Effect of Heat Treatment on Co-Cr-W Wear-Resistant Overlay Welds

Literature Overview and Research Context

The paper by Hu Shijie and Shi Haifang (2008), published in Hot Working Technology, addresses a critical industrial challenge in the manufacture of 20CrMo steel toothed claw components used in heavy-duty mining and earth-moving equipment. These toothed claws experience severe abrasive wear at their thrust surfaces, prompting the widespread application of cobalt-based hardfacing alloys as a surface reinforcement strategy. The study systematically investigates two heat treatment routes applied to the overlay weld layer and the substrate, evaluating their influence on microstructural evolution, hardness, and wear resistance. This work is particularly relevant to engineers working in surface engineering, hardfacing, and wear-resistant component design, as it bridges the gap between metallurgical theory and practical heat treatment selection.

Core Findings and Technical Analysis

The two heat treatment routes examined are:

The key metallurgical insight is that quench and temper treatment causes fragmentation of alloy carbides within the Co-Cr-W overlay layer. In cobalt-based hardfacing alloys, carbides such as Co3W, Co2W, Co3C, and Co6W2C form during solidification and are typically coarse and irregularly shaped in the as-welded condition. The quench and temper cycle promotes the break-up of these carbides into finer, more uniformly distributed particles, which directly enhances the wear resistance of the overlay. This is consistent with the well-established principle that finer carbide dispersion impedes abrasive removal more effectively than coarse carbides, which can act as weak points and initiate material loss during sliding or impact-abrasion.

Route B, involving carburization prior to quench and temper, yields even higher hardness and wear resistance. The carburization step at 920°C for 12 hours allows carbon atoms to diffuse into the overlay layer, increasing the overall carbon activity and promoting the formation of additional carbide phases. This results in a higher volume fraction of hard carbides within the binder matrix, thereby improving the load-bearing capacity of the overlay under abrasive conditions.

Engineering Practice Implications

For engineers specifying hardfacing and heat treatment processes for wear-critical components, this study provides actionable guidance. The carburizing route, while more energy-intensive and time-consuming, delivers superior wear performance and should be considered for applications subject to severe abrasive or erosive wear, such as toothed claw thrust surfaces, excavator bucket teeth, and mining shovel components. The normalizing-plus-quench-and-temper route offers a more economical alternative with moderate improvement in wear resistance, suitable for less demanding service conditions.

A critical practical consideration is the compatibility between the overlay layer and the substrate during the heat treatment cycle. The 20CrMo substrate undergoes carburization and subsequent quench and temper, which modifies its own microstructure and hardness profile. Engineers must verify that the thermal expansion mismatch between the overlay and substrate does not generate excessive residual stresses that could lead to overlay spallation or cracking. Pre-heat control, interpass temperature monitoring, and post-weld stress relief are essential quality control measures.

Study Insights and Reflections

This study underscores the importance of post-weld heat treatment in unlocking the full wear resistance potential of cobalt-based hardfacing alloys. In many industrial applications, hardfacing deposits are left in the as-welded condition, which represents a significant underutilization of the alloy system's capabilities. The fragmentation of carbides through controlled heat treatment is a relatively straightforward intervention that can yield substantial performance gains. Future work should explore the effect of tempering temperature on carbide morphology and the long-term wear behavior under cyclic loading conditions, which are more representative of real service environments.

Summary

The research demonstrates that both quench and temper treatment and carburizing plus quench and temper treatment improve the wear resistance of Co-Cr-W overlay welds on 20CrMo substrates, with the carburizing route providing superior hardness and wear performance due to increased carbide volume fraction and finer carbide dispersion.