Effect of Heat Treatment on Inconel 625/X90 Overlay Layer Microstructure and Properties

Literature Overview

The research by Liu Wei and colleagues from Southwest Petroleum University and Sichuan Petroleum and Natural Gas Construction Engineering Co., Ltd. (2018, Surface Technology, Vol. 47, Issue 6, pp. 83–88), supported by multiple funding sources including the Sichuan Provincial Key Laboratory of Oil and Gas Field Materials, investigates the effect of solution heat treatment on Inconel 625 overlay layers deposited on X90 pipeline steel. This work is directly relevant to the oil and gas industry where high-strength pipeline steels require corrosion-resistant overlay protection at weld joints and damaged sections.

As-Welded Microstructure Challenges

The as-welded Inconel 625 overlay on X90 steel exhibits several metallurgical challenges that compromise its performance:

Widmanstätten structure: A coarse, acicular microstructure forms in the heat-affected zone near the fusion line due to the rapid cooling rate and the high carbon content of the X90 base metal.
Martensite layer: A thin layer of martensite forms at the fusion line due to the dilution of carbon and alloying elements from the X90 base metal into the weld pool.
Element concentration gradient: Significant differences in alloy element content exist between the base metal and the deposited metal, creating a steep concentration gradient that results in highly non-uniform hardness distribution.

Microstructural Features and Their Consequences

Feature	Location	Cause	Consequence
Widmanstätten structure	Near fusion line	Rapid cooling + high carbon	Reduced ductility, crack susceptibility
Martensite layer	At fusion line	Carbon dilution from X90	High hardness, low toughness
Element concentration gradient	Base/deposit interface	Dilution during welding	Non-uniform hardness, stress concentration
V-shaped hardness profile	Across overlay	Combined effects of above	Stress concentration, fatigue initiation

The untempered hardness distribution follows a characteristic V-shaped profile: hardness decreases from the deposit material toward the fusion line (due to dilution and phase transformation), reaches a minimum in the transition zone, and then increases in the base metal heat-affected zone (due to martensite formation). This non-uniform hardness distribution creates internal stress concentrations that can initiate cracking under cyclic loading or thermal cycling.

Solution Heat Treatment Effects

The study evaluated solution treatment at three temperatures: 850 °C, 910 °C, and 980 °C. The results demonstrate that solution treatment effectively addresses all of the as-welded microstructural challenges:

Treatment Condition	Widmanstätten	Martensite Layer	Element Distribution	Hardness Uniformity
As-welded (no treatment)	Present	Present	Highly non-uniform	V-shaped profile
850 °C solution	Eliminated	Partially dissolved	Improved	Improved
910 °C solution	Eliminated	Mostly dissolved	Good	Good
980 °C solution	Eliminated	Eliminated	Most uniform	Near-uniform

At 850 °C solution treatment, the diffusion zone shows Ni, Cr, and Fe mass fractions of 36.14%, 28.31%, and 18.27% respectively, indicating significant element diffusion from the as-welded state. The hardness improvement in the heat-affected zone is 24 HV compared to the untreated condition.

At 980 °C solution treatment, the element distribution achieves the most uniform state. Compared to the as-welded condition, the Ni content decreases by approximately 16.27%, the Cr content decreases by approximately 8.32%, and the Fe content increases by approximately 37.76%. The hardness improvement in the heat-affected zone reaches 32 HV. The grain size increases progressively with increasing solution treatment temperature, which is consistent with grain growth kinetics.

Engineering Practice Integration

For Inconel 625 overlay applications on X90 pipeline steel, the selection of solution treatment temperature involves a trade-off between microstructural uniformity and potential grain coarsening. The 980 °C treatment provides the most uniform element distribution and hardness profile, but the associated grain growth may reduce the high-temperature strength and creep resistance of the overlay. For applications where high-temperature performance is critical, the 850 °C or 910 °C treatments may be preferable despite the slightly less uniform microstructure.

The elimination of the martensite layer through solution treatment is particularly important for preventing cracking in the overlay/base metal interface region. The martensite layer, being hard and brittle, is susceptible to cracking under residual stress or thermal cycling. Its dissolution through solution treatment replaces it with a more ductile austenitic structure that can accommodate deformation without fracture.

Key Reflections and Study Insights

This work provides valuable guidance for the optimization of post-weld heat treatment of dissimilar metal overlay welds in the oil and gas industry. The systematic evaluation of solution treatment temperatures demonstrates that even relatively modest solution temperatures (850 °C) can significantly improve the microstructural quality of Inconel 625 overlays on high-strength pipeline steels. The quantification of element diffusion and hardness improvement provides engineering criteria for selecting the appropriate solution treatment temperature based on the specific performance requirements of the application. The findings also highlight the importance of post-weld heat treatment in achieving the full performance potential of overlay welds, particularly for applications involving dissimilar metals with significant differences in thermal expansion coefficient and alloy composition. Future work should investigate the long-term stability of the solution-treated microstructure under service conditions, including exposure to high-temperature hydrogen environments and cyclic loading, to validate the durability of the heat treatment improvements in real-world applications.