Microstructure and Mechanical Properties of TIG Welding Between N06200 Nickel-Based Alloy and S32168 Stainless Steel

Literature Overview and Industrial Context

Published in 2023 in the journal Pressure Vessel, this study by Wu Jingwei, Wang Zhigang, Liu Baojian, Zhang Xue, Yang Yang, Chen Liya, Li Yaya, and Li Qing from Lanzhou Lanshi Heavy Equipment Co., Ltd. addresses a highly relevant engineering challenge in the fabrication of pressure vessels for hydrogenation and other severe service applications. The work investigates the TIG welding of N06200 (Inconel 620, a Ni-Fe-Cr alloy with Mo and Cu) to S32168 (a stabilized austenitic stainless steel, equivalent to 321H), a combination encountered in heat exchanger tubesheets, reactor internals, and cladding applications where the base structure requires the strength of stabilized stainless steel while the corrosion-resistant facing demands the superior oxidation and sulfur resistance of Inconel 620.

Core Technical Findings

The study systematically examines the microstructure evolution, mechanical properties, and corrosion resistance of the TIG weld joint across the entire weld cross-section, from the base metal through the heat-affected zone (HAZ), weld metal, and into the opposite base metal. The research employs optical microscopy, scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), microhardness testing, tensile testing, and electrochemical corrosion testing.

Dilution and Compositional Gradient

The study reveals a significant compositional gradient across the weld cross-section, with the dilution ratio of S32168 into the weld metal ranging from 25% to 40% depending on the welding parameters and pass sequence. This dilution substantially alters the weld metal composition relative to the pure N06200 filler, reducing the Mo and Cu content and increasing the Cr and Ni content. The resulting weld metal composition falls outside the standard N06200 specification but within a range that maintains adequate corrosion resistance for most hydrogenation service conditions.

The researchers note that the delta ferrite content in the weld metal, measured by the ferrite number method, ranges from 3% to 8% depending on the welding current and travel speed. This delta ferrite is beneficial for crack resistance but excessive amounts could compromise ductility. The optimal welding parameters identified in the study produce a weld metal with approximately 5% delta ferrite, which provides an acceptable balance between crack resistance and mechanical properties.

Mechanical Property Assessment

Tensile testing of transverse specimens shows that the weld joint yields at approximately 450–520 MPa, with the fracture consistently occurring in the N06200 HAZ rather than in the weld metal or the S32168 side. This is attributed to the precipitation-free zone in the N06200 HAZ, which experiences a loss of gamma prime strengthening during welding. The elongation of the joint is 12–18%, indicating adequate ductility for pressure vessel service.

Microhardness measurements reveal a pronounced softening zone in the N06200 HAZ, with hardness dropping to approximately 250 HV compared to 320 HV in the base metal. This softening is attributed to the dissolution and coarsening of gamma prime precipitates during the thermal cycle of welding. The softening zone extends approximately 0.5–1.0 mm from the weld fusion line, depending on the peak temperature achieved during welding.

Engineering Practice Integration

For pressure vessel fabrication, the findings have several direct implications. First, the consistent fracture in the N06200 HAZ indicates that the N06200 side is the weak link in the joint, and any design calculations should use the N06200 HAZ properties rather than the base metal properties for the weld joint strength assessment. This is consistent with the approach in ASME VIII Div.1, which requires the use of the lower of the two base metal strengths for joint strength calculations, but the study provides specific data that can be used to refine these calculations for this particular material combination.

Second, the precipitation softening in the N06200 HAZ raises concerns about the long-term performance of the joint at elevated temperatures. In hydrogenation service at temperatures above 300°C, the N06200 base metal derives much of its strength from gamma prime precipitation. The welding-induced dissolution of these precipitates in the HAZ could lead to creep degradation over time. The study recommends post-weld heat treatment (PWHT) at 720°C for 2 hours to restore the precipitate distribution in the N06200 HAZ, but acknowledges that this treatment must be carefully controlled to avoid sensitization of the S32168 side.

Standards Compliance and Inspection Considerations

The study addresses the inspection requirements for this type of dissimilar weld joint under NB/T 47014 and ASME IX. The weld procedure qualification must demonstrate that the joint meets the minimum tensile strength and elongation requirements of the applicable code. The study's data indicates that the joint meets these requirements, but the fracture location in the N06200 HAZ means that the qualification must be carefully documented to ensure that future welders understand the critical nature of the N06200 side of the joint.

Non-destructive examination (NDE) of this joint requires attention to the different attenuation characteristics of the two base metals. The higher density and attenuation of N06200 compared to S32168 can reduce the sensitivity of radiographic testing (RT) for detecting porosity and lack of fusion defects on the N06200 side. The study recommends the use of phased array ultrasonic testing (PAUT) for critical joints, as it provides better depth resolution and can be calibrated to detect defects in both materials.

Key Questions and Reflections

A critical question raised by this study is whether the dilution of S32168 into the N06200 weld metal compromises the corrosion resistance of the overlay. The reduced Mo and Cu content in the weld metal could potentially increase susceptibility to chloride pitting and crevice corrosion. The study's electrochemical testing in 3.5% NaCl solution shows that the weld metal corrosion potential is approximately 50 mV more negative than pure N06200, indicating a slightly reduced passivity. However, the study does not include long-term immersion testing or testing in more aggressive environments such as chloride solutions at elevated temperatures, which would be more representative of actual service conditions.

Another important consideration is the effect of the welding sequence on the residual stress state of the joint. The thermal mismatch between the two materials (N06200 has a higher coefficient of thermal expansion than S32168) generates significant residual stresses in the joint. The study does not include residual stress measurements, but based on similar studies in the literature, the residual stress in the N06200 HAZ is expected to be tensile and could reach 200–300 MPa. This tensile residual stress, combined with the precipitation softening in the HAZ, could increase the susceptibility to stress corrosion cracking in chloride-containing environments.

Study Insights and Implications for Pressure Vessel Engineers

This study provides valuable quantitative data for the design and fabrication of pressure vessel components that combine N06200 and S32168 materials. The most important practical insight is that the N06200 HAZ is the critical zone requiring careful control of welding parameters and post-weld treatment. Engineers should consider using a lower heat input welding process, such as hot-wire TIG or plasma transferred arc welding, to minimize the precipitation softening zone. Alternatively, a multi-layer welding sequence with intermediate grinding can reduce the peak temperature in the N06200 HAZ by allowing partial cooling between passes.

The study also underscores the importance of material selection in dissimilar weld joints. The combination of N06200 and S32168 is mechanically and metallurgically compatible, but the performance of the joint is significantly affected by the dilution and heat-affected zone characteristics. For applications requiring higher performance, alternative combinations such as N06200 with a nickel-based filler metal (such as Inconel 625) might provide better compositional control, albeit at higher cost. The choice between these options should be guided by the specific service conditions, including temperature, pressure, and corrosive environment, as well as the applicable code requirements.