Overlay Welding Repair Process for phi1550 Backup Roll

Literature Overview

This study, authored by Zhao Hui of Shenyang University of Technology and Han Zhong of the Institute of Metal Research, Chinese Academy of Sciences, and published in the Journal of Shenyang University of Technology (1997, Vol. 19, Issue 5, pp. 101-103), presents a systematic approach to the overlay welding repair of a phi1550 backup roll. The paper integrates materials analysis, failure assessment, filler metal selection, welding process design, and post-weld heat treatment into a comprehensive repair methodology. The collaborative approach between academic research and industrial application exemplifies best practices in engineering metallurgy.

Background and Failure Analysis

Backup rolls in hot strip rolling mills are subjected to extreme combined loading conditions including high contact stresses, thermal cycling, and mechanical impact. The phi1550 backup roll in question experienced service degradation that necessitated repair rather than replacement. The paper begins with a thorough analysis of the roll material composition, service conditions, and failure modes, establishing a scientific basis for the repair strategy.

The failure analysis methodology employed follows a logical progression:

Material characterization: Examination of the original roll material composition, microstructure, and mechanical properties to understand the baseline metallurgical condition.
Failure mode identification: Determination of whether the damage was due to surface wear, fatigue cracking, plastic deformation, thermal damage, or a combination of mechanisms.
Repair feasibility assessment: Evaluation of whether overlay welding could restore the roll to functional specifications while maintaining or improving performance.

Filler Metal Selection and Welding Process Design

The selection of filler metal for backup roll repair is governed by several critical requirements:

Requirement	Rationale	Selection Criteria
Hardness matching	Must resist contact pressure and wear	High-carbon or alloyed composition
Thermal compatibility	Must withstand thermal cycling without cracking	Appropriate CTE matching
Bond strength	Must resist spalling under impact loading	Good metallurgical bonding
Processability	Must be suitable for automatic welding	Consistent arc stability and deposition rate
Post-weld treatment compatibility	Must respond favorably to heat treatment	Hardenable or temper-stable composition

The authors selected submerged arc automatic welding (SAW) as the primary welding process, which is well-suited for overlay welding applications requiring high deposition rates, consistent quality, and minimal operator variability. The submerged flux provides excellent protection against atmospheric contamination and contributes to favorable weld metal composition through alloying element transfer.

The welding process parameters would typically include:

Welding current: Optimized for adequate penetration and deposition rate without excessive dilution.
Welding voltage: Controlled to achieve proper arc stability and bead profile.
Welding speed: Balanced between deposition efficiency and heat input management.
Flux type: Selected for appropriate alloying contribution and slag properties.
Number of passes: Designed to achieve required overlay thickness with proper interpass conditions.

Post-Weld Heat Treatment Strategy

The post-weld heat treatment is a critical element of the repair process, serving multiple purposes:

Residual stress relief: Reduction of welding-induced tensile stresses that could initiate fatigue or brittle fracture.
Microstructural homogenization: Equalization of microstructural differences between the weld overlay, heat-affected zone, and base metal.
Hardness optimization: Achievement of target hardness profile through controlled tempering or normalizing.
Crack prevention: Elimination of hydrogen-induced cracking susceptibility in the heat-affected zone.

The specific heat treatment parameters would be determined based on the base metal composition, weld metal composition, section thickness, and required mechanical properties. For a roll of this diameter, the thermal mass is substantial, and the heat treatment must be designed to achieve uniform temperature distribution throughout the cross-section.

Integration with Engineering Practice

The repair of large rolling mill components through overlay welding offers significant economic advantages over replacement, particularly for expensive backup rolls where manufacturing lead times are long. However, the repair must be executed with rigorous quality control to ensure that the repaired roll performs equivalently to a new roll throughout its service life.

Quality assurance measures should include:

Pre-repair dimensional inspection and mapping of damage areas.
Visual examination and non-destructive testing of the prepared surface.
In-process monitoring of welding parameters and bead appearance.
Post-weld dimensional verification to confirm restoration to specified geometry.
Hardness testing across the overlay layer to verify uniformity and adhesion.
Final surface finish verification to meet rolling mill operational requirements.

Key Questions and Reflections

This paper exemplifies the rigorous engineering approach required for critical component repair. The systematic progression from materials analysis through process design to quality verification reflects mature engineering practice. A particularly valuable aspect is the collaboration between academic researchers and industrial practitioners, which ensures that the repair methodology is both scientifically sound and practically implementable.

The use of submerged arc welding for this application highlights the advantages of automated processes in achieving consistent weld quality on large cylindrical geometries. The circular geometry of the backup roll presents unique challenges in terms of weld seam continuity, geometric distortion control, and uniform deposition, all of which are addressed through careful process design.

Summary

The overlay welding repair of the phi1550 backup roll demonstrates that systematic materials analysis, rational filler metal selection, and well-designed welding and heat treatment processes can effectively restore critical rolling mill components to full service capability. The collaborative research approach and rigorous quality control methodology presented in this paper provide a valuable template for similar repair applications in heavy industrial equipment. The successful application of submerged arc automatic welding for this purpose underscores the importance of process selection in achieving reliable repair outcomes for large, safety-critical components.