Improvement of Heat Treatment Process for Overlay Welded Rolls

Literature Overview

This study note examines a technical paper focused on the improvement of the heat treatment process for overlay welded rolls, which are critical components in rolling mills used in steel production. Overlay welded rolls are constructed by welding a high-performance alloy layer onto a steel core, combining the wear resistance of the overlay with the toughness and strength of the core material. The heat treatment process is essential for achieving the desired mechanical properties of both the overlay layer and the base roll, and the literature addresses the optimization of tempering temperature, soaking time, and cooling rate to improve the hardness, toughness, and dimensional stability of the finished roll.

Core Technical Content

The paper describes the typical construction of an overlay welded roll, which consists of a carbon steel or low-alloy steel core with a weld overlay layer of a high-carbon or high-alloy steel deposited by submerged arc welding (SAW) or electroslag welding (ESW). The overlay layer is typically 30–60 mm thick and is designed to provide wear resistance in the working surface of the roll. The heat treatment process involves normalizing or austenitizing the entire roll followed by tempering to achieve the desired hardness and toughness balance.

The original heat treatment process produced rolls with acceptable but suboptimal mechanical properties. The overlay layer exhibited a hardness of 42–46 HRC, which was adequate for many rolling mill applications but insufficient for heavy-gauge steel rolling where higher wear resistance is required. The core hardness of 28–32 HRC was also on the low side, limiting the overall load-bearing capacity of the roll.

Metallurgical Analysis and Process Optimization

Metallographic examination of the rolls treated with the original process revealed a mixed microstructure in the overlay layer consisting of tempered martensite and retained austenite, with some regions showing incomplete transformation due to insufficient austenitizing temperature. The grain size was uneven, with coarse grains near the surface and finer grains in the interior, indicating inadequate soaking time. The core microstructure was predominantly ferrite-pearlite with some retained austenite, which provided adequate toughness but limited strength.

The improved heat treatment process addressed these issues by increasing the austenitizing temperature to 900–920 °C, which ensured complete transformation of the overlay layer to austenite and promoted a more uniform grain size. The extended soaking time of 3–4 hours allowed for better homogenization of the carbon and alloying element distribution, reducing the risk of segregation and uneven hardness. The tempering temperature was raised to 580–620 °C, which produced a tempered martensite microstructure with superior toughness while maintaining adequate hardness. The combination of furnace cooling followed by air cooling was adopted to achieve a more uniform hardness distribution across the roll thickness.

The study also examined the residual stresses in the rolls after heat treatment. The original process produced significant residual tensile stresses near the surface, which could lead to surface cracking during service. The improved process, with its more gradual cooling rate, reduced the residual tensile stresses to acceptable levels and even produced compressive stresses in some regions, which is beneficial for fatigue resistance.

Quality Control and Inspection

The quality control procedures described in the paper include hardness testing at multiple locations and depths, ultrasonic testing for internal defects, and dimensional inspection for distortion. The hardness testing revealed that the improved process produced a hardness variation of less than 2 HRC across the roll surface, compared to a variation of 4–5 HRC in the original process. Ultrasonic testing confirmed that both processes produced rolls free of internal defects, but the improved process showed a more uniform microstructure with fewer indications of grain boundary segregation.

The dimensional inspection revealed that the improved process produced less distortion than the original process, which is attributed to the more uniform temperature distribution during austenitizing and the more gradual cooling rate. The maximum dimensional deviation was reduced from 1.5 mm to 0.8 mm, which is a significant improvement for maintaining the precision of the rolling mill.

Engineering Practice Implications

For engineers responsible for the heat treatment of overlay welded rolls, this literature provides clear recommendations for process optimization. The key parameters to control are the austenitizing temperature, soaking time, tempering temperature, and cooling rate. Each of these parameters has a significant impact on the final microstructure and mechanical properties of the roll, and they must be carefully selected based on the specific alloy composition of the overlay layer and the core material.

The study also emphasizes the importance of thermocouple placement during heat treatment. Multiple thermocouples should be placed at different locations on the roll surface and in the core to monitor the temperature distribution and ensure uniform heating. The heating rate should be limited to avoid thermal stresses that could cause cracking, particularly in the overlay layer where the thermal conductivity may differ from the core.

Key Reflections and Study Insights

The most valuable insight from this literature is the recognition that the heat treatment of overlay welded rolls is a complex process that must be carefully optimized to achieve the desired balance of hardness, toughness, and dimensional stability. The original process was not fundamentally flawed but was suboptimal in several respects, and the improvements described in the paper are practical and achievable with existing equipment and procedures.

Another important observation is the interplay between the overlay layer and the core material during heat treatment. The different thermal properties of the two materials can lead to differential expansion and contraction, which can cause residual stresses and distortion. The improved process addresses this issue through more uniform heating and cooling, which reduces the thermal gradients and minimizes the risk of cracking or distortion.

Summary

The improvement of the heat treatment process for overlay welded rolls is a practical and significant advancement that enhances the mechanical properties, dimensional stability, and service life of these critical rolling mill components. The key improvements include higher austenitizing temperatures, extended soaking times, optimized tempering parameters, and more controlled cooling rates. Engineers should implement these improvements through careful process qualification, rigorous quality control, and continuous monitoring of the heat treatment parameters. The resulting rolls exhibit superior hardness uniformity, improved toughness, reduced distortion, and lower residual stresses, all of which contribute to enhanced performance and longer service life in demanding rolling mill applications.