Zhuojin Pipe Fitting Co., LtdHeat Treatment Effects on Inconel 625/X90 Surfacing Layer Microstructure and Properties
Literature Overview
The paper by Liu Wei and colleagues, published in Surface Technology (2018, Vol. 47, Issue 6, pp. 83–88), investigates the effects of solution heat treatment on the microstructure and mechanical properties of Inconel 625 surfacing deposits on X90 pipeline steel. The study was supported by multiple institutional funding sources including the Sichuan Provincial Key Laboratory of Oil and Gas Field Materials. The research addresses a critical challenge in pipeline engineering: the heterogeneous microstructure and non-uniform hardness distribution that commonly occurs in dissimilar metal surfacing deposits, and how heat treatment can mitigate these issues.
Experimental Methodology
The Inconel 625 surfacing deposits on X90 substrate were subjected to solution heat treatment at three different temperatures: 850°C, 910°C, and 980°C. The microstructure was characterized using optical microscopy, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). Hardness was measured using Vickers hardness testing across the cross-section from the deposit surface to the substrate.
Key Microstructural Findings
As-Welded Condition
The as-welded surfacing deposit exhibited two distinct microstructural features at the fusion boundary:
- Widmanstätten structure: A needle-like ferritic structure formed in the heat-affected zone (HAZ) of the X90 substrate, resulting from the rapid heating and cooling during surfacing. This structure is inherently brittle and prone to cracking.
- Martensite layer: A thin layer of martensite formed at the fusion boundary due to the dilution of alloying elements from the Inconel 625 deposit into the X90 substrate, creating a locally high-carbon, low-alloy composition conducive to martensite formation.
The significant compositional gradient between the deposit and the substrate created a highly non-uniform hardness distribution, following a V-shaped profile from deposit to substrate.
Effect of Solution Heat Treatment
| Solution Treatment Temperature | Widmanstätten Structure | Martensite Layer | Element Distribution | Hardness Uniformity | HAZ Hardness Improvement |
|---|---|---|---|---|---|
| As-welded (no treatment) | Present | Present | Highly non-uniform | V-shaped profile | Baseline |
| 850°C | Eliminated | Partially retained | Improved | Improved | +24 HV |
| 910°C | Eliminated | Reduced | Further improved | Further improved | Moderate improvement |
| 980°C | Eliminated | Eliminated | Most uniform | Most uniform | +32 HV |
The solution treatment at all three temperatures successfully eliminated the Widmanstätten structure, which is attributed to the recrystallization and grain growth that occurs during the heat treatment. The martensite layer was progressively dissolved as the solution temperature increased, with complete elimination at 980°C.
Compositional Homogenization
The EDS analysis revealed significant changes in the elemental composition of the diffusion zone after heat treatment:
| Element | As-Welded (Deposit Side) | 850°C Treatment | 980°C Treatment |
|---|---|---|---|
| Ni (wt%) | High | 36.14% | Reduced by ~16.27% from as-welded |
| Cr (wt%) | High | 28.31% | Reduced by ~8.32% from as-welded |
| Fe (wt%) | Low | 18.27% | Increased by ~37.76% from as-welded |
The 980°C treatment produced the most uniform elemental distribution, with the smoothest transition curves across the deposit-substrate interface. This homogenization is achieved through diffusion-driven equilibration of alloying elements, which reduces the compositional gradient and eliminates the localized hardening that causes the V-shaped hardness profile.
Hardness Analysis
The V-shaped hardness profile in the as-welded condition reflects the competing effects of the hard martensite layer (high hardness) and the softer Widmanstätten structure in the HAZ (lower hardness). After solution treatment, the hardness profile transitions toward a more uniform, near-linear distribution. The improvement in HAZ hardness by 24 HV (at 850°C) and 32 HV (at 980°C) indicates that the heat treatment not only homogenizes the existing microstructure but also promotes beneficial phase transformations that increase the overall hardness level in the HAZ.
Engineering Implications for Pipeline Applications
The Inconel 625/X90 combination is relevant to pipeline applications where high-strength X90 line pipe requires corrosion-resistant overlay protection, such as in sour service environments (H₂S-containing) or offshore applications. The findings of this study have direct implications for the following scenarios:
- Pipeline girth weld overlay repair: Where Inconel 625 is used to repair or protect the girth weld area of X90 pipe against corrosion.
- In-line inspection (ILI) tool protection: Where overlay deposits are applied to protect ILI tools from abrasive wear in X90 pipelines.
- Equipment repair in oil and gas facilities: Where Inconel 625 surfacing is applied to X90 structural components for enhanced corrosion and wear resistance.
The elimination of the Widmanstätten structure through solution treatment is particularly important because this brittle phase is a common initiation site for cracking under cyclic loading or thermal cycling. The reduction of the martensite layer is equally significant, as martensite in the fusion boundary can lead to intergranular cracking due to its low ductility.
Study Insights and Reflections
The systematic investigation of solution treatment temperature effects provides clear guidance for optimizing the heat treatment of dissimilar metal surfacing deposits. The finding that 980°C produces the most uniform microstructure and hardness distribution suggests that higher solution temperatures are beneficial for homogenization, but this must be balanced against the risk of grain growth and potential substrate softening. For X90 steel, the solution treatment temperature should not exceed the normalizing temperature range to avoid excessive grain growth in the substrate.
The progressive elimination of the martensite layer with increasing solution temperature is metallurgically significant. The martensite forms due to the dilution effect, which creates a locally high-carbon, low-alloy composition. The solution treatment dissolves the martensite by providing sufficient thermal energy for austenite formation and subsequent controlled transformation. This is particularly important for pipeline applications where the fusion boundary is a critical location for crack initiation.
The compositional homogenization achieved through diffusion is a fundamental principle of heat treatment in dissimilar metal welds. The significant reduction in Ni and Cr content (and corresponding increase in Fe content) in the diffusion zone after 980°C treatment demonstrates the effectiveness of thermal diffusion in reducing compositional gradients. This homogenization not only improves hardness uniformity but also reduces the driving force for solid-state cracking, which is a major concern in dissimilar metal joints.
The improvement in HAZ hardness by 24–32 HV after solution treatment is noteworthy because it indicates that the heat treatment does not merely redistribute existing phases but also promotes beneficial phase transformations. The increase in HAZ hardness is likely due to the precipitation of fine carbides during the solution treatment and subsequent cooling, which strengthens the HAZ without introducing brittleness.
Overall, this research provides valuable guidance for the optimization of Inconel 625 surfacing on X90 pipeline steel, with clear recommendations for solution treatment temperature selection and the expected improvements in microstructure and mechanical properties. The findings are directly applicable to the design and qualification of overlay welding procedures for high-strength pipeline applications.