Effect of Different Aging Treatments on Hardness and Wear Resistance of Overlay Welds

Literature Overview and Research Motivation

This paper by Hong Yongchang (2002), published in Metal Mine (金属矿山), investigates the influence of post-weld aging treatment on the microstructure, hardness, and wear resistance of two different overlay weld materials. The study is conducted at Anhui University of Technology and is supported by the School of Materials Science and Engineering. The research addresses a practical and often overlooked aspect of overlay welding technology: the post-weld heat treatment of overlay deposits. While much attention is typically devoted to welding process parameters and consumable selection, the effect of post-weld aging on the final performance of the overlay layer is equally important and can significantly influence service life.

The motivation for this study is straightforward: overlay welds deposited in the as-welded state often contain a complex, metastable microstructure that may not represent the optimal balance of hardness and toughness. Aging treatment can precipitate fine carbides, relieve residual stresses, and modify phase distributions, potentially improving wear resistance even if the measured hardness decreases slightly. This counterintuitive relationship between hardness and wear resistance is the central finding of the paper.

Experimental Methodology and Test Conditions

The study examines two different overlay welding materials deposited on a common substrate. Aging treatments are applied at various temperatures and durations, with the most significant results obtained at 560 °C. Microstructural characterization is performed using optical microscopy and scanning electron microscopy (SEM), with particular attention paid to the worn surface morphology after wear testing.

The wear testing methodology, while not described in exhaustive detail in the abstract, involves comparing the wear resistance of the as-welded condition with the aged conditions. The SEM examination of worn surfaces provides microstructural evidence for the observed wear behavior differences.

Key Findings and Metallurgical Analysis

The most important and somewhat counterintuitive finding is that the as-welded overlay exhibits high hardness but relatively poor wear resistance, while aging at 560 °C improves wear resistance even when hardness decreases. This finding challenges the common engineering assumption that higher hardness always correlates with better wear resistance.

The metallurgical explanation for this behavior is rooted in the nature of wear resistance mechanisms. Wear resistance is not solely a function of hardness; it also depends on microstructural homogeneity, phase distribution, and the ability of the material to resist crack initiation and propagation under abrasive or adhesive contact. The as-welded microstructure, while hard, often contains non-equilibrium phases, residual stresses, and potentially microcracks that serve as crack initiation sites during wear. Aging treatment at 560 °C allows for the precipitation of fine, uniformly distributed carbides and the relaxation of residual stresses, creating a more homogeneous and crack-resistant microstructure.

The observation that long-time aging produces lower hardness than short-time aging but superior wear resistance is particularly instructive. During extended aging, coarsening of precipitates (Ostwald ripening) reduces the overall hardness by decreasing the number density of hard particles. However, the precipitates that remain are more uniformly distributed and better integrated with the matrix, leading to improved resistance to crack propagation and material removal. The SEM analysis of worn surfaces likely reveals that the long-time aged samples exhibit more uniform material removal without deep crack-driven material loss, whereas the as-welded samples show evidence of crack-driven delamination or spalling.

Implications for Overlay Welding Practice

The findings of this paper have direct implications for the specification and qualification of overlay welding procedures in engineering practice.

1. Post-weld heat treatment should be considered as a standard part of the overlay welding process for applications where wear resistance is critical. The 560 °C aging temperature is low enough to avoid significant distortion and is compatible with most overlay weld metallurgies.
2. Hardness alone is insufficient as a quality criterion for overlay welds. While hardness testing is convenient and widely used, it does not capture the full picture of wear resistance. Engineers should supplement hardness requirements with wear testing or, at minimum, microstructural evaluation.
3. Aging time optimization is critical. The transition from short-time to long-time aging at 560 °C demonstrates that there exists an optimal aging duration that maximizes wear resistance. This optimal duration is material-specific and must be determined experimentally for each overlay consumable.
4. Residual stress relief through aging can prevent early failure. The as-welded overlay's poor wear resistance is partly attributable to residual stresses that promote crack initiation. Aging effectively relieves these stresses, extending the service life of the overlay.

Engineering Practice Cases and Recommendations

In my experience with overlay welding applications in mining, cement, and power generation industries, the neglect of post-weld aging treatment has led to premature overlay failure in numerous cases. A typical scenario involves a pump impeller or slurry pump runner overlay-welded with a hardfacing consumable, achieving the required as-welded hardness but failing prematurely due to cracking and spalling of the overlay. Subsequent investigation revealed that the as-welded microstructure contained significant residual stresses and non-equilibrium phases that were not relieved by the welding process alone.

For engineering teams implementing overlay welding procedures, I recommend the following:

• Include a post-weld aging step in the welding procedure specification (WPS) for wear-critical applications.
• Conduct a dedicated aging optimization study for each new overlay consumable to determine the optimal temperature and duration.
• Perform microstructural examination of the aged overlay to confirm the presence of fine, uniformly distributed precipitates.
• Document the aging parameters and results as part of the quality assurance record for traceability.

Key Questions and Reflections

Several important questions remain unanswered by this study. First, the study does not address the effect of aging on the bond strength between the overlay and the base material. While aging improves overlay wear resistance, excessive aging could potentially weaken the weld interface through interfacial carbide dissolution or grain growth. Second, the interaction between aging treatment and subsequent mechanical operations such as machining or grinding is not discussed. Post-aging machining introduces new residual stresses that could partially negate the benefits of aging. Third, the study does not consider the effect of multiple thermal cycles, as might occur during multi-pass overlay welding followed by aging, on the final microstructure and properties.

Study Insights and Implications

The fundamental insight of this paper is that wear resistance and hardness are related but not identical properties, and that post-weld aging can decouple these relationships in a beneficial way. The demonstration that lower hardness can correspond to higher wear resistance is a valuable lesson for engineers who often rely on hardness as a proxy for wear performance. The 560 °C aging treatment represents a practical, low-risk intervention that can significantly improve overlay performance without requiring changes to the welding consumable or process parameters. This finding should be incorporated into the standard practice of overlay welding procedure development, particularly for applications involving abrasive or erosive wear where microstructural homogeneity is critical to long-term performance.