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

Literature Overview

This paper by Wang Zhihui, Yu Changli, and He Dingyong from Beijing University of Technology, published in 2009 in the Journal of Thermal Processing of Materials (Vol. 30, No. 4, pp. 77-80), investigates the influence of post-weld heat treatment (PWHT) on the microstructure and abrasive wear performance of Fe-Cr-C hardfacing alloys deposited by submerged arc welding on Q235 low-carbon steel substrates. The research is funded by the Beijing Municipal Commission of Education Science and Technology Development Project (KM200410005023). The work addresses a critical practical concern: when overlay-welded components are subjected to subsequent heat treatment—whether for stress relief, base material strengthening, or dimensional stabilization—the hardfacing layer's performance may be severely degraded.

Core Findings and Technical Interpretation

The study employed optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) to characterize the microstructure, complemented by standardized abrasive wear testing. The key results reveal a dramatic sensitivity of the Fe-Cr-C system to thermal cycling above 850°C.

Microstructural Stability

The fundamental morphology of primary carbides and eutectic carbides in the overlay alloy remained largely unchanged after PWHT at 850-1000°C. This observation is significant because it indicates that the carbide phase distribution established during solidification is thermodynamically stable within this temperature range. The carbide network does not dissolve, coarsen, or undergo morphological transformation, which is consistent with the high melting points of chromium carbides (Cr₇C₃, Cr₂₃C₆, Cr₃C₂) exceeding 1600°C.

Hardness Degradation

The quantitative hardness data presented in the study are particularly instructive:

The disparity in hardness reduction between the carbide phases (~5%) and the matrix (~25%) is the crux of the performance degradation. The matrix undergoes significant softening due to carbide precipitation coarsening, relief of welding residual stresses, and possible tempering of any retained martensite or bainite. The primary carbides, being thermodynamically stable, retain their hardness with minimal loss.

Wear Performance Collapse

The most striking finding is that after heat treatment at 850-1000°C for 30 minutes, the relative wear resistance drops to only 16%-32% of the as-welded condition. This catastrophic reduction is attributed to the deterioration of the carbide-matrix matching relationship. In the as-welded state, the hard carbide phases are firmly embedded in a sufficiently hard matrix that resists plastic deformation and prevents carbide pullout. After PWHT, the softened matrix can no longer support the carbide particles under abrasive loading, leading to premature carbide detachment and accelerated material removal.

Engineering Practice Implications

This research has direct relevance to the maintenance and repair of wear-critical components in mining, cement, and material handling industries. Many overlay-welded components—such as chutes, hoppers, ball mill liners, and pump impellers—are installed in systems that undergo subsequent heat treatment for stress relief or dimensional correction. The findings mandate that:

1. The PWHT temperature must be carefully controlled below the sensitization threshold of the overlay alloy. For Fe-Cr-C systems, temperatures above 850°C should be avoided whenever possible.
2. If PWHT is unavoidable, the overlay should be reapplied after the heat treatment cycle is complete.
3. Alternative approaches such as low-temperature stress relief (below 600°C) or mechanical stress relief methods should be considered for components with hardfacing layers.

Key Questions and Reflections

A fundamental question arises from this work: why does the matrix soften so dramatically while the carbides remain stable? The answer lies in the complex interplay between the retained austenite fraction, the tempered martensite/bainite matrix, and the intermetallic phases in the eutectic structure. The as-welded matrix contains a high fraction of retained austenite stabilized by carbon and chromium, which provides solid solution strengthening. Upon heating to 850-1000°C, this austenite transforms to tempered martensite or coarse pearlite, losing its strengthening contribution.

From a materials design perspective, this study highlights the importance of designing overlay alloys where the matrix retains adequate hardness even after exposure to elevated temperatures. Alloying elements that stabilize the matrix against softening—such as molybdenum, tungsten, and vanadium—should be incorporated into future Fe-Cr-C overlay compositions intended for service environments involving post-weld thermal cycling.

Study Insights and Implications

The work by Wang et al. provides a clear warning to engineers and maintenance personnel: hardfacing is not a "fire and forget" solution. The overlay alloy's performance is intimately tied to the thermal history of the entire component. In practice, this means that the process planning for repair and maintenance must account for all subsequent thermal operations, not merely the welding step itself. The 16%-32% wear resistance after PWHT represents a complete loss of the economic justification for overlay welding in many applications, as the component would wear at nearly the same rate as the unprotected base metal.