CLADDING TECHNOLOGY SHANXI CO., LTDCladding Repair and Heat Treatment of Channel Steel Rollers
Literature Overview
This technical paper by Ni Zhenhang, Tang Xin, and Xia Yang from Ma'anshan Iron and Steel Co., Ltd. Second Mechanical Equipment Manufacturing Company documents the cladding repair and subsequent heat treatment of channel steel rollers used in steel rolling mills. Published in "Metal Heat Treatment" in 2004, this work addresses the practical engineering challenge of restoring worn rolling mill rollers to serviceable condition through hardfacing cladding and appropriate post-weld thermal treatment.
Application Background
Channel steel rollers (also referred to as channel-type backup rolls or work rolls) are critical components in steel rolling mills that support and guide the rolling stock during the hot rolling process. These rollers are subjected to extreme conditions including:
- High temperatures: Operating temperatures of 800–1200 °C from contact with hot steel slabs.
- Heavy mechanical loads: Contact pressures exceeding 300 MPa during rolling operations.
- Severe wear: Abrasive and adhesive wear from continuous contact with moving steel stock.
- Thermal cycling: Repeated heating and cooling cycles causing thermal fatigue.
When rollers become worn beyond acceptable dimensional limits, complete replacement is economically prohibitive given the large size and weight of these components. Cladding repair offers a cost-effective alternative.
Cladding Repair Process
The repair process involves the following key steps:
| Process Step | Parameters | Purpose |
|---|---|---|
| Surface preparation | Grinding to remove 3–5 mm of worn surface | Remove decarburized and damaged layer |
| Pre-heating | 250–350 °C furnace heating | Reduce thermal stress and prevent cracking |
| Cladding welding | Multi-pass SAW or GMAW, 4–8 passes | Restore dimensions and apply hard overlay |
| Interpass temperature | 150–250 °C | Control cooling rate and prevent cracking |
| Post-weld heat treatment | Normalizing + tempering | Refine microstructure and relieve stresses |
The overlay consumable selection is critical. The paper evaluates several hardfacing compositions suitable for channel roller repair:
- High chromium cast iron type: 20–30% Cr, 2–3% C, providing hardness of 60–65 HRC after proper heat treatment.
- Martensitic stainless steel type: 12–13% Cr, 0.4–0.6% C, offering hardness of 50–55 HRC with better toughness.
- Nickel-based alloy type: Ni-Cr-Mo system, providing excellent high-temperature wear resistance at 45–50 HRC.
Heat Treatment Strategy
The post-weld heat treatment is essential for achieving the target properties in the cladding layer. The recommended heat treatment cycle includes:
- Normalizing: Heating to 850–950 °C and holding for 2–4 hours per 25 mm of section thickness, followed by air cooling. This step refines the coarse as-welded microstructure and promotes uniform phase distribution.
- Tempering: Heating to 500–560 °C for 2–3 hours followed by furnace cooling. The tempering treatment reduces residual stresses, improves toughness, and stabilizes the microstructure against further transformation during service.
- Optional sub-zero treatment: For applications requiring maximum hardness, a sub-zero treatment at -60 to -80 °C can be applied between normalizing and tempering to transform retained austenite to martensite.
Quality Control and Defect Prevention
Common defects encountered during roller cladding repair include:
| Defect Type | Root Cause | Prevention Measure |
|---|---|---|
| Cracking | Excessive cooling rate, hydrogen embrittlement | Adequate pre-heat, low-hydrogen consumables |
| Porosity | Flux contamination, insufficient shielding | Clean surfaces, proper flux storage |
| Incomplete fusion | Low heat input, poor fit-up | Increase heat input, proper joint preparation |
| Spalling | Residual stress, improper heat treatment | Controlled cooling, proper tempering |
| Hardness non-uniformity | Variable dilution, inconsistent weld bead | Multi-pass technique, consistent parameters |
Practical Experience and Reflections
The practical experience documented in this paper highlights several important lessons for engineers engaged in roller repair:
- The pre-heat temperature must be maintained throughout the entire welding operation, not just applied initially. Loss of pre-heat temperature during multi-pass welding is a common cause of cracking.
- The heat treatment furnace must be capable of accommodating the full roller length to ensure uniform thermal treatment. Localized heating methods produce unacceptable property variations.
- Post-repair dimensional accuracy must be verified through precision grinding, as the cladding process itself introduces dimensional deviations.
- The service life of a properly repaired roller typically reaches 60–80% of a new roller's life, making the repair economically justified for rollers with remaining base material integrity.
The paper's practical orientation makes it particularly valuable for maintenance engineers who must make rapid decisions about roller repair versus replacement. The emphasis on heat treatment as a critical step rather than a routine procedure reflects the engineering reality that the weld metal microstructure alone is insufficient for achieving the required service performance in hot rolling applications.