CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Preheating on Cladding Cracks in Guide Rolls
Literature Overview
This study examines the influence of preheating temperature on the formation of cracks during the weld overlay cladding of guide rolls used in hot strip rolling mills. Guide rolls are subjected to high temperatures, thermal shock, and mechanical contact forces, necessitating a hard and wear-resistant overlay layer. However, the cladding process is prone to cracking due to the high carbon equivalent of the base steel and the restricted cooling conditions imposed by the thick roll geometry. The research systematically varies the preheating temperature from 0 to 400 degrees Celsius and evaluates the resulting crack density, crack morphology, and overlay layer integrity through visual inspection, magnetic particle testing, and metallographic examination.
Crack Mechanisms in Guide Roll Cladding
The primary cracking mechanisms identified in guide roll cladding include hot cracking, reheat cracking, and cold cracking. Hot cracking occurs in the solidification zone of the overlay layer and is driven by low melting point impurities (such as sulfur and phosphorus) segregating to grain boundaries during solidification. Reheat cracking develops in the heat-affected zone during post-weld heat treatment or during the early cooling stage when the temperature is between 500 and 700 degrees Celsius. Cold cracking, also known as hydrogen-induced cracking, occurs at temperatures below 200 degrees Celsius and is driven by the combined effects of high hardness martensite, hydrogen diffusion, and tensile residual stress.
The following table presents the preheating temperature ranges and their effects on different crack types:
| Preheat Temperature (°C) | Hot Cracking Susceptibility | Reheat Cracking Susceptibility | Cold Cracking Susceptibility | Overall Crack Density |
|---|---|---|---|---|
| 0 (No preheat) | High | High | Very High | Severe |
| 100 | Moderate | Moderate | High | Moderate |
| 200 | Low | Low | Moderate | Low |
| 300 | Very Low | Low | Low | Minimal |
| 400 | Very Low | Very Low | Very Low | None observed |
The data clearly demonstrate that increasing the preheating temperature from 0 to 300 degrees Celsius significantly reduces all types of cracking. At 400 degrees Celsius, no cracks were observed in any of the test specimens, although this temperature is impractical for large roll components due to energy consumption and distortion concerns.
Experimental Results and Microstructural Analysis
The experimental specimens were fabricated from 42CrMo steel roll blanks with a nickel-based alloy overlay applied by submerged arc welding. The welding consumable was a nickel-copper alloy wire with a nominal composition of 70 percent nickel and 30 percent copper. The heat input was maintained at 18 kJ/mm, and the welding speed was 0.6 m/min. Metallographic examination of the overlay layer revealed that at low preheat temperatures, the microstructure consisted of hard martensite with extensive microcracking along grain boundaries. At 200 degrees Celsius preheat, the microstructure transitioned to a tempered martensite with reduced microcrack density. At 300 degrees Celsius, the microstructure was predominantly bainitic with excellent toughness.
The hardness profile across the overlay layer showed that preheating at 200 degrees Celsius produced an overlay hardness of approximately 320 HV, while at 300 degrees Celsius, the hardness decreased to approximately 280 HV. The reduction in hardness at higher preheat temperatures is attributed to the slower cooling rate, which allows for more complete austenite decomposition into bainite rather than martensite. Despite the lower hardness, the overlay layer at 300 degrees Celsius preheat exhibited superior resistance to spalling and delamination due to the absence of cracking and the more ductile microstructure.
Preheating Practice and Process Control
The study recommends a preheating temperature of 200 to 250 degrees Celsius for guide roll cladding as the optimal balance between crack prevention and maintaining adequate overlay hardness. The preheating should be applied uniformly across the entire roll surface, not just locally around the weld area, to minimize thermal gradients. For large-diameter rolls exceeding 500 mm, a gradual preheating rate of 100 degrees Celsius per hour is recommended to avoid thermal shock in the base metal.
The following process control recommendations are derived from the study findings:
- Preheat the entire roll surface to the target temperature using induction heating or gas heating.
- Maintain the preheat temperature for at least 30 minutes per 25 mm of thickness to ensure uniform temperature distribution.
- Monitor the preheat temperature at multiple locations using thermocouples to verify uniformity.
- Control the interpass temperature to remain within 150 to 300 degrees Celsius throughout the multi-pass welding sequence.
- Apply a post-weld stress relief treatment at 600 degrees Celsius for 2 hours per inch of thickness to further reduce residual stresses.
Key Reflections and Engineering Recommendations
The most important takeaway from this study is that preheating is not merely a precautionary measure but a fundamental process parameter that directly controls the metallurgical outcome of the cladding process. The study demonstrates that a systematic approach to preheating, guided by an understanding of the underlying crack mechanisms, can virtually eliminate cracking in guide roll cladding. Engineers should incorporate preheat temperature into their welding procedure specifications as a critical process variable with defined upper and lower limits.
The study also highlights the importance of considering the entire welding sequence, including preheat, welding, interpass temperature, and post-weld heat treatment, as an integrated process. Focusing on any single parameter in isolation may lead to suboptimal results. The recommended practice is to develop a comprehensive welding procedure qualification (WPQ) that documents the full thermal history and validates the crack-free performance of the overlay under realistic service conditions.