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

Literature Overview

This study by Wang Zhihui, Yu Changli, and He Dingyong from Beijing University of Technology investigates the influence of post-weld heat treatment (PWHT) on the microstructure and abrasive wear resistance of Fe-Cr-C hardfacing alloys deposited via submerged arc welding (SAW) on Q235 low-carbon steel substrates. Published in the Journal of Thermal Processing of Materials in 2009 (Vol. 30, No. 4, pp. 77-80), this work is particularly relevant to engineers who must balance the residual stress relief requirements of large structural weldments against the hardfacing performance of wear-resistant overlay layers.

Core Technical Findings

The researchers examined overlay deposits after PWHT at temperatures ranging from 850°C to 1000°C for 30 minutes and compared the results with as-welded conditions. The key findings can be summarized as follows:

• The fundamental morphology of primary carbides and eutectic carbides remained unchanged after PWHT, indicating that the carbide phase distribution established during solidification is thermally stable within this temperature window.
• The macro-hardness of the overlay layer decreased by approximately 25% compared to the as-welded condition after PWHT.
• Primary carbide hardness decreased only slightly, by an average of about 5%.
• The matrix hardness decreased significantly, by an average of approximately 39%.
• Relative wear resistance after PWHT at 850-1000°C for 30 minutes dropped to only 16%-32% of the as-welded value.

Technical Parameter Analysis

Microstructural Mechanism Interpretation

The critical insight from this work is that the degradation of wear resistance after PWHT is not driven by carbide dissolution or morphological change but rather by the softening of the matrix phase. The carbide-matrix matching relationship, which is fundamental to the tribological performance of hardfacing alloys, deteriorates significantly when the matrix softens while the carbides retain most of their hardness.

In as-welded Fe-Cr-C alloys, the rapidly solidified matrix provides a hard, transformation-toughened microstructure that effectively supports the hard carbide particles during abrasive contact. The residual compressive and tensile stresses from rapid solidification contribute to work-hardening capacity. When PWHT at 850-1000°C is applied, the matrix undergoes significant recovery and recrystallization, reducing its hardness by nearly 40%. The carbides, being thermodynamically stable compounds, resist this softening. This creates a mismatch where the soft matrix can no longer effectively retain the hard carbides under abrasive loading, leading to carbide pull-out and accelerated material loss.

Engineering Practice Implications

This finding carries profound implications for the design and fabrication of wear-critical components that require both hardfacing and PWHT. In piping systems where overlay-welded spools must undergo PWHT for stress relief (e.g., in power plant steam lines or refinery pressure vessels), the following considerations are essential:

1. PWHT avoidance strategies: Where possible, design the fabrication sequence so that hardfacing is applied after PWHT of the base structure. This is feasible when hardfacing is applied to machined surfaces of pre-stress-relieved components.
2. PWHT temperature optimization: If PWHT is unavoidable after hardfacing, the temperature should be minimized. The study shows degradation begins at 850°C; lower temperatures (e.g., 600-700°C) may provide partial stress relief while preserving more matrix hardness.
3. Alternative overlay compositions: Alloy systems with higher carbide volume fractions or those relying more on solid solution strengthening (such as high-chromium austenitic alloys) may be more tolerant of PWHT-induced matrix softening.
4. Post-PWHT hardening treatments: Subsequent cold working or low-temperature tempering (below 500°C) could partially restore matrix hardness without significantly affecting carbide stability.

Key Questions and Reflections

This study raises an important design conflict: the same thermal cycle that relieves harmful welding residual stresses in the base material simultaneously degrades the very property that makes hardfacing valuable. In practice, this dilemma is encountered frequently in components such as ball mill liners, mining equipment, and wear plates on structural steel assemblies. The engineer must determine whether the stress relief benefit to the base material outweighs the wear resistance loss in the overlay.

A practical approach would be to quantify the required residual stress level for the specific application. If the component operates under predominantly static loading with low fatigue sensitivity, a lower PWHT temperature or even no PWHT may be acceptable. Conversely, if the component is fatigue-critical, PWHT becomes mandatory, and the overlay material selection must be reconsidered.

Summary and Conclusions

The study conclusively demonstrates that PWHT at 850-1000°C for 30 minutes causes severe degradation of abrasive wear resistance in Fe-Cr-C hardfacing alloys, reducing relative wear life to only 16-32% of the as-welded value. The mechanism is the disproportionate softening of the matrix (39% hardness reduction) relative to carbides (5% hardness reduction), which destroys the critical carbide-matrix matching relationship. Engineers working on hardfaced components that require post-weld stress relief must carefully evaluate fabrication sequencing, consider overlay composition modifications, or accept reduced service life in the overlay zone.