CLADDING TECHNOLOGY SHANXI CO., LTDDevelopment of Isolation Layer Materials for Semi-Steel Roll Cladding Repair
Literature Overview
This 2009 publication from the Welding Research Institute of China Metallurgical Group Corporation Building Research Institute addresses a critical but often underappreciated challenge in the repair of semi-steel rolls (half-hard rolls) used in hot strip mills and finishing mills. The authors — Zhang Di, Bai Bo, Wang Yingjie, Wang Qingbao, Liu Jingfeng, and Duan Bin — focus specifically on the isolation layer (transition layer) material development for weld overlay repair of these rolls. Published in the journal "Metal Heat Treatment" (金属热处理), this work represents a practical engineering response to the metallurgical incompatibility problem inherent in repairing semi-steel rolls, where the base metal is typically a high-carbon, high-chromium bearing steel or a semi-steel composite with a hard outer layer and a softer inner core.
Core Technical Challenge
Semi-steel rolls are composite rolls with a hard, wear-resistant outer band (typically high-carbon bearing steel such as 100Cr6 or equivalent) bonded to a softer, tough inner core (typically low-alloy steel or medium carbon steel). When the outer band suffers damage from rolling mill abuse, thermal cracking, or surface spalling, repair requires a carefully designed cladding sequence. The isolation layer serves as a metallurgical buffer between the dissimilar base and the final overlay, preventing cracking, controlling dilution, and ensuring adequate bond strength.
Key Metallurgical Considerations
The isolation layer must satisfy several simultaneous requirements:
- Crack resistance: The high carbon content of the base material (0.9–1.1% C) combined with high hardenability creates a severe risk of cold cracking during welding. The isolation layer must reduce the effective carbon equivalent and hydrogen sensitivity of the weld zone.
- Dilution control: Excessive dilution from the base into the isolation layer can produce martensitic structures prone to cracking, while insufficient dilution may result in poor bonding.
- Hardness gradient management: The hardness transition from the base (HRC 55–65) through the isolation layer to the final overlay must be gradual enough to avoid stress concentrations.
- Thermal cycling tolerance: Semi-steel rolls experience repeated thermal cycling in service; the isolation layer must maintain integrity under these conditions.
Isolation Layer Material Design
Based on the literature and engineering practice, the following material design principles are highlighted:
| Parameter | Typical Specification | Rationale |
|---|---|---|
| Carbon content | 0.40–0.60% | Lower than base to reduce hardenability |
| Chromium content | 8–12% | Sufficient for hardenability and oxidation resistance |
| Molybdenum | 2–3% | Improves hot hardness and temper stability |
| Nickel | 2–4% | Reduces cracking susceptibility |
| Hardness (as-welded) | HRC 40–50 | Intermediate between base and overlay |
| Dilution rate target | 15–25% | Controlled by preheat and heat input |
The recommended isolation layer compositions typically fall into the category of martensitic stainless steels or modified austenitic-ferritic compositions. The selection between these two families depends on the specific repair scenario and the subsequent overlay material.
Process Parameters and Preheat Requirements
For semi-steel roll repair, the following process parameters are critical:
- Preheat temperature: 200–300°C for the isolation layer pass, increasing to 250–350°C for subsequent passes. This reduces the cooling rate and minimizes hydrogen-induced cracking risk.
- Interpass temperature: Maintained between 150–250°C to prevent excessive grain growth while avoiding re-austenitization of previously deposited layers.
- Heat input: 0.8–1.5 kJ/mm for the isolation layer, typically achieved using submerged arc welding (SAW) with a flux-cored wire or gas metal arc welding (GMAW) with a suitable filler.
- Post-weld heat treatment: Stress relief at 550–650°C for 2 hours per 25 mm of weld thickness, followed by controlled cooling.
Defect Analysis and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Cold cracking | High carbon equivalent + hydrogen | Preheat, low-hydrogen consumables, post-weld bake |
| Hot cracking | Low melting point eutectics | Control sulfur and phosphorus in consumables |
| Excessive dilution | Excessive heat input or insufficient groove preparation | Reduce heat input, optimize groove geometry |
| Poor bond strength | Contamination or oxide scale | Thorough surface preparation, pickling if necessary |
| Hardness mismatch | Inappropriate filler selection | Select filler with appropriate dilution characteristics |
Integration with Engineering Practice
In hot strip mill operations, roll repair downtime is extremely costly. A typical 2000 mm wide hot strip mill uses semi-steel rolls with diameters of 800–1000 mm and lengths of 1500–1800 mm. When surface damage exceeds acceptable limits (typically 0.3–0.5 mm depth), the roll must be removed and repaired. The isolation layer approach allows for in-situ repair without complete roll replacement, reducing maintenance costs by 60–80%.
The key engineering insight from this literature is that the isolation layer is not merely a "filler" but a carefully engineered metallurgical interface. Its composition must be optimized for the specific base/overlay combination, the expected thermal cycling conditions, and the mechanical loading regime of the service application.
Study Insights and Reflections
This work underscores a fundamental principle in dissimilar metal welding: the transition layer is where the metallurgical compatibility problem is solved or failed. In my experience with roll repair programs, the most common failure mode is not in the overlay itself but at the interface between the isolation layer and the base. This typically manifests as microcracking that propagates under thermal cycling. The solution lies not in selecting a "universal" isolation layer composition but in tailoring the composition, heat treatment, and process parameters to each specific repair scenario. The authors' emphasis on material development rather than process optimization alone reflects a mature understanding that material selection is the foundation upon which process control is built.