Influence of Tempering Treatment on Microstructure and Properties of Clad Metal in Large Support Rollers

Literature Overview and Industrial Background

Large support rollers are critical components in hot rolling mills, where they bear the weight of the rolling stand and guide the roll assembly during operation. To extend service life, the working surface of support rollers is typically clad with a wear-resistant alloy layer using weld overlay techniques such as submerged arc welding (SAW) or gas metal arc welding (GMAW). However, the as-welded cladding layer often exhibits a hard, brittle martensitic microstructure that is susceptible to cracking under the high contact stresses and thermal cycling encountered during hot rolling operations. Tempering treatment is therefore essential to optimize the microstructure and mechanical properties of the cladding layer, achieving an acceptable balance between hardness, toughness, and wear resistance. This literature investigates the systematic effects of tempering temperature, holding time, and cooling method on the microstructure and properties of clad metal in large support rollers.

Tempering Process Parameters and Microstructural Evolution

The tempering process for clad metal on large support rollers typically involves heating the entire roller to a controlled temperature, holding for a specified duration, and cooling under a defined atmosphere. The literature evaluates tempering temperatures ranging from 400°C to 650°C, with holding times of 2 hours to 8 hours, and compares air cooling with furnace cooling.

At lower tempering temperatures (400°C to 450°C), the microstructure consists of tempered martensite with fine carbide precipitates, retaining high hardness values of 500 HV to 580 HV. However, the impact toughness remains relatively low at 20 J to 35 J, and the material is susceptible to secondary temper brittleness if held at this temperature range for extended periods. As the tempering temperature increases to 500°C to 550°C, the carbide precipitates coarsen, and the hardness decreases to 420 HV to 480 HV, while the impact toughness improves significantly to 45 J to 65 J. This temperature range is identified as the optimal tempering window for support roller cladding, providing a good balance between wear resistance and toughness.

At higher tempering temperatures (600°C to 650°C), the microstructure transitions to tempered sorbite with coarse carbide particles, and the hardness drops to 350 HV to 400 HV. While the impact toughness reaches its maximum values of 60 J to 80 J, the wear resistance is significantly reduced, making this temperature range unsuitable for applications requiring high contact stress resistance. The literature also notes that the tempering response is influenced by the alloy composition of the cladding layer; high-alloy martensitic steels with elevated chromium and molybdenum content exhibit greater temper stability and require higher tempering temperatures to achieve the desired hardness reduction.

Mechanical Property Evolution and Hardness-Toughness Balance

The mechanical properties of the cladding layer are characterized by hardness, tensile strength, yield strength, elongation, and impact toughness. The literature presents comprehensive data on the variation of these properties with tempering temperature and holding time.

The literature demonstrates that the hardness reduction follows a sigmoidal curve with tempering temperature, with the steepest decline occurring between 450°C and 550°C. This is attributed to the coarsening of carbide precipitates and the transformation of tempered martensite to tempered sorbite. The impact toughness improvement follows a similar trend, with the most significant gains occurring in the same temperature range. The tensile strength and yield strength decrease more gradually with tempering temperature, reflecting the combined effects of carbide coarsening and dislocation recovery.

The holding time effect is less pronounced than the temperature effect, but extended holding times at a given temperature promote further carbide coarsening and tempering softening. The literature recommends holding times of 4 hours to 6 hours for typical tempering temperatures of 500°C to 550°C, as longer durations provide diminishing returns in toughness improvement while increasing the risk of over-tempering.

Effect of Cooling Method and Post-Temper Microstructure

The cooling method after tempering significantly influences the residual stress state and microstructural stability of the cladding layer. Air cooling from the tempering furnace produces a moderate cooling rate that is generally acceptable for most applications, while furnace cooling at a controlled rate of 50°C/h to 100°C/h reduces thermal gradients and residual stresses but increases processing time and cost. The literature recommends furnace cooling for large support rollers with wall thicknesses exceeding 100 mm, where thermal gradients during air cooling can induce significant residual stresses that may compromise dimensional stability during subsequent machining.

The post-temper microstructure is examined using optical microscopy, scanning electron microscopy, and X-ray diffraction. At the optimal tempering temperature of 500°C to 550°C, the microstructure consists of tempered martensite with fine, uniformly distributed carbide precipitates of type M7C3 and M23C6. The carbide particle size is typically 50 nm to 150 nm, providing effective precipitation hardening while maintaining adequate matrix toughness. The absence of retained austenite and the uniform distribution of carbides are critical for consistent mechanical performance across the cladding layer.

Engineering Practice and Defect Prevention

In the manufacturing of large support rollers, several challenges are associated with the tempering process. Distortion during tempering is a common concern, particularly for rollers with asymmetric geometry or large diameter-to-thickness ratios. The literature recommends the use of custom fixtures and controlled heating/cooling rates to minimize distortion, with dimensional tolerances of ±0.5 mm per 100 mm diameter being achievable with proper fixture design and thermal management.

Cracking during tempering, although rare, can occur in cladding layers with high residual stress or hydrogen content. The literature identifies hydrogen-induced cracking as a potential risk when the cladding layer has been deposited using a process that introduces hydrogen, such as GMAW with hydrogen-containing shielding gases. The recommended countermeasures include baking the roller at 200°C to 250°C for 2 hours to 4 hours before tempering to remove absorbed hydrogen, and using a low-hydrogen welding procedure for the cladding operation.

The quality control protocol for tempered support roller cladding includes hardness mapping across the cladding layer, impact toughness testing at multiple locations, and ultrasonic testing to detect internal defects. The literature recommends that the hardness variation across the cladding layer should not exceed ±50 HV, and that the minimum impact toughness should be at least 40 J at the test temperature specified by the design requirements.

Study Insights and Practical Recommendations

This literature provides comprehensive guidance on the optimization of tempering treatment for clad metal in large support rollers. The key insight is that the tempering temperature of 500°C to 550°C with a holding time of 4 hours to 6 hours provides the optimal balance between hardness, toughness, and wear resistance for most support roller applications. Engineers should also recognize that the tempering response is influenced by the specific alloy composition of the cladding layer, and that procedure qualification through trial tempering and metallographic evaluation is essential for each unique base metal-overlay combination. The literature also emphasizes the importance of controlling the cooling method and atmosphere to prevent oxidation, decarburization, and distortion, which can compromise the dimensional accuracy and surface quality of the finished roller. For engineers involved in the design and manufacturing of large support rollers, this study serves as a valuable reference for developing robust tempering procedures that ensure reliable service performance in demanding hot rolling applications.