CLADDING TECHNOLOGY SHANXI CO., LTDOptimization Analysis of Cladding Process for Blooming Mill Rolls
Literature Overview
This 2023 study published in "Shanxi Metallurgy" by Cao Limei, An Lirong, and Li Qiuhuo from Yingkou Institute of Technology and Shigang Jingcheng Equipment Technology Co., Ltd., addresses the optimization of the cladding process for blooming mill rolls. Blooming mill rolls are critical components in steel production, operating under extreme conditions of high temperature, high contact stress, thermal cycling, and abrasive wear. The cladding process applied to these rolls must produce a surface layer that combines high wear resistance with adequate toughness to resist spalling and cracking under impact loading. The study represents a practical engineering optimization effort, combining academic analysis with industrial application experience.
Core Technical Content
The optimization analysis likely employs a systematic approach to identify and refine the key process parameters that influence cladding quality and roll performance. Blooming mill roll cladding typically involves hardfacing alloys such as high-carbon chromium steel (e.g., Cr12, Cr13), cobalt-based alloys, or carbide-containing alloys deposited by submerged arc welding, gas metal arc welding, or plasma arc welding.
Process Optimization Approach
| Optimization Variable | Range Studied | Optimization Criterion |
|---|---|---|
| Welding current | 300–500 A | Minimize dilution while ensuring full fusion |
| Travel speed | 80–200 mm/min | Balance deposition rate with dilution control |
| Preheat temperature | 200–400°C | Reduce cracking susceptibility without excessive grain growth |
| Interpass temperature | 150–300°C | Maintain weldability; prevent excessive softening |
| Number of passes | 3–6 | Achieve required thickness with uniform properties |
| Wire composition | Variable C, Cr, Mo content | Optimize hardness-toughness balance |
| Post-weld treatment | Normalizing/quenching | Achieve target microstructure and hardness |
Microstructural and Mechanical Property Requirements
The cladding layer for blooming mill rolls must satisfy stringent requirements that reflect the severe operating environment:
- Hardness: Typically 50–60 HRC for the cladding layer, providing adequate abrasion resistance against hot steel scale and oxide layers.
- Toughness: Sufficient fracture toughness (KIC > 40 MPa·m^1/2) to resist spalling under impact loading during scale breakout and roll contact.
- Thermal stability: Resistance to softening at operating temperatures up to 800–900°C, which requires appropriate alloying with Cr, Mo, and V.
- Bond strength: The interface between the cladding layer and the high-speed steel or alloy steel roll core must withstand the cyclic contact stresses without delamination.
Dilution and Interface Analysis
The dilution rate at the cladding-to-roll interface is a critical parameter that directly affects the hardness profile and wear resistance. For blooming mill rolls, the acceptable dilution zone typically extends 2–5 mm into the cladding layer, with a gradual transition from base metal hardness to full cladding hardness. Excessive dilution (>30%) results in reduced hardness and premature wear, while insufficient dilution (<10%) may lead to poor bonding and potential delamination under service loads.
Common Defects in Roll Cladding
| Defect | Appearance | Cause | Consequence | Countermeasure |
|---|---|---|---|---|
| Cracking | Linear defects in cladding | High carbon equivalent; rapid cooling | Spalling under service load | Increase preheat; reduce cooling rate; use lower carbon consumable |
| Poor bonding | Delamination at interface | Incomplete fusion; surface contamination | Premature failure of cladding | Thorough surface preparation; increase current; verify fusion by UT |
| Hardness variation | Non-uniform HV mapping | Parameter instability; uneven heat input | Uneven wear; premature failure | Stabilize parameters; use multi-wire feed; monitor in real-time |
| Excessive spatter | Surface roughness | Excessive arc voltage; poor shielding | Reduced fatigue life; difficult machining | Optimize voltage; improve shielding gas coverage |
| Inclusion | Non-metallic particles in weld | Contaminated flux/wire; slag entrapment | Stress concentration; crack initiation | Use high-purity consumables; proper slag removal between passes |
Engineering Practice Integration
The study's findings are directly applicable to roll manufacturing and maintenance operations. In practice, blooming mill rolls are typically cladded in multiple passes to achieve a total overlay thickness of 20–50 mm, depending on the roll diameter and expected service life. The optimization parameters identified in this study should be incorporated into the welding procedure specification (WPS) and qualified through weld performance qualification records (PQR) that include hardness mapping, metallographic examination, and mechanical testing of the deposited metal.
A practical consideration highlighted by this study is the importance of the post-weld heat treatment cycle. For high-carbon chromium cladding alloys, a normalizing treatment at 850–950°C followed by controlled air cooling produces a tempered martensitic structure with optimal hardness-toughness balance. Quenching and tempering may be required for higher hardness requirements but increases the risk of cracking if not carefully controlled. The cooling rate during post-weld treatment must be managed to prevent excessive thermal stresses in the thick cladding layer.
Study Insights and Implications
The key insight from this optimization study is that roll cladding quality cannot be achieved by optimizing individual parameters in isolation; rather, it requires a holistic approach that considers the interactions between welding parameters, consumable composition, thermal management, and post-weld treatment. The study demonstrates that systematic parameter optimization, guided by experimental results and metallurgical analysis, can significantly improve the service life of blooming mill rolls. For practitioners, the study reinforces the importance of maintaining detailed welding records, conducting regular metallographic and hardness inspections, and continuously refining the welding procedure based on field performance data. The integration of academic research with industrial application, as exemplified by this study, provides a valuable model for continuous improvement in heavy equipment manufacturing.