Zhuojin Pipe Fitting Co., LtdEffect of Heat Treatment on Inconel 625/X90 Hardfacing Layer Microstructure and Properties
Literature Overview
Published in Surface Technology (2018, Vol. 47, No. 6, pp. 83–88), this paper by Liu Wei, Kang Quan, and colleagues from Southwest Petroleum University and Sichuan Petroleum and Natural Gas Construction Engineering Co., Ltd. investigates the effect of solution heat treatment on the microstructure and mechanical properties of Inconel 625 hardfacing layers deposited on X90 pipeline steel. Funded by multiple university and provincial research programs, the study addresses a practically important issue in oil and gas pipeline construction: the metallurgical compatibility between nickel-based hardfacing overlays and high-strength pipeline steel.
Core Technical Approach
The study deposits Inconel 625 hardfacing layers on X90 steel substrates and subjects the specimens to solution heat treatment at three different temperatures: 850°C, 910°C, and 980°C. The microstructure is characterized using optical microscopy, scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). Hardness profiles are measured across the cross-section from the weld metal to the base metal. The results reveal significant changes in microstructure, elemental distribution, and hardness uniformity as a function of heat treatment temperature.
Hardfacing Layer Microstructure Before and After Heat Treatment
| Condition | Fusion Zone Microstructure | Elemental Distribution | Hardness Profile |
|---|---|---|---|
| As-deposited (no HT) | Widmanstätten + martensite layer | High concentration gradient | V-shaped (non-uniform) |
| 850°C solution treatment | Widmanstätten eliminated, martensite reduced | Improved uniformity | Flattened, +24 HV in HAZ |
| 910°C solution treatment | Further improvement | More uniform | Further flattened |
| 980°C solution treatment | Widmanstätten and martensite eliminated | Most uniform | Most uniform, +32 HV in HAZ |
Elemental Distribution in Diffusion Zone After 850°C Treatment
| Element | Mass Fraction (%) | Change Relative to As-Deposited |
|---|---|---|
| Ni | 36.14 | Significant diffusion from weld metal |
| Cr | 28.31 | Significant diffusion from weld metal |
| Fe | 18.27 | Diffusion from base metal into weld zone |
Elemental Distribution Change After 980°C Treatment (Relative to As-Deposited)
| Element | Change in Mass Fraction (%) | Direction |
|---|---|---|
| Ni | -16.27 | Decrease (diffusion into base metal) |
| Cr | -8.32 | Decrease (diffusion into base metal) |
| Fe | +37.76 | Increase (diffusion from base metal) |
Interpretation of Technical Findings
As-Deposited Microstructure: Widmanstätten and Martensite Formation
The as-deposited Inconel 625 hardfacing layer on X90 steel exhibits Widmanstätten organization and a martensite layer near the fusion line. This is a direct consequence of the significant dilution between the nickel-based weld metal and the ferritic-pearlitic X90 base metal. The high carbon and alloy content of the base metal dilutes into the weld pool, creating a local composition that is susceptible to martensitic transformation upon rapid cooling. The Widmanstätten structure forms due to the specific thermal gradient and cooling rate at the fusion boundary.
The elemental analysis reveals extreme concentration differences between the weld metal and base metal regions. This concentration gradient creates a highly non-uniform hardness profile, with the characteristic V-shaped distribution: hardness decreases from the weld metal toward the fusion line, reaches a minimum, and then increases in the base metal. This non-uniformity is detrimental because it creates regions of contrasting mechanical properties that can act as stress concentrators under service loading.
Effect of Solution Heat Treatment
Solution heat treatment at 850°C, 910°C, and 980°C progressively eliminates the undesirable microstructural features and homogenizes the elemental distribution.
At 850°C, the Widmanstätten organization is eliminated through recrystallization and grain growth. The martensite layer is partially transformed, and significant elemental diffusion occurs, as evidenced by the measured composition in the diffusion zone (Ni 36.14%, Cr 28.31%, Fe 18.27%). The hardness profile begins to flatten, with a 24 HV improvement in the heat-affected zone compared to the as-deposited condition.
At 980°C, the treatment is most effective. Both the Widmanstätten organization and the martensite layer are completely eliminated. The elemental distribution achieves the highest degree of uniformity, with Ni decreasing by approximately 16.27%, Cr decreasing by approximately 8.32%, and Fe increasing by approximately 37.76% relative to the as-deposited condition. This indicates extensive diffusion of nickel and chromium into the base metal and iron into the weld metal, effectively creating a more homogeneous microstructure across the interface. The hardness profile is the most uniform, with a 32 HV improvement in the HAZ.
Grain Size Evolution
As the solution treatment temperature increases from 850°C to 980°C, the grain size progressively increases. This is consistent with the fundamental principles of recrystallization and grain growth kinetics: higher temperatures provide greater thermal activation for grain boundary migration. While larger grains may slightly reduce yield strength, they improve ductility and toughness, which is beneficial for the overall mechanical performance of the hardfacing joint.
Engineering Practice Integration
Application Context: Oil and Gas Pipeline Construction
X90 steel is a high-strength line pipe grade widely used in long-distance oil and gas pipelines. The requirement for corrosion-resistant overlays or dissimilar metal joints in pipeline construction necessitates the use of nickel-based hardfacing alloys such as Inconel 625. The findings of this study are directly relevant to pipeline welding procedures where Inconel 625 is used for:
- Transition joints between carbon steel and stainless steel piping
- Corrosion-resistant overlays on pipeline components
- Repair of localized corrosion damage on pipeline surfaces
- Girth welds in dissimilar metal pipeline connections
Heat Treatment Procedure Development
The study provides clear guidance for developing post-weld heat treatment procedures for Inconel 625 hardfacing on X90 steel:
- 850°C solution treatment is suitable for applications where moderate microstructural improvement is acceptable and the component geometry or material constraints limit the maximum allowable temperature.
- 980°C solution treatment provides the best microstructural and mechanical performance but requires careful evaluation of the effect on the base metal properties. The higher temperature may affect the mechanical properties of the X90 steel, particularly its yield strength, which is a critical parameter for pipeline design.
Quality Control and Acceptance Criteria
For pipeline applications, the following quality control measures should be implemented:
- Metallographic examination of the fusion zone to confirm the absence of Widmanstätten and martensite after heat treatment
- Hardness profiling across the cross-section to verify uniformity and compliance with specification requirements
- Elemental analysis at the fusion boundary to confirm adequate homogenization
- Mechanical testing (tensile, impact) to verify that the heat-treated joint meets the required performance criteria
Study Insights and Implications
This paper provides valuable quantitative data on the effect of solution heat treatment temperature on the metallurgical quality of Inconel 625 hardfacing on X90 steel. The progressive improvement in microstructure, elemental distribution, and hardness uniformity with increasing treatment temperature demonstrates the importance of post-weld heat treatment in achieving acceptable performance of dissimilar metal hardfacing joints.
The key insight for pipeline engineers is that the as-deposited condition of Inconel 625 hardfacing on X90 steel is metallurgically unacceptable due to the presence of brittle Widmanstätten and martensite phases and extreme elemental concentration gradients. Solution heat treatment is not optional but mandatory for achieving a reliable, durable joint. The selection of the optimal treatment temperature requires balancing the degree of microstructural improvement against the potential degradation of base metal properties at elevated temperatures.
The study also highlights the importance of elemental diffusion in achieving metallurgical compatibility at dissimilar metal interfaces. The extensive diffusion observed at 980°C, with Ni, Cr, and Fe redistributing across the interface, effectively creates a graded transition zone that accommodates the thermal expansion mismatch between the nickel-based overlay and the ferritic base metal. This graded transition is essential for long-term durability under thermal cycling and mechanical loading in pipeline service.