NiCrSiB Coatings Deposited by Plasma Transferred ... - Springer Link

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JMEPEG (2013) 22:590–597 DOI: 10.1007/s11665-012-0271-7

NiCrSiB Coatings Deposited by Plasma Transferred Arc on Different Steel Substrates P.R. Reinaldo and A.S.C.M. DOliveira (Submitted February 24, 2011; in revised form May 21, 2012; published online June 29, 2012) Colmonoy 6 (NiCrSiB) is a Ni-based alloy recognized for its superior mechanical properties, attributed to the presence of a dispersion of hard carbides and borides, which is strongly dependent on processing technique. This work gathered microstructure data from the literature and analyzed Colmonoy 6 coatings deposited by plasma transferred arc hardfacing. The aim of the study was to determine the influence of PTA deposition parameters and substrate chemical composition on NiCrSiB coating characteristics. Coatings were characterized in terms of their hardness, dilution, and microstructure, as well as mass loss during abrasive sliding wear tests. The results showed that coating performance is strongly dependent on the chemical composition of the substrate. Carbon steel substrate yielded coatings with greater wear resistance. Processing parameters also alter the performance of coatings, and the lower current and lower travel speed result in reduced mass loss.

Keywords

coatings, Ni alloys, PTA hardfacing

1. Introduction There is a growing demand for materials with high wear resistance for use in components of industrial equipment. Materials that can be used to improve the wear resistance of parts include Ni (nickel)-based alloys and most notable among these is Colmonoy 6 (NiCrSiB). Its excellent wear resistance is associated primarily with the presence of borides and carbides distributed in a Ni matrix (Ref 1, 2). This superalloy competes with cobalt-based alloys that have high abrasion resistance (Ref 1, 3, 4). The attractive properties of Colmonoy 6 opened the way for new research on the deposition of this alloy as a coating. Sudha et al. (Ref 5), who deposited Colmonoy 6 on AISI 304L using plasma transferred arc (PTA), showed that the resulting microstructure was heterogeneous and described it as consisting of three regions with different morphologies: a lamellar structure enriched with Cr and B; a region containing carbides and chromium borides; and a third region in which needle-shaped chromium carbides and chromium borides predominate. According to Sudha et al. (Ref 5), the evolution of the microstructure is determined by the eutectic transformation, temperature gradient, and redistribution of the elements in the alloy. Conde et al. (Ref 6) processed Colmonoy 6 alloy using high-power diode laser (HPDL) hardfacing and found that as Cr and B were added, the hardness of the coating increased as a result of the formation of Ni3B and Cr23C6 phases. Using laser cladding, Navas et al. (Ref 7) showed the role played by the microstructure in the wear

response of Colmonoy 6 coatings. According to these authors, the layers deposited during the laser cladding process exhibit a heterogeneous distribution of carbides and borides in the matrix, accounting for the different wear rates measured. Ming et al. (Ref 8), who also used laser cladding to deposit Colmonoy 6, showed that the wear resistance of this alloy depends more on the type and amount of the main hard phases distributed in the matrix than on the average hardness of the deposits. Das et al. (Ref 9), studying Colmonoy 6 deposited by gas tungsten arc welding (GTAW) hardfacing, showed that the very high hardness of Colmonoy 6 deposits is a result of the formation of chromium borides and carbides. Studies of Colmonoy 6 alloy coatings carried out to date have used different processing techniques as well as different substrate/ coating combinations, making it difficult to compare the results or draw any general conclusions from them. Table 1 summarizes the phases identified in different studies of the NiCrSiB alloy, showing its complexity. As industrial components can be made of a wide range of steels and since each chemical composition can affect coating properties differently, it is of great relevance to understand the effects of different substrates and processing parameters on coating properties. Moreover, the challenge is even greater in advanced hardfacing processes, such as PTA, as each processing parameter can be controlled individually as opposed to conventional hardfacing processes, which compromise a set of dependent parameters. In the present study, PTA was used to deposit Colmonoy 6 alloy on carbon steel and stainless steel substrates, for a systematic assessment of the impact of the chemical composition of the substrate and changes in the PTA processing parameters, on coating characteristics (dilution, phases formed, hardness, and abrasive wear resistance).

2. Materials and Methods P.R. Reinaldo, PIPE, Federal University of Parana´ (UFPR), Curitiba, Brazil; and A.S.C.M. DOliveira, Mechanical Engineering Department, Federal University of Parana´ (UFPR), Curitiba, Brazil. Contact e-mails: [email protected] and [email protected].

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The atomized NiCrSiB alloy, with a particle size between 50 and 200 lm, was deposited by PTA on 100 9 100 9 12 mm

Journal of Materials Engineering and Performance

Table 1 Summary of the phases identified in Colmonoy 6 alloy processed with different techniques as reported in the literature Author

Process

Kaul et al. (Ref 2) Sudha et al. (Ref 5) Conde et al. (Ref 6) Navas et al. (Ref 7) Paul et al. (Ref 4) Crook and Farmer (Ref 1)

Laser (LC) (PTA) Laser (HPDL) Laser (LC) Laser (LRM) Cast

Substrate AISI AISI AISI AISI AISI

Phases

304 304L 1020 1015 316L

CrB, CrB4, Cr3B4 Cr5B3, Cr7C3 Fe1.1 Cr0.9 B0.9, and Ni solid solution Cr2B, Cr7C3, Cr3C2, and Ni solid solution Ni3B, Ni3Si, Ni3Fe, Cr23C6, (Fe,Ni)23C6, and Ni solid solution CrB, Ni3B, (Cr,Fe)7C3, and Ni solid solution CrB, Cr2B, Cr5B3, Ni2B, Ni3B, Ni4B3, Cr7C3 and Ni solid solution CrB, Cr2B, Cr3B2, Cr5B3, Ni3B, Ni3Si, M23C6, M7C3, and Ni solid solution

Table 2 Chemical composition of the NiCrSiB alloy and substrate steels (wt.%) (Ref 10, 13) Materials Colmonoy 6 AISI 1020 AISI 304

C

Si

Mn

P

S

Cr

B

Ni

Fe

0.7 0.23 0.08

2.0-4.5 … 1.0

… 0.3-0.7 2.0

… 0.4 0.04

… 0.05 0.03

14-15 … 18-20

3.1-3.5 … …

Bal … 8.0-10.5

4.0 Bal Bal

Table 3 Processing parameters used for PTA deposition Parameter Electrode Travel speed Plasma gas flow rate Shielding gas flow rate Powder feed gas flow rate Stand-off distance Feed rate Current Travel speed

Values 3/16 in 100 mm/min Ar 2.0 L/min Ar 15 L/min Ar 2.0 L/min 10 mm Constant volume 130 and 170 A 5, 15, and 20 cm/min

AISI 1020 carbon steel and AISI 304 stainless steel plates. The chemical composition of Colmonoy 6 alloy and the substrate steels used is shown in Table 2. The Ni-based alloy was processed using the Starweld 300 from Stellite Deloro with the 600 torch (20-250 A) and the set of parameters are shown in Table 3, three torch travel speeds for each of the two current (130 and 170 A) were the variables studied. Deposits were inspected visually for porosity and cracks. Dilution, describing the mixture of a deposited alloy and the substrate, in a welded coating, was determined by two procedures: one using a ratio based on areas (D) and the other a ratio based on Fe content (d). In the first procedure, the dilution was calculated using Eq 1, where A is the overlay area of the deposited coating and B the area corresponding to the melted substrate, Fig. 1 Dð%Þ ¼

B :100 AþB

ðEq 1Þ

In the second procedure, the dilution was calculated according to the method adopted by Yaedu and dOliveira (Ref 10), using Eq 2 d ð%Þ ¼

Fecoat  Fealloy :100; Fesubst

ðEq 2Þ

where Fecoat is the Fe content in the coating, Fealloy the Fe content in the atomized NiCrSiB alloy, and Fesubst the iron content in the substrate steel. Iron content was measured by EDS over an area of 1 mm2 for the elements listed in Table 2. In the first procedure, it is assumed that the ruling

Journal of Materials Engineering and Performance

Fig. 1 Schematic drawing of a transverse cross section of a deposited single bead

event determining dilution (D) is the formation of the melt pool when the plasma arc reaches the substrate; whereas the second procedure (d) considers the chemical composition changes that might occur during the melting of the substrate and subsequent solidification and cooling of the coating. The deposited bead geometry was characterized by the width and thickness of the overlay and the penetration depth, as described in Fig. 1. The influence of processing parameters and substrate chemical composition on coating hardness was determined by measuring Vickers microhardness profiles under a 0.5 kgf load on the transverse cross section of the coating. Preparation of the samples for microstructure analysis followed standard procedures and included wet grinding with silicon carbide abrasive papers and polishing with 1 lm grainsize alumina. The microstructure was revealed by electrochemical etching in a solution of 15 mL of hydrochloric acid and ethanol using a voltage of 6 V for up to 90 s and was analyzed using scanning electron microscopy. The phases were identified by x-ray diffraction on the top surface of the deposits using a Cu Ka radiation. Coating wear performance as a function of substrate chemical composition and processing parameters was determined using abrasive sliding wear tests of the pin on abrasive paper type. In this set-up, an adaptation from the ASTM G9995 standard, the pins machined from the coating were slid over a hard metal disc covered with 220-mesh silicon carbide abrasive paper under a 10 N load at a tangential speed of 1.5 m/s. To avoid excessive wear of the abrasive paper track, which could affect the test conditions, each test was carried out in 500 m stages to give a total sliding distance of 5000 m. To

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calculate mass loss during the test, the pins were weighed before the test and after each 500 m stage. The wear rate was calculated by dividing the overall mass loss by the total sliding distance used in the abrasive sliding wear test, i.e., 5000 m.

and a consequent reduction in dilution. The heat input, H (kJ/mm), which is a relative measure of the energy transferred per unit length, is given by Eq 3 H¼

3. Results and Discussion 3.1 General Features of Coatings Coatings processed on the two substrates are free of defects such as cracks and porosity. Both the processing parameters and chemical composition of the substrate affected the geometry of the coatings. The width and thickness of the overlay and the penetration depth of the coatings deposited on the two substrate metals (AISI 1020 and AISI 304 steel) and the respective processing parameters are shown in Table 4. For each of the travel speeds used, the width of the coatings was found to increase as current increased, and for each of the currents used, a reduction in torch travel speed caused a greater buildup of material in the melt pool, leading to a significant increase in overlay thickness. These findings were also confirmed after processing on AISI 304 stainless steel plates; although, the use of this substrate did result in coatings with a greater penetration depth and/or reduced total thickness (overlay thickness + penetration depth). This fact can be explained by the thermal conductivity of the austenitic stainless steel—at 16.2 W/m Æ K it is around one-third that of carbon steel 51.9 W/m Æ K (Ref 11)—as this causes more energy to be concentrated in the melt pool, leading to an increase in its size and, consequently, greater dilution, Fig 2(a). As the heat input was the same, the change in dilution can be attributed to the different substrate properties. Dilution based on the Fe content ratio showed larger values than those obtained with the areas ratio, Fig. 2(b), in agreement with the literature (Ref 10), but confirmed the lower values measured for the coatings processed on AISI 1020 carbon steel substrate. It also confirmed that, regardless of the chemical composition of the substrate, processing with the lower travel speeds resulted in lower dilution. It is worth mention that although the lower travel speed and the thermal conductivity of the substrate are expected to affect diffusion behavior during hardfacing, d and D showed a similar response to the processing parameters tested. It should also be taken into account that in PTA deposition, unlike other hardfacing techniques, a very significant amount of heat is absorbed by the atomized feedstock, as described by Wu and Wu (Ref 12), resulting in less heat reaching the substrate

I  E  60 ; v

ðEq 3Þ

where I the depositing current (A), E is the arc voltage (V), and v the travel speed (cm/min) and 60 standardizes the units for I and v. For the same processing parameters reducing the travel speed from 20 to 5 cm/min corresponds to a heat input four times higher. Despite the greater heat input when a lower travel speed is used, more material passes through the arc every minute

Table 4 Coating geometry as a function of current at the highest and lowest travel speeds tested Travel Width, Overlay Penetration Current, A speed, cm/min mm thickness, mm depth, mm AISI 1020 130 170 AISI 304 130 170

5 20 5 20

14.4 8.6 21 10.1

5.1 2.5 4.7 2.2

0.35 0.42 1.12 1.24

5 20 5 20

13.86 7.65 16.75 8.56

2.7 1.25 2.39 1.22

0.99 0.73 1.53 1.15

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Fig. 2 Dilution measurements for coatings deposited on AISI 1020 and AISI 304 using different currents and travel speeds, based on the (a) areas ratio, Eq 1, and (b) Fe content ratio, Eq 2

Journal of Materials Engineering and Performance

because the feeding rate is kept constant, as it is an independent variable in PTA processing. This, together with the low thermal conductivity of Colmonoy 6 (15.01 W/m Æ K) (Ref 13), means that the atomized alloy absorbs a significant amount of heat as it passes through the plasma arc, i.e., it takes energy from the arc and therefore less energy reaches the substrate. For each of the substrates, the increased dilution with increasing current is a function of the greater heat input, which causes the melt pool to penetrate farther into the substrate, leading to greater dilution (Ref 12, 14). A similar trend is observed when the travel speed is reduced from 20 to 15 cm/ min. In this case, the variation in the amount of material that builds up in the melt pool is not significant, and therefore the effects of the increased heat input predominate, causing an increase in the size of the melt pool. However, a reduction in travel speed to 5 cm/min, which corresponds to a heat input four times higher than that at 20 cm/min, led to a reduction in the dilution measured in coatings processed on both AISI 1020 and AISI 304 steel. This behavior can be explained by the greater buildup of material in the melt pool under these processing conditions, resulting in a significant increase in the overlay thickness. During deposition, Colmonoy 6 alloy is melted in the arc and, as the torch moves, builds up behind the plasma arc. The deposited alloy is then pushed underneath the plasma arc by convection to the front of the melt pool, creating a film of liquid metal, whose composition is similar to that of the deposited NiCrSiB alloy. It is on this layer that the plasma arc then acts, reducing direct interaction with the substrate. Hence, although a reduction in travel speed from 20 to 5 cm/min results in increased heat input, this is not reflected in greater interaction with the substrate. Accordingly, the behavior of the melted material deposited has an influence on the characteristics of the coatings. This was demonstrated by Almeida et al. (Ref 15), who, when processing intermetallic coatings, showed that when a liquid film similar in composition to the deposited material is not formed, the interaction between the arc and substrate is very intense, resulting in a large dilution.

3.2 Coatings Hardness and Microstructure The impact of processing parameters and substrate chemical composition on coating hardness is shown in Fig. 3. The microhardness profiles measured along the transverse cross section of the coatings show that the chemical composition of the substrate was one of the factors determining coating hardness, which decreased with increasing current for each of the travel speeds used. The hardness of coatings deposited on the AISI 1020 carbon steel substrate was almost twice that of coatings deposited on AISI 304. This difference can be explained by the higher dilution, which results in the incorporation of elements from the substrate and a consequent change in the composition of the coating compared with that of the deposited NiCrBSi alloy. For each of the substrate steels used, the average hardness varied according to the current used. Colmonoy 6 coatings deposited on AISI 1020 steel with a travel speed of 5 cm/min and current of 130 A resulted in an average hardness value of 801 HV, while the hardness value for the same speed and a current of 170 A was 593 HV. It is interesting to note that the lower torch travel speed resulted in profiles with a greater spread of values, suggesting that a more heterogeneous microstructure was formed. A better understanding of the coating hardness measured for the different processing conditions was gained from x-ray

Journal of Materials Engineering and Performance

Fig. 3 Hardness profiles of NiCrSiB coatings deposited on the two substrates using 130 and 170 A currents and travel speeds of (a) 5 cm/min and (b) 20 cm/min

diffraction analysis, Fig. 4 to 7. It is observed that the phases present in the coatings were determined by the chemical composition of the substrate, explaining the strong dependence of the (coating/substrate) system hardness on this factor. The results presented above, as well as those reported in the literature and summarized in Table 1, confirm the complexity of the microstructure formed in NiCrSiB coatings under different conditions although the same processing technique is used. In addition to the complexity of the alloy, it is important to bear in mind that the identification of phases by x-ray diffraction depends on the particular phases formed. This in turn depends on the substrate, processing parameters, and test conditions used, as the greater the number of peaks, the greater the possibility of overlap, making it difficult to identify individual phases. Sudha et al. (Ref 5) reported that they had failed to detect Ni3Si by x-ray diffraction, probably because of multiple peaks and overlaps. Figures 4 and 5 show the x-ray diffraction results for Colmonoy 6 alloy coatings deposited on the AISI 1020 and AISI304 steel substrate with a 170 A current and 5 cm/min travel speed. A variety of phases is identified confirming the complexity of the alloy previously mentioned and the strong influence of the chemical composition of the substrate steel is shown by the smaller number of peaks of the x-ray diffraction analysis of the surface when AISI 304 was used as the substrate. The same carbides (Cr7C3, Cr23C6) formed irrespective of the

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Fig. 4 X-ray diffraction results for NiCrSiB coatings deposited by PTA on an AISI 1020 steel substrate with a 170 A current and travel speed of 5 cm/min. Inset shows a view of coatings microstructure

Fig. 5 X-ray diffraction results for coatings deposited by PTA on an AISI 304 steel substrate using a current of 170 A and travel speed of 5 cm/min. Inset shows a view of coatings microstructure

substrate, but only with AISI 304, Ni3Si, and Ni3B were observed. The dendritic structure observed in these coatings, without any clear presence of dispersed borides and carbides acting as reinforcing particles, confirms the major impact that the chemical composition of the base metal has on coating microstructure. Although a change in the processing parameters alters the variety of phases formed, the determining role of the substrate chemical composition persisted as shown in Fig. 6 and 7 for coatings processed on both substrates with 130 A and a travel speed of 20 cm/min. With a lower heat input, dendritic

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structures can no longer be observed and acicular precipitates not previously present in coatings deposited on AISI 304 can be seen. The morphology of the phases found in Colmonoy 6 coatings deposited on AISI 1020 carbon steel is similar to that observed by Conde et al. (Ref 6) in coatings deposited using HPDL on a carbon steel substrate despite the higher solidification rates imposed by HPDL. These results suggest that the atomized alloy melted completely in the plasma arc during deposition, mixed with the melt pool and solidified according to the conditions

Journal of Materials Engineering and Performance

Fig. 6 X-ray diffraction results for NiCrSiB coatings deposited by PTA on an AISI 1020 steel substrate with a 130 A current and 20 cm/min travel speed. Inset shows a view of coatings microstructure

Fig. 7 X-ray diffraction results for NiCrSiB coatings deposited by PTA on an AISI 304 steel substrate with a 130 A current and 20 cm/min travel speed. Inset shows a view of coatings microstructure

determined by the substrate characteristics, resulting in a variety of phases with different morphologies being formed even though the same processing parameters were used.

3.3 Wear Performance The wear performance of NiCrSiB alloy coatings deposited by PTA was investigated by Ramachandran et al. (Ref 16) in a

Journal of Materials Engineering and Performance

study of the effect of test parameters on abrasive sliding wear. They reported that the wear resistance of the coatings they used was superior to that of the substrate. In the present work, abrasive sliding wear test parameters were kept constant in order to rank the performance of the deposited Colmonoy 6 coatings. The excellent abrasive wear performance of Colmonoy 6 alloy (Ref 1) has been explained by the presence of hard phases, such as borides and carbides, dispersed in a Ni matrix.

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Fig. 8 Mass loss (g) of NiCrSiB coatings in the abrasive sliding wear test. AISI 1020 and AISI 304 steel substrates, 5000 m sliding distance, (a) 170 A current and (b) 130 A current

However, the microstructure and, consequently, performance of this Ni alloy depend very much on how it is processed. Navas et al. (Ref 7), using laser cladding to deposit Colmonoy 6 on AISI 1015 steel, found that abrasive wear resistance is lower when there is a large proportion of small hard phases and associated this behavior with the ease with which these small hard particles can be extracted from the matrix. However, although the large particles dispersed in the matrix can contribute to higher wear resistance as they are not easily removed from the matrix, they can suffer brittle fracture and adversely affect wear performance. The abrasive sliding wear performance of NiCrSiB coatings deposited by PTA under the conditions studied is illustrated in Fig. 8, which shows the variation in mass loss as a function of sliding distance and corresponding wear rate. It can be observed that deposition on AISI 1020 carbon steel, which resulted in the coatings with the highest hardness, produced the coatings with the lowest mass loss and wear rate. This behavior can be explained by the lower dilution and consequent conservation of coarse phases with high hardness seen in coatings deposited on a carbon steel substrate.

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For each of these material systems, it can be seen that the parameter that determines the variation in mass loss with sliding distance and wear rate was the deposition current. The lower current used (130 A) resulted in the lowest wear rates. It can also be observed that the lowest travel speed used (5 cm/ min) resulted in lower wear rates. This behavior is associated with greater buildup of material during deposition and consequently the lower dilution produced, which reduces incorporation of the elements from the substrate, better preserving the characteristics of the original alloy in the coating and causing the carbides and borides to be dispersed. The dendritic structure of coatings deposited on AISI 304 steel with a 170 A current and even the fine dispersion of carbides and borides observed after deposition on this steel with a lower heat input (130 A) adversely affected the performance of these coatings. This corroborates the findings reported by Navas et al. (Ref 7), according to which a fine dispersion of particles does not ensure wear resistance in Colmonoy 6 alloy coatings. Processing parameters must thus be optimized for each coating/substrate pair. This is confirmed by the similarity between the wear rate of coatings deposited under different

Journal of Materials Engineering and Performance

conditions (170 A, 5 cm/min, AISI 1020 and 130 A, 5 cm/min, AISI 304). Processing parameters also have a significant effect on coating thickness and can compromise wear performance. This was the case with coatings deposited on AISI 304 stainless steel at 170 A and 20 cm/min, for which the wear performance was determined by the coating geometry rather than coating microstructure and the thin overlay wore down completely before the end of the test (sliding distance 5000 m).

4. Conclusions This study strengthens the relevance of processing parameters optimization for each industrial component with different chemical compositions, particularly when working with complex alloys such as the NiCrSiB. Conclusions to be drawn from this study on the effect of processing parameters and substrate chemical composition on the characteristics and abrasive sliding wear performance of Colmonoy 6 alloy coatings deposited by plasma transferred arc are: 1. Coating characteristics are mainly determined by the chemical composition of the substrate, but processing parameters also play an important role and the increased heat input caused by an increase in current leads to greater dilution. However, the buildup of material during deposition at low travel speeds counteracts the increase in heat input, producing thicker coatings with lower dilutions. 2. Deposition on an AISI 304 stainless steel substrate produces shallow and wider coatings with lower hardness than deposition on a carbon steel substrate. This is a result of the greater dilution, assessed by d or D, and slower solidification conditions imposed by the low thermal conductivity of AISI 304 steel. 3. The chemical composition of the substrate also determines the wear behavior of coatings and the lowest wear rates were measured on coatings processed on carbon steel plates. For each substrate, the microstructure of NiCrSiB alloy coatings deposited by PTA is strongly dependent on processing conditions as these determine the magnitude of dilution. Subsequently, the lower the

Journal of Materials Engineering and Performance

dilution, the more complex the microstructure formed, and consequently the harder the coatings and the better the wear performance.

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