Microstructure and Mechanical Properties of N06200 and S32168 Dissimilar TIG Welding Joint

Literature Overview

This study, published in the journal "Pressure Vessel" (2023, Vol. 40, No. 6), was conducted by researchers at Lanzhou Lanshi Heavy Equipment Co., Ltd. The work addresses a critical engineering challenge in pressure vessel manufacturing: the reliable TIG welding of N06200 nickel-based alloy to S32168 stainless steel. Dissimilar metal welding joints are ubiquitous in chemical processing equipment, heat exchangers, and cryogenic pressure vessels where different corrosion resistance requirements exist across the vessel wall. The authors systematically investigated the weld joint microstructure and mechanical properties under varying TIG welding parameters, providing valuable guidance for production welding procedures.

Core Technical Findings

Microstructure Characterization

The metallographic analysis revealed three distinct microstructural zones in the weld joint:

Base metal zone: Equiaxed grain structure characteristic of both parent materials
Heat-affected zone (HAZ): Cellular grains growing perpendicular to the fusion boundary, indicating directional solidification under the thermal gradient
Weld pool center: Dendritic structure formed under high cooling rates

The presence of Nb-rich phases in the weld metal region is particularly noteworthy. S32168 stainless steel is a niobium-stabilized austenitic grade (equivalent to 321H or 321), and the Nb-rich precipitates contribute significantly to the elevated microhardness observed in the weld metal compared to either parent material. This is consistent with the precipitation hardening mechanism where NbC and Nb2C carbides form during solidification and subsequent cooling.

Mechanical Property Results

Parameter	Value	Remarks
Optimal welding current	130–150 A	TIG DC
Optimal welding voltage	15–17 V	Arc voltage
Optimal welding speed	80–100 mm/min	Travel speed
Tensile strength (optimal)	675 MPa	Weld joint
Tensile strength (N06200 BM)	780 MPa	Parent material
Fracture mode	Mixed (ductile + slight brittle)	On S32168 side

The tensile strength of the weld joint (675 MPa) is approximately 13.5% lower than that of the N06200 parent material (780 MPa). This strength reduction is attributed to the lower yield strength of S32168 stainless steel (typically 205–310 MPa) compared to N06200 (approximately 240–275 MPa minimum). The fracture occurring on the S32168 side confirms that the weaker material governs the joint failure, which is a conservative and predictable behavior from a design standpoint.

The absence of intergranular fracture or cleavage features is a positive finding, indicating that the welding procedure does not induce significant sensitization or brittle phase formation in the HAZ. This is particularly important for S32168, which is specifically designed to resist intergranular corrosion through Nb stabilization.

Engineering Practice Implications

Welding Procedure Specification Considerations

For engineers developing welding procedure specifications (WPS) for N06200-to-S32168 joints, the following observations from this study are directly applicable:

Current range selection: The optimal current window of 130–150 A for typical plate thicknesses suggests that lower penetration is achievable with good wetting, which is appropriate for thin-wall pressure vessel applications.
Travel speed optimization: The 80–100 mm/min range provides a balance between heat input and dilution. Excessive travel speed would increase dilution of S32168 into the weld, potentially introducing chromium carbide precipitation risks.
Filler metal selection: Although not explicitly detailed in the abstract, the presence of Nb-rich phases suggests that a nickel-based filler (such as ERNiCrMo-3 or similar) was likely used, which provides good compatibility with both parent materials.

FMEA Analysis for Dissimilar Metal Welding

Applying Failure Mode and Effects Analysis (FMEA) to this welding application:

Potential Failure Mode	Cause	Effect	Mitigation
Cracking in HAZ	High thermal conductivity mismatch	Cold cracking on S32168 side	Preheat control, low hydrogen filler
Intergranular corrosion	Sensitization of S32168	Loss of corrosion resistance	Nb stabilization, low heat input
Strength mismatch	Dilution imbalance	Joint failure at weaker side	Filler selection, parameter optimization
Hot cracking	Sulfur/phosphor segregation	Weld cracking	Low S, P filler; controlled solidification

Key Reflections and Study Insights

The most significant contribution of this study is the confirmation that reliable dissimilar welding between a nickel-based alloy and a niobium-stabilized stainless steel is achievable through conventional TIG welding without exotic techniques. The mixed fracture mode (predominantly ductile with minor brittle contribution) suggests that while the joint is not as strong as the N06200 parent material, it maintains adequate toughness for most pressure vessel applications.

One area that warrants further investigation is the long-term creep and stress corrosion cracking behavior of such joints at elevated temperatures. The Nb-rich phases in the weld metal could potentially affect high-temperature performance, and the microstructural stability of the HAZ under sustained thermal cycling deserves attention for applications involving thermal fatigue.

For production implementation, I recommend that engineers conducting qualification tests for similar dissimilar joints should pay particular attention to the dilution ratio and its effect on weld composition. The transition from equiaxed to cellular to dendritic structure across the joint provides a clear indicator of the thermal history, and monitoring this transition through microstructural examination can serve as a non-destructive proxy for process control verification.