Effect of welding heat input on the microstructure of dissimilar metals: Inconel 625 and. 316L stainless

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1 Effect of welding heat input on the microstructure of dissimilar metals: Inconel 65 and 36L stainless Esmail Ahmadi Zadeh, Mohammad Masaeli, Reza Dehmolaye Advanced Materials Research Center, Faculty of Materials Engineering, Najafabad branch, Islamic Azad University, Najafabad, Isfahan, Iran Abstract In dissimilar joining, the correct selection of filler metal and appropriate joining heat input is critical. In the current study, two dissimilar alloys (Inconel 65, 36L stainless steel) and a super alloy of Inconel 65 were welded using the tungsten arc method under inert gas protection. Welding was performed using three filler metals (Inconel 65, 8 and 39 L stainless steel) and three different heat inputs (.5,.9,.3 kj/mm) under the protection of argon gas. Microstructures of different areas of welding joints were investigated under all welding conditions using optical microscopy and a scanning electron microscope equipped with energy dispersive spectroscopy (EDS). The results showed that all joining have a good continuity with no splits or discontinuity at the joint point. All filler metals microstructures were observed in austenitic form with frozen dendrite structure. This investigation showed the presence of an unadulterated region in some joining, and it became clear that this area increased with increased heat input. Keywords: Dissimilar welding, microscopic microstructure, filler metals, Inconel 65, 36L stainless steel Introduction Welding is commonly used to connect a wide range of metals and alloys with different mechanical and physical properties. In recent years, many studies have been conducted on welding nickel-based alloys to stainless steels with a focus on finding the ideal filler metal. Studies have demonstrated that nickelbase filler metals show superior properties compared with austenitic stainless filler metals; cracking caused by freezing occurs when stainless steel filler metals are used for these joining [-]. Sireesha et al. evaluated dissimilar welding between austenitic stainless steel 36 and alloy 8 using four types of filler metals. Their results showed that the nickelbased filler metals had a higher tensile strength and thermal stability than the austenitic stainless filler metals [3-5]. Lee et al. examined the effect of various amounts of titanium in filler metals on the weldability and mechanical properties of dissimilar welds between nickel-based ally 69 and austenitic stainless steel L34. The results showed that as the titanium content increased in the chemical composition of the filler metal, the microstructure changed from columnar dendrite to coaxial dendrite [6]. In another study, Nafakh et al. studied the microstructure and dissimilar joining weldability between Inconel 657 and austenitic steel 3 Their results showed that the filler metal Inconel A (a nickel-base filler metal) had an optimal weldability at room temperature [7-8]. However, a study of literature in the area of dissimilar metal joining indicated that there is no systematic study of fusion welding between alloy 65 and austenitic steel 36L, nor is there an ideal filler metal for a joining between them. Therefore, we examined the microstructures of welded metals, and heat-affected zones were evaluated using different filler metals. The effects of the welding process and heat input on microstructure and metallurgical properties of the welding metals were also investigated. Methodology A super-alloy of Inconel 65 (nickel-based) and austenitic stainless steel 36L were used as base metals, and filler metals Inconel 8, Inconel 65, and

2 stainless steel 39L were used for better resistance against splits. Table shows the chemical composition of the base and filler metals. Base and filler metal s 36L Incon el 65 ERNi CrMo -3 ERNi Cr-3 39L Table : Chemical composition of base and filler metals used (wt percentage) Fe Rem ainin g Rem ainin g Nb +T a N - - >. M o Ni 4 Rem ainin g 63 Rem ainin g 3 C r M n Si Base alloys were chosen from sheets with a thickness of 4 mm, and, in accordance with the standards of electrodes and welding wire (AWS), welding wire with a high nickel content was used for sample welding. To establish the joining between the base metals, sheets with a length of 3 mm, width of 5 mm, and thickness of 4 mm were prepared. To ensure fusion welding operation and proper penetration, the sheet was prepared in accordance with the joining plan. The joining was created under gas (for butt) using the arc tungsten welding process. The sample joining plan was prepared in a one-way zigzag form with an angle of 7 C. Sample chamfering operations were performed using a milling machine with the joining design shown in Figure. Root Face (mm Groove Angle ( ) 7 Root Opening (mm) Thickness (mm) 4 C 4 3 Figure : Joining design and its dimensional specifications To perform the welding, samples of both base alloys were assembled together (for butt) with a distance of 4. mm (equivalent diameter of welding wire used) using welds. Welding of samples was performed under different conditions without preheating and using the tungsten arc-electrode welding method with shielding gas and electrode negative polarity (DCEN) (two filler metals of Inconel nickel base 8.65, and austenitic stainless filler metal 39L). Table shows the sample specifications. One of the most important parameters in welding is the heat input because it impacts preheat and inter-pass temperatures, thus affecting the structure and properties of the weld metal and the HAZ area. The heat input cannot be directly measured; therefore, equation () is () Heat input = µ (UI / V). In this relationship, µ is the welding efficiency, which is equal to 65. Welding voltage, current strength, and welding speed are represented by V, ma, and mm/s, respectively. Table shows the variables used to calculate the heat input for the three welding filler metal (Inconel 65, Inconel 8, and austenitic stainless filler metal). Table : Welding parameters and filler metals used Mean Curre weldi Sample nt Volta Heat Sam ng characteri stren ge input ple speed stics gth (V) (KJ/m (mm/ (A) m) S) Inconel 8 Inconel 65 stainless 39L Inconel 65 Inconel 8 stainless 39L stainless 39L Inconel 65 Inconel

3 To study the microstructures of the base alloys, welding, and cracks in the heat-affected zone, metallography was used. For this purpose, samples of welded base alloys with dimensions of 4 cm were made from welds under different conditions. The samples were mounted hot, and then, using the silicon carbide emeries, were struck from No. to After this, with the help of alumina powder (particle size 3 micrometers) and diamond powder, the samples were polished in two stages. The samples were etched using a marble solution ( gr CuSO4 + 5cc HCl + 5cc HO) for 35 seconds. Then, the microstructures of the different weld areas, base metals, and heat-affected zones were analyzed using an optical microscope. Additionally, a scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS) was used to semiquantitatively and comparatively study and determine the chemical compositions, and to more accurately identify phases and different structural areas. Results Welding metal microstructures Based on the chemical composition of the base and filler metals, and given that all have a cubic crystal structure with coaxial aspects; we found that the weld metal has a cubic crystal structure with a centered aspect. Since the ratio of nickel in this area is higher than that of iron, and since it has a FCC structure, we can use the term austenite. However, the solid nickel solution has iron and chromium that embody other alloying elements, such as titanium, niobium, manganese, silicon, and carbon in its place (due to the chemical composition of filler metal). Other studies have shown such elements in weld metals [9]. The influence of microstructure on filler metals Fine microscopic microstructures of the weld metals resulting from three different filler metals at a heat input of.5 KJ/mm were evaluated. Figure shows the microscopic metal microstructure of Inconel filler metal 8. In Figure -A, the dendrite microstructure of the coaxial weld grains is clearly visible, and the figure shows that the microstructure in the form of dendrite structure is axial, and secondary dendrite branches are also detectable. The figure also shows that the weld metal structure is fully austenitic because no transformation occurred during its freezing. In this figure, the grains at the cellular or dendrite levels are variable depending on their location. Near the fusion line, the microstructures of columnar grains are more cellular-dendrite, and secondary dendrite branches nearly absent. In areas close to the welding line, the microstructures of axial grains are more dendrite axial, and secondary dendrite branches are detectable in these areas. The change of the frozen state (by moving from the sides towards the welding center) is schematically depicted in Figure -B. In general, the content under combined freezing in alloys is largely determinant of the microstructure type created. The ratio of G / R is used as a measure to describe the combination of contents under combined freezing, where G is the thermal gradient and R is the growth rate. On the sides of the welds, R is the lowest and G is the highest value. Therefore, the X / R ratio reflects the low combined freezing in these areas. This leads to the formation of cellular or cellular dendrite structures on the welding sides or near the fusion line. As we move towards the welding center, the numerical value of R increases and the numerical value of G decreases. Therefore, the (G / R) ratio also decreases, resulting in more central welding points under combined freezing. This leads to the creation of the axial dendrite microstructure in this region. Moving from the edges towards the welding center, the microstructure becomes finer, in addition to changing the freezing state. This can be discriminated both visually and through measuring the distance between the dendrite arms. Microstructure crashing results from the cooling rate and more germination in the central areas. The multiplication of G R represents the cooling rate. As mentioned previously, R increases in the central parts of the welding, and G decreases. Since the increase in the R value (increased from zero at the edge of the RCL in the weld center line) is higher than the decrease in the G value, the (G R) multiplication increases moving towards the welding center and the cooling rate will be higher in these areas. In addition, more offshoots will appear in the central part of the weld. As the cooling rate in the freezing temperature range gets higher, the freezing time will be shorter, which gives less opportunity for the growth of dendrites and dendrite branches. Increases in grain number due to this higher germination results in less time for dendrite formation and growth from any grain. As the cooling rate in the freezing temperature range will be lower, there is more time for freezing, and this causes the smaller dendrite arms to be replaced by larger arms. This phenomenon is due to a reduction in the total surface energy. Smaller dendrite arms have a greater surface energy per unit volume; however, their level increases as their branches get smaller. Thus, the total surface energy reduction occurs due to the loss of dendrite small arms (if time remains) and this results in dendrite microstructure magnification and increases in inter-dendrite distances with decreasing cooling rate in the freezing temperature range. Other studies have produced similar results []. Figure (c) shows the Inconel 8 welding metal microstructure electron micrograph where the 3

4 austenitic and sediment field is visible in the background. The dendrites have been drawn from the austenitic field border to the grains center. The pond turbulence and cooling rate was slow, and dendrites formed in a particular order. No split was observed in the welding metal. Figure : Inconel 8 welding metal microstructure (a) optical microscope image, (b) schematic of freezing state changes (moving from the sides towards the center of welding), and (c) scanning electron microscopy (SEM) The microscopic structure of welding metal Inconel 65 is shown in Figure 3 (a). Inconel 65 has an austenitic field with deposits scattered in the field. This figure shows that the welding metal is frozen in the dendrite form. Given that the chemical composition of the filler metal and that of the base metal are nearly identical, the welding freezing structure is close to the freezing structure of Inconel 65, namely in its austenitic structure. There are continuous dendrites, somewhat similar to column dendrites, distributed equally around the welding, which demonstrates the uniformity of the chemical composition in the welding metal. In addition, as the cooling rate is greater, the distance between the dendrite arms will be less and the strength and toughness improve. Figure 3 (b) is an electron micrograph image of a weld with the filler metal Inconel 65. Inconel 65 is shown with an austenitic field with deposits scattered in the field, and the welding metal is frozen in the dendrite form. The deposits belong to molybdenum carbide and chromium M6C (where M means molybdenum and chromium is C). Due to the high content of molybdenum in the composition of Inconel 65 (Table ), the carbide formation is expected to 4

5 contain a high content of molybdenum. No splits were seen on the welding metal. B A Figure 3: Welding metal microstructure of Inconel 65, A- optical microscope image, (b) scanning electron microscopy (SEM) The microstructure of 39L stainless steel welding metal is shown in Figure 4. The figure shows that the welding metal is a fully austenitic structure with dendrite morphology. In these areas, the welding metal is solidified in an austenitic-ferrite structure. Structures of the areas solidified in the (AF) state are have some delta ferrite (δ) that is separated in the boundaries between dendrites or cells. The freezing types of austenitic stainless steel are sensitive to composition (ratio, Creq / NieqNieq = Ni% + 3C% + 5Mn%, Creq = Cr% + Mo% +.55Si% + 5Nb%) and kinetic parameters (welding speed). Other studies have shown similar results [-]. The ratio of Creq / Nieq in stainless steel 39L is equal to.7 (Table ), which is a high value, and the mean welding speed for this is equal to.86 mm/s, which is relatively low. With high ratios of Creq / Nieq and lower welding speed, the freezing type orients towards the AF type. Therefore, the dominant freezing type of welding metal stainless steel 39L is AF. Other studies have reported similar results [9, 3]. Figure 4: Optical microscope image of the microstructure of the welding metal 39L Effect of heat input on the microstructure of different parts of welding metal Effect of heat input on welding metal microstructures The microstructure of Inconel 8, resulting from welding with three different heat inputs, will be discussed in this section. Figure 5 shows the microstructure of the welding metal and heat input. In each of the three heat inputs, the welding metal has an austenitic field with sediment particles, and the welding metal for each of the three heat inputs is in the form of dendrite freezing. Comparison of the microstructures of the welding metal with different heat inputs suggests that the dendrite growth increases considerably with increasing heat input and melt turbulence, and the highest growth of deposits in the dendrite-based form was at the heat input of.3 5

6 kj. Dendrite growth with heat input increase has also been reported by other researchers [3, 9]. A..5, B..9, C..3 kj/mm Figure 5: Microscopic structure of Inconel 8 with different heat inputs The microstructures of Inconel 65 with different heat inputs are shown in Figure 6. The welding metal for all three heat inputs has an austenitic field with sediment particles, and the welding metal is solidified in the dendrite form. Comparison of the microstructures of the welding metal with different heat inputs suggests that the melt turbulence increases with increasing heat input, and the deposit growth increases. The deposits get longer, and the cooling rate is smoothed as the heat input increases. The dendrite structures change to columnar state, and the highest growth rate occurs at the heat input of.3 kj. 6

7 A..5, B..9, C..3 KJ/mm Figure 6: Microscopic structure of Inconel 65 with different heat inputs The microstructures of Inconel 39L with different heat inputs are shown in Figure 7. In all three heat inputs the welding metal has an austenitic field with a small amount of ferrite, and the welding metal is frozen in dendrite form. Comparison of the microstructures of the welding metal with different heat inputs suggests that with an increase in heat input, freezing has increased, and consequently, the cooling rate get smother and the ferrite transformation to austenite decreases. The deltaferrite is high, and grains have the opportunity to grow. Since the melting point of the filler metal is very similar to the base metal 36L, the melt turbulence is carried out slowly, and dendrites are grown in the same proportion. As the heat input increases, molybdenum, which is a ferriteencouraging element, this leads to the ferrite formation and higher stabilization. A high content of ferrite leads to frangibility of the welding metal and freezing cracks on the grain boundaries. The prospect of forming compressed phases topology (Sigma, Lave, Chi) has increased because these brittle phases will significantly reduce the mechanical properties of the welding metal. 7

8 A. 5. B. 9. C. 3. KJ/mm Figure 7. Microscopic structure of Inconel 39L with different heat inputs The effect of heat input on the microstructure of the heat-affected zone Heat input is one of the significant factors on the weldability of metals and alloys, particularly in dissimilar metals welding. To study the effects of heat input on the dissimilar joining of Inconel 65 alloy to austenitic stainless steel 36L, dissimilar welding between these two alloys were performed using Inconel 8 filler metal at different heat inputs of.5,.9, and.3 kj/mm. The interface between the welding metal and stainless steel 36L with the three different heat inputs is shown in Figure 8. Welding under all the heat inputs had good continuity, and there were no splits at the intersections. There is a small, unadulterated area between the welding metal and stainless steel 36L in each heat input that increased with increasing heat input in this area. Increase of the unadulterated area with the increase of heat input is due to the movement of the molten welding metal and the melted border area from the base metal and, consequently, more mixing of the base and welding metals. 8

9 A..5, B..9, C..3 KJ/mm Figure 8: Structure of heat affected by austenite Stainless Steel 36L with different heat inputs The Inconel 65 intersection with three different heat inputs with the filler metal Inconel 8 is shown in Figure 9. In all three heat inputs, the joining had good continuity and no splits were observed. Comparison of these figures and the intersection of welding metal with Inconel 65 did not show an unadulterated area. Based on Figure 9, sediments in the heat-affected zone near the fusion boundary have been largely solved. The figure also shows that, with an increase in heat input, the sediments are resolved to a greater especially in higher heat inputs, is due to a sharp increase in temperature in the heat-affected zone due to different temperature cycles of different passwelding. Figure (A & B) is an electron micrograph image and carbide point analysis, showing a large amount of sediment in the area affected by Inconel 65 heat. According to the elemental analysis characteristics of point B in Table 3, the high content of elements, such as Ti and Nb, and C, cause carbides of titanium and niobium to be formed at the extent, and sediments solution areas developed in the vicinity of the fusion line. The deposit solutions, temperature of 65 to 87 C. 9

10 A..5, B..9, C..3 KJ/mm Figure 9: Structure of the area affected by Inconel 65 heat with different heat inputs A. B. Figure : Area affected by Inconel 65 heat, A. scanning electron microscopy (SEM), B. energy dispersive spectroscopy (EDS) Table 3: Elemental analysis characteristics of sediment B

11 Elements Nickel Niobium Carbon Titanium Electron layer Ka L Ka Ka Weight percentage /67 9 /7 /7 3 /56 Atomic percentage /7 /3 6 /59 4 /35 Figure shows the intersection of Inconel 65 filler metal with three different heat inputs. According to the figure, the intersections of the base and welding metals are completely continuous and without any cracks. Due to the use of Inconel 65 and a heat input increase in each of the three areas affected by Inconel 65 heat, the unadulterated impact is not shown. This figure shows that the secondary deposits (dark) are established at the grain boundaries in the area affected by Inconel 65 heat, with the heat raise further away from the fusion line. Inconel 65 weld metal has high levels of chromium and molybdenum, and the base metal has carbon (4%), chromium, and molybdenum. An increase in heat input can penetrate from the molten and even solid welding metal towards the base metal, react with carbon, and form the secondary sediments, such as chromium and molybdenum carbides. Carbon also can penetrate from the base metal to the welding metal and form sediments near the fusion line in the welding metal. The highest deposits are seen at the heat input of.3 kj/mm. A linear analysis of carbide elements, such as Cr, Mo, and Nb, in the area affected by heat increases and causes the formation of carbides M3C6 and M6C (Figure ). A..5, B..9, C..3 KJ/mm Figure : Microscopic structure of the area affected by Inconel 65 heat with different heat inputs

12 Figure : Carbide-forming element changes in the area affected by Inconel 65 heat Figure 3 shows the intersection of Inconel 36L with Inconel 65 filler metal for three different heat inputs. The heat input increase caused many changes in the intersection and no cracks were seen in the heat-affected area. A partial melting area is clearly visible between the welding fusion line and the base metal. As the heat input increased in all three partial areas, metal freezing changed from page to cellular. The cause for the partial area formation is that stainless steel 36 L has a melting point (45 C) higher than the melting point of the filler metal of Inconel 65 (35 C). This causes the areas near the fusion line to be melted due to the high temperature of the melted welding metal, but not mixed with the welding metal. Growth in the area will occur in cellular form due to rapid cooling. With an increase in heat input, the welding metal, which has a melting point lower than that of the base metal, freezes faster than the base metal at the freezing time. Thus, the area of the base metal that is near the welding metal is in the molten state, while the welding metal and base metal are solid. With the increase of heat input, dendrite growth increased up to.3 kj/mm.

13 A..5, B..9, C..3 KJ/mm Figure 3: Microscopic structure of the area affected by 36L stainless steel heat with different heat inputs Figure 4 shows the intersection of stainless steel were observed. Figure 5 shows the electron 36L with three different heat inputs and the stainless micrograph image (SEM) of the area affected by filler metal 39L. The microstructure for all three stainless steel 36L heat. In this area, no track was heat inputs had an austenitic field with some ferrite observed, and the area that was heat-affected content. With an increase in heat input, the area expanded with increased heat input. Dendrite growth affected by stainless steel 36L heat expanded. can be seen from the fusion line towards the welding Freezing of the heat-affected area is softened by the metal. heat input increase, but no other important changes 3

14 A.5, B.9, C.3 KJ/mm Figure 4: Microscopic structure of the area affected by stainless steel 36L heat with different heat inputs Figure 5: Scanning electron microscopy (SEM) of microstructure of the area affected by stainless steel 36 L heat with filler metal 39L stainless filler metal 39L. With an increase in heat Figure 6 shows the intersections of Inconel 65 input, the intersection of Inconel 65 and welding welding metal with three different heat inputs and the metal in each of the three heat inputs is completely 4

15 continuous with no cracks. No significant change was observed, except for minor melting near the fusion line in the area affected by Inconel 65 heat. This partial melting area is visible due to the difference in the melting point of the filler metal with that of the base metal in the area affected by Inconel 65 heat. In this area, the borders were melted and thickened, and the melt area increased slightly with the increase of heat input. Figure 7 shows the linear analysis of the area affected by Inconel 65 heat. In this area, the nickel has decreased and the iron has increased. The reason is that the iron content is high in the chemical compositions of the 39L filler metal, and the Inconel 65 base metal is nickel-based (Table ). A.5, B.9, C.3 KJ/mm Figure 6: Microstructure of the area affected by Inconel 65 with different heat inputs 5

16 Figure 7: Linear analysis of the area affected by Inconel 65 heat with the filler metal 39L Materials Science and Engineering A, vol. 9, Conclusions pp.74-8, The most optimal heat input for the dissimilar joining [4] M. Sireesha, V. Shankar, S. Sundaresan, A was recorded as.5 kj/mm. With an increase in heat comparative evaluation of welding consumables for input, the HAZ and partial melting area increased, but the synthetic gradient decreased. The welding dissimilar welds between 36LN austenitic stainless structure was fully austenitic for all filler metals. steel and Alloy 8, Journal of Nuclear Materials, Cellular-dendrite structure occurs for L39, while vol. 79, pp , dendrite freezing occurs in Inconel 65 and 8 filler [5] M. Sireesha, S.K. Albert, S. Sundaresan, metals. Inconel 65 filler metal microstructure is "Metallurgical changes and mechanical behavior smaller than the grain of Inconel 8 filler metal. during high temperature aging of welds between When comparing the ultimate strength of the filler alloy 8 and 36LN austenitic stainless steel", metals, Inconel 65 was highest. The fully austenitic microstructure of Inconel 8 had coaxial grains. The Materials Science and Technology, Vol.9, No., sediment analysis results show that the grain pp. 4-47, 3. boundary is the NBC carbide type. [6] H. Y. Lee, S. H. Lee, J. B. Kim, Creep fatigue damage for a structure with dissimilar metal References welds of modified 9Cr Mo steel and 36L stainless [] C. E. Cross, N. Coniglio, Weld steel, International Journal of Fatigue, Vol. 9, pp. Solidification Cracking: Critical Conditions for Crack , 7. Initiation and Growth, Hot Cracking Phenomena in [7] H. Naffakh, M. Shamanian, F. Ashrafizadeh, Welds II, Springer-Verlag Berlin Heidelberg, pp. 39- Microstructural evolutions in dissimilar welds 58, 8. between AISI 3 austenitic stainless steel and [] A. Mitchell, Manufacture of Specialty Inconel 657, Journal of Materials Science, Vol. 45, Alloys, Lecture Presentation Slides, The Royal pp , Institute of Technology, Stockholm, Sweden, June, [8] H. Naffakh, M. Shamaniyan, F. Ashrafi 8. Zadeh, Dissimilar welding of AISI 3 austenitic [3] M. Sireesha, V. Shankar, S. Sundaresan, stainless steel to nickel-based alloy Inconel 657, Microstructural features of dissimilar welds between Journal of materials processing technology, vol. 9, 36L Austenitic stainless steel and alloy 8, pp , 9. 6

17 [9] J. C. Lippold, D. J. Kotecki, Welding metallurgy and weldability of stainless steels, A. John Wiley & Sons, Inc., 5. [] S. Kou, Welding metallurgy, John Wiley, New Jersey, 3. [] Suutala, N., Effect of solidification condition on the solidification mode in stainless steels, Metallurgical Transactions A., Vol. 4A, pp. 9-97, 983. [] C. D. Lundin, Unmixed zone in arc welds: Significance on corrosion resistance, Welding Journal, Vol. 5, No. 5-6, pp. 3-, 997. [3] Bhadeshia, H. K. D. H, David, S. A and Vitek, J. M., Solidification sequences in stainless steel dissimilar alloy welds, Materials Science and Technology., Vol. 7, No., pp. 5-6, 99. 7