Characterization of microstructures and mechanical properties of Inconel 617/310 stainless steel dissimilar welds

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1 available at Characterization of microstructures and mechanical properties of Inconel 617/310 stainless steel dissimilar welds H. Shah Hosseini, M. Shamanian, A. Kermanpur Department of Materials Engineering, Isfahan University of Technology, Isfahan, , Iran ARTICLE DATA Article history: Received 30 October 2010 Received in revised form 3 February 2011 Accepted 8 February 2011 Keywords: Inconel 617 AISI 310 austenitic stainless steel Dissimilar welding Filler material ABSTRACT The microstructure and mechanical properties of Inconel 617/310 austenitic stainless steel dissimilar welds were investigated in this work. Three types of filler materials, Inconel 617, Inconel 82 and 310 austenitic stainless steels were used to obtain dissimilar joint using the gas tungsten arc welding process. Microstructural observations showed that there was no evidence of any possible cracking in the weldments achieved by the nickel-base filler materials. The welds produced by 617 and 310 filler materials displayed the highest and the lowest ultimate tensile strength and total elongation, respectively. The impact test results indicated that all specimens exhibited ductile fracture. Among the fillers, Inconel 617 exhibited superlative fracture toughness (205 J). The mechanical properties of the Inconel 617 filler material were much better than those of other fillers Elsevier Inc. All rights reserved. 1. Introduction Inconel 617 (UNS N06617: Ni 22Cr 12Co 9Mo) is primarily a solid solution nickel-base superalloy with superior engineering properties. This alloy has been widely employed in power plants, aerospace, chemical, and nuclear industries because of its exceptional properties of high temperature strength and creep resistance. In addition, this alloy is extensively used in many reducing and oxidizing environments due to its excellent hot corrosion behavior derived from the simultaneous presence of chromium, aluminum and molybdenum in the alloy composition [1 4]. As Inconel 617 is a relatively expensive alloy, a cheaper material with good properties can be used in lower risk conditions to reduce material costs. AISI 310 austenitic stainless steel (SS), is a prevalent material used in high temperature applications (about 1150 C). This alloy would be a good alternative for Inconel 617. Due to the presence of chromium, an adherent oxide layer is formed on the surface of 310 SS. This layer can protect the alloy in oxidizing conditions [5,6]. The application of dissimilar welding processes such as gas tungsten arc welding may be inevitable to alternate Inconel 617 with AISI 310 stainless steel. Generally, the selection of a suitable filler material in dissimilar welding is one of the most important concerns. In recent years, extensive investigations have been conducted for introducing suitable filler materials in dissimilar welding processes. Sireesha et al. [7] used four types of filler metals to joint 316LN SS and alloy 800. Their results showed that the Inconel 82/182 filler materials exhibited the best properties. Dupont et al. [8] investigated the effect of processing parameters and filler metal chemistry on the microstructure and weldability of dissimilar welds between AL-6XN super SS and two nickel-base alloys, Inconel 625, and Inconel 622. Lee et al. [9] showed that the increase of Ti in filler metal composition led to the formation of equiaxial dendrites and thereby, increased elongation in the welding of nickel-base alloy 690 to SUS 304L. Weldability and joint properties between Inconel 657 and 310 SS were investigated by Naffakh et al. [10]. They illustrated that the Inconel A (a nickel base filler metal) was the best choice among four filler metals. Corresponding author. Tel.: ; fax: address: h.shahhosseini@ma.iut.ac.ir (H. Shah Hosseini) /$ see front matter 2011 Elsevier Inc. All rights reserved. doi: /j.matchar

2 426 MATERIALS CHARACTERIZATION 62 (2011) Table 1 The chemical composition of the base metals and the filler materials used in this study. Elements (wt.%) Base metals Inconel SS Inconel 617 Based on a literature survey, no previous work has been reported on the dissimilar welding between Inconel 617 and 310 SS. The aim of this study was to investigate the influence of filler materials in order to achieve the best mechanical properties in severe conditions. 2. Experimental Procedures Filler materials Inconel SS C Max 0.1 Max 0.1 Max 0.1 Si Mn Cu Cr Co Ni Bal Bal. 67 min 21 Fe 1.35 Bal. 3 3 Bal. Mo Al Ti Nb The base materials used in this work were nickel-base superalloy Inconel 617 and 310 SS under the rolled and solution annealed condition in the form of 12 mm thick plates. Three types of filler materials, Inconel 617, Inconel 82, and 310 SS were employed. The chemical compositions of the base and filler materials are given in Table 1. The base metal plates were cut and machined to the size of 195 mm 32 mm 12 mm. The specimens were machined to make a single V groove butt joint configuration with 70 groove angle. The root face and the root opening were 1.5 and 1 mm, respectively. Welding procedure was performed in four passes by the manual gas tungsten arc welding process with Direct- Current Electrode Negative. The welding parameters and the heat input in each welding pass are given in Table 2. For metallographic examinations, several specimens were prepared from the transverse cross section of the weldment. The specimens were prepared by grinding using 120, 240, 320, 600, 800 and 1200 grits of SiC paper, followed by the final polishing with 5 μm alumina powders. Then, the specimens were etched in Marbel solution (10 g of CuSO cc of HCl+ 50 cc of H 2 O). The microstructural features were investigated using an optical microscope and a scanning electron microscope (SEM Philips XL30) equipped with energy dispersive spectroscopy (EDS) point analysis. Tensile tests were carried out on the round transverse weld specimens using a universal Hounsfield H50KS tester at room temperature with a nominal strain rate of 1 mm/min. The tensile test specimens were prepared according to AWS-B4 specifications. They are schematically shown in Fig. 1. The Charpy V-notch impact test was conducted on the welds at room temperature by using an Amsler impact tester with the standard 55 mm 10 mm 10 mm specimens. The specimens were machined perpendicular to the welding direction with the notch in the center of the weld metal. The tensile and impact tests were performed on three specimens per condition to increase the results of the degree of precision. Microindention hardness measurements were performed across the welds to obtain the hardness profiles in the weld metal, heat affected zone, and the base metal at a load of 300 g using a Leitz microindention hardness tester. 3. Results and Discussion 3.1. Microstructural Features Base Metal Microstructures The microstructure of the 310 SS base metal is shown in Fig. 2. The microstructure consisted of fine equiaxed grains of austenite. Annealing twins can be observed in the austenitic matrix. There does not appear to be any delta ferrite or carbide precipitates in the microstructure due to a proper annealing treatment after cold rolling. The fully austenitic structure improves toughness and corrosion properties especially at high temperatures, but encourages the sensitivity to solidification cracking [5,6,12]. The Inconel 617 alloy was supplied in the form of a solution treated and water-quenched plate. The typical metallographic Table 2 The GTAW welding parameters used in this study. Filler metal Pass number Current (A) Welding parameters Voltage (V) Welding speed (mms 1 ) Heat input (kj mm 1 ) Total heat input (kj mm 1 ) 310 SS Inconel Inconel

3 427 Fig. 1 The specification of the tensile test specimens. features of the alloy in the as-delivered condition are shown in Fig. 3. The microstructure consisted of an austenitic matrix with some inter- and intra-granular precipitates. The EDS results showed that these blocky or stringer shaped particles are Cr-rich (M 23 C 6 ), Mo-rich (M 6 C) carbides or a combination of both, while the bright particles are Ti(C,N) [2,3]. Fig. 3 The microstructure of the as-received Inconel 617. Fig. 5a shows the microstructure of Inconel 617 weld metal. The microstructure is completely austenitic with a dendritic structure and contains particles dispersed in the matrix. The major part of the microstructure contains fine equiaxed dendrites. In comparison to previous works [10,13], a lower heat input was used in this study which resulted in a higher Weld Metal Microstructures The microstructure of Inconel 82 weld metal is shown in Fig. 4a and b. The weld metal contains about 67 wt.% nickel and is fully austenitic. Due to the presence of 3% Nb, the austenite is stabilized at a high temperature. In addition, the solidification mode is changed from cellular to dendritic as Nb has an intense tendency to increase the degree of constitutional undercooling [11,12]. The solidification grain boundaries and migrated grain boundaries are distinct in figure. The solidification grain boundaries result from the intersection of packets, or groups, of subgrains. In addition, the solidification grain boundary also exhibits a compositional component resulting from solute redistribution during solidification [7,12]. The equilibrium distribution coefficient of Nb is less than 1; therefore, it is easily redistributed in to interdendritic regions during solidification and produces Nb enriched carbides. It is revealed that NbC precipitates are formed in the interdendritic regions (Fig. 4b) [10]. Therefore, it is expected that this microstructure shows a high hot cracking sensitivity [8,12]. Fig. 2 Microstructure of the 310 SS base metal in the as-received condition. Fig. 4 (a) Optical and (b) SEM micrographs of Inconel 82 welding metal microstructure.

4 428 MA TE RI A L S CH A R A CT ER IZ A TI O N 62 ( ) Fig. 7 Interface between the filler 82 weld metal and the Inconel 617 base metal. cooling rate and consequently a finer microstructure. This fine microstructure has a lower segregation ratio of Mo which makes the welds brittle at room temperature. Therefore, the solidification cracking tendency is reduced. This weld metal displays a lower number of migrated grain boundaries than the Inconel 82 weld metal. This can be attributed to the formation of precipitates along the solidification grain and subgrain boundaries as delineated in Fig. 5b. These particles have an effective role in pinning crystallographic component of the solidification grain boundaries [12]. Fig. 6 illustrates the cellular and dendritic microstructure of the weld zone of the 310 SS filler metal. With the lack of segregated elements like Nb and Mo, the major elements present Fig. 5 (a) Optical and (b) SEM micrographs of Inconel 617 weld metal microstructure. Fig. 6 Micrographs of 310 SS weld metal microstructure. Fig. 8 Interfaces between the filler 617 weld metal and the 310 SS base metal (a) and between the filler 617 weld metal and the Inconel 617 base metal (b).

5 429 Fig. 9 Interfaces between the filler 310 SS weld metal and the 310 SS base metal (a) and between the filler 310 weld metal and the Inconel 617 base metal (b). in the composition of this filler metal are Fe, Cr and Ni that are well known for their low tendency to segregate in the interdendritic and intergranular regions. Hence, there is a minimal driving force (constitutional undercooling) to change the solidification mode from cellular to dendritic. Thus, from the standpoint of solidification mode, it can be expected that the solidification cracking sensitivity of 310 SS weldment is less than that of the prior weld metals. However, the presence of Cu in the weld metal composition leads to the formation of low melting point secondary phases. This phenomenon increases the susceptibility of the weld metal to solidification cracking [10] Interfacial Microstructures The interface between filler 82 weld metal and 617 base metal is displayed in Fig. 7. There is a considerable unmixed zone in the form of laminar layers between Inconel 82 weld metal and the base metal along the welding line. When the melting range of filler materials is similar to or higher than that of the base metal, only a small fraction of the base metal can be melted and no dilution occurs in the re-solidification stage; therefore, an unmixed zone is formed between two regions [7,13,15]. A similar trend was seen in the interface of the weld metal/310 SS base metal, but more grain growth occurred in the 310 SS heat affected zone due to the temperature increase during various welding passes. On the other hand, grain boundary thickening can be seen in the heat affected zone of the 617 base metal. The precipitation of titanium at these boundaries and the formation of low melting point particles may be the reasons for this phenomenon. Epitaxial growth also occurred in both sides of the weld metal vertical to the fusion line, because both filler and base metal have a FCC structure providing proper condition for this growth type [14]. The interfacial regions of the filler 617 weld metal and the base metals of 310 SS and Inconel 617 are shown in Fig. 8a and b, respectively. There is an unmixed zone between the weld metal and 310 SS base metal similar to that observed for Inconel 82 filler; however, no considerable unmixed zones can be observed in the Inconel 617 base metal showing more dilution It can be attributed to the similarity of filler and base metal from the viewpoint of the melting temperature and chemical composition. In addition, the columnar growth occurs near the boat fusion line. Nonetheless, it varies to equiaxed type in the welding pool center. The weld metal interfaces for the 310 SS filler metal with the 310 SS and Inconel 617 base metals are shown in Fig. 9a and b, respectively. A wide unmixed zone width can be seen for the Inconel 617 base metal, while a crack is distinguished in the unmixed zone of the 310 SS base metal between the heat affected zone and the weld metal adjacent to the fusion boundary running parallel to it. The crack is initiated due to the formation of low melting point Cu precipitates in the partially melted zone along the grain boundaries. The weld and base metals have the same melting point. Therefore, partially the melted zone is still liquid while, the weld and base metals are solid. Solidification of this liquid causes tensile stress on the solidified partially melted zone that can form the crack. These cracks will influence mechanical properties of the weldments. Several grain boundaries are formed from the fusion line which perpendicularly grows to a weld metal (Type I boundary). This type of grain boundary is obviously observed when the base and filler materials have the same structure. In the present dissimilar welds, as no allotropic transformation occurred during the cooling of the two base metals, therefore no type II grain boundary was observed in the microstructure [7,12]. Table 3 Tensile properties of the base and weld metals at room temperature. Filler and base metal type Yield strength (MPa) Ultimate tensile strength (MPa) Total elongation (%) Reduction in area (%) Location of failure 310 SS base metal 272±5 565±20 72±7 68±3 Inconel 617 base metal 425±16 837±27 61±4 45±2 Inconel 82 filler metal 405±14 617± ± ±3 310 SS base metal Inconel 617 filler metal 420±8 644±22 48± ±2 310 SS filler metal 440±25 615±12 24± ±1

6 430 MATERIALS CHARACTERIZATION 62 (2011) Table 4 The Charpy V-notch impact energy at room temperature. Filler and base metal type Impact energy ( J) 310 SS base metal 126±3 Inconel 617 base metal 225±7 Inconel 82 filler metal 164±6 Inconel 617 filler metal 206±4 310 SS filler metal 144± Mechanical Properties The results of the tensile test conducted on the dissimilar welded joints are listed in Table 3 along with the properties of the two base materials. It can be seen that all specimens were ruptured within the heat affected zone of the 310 SS base metal. This behavior is in agreement with the microstructural features discussed in Section 2. These observations confirm the suitable strength for the produced weldments. In the case of the 310 SS base metal, the grain growth and crack formation in the heat affected zone (especially when the 310 SS filler metal was used) may be responsible for the downtrend of the mechanical properties. The 617 filler material showed maximum ultimate tensile strength and total elongation compared with the other fillers. This can be related to the fine dendritic microstructure of this filler metal. Table 4 shows the results of the Charpy impact test performed on the weldments and base materials. It can be seen that the toughness of the weldments is between the values pertaining to the base metals and is better than that of similar prior studies egregiously. Among the weldments, the toughness of the alloy 617 is the highest. SEM micrographs from the fracture surfaces illustrated a fully ductile fracture in all the cases. The microindention hardness profiles across the welds for Inconel 82, Inconel 617, and 310 SS filler metals are shown in Fig. 10. For all specimens, the weld hardness is between the hardness of the base metals, showing a gradual increase in hardness from the 310 SS to the Inconel 617. This indicates a compositional gradient in the weldments. It could be related to the dilution occurring between two different base materials having different hardness. The microstructural non-uniformity can influence the hardness variation. The maximum and minimum hardness values of the weldments belong to the Inconel 617 and 310 SS filler metals, respectively. The solid solution strengthening and formation of precipitates provide higher hardness for the two nickel-base filler metals. There is a tiny decrease in the hardness of heat affected zone of the 310 SS that is due to grain growth and the presence of low hardness particles. 4. Conclusions Three types of filler materials, Inconel 617, Inconel 82 and 310 austenitic stainless steels were used to obtain dissimilar Inconel 617/310 austenitic stainless joint using the gas tungsten arc welding process. The following conclusions can be drawn from the results: The weld microstructure was fully austenitic for all filler materials. A cellular structure was seen for the 310 SS, while the dendritic solidification occurred for the Inconel 617 and 82 filler metals. The microstructure of the Inconel 617 filler metal was finer than that of the Inconel 82. Due to the formation of low melting point Cu precipitates in the partially melted zone, a number of cracks were initiated in the 310 SS unmixed zone when the 310 SS filler metal was used. All tensile specimens were broken in the 310 SS base metal. The Inconel 617 filler metal showed the superlative yield strength, ultimate tensile strength, and total elongation compared with the other filler metals. All welded specimens showed ductile fracture. The toughness value for the weld metals was within those for base Fig. 10 The microindention hardness profiles across the welds.

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