Microstructure and Mechanical Properties of TIG Welded Joints Between N06200 Nickel Alloy and S32168 Stainless Steel

Literature Overview

The study published in the journal Pressure Vessel (Vol. 40, Issue 6, 2023, pp. 15–21) by Wu Jingwei and colleagues from Lanzhou Lanchi Heavy Equipment Co., Ltd. addresses a critical engineering challenge: the reliable TIG welding of N06200 (Hastelloy C-276) nickel-based alloy to S32168 (a modified 321-type stabilized austenitic stainless steel). This dissimilar metal joint is frequently encountered in pressure vessels and heat exchangers operating under high-temperature, corrosive, or reducing environments where the nickel alloy provides superior corrosion resistance while the stainless steel serves as the structural body. The authors employed conventional TIG welding and conducted comprehensive metallographic and mechanical characterization of the resulting weldments.

Core Technical Findings

Microstructural Characterization

The weld joint exhibits three distinct metallurgical zones, each with characteristic crystal morphologies:

Base metal zones: Both N06200 and S32168 retain their original equiaxed grain structures away from the weld influence.
Heat-affected zone (HAZ): Cellular grains grow perpendicular to the weld axis, indicating directional solidification driven by the thermal gradient.
Weld fusion zone: Dendritic structures dominate the weld center, with the Nb-rich phase segregating at interdendritic regions.

The Nb-rich phase in the weld zone is particularly noteworthy because it contributes significantly to the local hardness enhancement. In S32168, niobium acts as a carbon stabilizer, but in the weld dilution zone, the interaction between Nb from the stainless steel and the Ni-Cr-Mo matrix of N06200 produces carbides and intermetallic compounds that elevate microhardness above both parent materials.

Mechanical Performance

The optimal welding parameter window identified is as follows:

Parameter	Range
Welding current	130–150 A
Welding voltage	15–17 V
Travel speed	80–100 mm/min

Under these conditions, the tensile strength reaches 675 MPa, which represents a reduction of approximately 13.5% relative to the N06200 base metal strength of 780 MPa. This strength loss is expected in dissimilar metal joints where the weaker side (S32168, typically 550–620 MPa) governs the joint behavior. The fracture analysis reveals a predominantly ductile fracture on the S32168 side with minor brittle fracture features, and no intergranular or cleavage fracture was observed. This is a favorable outcome, as it indicates that the weld metal and HAZ did not become the weakest links in the joint.

Engineering Practice Implications

Process Window Selection

The identified parameter range is relatively narrow, which demands careful process control. In practice, the following considerations should be applied:

Heat input management: The combination of 130–150 A and 80–100 mm/min yields a heat input of approximately 1.4–1.8 kJ/mm. This moderate heat input minimizes excessive grain coarsening in the S32168 HAZ while ensuring adequate fusion of the N06200 side.
Preheat and interpass temperature: For thick-section joints, a controlled preheat of 50–100°C is advisable to reduce thermal stresses, but excessive preheat must be avoided to prevent sensitization of the S32168 base metal.
Filler metal selection: Although not explicitly detailed in the abstract, the use of a nickel-based filler (such as ERNiCrMo-3 or equivalent) is essential to bridge the compositional gap and prevent cracking.

Defect Prevention

Based on the microstructural findings, the following defects are of particular concern in this dissimilar joint:

Cracking in the weld zone: The Nb-rich phases can form brittle intermetallics if the cooling rate is too slow or too fast. The dendritic structure in the weld center is susceptible to solidification cracking, especially if sulfur or phosphorus impurities are present.
HAZ softening or embrittlement: The cellular grain growth in the HAZ suggests a rapid cooling regime. If the cooling rate exceeds 50°C/s, martensite may form in the S32168 side, leading to reduced toughness.
Dilution mismatch: Excessive dilution of N06200 into the weld pool can lower corrosion resistance, while over-dilution of S32168 into the N06200 side can introduce carbon that forms chromium carbides, degrading pitting resistance.

Quality Control Recommendations

Inspection Method	Purpose	Acceptance Criteria
Visual inspection	Surface defects, undercut, reinforcement	ASME B31.3 or EN 10204
Dye penetrant testing (PT)	Surface cracks, especially at weld toe	Level II per ASTM E709
Radiographic testing (RT)	Internal porosity, incomplete fusion	Level II per ASME Section V
Hardness mapping	HAZ softening, weld metal over-hardening	Within ±15% of base metal
Tensile testing	Joint strength verification	≥675 MPa minimum

Study Insights and Reflections

This work demonstrates that reliable TIG welding of N06200 to S32168 is achievable with conventional equipment, provided the process parameters are tightly controlled within the identified window. The absence of intergranular fracture and the predominantly ductile failure mode are encouraging indicators that the joint can withstand cyclic loading and thermal fatigue. However, the 13.5% strength reduction relative to N06200 base metal must be accounted for in design calculations, particularly in pressure vessel applications governed by ASME VIII Div. 2 or NB/T 47003.

The Nb-rich phase observation provides a valuable metallurgical insight: niobium, while beneficial for carbon stabilization in austenitic stainless steels, can form hard intermetallic compounds in nickel-based weld metals. Engineers designing similar dissimilar joints should consider post-weld heat treatment (PWHT) at 800–900°C for 1–2 hours to homogenize the weld metal and reduce residual stresses, provided the corrosion resistance requirements permit such treatment.

Reference Value and Outlook

This study provides a solid empirical foundation for engineers dealing with Hastelloy C-276 to stabilized austenitic stainless steel joints in high-pressure, high-corrosion applications. The parameter window and mechanical data can be directly incorporated into welding procedure specifications (WPS) for similar materials. Future work should extend to creep testing at elevated temperatures (400–600°C) and stress corrosion cracking (SCC) evaluation in chloride-containing environments, as these are the primary degradation mechanisms for such joints in service.