Effect of Nickel Powder Buffering Layer on Microstructure and Hardness Properties of High Carbon Steel / Stainless Steel Arc Stud Welding

29, 2020 In this study, arc stud welding process was employed for welding AISI 316 stainless steel studs to AISI 1060 high carbon steel plates. A disc of Ni powder prepared and used as a buffering layer to enhance the properties of welding area. Optical and scanning electron microscopy were used to examine the microstructure. Energy-Dispersive X-ray (EDX) and X-Ray Diffraction (XRD) tests were performed to analyse and identify elements and phases, respectively in the weld region. The results observed that Ni powder prevented the direct contact between the dissimilar base metals. Existing of Ni altered the microstructure of the weld zone and encouraged dendritic type over cellular. Hardness reduced in the weld region from 600 HV to 200 HV due to the effect of Ni powder which prevented the formation of brittle


Introduction
In a process system that contains multiple parts, every part might work at a specific condition. Consequently, combination of different materials is necessary in design, especially in applications that require a certain combination of properties 1,2 . Welding technique can be employed to join between dissimilar metals. The properties of the resultant weld are governed by base metal (BM), filler metal, microstructural development in fusion zone (FZ) and type of implemented welding process 3 .
Many factors should be considered during dissimilar welding such as physical and mechanical properties of BM and possibility of alloying and compounds in the FZ. However, the employed welding technique might be the most effective factor. Based on literature, fusion and solid-state welding techniques have been used in joining between dissimilar metals [4][5][6][7] .
Welding carbon steel to stainless steel has a wide application in nuclear power plant, oil and petrochemical industries in components such as pressure vessels, heat exchanger and boilers. However, high carbon steel have poor weldability due to the high carbon content, which raises the hardenability and causes reduction in ductility and toughness 8,9 . Other issues included in fusion welding of high carbon steel are migration of carbon and possibility of formation of hard and brittle martensite phase in the heat affected zone (HAZ) 10,11 .
Conventional fusion welding processes such as shielded metal arc welding and gas metal arc welding techniques, are not feasible for joining many dissimilar metals due to metallurgical problems during welding, e.g. formation of intermetallic compounds and solidification cracking that lead to failure of the welded part during service 12,13 . Arc stud welding (ASW), which specialises in welding studs to plates is characterised with localised heating capability and performing welding in very short time [14][15][16] . Therefore, when ASW process is used for welding high carbon steel, the carbon diffusion might be restricted, and a very small HAZ can be produced. This offers a possibility to overcome the carbide formation in FZ and control martensitic transformation issue in HAZ 17 .
Another method that is used to improve the weldability of high carbon steel and increase the feasibility of joining high carbon steel to stainless steel is using of buffering layer. This can be accomplished by deposition a layer of an alloy, such as Ni base alloy onto the weld area before welding 2,11 . The deposition of Ni base alloy can significantly reduce the carbon migration and mitigate change in chemical, physical and mechanical properties throughout the weld 10,18 . Moreover, Ni has an intermediate thermal expansion coefficient that lays between carbon steel and austenitic stainless steel, Table 1, thereby decreases thermal stresses across the weld interface and consequently reduces internal tension stresses and improves life of the dissimilar joint 19 .
In gas power plants, the exhausts are usually made of high carbon steel and welded with stainless steel studs using shielded metal arc welding (SMAW) process. The high heat input of SMAW could affect negatively on the joint properties. Therefore, in this research, ASW process was performed instead of conventional arc welding process, and the impact *e-mail: mowahid.alali@uokufa.edu.iq of Ni powder buffering layer on the microstructure and hardness properties of the dissimilar joint was investigated.

Experimental Procedure
In this investigation, full threaded (M12 mm, 80 mm length) studs of AISI 316 stainless steel and plates of AISI 1060 carbon steel with dimensions of 5×5×8 mm were used as a BM. The carbon steel plates received in as-rolled condition. Commercially pure Ni powder with particle size of 50 μm was used for making the buffering layer. Table 2 shows the chemical composition of the materials used in this study. Ni powder was used as a buffering layer in a shape of disc with dimensions of 12 mm diameter and 1 mm thickness. The disc-shape was produced by compressing the Ni powder with 10-ton press in a suitable die. Prior welding, the disc placed in a solid hole with 12 mm diameter and 1 mm depth made in the centre of carbon steel plate. A schematic diagram of weld assembly presented in Figure 1.
ASW process was used to perform the joint between studs and plates using DABOTK (1000) welding machine. Welding parameters utilised in this study were 600A and 800A welding current and 0.2s, 0.3s and 0.4s welding time. Carbon steel plates were annealed by heating to about 850°C, holding for 20 min and left to cool in furnace. Before welding, the carbon steel plates were preheated at 350°C to prevent the heat sink during welding. The weldments then cut and prepared for metallographic examination and microhardness test using electrical discharge machining (EDM) technique following by standard grinding and polishing methods.
Microstructure was examined by optical microscope. To reveal microstructures at better details, the samples were etched with fresh mixed acids (two parts of acetic acid, two parts of HNO 3 and three parts of HCL) at stainless steel side and Nital solution at carbon steel side. The elemental distribution was studied by EDX analysis using Inspect F device. The scan performed in secondary electron mode with accelerating voltage of 20kV and spot size of 5. XRD test was carried out to identify phases and compounds. SHMADZU diffractometer model XRD 6000, Japan was used to perform the XRD scan. Vickers microhardness test was conducted according to ASTM 384 standard using LARYEE microhardness testing device. The measurements were engaged along the BM, HAZ and FZ, using 500g load and 15s dwell time.

Macrostructure and microstructure examination
Welding samples were subjected to visual test according to ISO 14555 to evaluate welding parameters with respect to weld appearance. The Welds should be free of welding defects such as lack of fusion and undercut. Only passed samples (Table 3) were subjected to further microstructure and hardness tests. Figure 2 shows the macrostructure of weldments prepared at different welding conditions. Apparently, the structure consists of three regions. The top region represents the stud, the fusion zone (FZ) is located in the middle and the region at the bottom is the carbon steel plate. An increment in the thickness of FZ can be noticed when Ni powder was added. Existing and distribution of Ni through the FZ was detected and confirmed by EDX analysis as shown in Figure 3 and Table 4. Ni wt% increase towards the centre of the FZ as shown in spectrum 20 and 21 in Figure 3 resulting from Ni powder addition. The increment of Ni wt% in FZ played an important role in controlling microstructure and mechanical properties.    Microstructure of the welded region adjacent to carbon steel is shown in Figure 4. Lower melting temperature and thermal conductivity of AISI 316 than AISI 1060, Table 1, causes FZ to be completely consisted of re-solidified microstructure of austenitic stainless steel that is characterised with ferrite-austenite (FA) solidification mode with ferrite as primary phase and austenite as a secondary. This solidification mode can be predicted using WRC-1992 constitutional diagram that introduced by welding research council in 1992 ( Figure 5), which depends on Cr and Ni equivalents of the steel. At the region near fusion boundary for the all welding conditions, dendritic structure consists of skeletal columnar ferrite in austenite matrix ( Figure 6) was mainly observed. It is believed that the dendritic mode of solidified structure is resulted from high speed, localised heat of ASW process and existing of relatively high thermal conductive Ni powder ( Table 1). All these factors led to decrease the G/R ratio, where, G is thermal gradient and R is solidification rate and encourage dendritic over cellular structure 21,22 .
Many locations in the welding joints without Ni powder featured with presenting of cellular-type structure as shown in Figure 7. The absence Ni increases the G/R ratio and encouraged cellular structure over dendritic 14,23 . At the weld centerline (Figure 8), columnar dendritic structure continued dominating at all weld conditions However, at 800A, the grains showed a noticeable growth and larger spacing between dendrites as presented in Figure 8c. The grain growth could be due to the relatively higher heat input of this condition. At the region adjacent to stainless steel, skeletal ferrite was mainly observed for the different welding conditions as shown in Figure 9. The columnar dendritic structure formed due to the high solidification rate that overcomes thermal gradient in this region due to the high speed and localised heat of ASW process.       Figure 10 shows distribution of hardness reading across the weld, covering BM, HAZ and FZ of the weld conditions (600 A and 0.3 s, with and without Ni powder). At carbon steel side, significant increase in hardness at HAZ was obtained with more than 750 HV. The high hardness number can be explained in terms of the effect of high carbon content that increases hardenability and consequently susceptibility of carbon steel to cooling rate or on the other word, welding speed. In FZ, the effect of Ni powder was clearly observed. Hardness of the weld without Ni powder reached to 600 HV. While with Ni addition, hardness decreased to 200 HV. Formation of brittle Fe-Cr phase, which was detected by XRD in FZ, Figure 11, could be the main reason for

Conclusions
In the present work AISI 316 stainless steel studs were welded to AISI 1060 high carbon by ASW process. Ni powder disc was utilised as a buffering layer. The following points can be concluded from this study: • Lower melting temperature and thermal conductivity of AISI 316 than AISI 1060, causes FZ to be completely consisted of re-solidified microstructure of austenitic stainless steel FA solidification mode.

•
Many locations in the welding joints without Ni powder featured with presenting of cellular-type structure. • Addition of Ni powder affected the microstructure of the FZ by encouraging growth of dendritic solidification type over cellular.

•
Hardness significantly reduced in the weld region from 600 HV to 200 HV once Ni powder disc was applied and formation of brittle Fe-Cr phase was eliminated.