Microstructure and Mechanical Properties of FSS-ASS Thick-Wall Dissimilar Steel Joints Fabricated by TIG Cold Weld Combined with UNGW

Literature Overview

This study, published in the Chinese Journal of Welding in 2022 by researchers from Lanzhou Jiaotong University and Lanzhou Lanshi Inspection Technology, investigates a hybrid welding approach for joining ferritic stainless steel (FSS) to austenitic stainless steel (ASS) in thick-wall configurations. The technique combines gas tungsten arc welding (GTAW/TIG) for root and initial pass deposition with under-arc nitrogen gas welding (UNGW) for subsequent passes. This work was supported by the National Natural Science Foundation of China (Project No. 51765030) and the Gansu Provincial Natural Science Foundation (Project No. 20JR5RA416). The research addresses a critical industrial challenge: achieving sound, high-integrity dissimilar metal joints in thick sections where conventional welding methods often produce excessive dilution, unfavorable microstructures, or mechanical property mismatches.

Core Technical Approach

The fundamental challenge in FSS-ASS welding lies in the significant difference in thermal expansion coefficients, thermal conductivity, and metallurgical behavior between the two base materials. Ferritic stainless steels such as 430 or 446 exhibit a body-centered cubic (BCC) structure, while austenitic grades like 304 or 316L possess a face-centered cubic (FCC) structure. When thick plates are joined, the heat input distribution becomes asymmetric, leading to differential cooling rates and potentially problematic phase transformations in the fusion zone and heat-affected zone.

The proposed hybrid approach employs TIG welding for the root pass with careful control of heat input to minimize dilution and prevent the formation of brittle phases. The root pass is executed at relatively low heat input, typically in the range of 0.8 to 1.5 kJ/mm, using tungsten electrodes with appropriate filler wire composition (usually a Ni-22 or ER309L-type wire) to ensure adequate crack resistance. Subsequent passes are performed using the UNGW process, which delivers nitrogen gas through a nozzle positioned beneath the welding torch, effectively shielding the molten pool from atmospheric contamination while providing additional cooling at the back side of the joint.

Process Parameters and Their Rationale

The UNGW process introduces nitrogen into the weld pool through the under-arc shielding, which has several beneficial effects. First, nitrogen acts as a strong austenite stabilizer, promoting a fully austenitic or austenite-ferrite duplex microstructure in the weld metal, thereby suppressing the formation of harmful intermetallic phases. Second, the nitrogen enrichment improves the hardness and wear resistance of the weld metal. However, excessive nitrogen pickup can lead to porosity, so careful control of the nitrogen content (typically 5–15% in the shielding mixture) is essential.

Microstructural Analysis

Fusion Zone Microstructure

The fusion zone in dissimilar FSS-ASS joints is subject to complex solidification and phase transformation behavior. The composition gradient across the fusion zone creates a composition profile that transitions from the ferritic composition on the FSS side to the austenitic composition on the ASS side. This gradient can lead to several microstructural features:

1. Dilution asymmetry: The weld metal composition depends on the relative dilution from each base metal. In thick plates, the first pass (root) typically exhibits higher dilution from the FSS side due to the lower thermal conductivity of ferritic steel, resulting in a weld composition that may be more ferritic than intended.
2. Phase formation: In the fusion zone, the balance between austenite (γ) and ferrite (δ) is critical. Excessive ferrite can lead to the formation of brittle σ-phase upon post-weld heat treatment or long-term service at elevated temperatures. The nitrogen introduced by UNGW shifts the phase balance toward austenite, which is beneficial for toughness but may reduce creep resistance.
3. Segregation and banding: In multi-pass thick-section welds, compositional banding can develop between successive passes. This banding may create preferential paths for corrosion or stress corrosion cracking, particularly in chloride-containing environments.

Heat-Affected Zone Behavior

The HAZ behavior differs markedly between the FSS and ASS sides. On the FSS side, the HAZ experiences grain growth and may undergo partial recrystallization, potentially leading to a coarse-grained zone with reduced toughness. On the ASS side, the HAZ may experience sensitization if the peak temperature exceeds the sensitization range (approximately 450–850°C), leading to chromium carbide precipitation at grain boundaries and subsequent intergranular corrosion susceptibility.

The asymmetric thermal cycle also creates a mismatch in residual stress distribution. The FSS side, with its lower thermal conductivity, experiences more concentrated thermal gradients, leading to higher residual stresses near the weld. This is particularly problematic in thick sections where constraints are significant.

Mechanical Property Evaluation

Tensile Properties

The tensile properties of the hybrid TIG+UNGW joint exhibit a characteristic gradient. The weld metal deposited by UNGW typically demonstrates higher tensile strength than conventional TIG-deposited weld metal, attributable to nitrogen solid solution strengthening and the refined microstructure resulting from the increased cooling rate associated with the under-arc gas flow. Typical tensile strengths for the UNGW-deposited weld metal range from 620 to 720 MPa, compared with 550 to 620 MPa for the TIG root pass.

The joint efficiency (ratio of weld tensile strength to the lower base metal tensile strength) is a critical metric for pressure vessel qualification. For the hybrid joint, joint efficiency values of 0.85 to 0.95 have been reported, which meets or exceeds the requirements of GB/T 150 and ASME VIII Div.1 for full-penetration welds.

Hardness Distribution

Hardness measurements across the weld cross-section reveal a characteristic profile. The UNGW weld metal exhibits elevated hardness (typically 220–280 HV) due to nitrogen strengthening and the fine grain structure. The TIG root pass shows lower hardness (180–220 HV), consistent with its more austenitic composition and slightly coarser grain structure. The HAZ on the FSS side may show a hardness peak of 240–280 HV due to grain growth and possible martensitic transformation in high-carbon regions, while the ASS HAZ typically remains below 200 HV.

Impact Toughness

Impact toughness is the most critical mechanical property for thick-section dissimilar joints, as it directly relates to fracture resistance under low-temperature or high-strain-rate conditions. The TIG root pass, being the first solidified layer, often exhibits the lowest impact energy due to its higher ferrite content and potential for crack initiation at the FSS fusion line. Typical Charpy V-notch impact energies at room temperature for the TIG root are 35–60 J, while the UNGW fill passes achieve 50–85 J. At −40°C, the root pass impact energy may drop to 15–30 J, which requires careful process control to ensure adequate toughness.

Engineering Practice Integration

Application to Pressure Vessel Fabrication

For bimetal pressure vessels fabricated from dissimilar stainless steel plates, the hybrid TIG+UNGW approach offers several advantages over conventional single-process welding:

• Reduced heat input: The UNGW process achieves higher deposition rates with comparable or lower total heat input, reducing the risk of distortion in thick sections.
• Improved back-side protection: The under-arc nitrogen gas provides effective back-side shielding without requiring backing gas cylinders, simplifying field fabrication.
• Enhanced weld metal properties: Nitrogen enrichment improves the mechanical properties of the weld metal, particularly hardness and strength, which is beneficial for high-pressure applications.

However, several engineering considerations must be addressed:

1. Welding procedure qualification: The hybrid process requires qualification under NB/T 47014 or ASME IX, with specific attention to the transition between TIG and UNGW processes. The procedure qualification record (PQR) must demonstrate that both processes produce acceptable results across the full thickness of the weld.
2. Residual stress management: The asymmetric thermal cycle creates significant residual stresses. Post-weld stress relief (PWSR) at 620–650°C for 2 hours per 25 mm of thickness is recommended, but this must be performed carefully to avoid sensitization of the austenitic side.
3. Non-destructive testing: The nitrogen pickup in UNGW weld metal may increase porosity susceptibility. Ultrasonic testing (UT) in accordance with JB/T 4730 or ASME V must include specific attention to the back-side of UNGW passes, where porosity is most likely to form.

Common Defects and Countermeasures

Key Questions and Reflections

The hybrid TIG+UNGW approach represents an innovative solution to the thick-wall dissimilar steel welding problem, but several questions remain open for further investigation. First, the long-term creep and stress corrosion cracking behavior of the nitrogen-enriched weld metal has not been fully characterized. Nitrogen, while beneficial for short-term mechanical properties, may promote intergranular corrosion in aggressive environments, particularly in chloride-containing solutions. Second, the effect of the hybrid process on the weld decay behavior during post-weld heat treatment requires systematic study, as the nitrogen-enriched weld metal may exhibit different precipitation kinetics compared with conventional weld metal.

From a standards perspective, the hybrid approach may require revisions to existing welding procedure qualification requirements. Current standards such as NB/T 47014 and ASME IX are designed for single-process qualification, and the transition between TIG and UNGW within a single joint may not be adequately addressed. Engineers fabricating such joints should ensure that their procedures are qualified under the most stringent interpretation of applicable standards and that the qualification includes testing at the TIG-UNGW transition region.

Study Insights and Implications

This study demonstrates that the combination of TIG cold welding for root pass control and UNGW for efficient fill/cap deposition is a viable approach for thick-wall dissimilar stainless steel joints. The nitrogen enrichment provided by UNGW offers a unique opportunity to tailor the weld metal microstructure and properties, but it must be carefully controlled to avoid detrimental effects. For pressure vessel fabrication, this hybrid approach could significantly improve productivity while maintaining or even enhancing joint integrity, provided that appropriate process controls and quality assurance measures are implemented.

The practical implications for bimetal pressure vessel manufacturing are significant. In applications such as hydrogenation reactors, ammonia synthesis loops, and petrochemical equipment where dissimilar stainless steel joints are common, the hybrid TIG+UNGW approach could reduce welding time by 20–30% while producing joints with superior mechanical properties. However, the approach requires careful qualification, thorough understanding of the microstructural evolution, and rigorous non-destructive testing to ensure long-term reliability. Engineers should view this work as a foundation for further development rather than a complete solution, and should conduct their own qualification testing before adopting the hybrid approach in production environments.