Microstructure Evolution and Properties of Inconel 625 Cladding on X90 Steel at Different Solution Treatment Temperatures

Literature Overview

This study investigated the microstructural evolution and mechanical properties of Inconel 625 weld overlay layers deposited on X90 pipeline steel substrate, with particular focus on the effect of post-weld solution treatment (annealing) at varying temperatures ranging from 950 °C to 1200 °C. X90 steel is a high-strength low-alloy (HSLA) pipeline grade with a minimum yield strength of 620 MPa, and Inconel 625 is a nickel-chromium-molybdenum alloy renowned for its excellent corrosion resistance and mechanical properties across a wide temperature range. The combination is commonly used in the fabrication of hydrogenation reactors, ammonia synthesis loops, and high-pressure hydrogen service equipment where both structural integrity and corrosion resistance are paramount.

Core Technical Findings

The as-welded overlay exhibited a columnar dendritic microstructure with primary phases of γ (Ni solid solution) and δ (Ni₃Nb) precipitates. The solution treatment temperature significantly influenced the dissolution and reprecipitation behavior of δ phase and secondary carbides. At 950 °C, a substantial fraction of δ phase remained in the microstructure, contributing to a hardness of approximately 280 HV30. As the solution treatment temperature increased to 1100 °C, the δ phase fraction decreased markedly, and the hardness dropped to about 220 HV30. At 1200 °C, nearly all δ phase was dissolved, resulting in a fully austenitic microstructure with a hardness of 195 HV30.

The corrosion resistance improved progressively with increasing solution treatment temperature, as evidenced by the decreasing corrosion rate in 10% HCl. This improvement is attributed to the elimination of δ phase, which is known to be susceptible to intergranular corrosion due to chromium depletion at grain boundaries. The fully solution-treated condition at 1200 °C exhibited the best corrosion resistance but at the expense of reduced strength and hardness.

Microstructure-Property Relationships

The δ phase plays a dual role in the Inconel 625 overlay. On one hand, it acts as a strengthening precipitate that contributes to solid solution and precipitation hardening, enhancing yield strength and hardness. On the other hand, δ phase at grain boundaries creates paths for intergranular attack, particularly in chloride-containing environments. The study confirmed this through intergranular corrosion testing in ASTM G155 conditions, where the as-welded and 950 °C annealed samples showed intergranular attack, while the 1100 °C and 1200 °C annealed samples passed without significant attack.

The dilution from the X90 substrate was measured to be approximately 8–12% in the first pass and decreased to 5–7% in subsequent passes. The dilution introduced carbon and manganese into the overlay, which promoted the formation of M₇C₃ and M₂₃C₆ carbides at the interface region. These carbides were particularly problematic in the first pass, where the dilution was highest and the microstructure was most heterogeneous.

Engineering Practice and Standards Considerations

For hydrogenation reactor applications governed by NB/T 47002 and GB/T 150, the overlay layer must meet specific requirements for bond strength, hardness, and corrosion resistance. The typical specification calls for a minimum bond strength of 150 MPa (measured by shear test) and a hardness within the range of 180–250 HV30. Based on the study results, a solution treatment temperature of 1100 °C for 2 hours provides the optimal balance, meeting both the mechanical and corrosion resistance requirements.

The welding process parameters for Inconel 625 overlay on X90 steel typically involve GTAW or PTA with the following ranges:

The interpass temperature is critical; exceeding 200 °C can promote δ phase precipitation and reduce the effectiveness of subsequent solution treatment. Engineers should monitor the interpass temperature rigorously during multi-pass overlay welding.

Defect Analysis and Countermeasures

Common defects observed in Inconel 625 overlays on X90 steel include hot cracking, lack of fusion at the interface, porosity, and excessive dilution. Hot cracking is the most prevalent defect, occurring primarily in the first pass where the dilution is highest and the solidification range of the weld metal is widest. The countermeasures include: (1) reducing the travel speed to increase heat input and promote more complete melting; (2) using a lower current to reduce the thermal gradient; (3) applying a preheat of 100 °C to reduce the cooling rate; and (4) ensuring thorough surface preparation to minimize contamination-induced cracking.

Lack of fusion at the steel-overlay interface is another critical defect that can compromise bond strength. This is often caused by inadequate surface preparation or insufficient penetration. The recommended surface preparation involves grinding the substrate to a roughness of Ra 6.3–12.5 μm to provide adequate mechanical keying, followed by cleaning with acetone or alcohol to remove all contaminants.

Study Insights and Reflections

The study underscores the importance of post-weld heat treatment in achieving the desired balance of mechanical properties and corrosion resistance in Inconel 625 overlays. The solution treatment temperature is a critical variable that must be optimized based on the specific application requirements. For high-stress applications where strength is paramount, a lower solution treatment temperature (950–1050 °C) may be appropriate, provided that the corrosion environment is not aggressive. For applications in chloride-containing or hydrogen service, a higher solution treatment temperature (1100–1200 °C) is recommended to eliminate δ phase and maximize corrosion resistance.

A practical consideration is the feasibility of solution treatment for large-scale components such as hydrogenation reactor shells. Achieving uniform heating to 1100 °C for a large vessel is challenging and expensive. In such cases, alternative approaches such as hot isostatic pressing (HIP) or selective laser remelting may be considered to achieve similar microstructural benefits without the need for full-scale furnace treatment.

This study provides a valuable quantitative basis for specifying post-weld heat treatment parameters in Inconel 625 overlay applications, and the methodology can be adapted for other nickel-based alloy overlays on high-strength steel substrates.