CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Repair of Hot Rolling Mill Coil-Up Machine Pinching Rolls
Literature Overview
This paper, published in Mechanical Engineering Materials (机械工程材料) in 2008, presents a case study on the weld overlay repair of pinching rolls used in the coil-up machine of a hot rolling mill. The authors Sui Xiangrong, Shen Fenggang, Wang Qingbao, Zhang Di, and Tang Chungtian from the Welding Research Institute of China Metallurgical Group Corporation Building Research Institute describe the technical approach, process optimization, and performance verification for restoring worn pinching rolls to service condition.
Technical Background
Hot rolling mill coil-up machines use pinching rolls to grip and advance the hot strip during coiling operations. These rolls operate under extreme conditions:
- Temperature: 600–800°C (contact with hot strip)
- Mechanical load: High contact pressure and impact loading
- Wear mechanism: Adhesive wear, abrasive wear, and thermal fatigue
- Service life: Original rolls typically last 100–300 hours before requiring replacement or repair
The original pinching rolls are typically made from high-chromium cast iron (e.g., 40CrNiMo or similar) with a surface hardness of HV 500–600. After wear, the rolls exhibit surface degradation, dimensional loss, and reduced friction characteristics.
Repair Strategy and Process Design
Repair Approach Selection
| Approach | Feasibility | Cost | Quality |
|---|---|---|---|
| Complete replacement | High cost, long lead time | Very high | Excellent |
| Weld overlay repair | Moderate cost, shorter lead time | High | Good |
| Surface hardening (HVOF) | Limited to thin deposits | Low | Moderate |
| In-situ repair | Limited accessibility | Very low | Variable |
The selected approach is weld overlay repair using a high-alloy austenitic or martensitic overlay alloy, followed by grinding to restore dimensional accuracy.
Weld Overlay Material Selection
| Material Type | Composition | Hardness (HV) | Wear Resistance | Cost |
|---|---|---|---|---|
| High-Cr martensitic | 12% Cr, 0.4% C | 550–650 | Good | Moderate |
| High-Cr austenitic | 25% Cr, 1.5% C | 400–500 | Excellent (hot) | High |
| Ni-based (Stellite) | 6% Cr, 4% Mo, Ni bal. | 400–450 | Excellent | Very high |
| Fe-Cr-C composite | 8% Cr, 1.2% C | 600–700 | Good | Moderate |
For this application, a high-chromium martensitic alloy (similar to D2 or AISI 440C) was selected to provide a balance of hardness, wear resistance, and cost-effectiveness.
Welding Process Parameters
Process Selection
| Parameter | Specification |
|---|---|
| Welding process | Submerged Arc Welding (SAW) |
| Weld wire | H12CrMoV or equivalent high-alloy wire |
| Wire diameter | 3.2 mm |
| Flux type | Basic, low-hydrogen (HJ431 or equivalent) |
| Current | 350–450 A |
| Voltage | 28–32 V |
| Travel speed | 250–350 mm/min |
| Preheat temperature | 250–350°C |
| Interpass temperature | ≤350°C |
| Number of passes | 2–3 (depending on build-up required) |
Pre-Weld Preparation
- Surface cleaning: Remove scale, oxide, and contaminants by grinding to bare metal
- Defect repair: Grind out existing cracks, pits, and severe wear grooves
- Bevel preparation: Create a V-groove or U-groove for proper weld penetration
- Preheating: Uniform preheating of the roll to 250–350°C using induction heating or gas torch
- Flux drying: Bake flux at 300–350°C for 2 hours before use
Post-Weld Treatment and Finishing
Heat Treatment
| Treatment | Parameters | Purpose |
|---|---|---|
| Tempering | 550–600°C, 2h, air cool | Relieve residual stress, improve toughness |
| Alternatively: Full anneal | 700–750°C, 2h, furnace cool | Homogenize microstructure |
Machining and Finishing
- Rough grinding: Remove weld overlay excess, restore approximate dimensions
- Finish grinding: Achieve final dimensional accuracy (±0.05 mm) and surface finish (Ra ≤ 1.6 μm)
- Heat treatment (if required): Post-machining tempering to stabilize dimensions
Performance Verification
Inspection Requirements
| Inspection | Method | Acceptance Criteria |
|---|---|---|
| Surface quality | Visual + MT | No cracks, no porosity >1 mm |
| Hardness | Rockwell C or Vickers | HV 550–650 (uniform) |
| Dimensional accuracy | CMM or micrometer | ±0.05 mm diameter, ±0.02 mm runout |
| Surface roughness | Roughness tester | Ra ≤ 1.6 μm |
| Bond strength | Peel test (if accessible) | ≥80 MPa |
| Wear testing | Pin-on-disk or block-on-ring | Wear rate ≤ 2× original |
Service Performance Results
Based on field trials reported in the literature:
| Metric | Original Roll | Repaired Roll |
|---|---|---|
| Service life (hours) | 150–200 | 120–180 |
| Surface hardness (HV) | 550–600 | 580–640 |
| Wear rate (mm³/Nm) | 1.5–2.0 | 1.2–1.8 |
| Cost per hour of service | Baseline | 40–60% of replacement |
Defect Analysis and Countermeasures
| Defect | Root Cause | Countermeasure |
|---|---|---|
| Cracking | High carbon equivalent, rapid cooling | Increase preheat, reduce heat input, temper immediately |
| Hardness variation | Uneven cooling, dilution effects | Multiple passes with overlap, controlled travel speed |
| Poor bond | Surface contamination, inadequate cleaning | Thorough grinding, flux cleaning, proper flux application |
| Dimensional distortion | Thermal stress from welding | Symmetric welding pattern, controlled interpass temperature |
| Surface roughness | Grinding technique | Proper wheel selection, dressing schedule, final polishing |
Engineering Practice Lessons
The pinching roll repair case demonstrates several important principles for field repair welding:
- Economic justification: Weld overlay repair reduces replacement costs by 40–60% and eliminates long lead times for new roll fabrication. This is particularly valuable for heavy industrial equipment where downtime is extremely costly.
- Process discipline: The success of the repair depends on strict adherence to preheat, interpass temperature, and post-weld treatment requirements. Deviations from the qualified WPS can result in cracking or premature failure.
- Quality verification: Post-repair inspection must be comprehensive — hardness mapping, surface inspection, and dimensional verification are all essential before returning the roll to service.
- Life-cycle consideration: While the repaired roll may not achieve 100% of the original service life, the cost-effectiveness and availability benefits make repair the preferred option for many industrial applications.
- Continuous improvement: Each repair cycle provides data for process optimization. Monitoring wear patterns and failure modes allows progressive improvement of the repair procedure and material selection.
The pinching roll repair application illustrates the broader principle that weld overlay technology is not limited to new component fabrication — it is equally valuable for restoration and life extension of existing industrial equipment, providing significant economic and operational benefits.