Overlay Welding Repair and Heat Treatment of Channel Steel Rolling Mill Rolls

Literature Overview

This 2004 publication in "Heat Treatment of Metals" by Ni Zhenhang, Tang Xin, and Xia Yang from Maanshan Iron and Steel Group's Second Machinery Equipment Manufacturing Company documents a practical engineering solution for the repair and restoration of channel steel rolling mill rolls. The study addresses the intersection of overlay welding technology and post-weld heat treatment, two disciplines that must be harmonized to produce a functional roll surface that can withstand the extreme thermal and mechanical loading of hot rolling operations.

Technical Background

Channel steel (U-channel) rolling mills operate under severe conditions: the roll surface is continuously exposed to red-hot steel (typically 900-1100°C), subjected to rolling contact stresses exceeding 1000 MPa, and experiences cyclic thermal loading that induces thermal fatigue cracks. The roll surface layer undergoes progressive degradation through surface fatigue, wear, oxidation, and thermal cracking, necessitating periodic repair and restoration.

The traditional repair approach involves grinding the damaged surface and regrounding to restore dimensional accuracy. However, when the accumulated wear depth exceeds acceptable limits, material removal by grinding alone is insufficient, and overlay welding becomes necessary to add material back to the roll surface before final grinding.

Overlay Welding Strategy

The repair overlay welding of rolling mill rolls presents unique challenges compared to general industrial overlay welding:

The overlay alloy selection for roll repair typically involves high-carbon high-chromium martensitic systems (Cr12, Cr12MoV, or similar) that produce a hard, wear-resistant surface after appropriate heat treatment. The deposit must be hard enough to resist wear from the hot steel strip but tough enough to resist thermal fatigue cracking during thermal cycling.

Post-Weld Heat Treatment

The heat treatment cycle following overlay welding is critical to achieving the target microstructure and properties. For high-carbon high-chromium overlay deposits on roll surfaces, the typical treatment involves:

1. Stress relief: 550-650°C for 2-4 hours to relieve welding residual stresses and prevent delayed cracking in the heat-affected zone.
2. Austenitization: 1000-1050°C to dissolve carbides and homogenize the carbon distribution.
3. Quenching: Oil quench or air quench (depending on section thickness) to transform the austenite to martensite.
4. Tempering: 180-250°C for 2-4 hours to reduce brittleness while maintaining hardness above 58 HRC.

The tempering temperature represents a critical balance: too low and the retained austenite fraction remains high, reducing hardness and dimensional stability; too high and the hardness drops below the required threshold while carbide coarsening begins.

Engineering Practice Considerations

From a rolling mill operations perspective, the overlay welding repair must be integrated into the maintenance schedule. The key practical considerations include:

• Pre-weld preparation: The damaged area must be ground smooth with a transition taper to avoid stress concentration at the weld toe. The base metal surface must be cleaned to remove oxide scale and contamination.
• Welding sequence: For large rolls, the welding sequence must be planned to minimize distortion. A symmetrical, balanced welding pattern around the roll circumference is essential.
• Post-weld inspection: Hardness mapping across the overlay surface, visual inspection for surface cracks, and dimensional verification are mandatory before returning the roll to service.
• Service life prediction: The overlay deposit thickness and the rate of wear consumption determine the service interval between repairs. Typically, 3-5 mm of overlay material provides sufficient life for 6-12 months of continuous rolling operation.

Key Reflections

This case study underscores the principle that overlay welding repair is not merely a welding operation but an integrated metallurgical process that includes base preparation, welding execution, heat treatment, and finishing. The quality of the repair depends on the weakest link in this chain. In practice, I have observed that many roll repair failures trace back not to the welding process itself but to inadequate preheating, insufficient stress relief, or improper tempering temperatures.

The Maanshan Steel approach reflects the pragmatic engineering philosophy common in Chinese heavy industry: proven technology, optimized parameters, and rigorous process discipline. The study's value lies in documenting specific process parameters that have been validated in production, providing a reliable reference for similar repair operations at other facilities.

A critical insight from this work is that the overlay alloy composition must be matched not only to the wear mechanism but also to the thermal cycling conditions. A deposit that excels in dry sliding wear tests may fail prematurely in thermal fatigue service if its thermal conductivity, thermal expansion coefficient, and thermal fatigue crack resistance are not optimized for the specific rolling application.

Summary

The overlay welding repair and heat treatment of channel steel rolling mill rolls exemplify the practical application of hardfacing technology in heavy industry. The success of such repairs depends on careful integration of welding metallurgy, heat treatment science, and mechanical engineering principles. Engineers responsible for such operations must understand that the overlay weld metal is not an independent component but a functionally graded interface between the roll core and the rolling environment, and its performance must be evaluated in the context of the complete roll assembly and its operating conditions.