Microstructure and Mechanical Properties Assessments of 304 Austenitic Stainless Steel and Monel 400 Dissimilar GTAW Weldment

Cupper-Nickel alloy Monel 400, AISI 304 austenitic stainless steel provided from Iraqi Oil Ministry and ENiCrFe-2 filler metal from ESAB company chemical compositions represented in Table 1. Welding samples with 100 mm × 50 mm × 4 mm dimensions and square butt joint configurations with 2 mm root space fabricated and welded with Diversion™ 180 TIG welding technology from Miller as shown in Figure 1.

Welding samples clamped strongly in fixture with copper back plate to distract harmful welding heat from weldment and avoid distortion during welding. Welding parameters established from earlier studies and from trial and error experiments and illustrated in Table 2. After welding, weldments shaped into required dimensions using wire cut machine EDM (Electrical Discharge Machining) to investigate welding joint metallurgical and mechanical characters.

Welding zone, HAZ and parent metals microstructures metallographic exanimated. Metallographic test standard procedure applied to investigate weldments various zones microstructure. Weldments samples prepared using SiC emery sheets with 220 to 1000 grit size, then followed by alumina disc polishing to obtain final mirror finish of 1μm. Monel 400 nickel alloy HAZ and welding joint microstructures detected with (50 ml HCl, 10 grams of CuSO4 and 50 ml distilled water) etching reagent, while (15cc HCl, 10 cc HNO3 and 10 cc CH3COOH) etching reagent used on AISI 304 austenitic stainless steel HAZ and welding zone. ASTM E8 standard applied in tensile studies of the weldment’s samples. Three samples test performed to ensure authenticity of results. 2 mm/min strain rate used for the three test samples.

Table 1. Experiment base metals and welding filler metal chemical compositions

Vickers Microhardness tester conducted to study microhardness on all three regions of weldments by keeping weld in center and test from the both sides. Microhardness test employed 500 gf load and 10 second dwell time, microhardness measurements carried out with regular 1 mm spacing and more than 20 microhardness points was taken (10 point from each side). SEM and EDS analysis employed on weldments various zones to determine the elements diffusion between the welding zone and HAZ and also to assess welding zone structure correlations with weldment mechanical properties changing.

Table 2. GTAW welding parameters

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Figure 1. Monel 400 and AISI 304 weldment configuration

4.1 Vickers micro hardness measurement

Monel 400 to AISI 304 dissimilar welding by using ENiCrFe-2 filler metal hardness profile clearly demonstrated maximum hardness value was recorded at welding zone, hardness value average at welding zone founded 202.2 HV while hardness average value of 192.6 HV recorded in Monel 400 HAZ and 134.6 HV for 304 austenitic stainless steel side as in Figure 5.

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Figure 5. AISI 304 and Monel 400 dissimilar GTAW weldment micro hardness profile

4.2 Weldments tensile test

ASTM E8 standard procedure applied to prepare samples to record accuracy tensile test values of dissimilar welding joints. Three tensile tests were prepared to record the average value of the test, in all three samples; samples failure occurred at AISI 304 base metal, as shown in Figure 6. Samples before fracture suffered significant amount of plastic deformation reflecting failure in ductile style. Tensile test average properties recorded from tests include tensile strength of 503.6 MPa and ductility of 29.75%.

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Figure 6. ASTM E8 standard tensile samples (fracture sample)

4.3 Weldments impact test

Three standard V-notch Charpy test specimens prepared from dissimilar weldments according to ASTM E23 code to analyze weldments strength for different suitable applications. Cumulative V-notch Charpy test results for three specimen tests are 72 J for first specimens, 74 J for second specimen and 78 J for third specimen.

6.1 Microstructure results discussion

Monel 400 HAZ microstructure in Figure 2(a), (b), (c) and (d) demonstrated coarse grains size at (HAZ) microstructure with Ni-Cu second phase formation, grains size coarsening can be observed in structure due to GTA welding heat. Also, partial melting zone (PMZ) formation can be easily observed in Monel HAZ and weld side of joint interface inside the HAZ. This zone formed when there is a zone of Monel parent metal heated just below liquid temperature but at the same time, above metal solid temperature, this means this zone only partially melted and called partially melted zone (PMZ). Because of good thermal conductivity and low heat capacity of monel 400 alloy, relatively big, about 2 mm, PMZ zone created adjacent to welding fusion line. In PMZ most of interdendritic solidification mode which observed in welding zone just after fusion line and the grain boundaries fully melted and performed as channels to convey welding melt metal from welding pool to the root face, and creating PMZ melted and resolidified layers [17]. PMZ phenomenon is considerable for fusion welding technology, like GTAW, when the input heat is high to melting and joining the parts. Welding procedure for this reason is very critical to successfully joining Monel 400 with AISI 304 alloys to avert shrinkage porosity or holes in welding microstructure solidification in weld pool. Therefore, it’s very important to understanding PMZ formation mechanism, And as a result understanding loss of ductility and hot cracking formation and phenomena in dissimilar welded joints [18]. Partially melted zone in Monel joint also formed low melting austenite eutectic carbide during solidification process and elements like Mn and Cr will be depleted from the welding zone toward monel interface side. Nickel reduction in PMZ weaker stabilization driving force of austenite on low temperatures during cooling, which when associated to high defunded carbon content from base metal consider a perfect condition for martensite creation, even with slower cooling rate conditions. However, due to high nickel concentration on PMZ of welds deposited when employed nickel alloys electrodes, austenite stabilizes on larger area inside PMZ, and because that only thin martensite layer expected to formed in welding zone deposited metal welded by these alloys. 

AISI 304 HAZ side microstructure examination reveals changes in grain size, but no precipitation and sensitization evidences can be detected. Also, fine austenitic grains size noticed near fusion line and twinning randomly grain structure observed in little amount. Figure 3(a), (b), (c) and (d) shown formation of welding filler metal debilitation area or unmixed zone (UZ), intermediate mixed zone and hard zone often created in like these joints. The unmixed zone (UZ) had laminar layer microstructure and formation when small fractions of base metal completely melted and resolidified without undergo filler metal dilution. Unmixed zone (UZ) created when fusion zone has a thin completely melted and re-solidified region adjacent to fusion line, where base metal quickly melted and solidified produced composition similar to parent metal known as unmixed or (chilled) zone [19], for example, when AISI 304 st.st welded using high chromium-nickel filler like ERNiFeCr-2, chromium and nickel concentration gradients founded in fusion zone, where unmixed zone has a structure with chemical composition similar to parent metal. No evidence of carbide precipitation was observed in AISI 304 HAZ and base metal from Figure 3(a), (b) and (c), but clearly grain growth observed due to the welding heat effects. UMZ formation causes in segregation during weld metal solidification and transformation of fractions from weld metal side which smaller than that in AISI 304 HAZ side, this process makes weld metal suffer dense segregation. In welding zone neighbor, the cooling rate in AISI 304 side is higher than fusion line and welding zone, resulting in longer exposure time to the sensitive temperature range for welding zone and fusion line. This exposure causes segregation of important elements like Mo and precipitations of other elements in weld zone side, a chromium-depleted zone may appear around these precipitations, which created soft zone and activated surface to pitting corrosion, some pits can rapidly form in this zone and propagate in very fast rate, which deeply affected pitting corrosion resistance and weldment toughness. El-Aziz et al. [20] and Liou et al. [21] results found similar pitting corrosion holes from precipitations in welding process.

Welding zone microstructure in Figure 4(a), (b), (c) and (d) shows interdendritic solidification microstructure formation in weld region just near the fusion line, also the microstructure demonstrated clearly partial melting zone (PMZ) formation just after the fusion line. Interdendritic network consisting elements like Ni, Cr, Fe, Cu as in Figure 4(a), (b). The region far from the fusion line and welding zone center demonstrated clearly epitaxial solidification as in Figure 4(c) and (d), migrated grain boundaries also observed clearly at welding zone.

Since welding metal in Figure 4(c) and (d) illustrated existence of cellular solidification structure of welding zone contains some copper, secondary phases formation also can be observed; secondary phases formation makes welding metal more suspected for hot cracking defect. The globular precipitates and oxide inclusions distributed in welding zone matrix will affect joint properties. Formation of migrated grain boundaries (MGBs) can be observed clearly in Figure 4(c), (d) which most preferred austenitic welding metals. Weld metals with migrated grain boundaries (MGBs) as reported by other research papers, are more sensitive to the ductility dip cracking (DDC) issues which formed all over migrated grains boundaries.

Migrated grains boundaries existence in solidified weld metal structure produced welding zone with good strength and strain hardening capacity. The increasing of yield strength in welding zone attributed to the high stresses needed to innated slip transmission process. Dislocation reactions that operate at grain boundary affects both strength and deformation behavior. high strength and good mechanical properties obtained when interfaces act as dislocations movement obstacles, but no significant changes expected in welding zone mechanical properties when welding interface microstructure acts as a sink or its transparent to dislocations movements cross the structure grains boundaries. Obvious studies have reported that grains boundary migration is possible during reheating processes sequence, like what happen in welding procedures with multipass techniques, migrated grain boundaries formation phenomena strongly related to the element’s diffusion and its effects in microstructure crystal lattes and crystal misorientation across grains boundaries and inclination of boundary plane [22], which effected by welding procedure heating cycle. Boundary migration initiated in welding zone solidified structure by bulges formation and expansion in boundary which grow into adjacent grains in another direction, spatial distribution and directions of these bulges occurred in random mode [22]. In this study, GTAW multipass procedure used to join these dissimilar metals, multipass procedure will normally produce high heat input value to the welding zone cause migration of grain boundaries phenomena. However microscopic and SEM images state that ductility dip cracking is not being recorded because of high amounts of Nb and Mo presence in unmixed zone (UZ) which formed adjacent to fusion line of AISI 304 when base metal melted and then quickly solidified, dynamic recrystallization classified as one of most important originators of DDC.

Dynamic recrystallization can define as a phenomenon that occurs when material strained in high temperatures range. When that happen new grains formation in the high stress concentration locations like grains boundaries triple points, but the DDC agreed mechanism upon most researcher’s state on that DDC is grains boundaries sliding phenomenon. Grains boundaries sliding generated stresses on grains boundaries triple points in weld metal microstructure [23], Nb and Mo had master effect in reduced weld met DDC susceptibility because two reasons, the first one because Nb addition to weld metal structure formed NbC at solidification end. NbC formation pinned welding metal microstructure grains boundaries and tortuous the dislocations movements bath, also grains boundaries pinning created mechanical locking inhibited grain boundary sliding and therefore DDC.

The second reason related with Mo addition to welding zone microstructure, Mo addition forms M23C6 which precipitate in grains boundaries, carbide precipitate effects are uncertain whether helps with DDC mitigate, or it contributes to other effect. Noecker and DuPont [24] hypothesized effects of localized stress which forms at grains boundaries carbide interface and locking boundaries slipping.

Young et al. [25] proposed that (Fe,Cr)23C6 carbides formation is the major contributor in DDC initiation, because its generated localized solidification stresses during cooling. Nb or Ti additions forms MC carbides and reduce (Fe,Cr)23C6 formation and decreased welding microstructure DDC susceptibility, because DDC is precipitation cracking mechanism according to his work. This is not always true because austenitic alloys like AISI304 stainless steel can be susceptible to DDC even when no carbides precipitation occurred and unmixed zone microstructure with high chromium and nickel concentration and elements’ gradients from fusion zone to UZ and HAZ in this research prove this fact.

6.2 SEM and EDS results discussion

SEM in Figure 7(a) and (b) of AISI 304 unmixed zone (UZ) is founded to have large sized voids disclose the fact of these weldments expected to failed with ductile mode fracture owing to micro and macro-voids existence, these large sized voids as discussed earlier in UZ formation mechanism could form from the segregation phenomena which cause very sensitive structure to pitting corrosion.

SEM in Figure 8 of PMZ in Monel side shown different uniform texture with no or few voids which increased welding metal mechanical properties. Welding zone SEM in Figure 9 has an austenitic texture with carbide, which is the favor of MGBs phenomena as reported by other studies [22]. EDS Analysis in Table 3 for Monel 400 HAZ and PMZ shows high nickel concentration on PMZ, this effects austenite stabilizes on larger area inside PMZ, and because that only thin martensite layer expected to formed in welding zone deposited metal. Also, chemical profile of PMZ on Monel interface which obtained from EDS analysis shows highlighting Fe, Cr and Ni percentages. From welding zone to PMZ planar region a gradual increasing in Ni and Cr content and reduction of Fe content can be observed. AISI 304 HAZ and PMZ EDS Analysis in Table 4 reflected the effects of unmixed zone UZ and soft zone formation EDS demonstrated the formation of soft and brittle zone in HAZ due the segregation of Mo from welding zone to fusion line and HAZ direction, and depleting of Cr toward welding zone direction which resulted in toughness and ductility leakage with reducing in pitting corrosion resistance in AISI 304 HAZ and fusion line.

Welding zone EDS shown high percentage of important elements like Ni, Cr, Fe and C which produced austenitic microstructure with MGB and carbide precipitation on grains boundary, this texture resulted in good mechanical properties to the welding zone compering with the neighbor zones.

6.3 Mechanical tests results discussion

Microhardness readings of weldments in Figure 5 showed that the welding zone recorded a maximum value compared to other zones of weldment. Presence of hard phases like NbC, (Nb, Mo) C formation evidence from SEM/EDS analysis and MGB formation in welding zone microstructure can be proofed clearly in microhardness profile values. highest hardness recording in welding zone because of metallic carbides presence which enhanced strength of weldments, AISI 304 unmixed zone and soft zone formation strongly involved in tensile test fracture location which marked in stainless steel side of weldments as show in Figure 6. As discussed, and explained in UZ formation in AISI 304 side, this zone will suffer from strong leakage in toughness and ductility with reduction in pitting corrosion resistance which forms voids in HAZ microstructure weaken this zone. V-notch Charpy test results certificated the microstructure and SEM results which reflected the highest toughness value of welding zone structure comparing with the other zones. Hardness test and V-notch Charpy test results well agreed with tensile test results and can certificated the SEM/EDS and microstructures results.

The most important features of the proposed method of this research are the ability to predicated welding zone and HAZ microstructure and mechanical properties connected to it by applying this study results which demonstrated that successful dissimilar joints of Monel 400 nickel alloy and 304 stainless steel can be produced by employing GTAW technology and ENiCrFe-2 welding filler. In another hand, the amount of input heat to welding zone is essential factor in dissimilar fusion welding and it’s very difficult to precise control of welding heat in like this method which consider the main limitation of this study beside other limitations like the needed to professional TIG welder, which can be eliminated by automation welding process.

Development of this research in future work can be done by studying the impact of Mo and Ni nano particles addition to welding zone and study welding zone alteration according to that addition or changing welding current mode in TIG technology to pulse mode and control the time between current pulse to control the amounts of heat input.