Effect of Preheating on Cracking in Clamp Roll Overlay Welding

Literature Overview

Published in Hot Working Technology in 2006, this paper by Wang Lihua and Hui Yuanbo investigates the influence of preheating temperature on the cracking susceptibility of clamp rolls during overlay welding operations. The research, conducted with input from Shaanxi Rolling Equipment Factory, addresses a persistent quality challenge in the maintenance and refurbishment of rolling mill equipment where overlay welding is used to restore worn clamp roll surfaces.

Technical Background

Clamp rolls in rolling mills are subjected to cyclic mechanical loading, thermal cycling, and abrasive wear. When the roll surface becomes worn or damaged, overlay welding is employed to restore the original dimensions and surface properties. However, the overlay welding of clamp rolls is prone to cracking due to:

• High carbon equivalent of the base material (typically 45# steel or similar medium-carbon steel)
• Thick section geometry that restricts strain accommodation
• Rapid cooling rates that promote martensitic transformation in the HAZ
• Hydrogen pickup from the welding process
• Pre-existing residual stresses from prior service

Preheating Temperature Investigation

The study systematically varies preheating temperature and evaluates its effect on cracking propensity:

Mechanism of Preheating Effect on Crack Prevention

Preheating reduces cracking susceptibility through multiple mechanisms:

1. Reduced cooling rate: Slower cooling allows diffusion-controlled transformations rather than diffusionless martensitic transformation
2. Lower thermal gradient: Reduced temperature difference between the hot weld zone and the cooler base metal decreases thermal stress magnitude
3. Hydrogen diffusion: Elevated temperature promotes hydrogen escape from the weld metal and HAZ during cooling
4. Reduced HAZ hardness: Softer HAZ can accommodate plastic strain without cracking
5. Reduced residual stress: Lower thermal gradients generate lower peak residual stresses

Critical Preheating Temperature Determination

The research identifies a critical preheating temperature range of 250-350°C below which cracking remains a significant risk. Below 200°C, the cooling rate in the critical temperature range (800-500°C) remains too high to prevent martensite formation. Above 350°C, the benefits of preheating diminish while the energy cost increases significantly.

Welding Procedure Recommendations

Based on the research findings, the following welding procedure is recommended for clamp roll overlay welding:

• Preheat to 300°C uniformly over a 150 mm radius from the weld zone
• Maintain interpass temperature between 250-350°C throughout multi-pass welding
• Use low-hydrogen electrodes (E5015 type) with controlled moisture content below 5 g/kg
• Apply moderate heat input (2.0-3.0 kJ/mm) to balance dilution and cooling rate
• Perform post-weld stress relief at 550-600°C for 2 hours per 25 mm of roll thickness
• Inspect for cracks using magnetic particle testing before and after stress relief

Engineering Significance

This research provides practical guidance for maintenance engineers responsible for roll refurbishment programs. The systematic relationship between preheating temperature and cracking propensity enables the establishment of clear acceptance criteria for preheat application. In industrial settings where roll availability directly impacts production scheduling, minimizing rework due to overlay cracks is essential for cost-effective maintenance.

The findings also highlight the importance of integrating metallurgical understanding into welding procedure development. Simply applying a higher preheat temperature without understanding the underlying mechanisms can lead to other quality issues such as excessive grain growth, reduced hardness in the HAZ, or distortion. The optimal preheating temperature must balance crack prevention with the maintenance of mechanical properties required for service.

Study Insights and Practical Implications

The research demonstrates that preheating is not merely a conventional practice but a scientifically justified process parameter that directly controls the metallurgical transformation behavior in the heat-affected zone. For engineers developing welding procedure specifications for overlay operations on high-carbon or high-carbon-equivalent base materials, the determination of minimum preheating temperature should be based on the carbon equivalent of the base material and the required cooling rate to avoid martensite formation.

The practical implication is that welding procedure qualification should include systematic evaluation of preheating temperature effects on both cracking and mechanical properties, rather than simply adopting preheat values from generic references. This approach, while requiring additional development effort, ultimately leads to more reliable procedures and reduced production losses from cracking-related rework.