big attention is currently being paid to the surface treatment of materials, where good
material properties are combined with the properties of surface layers, protecting particularly the base material from the adverse influence of atmosphere (corrosion) or aggressive environs, where the chemical stability of parts is necessary. The practicability and total costs of manufacturing the parts make manufacturers replace expensive materials with those that have suitable surface treatment, good performance properties and lower costs of manufacturing. Surface treatment generally provides enhanced quality characteristics of material surface, while preserving the original material properties (strength, toughness, etc.). 1.1
Nitridation
Nitridation belongs to the field of thermo-chemical treatment of steels, applied with the aim of improving the surface quality of steel products, in particular their hardness, abrasionresistance, cyclic strength of parts, resistance to crack propagation, thermal fatigue etc. Besides the aforementioned properties, the nitrided surface of steels and alloys is distinguished by high corrosion resistance, particularly in the conditions of atmospheric corrosion, flowing water and combustion gases. The protective efficiency of nitride surfaces is substantially lower in the conditions of chlorides; the layer is not acid-resistant (4). Nature of nitriding Nitrogen on the surface immediately combines with iron, thus yielding the nitride, which transports the nitrogen under the surface by diffusion. Nitriding is usually carried out at the temperatures from 450°C to 600°C (7). The molecular form of nitrogen cannot be used for nitriding, as in contact with iron at elevated temperatures it becomes inactive and does not form a nitride. Therefore, to combine iron with nitrogen, the atomic nitrogen has to be used. The latter occurs transiently in the reactions where nitrogen is formed by splitting. The simplest reaction like that is the decomposition of ammonia: NH3 → N + 3H. Nitridation process The results of nitriding may be influenced by the steel composition, temperature, period, pre-heating treatment, dissociation of ammonia and pressure. In a regular process, conditions 7
are constant, while in special processes, the combinations of circumstances are more complicated. In a regular process, the working temperature ranges between 480 and 530°C, most frequently 480°C and 510°C, while extremely pure ammonia is used and the most appropriate degree of association (approximately 20 – 30% dissociated gas in the departing ammonia) is kept in the course of the whole process. Duration of the process ranges from 12 to 96 hrs., yet most frequently from 24 to 72 hrs. (7). Equipment for nitridation Furnaces can be classified into three categories on the basis of manipulation, construction and the method of heating. The first category comprises the furnaces for individual batches and the furnaces of semi-continual and continual performance. The second group involves the chamber, shaft and bell nitridation furnaces. The last group contains the electric resistance furnaces and the furnaces for liquid or gaseous fuel. Induction furnaces for solid fuel are not suitable for nitriding (7). 1.2
Nitro-oxidation
Nitro-oxidation (NO) in a fluid layer consists of the process of nitridation followed by the surface oxidation. The processes of nitridation and the following oxidation of construction steels are quite well-known; Degussa, a German company, provides the procedure known as TENIFER QPQ, together with the application in ammonium salts (5). The salt solution generally consists of alkali-kyanite and alkali-carbonate. It is placed in a vessel made of a special material and fitted with a venting mechanism. The active constituent in the TF 1 bath is alkali-kyanite. In the process of nitridation, the part surface reacts with alkali-kyanite, while giving alkali-carbonate. After adding an exact amount of non-toxic REG 1 regenerator, active nitridation constituents in a salt bath are activated again, while the activity of the TF 1 bath is held in very precise tolerances (16). More environment-friendly and affordable is the use of fluidized technology, which is less known. The nature of nitro-oxidation dwells in the unconventional technology of thermochemical treatment, developed in the connection with fluid technology. In nitro-oxidation, a diffusion layer containing nitrogen (so called ε – phase) is first formed on the surface. This process is followed by further oxidation in the atmosphere of distilled water vapour (or plasma). The extensive process of affecting the material surface by oxygen results into the 8
formation of an oxidic surface layer. At the end, it is possible to achieve a high degree of corrosion resistance, even in tropical conditions and the environment of salts. Nitro-oxidation has a positive impact on corrosion resistance and wear resistance of materials, while enhancing their mechanical and retaining their original forming properties. It enables non-alloyed low-carbon steels to retain their formability and enhanced strength along with the corrosion resistance, which may be acquired only by using expensive alloying materials with limited formability. Nitro-oxidation is a simple and clean process producing no waste and poisonous vapours, which is its major advantage (6). One of the advanced procedures is plasma nitridation, offered by RÜBIG, a company with its headquarters in Austria. The process is digitally controlled, which helps provide repetitiveness of the surface layers production.
9
2.
WELDING STEEL SHEETS TREATED BY NITRO-OXIDATION Nitro-oxidation-treated sheets are characterized by high corrosion resistance and
enhanced mechanical properties (surface hardness, wear-resistance and others). It is therefore desirable to preserve those material properties as much as possible in the process of welding, with a minimum influence on surface layers. 1.3
Experimental material and its properties
The
base
material
for
the
dissertation
experiment
was
DC 01 EN 10130-91
(Cr 01 ISO 17/12N49-69, 11 321.21, Tab. 1) deep drawing steel used in the automotive industry, chosen for its very good mechanical properties. The nitro-oxidation of the tested steel sheets 1 mm thick took place in a fluid furnace (fig. 1) in Kaliareň, s.r.o., Považská Bystrica. CHEMICAL COMPOSITION OF BASE MATERIAL
Table 1
Classification according to EN standard
C [max %]
Mn [max %]
P [max %]
S [max %]
Si [max %]
Al [min %]
DC 01
0.12
0.60
0.045
0.045
0.1
-
Fig. 1 Furnace used for nitridation of specimens
10
The medium used in the fluid furnace was Al2O3 with the grain size 120µm, and ammonium as the fluid environment for nitridation. Surface oxidation was carried out in the conditions of distilled water vapour.
1.3.1
Preparing and analysing the material treated by nitro-oxidation
Table
table 2 shows various parameters of thermo-
chemical treatment with varying temperature and the duration of individual operations, with or without inter-annealing and under various conditions of cooling. The data used in the initial stage of the research were later reviewed regarding weldability. HEAT TREATMENT OF SHEET SAMPLES NITRIDATION Mode
1
Oil/ air
Table 2
INTERANNEALING
OXIDATION
COOLING CONDITIONS
T
t
T
t
T
t
Air
Oil “O”
[°C]
[min]
[°C]
[min]
[°C]
[min]
[ks]
[ks]
O
540
45
-
-
380
5
5
5
V, V
540
45
-
-
380
5
3
3
O
580
45
-
-
380
5
5
5
V
580
45
-
-
380
5
2
3
O
580
45
580
60
380
5
5
5
V
580
45
580
60
380
5
2
3
O
620
45
-
-
380
5
5
5
V
620
45
-
-
380
5
2
2
A
A 2 B
3
A
The micro-structure of the specimens was affected into the depth of approximately 300µm. The affected zone can be divided into two basic sections: the compose layer (CL) about 70µm thick, consisting of a very continuous oxide layer of Fe3O4 and Fe2O3, 0.3 to 0.6µm thick (Figure 3), and the adjoining continuous layer of ε - phase (Fe2-3N) approximately 6 to 7µm thick (Figure 2). These parameters were measured on a nitride specimen at 540°C and oxidised for 5 minutes, and then air-cooled. The depths of individual layers were affected by thermo-chemical treatment, or the selected parameters of nitridation and oxidation. Fastening on ε –phase was the ferritic matrix with massive precipitations of nitrides γ´ Fe4N of acicular shape. Its thickness ranged from 25 to 30µm. Another section of the affected 11
zone was a diffusion zone (DZ) by the thickness of 100µm, formed by the ferritic matrix with precipitated fine nitrides α´´- Fe16N2 of irregular geometric shape.
Fig. 2 Nitridic ε phase on the surface of the analysed material
Fig. 3 Presence of an oxidic layer on the surface of the nitridic one
Figure 4 illustrates the interaction of dislocations and the secondary precipitated phases in CL of nitride layer, γ´ - Fe4N nitride in particular. In most cases dislocations pass through the secondary precipitated particles. However, the dislocations anchoring on the particles may be observed as well (5). 12
Micro-structure of DC 01 material post the process of nitro-oxidation
Substructure of CL–a detail of massive particle of nitride γ´ - Fe4N during interactions with dislocations
Fig. 4 Micro-structure of surface layer
Various nitridation treatments with following oxidation were reviewed after the first test of welding by CO2 laser (Tab. 30), regarding the results of welding. The behaviour of material then helped determine the material for further tests. In order to preserve uniformity and continuity of the surface layer (its repeatability), the oxidation temperature was reduced from 380 °C to 350 °C, as it was the limit for the equipment used. Nitridation was carried out at 580 °C for 45 minutes, while oxidation at 350 °C for 5 minutes with air-cooling. When observing the micro-structure of nitro-oxidised material, abnormalities in the continuity of ε phase, markedly affecting corrosion resistance, were discovered. Along with securing the continuity of ε phase, it was essential to also test the influence of the oxidic layer thickness on corrosion resistance typical for the material.
Fig. 5 Continuity of ε phase on the surface of welded material (discontinuous on the left, continuous on the right)
After enlarging the gap between individual sheet samples, the problem was eliminated. When examining the consequences of defects in the continuity of ε phase, we found it necessary to observe certain principles related to the insertion of specimens into the furnace 13
where nitridation took place. Ε phase discontinuity was due to the small gap between the sheets during nitro-oxidation, because the fluid environment was not uniformly distributed in the material surface. Magnification of the gap between the metal specimens eliminated the problem. GDOES (Glow Discharge Optical Emission Spectrometry) technology was used to measure evaporation of the material surface in the depth defined by a plasma beam, while continuously measuring the composition of individual newly revealed layers of material in very short time intervals. GDOES analysis was performed in order to check the composition of the individual elements’ content. It helped detect the changes in concentrations of the elements present from the surface of material to the depth of a few nanometres. GDOES analysis (Figure 6) confirmed the high content of oxygen on the surface, indicating the presence of the oxide layer to a thickness of approximately 0.5 mm. At this point, a noticeable increase in nitrogen content near the surface of the material in the form of nitrides was observed. High nitrogen content was observed up to the thickness of about 6-7µm, which correlates with the identified ε phase. These results confirm earlier assumptions regarding the presence of nitride and oxide layers on the surface of the original material.
14
Fig. 6 Results of GDOES analysis of the surface of nitrooxidised material
1.3.2
Mechanical properties
The static tensile test was made according to EN895 and BSEN 10002-1:2001 standards. A total of 12 specimens were divided into four groups (three specimens in each group) undergoing different treatment. Group 1 represented the pure, untreated material. Group 2 comprised of the specimens nitrided at 540°C for 45 minutes, and then oxidised at 380°C for
15
5 minutes. The specimens in group 3 were treated by oxidation for 10 minutes, while those in group 4 for 15 minutes. The test results (Table 3) showed a 52 to 54% increase in yield strength of group 2 (540 °C/45 min nitriding, oxidation 380 °C/5 min).
VALUES MEASURED IN STATIC TENSILE TEST
Table 3
Group
Re [MPa]
Rm [MPa]
A [%]
1
200
282.3
31.2
2
308
377.3
23.7
3
304
400.7
23.7
4
307
383
23.3
Unlike in the case of raw material, the course of the tensile test of nitro-oxidised material exhibits a significant yield value, as shown in Figure 7. The value of the yield strength increased from the original value of the basic raw material by 34 to 42%. The ductility of the material decreased after nitro-oxidation by about 7.5%. The differences in ductility between the heat-treated groups were insignificant, in the range of 0.4 %.
Fig. 7 Courses of the static tensile test in dependence on the surface treatment (41) Also observed was a reduction of deepening “h” by 15% in the Erichsen test (Figure 8), and a slight increase in the deep drawing coefficient “m” of 1.5% in the Fukui test. Both the Erichsen and the tensile tests were performed on several treated surfaces (non-
16
treated, treated by sand-blasting and thermo-chemical treated by nitro-oxidation), to compare the strength of steel, depending on the treatment process (Figures 9 and 10).
Fig. 8 Fracture of sand-blasted specimen in the Erichsen test Figures 9 and 10 graphically summarize the results of the Erichsen test and the static tensile test on experimental specimens after the application of various surface treatments (sand-blasting, nitro-oxidation, sand-blasting + nitro-oxidation). It was found that sandblasting increased the strength by 6.5%, while nitro-oxidation increased the strength by 15%.
Fig. 9 Results of the Erichsen test in dependence of the surface treatment of a specimen
17
Fig. 10 Comparison of the mechanical properties of specimens treated by various surface treatment
More intensive deformation led to the increased damage of the ε - phase layer (yet only the porous surface part of the phase exhibited increased brittleness). The occurrence of microcracks or other imperfections (that might be supposedly due to the presence of hard phases) was not detected in the NO layer formed by a ferritic matrix with excluded γ'- Fe4Nor α'' - Fe16N2 nitrides. This suggests that the precipitated nitrides neither had a significant impact on the change of micro-structure in forming processes, nor initiated the distortion in the surface area of the material after NO. The nitride layer is in a state of intensive inner compressive stress which, acting in the direction opposite to tensile strain, results in delayed cracking in the process of forming (5). The surface layer was further analyzed in order to determine the hardness of a very thin oxide layer. The results of micro-hardness measurements of HVM 0.01 steel after nitrooxidation are shown in Figures 11 and 12. The maximum value of 1130 HV 0.01 was measured in the highest possible measurable position. Owing to the thickness of the oxide layer, the measurement should be repeated by using the device of a lower load and a higher
18
accuracy. When compared to the material without thermo-chemical treatment, the micro-
HVM0.01
hardness on the material surface increased by 653%.
Distance from surface [µm]
Fig. 11 The course of hardness from the surface towards the core of the material
Fig. 12 The location of imprints at measuring the micro-hardness of NO material
1.3.3
Corrosion resistance
Examined were six specimens of metal sheets 50x100x1 mm of all the above-mentioned types. The specimens marked by numbers from 1 to 5 differed in the way of surface treatment, i.e. their pre-treatment prior to chemical heat treatment. The specimens marked by number 6 were untreated.
19
Treatment of individual specimens was as follows: 1. 2. 3. 4. 5. 6.
Sand-blasted Nitrided Nitro-oxidised (oxidation for 5 min) Nitro-oxidised (oxidation for 10 min) Nitro-oxidised (oxidation for 15 min) Base material
The corrosion properties of the materials supplied were examined by an EPR test, a resistance test in the condensation chamber without the presence of sulphides, and by an immersion test in 3% NaCl. The specimens were cleaned by methyl alcohol. 1.3.3.1 The method of electrochemical potentio-kinetic reactivation (EPR)
Corrosion resistance of the samples was examined by measuring the corrosion potential in three model environments of graded aggressiveness. Using the AGILENT 344406A digital multimeter, the free potential against the saturated calomel electrode in different environments was measured. The values were recorded after 30 and 60 minutes of immersing the specimens in the related electrolyte. The tests were carried out at the laboratory temperature, without the movement of the corrosion environment. The potentials measured in distilled water are shown in Figure 13, those in 1% Na2SO4 in Figure 14, and in 3% NaCl in Figure 15. All the measured values exhibited a slight tendency in time towards the negative values. The least resistant surface in all three applied environments was that of specimen No. 6. The surfaces of the specimens after thermochemical treatment were cathodic regarding the substrate. The corrosion potentials measured after 60 minutes of exposition allow us to determine the galvanic series of the examined specimens as follows: distilled water: No. 4, 1, 5, 3, 2, 6 1 % Na2SO4:
No. 4, 3, 5, 1, 2, 6
3 % NaCl:
No. 2, 4, 3, 5, 1, 6
20
Fig. 13 Results of the potential analysis of specimen in distilled water
Fig. 14 Corrosion potentials of specimen in 1% Na2SO4
21
Fig. 15 Corrosion potentials of specimen in 3% NaCl
In the case of conductive connection between materials with different corrosion resistance, the value of ∆E of the difference in their corrosion potentials in a given ∆E environment is critical from the point of possible damage of a less resistant component of the resulting corroded bimetallic element. It is well-known that the technically significant corrosive damage to the anode of the element occurs if ∆E ≤ 500 mV (in a non-aggressive electrolyte), if ∆E ≤ 250 mV (in a slightly aggressive electrolyte), and if ∆E ≤ 150 mV (in a highly aggressive electrolyte). OVERVIEW OF MEASURED ∆E OF BIMETALLIC COUPLES OF SPECIMENS Table 4 ∆E [mV] Bimetallic couple of specimens distilled water
1 % Na2SO4
3 % NaCl
6-1
350
355
211
6-2
203
356
376
6-3
270
458
396
6-4
391
464
433
6-5
293
388
344
≤ 500
≤ 250
≤ 150
Safe bimetallic contact
22
Table 4 shows the ∆E bimetallic couples composed of specimens 6 and 1 – 5, as determined on the basis of the potential analysis of individual specimens. Specimen No. 6, i.e. anode, always seems to be less resistant. The measurements also indicate that the resulting bimetallic elements in 1% Na2SO4 and in 3% NaCl are not “corrosion safe”. In the case of surface treated specimens, the corrosion attack could be observed in the parts where the surface layer discontinuity was observable. This confirms our supposition regarding the poor corrosion resistance of the weld joints. 1.3.3.2 Test of corrosion resistance in the condensation chamber
The test was carried out in the KB 300 condensation chamber of 43096101 type at the temperature of 35 ± 0,1 °C and in the conditions of 100% humidity (distilled water). The corrosion damage was assessed after 16, 48, 72, 144 and 240 hours of exposition of the specimens by using gravimetric analysis. The analytic scale KERN ATL 220-5 DAM was used. The weight was determined with the accuracy of ± 0.00001g. After 240 hours of testing, the specimens were evaluated regarding the degree of corrosion attack according to STN EN ISO 10289/C standards. Five pieces of each type of the supplied specimens were tested and analyzed using the above-mentioned procedure. The results of gravimetric analysis proved that, within the course of the test, the specimens recorded the weight increase which was due to the formation of a corrosion layer on their surface. Table 5 shows the data calculated for a surface unit. RESULTS OF GRAVIMETRIC ANALYSIS OF THE SAMPLES EXPOSED IN THE CONDENSATION CHAMBER
Table 5
Specimen No. Exposition [h]
1
2
3
4
5
6
Weight increase [g.m-2] 16
0.0388
0.0854
0.0757
0.0815
0.2039
0.0505
48
0.0698
0.0951
0.0834
0.1125
0.2719
0.1804
72
0.1106
0.1183
0.1086
0.1533
0.3107
0.6383
144
0.2134
0.1339
0.1242
0.1707
0.3592
6.9918
240
0.2755
0.1377
0.1280
0.3026
0.3690
8.4890
23
The determined changes in weight revealed that, except for shorter expositions, specimen No. 6 exhibited the greatest change in the course of the test. The weight changes (expressed in %) caused by corrosion of samples 1 to 5 after 240 hours of exposition are summarized in Table 6. RELATIVE WEIGHT INCREASE OF THE SPECIMENS, EXPOSITION 240 HOURS
Table 6
Specimen No. 1
2
3
4
5
6
1.51
3.56
4.35
100
Weight increase [%] 3.25
1.62
The degree of the corrosion attack which was manifested by the occurrence of a red corroded
material
on
the
surface
of
specimens
was
determined
regarding
STN EN ISO 10289/C standards. Results are shown in Table 7. EVALUATION OF SPECIMENS AFTER THE TEST IN THE CONDENSATION CHAMBER Table 7 Specimen No.
Scale of corrosion attack [%]
Degree of corrosion attack[C]
1
16
3
2
0.25
9
3
0
10
4
7
4
5
0.25
9
6
82
0
1.3.3.3 Test of corrosion resistance in 3% NaCl
Specimens were tested by immersion in 3% NaCl solution at room temperature and with no change of the corrosive environment. After 24, 48, 72, 96 and 120 hour exposure, the corrosion attack was evaluated by gravimetric analysis, and, after the completion of the test, also according to EN ISO 10289 / C standards.
24
In the course of the test in 3% NaCl, all exposed specimens exhibited weight losses. The results showing the loss of weight are listed in Table 8. Specimen No. 6 exhibited the least corrosion resistance in this test. Table 9 shows that the differences between the surface-treated specimens No 1-5 on one hand, and sample No. 6 on the other hand were less distinct than in the case of their exposure in condensation chamber, which is attributed to the aggressiveness of the electrolyte. RESULTS OF GRAVIMETRIC ANALYSIS OF SPECIMENS EXPOSED TO 3% NaCl Table 8 Specimen No. 1
Exposition
2
3
4
5
6
Weight loss [g.m-2] 24
0.0291
0.0291
0.1068
0.1457
0.0097
0.6117
48
0.2622
0.2136
0.2039
1.0196
0.2330
1.7866
72
1.3594
1.2526
1.6604
2.0488
0.9613
2.8742
96
2.3207
2.1750
3.2722
3.7675
1.5924
5.2046
120
2.8742
2.7868
3.6121
3.8160
2.2916
6.3115
RELATIVE WEIGHT LOSS OF SPECIMENS, EXPOSITION FOR 120 HOURS Table 9 Specimens No. 1
2
3
4
5
6
44.15
57.23
60.46
36.31
100
Weight loss [%] 45.54
The appearances of specimens 1 – 6 after the test are documented in Figure 16. The results quantifying the corrosion attack on both sides of each specimen are summarised in Table 10. Note that, given the number of samples tested, the abovementioned results can be regarded as tentative.
25
Fig. 16 Appearance of specimens after the test in 3% NaCl. From left to right specimens No. 1 – 6
EVALUATION OF SPECIMENS AFTER THE TEST IN 3% NaCl Table 10 Scale of corrosion
Degree of corrosion
attack
attack
[%]
[C]
1
27
2
2
0
10
3
0
10
4
24
2
5
0.25
9
6
100
0
Specimen No.
In conclusion we can say that the highest corrosion resistance can be assigned to specimen No.3. The parameters of its nitro-oxidation (nitridation 580°C/45 min, oxidation 350°C/5 min) proved to the best surface treatment for corrosion resistance of all the specimens tested. When comparing corrosion properties of the thermo-chemical treated specimens with those of base/untreated material, we can state that: - Specimens with thermo-chemical treatment exhibited higher electro-chemical resistance than those untreated, in all the model testing situations; - The differential value of corrosion potentials of samples 1 – 6, 2 – 6, 3 – 6, 4 – 6, 5 – 6 determined for aggressive model electrolytes was higher than that acceptable for a safe bimetallic joint. To eliminate the bimetallic corrosion damage of steel, it is therefore necessary to provide a perfect continuity and failure-free performance of the applied surface treatment; 26
- All surface-treated specimens showed a corrosion resistance higher than that of untreated samples. When tested in the condensation chamber, their weight gain represented from 1.51 to 4.35% of the weight gain of untreated samples. The screening test in 3% NaCl revealed a lower difference, yet exhibiting 36.31 to 60.46% of the weight loss of the untreated sample; - The instant corrosion rate in various corrosive conditions can be determined by measuring and evaluating the potentio-dynamic polarisation curves, quantified corrosion flows and polarisation resistance of samples, or using other electrochemical procedures.
1.3.4
Physical and thermo-physical properties
For more thorough knowledge of basic material, some physical properties were also studied. Thermal conductivity was investigated, volume specific heat capacity and thermal diffusivity. The measurement was conducted on a metal sheet by the thickness of 1mm, using the ISOMET model 104 device made by the Applied Precision Company. Five measurements were made on each specimen. The differences in values of the individual specimens were insignificant. Table 11 shows the mean values recorded in all measurements. This measurement is mainly used in the development and production of new materials, control of process parameters etc. The measured values also affect the interaction of material with a laser or electron beam during the welding process. RESULTS OF THERMO-PHYSICAL TESTS
1.4
Table 11
λ
Thermal conductivity
3.595 [W.m-1.K-1]
c
Volume specific heat capacity
2.411 [J/m3.K]
a
Thermal diffusivity
1.716 [m2.s-1]
T
Mean temperature during the measurement
26.75 [°C]
MAG welding Since the examined material was a commonly used construction steel with surface
treatment, the initial tests focused on traditional welding methods such as MAG (Metal Active Gas) welding. This technology has been on the market for a very long time, and is supposed
27
to have been thoroughly studied, which can help in the identification of suitable welding parameters.
1.4.1
Parameters of welding
The welding was conducted according to the parameters listed in Table 12 using the equipment made by the Fronius Company with the specifications as shown in Table 13. The power source is commonly used for CMT welding, but can be adjusted to the standard MAG welding regime. PARAMETERS OF MAG WELDING
Table 12
Specimen
Welding speed (mm/s)
Voltage (V)
Current (A)
Feed rate of wire (m/min)
Correction of the arc length (%)
Welding gap (mm)
R 01
15
14.8
36
2.3
0
0
R 02
10
14.8
36
2.3
0
0.4
R 03
7
14.8
36
2.3
0
0.4
R 04
10
14.8
36
2.3
0
0.6
R 05
10
15.8
53
3
0
0.4
R 06
15
16.2
53
3
0
0.4
R 07
10
17.4
48
3
+10
0.4
R 08
15
17.7
51
3
+10
0.4
R 09
15
20.6
51
3
+30
0.4
R 10
10
14.3
51
3
-10
0.4
R 11
10
15.8
53
3
0
0.4
EQUIPMENT USED FOR MAG WELDING Welding device
Fronius VR 7000 CMT
Manipulator of torch
Plasma cutter 6020 Combi
Position of welding
Table 13
PA
Gas flow rate
Corgon 18 (82 % Ar, 18 % CO2 recommended by the manufacturer of filler metal) 13l/min
Filler metal
SG2
Wire diameter
0.8 mm
Fore-blow, after-blow
0.3 s
Protective gas
28
1.4.2
Analysis of the weld joints In most cases, a root with incomplete penetration was observed, which was due to the
insufficient energy supplied to the welding spot; the joints were therefore discarded. The other observed defects of weld joints were irregularities in the bead appearance and increased porosity observable with the naked eye. SURFACE OF SPECIMENS PREPARED BY MAG WELDING Code of specimen
Surface of the weld joint
Table 14
Note Weld joint exhibited increased porosity of the weld metal and the lack of penetration in the weld root.
R06
The specimen exhibited irregular appearance of the weld bead, increased porosity of the weld metal and the lack of penetration in the weld root. R08
The specimen exhibited excessive spatter (easily removable thanks to surface layer), irregular appearance of the weld bead and burn through. R09
All samples exhibited increased porosity observed on the surface of the weld in the weld metal. Samples with irregular bead appearance showed an enormous damage of the surface layer. After determining the parameters at which the desired penetration and a continuous
29
weld surface without significant porosity were achieved, the sample was subjected to macroscopic and microscopic analyses. APPEARANCE OF R 07 WELD JOINT SUBJECTED TO MACRO-ANALYSIS AND MICROANALYSIS
Table 15
Weld surface
5 mm
Weld root
5 mm
Weld macroscopy
1 mm
The Table above proves the existence of welding parameters enabling the achievement of a suitable appearance of both the weld bead and the weld root. The surface of weld bead indicated the increased porosity which was proved in the majority of cross-sections. The width of HAZ was approximately 1 mm from both sides of the joint, while the total joint width (together with HAZ) was approximately 5.7mm.
30
Fig. 17 Micro-structure of weld metal
Fig. 18 Micro-structure of weld metal
Figures 17 and 18 characterise the micro-structure of WM consisting of mainly acicular and polygonal ferrite precipitated along the boundaries of columnar grains. Sporadically, coarse acicular ferrite and upper bainite occurred.
Fig. 19 Transition of WM-HTHAZ-HAZ
Fig. 20 Micro-structure of HTHAZ
In HT HAY a significant increase in the size of grain was observed (Figure 19). Micro-structure (Figure 20) consisted mainly of acicular and polyhedric ferrite, while coarse acicular ferrite was observed occasionally.
Fig. 21 Micro-structure of LTHAZ
Fig. 22 Transition of LTHAZ-BM
31
LTHAZ has a fine-grained structure (Figure 21) consisting of polygonal grains of ferrite, while the annealing of preferentially oriented grains of the base material (after rolling) occurred. Figure 22 documents the transition between LTHAZ and BM. The BM microstructure consists of polyhedric ferrite, while tertiary cementite is precipitated along the grain boundaries. The structure is of the deformation texture, which is associated with the technology of the steel sheets’ production. The structure of the joint is similar to the untreated weld joints prepared by MAG technology. However, the appearance of macro-structure exhibits enlarged grains in the weld metal, which is likely due to the lower welding speed. Evidently, it is necessary to supply more heat to the weld, as the heat in the initial phase is consumed to distort the resistant nitrooxidised layer. Since in the majority of cases the samples showed irregular bead shape, excessive distortion of surface layer in HAZ area, and spatter and increased porosity observed on all samples, the findings indicate that MAG technology is unsuitable for welding. The hardness measurement identified the maximum hardness of 257.2 HV 0.1 measured in the weld metal, which is almost double the hardness of the base material, where the lowest hardness of 134.7 HV 0.1 was measured.
Fig. 23 Measurement of hardness on the specimen R 07
32
1.5
CMT welding, CMT brazing, hybrid welding
Based on the results of previous technologies, we tested the technology of “Cold Metal Transfer” with the supposed reduction of the heat supplied into the weld. CMT technology was employed to prepare three sets of samples: the first group of samples was welded/soldered in the Fronius Slovakia s.r.o. company, while the other two in the First Welding Company, a.s., a total of 56 samples were prepared. The most frequent defects detected by visual inspection were porosity on the surface, the joint root, as well as across the joint, and incomplete penetration and irregular surface of both the weld joint and the root of the weld joint. The samples matching the visual inspection were subjected to macroscopic and microscopic analyses. For the reason of comparison, the untreated sample was also analysed.
1.5.1
Welding and brazing
The prepared samples can be divided into three groups. Group 1 comprised of the samples prepared by the CMT method (Fig. 24). Group 2 contained the samples made by the CMT brazing, and Group 3 comprised of samples made by hybrid arc brazing with laser support.
Fig. 24 CMT welding/brazing in Fronius s.r.o. company
The geometric configuration of the hybrid CMT - laser brazing is shown in Figure 25. Since the available CO2 laser was not suitable for melting the copper and copper alloys, the main objective was to use a laser beam to damage the surface layer. The setup of the laser head jacked with the CMT welding torch can be seen in Figure 25. The welding direction is given by the procedure of individual technologies, i.e. the laser beam always proceeds first in 33
welding direction. Figure 25 also illustrates the angle of the laser beam incidence, the slope of the CMT nozzle and the distance between the laser beam interaction with the material from the welding spot.
Fig. 25 Geometric configuration of laser technology head and CMT burner in hybrid brazing, inclination angle of CMT nozzle α=50°, inclination angle of laser head β=30°
The parameters of welding for all the samples are given in Tables 18 – 20. Subsequently, the specimens with a positive evaluation were marked green in the tables. For discarded specimens, reasons of rejection were provided. A total of five samples were prepared by CMT brazing, four by CMT methods and two by hybrid CMT - laser brazing. A wire OK AristoRod 12:50 diameter 1mm was used as the alloying material in case of the CMT method, while the bronze CuSi3 braze was used in the case of the CMT brazing and the hybrid CMT - laser brazing. The chemical composition of the alloying materials is given in Tables 16 and 17.
CHEMICAL COMPOSITION OF OK ARISTOROD 12.50 WIRE C [%] 0.10
Si [%] 0.90
Mn [%] 1.50
CHEMICAL COMPOSITION OF CuSi3 WIRE Cu [%] 96.0
Si [%] 3.0
Mn [%] 1.0
34
Table 17
Table 16
WELDING PARAMETERS OF SPECIMENS
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 17 18 19 20 21 22 23 24 25 26
Parameters of thermo-chemical treatment No surface treatment No surface treatment No surface treatment N:580/45 O:350/10 N:580/45 O:350/10 N:580/45 O:350/10 N:580/45 O:350/10 N:580/45 O:350/10 N:580/45 O:350/5 N:580/45 O:350/5 N:580/45 O:350/5 N:580/45 O:350/15 N:580/45 O:350/15 N:580/45 O:350/15 N:580/45 O:350/5 N:580/45 O:350/5 N:580/45 O:350/5 N:580/45 O:350/5 N:580/45 O:350/5 N:580/45 O:350/15 N:580/45 O:350/15 N:580/45 O:350/5 N:580/45 O:350/5 N:580/45 O:350/15 N:580/45 O:350/15
39 40 41 44 45
N:580/45 O:350/15 N:580/45 O:350/15 N:580/45 O:350/5 N:580/45 O:350/15 N:580/45 O:350/10
Spec. No.
Table 18
35
9.2 9.2 9.2 9.2 8.9 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.3 9.8 9.8 9.8 9.8 10.9 10.9 10.9 10.9
Wire feed [m.min-1 ] 5.5 5.5 5.5 5.5 5.0 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.2 5.5 6.0 6.0 6.0 6.0 4.5 4.5 4.5 4.5
Welding speed [mm.s-1 ] 16.67 20.00 20.00 20.00 20.00 20.00 20.00 30.00 25.00 20.00 25.00 30.00 20.00 25.00 30.00 20.00 20.00 20.00 20.00 20.00 20.00 20.00 25.00 25.00 30.00
10.9 11.5 11.5 11.1 12.2
4.5 5 5 5 6
20.00 20.00 20.00 20.00 20.00
Current [A]
Voltage [V]
103 103 103 103 82 86 86 86 86 86 86 86 86 86 86 86 94 103 103 103 103 107 107 107 107 107 120 120 158 179
Way of welding/brazing
Protective gas
Additive material
MIG brazing MIG brazing MIG brazing MIG brazing MIG brazing MIG brazing MIG brazing MIG brazing MIG brazing MIG brazing MIG brazing MIG brazing MIG brazing MIG brazing MIG brazing MIG brazing MIG brazing MIG brazing MIG brazing MIG brazing MIG brazing MAG welding MAG welding MAG welding MAG welding
Argon Argon Argon Argon Argon Argon Argon Argon Argon Argon Argon Argon Argon Argon Argon Argon Argon Argon Argon Argon Argon Corgon18 Corgon18 Corgon18 Corgon18
CuSi3 CuSi3 CuSi3 CuSi3 CuSi3 CuSi3 CuSi3 CuSi3 CuSi3 CuSi3 CuSi3 CuSi3 CuSi3 CuSi3 CuSi3 CuSi3 CuSi3 CuSi3 CuSi3 CuSi3 CuSi3 OK 12.50 OK 12.50 OK 12.50 OK 12.50
MAG welding MAG welding MAG welding MAG welding MAG welding
Corgon18 Corgon18 Corgon18 Corgon18 Corgon18
OK 12.50 OK 12.50 OK 12.50 OK 12.50 OK 12.50
Note: discard reason – Irregular shape of braze Excessive braze reinforcement – Excessive braze reinforcement Excessive braze reinforcement Low parameters Low parameters Pores Pores Pores – Irregular shape - wettability Irregular shape - wettability Pores Unsuitable parameters Wrong setup of sheets – – – – – Incomplete penetration Incomplete penetration + Pores Incomplete penetration + micro pores Pores Pores Pores Excessive weld reinforcement Pores
Spec. No. 46
Wire Parameters of Current Voltage feed thermo-chemical [A] [V] treatment [m.min-1 ] N:580/45 O:350/10 158 11.1 5 – specimens selected for evaluation N: – nitridation (°C/min.), O: – oxidation (°C/min.)
Welding speed [mm.s-1 ] 20.00
Way of welding/brazing
Protective gas
Additive material
MAG welding
Corgon18
OK 12.50
PARAMETERS OF WELDING THE SPECIMENS (CMT AND CMT BRAZING)
10.1
Wire feed [m.min-1 ] 3.6
Welding speed [mm.s-1 ] 7
08/1
08/5
N:580/45 O:350/5
92
10.1
3.6
7
MAG welding
Corgon18
OK 12.50
2/1 3/1
N:580/45 O: – N:580/45 O: –
98 98
12.8 12.8
5.0 5.0
21 20
MAG welding MAG welding
Corgon18 Corgon18
OK 12.50 OK 12.50
3/2 4/1 5/1
N:580/45 O: – N:580/45 O: – N:580/45 O: –
98 120 120
12.8 11.5 11.5
5.0 5.0 5.0
21 20 20
MIG brazing MAG welding MAG welding
Argon Corgon18 Corgon18
CuSi3 OK 12.50 OK 12.50
6/1 2/2 4/2
N:580/45 O: – N:580/45 O: – N:580/45 O: –
92 72 100
10.1 11.6 16.5
3.6 4.3 5.0
20 20 20
MAG welding MIG brazing MIG brazing
Corgon18 Argon Argon
OK 12.50 CuSi3 CuSi3
5/2 13/1
N:580/45 O: – N:580/45 O:350/5
88 90
9.0 11.4
5.3 3.6
20 20
MIG brazing MAG welding
Argon Corgon18
CuSi3 OK 12.50
15/1
N:580/45 O:350/5
89
10.4
3.6
33
MAG welding
Corgon18
OK 12.50
Current [A]
Voltage [V]
92
Pores
Table 19
Parameters of thermo-chemical treatment N:580/45 O:350/5
Spec. No.
Note: discard reason
Way of welding/brazing
Protective gas
Additive material
MAG welding
Corgon18
OK 12.50
Note: discard reason
36
Increased porosity in the region of weld root and excessive amount of weld metal Increased porosity in the region of weld root and excessive amount of weld metal Extreme porosity Increased porosity in the region of weld root Increased porosity of joint – Incomplete penetration of root section – Increased porosity of joint Porosity in the region of weld root, discontinuity of weld bead – Increased porosity in the region of weld root Incomplete penetration of root section (due to big weld gap). Porosity
16/1
Parameters of thermo-chemical treatment N:580/45 O:350/5
04L 24/2
N:580/45 O: – N:580/45 O:350/10
88 88
9.0 9.0
5.3 5.3
20 20
MAG welding MAG welding
Corgon18 Corgon18
OK 12.50 OK 12.50
23/1
N:580/45 O:350/10
88
9.0
5.3
20
MAG welding
Corgon18
OK 12.50
24/1 05
N:580/45 O:350/10 N:580/45 O: –
88 92
9.0 10.1
5.3 3.6
20 15
MAG welding MAG welding
Corgon18 Corgon18
OK 12.50 OK 12.50
N:580/45 O: –
81
11.3
3.9
7
MAG welding
Corgon18
OK 12.50
Spec. No.
09
10.1
Wire feed [m.min-1 ] 3.7
Welding speed [mm.s-1 ] 10
Current [A]
Voltage [V]
90
Way of welding/brazing
Protective gas
Additive material
MAG welding
Corgon18
OK 12.50
Note: discard reason Incomplete penetration of root section – Discontinuity and porosity of weld bead Increased porosity of weld, irregular appearance of bead Increased porosity of weld Increased porosity of weld, irregular appearance of bead Excesssive weld reinforcement and high amount of filler material
– specimens selected for evaluation N: – nitridation (°C/min.). O: – oxidation (°C/min.)
37 WELDING PARAMETERS OF SPECIMENS (HYBRID CMT – LASER BRAZING) Spec. No.
Parameters of thermo-chemical treatment
Curre nt [A]
Voltage [V]
Wire feed [m.min-1 ]
CMT Welding speed [mm.s-1 ]
Table 20 Laser
Protectiv e gas
Additive material
P [kW]
Protective gas
Welding speed [mm.s-1 ]
11/1
N:580/45 O:350/5
72
8.3
3.6
45
Argon
CuSi3
4
Helium
45
11/2
N:580/45 O:350/5
72
8.3
3.6
45
Argon
CuSi3
4
Argon
45
12/1 13/1
N:580/45 O:350/5 N:580/45 O:350/5
72 72
8.3 8.3
3.6 3.6
45 45
Argon Argon
CuSi3 CuSi3
4 4
Argon Argon
45 35
14/2
N:580/45 O:350/5
72
8.3
3.6
35
Argon
CuSi3
4
Argon
35
18/1
N:580/45 O:350/5
88
9.0
5.3
30
Argon
CuSi3
3
Argon
30
Note: discard reason Nitro-oxidation-treated weld surfaces Incomplete penetration of root section – Discontinuity of weld bead Discontinuity of weld bead, Increased porosity b=5.increased porosity of weld, irregular appearance of weld bead
38
19/1
N:580/45 O:350/5
88
9.0
5.3
30
Argon
CuSi3
3
Argon
30
19/2
N:580/45 O:350/5
88
9.0
5.3
30
Argon
CuSi3
3
Argon
30
20/1
N:580/45 O:350/5
88
9.0
5.3
25
Argon
CuSi3
3
Argon
25
21/1
N:580/45 O:350/5
88
9.0
5.3
20
Argon
CuSi3
2
Argon
20
21/2
N:580/45 O:350/5
82
8.9
5.0
25
Argon
CuSi3
2
Argon
20
25/2
N:580/45 O:350/15
88
9.0
5.3
30
Argon
CuSi3
4
Argon
30
20/2
N:580/45 O:350/5
88
9.0
5.3
30
Argon
CuSi3
3
Argon
30
17/1
N:580/45 O:350/5
72
8.3
3.6
35
Argon
CuSi3
3
Argon
35
18/2
N:580/45 O:350/5
72
8.3
3.6
45
Argon
CuSi3
4
Argon
30
– specimens selected for evaluation N: – nitridation (°C/min.). O: – oxidation (°C/min.)
b=5, discontinuity of weld bead, increased porosity b=5, CMT b=5, increased porosity of weld, irregular appearance of weld bead b=5, irregular appearance of weld bead, increased porosity b=5, Incomplete penetration of root section, porosity of weld bead Increased porosity of weld, irregular appearance of weld bead b=5, irregular appearance of weld bead, local incomplete penetration of root section Incomplete penetration of root section increased porosity, irregular appearance of weld bead b=5, increased porosity of weld, irregular appearance of weld bead
JOINTS PREPARED BY CMT WELDING AND CMT BRAZING
Joint prepared by CMT brazing (specimen No. 5/2)
Table 21
Joint prepared by CMT method (specimen No.23)
Joint prepared by CMT method (specimen No. 4/1)
Joint surface
5 mm
2 mm
5 mm
4 mm
39
3mm
Joint root
Joint macroscopy
4 mm
JOINTS PREPARED BY CMT BRAZING AND HYBRID METHOD OF JOINING Joint prepared by CMT method (specimen No. 6/1)
Table 22
Joint prepared by hybrid CMT - laser brazing (specimen No. 12/1)
Joint prepared by hybrid CMT - laser brazing (specimen No. 11/1)
Joint surface
4 mm
3 mm
4 mm
40 Joint root
4 mm
Joint macroscopy
3 mm
4 mm
1.5.2
Micro-structural analysis and mechanical properties of joints
Micro-structure of the CMT weld joint of 4/1 specimen shown in Fig. 26 consists of acicular ferrite, with polygonal-ferrite precipitated along the columnar grains boundaries. In rare cases, Widmanstätten ferrite was observed.
acicular ferrite
polygonal ferrite Widmanstätten ferrite Fig. 26 Micro-structure of weld metal (specimen No. 4/1)
Figure 27 illustrates transition of the weld metal into the ultra-heat affected zone. HT HAZ consists of upper bainite with precipitated Fe3C icicles, coarse acicular ferrite and proeutectoid ferrite precipitated along the boundaries of the original austenitic grain. In the HT HAZ, coarsening of the original austenitic grain could be observed, which is typical for all conventional joining technologies; it occurs owing to the longer cooling of the joint after the process of welding.
41
Fe3C
HT HAZ
WM
upper bainite
proeutectoid ferrite
thick acicular ferrite Fig. 27 HT HAZ – WM transition (specimen No. 4/1) Transition between the inter-critical HAZ and the base material is shown in Figure 28. HAZ consists of a fine-grain polyhedric structure of ferrite. It is an annealed zone, where grain refinement and re-orientation of the originally oriented grains (after rolling) to a regular polygonal shape can be observed. BM
HAZ
Fig. 28 BM – HAZ transition (specimen No. 4/1)
42
1
3
5
7
2
4
6
8
9 10
11 12
Fig. 29 Zone of micro-hardness measurement (specimen No. 4/1)
Figure 29 illustrates the arrangement of imprints in the specimen cross-section. The values measured by Buehler IndentaMet 1100 Series durometer are shown in Table 23. Loading force of the diamond pyramid with the top angle 136° was 0.98 N for all cases, with the holding time of 10 s. VALUES OF MICRO-HARDNESS (specimen No. 4/1) No. of measurement
Table 23
1
2
3
4
5
6
7
226.7
221.3
225.9
194.8
157.6
162.3
136.9
0.00
0.25
0.50
0.75
1.00
1.25
1.50
8
9
10
11
12
13
Measured hardness HV 0.1
131.9
127.1
129.8
116.8
120.1
122.2
Distance of imprint from the centre [mm]
1.75
2.00
2.25
2.50
2.75
3.00
Measured hardness HV 0.1 Distance of imprint from the centre [mm] No. of measurement
Fig. 30 Course of micro-hardness (specimen No. 4/1)
43
The highest micro-hardness value (226.7 HV 0.1) was measured in the middle of the weld joint (in weld metal), while the lowest one (116.8 HV 0.1) on the HAZ/base metal interface. Strength of the joints welded by CMT brazing (Figure 31) was found to be higher (312MPa) than that of the base material, even despite the porosity occurring in the joint. Higher strength of the joint is probably associated with the excessive amount of braze. Laser beam made the braze bind better to the base material in comparison with the CMT brazing (Figure 32), where the joint disrupted (300MPa).
Fig. 31 Specimen No. 5/2 after static tensile test
Fig. 32 Specimen No. 12/1 after static tensile test
1.5.3
Evaluation of the joints made by CMT technology
When compared with conventional methods, CMT welding (brazing) was preferred for its technological capability to produce less heat in the spot of welding, which might reduce the distortion of nitro-oxidic surface layer and improve corrosion resistance of the joints welded. All the above mentioned three joining processes (welding, brazing, hybrid joining methods) exhibited imbalanced results. First, excluded were the joints that did not meet the
44
criteria of visual inspection. Many joints were unsuitable mainly due to increased porosity. Tables 21 and 22 list the specimens that passed the visual test. Macro-structures of the joints exhibited excessive occurrence of pores. Another defect was due to the excessive weld reinforcement, i.e. excessive amount of filler material supplied to the weld. To eliminate the defects, we adjusted the parameters of welding speed and wire feed, and used a thinner wire (Ø 0.8 mm instead of Ø 1.0 mm). The tests however did not approve the application of the technology, as it was impossible to determine stable parameters for the process of joining that would provide a weld with suitable bead appearance, acceptable form of the weld joint cross-section and eliminated porosity. One of the undesirable effects of the welding process was the distortion of surface layer (peeling), which was due to thermal impact on the surface of the base material (Table 21, specimen 5/2). Since this defect was occurring repeatedly in several joints, we assumed that CMT brazing insufficiently limits the amount of heat introduced into the weld, thereby peeling the coating and increasing thus the areas susceptible to corrosive attack. 1.6
Hybrid welding by TIG and laser beam
Another conventional welding technology is TIG (Tungsten Inert Gas), i.e. welding by using a tungsten electrode that does not melt. Since there were no substantial differences between TIG technology and MAG welding, the former was combined with welding by using CO2 laser beam. 1.6.1
Parameters of welding
Hybrid TIG-laser welding technology was applied with the parameters as shown in Table 24. The samples for further analyses were chosen by visual inspection. PARAMETERS USED IN HYBRID GTAW WELDING + LASER WELDING
Table 24
Specimen No.
Izv. TIG [A]
Uzv. TIG [V]
Laser power [kW]
vz [mm/s]
focus
Note
39/2
115
21.5
1.5-1.6
50
2
Unsuitable joint
32/1
120
22
1.5-1.6
50
2
Unsuitable joint
32/2
120
22.8
1.5-1.6
45
2
Suitable joint
32/3
118
21.5
1.5-1.6
45
2
Unsuitable joint
32/4
116
21.8
1.5-1.6
45
2
Suitable joint
45
Specimen No.
Izv. TIG [A]
Uzv. TIG [V]
Laser power [kW]
vz [mm/s]
focus
Note
T+L/1
116
23.5
1.5-1.6
45
2
Suitable joint, untreated sheet
22/1
116
22.3
1.5-1.6
45
2
Unsuitable joint
17/1
112
23
1.5-1.6
45
2
Unsuitable joint
17/2
116
21.8
1.5-1.6
45
2
Suitable joint
1.6.2
Analysis of joints As can be seen from Table 25, the samples selected for analysis of the macro-structure
exhibited visual defects observed also in other technologies. After cutting, grinding, polishing and etching by standard procedures, observed was increased porosity in the weld. Macroscopic analysis also proved that the width of HAZ achieved 5.3 times of the base material’s thickness. MACRO-STRUCTURAL ANALYSIS OF SELECTED SPECIMENS Specimen No.
Weld surface
Weld root
Table 25 Macro-structure
17/2
3mm
3mm
1mm
3mm
3mm
1mm
32/3
46
Specimen No.
Weld surface
Weld root
Macro-structure
32/4
1mm
Untreated sheet
3mm
1mm
Regarding the wide HAZ and peeling surface layer near the bead, a significant reduction in corrosion resistance is presumed. It was therefore decided not to test the joints, and proceed to other technologies of joining the nitro-oxidised thin steel sheets. 1.7
Electron beam welding
This technology is typical for vacuum protection which might help prepare high-quality weld joints. It is also supposed that when replacing the protective atmosphere, the vacuum might markedly influence both the heat distribution in welded material as well as the shape of the weld surface and root of the weld joint, since the blowing of protective atmosphere was missing and the shape of the surface and weld root were influenced only by the flow due to the electron beam effect.
47
The nitro-oxidised sheet was welded by using PZ EZ ZH1 equipment in Prvá zváračská (the First Welding Company), a.s. The working pressure in the electron cannon was 4.10-4 Pa, and in welding chamber 4.10-2Pa. The first tests proved increased porosity of welds and formation of spatter. The further tests therefore used front-to-back oscillation of the electron beam. Welding parameters were as follows: •
Length of oscillation: 4 and 5.5 mm
•
Frequency of oscillation: 40, 60 and 120 Hz
•
Focusation current: 720, 726, 736 mA
•
Welding speed: 5, 10, 15, 20 mm/s
•
Welding current: 3, 5, 10, 12, 15 mA
Trying to reduce porosity and spatter, we applied welding by several passes. The role of the first pass was to clean the material surface from the oxidic layer causing the weld porosity in the vacuum chamber. The second pass assured the required penetration, while the third was a “cosmetic one” aimed at the improvement of the weld surface. Individual welding parameters of multi-pass welds are listed in Table 26. PARAMETERS OF ELECTRON BEAM WELDING
1.7.1
Table 26
No. of pass
Welding voltage[kV]
Welding current[mA]
Welding speed[mm/s]
Focusation current [mA]
Frequency [Hz]
1
40
3
10
736
60
2
40
10
10
736
60
3
40
5
10
736
60
Preparation of joints
Table 27 lists the appearance of surface, root and cross-sections of the welds of selected specimens which exhibited the lowest spatter and porosity. The figures document the problems with regularity of bead appearance and increased spatter in all specimens. Table 27 also justifies the application of several passes, as multi-pass welds exhibited less irregularities in the appearance of weld bead. A characteristic feature of all the weld joints cross-sections was the convexity of their surface. The micro-structure of the weld joints prepared by electron beam welding depended on the number of passes. The weld joints prepared by one and three 48
passes (Tab. 27, cross-section of specimens 502 and 606, Fig. 33) were different, since in the case of multi-passes, each consequent pass effected the structure created by the previous one. SURFACE, ROOT AND CROSS-SECTION OF THE JOINTS WELDED BY ELECTRON BEAM (39) Specimen No. No. of passes
503
502
606
1
3
3
Table 27
Weld surface
2 mm
2 mm
2 mm
2 mm
2 mm
2 mm
Weld root
Crosssection weld
of
49
Fig. 33 Macro-structure of a weld joint prepared by three passes (39) When measuring micro-hardness, the highest value was observed in the region of the weld root (180 HV 0.1). Micro-hardness values of individual regions of weld joints are shown in Table 28, which suggests good plastic properties of the weld joint. AVERAGE VALUES OF MICRO-HARDNESS IN INDIVIDUAL PARTS OF WELD JOINT
Table 28
Surface
Centre
Root
Weld metal
152.7
159.1
185.7
HAZ
161.3
134.5
181.6
Base material
154.8
124.8
180.2
1.7.2
Micro-structural analysis of joints
Figure 34 documents the micro-structure of the weld metal analysed in three basic lines – surface layer, middle section and root section. Micro-structure of the surface layer of the weld metal is formed by a coarse acicular ferrite and polygonal ferrite, which acquires the character of a columnar one in certain regions. Sporadically, some grains of upper bainite occurred in the weld metal. The chemical composition of the secondary precipitated particles observed in the ferritic matrix indicates carbides and nitrides (Figure 34 a). A very similar character of the micro-structure was observed in the root of the weld metal (Figure 34 c). The micro-structure in the mid-line of the weld metal gained a polygonal character (Fig. 34 b) consisting of fine ferrite and moderate heterogeneity in grain size.
50
a)
b)
c)
Fig. 34 Micro-structure of weld metal prepared by three passes a) surface, b) centre, c) root
The following micro-structures (Figure 35a) document a HT HAZ in the region of surface layer. The micro-structure is formed by ferrite of polygonal morphology, a very small grain size and a relatively homogeneous size. No adverse coarsening of the original austenite grain was observed. A very similar character of micro-structure was observed even in the middle line of the weld joint (Figure 35 b). Figure 35c characterizes the HT HAZ micro-structure at the root of the weld joint. The micro-structure was again fine-grained and polyhedral, consisting of ferrite. We can state that no substantial differences in the character of microstructure were observed in all three analyzed lines.
a)
b)
c)
Fig. 35 Micro-structure of HT HAZ of the weld made by three passes a) surface, b) centre, c) root
The micro-structure of the HAZ-BM transition is documented by the individual analyzed lines in Figure 36. The micro-structure of the HAZ-BM transition near the surface layer (Figure 36 a) is of polygonal character, made up of ferritic particles and precipitated particles of secondary phases observed in the matrix and along the boundaries of ferritic grains. Comparison of the micro-structures of HAZ and BM reveals a relatively large difference in grain size. Very similar character of the micro-structure was observed in the mid-line and at the root of the weld joint (Figures 36 b, c). As mentioned above, observed was a relatively strong precipitation in the ferritic matrix. The comparison of the analyzed lines revealed a slight difference in the density of precipitates. Density of precipitates near the surface layer and the root was higher than that in the mid-line of the weld joint. The difference might be
51
due to the initial state of the base material treated by nitro-oxidation. Owing to the re-melting process and the thermal effect of electron beam, nitrides and oxides in the surface layers dissolved, while precipitating again in the phase of the weld joint‘s cooling.
a)
b)
c)
Fig. 36 Micro-structure of HAZ-WM transition in the weld made by three passes a) surface, b) centre, c) root
1.7.3
Micro-hardness of joints
The weld joints exhibited a uniform hardness across the width of the joint from the centre of the weld to the base material. The highest measured value of micro-hardness was 192.3 HV 0.1 at the root of the weld joint, and the smallest decrease in micro-hardness was observed on the surface of the joint (120.1 HV 0.1). The average hardness measured from the centre of the weld joint to the base material (according to Figures 37 and 38) near the surface was 158.5 HV 0.1, in the centre of the weld 143.1 HV 0.1 and at the root of the weld 183.1 HV0, 1, while the last 5 points located in the base material were not taken into account.
Fig. 37 Measuring micro-hardness near the weld surface from the joint centre towards the base material
52
Fig. 38 Macroscopic picture of measuring micro-hardness in three lines from the joint centre towards the base material
When comparing the electron beam welding technology with other technologies, it is evident that the former one requires vacuum that might considerably limit its practical application in the case of o large-scale welds. Practical application also requires multiple electron beam passes in order to achieve the welds of acceptable quality. This multiplies the time and economic costs, making them unacceptable for practice. Given the dimensions of the weld joint (including the heat affected zone) which are approximately 3-fold with regard to the thickness of the welded specimens (1mm plate thickness), the technology cannot be recommended as suitable for welding thin steel sheets prepared by nitro-oxidation.
1.8
Welding by CO2 gaseous laser In order to eliminate the factors that could negatively affect the assessment of suitability
of individual technologies for welding the sheets treated by nitro-oxidation, penetration welds (hereinafter referred to as welds) were performed on the tested specimens in the first stage. Given the different types of laser (active medium, wavelength, etc.), the welds were made on a CO2 gaseous laser. Table 29 lists the welding equipment used and its parameters. PARAMETERS OF THE EQUIPMENT USED
Table 29
Type of laser
Ferranti Photonics AF 8 CO2
Shielding gas
Ar 99.996% (18 l/min)
Welding speed [mm/s]
30, 40, 50, 60
Laser power [W]
2000
Laser power at low welding speed was adjusted to 1.5kW; yet even at that speed and performance, the welds were unsatisfactory (also in terms of labour productivity, too low
53
speed is undesirable). Based on previous experience, Argon was used as a shielding gas. The tests using helium as a shielding gas produced a very irregular appearance of bead and other unacceptable weld defects. Argon was therefore preferred for economical reasons. When evaluating the cross-sections of the weld joints (Table 30) and appearance of weld beads prepared by other welding technologies, several defects were identified, e.g. unacceptable bead appearance, porosity and incomplete penetration in root section. Regarding the subsequent welding, 1AV specimen (Table 31) proved to be the best alternative of thermo-chemical treatment; it was therefore later analysed for the occurrence of internal defects.
EVALUATING VARIOUS MODES TREATED OF MATERIALS AFTER THE PROCESS OF WELDING BY CO2 LASER
Table 30
1.8.1 Macro-structural analysis of joints
The results of analyses confirmed the previous visual inspection which identified a defect in continuity of weld root, i.e. an un-penetrated root at the welding speed of 60mm/s. Welding speed 30mm/s caused a much wider heat affected zone as well as actual width of the weld. The samples produced at welding speed of 40mm/s and 50mm/s were satisfactory. The symmetry of the weld speaks in favour of the speed of 40mm·s-1. For further tests, the specimens will be therefore prepared in the range of the welding speed 40mm/s to 50mm/s.
54
WELD APPEARANCE OF 1AV SPECIMEN AT VARIOUS WELDING SPEEDS (38)
Table 31
Welding speed [mm/s] 40
50
60
Cross-section of weld
Weld root
Weld surface
30
1.8.2
Micro-structural analysis of joints
The micro-structure of the weld joint prepared at the welding speed of 40mm/s is shown in Figure 39. Laser welding made nitrides dissolve up to 1mm from the boundary of weld metal towards the heat-affected zone. The structure is formed by acicular ferrite and the ferrite precipitated along the boundaries of the columnar crystals. The micro-structure does not exhibit any abnormalities or unwanted changes in the phase composition that might be due to the nitrides comprised in the surface layer of the materials welded.
55
The micro-structure of the high-temperature HAZ consists of polygonal ferritic grains. HAZ micro-structure did not reveal any grain coarsening common with conventional welding methods.
Fig. 39 Micro-structure of laser joint (WM – HAZ –BM) (40)
1.8.3 Mechanical properties
Mechanical examinations proved that the strength of weld joints was higher than that of base material, since none of the examined specimens was disrupted in the spot of the weld joint. Average tensile strength was 434.1MPa.
Fig. 40 Static tensile strength test of a laser beam joint (19) Micro-hardness measurement (Figure 41) confirmed increased hardness in the weld metal. The micro-structure of weld joints did not exhibit grain coarsening in HT HAZ, which might decrease the joint strength.
56
Fig. 41 Course of micro-hardness of WM-HAZ-BM at high welding speed of 40 mm/s (40)
Figures 42 and 43 show the micro-hardness dependence of the distance of the imprint from the axis of the weld joint. This dependence was plotted for four different welding speeds (30, 40, 50, 60 mm/s). As both graphs show, the highest hardness 360 HV 0.1 of the joint was achieved, as supposed, in the area of weld metal. Another supposition concerning the fact that hardness depends on the welding speed was also confirmed. The highest hardness was observed at the welding speed of 60mm/s. This was due to higher cooling speed of the weld joint. The lowest micro-hardness was recorded at the welding speed of 30 mm/s. Hardness of the base material was 150 HV 0.1. Differences between the upper and lower parts of the weld joint were less distinct, as well as the differences between individual welding speeds.
57
Fig. 42 Measuring micro-hardness near the weld surface from the weld centre towards the base material at various welding speeds
Fig. 43 Measuring micro-hardness in the weld root from the weld centre towards base material at various welding speeds
1.9
Welding by solid-state disc laser
Regarding the results of CO2 welding by laser beam, the experiment proceeded by testing another type of laser, using a concentrated energy source.
58
Solid-state lasers are suitable for welding the materials of the thickness up to 7mm, where their efficiency is higher than those of gaseous CO2 laser, which are otherwise more efficient for materials of bigger thickness (Figure 44).
Fig. 44 Compared penetrations of the solid state and gaseous laser beams
Shapes of the penetration welds prepared by gaseous and solid-state lasers in dependence of welding speed are shown in fig. 45 illustrating that narrower penetration weld was achieved by solid-state laser at higher speeds.
59
Fig. 45 Comparison of penetration welds prepared by solid state and gaseous laser beams in dependence of welding speed
For welding, a disc laser of TRUMPF Co. was used. Its parameters are listed in Table 32. WELDING SOURCE AND ITS PARAMETERS Source type
TruDisc 1000 laser
Laser power
1000 W
Optics used
Ø 35 mm
Collimation distance
100 mm
Focusing distance
200 mm
Spot diameter
600 µm
Wavelength
1030 nm
Welding speed
20 mm/s
Protective gas
Argon (10 l/min)
Optical cable
Step Index Ø 300 µm
Table 32
Having considered previous experience in samples preparation, the power output was set up to 1 kW, while optimum welding speed was searched (Table 33). In addition, it was found that both focusation of laser beam and the direction of blowing the shielding gas have a
60
significant impact on the quality of weld joints. Several tests were therefore performed regarding the focusation and direction of blowing the protective atmosphere in the welding process. RESULTS OF THE INITIAL EXPERIMENTS OF WELDING BY SOLID-STATE LASER Specimen No.
Welding speed [mm/s]
Table 33
Evaluation
Burn through in several places pores on the weld surface and in the weld root Incomplete penetration in root section Incomplete penetration in root section Suitable Unsuitable Incomplete penetration in root section Incomplete penetration in root section
1
10
2
20
3
30
4
27
5
20
6
20
7
20
8
20
9
20
10
20
11
20
12
20
13
20
14
20
15
20
16
20
17
20
18
20
19
30
Suitable Incomplete penetration in root section Incomplete penetration in root section and pores from the side of root Unsuitable Unsuitable root on the weld end Unsuitable Incomplete penetration in root section Suitable Pores in the weld root Sporadic porosity
20
30
Sporadic porosity
Incomplete penetration in root section and several pores on the weld surface
The achieved results were better when compared with those achieved by welding the thermo-chemical untreated base material. A suitable weld in the case of untreated material was achieved at the welding speed 10mm/s. The speed of 20mm/s was determined as the most suitable (most stable) for welding nitro-oxidised metal sheets. When compared with common sheets, the welding speed grew due to the dark, dim and rough surface typical for nitrooxidised material.
61
Since the results varied at equal speeds, it was necessary to identify the factors influencing the welding process; we therefore decided to analyse the influence of the parameters directly affecting the welding process. 1.9.1
Influence of focusation on the process of welding
The laser beam was focused from the negative values (under the material surface) up to the positive ones (above the material surface), while quality of the point burnt by laser beam was monitored. As the laser beam head was mounted on a robotic arm, it was quite difficult and laborious to change the position of the nozzle blowing the shielding gas Argon into the spot of welding; shielding gas was not therefore blown to the welding spot in these experiments.
Fig. 46 Focusation of laser beam regarding the material surface
Table 34 illustrates the influence of the beam focusation on the shape of penetration weld and occurrence of pores. To set up the optimum focusation in terms of power, the same test was carried out with the same specimens at the same welding output (1 kW) and variable length of pulse.
62
SPOT PENETRATION WELDS OF THE BASE MATERIAL BY SOLID STATE LASER OF 1 KW POWER AND 5 MS PULSE Table 34 Focusation [mm]
-14
-13
-12
-11
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
+1
+2
+3
+4
+5
+6
+7
+8
+9
Specimen
Focusation[mm ]
Specimen
Focusation[mm ]
Specimen
Focusation [mm]
Specimen
Focusation [mm]
Specimen
Focusation [mm]
Specimen
63
As assumed, similar defects occurred more frequently at increased pulse duration (5ms, 10ms, 15ms, 20ms). The optimum setting was achieved when focusing the beam directly on the surface. Otherwise, the consequent increased turbulence of molten material lead to increased porosity. To sum up: insufficient energy supplied into the weld gives rise to porosity in the weld itself. 1.9.2
Blowing the protective atmosphere and the impact of its direction on the welding process
Regarding its reasonable price and quality of joint, Argon has been proved to be the most suitable shielding gas for the welding process. Helium is more expensive and without a distinct influence on the weld formation and appearance. Testing was conducted in two levels: blowing positions (concurrent, opposite and perpendicular, fig. 47a) regarding the direction of welding and movement of welding head, and the tilt of the blowing nozzle, i.e. the perpendicularity of blowing regarding the welding direction (Figure 47b).
a) Tested directions of blowing Argon
b) Testing the perpendicularity of blowing Argon
Fig. 47 Draft of the tested direction and perpendicularity of blowing the shielding gas
64
The examinations proved that the best quality of the weld joint surface and the weld joint itself was achieved when blowing the shielding gas into the welding spot in the direction parallel with the direction of welding and minimum tilting of the blowing nozzle from the material surface. Such an experimental setup (Figure 47) provided a satisfactory appearance of the weld free from spatter, pores and defects. Appropriate magnification revealed a casting structure (Figure 48) on the weld metal surface, which was formed during the solidification of the material. Special attention should be paid to the setup of the parts welded, in order to achieve a proper joint without displacement. Potential displacement was found to cause turbulent flow of the protective atmosphere, resulting in deterioration of the joint quality.
1 mm Fig. 48 A joint surface prepared by laser beam
Having eliminated the defects in welding technology, i.e. porosity, incomplete penetration in the root section etc., the prepared specimens were suitable for further analysis. The final specimens showed a regular appearance of surface and root of the weld joint (butt Ishaped weld). The appearance corresponded to that typical for welding thin steel sheets by laser beam, while the loss of the material burnt in the welding process expressed narrowing of the base material from the value of 1mm to 0.9mm, representing a 10% decline in the material thickness. Compared to traditional methods of welding, the resulting joints were characterized by a narrower heat-effected zone, which is also typical for laser beam welding.
65
Irregular appearance of a weld joint root
Burn Through
Incomplete penetration in root section Fig. 49 The most frequent defects observed in laser welding
Having finished the aforementioned tests, we proceeded with different lenses with a narrower intensity focused point in order to reduce thermal influence and reduce the width of the weld. Application of the narrower point of the laser beam posed further problems, particularly in terms of increased demands on the alignment of welded sheets. A minor yet not negligible role is played by the treatment of weld edges. Cutting the sheets with an insufficiently sharpened shear knife causes rounded sheet edges. To eliminate this factor, we set up an occasional rounded edge so that it was located in the root of the weld joints. This finding let us conclude that it was more reasonable to use the original lens with a larger focused point. 1.9.3
Macro-structural and micro-structural analyses of joints
Specimens welded at 20mm/s were analysed. The specimens for macroscopic and microscopic analyses were prepared in a standard way (grinding, polishing and etching in 3% Nital). Macroscopic analysis (Figure 50) showed neither porosity nor abnormalities in the shape of the weld. Joints were characterized by a very narrow heat-effected zone with regard 66
to the used power of average width 0.5mm. Unlike in CO2 laser beam welding, the joints were larger even when using a half-power source.
Fig. 50 Focusation of laser beam regarding the material surface
The micro-structure of the weld metal of the specimen treated by nitro-oxidation (fig. 51) is formed mainly by acicular ferrite and the ferrite precipitated along the boundaries of columnar grains.
Fig. 51 Micro-structure of weld metal
Transition between the weld metal and heat-effected zone (Figure 52) did not exhibit any adverse coarse grains. The micro-structure of the heat-effected zone (Figure53) is composed of polygonal ferrite. Preferentially oriented grains were re-oriented to a regular polygonal shape.
67
Fig. 52 Micro-structure of HAZ transition in weld metal
Fig. 53 Micro-structure of heat-affected zone
The base material (fig. 54) is composed of polygonal ferrite and tertial cementite precipitated on the grain boundaries. As a result of rolling, the orientation of primary grains is clearly visible.
68
Fig. 54 Micro-structure of base material
1.9.4
Mechanical properties
Micro-hardness measurement was carried out by using a Buehler micro-hardness tester. An indentor in the shape of a tetrahedral pyramid was injected into the body to be measured under the test load of 1N, which acts in the perpendicular direction for a fixed period of 10s. Measurement started at the weld centre line and proceeded across the weld metal, heat affected zone and the base material. Spacing between the imprints had to be 3.5 ÷ 4 times the diagonal of the imprint to avoid biasing of the previous measurement. The values measured for steel treated by nitro-oxidation are shown in Figure 55.
Fig. 55 Course of micro-hardness of WM-HAZ-BM
69
The minimum value of micro-hardness in the base material was (143 HV 0.1), while the maximum one was observed in the weld metal (217 HV 0.1). These values are by 65% lower than those of CO2 laser welding, mainly due to lower cooling rate of weld joint. As illustrated in fig. 56, disruption of the weld joints during the static tensile test did not occur in the spot of weld, but in the base material far from the weld, thus proving that the weld must be stronger than the base material itself.
Fig. 56 Specimens after static tensile test
Table 35 shows the measured values of the static tensile test; results are very similar in all cases. MEASURED VALUES OF STATIC TENSILE STRENGTH
Table 35
Code
L [mm]
F0.5 [kN]
Fm [kN]
R0.5 [MPa]
Rm [MPa]
A [%]
1
50
3.02
3.91
321
416
16
2
49.5
2.95
3.75
320
407
15.2
3
50
2.92
3.81
316
413
16
4
50.2
2.82
3.75
309
411
16.3
The Erichsen ductility test was conducted by injecting a Puncheons Round head from both the surface of the weld joint and the root joint. The Erichsen test also proved sufficient strength of joints in both cases.
70
Fig. 57 Perpendicular crack formed in Erichsen test
All three tested specimens (Table 36) exhibited perpendicular cracks in the weld joints (Figure 57), thus proving sufficient strength of the weld joints, as there were no disruptions along the joint. The measured values showed no striking differences.
VALUES MEASURED IN ERICHSEN TEST Specimen Indent Note No. depth [mm] Imprint from the side of 03 8.1 surface Imprint from the side of 06 8.1 root Imprint from the side of 20 8.0 root
Table 36
the weld the weld the weld
Provided that the defects occurring in the weld are not revealed by visual inspection as lack of fusion or porosity within the weld joint, this may lead to the formation of longitudinal cracks in the weld (Figure 58).
71
Fig. 58 Longitudinal crack formed in Erichsen test
72
3.
DISCUSSION Besides increased corrosion resistance, the process of nitro-oxidation also brings about a
marked enhancement of mechanical properties of materials. This can substantially contribute to decreasing the total weight of a product, and subsequently also power demands for its performance. Prior to practical application, it is however necessary to solve the problems connected with joining these materials, since it is rarely possible to manufacture a final product by a single technological process. One of the objectives of this monograph was to assess suitable technologies for welding surface-treated steel sheets, while defining the technological parameters that markedly influence the resulting quality of weld joints. This monograph assesses various technologies of welding the materials from the aspect of both, achieving a high-quality joint, and defining the parameters necessary to manufacture it. One of the key comparative criteria for the selection of the most suitable alternative of joining was the degree of affecting the base material in order to minimise the degradation impact of the joining process on the produced surface layer. Initial tests revealed the problems connected with the quality of surface layer, particularly regarding its discontinuity. It was therefore necessary to modify the regime of surface treatment. In order to achieve a high-quality and continuous joint, it was necessary to enlarge the gap between individual specimens during the processes of nitridation and oxidation. The specimens were joined by the following technologies: •
MIG welding,
•
Hybrid TIG – laser welding,
•
CMT welding,
•
CMT brazing,
•
Hybrid CMT and laser brazing,
•
Electron beam welding,
•
Gaseous (CO2) laser welding,
•
Solid-state (disc) laser welding.
Specimens prepared by MIG technology exhibited a wide HAZ and excessive spatter observed on all the prepared specimens, including those with continuous joints. Despite that, the joints did not achieve satisfactory strength in the static tensile test, and the micro-hardness
73
of the joints exhibited slightly increased values than the joints made of material without nitrooxidation. Regarding the goal to minimise the impact of the process of welding on the surface layer, we abandoned further suitability tests of this technology. The research also considered TIG welding technology; however, regarding the aforementioned results, there was a supposition that the joints might exhibit a similar defect (wide HAZ). We therefore tried to combine the mentioned technology with laser welding, in order to achieve higher welding speed and deeper penetration of the laser beam. Application of the laser beam also reduces the amount of heat introduced into the weld by TIG process. This assumption was finally confirmed, as the width of the HAZ was about 5 times wider than the thickness of the material welded, though the joints produced were continuous and free from porosity and spatter. Owing to the wide HAZ, neither this welding technology could be accepted. Another option for reducing specific heat input and thus the level of the surface layer distortion, was the application of the CMT process. This technology provides a number of modifications, e.g. CMT brazing. The results, however, did not meet our expectations. The surface layer of specimens was significantly affected; it was peeling off to a distance of approximately 2mm from the edge of the weld. The joints exhibited enormous porosity both on the surface and inside the weld metal in all cases of modifications (CMT welding, CMT brazing, CMT brazing in combination with laser beam). It was probably due to faster cooling, which did not allow enough time for pores to escape from the metal welded. Despite high porosity, the joints prepared by CMT brazing achieved sufficient strength and the disruptions in all samples occurring in the base material were caused by the excessive bead which was due to the elevation generated over the joint. In the case of hybrid CMT brazing, the disruption in the joint region resulted from the increased porosity. Regarding that the CMT method cannot be recommended as a suitable alternative for joining nitro-oxidised materials. As for concentrated energy sources, three were considered for application: plasma, electron beam and laser beam. The plasma arc was excluded since it might excessively affect the surface layer, thus missing the primary objective of minimising the impact on surface layer. A plasma arc would be applicable only in the form of a micro-plasma arc, which was not available. When testing the joining by electron beam in a vacuum environment, enormous defects in the formation of a joint surface and root were observed, as were spatter and porosity of the joint. Since the aforementioned defects could not be eliminated by adjusting the welding parameters, the preparation of weld joints by this technology was abandoned.
74
Welding by laser beam seemed to be very promising from the early stage of the research. Despite the high cost of welding equipment and the need for accurately jacked welded materials, only this technology enabled making the joints of outstanding quality. Welding by CO2 laser used for testing the first specimens indicated that, in case of correct adjustment of welding parameters, this technology could be recommended for welding nitrooxidised steel sheets. The application of a gaseous CO2 laser brought about a very narrow weld and HAZ. The following welding parameters can be considered suitable: material thickness 1mm, laser power 2kW, welding speed ranging from 40 to 50mm/s and focusation directly on the surface of the materials welded. Higher speeds resulted in the lack of fusion, while lower speeds caused an inappropriate shape in the weld cross-section. The joints exhibited enhanced hardness in the weld metal region, which was due to high cooling rates. Disruptions in all specimens occurred in the base material. It is worth noting that this technology induced sporadic porosity of joints, which could not be unambiguously justified. Regarding the small thickness of the material welded, it was necessary to limit the laser power to 25% of nominal power source. Setup in such a case is less accurate, which steered us to prepare further weld joints by using the equipment of lower nominal power. The aforementioned considerations led us to use a solid-state disc laser. Regarding the previous research into welding by gaseous laser, several factors affecting the welding process were analysed. However, the partial research results proved the importance of focusing the laser beam directly on the surface of the material welded, while simultaneously blowing the shielding gas as close as possible to the welding spot and in the direction parallel with the direction of welding. When compared to the CO2 laser, the aforementioned one provided more stable results with guaranteed repeatability of the joints produced. Since only a disc laser source of 1kW power was available, suitable welding speed ranged from 20 to 25mm/s. When compared to the ones made by CO2 laser, the joints made by disc laser exhibited regular bead appearance and were free from spatter and pores. A static tensile test confirmed disruption in the base material. An Erichsen test of the weld joint revealed the occurrence of a crack from the side of the weld surface and root in the direction perpendicular to the weld. The Erichsen test of the material treated by nitro-oxidation without a weld revealed the decrease in deep drawability by 11.3%, thus indicating excellent properties of the weld. The mentioned partial findings let us conclude that welding by solid-state disc laser is a suitable technology for welding thin steel sheets treated by nitro-oxidation. To reduce the weld shape, it is recommended to test a welding machine of higher output in order to achieve
75
higher welding speed, which is decisive in practical application regarding the automation of welding process. An overview of the research results achieved by using individual joining technologies is given in Table 37.
RESEARCH RESULTS
Table 37 Evaluation method
Technology
Visual examination
Macroanalysis
Tensile test
Erichsen test
Discard reason
GMAW (MIG)
Wide heat affected zone, spatter
Hybrid GTAW (TIG) and laser welding
Wide heat affected zone
CMT welding
High porosity
CMT brazing
High porosity
Hybrid CMT brazing and laser welding
Unsuitable appearance of joint, high porosity
Electron beam welding
Spatter, unsuitable appearance of joint, more passes necessary
Gaseous (CO2) laser welding
Suitable, 100% repeatability of making joints not guaranteed, sporadic porosity
Solid state (disc) laser welding
Suitable
positive results negative results conditional positive results
From the aspect of macroscopic analysis, the technologies utilising concentrated energy sources exhibit much narrower both HAZ and weld. The lowest widths of HAZ (250 µm) and weld joints (1.3 mm) was supposed to be achieved by the technology of welding by gaseous CO2 laser. Welding by solid-state disc laser produces a weld joint of bigger dimensions, yet increased output of laser beam is supposed to reduce the weld and the HAZ width. When
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using the methods without utilising concentrated energy sources, the HAZ width was larger by approximately 400% - 500%. A laser beam is therefore recommended for welding and joining thin steel sheets, in order to minimise the weld dimensions and affect the base material. Comparison of the maximum micro-hardness of joints made by individual technologies let us state that the highest micro-hardness as well as the highest differences in microhardness occurred in the process of welding by CO2 laser, which was mainly due to rapid cooling of weld joints. The lowest values (up to 87.5% decrease) were achieved in the welds made by electron beam, which was due to the application of several passes of electron beam and annealing, and the subsequent softening of weld joint in particular. A static tensile test proved satisfactory strength of the weld joints, since disruptions of the specimens occurred in the base material. In the majority of welding technologies, strength was nearly the same, with maximum deviation of 2.8% (8.5MPa). Suitable strength of weld joints in case of welding by solid-state disc laser was also proved by the Erichsen test, where distortion of weld joint occurred perpendicularly to the direction of welding. When compared to the measurement of original nitro-oxidised material without a weld, the difference represented 11.5%.
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CONCLUSION The decision on dealing with the topic of welding the steel sheets treated by nitrooxidation was made after the finding that the subject has not been covered in available sources. Literature in the field describes various technologies of nitro-oxidation either in salt baths, fluid layers or other conditions; yet the subject of welding steel sheets treated by nitrooxidation has not been covered. Several welding technologies were eliminated from the list of selected ones on the basis of visual inspection, as they caused massive porosity of weld joints. In a majority of cases, the parameters of the welding process were set up to avoid pore formation; repeatability of making weld joints was not guaranteed however, or the technologies were excluded owing to the excessive dimensions of the weld metal and HAZ. When applying the technologies utilising concentrated energy sources, a precise setup of the materials welded was necessary in order to avoid discontinuity of the weld resulting from the big weld gap (without using filler material). This monograph also discusses the direction of blowing the protective atmosphere which has a noticeable impact on forming the weld joint. In the case of laser beam welding, the welding speed could be increased while preserving the other parameters, which was mainly due to the change of the surface of the weld material induced by the process of nitro-oxidation: the surface gained a dim and dark appearance, causing the lower reflection and thus higher absorption of beam. In the initial phase of the research programme we assumed that the dissolving nitrooxidic layer would impact the structure of the weld joints. Structural analysis of the joints did not however prove sharp differences, when compared to the welding of materials without surface treatment. The recommended technology of welding by laser beam generated by solid-state disc laser source achieved stable and repeatable results at the power output of 1kW and welding speed of 20mm/s. A static tensile test of mechanical properties proved sufficient strength of joints, as the disruption occurred in the base material. Corrosion resistance of nitro-oxidised material was supposed to be disrupted by the process of nitro-oxidation. Corrosion attacked most of the surface in the spot of the distorted surface layer. After welding, it is therefore important to protect the surface layer or the narrow surface of the weld joint with a suitable anticorrosion coating, in order to preserve the excellent corrosion-resistance of the nitrooxidised material.
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Nitro-oxidised materials demonstrate high potential and induce plenty of practical applications. They can be used in the places where materials are expected to meet high demands. Nitro-oxidation combines enhanced corrosion resistance with excellent mechanical properties. We recommend that further research in the field is oriented on investigating the degree of degradation of the properties and weld joints of the metal sheets treated by nitro-oxidation in time, while determining the fatigue properties of the joints.
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ACKNOWLEDGEMENTS I would like to thank PhDr. Emília Mironovová for translating this monograph as well as for her assistance when publishing the research results. This research was conducted within the support of the Slovak Research and Development Agency grant No. 0057-07.
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CONTENTS LIST OF ABBREVIATIONS .................................................................................................................... 4 INTRODUCTION ....................................................................................................................................... 6 1
2
NITROGEN SATURATION OF MATERIAL SURFACE ....................................................... 7 1.1
NITRIDATION .........................................................................................................7
1.2
NITRO-OXIDATION .................................................................................................8
WELDING STEEL SHEETS TREATED BY NITRO-OXIDATION ................................... 10 2.1
EXPERIMENTAL MATERIAL AND ITS PROPERTIES .................................................. 10
2.1.1 Preparing and analysing the material treated by nitro-oxidation ........................ 11 2.1.2 Mechanical properties ........................................................................................ 15 2.1.3 Corrosion resistance ........................................................................................... 19 2.1.3.1 The method of electrochemical potentio-kinetic reactivation (EPR) ................ 20 2.1.3.2 Test of corrosion resistance in the condensation chamber............................... 23 2.1.3.3 Test of corrosion resistance in 3% NaCl......................................................... 24 2.1.4 Physical and thermo-physical properties ............................................................ 27 2.2
MAG WELDING ................................................................................................... 27
2.2.1 Parameters of welding ........................................................................................ 28 2.2.2 Analysis of the weld joints................................................................................... 29 2.3
CMT WELDING, CMT BRAZING, HYBRID WELDING .............................................. 33
2.3.1 Welding and brazing ........................................................................................... 33 2.3.2 Micro-structural analysis and mechanical properties of joints ........................... 41 2.3.3 Evaluation of the joints made by CMT technology ............................................. 44 2.4
HYBRID WELDING BY TIG AND LASER BEAM ....................................................... 45
2.4.1 Parameters of welding ........................................................................................ 45 2.4.2 Analysis of joints................................................................................................. 46 2.5
ELECTRON BEAM WELDING.................................................................................. 47
2.5.1 Preparation of joints ........................................................................................... 48 2.5.2 Micro-structural analysis of joints ...................................................................... 50 2.5.3 Micro-hardness of joints ..................................................................................... 52 2.6
WELDING BY CO2 GASEOUS LASER ...................................................................... 53
2.6.1 Macro-structural analysis of joints ...................................................................... 54 2.6.2 Micro-structural analysis of joints ...................................................................... 55 2.6.3 Mechanical properties ........................................................................................ 56 2.7
WELDING BY SOLID-STATE DISC LASER................................................................ 58
2.7.1 Influence of focusation on the process of welding ............................................. 62
2.7.2 Blowing the protective atmosphere and the impact of its direction on the welding process........................................................................................ 64 2.7.3 Macro-structural and micro-structural analyses of joints ................................... 66 2.7.4 Mechanical properties ........................................................................................ 69 3.
DISCUSSION ................................................................................................................................... 73
CONCLUSION .......................................................................................................................................... 78 ACKNOWLEDGEMENTS...................................................................................................................... 80 REFERENCES ........................................................................................................................................... 81
