Zhuojin Pipe Fitting Co., LtdEffect of Heat Treatment on Co-Cr-W Wear-Resistant Hardfacing Deposits
Literature Overview
This study by Hu Shiju and Shi Haifang, published in Hot Working Technology (2008, Vol. 37, No. 19, pp. 121–123), addresses a practical problem encountered in the manufacture of 20CrMo steel tooth-shaped components (toe claws) used in heavy-duty mining and construction equipment. The authors investigate how post-weld heat treatment and carburizing treatment influence the mechanical properties and wear resistance of Co-based hardfacing alloys deposited on thrust faces of these components. The work is grounded in an actual production scenario and provides direct engineering relevance.
Core Technical Points
Base Material and Hardfacing Composition
The base material is 20CrMo steel, a low-alloy steel with moderate carbon content and chromium-molybdenum alloying, commonly used for high-strength structural components subject to fatigue and wear. The hardfacing material is a Co-based cermetal alloy containing Cr and W, which is the industry standard for extreme wear applications due to the formation of hard carbide phases (Co3W, Co7W6, Cr7C3, Cr23C6) dispersed in a cobalt-rich binder matrix.
Heat Treatment Conditions Studied
| Treatment Route | Parameters | Purpose |
|---|---|---|
| Quenching and tempering | 920°C × 12h + 860°C quench + 200°C temper | Simulate post-forging heat treatment of the tooth claw |
| Carburizing + quenching + tempering | 920°C × 12h carburizing + 860°C quench + 200°C temper | Simulate surface hardening of the shaft journal |
Key Findings
The study reveals two critical mechanisms by which heat treatment improves hardfacing performance:
- Carbide fragmentation during quenching and tempering: The rapid cooling during quenching from 860°C causes the alloy carbides within the Co-Cr-W hardfacing layer to undergo fragmentation and redistribution. This refinement of the carbide network significantly enhances the wear resistance of the deposit, as finer and more uniformly distributed carbides provide superior abrasive resistance compared to coarse, clustered carbide structures.
- Carbon enrichment from carburizing: When the component undergoes carburizing treatment at 920°C for 12 hours, carbon diffuses into both the base metal and the hardfacing layer. This additional carbon content increases the hardness of the hardfacing deposit and further improves wear resistance by promoting the formation of additional carbide phases and increasing the volume fraction of the hard phase.
Microstructural Interpretation
The Co-Cr-W system is well known for its excellent wear resistance, but the as-deposited microstructure often contains coarse dendritic carbides and large intermetallic compounds that limit performance. The 860°C quench temperature is carefully selected to be below the austenitizing temperature of 20CrMo steel while being high enough to induce significant transformation within the hardfacing layer. The subsequent 200°C tempering serves to relieve residual stresses without causing over-aging of the carbide structure.
The carburizing treatment introduces a diffusion couple between the carbon-enriched base metal and the hardfacing layer. At the interface, this can lead to the formation of a transition zone with modified microstructure, which may actually enhance the bond strength between the hardfacing and the base material.
Engineering Practice Implications
Process Sequencing Considerations
A critical insight from this study is the importance of process sequencing in the manufacture of hardfaced components. The order of operations—hardfacing followed by heat treatment versus heat treatment followed by hardfacing—has profound implications for the final properties. In this study, the hardfacing was applied before the heat treatment, which means the hardfacing layer was subjected to the full thermal cycle. This approach is advantageous because:
- The hardfacing layer benefits from the same beneficial microstructural changes as the base metal.
- Thermal stresses at the interface are minimized because both materials experience the same thermal history.
- The carburizing treatment provides an additional hardening mechanism specifically for the hardfacing layer.
Practical Recommendations for Production
For manufacturers of tooth-shaped components or similar wear-critical parts:
- Hardfacing should be applied before final heat treatment whenever possible, to allow the heat treatment to optimize the hardfacing microstructure.
- Carburizing is particularly beneficial for Co-Cr-W hardfacing layers, as the additional carbon directly contributes to carbide formation and hardness.
- Quenching temperature selection should balance the need for carbide refinement in the hardfacing layer against the risk of cracking; 860°C appears to be a practical compromise for 20CrMo base steel.
- Post-hardfacing inspection should include hardness mapping across the deposit thickness to verify uniform carbide distribution.
Key Questions and Reflections
One question that arises from this study is whether the carburizing treatment could potentially lead to excessive carbon enrichment at the hardfacing/base metal interface, potentially causing interface cracking during quenching. The study does not address this concern directly, but from a practical standpoint, the carbon gradient at the interface could create localized high-carbon regions with increased brittleness. In production, this should be monitored through metallographic examination of the interface region.
Another consideration is the long-term stability of the refined carbide structure after extended service. The 200°C tempering temperature is relatively low, and prolonged exposure to elevated temperatures in service could potentially cause carbide coarsening. For applications involving sustained temperatures above 200°C, additional stability testing would be warranted.
Study Insights and Implications
This study provides a valuable demonstration of how heat treatment parameters can be leveraged to optimize hardfacing performance, rather than simply treating heat treatment as a base metal processing step. The finding that carburizing improves Co-Cr-W hardfacing properties is particularly significant, as it suggests a synergistic approach to surface engineering where multiple processes are combined for maximum benefit. For engineers involved in the design of wear-critical components, this work underscores the importance of considering the entire manufacturing sequence when specifying hardfacing and heat treatment processes.