Hard facing with Plasma Transferred Arc (PTA) welding technique can result ... microstructural properties of an high carbon cobalt based alloy PTA hard facing.
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Influence of dilution on microstructure and mechanical properties of a cobaltbased alloy deposited by Plasma Transferred Arc welding. A.E.Yaedu, P.S.C.P.da Silva, A.S.C.M.d’Oliveira Universidade Federal do Paraná, Curitiba (Brazil)
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Introduction
Selection of a coating material is a very important stage in manufacturing operations, ranging from the design of new components to maintenance of worn components. However due to limitations on processing techniques or even deposition procedures, after surface welding coating materials exhibit poorer properties compared to the original alloy. Dilution effects are the main responsibles for this properties degradation, in fact as elements from the substrate metal mix with the selected alloy, microstructural and performance changes should be expected. Cobalt-based alloys are known by their high resistance to wear and corrosion under severe conditions. These alloys have about 30% wt chromium, 4 to 17% wt tungsten and 0,1 to 3% carbon [1]. For these alloys presenting complex systems, like the quaternary Co-Cr-W-C system, pseudobinary diagrams are available, figure 1, enabling a better understanding of the behaviour of the alloy. The high carbon alloy, like the commercially known Stellite 1, has 27% of M7C3 and 1,5% of W6C, and according to figure 1 “fits over” the eutectic transformation. According to the literature this alloy has been described as exhibiting an hypereutectic structure [2] and also an hypoeutectic structure [3]. Although these variations could be due to different powders manufactures, dilution effects can play a major role as alloying occurs between the substrate and the coating alloy, during the metallurgical bonding of a surface welding procedure. Since wear resistance of cobalt based alloys depends on their microstructure (hardcarbides in a tough matrix), changes on the chemical composition of the alloy could affect their performance. 1700 Liquide
Temperature [K]
1650
Liq.+ M7C3
Liq. + γ
1600
1572 K 1566 K
1550
γ + Liq.+ M7C 3
1500 γ + M7C3 1450 1400 0
1
2
3
4
5
6
7
8
γ - Co
9
C(%pds)
M7C3
γ - Co:
74.0 Co
21.6 Cr
4.4 W
M7 C 3 :
12 Co
74 Cr
5W
9C
Fig 1 - Schematic representation of the pseudo binary diagram of the Co-Cr-W-C system [4]. Composition of the alloy used in this work is identified by the dotted line. Hard facing with Plasma Transferred Arc (PTA) welding technique can result on high quality deposits, with low dilution and high deposition rates [5]. PTA surface welding technique is an
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evolution of the GTAW. In PTA technique ionised gas is forced through a constrictor nozzle, which expands and accelerates, enhancing the energy transferred to the substrate. The PTA process uses two independently adjustable arcs a pilot arc and the main arc. Due to the concentrated energy, the plasma-transferred arc allows high deposition rates and produces high quality surface. In order to evaluate the potential of this technique in maintenance operations, where frequently only manual procedures are allowed due to geometrical limitation of the component to be recovered, a manual PTA torch was used in this work. The aim of this study is to evaluate the influence of the substrate on the mechanical and microstructural properties of an high carbon cobalt based alloy PTA hard facing. Three different steels were used as base materials - carbon steel, austenitic stainless steel and martensitic stainless steel- and two different powder-feeding rates were the main parameters tested. Microstructural examination by optical and scanning electron microscopy, microhardness, and dilution evaluation were performed to determine coating features.
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Experimental procedures
The high carbon cobalt based alloy, commercially known as Stellite 1, was supply by Deloro Stellite and deposited on plates (150x100x12mm) of three different steels, figure 2: A carbon steel – AISI 1020 (from now on referred as material C) An austenitic stainless steel – AISI 304 (from now on referred as material A) A martensitic stainless steel – AISI 410 (from now on referred as material M) AISI 1020
AISI304
AISI410
Fig 2 – Microstructure of the substrate steels used in this work Chemical composition of the as received materials is presented on table 1. Table 1 - Chemical composition of the as received materials
Co based alloy AISI 1020 AISI 304 AISI 410
Co Bal. -
Fe 3,0 Bal. Bal. Bal.
C 2,4
0,18 – 0,23 0,08 0,15
Cr 31,0 -
Ni 3,0 -
18 – 20 11,5 – 13,5
8,0 – 12,0 -
W 12,5 -
Mo 1,0 -
Si 2,0 -
1,0 1,0
Mn 1,0
0,3 – 0,6 2,0 1,0
For each substrate material two sets of specimens were processed in order to evaluate the effect of powder feeding rate. Hard facing was done by Plasma Transferred Arc process using a manual
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torch, under two different processing conditions, table 2. Single tracks and five parallel overlapped (~33%) tracks coatings were produced. Table 2 - PTA processing parameters. Parameter Plasma gas flux – Ar Shielding gas flux – Ar Feeding gas flux – Ar Main arc current intensity Voltage Powder feeding rate Welding speed
Set 1 5,0 l/min 5,0 l/min 5,0 l/min 100 a 110 A 30 V
Set 2 5,0 l/min 9,0 l/min 8,5 l/min 105 a 115 A 33 V
22 g/min
38 g/min
225 mm/min.
225 mm/min.
Characterization of the different specimens (lower powder feeding rate - C1, A1, M1 and higher powder feeding rate – C2, A2, M2) was undertaken with dye penetrant non-destructive test, to evaluate surface features like cracks and poros. Dilution levels were determined on the transverse cross section of the coated by two different procedures: as the participation of the substrate on the coating material, figure 3, and by semi-quantitative Energy Dispersion Spectroscopy (EDS) analysis of the iron profile. Measurements are the average of the evaluation made after cutting coated specimen at six different locations. Hardness profiles were done using a Vickers diamond pyramid under a 500g load.Profile were determined as a function of the distance to the fusion line according to the schematical representation on figure 4. Microstructure was evaluated by optical and scanning electronic microscopy.
A B
δ =
Substrate melted area B × 100 = Total melted area A+B
Fig 3 - Procedure used to evaluate dilution levels (areas determined by quantitative metalography)
Dureza Hardness
Average
(Média)
Di stância Distance from fusion line
Fig 4 – Schematical representation of the procedure used to determine hardness profiles
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Results and discussion
3.1.
Surface features
Specimens were first evaluated by visual inspection of the coated surfaces. Coatings from set 1 have a good surface appearance unlike those from set 2, which present a very poor appearance with high roughness and unmelted powder particles. Showing that it is possible to obtain a good surface appearance with a manual torch, provided the selection of processing parameters is done adequately. Although no porosities were observed some of the coatings exhibited cracks, no correlation with a specific substrate material was possible in spite of their distinct properties. As expected, specimens processed with the higher feeding rate (38g/min) are thicker. Welder skills are very important as one uses manual torch to deposit the coating material, and this could account for the non-uniformity thickness of the tracks produced. 3.2.
Dilution
Dilution levels determined as the participation of the substrate on the coating are presented on table 3. Table 3 - Dilution measurements Sample C1 A1 M1
Dilution (%) 18,0 29,3 26,5
Standard deviation 2,9 6,8 3,2
Sample C2 A2 M2
Dilution (%) 4,9 8,2 12,9
Standard deviation 0,8 1,6 1,8
The set of specimens processed with the lower powder-feeding rate presents an higher dilution level than the higher powder-feeding rate set. Carbon steel substrates exhibited the lowest dilution levels on both sets of specimens. No correlation between dilution level and the stainless steels substrates was possible as it varied with the powder-feeding rate. Previous work has shown [6] that the diffusion of iron from a substrate to the coating materials is also a good indicative of the dilution level. The iron profile, evaluated by EDS (Energy Dispersion Spectroscopy) results are presented on figure 5. For comparison purposes, the 3% line corresponding to the amount of iron on the as received material was included. Specimens from set 1 have the highest dilution levels, in agreement with previous dilution measurements from the areas relationship. Iron levels change through the coating thickness, decreasing from the fusion line to the external surface. Table 4 presents iron levels near the interface with the substrate and the external surface for the different conditions evaluated in this work. As before, the cobalt coating deposited on carbon steel substrates show the lowest dilution levels on both sets of specimens. However, a trend might be identified as the amount of iron near the external surface rises as the substrate material changes from carbon steel (AISI 1020), to austenitic stainless steel (AISI 304), to martensitic stainless steel (AISI 410). However, if one evaluates the iron levels near the interface, the chemical composition of the substrate material cannot be correlated to dilution levels measured by iron profile across the coating thickness.
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110
C1 A1 M1 C2 A2 M2 powder
90 70 50
Fe (%)
30 10 -6000
-4000
-2000
-10 0
2000
4000
Distance from the fusion line (um)
Fig 5 - Iron profile measured on the transverse section of the coated specimen Table 4 - Iron levels in the coating of the different specimens. Specimen C1 A1 M1 C2 A2 M2 3.3.
Iron near the interface 19,4 34,9 30,3 25,5 14,2 19,8
Iron at the external surface 16,5 20,1 23,4 4,2 6,9 10,9
Microhardness
Microhardness profiles obtain for the different conditions tested are presented on figure 6. Specimens processed with the higher powder-feeding rate have higher hardness, which can be related to the measured lower dilution levels of this set of specimens. This can be attributed to a more significant alloying phenomenon between the base materials and deposited alloy for the lower powder feeding rate set of specimens. Hardness increases from the fusion line to the external surface in agreement with ED’s profiles. Although it has been mentioned in the literature [7] that for laser coatings hardness can be affected by the chemical composition of the substrate, this was not the case in the present work.
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800
Vickers microhardness (HV 0,5)
700
C1 A1
600
M1 500
C2 A2
400
M2
300 200 100 0 -7000
-5000
-3000
-1000
1000
3000
5000
distance from the fusion line (um)
Fig 6 - Vickers Microhardness profiles The influence of the base material on the performance of a coated specimen must also be evaluated by its response to the thermal cycle of the deposition procedure. According to the hardness profiles determined, carbon steel and austenitic stainless steel are not affected by the imposed thermal cycle. However martensitic stainless steel has its features altered near the interface with the coating. Heat affected zone can be divided into two regions, an higher hardness region near the interface corresponding to an austenitised and quenched region, followed by a softer tempered region adjacent to which one finds the original steel hardness [8]. Depending on the operational conditions of the hardfaced components these alterations may play a major role on its service life. 3.5.
Microstructure
Microstructure of the coatings as observed under optical and scanning electronic microscope, are similar and independent from the substrate material. Figure 5 shows microstructures at the interface with the substrate and near the external surface. Near the fusion line an hypoeutectic solidification structure is observed, where primary dendrites of a cobalt solid solution are surrounded by a carbide net. Near the external surface microstructure is best described by a cobalt rich matrix (γ) with carbides. The observed change on the carbides morphology and distribution can account for the measured hardness variation across the coating thickness. The observed changes on the coating microstructure across its thickness should be associated with solidification kinetics, with dilution playing a minor role. Although theoretically the expected microstructure should present an hypereutectic feature, in this work all coatings present an hypoeutectic microstructure. This can be understood bearing in mind that the deposited alloy has a chemical composition very close to the eutectic transformation,
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therefore one could have expected dilution to have an important role determining the final coating microstructure.
(a)
(b) Fig 7 - Coating microstructure, (a) near the fusion line and (b) close to the external surface.
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Conclusions
For the conditions tested in this work: -
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Powder feeding rate has a significant role on the optimisation of coating features, ranging from thickness to its hardness Increasing dilution levels results on a coating hardness decrease but did not affect the observed microstructure Processing parameters should be optimised as a function of the substrate composition, as for the conditions tested the low carbon steel exhibited the lowest dilution level and the martensitic stainless steel is the most affected by the thermal cycle of the deposition process.
Acknowledgments
Thanks are due to Agência Nacional do Petróleo for funding this work and the scholarship of Mr Adriano E. Yaedu, and to Eng. Sérgio Simões from Deloro Stellite for processing arrangements and for supplying the coating material.
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References
(1) T. B. Massalski, Binary Alloy Phase Diagrams, ASM International, 1990. (2) B. de B. Foltran, “ Comparação dos revestimentos de superligas à base de cobalto (Co-Cr-W-C) depositados por eletrodo revestido, plsma por arco transferido e laser”, Master dissertation, Universidade Federal do Paraná, Brazil, 2000. (3) R. B Silvério and A. S. C. M. d’Oliveira, “ Cobalt based alloy coating deposited by PTA using powder and wire feeding”, Congresso Brasileiro de Engenharia de Fabricação, Uberlândia/MG, Brazil, 2003. (4) A. Frenk and W. Kurz, ‘ High speed laser cladding: solidification conditins and microstructure of a cobalt-based alloy” Materials Science and Engineering A173, p.339-342 (1993)
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(5) H.Hállen, E. Lugscheider, A.Ait-Mekideche “Plasma Transferred Arc Surfacing with High Deposition Rates”Fourth National Thermal Spray conference, Pittsburg, PA, USA, 4-10 May 1991 (6) X. Zhao, “ Effect of surface modification processes on cavitation erosion resistance” Ph.D. thesis, Universidade Federal do Paraná, Brazil, 2002. (7) R. Colaço, T. Carvalho and R. Vilar, “ Laser cladding of Stellite 6 on steel substrates”, High Temperature Chemical Processes, vol 3, p.21-29, 1994 (8) A.S.C.M d’Oliveira , R. Slud and R. Vilar , “Soldagem de superfícies por laser: A importância do substrato”, Congresso Nacional de Soldagem, Curitiba/PR, Brazil, 2000.
