Microstructure and Mechanical Properties of TIG Welded Joints of Inconel 625 Alloy

Literature Overview

This paper published in Welding Technology (Vol. 43, No. 6, 2014, pp. 20–22) by Wang Zeming, Tao Haiyan, Tang Bin, Fan Baoquan, Yu Dehuai, and Wang Shizhong from the Reactor Fuel and Materials Key Laboratory at the China Institute of Atomic Energy Research investigates the microstructure and mechanical properties of TIG welded joints of 6 mm thick Inconel 625 alloy plates. The study is supported by the China Institute of Atomic Energy Research Youth Fund (JJXM-QNJJ-11-11-04). Inconel 625 is a nickel-base superalloy widely used in nuclear, aerospace, and chemical processing applications due to its excellent combination of high-temperature strength, corrosion resistance, and resistance to stress corrosion cracking.

Material Background and Welding Challenges

Inconel 625 is a precipitation-strengthened nickel-base alloy with the nominal composition of Ni-21Cr-9Mo-3Nb-1Ti. The alloy derives its strength from gamma-prime (γ') and gamma-double-prime (γ'') precipitates that form during solution treatment and aging. Welding this alloy presents several challenges:

• Sensitization: The high chromium and niobium content makes the alloy susceptible to intergranular sensitization in the heat-affected zone (HAZ) if the thermal cycle is not properly controlled.
• Hot cracking: The wide solidification range and high ductility dip of the weld metal make it susceptible to solidification cracking.
• Precipitation coarsening: Excessive heat input can cause coarsening of strengthening precipitates in the HAZ, leading to localized softening.
• Residual stress: The high thermal conductivity and coefficient of thermal expansion of nickel-base alloys can generate significant residual stresses during welding.

Welding Process Parameters and Methodology

The study employs conventional TIG welding with varying process parameters to investigate the effect of heat input on microstructure and mechanical properties. The base material is 6 mm thick Inconel 625 plate, welded in a single pass. The following parameters were varied:

Microstructural Analysis

The microstructural examination reveals the following key features:

Weld metal zone: The weld metal exhibits a typical cast structure characteristic of solidification from a fully molten state. The center of the weld contains a small region of equiaxed grains, flanked on both sides by columnar grain regions. This microstructural pattern is consistent with the directional solidification expected in TIG welding, where the heat extraction direction is primarily away from the weld center toward the cooler base metal.

The equiaxed grain region in the center of the weld is attributed to constitutional supercooling and grain detachment from the columnar front. The presence of equiaxed grains is generally beneficial for mechanical properties, as they provide better ductility and crack resistance compared to fully columnar structures.

Heat-affected zone (HAZ): A critical finding is that the HAZ microstructure does not exhibit grain coarsening. This is significant because grain coarsening in the HAZ of nickel-base alloys is a common concern that can lead to localized softening and reduced creep resistance. The absence of grain coarsening indicates that the thermal cycle applied during welding was sufficiently controlled, with peak temperatures not exceeding the grain growth threshold for Inconel 625.

The HAZ microstructure likely consists of a narrow band of sensitized regions near the fusion line, where carbide precipitation may occur due to chromium and niobium depletion. However, the paper does not provide detailed information on carbide precipitation or sensitization resistance testing, which represents a gap in the analysis.

Mechanical Property Results

The tensile strength of the welded joint is reported to be comparable to the base metal tensile strength. For Inconel 625 in the solution-treated condition, the typical tensile strength is approximately 1000–1150 MPa. The welded joint achieving comparable strength indicates that:

1. The weld metal composition is appropriate, without excessive dilution from the base metal or contamination from the welding process.
2. The microstructure of the weld metal, despite being in the as-welded cast condition, provides adequate strength.
3. The HAZ has not been significantly softened by precipitate coarsening or phase transformation.

However, the paper does not report data on yield strength, elongation, or hardness profiles across the weld cross-section, which would provide a more complete mechanical characterization. In particular, the hardness profile is critical for assessing HAZ softening and weld metal strength uniformity.

Engineering Practice Implications

For nuclear and aerospace applications where Inconel 625 is used, the following engineering considerations arise from this study:

1. Post-weld heat treatment (PWHT): The as-welded condition of Inconel 625 welds typically requires a solution treatment at 1050–1150°C followed by aging at 870°C for 8 hours to restore the full mechanical properties. The study does not report PWHT data, which limits the applicability of the findings to as-welded conditions only.
2. Fatigue and creep resistance: The cast microstructure of the weld metal, with its columnar grains and potential for segregation, is generally inferior to the base metal in terms of fatigue and creep resistance. Long-term service at elevated temperatures may require additional qualification testing.
3. Corrosion resistance: The weld metal microstructure, particularly the presence of carbides and the distribution of strengthening phases, directly affects corrosion resistance. The absence of intergranular corrosion testing in this study is a notable omission for a material specified for nuclear and chemical applications.
4. Welding procedure specification (WPS): The successful single-pass welding of 6 mm thick Inconel 625 plate with TIG welding suggests that this thickness range is feasible with appropriate parameter control. For thicker sections, multi-pass welding with interpass temperature control and possibly pulse TIG or alternating current TIG would be required.

Key Technical Observations

The absence of grain coarsening in the HAZ is the most significant positive finding of this study. In nickel-base superalloy welding, HAZ grain coarsening is a persistent challenge that can compromise creep strength and fatigue life. The fact that conventional TIG welding with appropriate parameter selection can avoid grain coarsening in 6 mm thick material suggests that the thermal cycle is sufficiently rapid to limit grain growth. This finding is particularly encouraging for applications where the weldment will be subjected to elevated temperature service.

The presence of equiaxed grains in the center of the weld metal is also a positive microstructural feature. Equiaxed grains provide better ductility and crack resistance than columnar grains, and their presence indicates that the solidification conditions promote sufficient constitutional supercooling to enable grain detachment and nucleation. The columnar-to-equiaxed transition (CET) ratio should be quantified in future work to provide a more precise assessment of weld metal quality.

Study Insights and Limitations

This study provides useful baseline data on the microstructure and mechanical properties of TIG welded Inconel 625 joints, but several limitations must be acknowledged. The lack of PWHT data, hardness profiling, and corrosion testing significantly limits the engineering utility of the findings. For nuclear applications, qualification testing in accordance with relevant standards such as ASME Section III or RCC-M is mandatory, and the data presented in this paper would constitute only a preliminary investigation.

The single-pass welding of 6 mm thick material is a reasonable demonstration of TIG welding capability, but industrial applications often involve thicker sections and more complex geometries. The extrapolation of these results to thicker weldments requires careful consideration of thermal cycle effects, residual stress distribution, and distortion control.

In conclusion, this research confirms that conventional TIG welding can produce acceptable welds in Inconel 625 alloy with appropriate parameter control, achieving tensile strength comparable to the base metal without HAZ grain coarsening. However, comprehensive qualification for engineering applications requires additional testing of post-weld heat treatment effects, corrosion resistance, fatigue performance, and long-term elevated temperature behavior.