Weld Overlay Repair and Strengthening of a 1550 mm Backup Roll

Literature Overview

This 2003 study by Huang Cheng, Gong Maoxiu, He Xuyou, He Bing, and Chen Longfen from Shandong Metallurgical Research Institute and Jinan Iron and Steel Group addresses the practical challenge of restoring a large-diameter (φ1550 mm) hot strip mill backup roll through weld overlay repair and subsequent strengthening. Backup rolls in hot rolling mills endure extreme thermal loads, mechanical contact stresses from the work roll, and abrasive wear from scale and oxide debris. Their service life directly impacts mill productivity and product quality, making efficient repair strategies economically critical.

Technical Background and Service Conditions

A φ1550 mm backup roll operates under conditions that impose severe demands on the roll surface material. The roll surface experiences temperatures exceeding 900–1100 °C during hot rolling operations, contact stresses of 2000–3500 MPa at the roll contact zone, and continuous abrasive action from iron oxide scale. The typical failure modes include surface spalling, indentation, thermal cracking, and excessive diameter wear. When these defects accumulate, the roll must be either repaired or replaced, with repair being far more economical for large-diameter rolls.

The repair strategy involves removing damaged material through grinding or machining, applying a high-strength overlay layer to restore the roll diameter and improve surface properties, and then performing post-weld heat treatment or surface hardening to optimize the mechanical properties of the overlay.

Repair Process Description

Process Step	Method	Parameters	Purpose
Surface preparation	Grinding / machining	Remove 5–15 mm of damaged material	Eliminate cracks, spalling, and oxidation
Preheating	Induction or furnace	200–350 °C	Reduce residual stress, prevent cold cracking
Overlay welding	Submerged arc welding (SAW) or MMA	Multiple passes, low heat input	Build up diameter, apply hardfacing alloy
Post-weld treatment	Controlled cooling or tempering	550–650 °C for 2–4 h	Relieve residual stress, stabilize microstructure
Final machining	Grinding to final dimensions	Surface roughness Ra ≤ 0.8 μm	Restore geometric accuracy
Surface hardening	Induction hardening or shot peening	Case depth 2–5 mm	Improve surface hardness and fatigue resistance

The overlay alloy selection is critical for backup roll repair. High-chromium white cast iron alloys (such as Cr18-Cr25 type) or nickel-based alloys are commonly used, providing hardness in the range of 50–60 HRC while maintaining adequate toughness to resist spalling under impact loading.

Microstructure and Performance Analysis

The overlay microstructure typically consists of a matrix of martensite or austenite-ferrite with dispersed carbides. The hardness distribution through the overlay cross-section is not uniform; the surface layers, which undergo the highest cooling rates, tend to develop finer microstructures and higher hardness, while deeper layers show coarser structures due to the thermal influence of previously deposited passes.

The strengthening treatment—whether induction hardening or shot peening—introduces compressive residual stresses at the surface, which counteract the tensile stresses imposed during rolling service. This compressive stress layer is particularly effective in delaying the initiation of surface fatigue cracks, extending the roll's service life between repairs.

Key Technical Challenges

Dilution control: The base roll material (typically low-alloy steel or alloy steel) dilutes into the overlay, reducing the effective alloy content. Multi-pass welding with careful parameter control is essential to maintain overlay composition.
Residual stress management: The large diameter and thick section of the backup roll create significant thermal gradients during welding, leading to high residual stresses that can cause cracking or distortion. Preheating and post-weld heat treatment are mandatory.
Dimensional accuracy: After overlay welding, the roll must be ground back to precise diameter and runout tolerances (typically ≤ 0.02 mm TIR). Excessive overlay thickness increases machining time and cost.
Thermal fatigue resistance: The overlay must withstand repeated thermal cycling between ambient temperature and 1000+ °C. This requires careful selection of alloy composition to balance hardness (for wear resistance) with thermal shock resistance.

Engineering Practice and Economic Considerations

For a φ1550 mm backup roll, the economic life extension achieved through weld overlay repair can be substantial. A typical roll may undergo 3–5 repair cycles before complete replacement is necessary. Each repair cycle involves overlaying 3–8 mm of material, with the total overlay thickness per cycle kept within limits to avoid excessive hardness gradient issues.

The study's findings contribute to the development of standardized repair procedures for large-diameter mill rolls, providing parameter recommendations that can be adapted to specific roll geometries and service conditions. The integration of surface strengthening after overlay welding represents a synergistic approach that combines the benefits of alloy composition (wear resistance) with surface engineering (fatigue resistance).

Concluding Remarks

This literature demonstrates the practical application of weld overlay technology in heavy industrial equipment repair, specifically for large-diameter hot mill backup rolls. The systematic approach of overlay repair combined with post-weld strengthening offers a cost-effective solution for extending roll life while maintaining rolling mill productivity. The technical insights regarding dilution control, residual stress management, and surface hardening provide a comprehensive framework for developing reliable repair procedures for similar heavy machinery components in the metallurgical and mining industries.