Overlay Welding Repair and Strengthening of 1550 mm Backup Rolls

Literature Overview

This paper by Huang Cheng and colleagues, published in Shandong Metallurgy in 2003, documents the overlay welding repair and strengthening of 1550 mm backup rolls in a medium plate mill at Jinan Iron and Steel Group. Backup rolls in medium plate mills operate under extreme conditions—high contact stress, thermal cycling, and mechanical impact—and are subject to surface degradation, spalling, and plastic deformation. The paper presents a systematic approach to extending backup roll service life through overlay welding and specialized heat treatment.

Core Technical Content

The study begins with a failure analysis of the backup rolls, identifying the primary degradation mechanisms and then developing a repair strategy that combines overlay welding with post-weld heat treatment.

Failure Analysis

The 1550 mm backup rolls in the medium plate mill experienced the following failure modes:

Surface spalling: Caused by cyclic contact stress exceeding the material's fatigue limit at the subsurface, leading to crack initiation and propagation.
Plastic deformation: Local yielding under excessive rolling force, resulting in loss of roll diameter and surface roughness.
Thermal cracks: Initiated at the roll surface due to rapid heating during hot rolling and subsequent thermal contraction during cooling between passes.
Wear: Abrasive wear from oxide scale transfer and mechanical abrasion from the strip being rolled.

Repair Strategy

Repair Step	Specification	Purpose
Surface preparation	Grind off damaged layer (3–5 mm), bevel edge preparation	Remove degraded material, create weldable geometry
Preheating	250–300 °C	Reduce cooling rate, minimize cold cracking risk
Overlay welding	Submerged arc overlay (SAW), low-crack-tendency wire	Build up surface layer with superior wear and fatigue resistance
Number of overlay layers	2–3 layers	Achieve required surface hardness and thickness
Post-weld heat treatment	Specialized tempering and stress-relief cycle	Optimize microstructure, relieve welding residual stress
Final grinding	Precision grind to dimensional tolerance	Restore geometric accuracy and surface finish

Material and Process Selection

The selection of overlay welding consumables was critical to the repair's success. The authors selected a low-crack-tendency submerged arc welding wire with a composition tailored to provide:

High hardness (target 45–55 HRC) for wear resistance.
Good toughness to resist spalling and impact loading.
Low carbon equivalent to minimize cold cracking susceptibility in the thick-section backup roll.
Compatibility with the base roll material (typically Cr-Mo or Cr-Ni-Mo alloy steel) to prevent intermetallic formation at the weld interface.

The specialized heat treatment cycle was designed to achieve two objectives simultaneously: relieving the high residual stresses induced by overlay welding (which can reach 300–400 MPa in the weld zone) and optimizing the microstructure of the overlay layer for the operating conditions. The heat treatment likely involved a two-stage process: an initial stress-relief anneal at 550–650 °C followed by a tempering cycle to achieve the target hardness while maintaining adequate toughness.

Performance Results

The repaired backup rolls demonstrated a 2.54× improvement in service life compared to new rolls. This remarkable result can be attributed to several factors:

The overlay layer provided a harder, more wear-resistant surface than the original roll material.
The optimized microstructure from the specialized heat treatment improved fatigue resistance.
The stress relief from heat treatment prevented premature spalling.
The precision grinding restored surface quality, reducing rolling force and contact stress.

Engineering Practice Considerations

For engineers managing backup roll maintenance in plate mills, the following considerations are important:

Timing of repair: Overlay welding should be performed before the roll reaches its end-of-life condition. Repairing heavily damaged rolls may require excessive material removal, compromising the roll's structural integrity.
Dimensional control: The overlay build-up must be carefully controlled to maintain the roll's diameter and concentricity within tolerance. Excessive build-up on one side can cause roll imbalance and vibration.
Residual stress management: The thick-section nature of backup rolls (typically 400–600 mm in diameter) makes them particularly susceptible to high welding residual stresses. The specialized heat treatment is not optional—it is essential for preventing delayed cracking and roll distortion.
Inspection protocols: Post-repair inspection should include hardness profiling across the overlay layer, ultrasonic testing for internal defects, and dimensional verification.

Study Insights and Reflections

This case study exemplifies the power of overlay welding as a cost-effective repair strategy for large, heavy-section components. The 2.54× life extension translates to significant economic savings, as backup rolls represent a major capital investment in plate mills. The author notes that the success of this repair hinges on the integrated approach—combining proper consumable selection, controlled welding parameters, and optimized post-weld heat treatment. Each element is necessary; omitting any one would likely result in premature failure. This holistic approach should serve as a model for other heavy-section component repairs in the metallurgical industry, where overlay welding combined with thermal post-treatment can extend component life while reducing downtime and capital expenditure.