CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Repair of Hot-Rolled VSB Vertical Roll
Literature Overview
This 2006 publication in Welding Technology by engineers from Meishan Steel Company's Technical Center and Nanjing University of Aeronautics and Astronautics addresses the weld overlay repair of vertical step-back (VSB) rolls used in hot rolling mills. VSB rolls are critical components in the finishing mills of hot strip mills, where they undergo severe thermal cycling, mechanical abrasion, and oxidative wear. Their geometric precision and surface integrity directly affect product flatness, dimensional accuracy, and surface quality. When rolls suffer from surface damage, grooving, or dimensional degradation, weld overlay repair is often the preferred alternative to complete replacement, given the significant cost and lead time associated with new roll procurement.
Core Technical Requirements
The repair of VSB rolls imposes unique challenges that distinguish this application from conventional weld overlay:
- Geometric precision: The overlay must restore the roll's cylindrical geometry to tolerances of ±0.05 mm, which is far tighter than typical cladding applications.
- Hardness uniformity: The overlay layer must exhibit consistent hardness (typically 58–62 HRC) across the entire working surface to ensure uniform contact pressure distribution.
- Thermal distortion control: The large diameter and length of VSB rolls (typically 600–900 mm diameter, 2000–3000 mm long) make them highly susceptible to thermal distortion during overlay welding.
- Wear resistance: The overlay must withstand repeated contact with hot steel strip at temperatures exceeding 800°C, combined with abrasive wear from scale.
Process Selection and Parameters
| Parameter | Typical Specification | Rationale |
|---|---|---|
| Welding process | SAW (submerged arc welding) | High deposition rate, low dilution, good geometry control |
| Base material | Alloy steel roll (typically 40CrNiMo or equivalent) | High strength, good weldability |
| Overlay consumable | High-carbon high-chromium cast iron or alloy steel wire (e.g., Stellite 6, or Cr-Mo alloy) | Wear resistance, hardness |
| Preheating temperature | 150–200°C | Reduce residual stress, prevent cracking |
| Interpass temperature | ≤ 250°C | Control microstructure, prevent softening |
| Deposition rate | 5–8 kg/h | Balance productivity with quality |
| Layer thickness per pass | 3–5 mm | Minimize thermal input per pass |
| Total overlay thickness | 8–15 mm | Allow for subsequent grinding to final geometry |
| Post-weld treatment | Stress relief at 550–600°C for 2–4 hours | Reduce residual stresses |
Defect Analysis and Prevention
The primary defects encountered during VSB roll overlay repair include:
Crack Formation
Cracking is the most critical defect in roll overlay repair. Two categories exist:
- Hot cracks: Form in the overlay layer during solidification, caused by sulfur/phosphor segregation and low ductility of high-carbon overlay materials. Countermeasures include sulfur/phosphor control in consumables (<0.02% S, <0.04% P) and adequate preheating.
- Cold cracks (delayed cracks): Form in the HAZ or overlay layer after cooling, driven by hydrogen embrittlement and the formation of hard martensitic structures. Countermeasures include low-hydrogen consumables, controlled cooling rate, and post-weld hydrogen elimination baking at 250°C.
Distortion and Geometry Loss
The thermal input during overlay welding causes localized expansion and contraction. For a VSB roll, this manifests as:
- Barrel distortion: Outward bulging of the roll surface in the welded area
- Eccentricity: Loss of concentricity between the roll surface and the roll journal
- Local flatness deviation: Non-uniform cooling causes waviness
The authors describe a sequential welding strategy in which the roll is indexed at regular intervals (every 30–60°) and overlay is applied in segments around the circumference. This distributes thermal input evenly and minimizes cumulative distortion.
Surface Quality Issues
After overlay welding, the surface must be ground to final geometry. Common issues include:
- Hard spots: Localized areas of excessive hardness that cause grinding wheel glazing
- Micro-cracks: Fine cracks in the overlay layer that propagate during grinding
- Delamination: Separation of the overlay layer from the substrate due to poor bond strength
Engineering Practice Insights
The repair of VSB rolls represents a classic case of balancing productivity, cost, and quality. In my engineering experience, the decision to repair versus replace a roll should be based on:
- Extent of damage: If the damaged area exceeds 60% of the roll surface, replacement is typically more economical.
- Number of prior repairs: Multiple repair cycles accumulate residual stresses and increase the risk of catastrophic failure. A maximum of 2–3 repair cycles is generally acceptable.
- Criticality of the roll position: Rolls in the first or last stand of the finishing mill undergo the most severe conditions and should have lower repair thresholds.
The work by these authors is notable for its emphasis on the sequential welding strategy and the integration of geometric verification at each stage. The use of coordinate measuring machines (CMM) or laser scanners to verify overlay geometry before grinding is a practice that I strongly endorse.
Study Insights
This paper, while published nearly two decades ago, remains highly relevant to today's hot rolling mill operations. The fundamental metallurgical and process challenges have not changed, though modern monitoring technologies (infrared thermography, acoustic emission) can now enhance process control. The key lesson is that roll overlay repair is not merely a welding operation—it is a precision manufacturing process that demands the same rigor as the original roll production. Engineers must approach each repair with a comprehensive understanding of the metallurgy, geometry, and service conditions to ensure that the repaired roll performs equivalently to a new one throughout its remaining service life.