Effect of Post-Weld Heat Treatment on Microstructure and Abrasive Wear Resistance of Fe-Cr-Mo-C Overlay Welding Alloys

Literature Overview

Published in China Surface Engineering (2025, Vol. 38, No. 3), this paper by researchers from Beijing University of Technology and Weihai Tianrun Jinyu New Materials Technology investigates the effect of post-weld heat treatment (PWHT) on the microstructure and abrasive wear resistance of Fe-Cr-Mo-C wear-resistant overlay welding alloys. The study examines three chromium content levels (14 wt.%, 18 wt.%, 23 wt.%) and evaluates the impact of a 900°C post-weld heat treatment. The research is motivated by the severe high-temperature abrasive wear experienced by single-tooth rollers in mining and material handling applications, where overlay welding is used to restore worn surfaces.

Core Technical Points

Base Microstructure of Fe-Cr-Mo-C Overlay Alloys

The Fe-Cr-Mo-C overlay welding alloys, produced using flux-cored wire arc welding, consist primarily of three phases:

The M7C3 carbide phase is the dominant contributor to abrasive wear resistance in these alloys. Its volume fraction, size, morphology, and distribution are the primary determinants of wear performance. The study systematically examines how chromium content and PWHT affect these carbide characteristics.

Effect of Chromium Content on Carbide Characteristics

The trend is clear: increasing chromium content from 14 to 23 wt.% increases the M7C3 carbide area fraction from 48.9% to 57.2% while simultaneously reducing the average carbide size from 27.9 μm to 23.7 μm. This dual effect—higher volume fraction combined with smaller particle size—is highly beneficial for abrasive wear resistance. The increased carbide content provides more hard phases to resist abrasive particle cutting, while the finer carbide size distributes the abrasive load more evenly across the surface, reducing the likelihood of localized carbide pull-out.

Effect of 900°C Post-Weld Heat Treatment

The application of a 900°C PWHT has a detrimental effect on the microstructure and wear resistance of all three Fe-Cr-Mo-C alloys:

The PWHT causes two adverse effects: (1) the M7C3 carbide area fraction decreases significantly across all compositions, indicating carbide dissolution and transformation, and (2) the remaining carbides coarsen substantially, increasing in size by 15-18%. Both effects reduce the alloy's ability to resist abrasive wear. The carbide dissolution is likely due to the high temperature promoting the transformation of M7C3 carbides to more stable phases such as M23C6 or even complete dissolution into the austenite or ferrite matrix. The coarsening is a classic Ostwald ripening phenomenon where smaller carbides dissolve and larger ones grow to reduce the total interfacial energy.

The reduction in hardness and wear resistance following PWHT is a direct consequence of these microstructural changes. The loss of hard M7C3 carbides and the coarsening of the remaining carbides both reduce the surface's ability to resist the cutting and ploughing actions of abrasive particles.

Engineering Practice Integration

In the context of single-tooth roller repair for mining equipment, the findings of this study have direct practical implications. Single-tooth rollers in crushers and conveyors experience severe abrasive wear from rock and ore particles, and overlay welding is the primary repair method. The study demonstrates that:

• Higher chromium content (23 wt.%) provides the best as-welded wear resistance and should be preferred when the base material can accommodate the residual stresses associated with high-carbon martensitic overlay welds.
• Post-weld heat treatment at 900°C is counterproductive for wear resistance and should be avoided unless there is a compelling need to relieve residual stresses.
• The as-welded condition should be maintained for maximum wear performance, with residual stress management achieved through process optimization rather than post-weld heat treatment.

For pipeline and piping system applications where Fe-Cr-Mo-C overlay alloys might be used for erosion-resistant coatings on pipeline internals, the study suggests that PWHT should be carefully evaluated. If residual stress relief is necessary to prevent cracking, a lower temperature PWHT (e.g., 600-700°C) might be more appropriate, as it would provide some stress relief while minimizing carbide coarsening and dissolution.

Key Questions and Reflections

The study raises several important questions for engineering practice. First, the study only evaluates a single PWHT temperature (900°C) and does not investigate the effect of lower temperature treatments. A systematic study of PWHT temperature effects on carbide stability would be valuable for establishing a practical PWHT window that balances stress relief against microstructural degradation. Second, the study focuses on abrasive wear resistance but does not address the effect of PWHT on other properties such as toughness, fatigue resistance, or corrosion resistance. In many service environments, a trade-off between wear resistance and other properties may be necessary.

The study also does not address the effect of PWHT on the retained austenite content. Retained austenite can transform to martensite during PWHT, which would increase hardness but also increase brittleness. The interaction between retained austenite transformation and carbide stability during PWHT is a complex phenomenon that warrants further investigation.

Study Insights and Implications

This study provides clear and actionable guidance for engineers working with Fe-Cr-Mo-C wear-resistant overlay alloys. The primary finding is that PWHT at 900°C is detrimental to wear resistance due to M7C3 carbide dissolution and coarsening, and that the as-welded condition should be maintained for maximum abrasive wear performance. The chromium content optimization (23 wt.% for best wear resistance) provides a clear alloy design guideline for single-tooth roller and similar applications.

For pipeline and equipment repair engineers, the key practical recommendation is to avoid high-temperature PWHT for wear-resistant overlay coatings unless absolutely necessary for stress relief. If stress relief is required, alternative approaches such as low-temperature stress relief (below 600°C), vibration stress relief, or process optimization to minimize residual stresses should be considered. The study underscores the importance of understanding the microstructural consequences of heat treatment before applying it to wear-resistant coatings, as conventional PWHT practices developed for structural welds may be inappropriate for surface engineering applications.