PTA WELDING OF DUPLEX STAINLESS STEEL USİNG Cu/Ni INTERLAYER

PTA WELDING OF DUPLEX STAINLESS STEEL USİNG Cu/Ni INTERLAYER Ihsan Kirik 1, Niyazi Ozdemir 1, Serdar Mercan 2, Zulkuf Balalan 1 1 University of Firat, Faculty of Technology, Department of Metallurgy and Material Engineering, Elazığ, Turkey. alihsankirik@gmail.com, nozdemir@firat.edu.tr, 2 University of Cumhuriyet, Faculty of Technical Education, Department of Machine, Sivas, Turkey. ABSTRACT In this study, commercial duplex stainless steel plate of 5 mm thick was welded by plasma transfer arc (PTA) welding technique both itself and using copper/nickel plate of 2 mm thick in interface. The effect of austeniticferrites interlayers on microstructure of AISI 2205 duplex stainless steel joined by PTA welding process was examinated. So, microstructure, mechanical behavior and interface layer effects of plasma welding were investigated. After plasma welding, microstructural analysis including metallographic examination, and tensile strength tests were performed. As a result of this study, it was seen that with this technique can be obtained penetration depth of 5 mm without any pretreatment of welding. Also, Ni interface layer was raised tensile strength of welding but, monel interface layer was decreased tensile strength values. 1. INTRODUCTION Plasma arc welding (PAW) can be defined as a gas-shielded arc welding process where the coalescence of metals is achieved via the heat transferred by an arc that is created between a tungsten electrode and a workpiece. The arc is constricted by a copper alloy nozzle orifice to form a highly collimated arc column. The plasma is formed through the ionization of a portion of the plasma (orifice) gas (Raymond and Slatter, 1998). The process can be operated with or without a filler wire addition. The PAW process can be used in two distinct operating modes, often described as the melt-in-mode and the keyhole mode. The melt-in-mode refers to a weld pool similar to that which typically forms in the gas-tungsten arc welding (GTAW) process, where a bowlshaped portion of the workpiece material that is under the arc is melted (Hsu and Rubinsky, 1988;Wang and Chen, 2002). In the keyhole mode, the arc fully penetrates the workpiece material, forming a nominally concentric hole, or keyhole, through the thickness. The molten weld metal flows around the arc and resolidifies behind the keyhole as the torch traverse the workpiece (Haris, 1993; Kurt et al., 2009). Duplex stainless steels (DSS) are finding increased application in aggressive marine environments because of their superior performance in comparison to traditional austenitic stainless steels (Sieurin and Sandstrom; 2006). DSSs received the name from the fact that their microstructure consists of a balanced mixture of ferrite and austenite phases after water quenching heat treatment (Charles and Bernhardsson, 1993). The ferrite/ austenite phase balance in duplex stainless steel is achieved primarily by adjusting chromium, nickel, and nitrogen contents, and by control of thermal history (Folkhard, 1988; Tavara, et al., 2001). Conventional fusion welding processes required for construction assembly have a considerable impact on duplex structure, both in the fusion zone (FZ) and in the heat affected zone (HAZ) (Elsawy, 2001). It is well known that impact toughness of the welds in duplex stainless steels decreases with the increase of δ-ferrite in the HAZ, since the local duplex

structure is severely ferritized by the high peak temperature and by the fast cooling rate of the thermal cycle. Another problem associated with fusion welding of these materials is their susceptibility to solidification cracking, which is greater than that of the 304 L austenitic stainless steels (Lippold and Kotechi, 2005; Davis,1993). The precipitation of undesirable phases such as intermetallic compounds, carbides and nitrides can cause a drastic deterioration in toughness and corrosion resistance, for instance in the case of γ-phase, which has very fast formation kinetics (Krysiak, et al.,1993). Therefore it is necessary to assure the continuity of duplex structure properties across the weld by controlling the phase balance both in the FZ and in the HAZ. For practical application of this kind of welded joints, an adequate proportion of ferrite in the FZ would be in the range of 30 70% (Martikainen, 1995). This ferrite content depends on the chemical composition of the FZ and the cooling rates of the weld, which are related to the input energy applied during welding (Irving, 1995). For this reason, the present research is intended to determine the optimal welding conditions for autogenous welding (with and without filler) of duplex stainless steels while controlling the input energy. In the literature, limited research has been made on the welding of duplex stainless steel. Duplex stainless steel was joined by gas tungsten-arc welding process using with and without nickel enhancement (Urena, et, al.,2007). Duplex stainless steel was joined by submerged arc welding and reported that cooling the weld in air provides a satisfactory amount of reformed austenite and prevents formation of sigma phase (Muthupandia, et al.,2003). SAE 2205 duplex stainless steel was welded by the electron-beam technique. They also revealed that the grain growth in the heat affected zone (HAZ) was restricted when welding with the electron beam technique (Ku, et al., 1997). In the present study, AISI 2205 duplex stainless steel plate of 5 mm thick was welded by plasma welding technique using Cu/Ni of 2 mm thick in interface. Therefore, austenitic interface layer selected to plasma welding of the duplex stainless steels. This application in plasma welding is a first. In this study, also the effects of austenitic and ferritic interlayer material on penetration, microstructure and mechanical behavior of plasma welding were investigated. 2. EXPERIMENTAL PROCEDURE AISI 2205 type duplex stainless steel plate of 5 mm thick was welded by plasma welding technique both itself and using copper/nickel plate of 2 mm thick interlayer in this research. The chemical composition of AISI 2205 and Cu/Ni interlayer was seen in Table 1. Specimens were prepared in dimensions 70x5x65 mm 3 (AISI 2205) and 70x5x2 mm 3 (Cu/Ni). The operating principle of plasma welding system was schematically illustrated in Fig. 1. After plasma arc welding, the welded specimens were examined by a scanning electron microscopy (SEM), and an energy dispersive spectroscopy (EDS). Intermetallic compounds were identified by X-Ray diffraction technique. Tensile test was carried out at room temperature after welding. The fracture surfaces were observed using scanning electron microscopy. Materials Table 1. Chemical compositions of test materials Alloying elements (wt%) C Mn P S Cr Mo Ni Cu Co Fe AISI2205 0.01-0.03 1.68-2.00 0.026 0.001 21-23 1.3320 3.374 - - Balance Nickel - 0.007 - - - - Balance 0.001 0.066 - Monel 0.1 1 0.045 0.02 - - Balance 32.5-1.6

Table 2. The processes parameters used in PTA welding of AISI 2205 with Cu/Ni interlayer Specimen no Welding current (A) Plasma gas flow (l/min) Shielding gas flow (l/min) Traverse speed (m/min) Orifice diameter (mm) Nozzle/workpiece distance (mm) Interlayer S1 130 1.1 27 0.01 2.4 2 - S2 130 1.1 27 0.01 2.4 2 nickel S3 130 1.1 27 0.01 2.4 2 monel S4 140 1.1 27 0.01 2.4 2 - S5 140 1.1 27 0.01 2.4 2 nikel S6 140 1.1 27 0.01 2.4 2 monel Figure 1. The operating principle of keyhole plasma welding system 3. RESULTS AND DISCUSSION 3.1. Macro and microstructure of the welded parts Fig.2 illustrates the width and top surface of PTA-welded joints of specimens S1, S2, S3, S4, S5 and S6 using two different welding currents, without and with an Cu/Ni interface layers, respectively. The widths of weld metal surface are seen as approximately 8,5 and 10 mm for the welding curent of 140 A, respectively. The volume of weld metal increases depends on increasing the current intensity, associated with the increase of heat input. As can be seen in Fig. 2, the penetration depths for the two different welding current without and with Cu/Ni interlayer were obtained between 2.5-5 mm, respectively. According to these results, the penetration depth of the specimens decreased with the decrease of welding current. With increase of the welding current, obtained higher heat input reasons to a greater increase in the penetration depth and volume of weld metal. Figure 2. The cross-sectional macroappearance of PTAwelded specimens. S4, S5 and S6 specimens

Fig. 3 shows the evaluation of the microstructure for specimen S1 (without interlayer) that occurred at the welding interface (adjacent to the weld zone, weld metal and parent metal). In the PTA welding of specimen S1, Cu/Ni interface layer was not used. As shown in the figure, the weld metal consisted of plenty acicular ferrite islands and numerous widmanstatten ferrite, lath and plate type martensite. The structures that occurred were observed to be quite different, and cracks, cavities and unconnected interfaces were not observed in both transition zones adjacent to the weld zone. Weld metal Weld metal Grain Boundary Ferrite Without interlayer Parent metal Transition zone Austenite Figure 3. SEM micrograph taken from the welding interface of S1 specimen without interlayer In Figure 4 the welded samples are shown for S3 specimen with monel interlayer. As shown in the figure, the main metal consisted of austenite and ferrite grains, the weld metal consisted of dendritic structure, grain coarsening and numerous widmanstatten ferrite. Dendritic structure and grain coarsening is thought to occur due to fast cold and increasing heat input. Also, this situation has been influenced by the use of monel interlayer. The structures that occurred were observed to be quite different, and cracks, cavities and unconnected interfaces were not observed.

Weld metal Transition zone Grain coarsening With monel interlayer Transition zone Parent metal Austenite Ferrite Figure 4. SEM micrograph taken from the welding interface of S3 specimen with monel interlayer In Figure 5 the welded samples are shown for S5 specimen with Ni interlayer. As shown in the figure, it was determined that the grains on the welding seam first occurred vertically to the main material beginning from the first seam side where the hardness began and then the first hardened grains did not occur vertically on the dendritic structure in the inner sides of the seam. It was observed that the width of the welding seam and HAZ became wider than that of the joints at 130 A welding power depending on the increased heat input. On both sides of the welding seam, there was no a specific grain hypertrophy, deformation and crack formation on the main materials. A homogeneous dispersion with small grains, beginning from the zone with coarse grains on both sides appropriate for the original structure of the materials was also observed due to austenitic Ni interlayer material.

Weld metal Transition zone With nickel interlayer Transition zone Parent metal Acicular ferrite Widmanstätten ferrite Figure 5. SEM micrograph taken from the welding interface of S5 specimen using nickel interlayer 3.2. Microhardness The cross-sectional microhardness measurement results of AISI 2205 PTA welded joints without and with nickel/monel interlayer are given in Fig. 6. As it is clearly seen, the hardness distribution curves of the welded joints of all specimens in the weld center are the lowest. The hardness increase towards both sides of the weld metal, in the non-affected region reaches the original hardness of the materials. The increase of hardness in the welding interface might be related directly to the microstructure formed in the welding interface. Chrome carbide phases and a martensitic structure are formed in the intermediate zone as a result of rapid cooling increased the hardness across the weld seam zone.

Figure 6. Microhardness distribution across the welding interface of PTA-welded specimens 3.3. Tensile test results Macro photo of tensile test specimens of PTA-welded joints after tensile tests are shown in Fig. 7.From macro photo of the tensile tests, it is seen that, all specimens are fractured in welding line, except specimens which welded with nickel interface layer; the others are fractured in welding line without necking. Figure 7. Optical photo of tensile test results of PTA-welded joints Figure 8 shows the results of the tensile test for specimens S1 S6 and AISI 2205. Examining the tensile test values of the specimens, a decrease of these values was observed depending on the decrease in the welding current. This decrease was directly associated with the penetration depth, the presence of unconnected zones and structural changes that took place within the weld zone. As it is known the presence and size of an unwelded zone would create a notch effect and have an impact on the fracture behavior of tensile tests. In the result of tensile tests of the specimens welded without and with monel/nickel interlayer, the results of between 421-780 MPa obtained. These results showed that nickel interface layer in the plasma welding of duplex stainless steel increased the tensile strength; on the other hand monel interface layer decreased tensile strength. This result can be explained with lower carbon content and other elements of fusion zone by nickel interface layer.

Figure 8. Tensile test results of PTA-welded joints 3.4. EDS analyses SEM micrographs and EDS analysis of the welded joints S1, S3 and S5 are presented in Fig. 9 and Table 3, respectively. Point 1 (weld metal center), point 2 (adjacent to transition zone), point 3 (transition zone), point 4 (adjacent to the transition zone AISI 2205 side) and point 5 (on AISI 2205 side) are marked as the EDS analysis points. Figure 9. The SEM micrographs and EDS analyses points across the welding interface of the PTA-welded specimens S1, S3 and S5. Table 3. EDS analyses results of S1, S3 and S5 specimens Alloying elements (wt%) Specimen EDS point no Mn Cr Si Ni Cu Fe S1 1. Point 0.75 19.4 1.02 3.71-67.72 2. Point 0.81 19.17 1.10 3.41-67.84 3. Point 0.83 19.14 1.30 4.13-67.89 4. Point 0.81 19.9 1.12 3.63-66.60 5. Point 0.78 20.3 1.28 3.64-66.93 6. Point 0.72 19.09 1.27 3.79-67.02 S3 1. Point 0.68 9.16 0.58 21.0 10.2 51.0 2. Point 0.70 9.37 0.73 20.3 9.78 50.6 3. Point 0.52 15.52 0.87 12.3 5.43 59.7 4. Point 0.87 21.3 1.23 4.23-70.4 5. Point 0.80 21.4 1.11 4.11-70.8 S5 1. Point 0.67 13.67 0.74 21.16 0.51 57.1 2. Point 0.62 13.96 0.65 20.13 0.65 56.4 3. Point 0.78 18.84 1.12 10.82 0.46 66.4 4. Point 0.83 21.44 1.12 4.32-70.5 5. Point 1.10 21.61 1.13 3.67-70.9

The results of the elemental analysis and microstructural examination in the interface region of PTA-welded joints (specimens S1, S3 and S5) clearly demonstrated that different amounts of Fe, Cr, Si, Ni, Cu and Mn were obtained in S3 and S5, but in S1 Cu not determined. Additionally, from the results of EDS analysis taken from the interface for the PTA-welded specimen S3, it was revealed that the elemental Cu and Ni transition occurred from monel interlayer to AISI 2205. Furthermore, elemental Cr and Si transition occurred the same distance from AISI 2205 duplex stainless steel to weld metal. Otherwise, in the EDS analysis of specimen S5 inconsiderable Cu was obtained. And in the EDS results of S5 only Ni transition occurred from nickel interlayer to AISI 2205 and elemental Cr, Si, Fe and Mn transition occurred the same distance from AISI 2205 duplex stainless steel to weld metal, as seen in the Table 3. 3.5. Fractography The fracture surface images of the tensile test for the PTA-welded specimens S1, S3 and S5 are given in Fig.10. In this micrograph, it can be seen that the fracture surface of S1 contains ductile dimples which form by the microvoid coalescence mechanism and facets associated with cleavage fracture. And fracture surface of the PTA-welded specimen S5 tested at room temperature also exhibits a similar mixed dimple and facetted appearance. However, the ductile mode of failure tends to predominate in the PTA-welded of AISI 2205 using nickel interlayer. But in SEM photograph of S3 it is seen that the fracture mode was brittle manner which separate cracks. Figure 10. SEM micrographs of fracture surfaces after tensile test of PTA-welded specimens. S1, S3 and S5 specimens

4. Conclusions AISI 2205 type duplex stainless steel plate was joined by plasma arc welding technique without and with nickel/monel plate of 2 mm thick in interface. The following results were obtained. AISI 2205 duplex stainless steel can be joined without and with nickel/monel interlayer by PTA welding process using on 130 and 140A welding current. The maximum hardness was observed in the transition zone that the hardness is due to the formation of martensite structure as a result of sudden cooling. Penetration depth increased the impact strength of keyhole like welding of AISI 2205 without and with nickel/monel interlayer. The welding current increased the tensile strength of the PTA welding of AISI 2205. In the result of tensile tests of the specimens welded without and with monel/nickel interlayer, the results of between 421-780 MPa obtained. These results showed that nickel interface layer in the plasma welding of duplex stainless steel increased the tensile strength; on the other hand monel interface layer decreased tensile strength. 5. References Charles, J., Bernhardsson, S., Proceedings Duplex Stainless Steels, 91, p. 3, 1993. Davis, J.R., Selection of wrought martensitic stainless steels, ASM Metals Handbook, V.6, 432-441, 1993. Elsawy, A.H., Characterisation of the GTAW fusion line Phases for superferritic stainless steel weldment, Journal of Materials Processing Technology, 118, 128-132, 2001. Folkhard E. Welding Metallurgy of Stainless Steels. Wien/New York: Springer Verlag, 1988. Harris, ID., Welding, brazing and soldering, ASM metals handbook, vol. 6. OH: Materials Park; p. 195, 1993. Hsu, YF., Rubinsky, B., Two-dimensional heat transfer study on the keyhole plasma arc welding process, Int J Heat and Mass Trans 31(7):1409 1421, 1988. Irving, B., Plasma arc welding takes on the advanced solid rocket motor, Weld J 71:49 50, 1995. Krysiak, K.F., Grubb, J.F., Campbell, R.D., Selection of Wrought Ferritic Stainless Steels, ASM Metals Handbook, 6, 443-454, 1993. Kurt B., Orhan, N., Somunkiran, I., Kaya, M., The effect of austenitic interface layer on micro structure of AISI 420 martensitic stainless steel joined by keyhole PTA welding process, Materials and Design, 30, 66, 1-664, 2009. Ku, J.S., Ho, N. J., Tjongb S.C., Properties of electron beam welded SAF 2205 duplex stainless steel, Journal ofmaterials Processing Technology, 63, 770-775, 1997. Lippold. JC., Kotecki. DJ., Welding metallurgy and weldability of stainless steels, Wiley, Hoboken, New Jersey, pp 88 135, 2005. Martikainen, J., Condition for achieving high-quality welds in the plasma arc keyhole welding of structure steels, J. Mater. Process Technol., 52(1), 68-75, 1995. Muthupandia, V., Bala Srinivasan P.,, Seshadri, S.K., Sundaresan, S., Effect of weld metal chemistry and heat input on the structure and properties of duplex stainless steel welds, Mater. Sci. Eng. A, 358, 9 16, 2003. Raymond T, Slatter, E., The plasma advantage in automated welding, Weld J 9:55 57, 1998.

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