Study Note on Microstructure and Property Comparison of Different Material Carrying Roll Overlay Layers After Annealing Treatment

Literature Overview and Industrial Context

Carrying rolls are critical components in hot rolling mills, where they guide and support hot steel slabs during the rolling process. The overlay layers applied to carrying rolls must withstand extreme thermal cycling, mechanical impact, and abrasive wear from steel scale and oxide debris. This paper investigates the microstructure and mechanical properties of overlay layers on carrying rolls fabricated from different base materials after post-weld annealing treatment. The study addresses a practical challenge: selecting the optimal combination of base material, overlay material, and heat treatment parameters to maximize roll service life while maintaining economic feasibility.

Base Materials and Overlay Configurations

The study evaluates three base material options for carrying rolls, each paired with a NiCrMoSiB-type overlay material applied by submerged arc welding (SAW). The following table summarizes the material configurations:

The NiCrMoSiB overlay material is selected for its excellent combination of high-temperature hardness retention, thermal shock resistance, and wear resistance. The overlay is applied in multiple passes to achieve the 12 mm thickness, with interpass temperature controlled below 150 °C to minimize thermal distortion and residual stress accumulation.

Annealing Treatment Parameters and Microstructural Response

Post-weld annealing is essential to relieve residual stresses, homogenize the microstructure, and optimize the mechanical properties of the overlay layer. The annealing parameters are critical, as excessive temperature can cause carbide coarsening and softening, while insufficient temperature leaves residual stresses that promote fatigue cracking. The study employs the following annealing conditions:

After annealing, the microstructure of the overlay layers exhibits significant differences depending on the base material. Configuration A (Q345 base) shows a microstructure dominated by martensite with dispersed carbides, with some retained austenite near the fusion boundary. The lower alloy content of the base material results in a slightly coarser microstructure in the overlay due to higher dilution. Configuration B (42CrMo base) exhibits a more refined martensitic structure with finer carbide distribution, benefiting from the higher alloy content that promotes nucleation during solidification. Configuration C (35CrMoA base) shows the finest microstructure with a complex mixture of martensite, bainite, and carbide phases, reflecting the synergistic effect of the medium-alloy base on overlay solidification.

Mechanical Property Comparison and Performance Analysis

The hardness profiles across the overlay thickness reveal that Configuration C achieves the highest hardness and the most uniform distribution. The lower dilution rate of Configuration C is a direct result of the closer metallurgical compatibility between the 35CrMoA base and the NiCrMoSiB overlay material. The reduced dilution preserves the high-alloy composition of the overlay, maintaining the carbide-forming elements (Cr, Mo) that contribute to hardness and wear resistance.

The thermal shock resistance, evaluated by thermal cycling between 800 °C and 20 °C in air, shows a clear trend with base material selection. Configuration C withstands 220 thermal shock cycles before cracking initiates at the fusion boundary, while Configuration A fails after only 120 cycles. This difference is attributed to the lower dilution and better metallurgical compatibility of Configuration C, which reduces the thermal mismatch stress at the fusion boundary. The higher carbon equivalent of the 35CrMoA base also promotes a more ductile transition zone, accommodating thermal strain without cracking.

Wear testing under simulated carrying roll conditions (abrasive wear with iron scale particles at 600 °C) demonstrates that Configuration C exhibits 45% better wear resistance than Configuration A and 20% better than Configuration B. The wear mechanism transitions from abrasive ploughing in Configuration A to micro-ploughing with matrix deformation in Configuration C, indicating a fundamental improvement in the wear resistance mechanism.

Engineering Practice and Selection Guidelines

For carrying roll applications, the selection of base material should be guided by the following criteria. When the roll operates at temperatures below 600 °C with moderate thermal cycling, Configuration A (Q345 base) provides adequate performance at the lowest cost. When thermal cycling is more severe or roll life extension is a priority, Configuration B (42CrMo base) offers a balanced combination of cost and performance. For demanding applications involving frequent thermal shock events or high-temperature service above 700 °C, Configuration C (35CrMoA base) is the recommended choice despite the higher material cost.

The annealing treatment is non-negotiable for all configurations. Skipping the annealing step results in residual stresses that can reach 400–500 MPa, which are sufficient to initiate cracking during the first thermal cycling event in service. The furnace cooling to 400 °C followed by air cooling is a critical step that balances stress relief against carbide coarsening. Cooling directly to room temperature from the annealing temperature would result in excessive hardness and brittleness, while air cooling from the annealing temperature would promote carbide precipitation and softening.

Study Insights and Reflections

The most valuable finding of this study is the demonstration that base material selection has a more profound impact on overlay performance than previously recognized. Many engineers focus exclusively on the overlay material and welding parameters while treating the base material as a secondary consideration. This research clearly shows that the base material composition influences the overlay microstructure through dilution effects, thermal conductivity differences, and metallurgical compatibility. The selection of a higher-alloy base material can yield better overlay performance even with the same overlay material and welding parameters.

The annealing temperature variation across configurations (650–700 °C) also deserves attention. The higher annealing temperature for Configuration C is justified by the higher alloy content of the 35CrMoA base, which requires more thermal energy to fully relieve stresses and homogenize the microstructure. Engineers should avoid applying a single annealing temperature to all configurations, as this would under-treat the higher-alloy base and over-treat the lower-alloy base, leading to suboptimal performance in both cases.

This research provides a practical framework for carrying roll material selection and heat treatment optimization. The key takeaway is that the base material-overlay material combination must be evaluated as a system, not as two independent components. Engineers should conduct systematic evaluations of base material options before committing to a single configuration, as the long-term performance gains from optimal selection can justify the initial investment in testing and qualification.