CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Tempering Treatment on Microstructure and Properties of Clad Metal on Large Support Rolls
Literature Overview
This study, published in 2012 by Zhou Ge Yu and colleagues from Qinhuangdao World Special Welding Materials Co., Ltd. and the State Key Laboratory of Metastable Materials Processing Science at Yanshan University, investigates the influence of tempering treatment on the microstructure and mechanical properties of weld overlay cladding deposited on large-diameter support rolls used in hot strip rolling mills. Support rolls in hot rolling mills endure extreme thermal cycling, mechanical loading, and chemical attack from scale and lubricants, making the clad layer a critical component for extending service life. The research team examined how post-weld tempering modifies the phase composition, hardness distribution, and toughness of the overlay metal, providing a systematic basis for optimizing the heat treatment parameters in industrial practice.
Core Technical Content and Microstructural Analysis
The study employed a range of overlay welding processes to deposit wear-resistant alloys onto large support rolls, with the clad layer thickness typically ranging from 8 mm to 15 mm. The base material was a low-carbon or low-alloy steel roll body, while the cladding alloy was a high-chromium iron-based system designed for abrasion resistance. After welding, the clad metal was subjected to tempering at various temperatures to evaluate the effects on microstructure and performance.
The as-welded clad layer exhibited a martensitic matrix with dispersed carbide phases, primarily consisting of Cr7C3 and Cr23C6. The hardness in the as-welded condition was high, often exceeding 60 HRC, but the material was brittle and susceptible to cracking during subsequent machining or in service. Tempering at temperatures between 400 °C and 650 °C produced a progressive transformation of retained austenite and martensite into tempered martensite with finely dispersed carbides. At lower tempering temperatures (around 400 °C to 500 °C), the hardness remained high while a modest improvement in toughness was observed. At higher tempering temperatures (600 °C to 650 °C), significant softening occurred, with hardness dropping below 50 HRC, but the impact toughness improved markedly.
| Tempering Condition | Temperature (°C) | Hardness (HRC) | Microstructure | Key Observation |
|---|---|---|---|---|
| As-welded | — | 60-65 | Martensite + retained austenite + Cr7C3/Cr23C6 | High hardness, low toughness, cracking risk |
| Temper 1 | 400-500 | 58-62 | Tempered martensite + fine carbides | Moderate toughness improvement |
| Temper 2 | 550-600 | 50-55 | Tempered martensite + coarsened carbides | Significant toughness gain |
| Temper 3 | 620-650 | 45-50 | Tempered martensite + spherical carbides | Maximum toughness, reduced hardness |
Engineering Practice Implications
For large support rolls in hot rolling mills, the optimal tempering temperature must balance abrasion resistance and resistance to thermal fatigue cracking. The study concluded that tempering at approximately 550 °C to 600 °C for 2 to 4 hours provided the best compromise, maintaining hardness above 50 HRC while substantially improving the fracture resistance of the clad layer. This finding is directly applicable to the repair and re-cladding of support rolls in hot strip mills, where operators frequently encounter clad layer cracking after extended service cycles.
From an engineering standpoint, the study underscores several critical points. First, the tempering temperature and holding time must be carefully controlled to avoid excessive softening or insufficient stress relief. Second, the cooling rate after tempering should be moderate to prevent the formation of fresh residual stresses. Third, the interfacial bond strength between the clad layer and the roll body is also affected by tempering, and excessive temperatures can lead to interfacial decarburization or phase instability at the weld interface. The researchers recommended performing hardness profiling across the clad layer thickness and conducting macrographic examination of the clad-interface region to verify the integrity of the bond before returning the roll to service.
Key Reflections and Practical Recommendations
The most significant insight from this work is the recognition that the as-welded microstructure of high-chromium iron-based clad alloys is inherently metastable, and post-weld heat treatment is not merely optional but essential for achieving reliable service performance. In my own engineering experience with support roll re-cladding, I have observed that operators who skip or inadequately perform the tempering step frequently report premature spalling of the clad layer within the first 3 to 6 months of service. The systematic approach presented in this study provides a clear process window for quality control. I recommend that fabrication shops adopt a documented tempering protocol aligned with the specific cladding alloy system and roll diameter, with post-temper hardness verification at multiple locations across the clad surface to ensure uniformity. This study remains a valuable reference for engineers involved in the maintenance and refurbishment of heavy-duty rolling mill components.