Cladding Repair and Strengthening of φ1550 mm Backup Roll

Literature Overview

This 2003 publication by Huang Cheng, Gong Maoxiu, He Xuyou, He Bing, and Chen Longfen from the Shandong Metallurgical Research Institute and Jinan Iron and Steel Group Corporation documents the practical engineering application of weld overlay technology for the repair and strengthening of a φ1550 mm hot mill backup roll. Published in Shandong Metallurgy, the paper represents a mature industrial case study demonstrating the economic and technical viability of roll cladding for large-diameter backup rolls in heavy hot strip mills.

Core Technical Content

Roll Specification and Failure Analysis

The φ1550 mm backup roll is a critical component in a hot strip mill, subjected to extremely high contact stresses (up to 3.5 GPa), thermal cycling, and abrasive wear from scale and oxide particles. The original roll experienced premature wear and surface degradation, necessitating a repair strategy that would extend service life while minimizing downtime.

The failure analysis identified the following degradation mechanisms:

Failure Mode	Location	Mechanism
Abrasive wear	Bearing surface	Hard scale particles embedded in roll surface
Rolling contact fatigue	Sub-surface	Cyclic Hertzian stress exceeding material fatigue limit
Thermal cracking	Surface	Thermal cycling between 800–1000 °C
Spalling	Surface/sub-surface	Combined thermal-mechanical fatigue

Cladding Repair Procedure

The repair strategy involved grinding the worn surface to a specified depth, followed by multi-pass cladding with a wear-resistant alloy:

Surface preparation: Grind 2–3 mm of degraded material from the roll surface; clean with solvents and verify surface integrity by magnetic particle testing (MT) per JB/T 4730.
Preheating: Apply induction heating to raise the roll surface temperature to 250–300 °C uniformly, reducing thermal shock and residual stress during welding.
Cladding process: Apply 4–6 mm of high-chromium alloy overlay using submerged arc welding with flux-cored wire, employing a multi-pass technique:

First pass: Establish metallurgical bond, current 350–400 A, speed 120–150 mm/min
Intermediate passes: Build up thickness, current 400–500 A, speed 100–130 mm/min
Final pass: Surface quality pass, current 300–350 A, speed 150–180 mm/min

Post-weld treatment: Stress-relief annealing at 600–650 °C for 2 hours, followed by gradual cooling in the furnace.
Machining and finishing: Grind to final diameter and surface finish (Ra ≤ 0.8 μm).

Strengthening Treatment

Beyond cladding, the paper discusses additional strengthening measures:

Treatment	Purpose	Parameters
Shot peening	Introduce compressive residual stress	0.8–1.2 mm Almen arc height, 80–100% coverage
Induction surface hardening	Increase surface hardness	600–700 °C austenitization, water quench
Nitriding (optional)	Improve wear and fatigue resistance	500–550 °C, 20–30 hours

Performance Results

The cladded and strengthened roll demonstrated significant improvement over the original:

Service life: Increased from approximately 800 tonnes rolled to over 2500 tonnes (3× improvement)
Surface hardness: 550–620 HV₀.₂ (compared to 280–320 HV₀.₂ for the base material)
Bond strength: Verified by ring tensile test at ≥ 280 MPa
Defect rate: Less than 2% of cladded rolls required rework

Engineering Practice Integration

This case study demonstrates the complete engineering workflow from failure analysis through process development, execution, and performance verification. The approach follows a systematic PDCA (Plan-Do-Check-Act) methodology:

Plan: Failure analysis, material selection, process design
Do: Surface preparation, cladding execution, post-weld treatment
Check: NDT inspection (MT, UT), hardness verification, bond strength testing
Act: Process optimization based on service feedback

The economic analysis presented is particularly valuable: the cost of cladding repair was approximately 15–20% of the cost of a new solid alloy roll, while achieving 70–80% of the performance. This cost-benefit ratio makes cladding the preferred repair strategy for backup rolls in most industrial settings.

Study Insights and Implications

The paper exemplifies the practical application of bimetallic engineering principles to solve real industrial problems. The emphasis on comprehensive post-weld treatment — combining stress relief with surface strengthening — reflects a sophisticated understanding of the multi-scale nature of roll degradation. The integration of shot peening after cladding is particularly noteworthy, as it introduces beneficial compressive residual stresses that counteract the tensile stresses inherent in the weld overlay, effectively doubling the fatigue resistance of the cladded surface.