Surfacing Repair Process for φ1550 Backup Roll

Literature Overview

This 1997 study by Zhao Hui (Shenyang University of Technology) and Han Zhong (Institute of Metal Research, Chinese Academy of Sciences) presents a systematic approach to the surfacing repair of a φ1550 backup roll in a hot rolling mill. Published in the Journal of Shenyang University of Technology, the paper demonstrates the integration of materials analysis, failure investigation, and welding process design for the restoration of critical rolling mill components.

Component Description and Failure Analysis

Backup Roll Function and Requirements

The φ1550 backup roll is a critical component in a four-high or six-high rolling mill configuration. It supports the work roll and transmits the rolling force to the mill housing. The operating conditions impose demanding requirements:

Failure Modes Observed

The paper analyzes the material, usage conditions, and failure characteristics of the backup roll. Typical failure modes for backup rolls include:

1. Surface wear and indentation: Progressive material loss from the bearing surface due to contact with the work roll and scale.
2. Cracking: Surface-initiated cracks propagating into the roll body, often at stress concentration points.
3. Spalling: Subsurface crack initiation and propagation leading to surface flaking.
4. Galling: Adhesive wear at the work roll interface under high contact stress.

Welding Consumable Selection

Material Selection Criteria

The selection of surfacing consumables for backup roll repair requires careful consideration of multiple competing requirements:

Consumable Selection Rationale

The authors selected submerged arc automatic welding (SAW) as the primary process, which is well-suited for the large, relatively flat bearing surface of a backup roll. The choice of SAW provides:

• High deposition rates for efficient restoration of worn dimensions
• Consistent weld quality through automated parameter control
• Deep penetration with good fusion characteristics
• Reduced operator variability

The specific consumable selection likely involved a multi-layer approach:

1. Transition layer: A low-carbon or medium-carbon alloy to reduce dilution effects and improve weldability
2. Surfacing layer(s): A high-alloy composition providing the required hardness and wear resistance

Welding Process Design

Process Parameters

Post-Weld Heat Treatment

The paper specifies a post-weld heat treatment method, which is critical for backup roll repair due to the high carbon equivalent of the base material and the significant residual stresses developed during welding. The PWHT likely involves:

1. Stress relief annealing: Heating to 550-650°C for 2-4 hours per 25 mm of section thickness, followed by controlled cooling
2. Tempering: If the base material is quenched and tempered, re-tempering at the original tempering temperature to restore core toughness
3. Grinding: Post-PWHT grinding to achieve final dimensional accuracy and surface finish

Quality Control Measures

Inspection Requirements

Critical Control Points

The repair of a backup roll represents a high-consequence welding application where failure can lead to mill downtime, product quality degradation, and safety hazards. Critical control points include:

1. Base material condition assessment: Pre-weld inspection to evaluate the extent of damage and determine if repair is feasible
2. Dilution monitoring: Cross-sectional hardness profiling to ensure the transition between base and weld metal is gradual
3. Residual stress management: Proper PWHT to prevent delayed cracking and ensure dimensional stability
4. Surface integrity: Post-grinding inspection to verify crack-free surface condition

Engineering Practice Implications

This study exemplifies the systematic approach required for critical component repair welding. The integration of materials science (from the Institute of Metal Research) with engineering practice (from Shenyang University of Technology) demonstrates the value of collaborative research in solving industrial welding problems. For engineers involved in rolling mill maintenance, the key lessons include:

• Always begin with thorough failure analysis before selecting repair methodology
• Consumable selection must balance hardness, toughness, and weldability requirements
• Automated welding processes provide superior consistency for large, repetitive surfacing applications
• Post-weld heat treatment is non-negotiable for high-carbon-equivalent repair welding
• Quality control must be comprehensive, covering both surface and subsurface integrity

The paper's methodology can be adapted to other large rotating component repairs, including mill housing repairs, gear ring surfacing, and large shaft restoration, provided the specific material and service conditions are properly evaluated.

Study Insights and Reflections

The backup roll repair study highlights a fundamental principle in engineering repair welding: the goal is not merely to restore dimensions but to restore or exceed the original functional performance of the component. This requires understanding the failure mechanism, selecting appropriate materials and processes, and implementing rigorous quality control. The involvement of a research institute in the consumable selection process underscores that successful repair welding often requires access to materials expertise beyond what is available in a typical maintenance workshop. For modern applications, this approach can be enhanced with advanced NDT techniques such as phased array ultrasonic testing and thermal imaging for residual stress assessment.