Microstructural Evolution and Performance Characteristics of 42CrMo Continuous Casting Roll Overlay Layers

Literature Overview

This study by Liu Zhen and colleagues from Ma'anshan Iron and Steel Heavy Machinery Manufacturing Co., Ltd. investigates the microstructural evolution and performance characteristics of overlay layers deposited on 42CrMo continuous casting rolls using submerged arc overlay welding. The overlay consists of one layer of 430 alloy followed by two layers of 414N alloy. The research focuses on dilution rates, alloy element burn-off, microstructural evolution, and hardness distribution, providing valuable insights for engineers designing surface hardening solutions for continuous casting equipment.

Overlay Welding Process and Layer Configuration

The overlay welding process employs a multi-layer approach with different alloy compositions to optimize both metallurgical bonding and surface properties. The first layer, deposited with 430 alloy wire, serves as a transition layer between the 42CrMo base material and the subsequent wear-resistant layers. The 430 alloy, being a martensitic stainless steel with good weldability, provides a metallurgically compatible interface that reduces the risk of cracking due to thermal expansion mismatch. The subsequent two layers of 414N alloy, a high-carbon, high-chromium alloy, provide the primary wear-resistant surface.

The radial structure of the overlay, from the core outward, consists of: 42CrMo base material, heat-affected zone (HAZ), transition layer, 430 overlay layer, inner 414N overlay layer, and outer 414N overlay layer. This multi-layer configuration is a common and effective approach for achieving both sound bonding and superior surface properties in overlay welding applications.

Dilution Rate and Element Burn-Off Analysis

The dilution rate decreases significantly from the first to the third layer, from 27.3% to 12.5%. This progressive reduction is expected because each subsequent layer is deposited on top of the previous overlay layer rather than directly on the base material, reducing the amount of base material melted and mixed with the new deposit. The dilution rate is a critical parameter because it directly affects the final composition of the overlay layer and consequently its microstructure and properties.

The chromium burn-off rates are relatively low, ranging from 0.82% to 1.55%, which is favorable for maintaining the intended alloy composition. The slightly higher burn-off in the second layer may be attributed to the interaction between the 414N alloy and the 430 transition layer, which has a different composition and melting behavior. The low burn-off rates indicate that the submerged arc welding process provides adequate shielding and minimizes atmospheric contamination of the weld pool.

Microstructural Evolution and Hardness Distribution

The overlay layers exhibit a microstructure of fine lath martensite, which is the primary source of the high hardness achieved. The 42CrMo base material, after overlay welding, undergoes thermal cycling that affects the HAZ microstructure. The HAZ experiences a partial recrystallization and grain growth, which can reduce the hardness in this region. However, the transition layer, composed of the 430 alloy, provides a gradual hardness transition between the base material and the high-hardness overlay layers.

The final overlay surface achieves a hardness of approximately 3 mm in thickness with uniform composition and hardness distribution. The fine lath martensite structure of the 414N layers provides excellent wear resistance while maintaining adequate toughness for the service conditions of continuous casting rolls. The hardness gradient from the base material through the HAZ, transition layer, and overlay layers is smooth, which is important for preventing interfacial cracking under the thermal and mechanical stresses experienced during continuous casting operations.

Engineering Practice and Quality Control Considerations

For continuous casting roll overlay welding, several quality control measures are essential. First, the dilution rate should be monitored and controlled by adjusting the welding parameters, including current, voltage, travel speed, and wire feed rate. Second, the interpass temperature should be maintained below 200°C to prevent excessive grain growth and phase transformation in the previously deposited layers. Third, the surface preparation of the base material is critical—any scale, rust, or contamination must be removed to ensure sound metallurgical bonding.

The multi-layer approach with a transition layer is particularly important for 42CrMo base materials, which have a different chemical composition and thermal expansion coefficient than the high-chromium overlay alloys. Without the transition layer, cracking at the interface is likely due to the high residual stresses and thermal mismatch. The 430 alloy serves as an effective buffer, reducing the compositional gradient and thermal stress at the interface.

Study Insights and Reflections

This study demonstrates the effectiveness of a carefully designed multi-layer overlay approach for continuous casting rolls. The progressive reduction in dilution rate from 27.3% to 12.5% illustrates the importance of layer sequencing in overlay welding design. Engineers should consider that the first layer, with the highest dilution rate, serves a dual purpose: it provides the metallurgical transition and it establishes the foundation for subsequent layers. The fine lath martensite structure achieved in the final layers is a result of the high carbon and chromium content combined with the rapid cooling rates typical of overlay welding, and this structure provides an excellent balance of hardness and toughness. The low chromium burn-off rates confirm that submerged arc welding is a suitable process for this application, provided that adequate shielding gas coverage is maintained.