Effect of Tempering Treatment on Microstructure and Properties of Overlay Weld Metal on Large Support Rollers

Literature Overview and Research Context

The research by Zhou Geyu, Zhu Maosheng, Yang Jian, Jiang Yongwen, Zhou Yefei, and Yang Qingxiang, published in Welding (2012, Issue 4, pp. 60-63), investigates the effects of tempering treatment on the microstructure and mechanical properties of overlay weld metal deposited on large support rollers. The study was conducted by researchers from Qinhuaide Wild Special Welding Materials Co., Ltd. and the State Key Laboratory of Metastable Materials Preparation Science and Technology at Yanshan University.

Large support rollers in steel rolling mills are subjected to extreme conditions including high contact stress, abrasive wear, thermal cycling, and impact loading. Overlay welding is a common method for refurbishing worn rollers, but the as-welded overlay metal often exhibits high hardness with poor toughness due to the presence of untempered martensite and retained austenite. Post-weld tempering treatment is therefore essential to achieve the required balance of hardness and toughness for service conditions.

Core Technical Content and Experimental Approach

The authors developed a flux-cored wire specifically for support roller overlay welding and investigated the microstructure and properties of the deposited metal under different tempering conditions. The experimental program included:

1. As-welded condition examination: Observation of the initial microstructure and properties after overlay welding.
2. Tempering temperature study: Tempering at various temperatures to determine the optimal tempering condition.
3. Tempering time study: Extended tempering at the optimal temperature to evaluate the effect of time on property stability.

The characterization methods included optical microscopy for microstructure observation, X-ray diffraction for phase analysis, and surface macrohardness testing for property evaluation.

Key Findings and Microstructural Evolution

The as-welded overlay metal microstructure consisted of martensite, retained austenite, and a small amount of carbides. The presence of retained austenite in the as-welded condition is typical for high-carbon or high-alloy martensitic overlay welds, where the austenite stabilization elements (such as nickel, manganese, and carbon) prevent complete transformation during the rapid cooling of the welding process.

As the tempering temperature increased, a critical transition was observed at 520°C. Below this temperature, retained austenite remained stable in the microstructure. At 520°C and above, the retained austenite underwent complete decomposition into martensite and carbides, resulting in a microstructure consisting of tempered martensite and carbides.

The hardness response to tempering temperature exhibited a characteristic secondary hardening behavior. As the tempering temperature increased from the as-welded condition, the hardness initially increased due to the precipitation of fine carbides (secondary hardening), reaching a maximum of HRC 54 at 520°C. Beyond this temperature, the hardness decreased due to the coarsening of carbides and the softening of the martensite matrix.

The temper stability study at 500°C revealed that the overlay metal maintained excellent resistance to over-tempering. Even after extended tempering for 48 hours at 500°C, the hardness remained at HRC 47, which still satisfies the service requirements for support roller overlay applications. This excellent temper stability is attributed to the fine carbide precipitation and the alloying elements that retard carbide coarsening.

Interpretation of Technical Points

The secondary hardening phenomenon observed in this study is a well-known metallurgical effect in martensitic steels and overlay welds. It occurs due to the precipitation of fine, coherent carbides during tempering, which provide additional strengthening beyond the solid solution strengthening of the tempered martensite matrix. The peak hardness at 520°C represents the optimal balance between matrix softening and precipitate strengthening.

The complete decomposition of retained austenite at 520°C is significant because retained austenite, while providing some toughness, can be detrimental in certain service conditions. Under cyclic loading or impact, retained austenite can transform to martensite, causing volume expansion and internal stresses that lead to cracking. The complete decomposition at 520°C eliminates this potential failure mechanism.

The excellent temper stability at 500°C is particularly valuable for support roller applications. In service, support rollers experience repeated thermal cycling, which can cause over-tempering of the overlay layer. The ability to maintain hardness after extended exposure at 500°C ensures that the overlay layer retains its wear resistance throughout the service life of the roller.

Process and Standards Analysis

The overlay welding of large support rollers is governed by several industry standards and specifications:

The tempering treatment is an integral part of the welding procedure and must be specified and qualified as part of the welding procedure specification (WPS). The tempering temperature, time, and cooling rate are critical process parameters that affect the final properties of the overlay weld.

For support roller overlay welding, the following process considerations are important:

1. Preheat temperature: To reduce the cooling rate and minimize the formation of untempered martensite, a preheat temperature of 200-300°C is typically recommended.
2. Interpass temperature: Maintaining an interpass temperature of 200-250°C prevents excessive cooling between passes and reduces the risk of cracking.
3. Post-weld heat treatment: The tempering treatment should be performed immediately after welding to prevent the formation of cold cracks in the as-welded martensitic structure.
4. Cooling rate control: Controlled cooling during tempering prevents the formation of new retained austenite and ensures uniform property distribution.

Integration with Engineering Practice

The findings of this study have direct practical applications in the refurbishment of large support rollers in steel mills and other heavy industries. The recommended tempering condition of 500°C for 48 hours provides a reliable process for achieving the required hardness and temper stability in overlay welds.

In practice, the tempering treatment can be performed in a furnace or by localized induction heating. Furnace tempering is preferred for large rollers where uniform heating is required, while induction heating is suitable for smaller components or localized repair work.

The developed flux-cored wire, combined with the recommended tempering treatment, provides a complete solution for support roller overlay welding. The wire composition is designed to produce a martensitic structure with sufficient retained austenite for toughness in the as-welded condition, which then transforms to a tempered martensite structure with optimal hardness and temper stability after heat treatment.

For steel pipe and pipe fitting applications, similar principles apply. Overlay welding of high-wear components such as pipe elbows, tees, and reducers can benefit from post-weld tempering treatment to achieve the optimal balance of hardness and toughness. The specific tempering parameters would need to be adjusted based on the overlay alloy composition and the service conditions.

Key Questions and Reflections

Several important questions arise from this research that warrant further investigation. First, the study does not evaluate the toughness properties of the overlay metal under different tempering conditions. While hardness is a critical property for wear resistance, toughness is equally important for resistance to impact and cyclic loading. A comprehensive evaluation of hardness-toughness balance would be needed to determine the optimal tempering condition for specific service applications.

Second, the study focuses on macrohardness but does not address the hardness distribution through the thickness of the overlay layer. In multi-pass overlay welds, the hardness can vary significantly from the surface to the root, and this gradient can affect the wear performance.

Third, the long-term wear performance under actual service conditions is not evaluated. Laboratory wear testing provides useful comparative data, but field performance can be influenced by environmental factors, thermal cycling, and cyclic loading that are not captured in bench-scale tests.

Study Insights and Implications

The key insight from this research is that tempering treatment is essential for optimizing the properties of overlay weld metal on large support rollers. The complete decomposition of retained austenite at 520°C and the excellent temper stability at 500°C provide a clear process window for achieving the required service properties.

For engineering practice, the recommended tempering condition of 500°C provides a practical solution that balances hardness retention with temper stability. This condition ensures that the overlay layer maintains its wear resistance throughout the service life of the support roller, even under repeated thermal cycling.

The study also highlights the importance of alloy design in overlay welding consumables. The developed flux-cored wire composition was specifically designed to produce the desired microstructure evolution during tempering, demonstrating that consumable development and process optimization must be considered together for optimal results.