Microstructural development on AISI 1060 steel by FeW/B 4 C composite coating produced by using tungsten inert gas (TIG) process

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 19, August 2012, pp Microstructural development on AISI 1060 steel by FeW/B 4 C composite coating produced by using tungsten inert gas (TIG) process Serkan Islak a *, Soner Buytoz b & Muzaffer Karagöz c a Cide Rifat Ilgaz Vocational High School, Kastamonu University, Kastamonu, Turkey b Department of Metallurgy and Materials Engineering, Faculty of Technology, Firat University, Elazig,Turkey c Department of Metallurgy and Materials Engineering, Faculty of Engineering, Bartin University, Bartin,Turkey Received 18 August 2011; accepted 16 July 2012 This study discusses the effects of B 4 C addition on the morphologies of primary carbides and eutectic colonies in FeW weld-surfacing alloys. Results reveal that a series of FeW weld-surfacing alloys with different B 4 C contents are successfully fabricated onto AISI 1060 steel by using tungsten inert gas (TIG) arc welding. SEM observations show that the obtained coating sheet has a smooth and uniform surface, as well as a metallurgical combination with the AISI 1060 steel substrate without cracks and pores at the interface. Fe 23 (C,B) 6, Fe 3 (C,B), FeW 2 B 2 and FeW 3 C boride and carbides are determined in the microstructure. The microhardness values of the coating results show a difference depending on the rate of B 4 C and coating conditions. The maximum microhardness value of the coating is about 1095 HV. Keywords: TIG process, Composite coatings, Microstructure Surface coating techniques have been successfully applied on different substrates to provide a better surface property in recent years. 1,2 The most common surface coating techniques are vapor deposition (PVD and CVD), sol-gel, electrolytic coating and hardfacing process (welding and thermal spray). Hardfacing primarily involves deposition of a hard and wear resistant material to the specific areas of the surface of the components by any of the techniques such as welding and thermal spraying. 3 Welding cladding processes contain oxyacetylene gas welding (OAW), tungsten inert gas welding (TIG), submerged arc welding (SAW), plasma transferred arc welding (PTA) and laser beam welding (LBW) methods. These techniques create an excellent bond between the coating and substrate. 4-7 Among the weld cladding procedures, tungsten inert gas (TIG) surfacing process is a cost-effective approach applied when reactive materials (as coatings or substrates) are involved. 8 In this method, substrate surface and powder or hard filler wire used for coating were melted down using arc source and then solidified, which resulted in the formation of a new composite layer Composite coating is generally made up from a ductile matrix and a hard ceramic material as *Corresponding author ( serkanislak@gmail.com) reinforcement. 13 While nickel, cobalt, iron and titanium based alloys are used as matrix in the production of these composite coatings, in terms of reinforcement elements, hard reinforcement particles such as SiC 14, TiC 15, B 4 C 16,17, WC 18,19, TiB 20 2 and 21,22 Cr 3 C 2 are preferred. In recent years, the application of B 4 C in the surface coating processes has attracted more and more attention for its super hardness, thermal stability, high melting point (2450 C), low density (2.52 g/cm 3 ) and super mechanical properties. 23 Boron carbide (B 4 C) is one of the hardest materials known, ranking third behind diamond and cubic boron nitride (cbn). Due to its high hardness, boron carbide powder is used as an abrasive material and it shows perfect wear resistance Chao and et al. 16 investigated the characterization of B 4 C particulate reinforced nickel composite coatings on A3 steel substrate by laser cladding. The composite coating gave very high hardness of 1400 HV which was seven times higher than that of A3 steel and excellent wear resistance. Meng et al. 27 produced TiB 2 /TiC Ni composite coatings by laser cladding of pre-placed B 4 C+NiCrBSi powders onto Ti 6Al 4V substrate. The composite coating mainly consists of γ-ni matrix phase, TiB 2, TiC and CrB reinforcements. The average hardness of the coating is four times as big as that of Ti 6Al 4V substrate.

2 254 INDIAN J. ENG. MATER. SCI., AUGUST 2012 In this study the microstructural characteristics of FeW overlays with B 4 C powder have been investigated. The results obtained from the deposition process and the investigation of the tracks and surfaces produced have been characterized by means of scanning electron microscopy (SEM), X-ray analysis (EDS), X-ray diffraction (XRD) and energy dispersive analysis, and by microhardness testing. Microstructural properties and change of hardness of samples which had different wt. % of B 4 C powders were discussed. Experimental Procedure In coating processes from an AISI 1060 steel produced samples were used, which had dimensions of 20 mm 10 mm 80 mm. In order to place coating powders, holes with 8 mm of width and 1.5 mm of depth were drilled on substrates. Prior to the modification of surfaces by using TIG method, surfaces of steel material were cleaned with acetone. Metallic powders were mixed, wetted and, then placed in the surface holes, thus metallic powders do not lifted from the surface by the shielding gas. Then, samples were left to dry at 50 C temperature for 30 min. Figure 1 illustrates the SEM images for every powder. Table 1 gives chemical compositions of the powders and the substrate used in coating process. FeW powder was modified by addition of B 4 C particles. B 4 C particles were added to the powder alloys used for obtaining coating layers, in a range of 10 wt% to 40 wt%. The average sizes of the FeW powder were from 10 to 75 µm, and the B 4 C particles were less than 10 µm. The coating powders with various B 4 C content are given in Table 2. Coating process was carried out using production parameters given in Table 3. Figure 2 shows principle scheme of surface coating process via TIG method. For microstructure examinations, 10 mm 10 mm 10 mm samples were extracted from the middle part of coating material. Coating materials undergoing metallographic procedures were etched in 5 ml HNO ml HCl + 65 g FeCl 3 solution used as etchant for microstructure examinations. While a scanning electron microscope (SEM) was used for microstructure examinations, X-ray diffraction (XRD) and X-ray energy dispersive spectrometer (EDS) were used for phase analyses. Hardness measurement was conducted throughout a line from superstrate of the coating through substrate at intervals of 250 µm, in waiting time of 10 s and with a load of 200 g using a Future-Tech FM 700 brand micro-hardness device. Fig. 1 SEM image of metal powders used in coating; (a) FeW powder and (b) B 4 C powder Table 1 Chemical compositions and physical properties of substrate and coating powders Substrate & Powder Composition (wt%) AISI 1060 FeW powder B 4 C powder C Mn Si Ni Cr Fe Balance Balance 0.05 B W Melting point ( C) Density (g/cm 3 ) Size (µm)

3 ISLAK et al.: MICROSTRUCTURAL DEVELOPMENT OF AISI 1060 STEEL 255 Fig. 2 Schematic diagram of TIG cladding system Table 2 The coating powders with various B 4 C contents Samples Powder ratios Production speed (mm/s) S 1 S 2 S 3 S 4 FeW FeW-10 wt% B 4 C FeW-20 wt% B 4 C FeW-40 wt. % B 4 C *Heat input (kj/cm) *Heat input Q = η.u.i.60/(ν.1000) U: voltage, I: current, ν: travel speed, η: productivity coefficient ( for TIG process η: 0.65) Electrode Protective gas Electrode diameter Gas flow rate Current Voltage Production speed Heat input Table 3 Production parameters W-2% ThO 2 electrode 99.9 % pure argon 2.4 mm 10 L/min 140 A 20 V mm/s kj/cm Results and Discussion In order to compare the microstructure of FeW/B 4 C coatings fabricated using TIG process, different powder mixtures were used. Coating thickness has been varied between 2000 µm and 2400 µm. The coatings were well formed without cracking. In Figs 4-10, the typical dendritic microstructure can be found clearly. According to the microstructure characteristics, the districts of the TIG process can be generally divided into deposited coating and substrate. The coating layer and the substrate were bonded metallurgical with interface layer each other. From Fig. 3 Ternary phase diagram of Fe-C-B 33 the interface to coating surface, cellular and dendritic solidification were observed in opposite direction to the heat flux. On the other hand, the planar solidification was observed at the interface. The structural change is associated with the solidification rate (V) of the metallic liquid in the PTA melt pool and the temperature gradient ( T). The T/V ratio of the interface between the coating layer and the substrate, whose temperature gradient is high and its solidification rate is low, is significantly high; hence, the interface displays a planar structure. The fact that the interface having a planar structure proves that the metallurgical bonding between the substrate and the coating layer is perfect. The solidification rate increases rapidly after planar solidification, and the temperature gradient that was at its maximum value at the beginning, starts to decrease rapidly. In conclusion, the T/V ratio decreases causing the formation of a cellular structure. Dendritic solidification occurs after cellular solidification. 16,31,32 Theoretically, the possible products of the reaction between liquid B 4 C and Fe may be Fe 3 (C,B), Fe 23 (C,B) 6, Fe 2 B, α-fe / Fe 3 (C,B), Fe 23 (C,B) 6, Fe 2 B according to the Fe C B ternary phases diagrams (Fig. 3).

4 256 INDIAN J. ENG. MATER. SCI., AUGUST 2012 The SEM image of FeW coating which was produced without B 4 C was given in Fig. 4. Generally, Fe 2 C, Fe 3 C and FeW 3 C carbides were formed in the microstructure of coating. This situation was supported by both EDS analysis (Table 4) and XRD pattern (Fig. 5). These carbides in α-fe matrix were found as reinforcement. FeW powder was converted into Fe and W with the given energy in the melt pool. A small amount of W 3 C hard phase was obtained from the combination of the carbon in the substrate and tungsten in the FeW powder (Fig. 5). The SEM image of FeW-10 wt% B 4 C composite coating was shown in Fig. 6 and its XRD pattern was given in Fig. 7. It was understood from the SEM image in Fig. 6b the borides and carbides formed with the dissolution of B 4 C with the given energy. The formation of Fe 3 (C,B), Fe 23 (C,B) 6 and FeW 2 B 2 phases prove the dissolution of B 4 C and this situation is supported with the XRD pattern in Fig. 7. In Fig. 6b, the EDS analysis of dark structure in SEM image is 5.81% B, 14.07% W and 80.12% Fe. This analysis demonstrates the formation of FeW 2 B 2 phase. The substrate, the interface and the coating layer regions of FeW-20 wt% B 4 C composite coating were given in Fig/. 8a. The center and upper regions of coating layer were shown in Figs 8b and 8c, respectively. The growth of most dendrites is observed to be upwards towards the alloyed layer surface, and their direction is almost perpendicular to the interface (Fig. 8b). However, in the upper region, due to the effect of polycrystalline, the microstructure becomes untidy dendrites (Fig. 8c). 16 The EDS analysis of A zone (Fig. 8a) of coating layer is 3.78% C, 9.79% B, 14.21% W and 72.22% Fe. The dendrites consist of Fe 2 W and interdendrites eutectics are composed of α-fe/fe 23 (C,B) 6 phases. The formation Table 4 Chemical composition (wt%) of coating layers Samples Microstructures S 1 S 2 S 3 S 4 Chemical composition (wt%) C B W Fe Cladding layer Balance Dendrite Balance Interdendritic Balance Cladding layer Balance Dendrite Balance Interdendritic Balance Cladding layer Balance Dendrite Balance Interdendritic Balance Cladding layer Balance Dendrite Balance Interdendritic Balance Fig. 4 SEM morphologies of specimen S 1 (a) general appearance and (b) interface (cross-section) and planar solidification Fig. 5 XRD pattern of S 1 sample (coated with FeW)

5 ISLAK et al.: MICROSTRUCTURAL DEVELOPMENT OF AISI 1060 STEEL 257 Fig. 6 SEM morphologies of specimen S 2 (a) general appearance and (b) top surface of clad Fig. 7 XRD pattern of S 2 sample (coated with FeW+10% B 4 C) of these phases is supported with the XRD analysis (Fig. 9). The EDS analysis of the coating layer deposited with the FeW + 40% B 4 C is 4.27% C, 11.16% B, 6.01% W and 78.56% Fe. The unremelted boron carbide (B 13 C 2 ) phase was observed in the coating layer (Fig. 10a). This situation showed that the energy Fig. 8 SEM morphologies of specimen S 3 (a) general appearance, (b) center region and (c) upper region of clad given was not sufficient for dissolving B 4 C totally. 23 The eutectics as a vein-like phase formed among the dendrites (Fig. 10b). The eutectic structure just in like other coatings is composed of α-fe/fe 23 (C,B) 6 phase. The formation of this phase is supported with the XRD pattern in Fig. 11. While the α-fe matrix

6 258 INDIAN J. ENG. MATER. SCI., AUGUST 2012 Fig. 9 XRD pattern of S 3 sample (coated with FeW+20% B 4 C) Fig. 11 XRD pattern of S 4 sample (coated with FeW+40% B 4 C) Fig. 10 SEM morphologies of specimen S 4 (a) upper region and (b) dendrites and interdendrites eutectics provides toughness, the Fe 23 (C,B) 6 phase enhances the strength to the coating layer. The tungsten inert gas (TIG) cladding of FeW-B 4 C powder with AISI 1060 steel is anticipated expected to significantly increase hardness in the cladding layer. The microhardness of the coating layer was Fig. 12 Microhardness profile of the cladding layer across the depth measured along the coating depth from the surface; all of the data were an average of five measurements. The microhardness profiles of the coatings across the depth are depicted in Fig. 12. Average hardness values of S 1, S 2, S 3 and S 4 specimens were measured as HV 480, 810, 880 and 1095, respectively. The FeW-B 4 C composite coating demonstrates a microhardness 4-5 times higher than the AISI 1060 steel (substrate). The carbide and boride formed in microstructure contributed to obtaining microhardness values stated above. The amount of carbide and boride increases with an increase in B 4 C content. The undissolved or resolidified B 13 C 2 and WC 1-X occured substantially in the microstructure of the cladding layer produced by increasing of B 4 C powder rate. B 13 C 2 and WC 1-X carbides contribute to a extra hardness and strength. 28,34 While hardness of coating area is in maximum level in the hardness change

7 ISLAK et al.: MICROSTRUCTURAL DEVELOPMENT OF AISI 1060 STEEL 259 between coating area and substrate, it reduces in small amount towards interface and reduces dramatically in the interface. This situation is associated with the fact that the Fe content is scarce in coating area, and that it is higher in interface area Conclusions The microstructural and compositional investigations have been carried out on cross-sections of FeW-B 4 C based coatings obtained by tungsten inert gas (TIG) process with different boron carbide (B 4 C) powder rate. The coating thickness produced on AISI 1060 steel with FeW-B 4 C powder by the TIG process was measured as µm using the optical microscope. The SEM examinations results showed that no evidence of microcrack and gas porosity formation was found in all production parameters. The microstructure of the coating surfaces consists of α-fe, Fe 3 (C,B), Fe 23 (C,B) 6 and FeW 2 B 2, Fe 2 W, WC 1-x and B 13 C 2 phases. The dendrite structures have been observed from the interface to the alloyed layer surface. The hardness of coating alloyed with B 4 C powder enhanced significantly. The FeW-B 4 C composite coating demonstrates microhardness 4-5 times higher than the AISI 1060 steel. The average hardness was between HV 480 and The maximum microhardness value was measured as 1095 HV in the coating which produced with 40% B 4 C powder rate. This is attributed to the presence of Fe 3 (C,B) and Fe 23 (C,B) 6 carbides dispersed in the eutectic matrix. References 1 Zhang D W & Lei T C, Wear, 255 (2003) Pang W, Man H C & Yue T M, Matter Sci Eng A, 390 (2005) Kumar S, Mondal D P & Jha AK, J Mater Eng Perform, 9 (2000) Islak S & Yildirim M M, e-j New World Sci Acad, 4(2) (2009) Jha A K, Prasad B K, Dasgupta R & Modi O P, J Mater Eng Perform, 8 (1999) Yildirim M M, Buytoz S & Islak S, Microstructure characteristics of SiC based coatings produced by means of TIG method on 45Mn5 steel, Proc 11 th Int Mater Symp, Denizli-Turkey, Yildirim M M, Islak S, Ulutan M, Karagöz M & Buytoz S, Microstructure properties of Cr 3 C 2 +SiC based coating produced by PTA process, Proc 13 th Int Mater Symp, Denizli-Turkey, Antony K C, Glenny J & Northwood J E, Hardfacing, Welding, Brazing and Soldering, vol 6, (Metals Handbook, American Society for Metals), Lin Y C & Wang S W, Tribol Int, 36 (2003) Buytoz S & Ulutan M, Surf Coat Technol, 200 (2006) Buytoz S, Surf Coat Technol, 200 (2006) Orlowicz A W & Trytek A, Wear, 254 (2003) Lu S, Kwon O & Guo Y, Wear, 254 (2003) Lou B, Chen Z, Bai W & Dong G, Trans Nonferr Met Soc China, 16 (2006) Candel J J, Amigo V, Ramos J A & Busquets D, Surf Coat Technol, 204 (2010) Chao M, Niu X, Yuan B, Liang E & Wang D, Surf Coat Technol, 201 (2006) Buytoz S, Ulutan M, Kurt B, Islak S & Somunkiran İ, e-j New World Sci Acad, 5(1) (2010) Huang S W, Samandi M & Brandt M, Wear, 256 (2004) Xu G, Kutsuna M, Liu Z & Sun L, Surf Coat Technol, 201 (2006) Du B, Zou Z, Wang X & Qu S, Appl Surf Sci, 254 (2008) Sun G, Zhang Y, Liu C, Luo K, Tao X & Li P, Mater Des, 31 (2010) Kathuria Y P, Surf Coat Technol, 140 (2001) Xibao W, Appl Surf Sci, 252 (2005) Rafainello W & Srivinasan M, Non-Oxide Materials: Applications and Engineering, in Carbide, Nitride, and Boride Materials Synthesis and Processing, (Chapman-Hall, London), Spohn M T, Miner Rev, 6 (1994) Pierson H O, Handbook of Refractory Carbides and Nitrides, (William Andrew Pub., Noyes), Meng Q W, Geng L & Zhang B Y, Surf Coat Technol, 200 (2006) Buytoz S, Yildirim M M & Eren H, Mater Lett, 59 (2005) Buytoz S, Mater Lett, 60 (2006) Nagarathnam K & Komvopoulos K, Metall Mater Trans A, 26 (1995) d'oliveira A S C M, Vilar R & Feder C G, Appl Surf Sci, 201 (2002) Li M, He Y & Sun G, Mater Des, 25 (2004) Vilars P, Prince A & Okomoto H, Handbook of Ternary Alloy Phase Diagrams, vol 4, (ASM), Kathuria Y P, Surf Coat Technol, 132 (2000) Thawari G, Sundarararjan G & Joshi S V, Thin Solid Films, 423 (2003) Jian, L N & Wang H M, Surf Coat Technol, (2005) Yang S, Liu W, Zhong M & Wang Z, Mater Lett, 58 (2004) Li Q, Song G M, Zhang Y Z, Lei T C & Chen W Z, Wear, 254 (2003)