Tempering Treatment Effects on Large Support Roller Overlay Metal Microstructure and Performance

Literature Overview

The study by Zhou Geyu, Zhu Maosheng, Yang Jian, Jiang Yongwen, Zhou Yefei, and Yang Qingxiang, published in Welding (2012, No. 4, pp. 60-63), investigates the microstructural evolution and mechanical property changes in overlay-welded support rollers subjected to various tempering treatments. The research involves a specially developed flux-cored wire for support roller overlay, with joint contributions from Qinhuangdao Wild Special Welding Materials Co. and the State Key Laboratory of Advanced Special Steel Processing Science and Technology at Yanshan University. This work is particularly relevant to the rolling mill industry, where support rollers are critical components subjected to severe wear and thermal cycling conditions.

Overlay Metal Microstructure in As-Welded Condition

The as-welded overlay deposit exhibits a microstructure consisting of martensite, retained austenite, and a small amount of carbides. This microstructure is characteristic of high-carbon, high-alloy hardfacing alloys that undergo rapid solidification during welding, promoting the formation of metastable phases. The retained austenite content in the as-welded condition is significant, as it plays a crucial role in the subsequent tempering response and the overall mechanical behavior of the deposit.

The presence of retained austenite in the as-welded condition has several implications. First, it provides some degree of ductility that helps accommodate welding residual stresses and reduces the risk of cracking. Second, it serves as a reservoir of carbon and alloying elements that can be released during tempering, contributing to secondary hardening effects. Third, the stability of retained austenite under service conditions is a critical concern, as its decomposition can lead to dimensional instability and property degradation.

Tempering Treatment Systematic Study

The authors conducted a systematic investigation of tempering temperature effects, ranging from lower temperatures through the critical range where retained austenite decomposition occurs. The key findings are summarized below:

The observation that retained austenite decomposition is complete at 520°C is metallurgically significant. The decomposition of retained austenite releases carbon into the surrounding martensite matrix, promoting the precipitation of fine secondary carbides. This secondary carbide precipitation is the primary mechanism responsible for the secondary hardening peak observed at 520°C, where the hardness reaches its maximum value of HRC 54.

Anti-Tempering Stability and Long-Term Performance

A critical finding of this study is the evaluation of anti-tempering performance at 500°C with extended holding times. When tempered at 500°C and held for 48 hours, the overlay metal retains a hardness of HRC 47, which the authors confirm is sufficient to meet the performance requirements for support roller applications. This result is of considerable practical importance, as support rollers in rolling mills are subjected to prolonged exposure at elevated temperatures during hot rolling operations.

The anti-tempering behavior at 500°C can be attributed to the combined effects of retained austenite stability at this temperature and the formation of fine, thermally stable carbides. At 500°C, the retained austenite has not yet fully decomposed, maintaining its contribution to hardness through solid solution strengthening. Simultaneously, the existing carbide population remains fine and dispersed, providing effective precipitation hardening. The extended 48-hour hold time at 500°C demonstrates that the microstructural stability is maintained even under prolonged thermal exposure, which is essential for the long service life of support rollers.

Engineering Practice Considerations

For rolling mill operations, the selection of tempering parameters for overlay-welded support rollers requires careful consideration of the service environment. The following decision framework can be applied:

1. High-temperature service (>520°C continuous exposure): The tempering treatment should be selected to maximize anti-tempering stability rather than peak hardness. A tempering temperature of 500°C provides the best long-term hardness retention, even though the initial hardness is lower than the peak value at 520°C.
2. Moderate-temperature service (<500°C): Tempering at 520°C to achieve the maximum hardness of HRC 54 is appropriate, as the retained austenite will not undergo significant decomposition under service conditions below 500°C.
3. Cyclic thermal loading: The retained austenite decomposition during cyclic heating and cooling may lead to progressive microstructural changes. The anti-tempering stability data at 500°C provides a conservative estimate of the microstructural stability under such conditions.

The flux-cored wire composition developed for this application is specifically designed to produce the desired as-welded microstructure with adequate retained austenite content and appropriate carbide formation. The alloy design philosophy balances hardness requirements with toughness and thermal stability, which is essential for the demanding service conditions of support rollers.

Study Insights and Reflections

This research provides valuable guidance for the metallurgical treatment of overlay-welded support rollers, addressing the critical need for long-term hardness stability under elevated-temperature service conditions. The identification of 520°C as the secondary hardening peak temperature and 500°C as the optimal anti-tempering temperature establishes clear guidelines for post-weld heat treatment procedures.

From a materials engineering perspective, the interplay between retained austenite stability, secondary carbide precipitation, and matrix softening during tempering represents a classic example of competing mechanisms that must be balanced through careful parameter selection. The finding that the hardness decreases after 520°C due to over-tempering and carbide coarsening underscores the importance of not exceeding the optimal tempering temperature.

For industrial implementation, I recommend that the tempering treatment be carefully controlled with precise temperature monitoring and adequate holding time to ensure uniform microstructural transformation throughout the deposit thickness. The thermal gradients across thick overlay deposits can lead to non-uniform tempering response, potentially creating zones of different hardness and microstructure that may affect the overall performance of the support roller. Future work should address the effect of tempering on the bonding interface between the overlay deposit and the base metal, as well as the residual stress state after tempering treatment.