Microstructure/Property Relationships in Dissimilar Welds Between Duplex Stainless Steels and Carbon Steels

Introduction 

     Duplex stainless steels have become increasingly attractive to a number of industry sectors due to their superior mechanical properties and corrosion characteristics relative to other stainless steels and structural steels. Although the joining of duplex stainless steels to themselves has been studied extensively, the increased application of these steels will require a better understanding of the issues associated with welds to dissimilar metals. The joining of dissimilar materials is generally more challenging than that of similar materials because of differences in the physical, mechanical and metallurgical properties of the base metals to be joined. These differences may also complicate the selection of filler metals compatible to both base metals. Therefore, filler metal selection is often a compromise between the two dissimilar metals. There are few guidelines for dissimilar metal joining and, in most cases, predicting the microstructure and resultant properties of the weld deposit can be difficult.

     Early investigations on the joining of dissimilar metals were primarily devoted to ferrous alloys; however, much of the emphasis was placed on the prevention of weld metal liquation cracking (often referred to as microfissuring), heat-affected zone cracking, carbon migration and oxide penetration, as discussed by Pattee, et al. (Ref. 1). In the 1940s, Schaeffler proposed a diagram for the selection of electrodes for the dissimilar joining of plain carbon and stainless steels that related the microstructural constitution of the weld deposit to its composition, as dictated by the relative proportion of filler metals and base metals (Ref. 2). This diagram (Fig. 1), commonly referred to as the Schaeffler Constitution Diagram, can be used as a means of predicting the weld metal microstructure of dissimilar metal welds in a select group of alloys.

 

Fig. 1 - Schaeffler Diagram (Ref. 2) showing the dissimilar combinations used in this investigation.

Fig. 2 - WRC-1992 diagram, extended version (Ref. 5)

     Recently, Odegard, et al. (Ref. 6), studied the joining of duplex Alloy SAF 2507 to carbon steels with respect to fusion zone mechanical properties. They reported that the phase stability and the overall properties of the fusion zone were influenced by the welding parameters and that low heat inputs were necessary to ensure structural integrity and solidification cracking resistance. Furthermore, they noted that a highly ferritic fusion zone resulted from high dilution by the carbon steel, making the weld metal microstructure susceptible to secondary austenite formation in multipass welding due to reheating of the deposited weld metal by subsequent passes. High heat input welding schedules were reported to increase the susceptibility to fusion zone solidification cracking. 
 Table 1 presents mechanical property data, collected by Odegard, et al. (Ref. 6), which includes tensile strength, yield strength and elongation values for dissimilar joints between the super-duplex Alloy 2507 and either carbon steel or Type 316 austenitic stainless steel. No other mechanical property data for dissimilar welds between duplex stainless steels and carbon steels was found in the literature. 

Experimental Procedure 

Materials 

 Welding trials were designed to study the dissimilar joining of duplex stainless steel Alloy 2205 to A36 carbon steel. This dissimilar combination was selected based on a survey of industrial users. Both duplex stainless steel and Ni-based filler metals were selected to join these metals. The chemical compositions for the base and filler metals are listed in Table 2. The base materials were supplied in the form of 12.5-mm (0.5-in.) plate. These plates were cut into 5 x 20 cm (2 x 7.8 in.) coupons with a 37-deg bevel on each plate to provide a 74-deg groove angle for a single-V-groove butt joint configuration. The root face was 1.6 to 2.4 mm (0.062 to 0.094 in.) with a root opening of 2.4 mm. 

     All welding was performed using the automatic gas tungsten arc welding (GTAW) process with cold wire feed of 1.14-mm (0.045-in.) diameter welding wire. ER2209 and Alloy 625 filler metals were used in the multipass welds to join duplex Alloy 2205 to A36 carbon steel. Two heat input levels were initially selected and used throughout the study, as listed in Table 3. The welding heat inputs used were 1.57 kJ/mm (39.8 kJ/in.) and 2.60 kJ/mm (66 kJ/in.), with the only exception being the root passes in each combination. Due to the difference in material properties (mainly thermal conductivity) the heat flow differed between the two filler metals, thus the root pass heat inputs were different. The 2205/2209/A36 combination utilized a root pass heat input of 1.05 kJ/mm (26.7 kJ/in.), while that of the 2205/625/A36 combination was 1.90 kJ/mm (48.3 kJ/in.). 

Metallurgical Characterization 

Optical Metallography 

     Weld cross sections were removed from each of the combinations and polished through 0.05-micron alumina. The samples were electrolytically etched in 10% oxalic acid at 5-7 V for 20-25 s. Also, color etching techniques were employed to provide better resolution between the ferrite and austenite. These techniques included a two-step modified Murakami's etch (Ref. 7), which produces a composition-sensitive film resulting in a variety of colors on the ferrite phase, while the austenite remains white (uncolored). A ferro-fluid colloidal suspension of Fe₃O₄ was also used to reveal the duplex microstructure (Ref. 7). Residual magnetism attracts Fe₃O₄ particles to the ferrite phase to provide a color of brown or dark blue in stark contrast to the austenite, which is white. 
 
 
 

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Fig. 3 - Cross section of 2205/2209/A36 weld combination: A - Root pass; B - cover pass. 

Fig. 4 - WRC-1992 diagram for A36/2209/2205 combination. 

Fig. 5 - FN vs. location for the 2205/2209/A36 weld combination. 

Fig. 6 - Representative micrograph of 2209/A36 fusion boundary region. Arrows indicate fusion boundary. 

Fig. 7 - Charpy V-notch impact toughness vs. temperature for the 2205/2209/A36 weld combination. 

Hardness testing was carried out on a Leco Model M-400 microhardness unit using a 1-kg load. Surveys were conducted both across the weld and from root to cover pass within the weld deposit. The distance between indents was determined by the weld dimensions in order to obtain appropriate data for each pass. Generally, the distance between indents was 0.5 mm (0.02 in.) in the lower passes of the weld, 1.0 mm (0.04 in.) in the central portions of the weld metal and 2.0 mm (0.12 in.) in the cover pass of the weld metal. Hardness was also measured along the fusion boundary and across the heat-affected zone (HAZ) into the base metal. 
     Hardness testing was also conducted at reduced loads (25 and 50 g) along the 2209/A36 and 625/A36 fusion boundaries. This was done to more closely examine the hardness in the microstructural transition region between the weld fusion zone and HAZ. 

Ferrite Number Measurement 

     The ferrite number (FN) in each weld layer was determined using a MagneGage calibrated per AWS A4.2-91 according to the method developed by Kotecki (Ref. 8). All measurements were obtained on longitudinal sections in the fusion zone rather than transverse sections in order to better quantify variation within a given pass. Four measurements were taken at four different locations within each pass. These measurements were then averaged to obtain the FN for each pass. 

Charpy Impact Toughness Testing 

     Charpy V-notch (CVN) specimens were machined from the welded coupons. Samples were prepared in the L-T orientation as per ASTM E-23. The L-T orientation represents a sample transverse to the welding direction with the notch located such that testing occurs through the thickness of the weld from the root to the cover passes. All notches were located in the center of the weld deposit. Charpy V-notch testing was performed at temperatures from -196C (-320.8F) to more than 100C (212F) in order to develop a complete ductile-to-brittle transition temperature (DBTT) curve for each combination. Scanning electron microscopy (SEM) was utilized to examine the fracture surfaces of the CVN impact samples. 

Pitting Corrosion Testing 

     Pitting corrosion tests were performed at 50C (122F) as described in ASTM G-48. Two types of samples were tested. The first type consisted of an entire weldment, including both base materials and the fusion zone. 

Results 

2205/2209/A36 Weldment 

Microstructure 

     The dissimilar 2205/A36 combination was welded with duplex filler metal ER2209. Figure 3A shows the root pass microstructure of this combination resulting from a heat input of 1.05 kJ/mm and consists of an austenite matrix (white) with both skeletal and acicular ferrite (dark etching). Ferrite measurements indicated 20-25 FN in the root pass for both welding heat inputs of 1.57 KJ/mm and 2.60 KJ/mm. Based on this FN level, the WRC-1992 diagram (Fig. 4) predicts that the dilution in the root pass was approximately 33%, which corresponds well with the 35-40% dilution that was estimated metallographically. Subsequent passes in these weldments exhibited increased FN from the root to the cover pass, ultimately reaching 92 FN. A micrograph of the cover pass is shown in Fig. 3B. The FN transition from the root to the cover pass is shown in Fig. 5. Note that the FN differential between the root pass and cover pass is roughly 70 FN and that a significant increase in FN occurs between the last fill pass and the cover pass for both heat inputs. 
     The filler metal composition, dilution and cooling rate are the principal factors that influence the weld metal ferrite/austenite balance in duplex stainless steels. The lower heat input (1.57 kJ/mm) weld passes undoubtedly resulted in higher cooling rates than the higher heat input (2.60 kJ/mm) weld. However, as shown in Fig. 5, the difference in heat input and cooling rate did not significantly affect the ferrite/austenite balance. 

Fusion Zone Hardness 

     The resultant weld deposit hardness of this combination varied with heat input. The lower heat input weld exhibited higher hardness in the weld metal and along the A36 fusion boundary than the high heat input weld. The hardness in the root pass varied from 261 to 287 DPH . The remainder of the weld metal exhibited hardness levels between 229 and 279 DPH. An average hardness of 260 DPH was found in the cover pass. Lower hardness was found in the center portions of the fusion zone (midway through the thickness), as average hardness was 250 DPH. The hardness along the fusion boundary of the carbon steel was found to be much higher than the bulk fusion zone hardness and ranged from 253-416 DPH. Generally, the hardness along the 2209/A36 fusion boundary was above 300 DPH, with average levels reaching 410 DPH near the root pass. Softer regions along this boundary were found to exist in the top third of the weld. Average hardness levels within and at the fusion boundary of the A36/2209/2205 weldments are summarized in Table 4. 
     The 2.60-kJ/mm weld exhibited a lower average hardness, with root pass values from 249 to 256 DPH and an average hardness of 250 DPH (Table 4). The remainder of the weld metal exhibited hardness levels between 223 and 265 DPH. An average hardness of 245 DPH was measured in the cover pass. Lower hardness was found in the center portions of the fusion zone (midway through the thickness), with an average hardness of 230 DPH. The hardness along the 2209/A36 fusion boundary in this weld was also significantly lower than the lower heat input weld.  The fusion boundary region associated with the root pass exhibited the highest hardness, 300 DPH. Hardness elsewhere along the boundary was below 300 DPH with the lowest hardness exhibited in the fusion boundary adjacent to the cover pass. 

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Fig. 8 - CVN fracture behavior for 2205/2209/A36 weldment: A - Lower shelf; B - fracture transition range. 
 

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(B)

Fig. 9 - Average pit depth vs. weld layer for 2205/2209/A36 combination. 

Fig. 10 - Pit density vs. weld layer for the 2205/2209/A36 pitting samples: A - Weldment samples; B - all-weld-metal samples. 

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Fig. 11 - Microstructure of 2205/625/A36 combination: A - Root pass; B - cover pass. 
 

Fig. 12 - CVN impact toughness vs. temperature for the 2205/625/A36 weld combination. 

 

Fig. 13 - SEM fractograph of 2205/625/A36 CVN specimen produced at 1.57 kJ/mm and tested at -196°C. 
 

Weld Metal Impact Toughness 

     The CVN impact toughness results for the 2205/2209/A36 weld combination at both heat input levels are shown in Fig. 7. The lower shelf impact energy was roughly 7.5 ft-lb at temperatures below -140C (-220F). The upper shelf of the DBTT curve was roughly 240 ft-lb (325 J) for the 1.57 kJ/mm heat input and 250 ft-lb (339 J) for the 2.60 KJ/mm heat input. These DBTT curves also show the transition impact energy, midpoint between the upper and lower shelf, to be 125-130 ft-lb (169-176 J) and the corresponding transition temperature to be between -45 to -50C (-49 to -58F) for both heat input levels. 
     Fracture surfaces from upper and lower shelf and transition region specimens were evaluated in the SEM. The upper shelf fracture surface exhibited ductile fracture, as exemplified by a dimple-type fracture morphology. The lower shelf exhibited a cleavage-type fracture showing a relatively flat fracture surface as shown in Fig. 8A. Fracture surfaces in the transition region exhibited mixed mode fracture as shown in Fig. 8B. Portions of the fracture surface resemble cleavage fracture, while other areas show ductile fracture. The transition region fracture behavior was similar for both heat inputs in the temperature range from -20 to -90C (-4 to -130F). 

Pitting Corrosion 

     Corrosion testing of the weld metal was conducted in two ways. Initial testing was conducted on the entire weldment by immersing the entire joint cross section, including the base materials, into 6% FeCl₃ solution at 50C for 72 h.

2205/625/A36 Weldment 

Microstructure 

     The dissimilar combination of Alloy 2205 and A36 was also welded using the Ni-based filler metal 625. The heat inputs utilized for fill and cover passes were the same as for the 2205/2209/A36 combination. The root pass heat input, 1.90 kJ/mm, was different from the 2205/2209/A36 combination due to differences in thermal properties of the filler alloys. 

Fusion Zone Hardness 

     The fusion zone hardness for the 2205/625/A36 welds was unaffected by weld heat input, ranging from 190-220 DPH. Similarly, the hardness along the carbon steel fusion boundary region was comparable to that of the fusion zone for both heat inputs, ranging from 180-220 DPH. The average hardness values for this combination are also summarized in Table 4. 

Weld Metal Impact Toughness 

     Charpy V-notch test results for the 2205/625/A36 dissimilar weld combination are shown in Fig. 12. As expected, a DBTT is not observed due to the fully austenitic structure of the fusion zone. The impact toughness decreases slightly with decreasing temperature. At -196C impact toughness was 62 ft-lb (84 J) and 57 ft-lb (77 J) for 1.57 kJ/mm and 2.60 kJ/mm heat inputs, respectively, relative to 88 ft-lb (119 J) and 76 ft-lb (103 J) at 25C (77°F). Note that the lower heat input, 1.57 kJ/mm, resulted in higher impact toughness values at any given temperature. A representative fracture surface for this combination is shown in Fig. 13. The fracture mode was ductile rupture over the entire range from -196C to 25C. Note the evidence of the underlying solidification substructure on this fracture surface, probably due to fracture initiation at the interdendritic eutectic constituent. 

     The weldment corrosion response of the 2205/625/A36 weldments also resulted, as expected, in general attack of the carbon steel base metal. Attack was also very pronounced in the root of the weld and resulted in the complete dissolution of the root pass. The root pass was the only pass attacked in this manner and probably resulted from higher dilution of this pass by the A36 base metal. Figure 14 shows increased pit depths with increased heat input for both corrosion sample types. However, the pit depth difference is much smaller in the all-weld-metal samples than the weldment samples when comparing the two heat inputs. The pitting density results vs. weld layer for the weldment and all-weld-metal samples are shown in Fig. 15. Pit density increased with increased heat input as shown for both corrosion samples. The only exception is weld layer 2 in the weldment sample, as the higher heat input resulted in lower pit density relative to the 1.57 kJ/mm weld. The pit densities of both corrosion sample types appear higher in the initial weld layers and generally tend to level out above the third weld layer. 

 The weight loss of the all-weld-metal corrosion samples was 1.402 g (21.5% of initial weight) and 2.212 g (20.6% of initial weight) for the 1.57 kJ/mm and 2.60 kJ/mm heat inputs, respectively. The corrosion rate, expressed as weight percent, indicates that no difference in pitting corrosion is evident between the two heat inputs as the weight loss percentages result in similar values. 
 

     The results of this investigation have shown that duplex stainless steel alloy 2205 can be joined to A36 carbon steel using either duplex filler material ER2209 or Ni-based Alloy 625. Both combinations exhibited good weldability and were free from fabrication-related defects such as solidification cracking, liquation cracking, porosity, incomplete fusion, etc. In general, from a procedural point of view, there were no problems welding the dissimilar combinations using either filler material. 

Microstructure Evolution 

Weld Metal 

     Table 4 compares the FN and hardness for each of the combinations studied relative to location in the weld, i.e. root pass, fill passes and cover pass. Dilution of the Alloy 625 filler metal is not sufficient to form ferrite within the weld deposit, and the entire fusion zone is austenitic. The 2205/2209/A36 weld exhibited 20-25 FN in the root pass, 25-45 FN in the fill passes and increased significantly to 95 FN in the cover pass. The fill passes showed a gradual increase in FN with each pass. This results from both a change in dilution from bottom to top in the multipass weld and the formation of secondary austenite in underlying weld passes that reduces the FN in the underlying passes. The WRC-1992 diagram can be used to estimate the dilution effects on FN, but the diagram is not effective in predicting secondary austenite formation in multipass welds. Thus, some deviation from the predicted WRC-1992 FN based strictly on dilution calculations would be expected. In addition, this diagram predicted lower FN for the cover pass than was actually measured. 
     The cover pass FN increased significantly relative to the last fill pass for the 2205/2209/A36 combination. It is interesting to note that the cover pass ferrite number (95 FN) is actually greater than the predicted FN of undiluted ER2209 filler metal. The reason for this high FN in the cover pass is not clear. The fact that the cover pass FN is comparable between heat inputs suggests that cooling rate is not a factor. 

Fusion Boundary Region Microstructure 

    The high hardness along the fusion boundary region of both weld combinations is attributable to the formation of a narrow band of martensite at the dissimilar interface. This martensitic region is predicted in both cases by the Schaeffler Diagram - Fig. 1. By drawing tie lines between the filler metal composition and midpoint of the base metal compositions, it can be seen that for the 2209/625/A36 combination, 15% of this tie line lies within a region where martensite is present. The 2205/2209/A36 tie line, on the other hand, has more than 65% of its length in a martensitic region. Thus, for the 2209 filler metal the composition transition region over which martensite can form will be much wider. This may explain the pronounced martensite formation along the 2205/2209/A36 fusion boundary region and the apparent martensite-free fusion boundary region in the 2205/625/A36 weld. Another factor that influences martensite formation is the difference in carbon diffusion rates in the different combinations. If carbon migration is reduced or restricted, the likelihood of martensite formation will be similarly reduced. 
     Gittos and Gooch (Ref. 12) studied the fusion boundary below stainless steel and nickel-alloy claddings on Cr-Mo steels. They reported narrow bands of martensite formation along the interfaces of both cladding alloys, with hardness levels in the as-welded condition ranging from 300-441 HV. This martensitic band was associated with the partial mixing region between the fusion zone and base metal HAZ. 
     It is interesting to note in Fig. 6 that Type II grain boundaries are present within the fusion zone. Wu and Patchett (Ref. 13) found similar Type II grain boundaries previously in austenitic stainless steel cladding. They reported that Type II grain boundaries were formed due to the crystallographic change from -ferrite, the existent phase of solid hypoperitectic base steel at the initial transient of solidification, to -austenite as a result of accumulation of nickel at the solidification front. They reported that disbonding (i.e., cracking) was more severe in nickel-based alloy deposits that solidified directly to austenite where high dilution from the base metal was incurred. However, they also claimed that the ferritic solidification of duplex stainless steels eliminates Type II grain boundaries from occurring. 

     The CVN results for both combinations are summarized in Table 5. In the 2205/2209/A36 combination, there was little apparent difference in toughness as a function of heat input, which reflects the similarity in microstructure of these weld deposits. Because of the L-T orientation of the test samples, the toughness values represent a composite of the entire weld deposit and are not indicative of local variations due to microstructure. For example, lower toughness might be expected in the higher FN cover pass. 
     It should be pointed out that the toughness of the 2205/2209/A36 may be significantly affected by the formation of martensite along the A36 fusion boundary region. This narrow band of martensite would undoubtedly result in reduced toughness along this particular region relative to the fusion zone. However, no fusion boundary CVN testing was performed to investigate any toughness anomaly in this region. 
     As expected, the fully austenitic fusion zone of the 2205/625/A36 combination exhibited good impact toughness over a wide range of temperatures. This microstructure does not exhibit upper/lower shelf transition behavior such as ferritic (or high ferrite) weld metals, and, instead, the impact toughness exhibits a gradual decrease with temperature. It is interesting that the upper shelf toughness of the duplex 2209 deposit far exceeds that of the Alloy 625 deposit and that the duplex filler material provides comparable toughness at temperatures down to -60C. 
    Table 6 compares fusion zone impact energies obtained in this study to previous work from duplex stainless base/filler metal combinations performed by Kotecki (Ref. 15) and Bonnefois, et al. (Ref. 16). The impact toughness data reported here are higher than from this previous work and may be attributed to higher austenite contents (lower FN) in the root and fill passes of the dissimilar welds. Most importantly, these comparisons suggest that dilution from a dissimilar carbon steel base metal does not compromise the toughness of the fusion zone relative to a similar metal combination. 

     The two types of corrosion samples (weldment and all weld metal) and their pitting corrosion data are summarized in Table 7. As shown for the 2205/2209/ A36 material combination, the weldment sample shows that for increasing heat input, the pitting corrosion resistance increased when considering the pit density. The average pit depth, on the other hand, increased with increasing heat input and may be due to higher concentration on fewer initiated pits. 

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Fig. 15 - Pit density vs. weld layer for 2205/625/A36 combination: A - Weldment sample; B - all weld metal. 
 
 
 
 

     The 2205/A36 base metal combination has been successfully joined with duplex stainless steel ER2209 and Ni-based Alloy 625 filler metals using multipass GTAW. Heat input had only a minor effect on the microstructure and toughness for the ER2209 combination. However, the corrosion behavior showed a marked improvement for higher heat input welding parameters relative to the lower heat input. 
     The choice of filler metals in joining 2205 to A36 will be primarily dependent on the service requirements needed. The strength and ductility of nickel-based Alloy 625 and duplex filler metal ER2209 are similar, being roughly 110 ksi and 30-40% elongation. Therefore, a balance between toughness and corrosion resistance will determine the use of either the duplex filler metal ER2209 or the nickel-based Alloy 625. The recommendation for cryogenic temperatures would be to utilize Alloy 625 over the duplex filler metal due the increased toughness at these temperatures. However, dilution should be minimized in the root pass to limit the carbon steel dilution in the fusion zone to prevent the reduction in corrosion resistance. On the other hand, in service environment temperatures greater than -50°C, the duplex filler metal ER2209 should be utilized due to higher toughness over the nickel-base Alloy 625. The heat input should be as high as possible in order to gain the maximum corrosion resistance of the fusion zone. 

Conclusions 

     1) The fusion zone microstructures of dissimilar weld combination 2205/2209/A36 resulted in a general increase in ferrite number with each subsequent pass and a large increase in FN occurred between the last fill pass and the cover pass. The measured FN in the cover pass was greater than that predicted by the WRC-1992 diagram. 
     2) The ferrite number and hardness of the 2205/2209/A36 combination were similar for both the 1.57 and 2.60 kJ/mm heat inputs, suggesting that heat input is a secondary factor relative to weld metal composition in controlling the ferrite/austenite phase balance. 
     3) The 2205/2209/A36 combination formed a narrow martensitic band adjacent to the A36 fusion boundary along the entire thickness of the weld. No martensite was observed in welds made with the Alloy 625 filler metal, possibly due to the smaller composition range over which martensite forms and lower carbon migration rates in Ni-based vs. Fe-based alloys. 
     4) The 2205/2209/A36 fusion zone exhibited similar upper shelf energies for the two heat inputs utilized, suggesting that heat input and dilution from the dissimilar base metal had little or no effect on the mechanical properties. 
     5) The 2205/625/A36 fusion zone toughness behavior was typical of face-centered cubic (FCC) materials as impact energies decreased 10-15 ft-lb (14-20 J) over a temperature range from 25 to -196C. 
     6) Pitting corrosion resistance of the weldment samples utilizing 2209 filler metal showed increased resistance as the heat input increased from 1.57 kJ/mm to 2.60 kJ/mm. The all weld metal samples resulted in a lower percentage weight loss for the higher heat input weld, signifying increased corrosion resistance with the 2.60 kJ/mm weld relative to the 1.57 kJ/mm weld. 
     7) The Alloy 625 fusion zone exhibited general attack in the root pass for each type of corrosion sample tested. It appeared that corrosion resistance of the 2205/625/A36 combination decreases with increasing heat input for the remaining fill and cover passes. 
 

     This work was supported by the members of Edison Welding Institute through the Cooperative Research Program. The technical support of Dr. Wangen Lin, formerly with EWI and currently with Pratt & Whitney, and other members of the EWI staff during the course of this investigation is greatly appreciated. 

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