Post-Weld Heat Treatment Effects on Microstructure and Abrasive Wear Resistance of Fe-Cr-Mo-C Surfacing Alloys

Literature Overview

The study by Li Erhu et al., published in China Surface Engineering (2025, Vol. 38, No. 3, pp. 152–160), addresses a critical industrial problem: the severe high-temperature abrasive wear experienced by single-tooth rollers in mining and material handling applications. The authors employed flux-cored arc welding (FCAW) to deposit Fe-Cr-Mo-C wear-resistant surfacing alloys with varying chromium contents (14 wt.%, 18 wt.%, and 23 wt.%), and then investigated the effects of post-weld heat treatment (PWHT) at 900 °C on the microstructure and abrasive wear performance. This work is particularly relevant to engineers dealing with the repair and refurbishment of heavy-duty wear components in the mining and cement industries.

Core Technical Findings

The base microstructure of the Fe-Cr-Mo-C surfacing alloys consists of martensite, retained austenite, and M₇C₃ carbides. The following table summarizes the key quantitative results:

Parameter	Cr = 14 wt.%	Cr = 18 wt.%	Cr = 23 wt.%
M₇C₃ area fraction (as-welded)	48.9%	—	57.2%
M₇C₃ size (as-welded)	27.9 μm	—	23.7 μm
M₇C₃ area fraction (after 900 °C PWHT)	41.4%	48.9%	50.8%
M₇C₃ size (after 900 °C PWHT)	32.1 μm	27.3 μm	26.4 μm

The as-welded alloys show a clear trend: increasing chromium content from 14 to 23 wt.% increases the M₇C₃ carbide area fraction from 48.9% to 57.2%, while simultaneously refining the carbide size from 27.9 μm to 23.7 μm. This combination of higher volume fraction and finer dispersion directly translates to improved abrasive wear resistance, as the hard carbide phase is better distributed to resist particle cutting and ploughing.

However, the 900 °C PWHT produces a detrimental effect. The carbide area fraction decreases significantly (e.g., from 48.9% to 41.4% for the 14 wt.% Cr alloy), while the carbide size increases (e.g., from 27.9 μm to 32.1 μm). This coarsening and partial dissolution of M₇C₃ carbides reduces the overall hardness and abrasive wear resistance of the surfacing layer.

Interpretation of Metallurgical Mechanisms

The adverse effect of 900 °C PWHT can be understood through classical carbide precipitation and coarsening kinetics. At 900 °C, which is close to the Ac₁ temperature for many Fe-Cr-C alloys, the following processes occur:

Carbide coarsening via Ostwald ripening: Smaller carbides dissolve preferentially due to their higher surface energy, while larger carbides grow at their expense. This explains the increase in average carbide size.
Carbide dissolution: The elevated temperature promotes the dissolution of metastable M₇C₃ carbides into the matrix, reducing the overall area fraction.
Matrix softening: The martensitic matrix undergoes tempering, reducing its hardness contribution.

The chromium content plays a dual role. Higher Cr increases the thermodynamic driving force for M₇C₃ formation (Cr is a strong carbide former with a high affinity for carbon), but it also raises the alloy's hardenability and the temperature at which carbide stability is maintained. Nevertheless, even the highest-Cr alloy (23 wt.%) cannot fully resist carbide coarsening at 900 °C.

Engineering Practice Implications

For single-tooth roller repair applications, the following practical conclusions emerge:

Avoid high-temperature PWHT: If the goal is to maximize abrasive wear resistance, the as-welded condition should be preserved, or a lower-temperature stress-relief treatment (e.g., 550–650 °C) should be employed instead of 900 °C.
Optimize Cr content: The 23 wt.% Cr composition provides the best as-welded wear performance, but the cost and weldability must be balanced against the benefit.
Multi-pass welding: Multi-pass welding with interpass temperature control can further refine the carbide distribution and reduce residual stress without requiring high-temperature PWHT.

Key Questions and Reflections

A notable concern is the lack of data on the residual stress state before and after PWHT. High residual tensile stresses in the as-welded condition can lead to cracking during service, and the decision to forgo PWHT must be weighed against this risk. Additionally, the study does not report the specific abrasive wear test parameters (e.g., test standard, abrasive type, load, sliding distance), which limits direct comparison with other literature. Engineers applying these findings should conduct their own wear testing under representative service conditions.

The study also raises the question of whether a two-stage PWHT (e.g., 600 °C stress relief followed by 700 °C carbide reprecipitation) could decouple the beneficial stress relief from the detrimental carbide coarsening. This warrants further investigation.

Study Insights and Practical Recommendations

This study provides a clear and actionable insight: for Fe-Cr-Mo-C surfacing alloys designed for abrasive wear resistance, the as-welded microstructure with its fine, high-volume-fraction M₇C₃ carbides is superior to the post-heat-treated condition. The optimal strategy involves selecting a high-Cr composition (20–23 wt.%) and avoiding high-temperature PWHT. Instead, engineers should focus on controlling welding parameters—particularly interpass temperature and heat input—to achieve the desired microstructure without relying on post-weld thermal processing. This approach aligns with the growing trend in wear-resistant surfacing of designing the microstructure during welding rather than modifying it afterward. The work by Li et al. serves as a valuable reference for practitioners in the mining, cement, and aggregate industries who face the recurring challenge of single-tooth roller refurbishment.