Forged Steel Cold Rolled Work Rolls: Q&A
What are the common quality issues in mill roll grinding? How to resolve them?
| Defect | Cause | Resolution Method |
|---|---|---|
| Straight wave (polygonal) | ① Dull or glazed grinding wheel; ② Unbalanced or poorly dressed wheel; ③ Damaged spindle or bearings; ④ Excessive wheel speed; ⑤ Wheel hardness too high. | ① Dress wheel promptly; ② Balance wheel; ③ Eliminate bearing and spindle clearance; ④ Adjust speed; ⑤ Select appropriate hardness. |
| Spiral marks | ① Axial play in grinding wheel spindle; ② Misalignment between wheel and workpiece axes; ③ Uneven wheel surface; ④ Excessive longitudinal feed. | ① Eliminate axial play; ② Align wheel and workpiece axes; ③ Dress wheel; ④ Adjust longitudinal feed. |
| Scratches & scoring | ① Loose abrasive grains between wheel and workpiece; ② Dirty coolant filter. | ① Clean and redress wheel; ② Filter coolant and clean filter. |
| Roll surface burn | ① Excessive grind volume, high wheel speed, low workpiece speed, or deep grind; ② Hard wheel with dull grains causing overheating; ③ Dense or clogged wheel; ④ Large contact area; ⑤ Poor thermal conductivity; ⑥ Inadequate cooling. | ① Increase roll speed, reduce depth; ② Select softer, finer-grain wheel; ③ Maintain wheel sharpness; ④ Reduce contact area; ⑤ Use suitable coolant. |
| Tapered roll body | ① Misaligned headstock and tailstock centers; ② Poor roll axis adjustment; ③ Wheel too soft. | ① Align centers with dial indicator; ② Adjust roll axis to match centerline; ③ Select appropriate wheel hardness. |
| Barrel-shaped roll | ① Vertical misalignment of headstock and tailstock; ② Incorrect roll axis adjustment in vertical plane. | ① Correct center alignment; ② Align roll axis with centerline. |
How to perform failure analysis on mill rolls?
Through roll failure analysis, identifying the root cause of damage is a shared goal between roll manufacturers and cold rolling mills. In cases of peeling, fracture, or surface cracks, users should:
(1) Protect the fracture surface
(2) Collect spalled fragments
(3) Gather and safeguard roll change records, eddy current inspection reports, and related documents
Macro-examination of spalling fractures allows experts to identify crack initiation, propagation, and final fracture zones. Special attention should be paid to the crack origin. Compare with roll change history to check for abnormal records or surface damage, helping identify the spalling cause.
If visual inspection is inconclusive, samples from the crack origin should be analyzed via metallography, acid etching, composition analysis, etc., to assess structural integrity, carbide distribution, and surface damage. This can be done jointly by manufacturer and user, or by a third party.
What are the main causes of mill roll spalling?
Roll spalling causes include:
(1) Surface damage-induced spalling: Caused by rolling incidents or improper handling, leading to thermal shocks, structural changes, and cracking. Most common in high-hardness rolls.
(2) Fatigue-induced spalling: Typical in intermediate and backup rolls; fatigue cracks propagate over long service cycles.
(3) Metallurgical defect-induced spalling: Due to excessive inclusions or poor carbide distribution.
How to identify roll spalling?
Spalling is highly detrimental. Surface-crack-initiated spalling often starts with small cracks that expand obliquely against roll rotation, forming “cat’s tongue” or “rail” patterns before final fracture.
| Type | Typical Features | Main Cause | Improvement Measures |
|---|---|---|---|
| Zoning | Smooth grey fatigue fracture zone | Untreated surface cracks from incidents or grinding | Reduce accidents; improve detection; optimize grinding |
| Conchoidal spalling | Concentric shell-like patterns | Subsurface inclusions acting as crack origins | Use ESR steel; ensure forging ratio; control process |
| Thermal shock | Peeling immediately after incidents | Localized thermal shock (e.g., cobbling) | Prevent rolling incidents |
| End spalling | Ring-shaped hardening layer separation | Excessive residual and operational stress | Reduce stress; improve end design |
| Areal pitting | Localized pitting | High contact fatigue; extended service cycle | Adjust roll cycle; revise technical requirements |
How to prevent spalling?
Prevention requires collaboration:
Manufacturers: Use electroslag remelting (ESR) material, improve carbide distribution, apply careful heat treatment, and conduct thorough inspections.
Users: Operate carefully, establish grinding and incident response procedures, promote surface inspection, and prevent damage.
(1) Reduce rolling incidents
Low-temperature tempered Cr alloy steel rolls are sensitive to thermal shock (e.g., cobbling). Local heating above 800°C decomposes martensite, causing cracks and reduced strength.
(2) Enhance inspection
Surface defects often require non-visual methods:
| Method | Scope | Defects Detected |
|---|---|---|
| Double etching | Surface | Cracks, heat-affected zones, segregation |
| Surface wave | Surface/subsurface | Cracks, pinholes |
| Eddy current | Surface/subsurface | Cracks, pinholes |
| Dye penetrant (PT) | Surface | Open cracks, pinholes |
| Magnetic particle | Surface/near-surface | Cracks |
(3) Manage incident rolls
Implement tracking and inspection procedures for rolls involved in incidents, with follow-up checks for at least three cycles.
(4) Roll grinding
Improper grinding causes burns and cracks. Post-grind inspection is recommended. Standard practice: remove ≥0.1mm per side per grind; remove all damage from incident rolls.
Grinding parameters:
| Process | Feed (mm) | Roll speed (m/min) | Axial feed (mm/min) | Wheel speed (m/min) | Wheel type | Coolant (L/min) |
|---|---|---|---|---|---|---|
| Roughing | <0.02 | 40–60 | 2000–3000 | 1500–1700 | Alumina, 60–100 grit, medium-soft resin | >30 |
| Finishing | <0.005 | 40–60 | 300–2000 | 1500–1700 | Alumina, 60–100 grit, medium-soft resin | >30 |
(5) Management system
Roll life cycle management should cover:
Design → procurement → production → storage → use → maintenance → data analysis → evaluation → redesign → reprocurement.
Key aspects:
a) Annual review and redesign based on performance and production plans.
b) Ensure adequate production time for quality.
c) Assign roll maintenance responsibility; keep historical records.
d) Provide training and technical exchange.
e) Set rolling/grinding cycles and parameters.
f) Monitor material quality and mill stability.
What are double etch and penetrant testing? How are they done?
These are simple, economical methods for grind shops.
Double etch test:
| Step | Action | Notes |
|---|---|---|
| 1 | Clean with methanol | |
| 2 | Etch with HNO₃: methanol (1:3) | |
| 3 | Wipe clean with methanol | |
| 4 | Etch with HCl: methanol (1:1) | |
| 5 | Clean again with methanol | |
| 6 | Inspect: darker areas = defects | |
| 7 | Rust-proof with acetone/oil |
Penetrant testing (PT):
| Step | Action | Notes |
|---|---|---|
| 1 | Clean with gasoline/acetone | |
| 2 | Apply cleaner | |
| 3 | Spray penetrant | |
| 4 | Remove excess after 10–15 min | |
| 5 | Apply developer | |
| 6 | Inspect after 5–10 min; avoid false indications | |
| 7 | Clean and rust-proof |
Grinding procedure for incident rolls
Q&A for forged steel cold rolled work rolls
How to prevent backup roll breakage?
Fractures often occur at the roll body–neck junction on small strip mills. Causes:
(1) Material strength: Low purity, high inclusions, poor forging/heat treatment.
(2) Design/machining: Small fillets, sharp corners, poor finish promote fatigue.
(3) Overloading: Common in small/medium mills.
Solutions: Improve design, reduce stress concentration, increase fillet radius, improve surface finish, select better material, avoid overloading.
How to inspect roll surfaces?
Common methods:
(1) Eddy current: Fast, detects surface/subsurface defects. Requires investment and expertise.
(2) Ultrasonic: Accurate but slower. Suitable for all mill sizes.
(3) Magnetic particle: Detects surface cracks. Low cost; may leave residue.
(4) Dye penetrant: Good for local inspection. Slow; risk of missed defects.
