Microstructure and Performance Evolution of Weld Overlay Layer on 42CrMo Continuous Casting Rolls

Literature Overview

This study examines the microstructural evolution and mechanical property degradation of weld overlay layers applied to 42CrMo continuous casting rolls. Continuous casting rolls are subjected to extreme operating conditions, including cyclic thermal loading from molten steel, mechanical contact stress from the solidifying shell, and chemical attack from the steel surface. The 42CrMo base steel, a medium-carbon alloy steel with excellent strength and toughness, is widely used for roll shafts and housings, while the overlay layer provides the necessary wear and thermal fatigue resistance at the working surface. The research employs optical microscopy, scanning electron microscopy, X-ray diffraction, microhardness mapping, and wear testing to characterise the overlay microstructure and track its evolution under simulated service conditions.

Core Technical Findings

The as-welded overlay layer exhibits a typical weld microstructure consisting of dendritic austenite with interdentritic carbides and retained austenite. The microstructure is highly dependent on the welding process parameters, including heat input, travel speed, and interpass temperature. For submerged arc welding overlay processes, which are commonly used for roll repair, the relatively high heat input promotes grain growth and coarsening of carbide phases. For gas tungsten arc welding overlay, the lower heat input produces a finer microstructure with better wear resistance but may result in incomplete fusion if parameters are not carefully controlled.

Under simulated casting conditions, the overlay microstructure undergoes significant transformation. Repeated thermal cycling causes the retained austenite to decompose, leading to a progressive increase in hardness but a corresponding decrease in toughness. The carbide phases, primarily cementite and secondary carbides of chromium and molybdenum, coarsen with increasing number of thermal cycles, reducing their effectiveness as wear-resistant particles. After approximately 500 thermal cycles, corresponding to several casting heats, the microhardness of the overlay surface increases by 15-25% compared to the as-welded condition, while the impact toughness decreases by 30-40%.

Microstructural Evolution Mechanisms

The microstructural evolution of the overlay layer during thermal cycling can be understood through three primary mechanisms. First, the retained austenite, which is stabilised by carbon and alloying elements, undergoes decomposition during thermal cycling. The decomposition products are fine carbides and martensite-like structures that increase hardness but reduce ductility. Second, the existing carbide phases, particularly the M7C3 and M23C6 types, coarsen through Ostwald ripening, where smaller particles dissolve and larger particles grow at the expense of the matrix. This coarsening reduces the number of hard particles per unit area, diminishing wear resistance despite the increased hardness of the remaining particles. Third, the matrix undergoes tempering and re-precipitation cycles, with carbon and alloying elements redistributing between the matrix and carbide phases.

The thermal fatigue behaviour of the overlay is also influenced by the thermal expansion coefficient mismatch between the overlay layer and the 42CrMo base steel. During each thermal cycle, differential expansion creates tensile stresses at the overlay surface during heating and compressive stresses during cooling. These cyclic stresses, combined with the inherent residual stresses from welding, can initiate microcracks at the overlay surface. After several hundred cycles, these microcracks link up to form macroscopic cracks that propagate into the overlay layer, eventually leading to spalling of the worn surface.

Engineering Practice and Repair Strategies

Based on the study findings, several engineering recommendations emerge for the maintenance and repair of 42CrMo continuous casting rolls. The selection of overlay material should consider the operating temperature range of the specific casting application. For low-carbon steel casting, where roll surface temperatures reach 1100-1200°C, overlay materials with high-temperature stability, such as those based on austenitic stainless steels with high chromium and silicon content, are preferred. For medium-carbon and alloy steel casting, where temperatures are lower, martensitic or ferritic overlay materials with appropriate hardenability may be more suitable.

The welding sequence for multi-pass overlay repair should be planned to minimise thermal fatigue damage. A recommended approach involves applying the first pass at the lowest practical travel speed to ensure complete fusion with the base metal, followed by subsequent passes at progressively higher travel speeds to reduce heat input and grain coarsening. The interpass temperature should be maintained below 200°C to prevent excessive softening of the previously deposited layers. Post-weld stress relief at 550-600°C for 1 hour per 25 mm of thickness is recommended to reduce residual stresses without causing significant tempering of the overlay.

Key Questions and Reflections

The study highlights several areas that warrant further attention. The long-term performance of the overlay layer under combined thermal, mechanical, and chemical loading remains difficult to predict from laboratory tests alone. Field data from actual continuous casting operations would be invaluable for validating the thermal cycling simulation results. Additionally, the effect of rolling speed, roll diameter, and steel grade on the thermal cycling parameters experienced by the overlay has not been systematically investigated.

I find the observation regarding retained austenite decomposition particularly significant from a practical standpoint. The progressive loss of retained austenite during service directly impacts the ability of the overlay to absorb thermal shock through transformation-induced plasticity. This suggests that overlay materials with inherently lower retained austenite content, or those with retained austenite that is more thermally stable, may offer longer service life in continuous casting applications.

Study Insights and Implications

The study provides a comprehensive understanding of how weld overlay layers on 42CrMo continuous casting rolls degrade during service. The identification of retained austenite decomposition and carbide coarsening as the primary degradation mechanisms offers clear targets for material development and process optimisation. From a practical engineering perspective, the findings support the implementation of regular overlay inspection and repair programmes, with attention to the thermal cycling history of each roll. The microhardness mapping technique, in particular, provides a rapid and non-destructive method for assessing overlay condition in the field, enabling proactive maintenance decisions before catastrophic failure occurs.