Effect of Microstructure and Composition on the Properties of Weld Overlay Layers on Hot Rolling Mill Rolls

Literature Overview

This 1991 study by Wang Jian, Xue Jin, Lou Baicheng, and Lang Yi from Xi'an Jiaotong University and the Ansteel Iron and Steel Research Institute represents one of the early systematic investigations into the relationship between microstructure, chemical composition, and the mechanical performance of weld overlay layers applied to hot rolling mill rolls. The research was conducted within the context of Ansteel's industrial practice, which gave it significant practical relevance for heavy-duty rolling mill operations in China during that era. At that time, hot rolling mill rolls suffered from severe wear, thermal fatigue, and surface cracking under extreme operating conditions involving temperatures above 900°C, high contact stresses, and continuous mechanical abrasion. Weld overlay cladding was introduced as a cost-effective repair and surface enhancement strategy, but the selection of appropriate overlay compositions and welding parameters remained largely empirical. This work sought to establish a more rational basis for overlay design by correlating microstructural features and chemical composition with hardness, wear resistance, and thermal stability.

Core Technical Content and Key Findings

The authors systematically examined several overlay alloys applied to cast steel and alloy steel roll substrates. The base roll materials typically had carbon equivalents in the range of 0.4–0.6%, with microstructures consisting of pearlite and ferrite with varying amounts of carbides depending on the casting and heat treatment history. The overlay alloys investigated included high-carbon high-chromium compositions, medium-carbon manganese alloys, and modified martensitic stainless compositions. The study revealed that the microstructure of the overlay layer was strongly influenced by the cooling rate at the overlay-substrate interface, the carbon and alloy content of the filler, and the welding thermal cycle.

A critical finding was that the hardness distribution within the overlay layer was highly non-uniform. The dilution zone near the substrate interface showed significantly lower hardness due to the mixing of the base metal with the overlay filler, often resulting in a soft band that became a preferential site for crack initiation and material loss during service. The authors recommended multiple thin pass cladding rather than single thick passes to minimize dilution effects and achieve more uniform hardness profiles throughout the overlay thickness.

Microstructural Analysis and Dilution Effects

The dilution phenomenon was identified as the single most important factor governing overlay performance. In single-pass cladding operations, dilution ratios as high as 40–50% were observed, particularly at the root of the overlay bead. This high dilution resulted in a coarse-grained, partially transformed microstructure with reduced carbide density and significantly lower hardness compared to the bulk overlay. The authors demonstrated that by adopting a multi-pass approach with controlled inter-pass temperatures, dilution could be reduced to below 20%, and the resulting overlay exhibited a more homogeneous martensitic or martensite-plus-carbide microstructure.

The microstructural evolution from the substrate interface outward followed a predictable pattern: a thin dilution zone with mixed ferrite-pearlite-martensite structure, followed by a transition zone with increasing martensite content, and finally the bulk overlay zone with predominantly martensite and dispersed carbides. The carbide morphology was also composition-dependent. In high-chromium compositions, M7C3 and M23C6 carbides were predominant, appearing as blocky or rod-shaped particles distributed along grain boundaries and within the martensite matrix. In medium-carbon manganese alloys, the carbide content was lower but the retained austenite fraction was higher, which provided some toughening at the expense of maximum hardness.

Engineering Practice Implications

For hot rolling mill roll applications, the operating environment imposes simultaneous demands for wear resistance, thermal stability, and resistance to thermal fatigue. The study concluded that high-carbon high-chromium overlay alloys offered the best combination of hardness and thermal stability for roll face applications, but required careful control of welding parameters to avoid excessive cracking. The recommended welding parameters included low heat input, small bead size, and controlled inter-pass temperatures to maintain a predominantly martensitic microstructure with fine carbide dispersion.

Post-weld heat treatment was found to be beneficial for reducing residual stresses and improving the toughness of the overlay. A tempering treatment at 200–300°C was recommended to relieve welding residual stresses without significantly softening the martensitic structure. The tempering treatment also converted any retained austenite to a more stable form, reducing the risk of delayed cracking during subsequent thermal cycling in service.

The study also addressed the practical issue of bonding quality between the overlay and the roll substrate. Surface preparation of the roll before cladding, including grinding to remove the decarburized layer and oxide scale, was essential for achieving sound metallurgical bonding. Incomplete removal of surface oxides led to lack of fusion defects and reduced bond strength, which could result in overlay spalling during roll mounting and operation.

Key Questions and Reflections

Several important questions arise from this study that remain relevant to modern cladding practice. First, the study was conducted in 1991, and many of the welding processes available today, such as plasma transferred arc (PTA) cladding and laser cladding, offer superior control over dilution and thermal input compared to the manual and semi-automatic submerged arc or shielded metal arc processes used in the study. However, the fundamental principles regarding dilution, microstructural evolution, and the trade-offs between hardness and toughness remain universally applicable.

Second, the study focused primarily on static properties such as hardness and microstructure but did not extensively address the dynamic performance of the overlay under actual rolling conditions, including rolling contact fatigue, thermal shock resistance, and spalling behavior. Modern studies have shown that the performance of roll overlays under rolling contact fatigue is significantly influenced by the subsurface microstructure and the presence of residual compressive stresses, which were not fully characterized in this early work.

Third, the question of overlay life prediction remains challenging. While the study provided valuable insight into the relationship between microstructure and static properties, translating these findings into reliable service life predictions requires additional testing under simulated rolling conditions, including rolling contact fatigue testing and thermal cycling testing.

Study Insights and Implications for Modern Practice

This 1991 study represents a foundational contribution to the understanding of weld overlay cladding for hot rolling mill rolls. Its emphasis on dilution control, multi-pass cladding strategies, and the importance of substrate surface preparation remains directly applicable to modern engineering practice. The systematic approach of correlating composition with microstructure and then with mechanical properties established a methodology that continues to guide overlay alloy selection and process optimization.

For contemporary engineers working on roll cladding applications, the key takeaways from this study include the importance of minimizing dilution through multi-pass strategies, the critical role of inter-pass temperature control in managing residual stress and microstructural evolution, and the necessity of thorough substrate surface preparation to ensure sound metallurgical bonding. Modern processes such as PTA and laser cladding can achieve dilution ratios below 10%, but the principles of dilution management and microstructural control remain the same. The study also underscores the importance of post-weld heat treatment in managing residual stresses and improving the overall performance of the cladding system.