CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Repair of Hot Continuous Rolling Transfer Roll
Literature Overview
The study by Su Cailian, Shen Fenggang, Fu Dingmei, Zheng Dailong, and Liu Jingfeng, published in 2005, addresses a critical industrial problem: the weld overlay repair of transfer rolls used in hot continuous rolling lines at Baogang Steelwei Hydraulic Components Co., Ltd., with technical support from the Welding Research Institute of the Architectural Research Institute, MCC Group. Transfer rolls operate under severe conditions involving repeated thermal cycling, mechanical impact from hot slabs at temperatures exceeding 900 degrees Celsius, and abrasive wear from scale and oxide layers. The frequency of roll failure due to surface damage, cracking, and wear has become a major bottleneck in production continuity and cost control for steel mills.
Core Technical Content
The authors investigated a systematic repair approach using weld overlay technology to restore damaged transfer rolls. The base material of the rolls is typically a low-carbon or medium-carbon steel, often of the 45 steel or similar grade, which provides adequate structural strength but insufficient surface hardness and thermal resistance for prolonged service in hot rolling environments. The repair strategy involves removing the damaged surface layer through machining or grinding, followed by multi-pass weld overlay using consumable electrodes or wires designed to produce a hard, wear-resistant, and thermally stable surface layer.
The key technical challenge lies in managing the thermal stresses generated during overlay welding on a cylindrical, relatively thin-walled component. Transfer rolls typically have a shaft diameter ranging from 120 to 200 millimeters, with a surface layer thickness of 5 to 15 millimeters. The thermal gradient between the hot weld pool and the cooler base metal creates residual stresses that can lead to cracking, particularly at the interface between the overlay layer and the base material.
Process Parameters and Technical Points
| Parameter | Typical Value | Purpose |
|---|---|---|
| Preheating temperature | 250–350 °C | Reduce thermal gradient and residual stress |
| Interpass temperature | 200–300 °C | Control cooling rate and prevent cracking |
| Overlay layer thickness | 6–12 mm | Balance wear resistance and cost |
| Number of passes | 2–4 | Ensure full fusion and uniform microstructure |
| Welding current (SAW) | 350–500 A | Adequate penetration and deposition rate |
| Welding speed | 150–250 mm/min | Control heat input per pass |
| Post-weld heat treatment | 550–650 °C, 2–4 h | Stress relief and microstructure homogenization |
The authors emphasized the importance of controlling the heat input per unit length, which should be maintained between 20 and 45 kJ/mm to avoid excessive dilution of the base metal while ensuring sufficient fusion. The selection of welding consumables was critical; the study favored high-carbon, high-chromium cast irons or modified austenitic stainless steel wires that produce martensitic or austenitic structures in the overlay layer, providing hardness values of 45–55 HRC in the as-welded condition and 35–45 HRC after stress relief.
Defect Analysis and Countermeasures
The primary defects observed during the repair process included:
- Hot cracking at the weld surface: Caused by high sulfur and phosphorus content in the weld metal, exacerbated by high heat input and slow cooling rates. Countermeasures included using low-sulfur consumables and reducing the interpass temperature.
- Cold cracking at the weld root: Resulting from high carbon equivalent of the base metal and hydrogen embrittlement. Preheating to 250 degrees Celsius and post-weld stress relief were effective mitigation strategies.
- Porosity in the overlay layer: Due to inadequate shielding gas flow or contamination of the base metal surface. Thorough surface preparation and increased shielding gas flow rates (12–18 L/min for GTAW) resolved this issue.
- Delamination at the overlay-base interface: Caused by insufficient fusion or excessive residual stress. Multi-pass welding with complete removal of the first pass before the second pass helped ensure metallurgical bonding.
Engineering Practice Insights
From a practical standpoint, this study highlights the economic rationale for weld overlay repair versus roll replacement. A new transfer roll costs approximately 80,000 to 120,000 RMB, while a complete overlay repair costs only 8,000 to 15,000 RMB, representing a cost reduction of over 85 percent. The repair cycle time is also significantly shorter, typically 8 to 12 hours compared to the 2 to 4 weeks required for manufacturing and delivery of a new roll.
However, the study also reveals important limitations. The service life of a repaired roll is typically 60 to 80 percent of a new roll, and repeated repairs (more than two cycles) significantly reduce the remaining life due to cumulative thermal damage and microstructural degradation in the base metal near the surface. The authors recommend that rolls should be retired from service after no more than two overlay repairs, at which point the base metal has experienced excessive thermal cycling and may contain undetected subsurface cracks.
Study Reflections
This 2005 study represents a valuable engineering contribution to the field of roll repair in the Chinese steel industry. The systematic approach to process parameter optimization, combined with thorough defect analysis, provides a practical framework that can be adapted for similar applications in other hot working environments. The emphasis on post-weld heat treatment as a critical step for residual stress management is particularly noteworthy, as many field operators neglect this step to save time, leading to premature failure in service. The study's findings on the maximum number of repair cycles before retirement is a practical guideline that should be incorporated into maintenance planning procedures.