Cladding Repair and Reuse of 1800x2740mm Medium Plate Hot Rolling Backup Roller Study Note

Literature Overview

This study note addresses the cladding repair and reuse of a Φ1800 mm × 2740 mm medium plate hot rolling backup roller. Backup rollers in medium plate mills experience extreme loads (typically 200-500 kN per meter of roller length), thermal cycling (surface temperatures of 400-600 °C), and abrasive contact with scale-laden strip. The literature examines the condition assessment, repair strategy, overlay process, and performance validation of the refurbished roller.

Original Roller Condition Assessment

Failure Analysis

The backup roller exhibited the following damage modes:

Surface wear: Uniform wear of 8-15 mm across the working surface
Surface cracking: Multiple radial and circumferential cracks up to 3 mm deep
Indentation: Localized depressions from scale adhesion and impact loading
Thermal fatigue: Fine crack network in the outer 5-10 mm of the surface

The original roller was a forged carbon steel roller (typically 42CrMo or similar) with a case-hardened surface layer. After 12-18 months of service, the surface layer was completely worn through, exposing the softer core material to rapid wear and cracking.

Condition Assessment Methods

Assessment Method	Purpose	Findings
Visual inspection	Surface damage extent	8-15 mm wear, multiple cracks
UT thickness measurement	Remaining core thickness	Adequate (minimum 120 mm)
MT/PT inspection	Surface and near-surface cracks	Cracks up to 3 mm deep
Hardness profile	Case depth and core hardness	Case worn through, core 250 HB
Metallographic examination	Microstructure assessment	Decarburized surface, no internal defects
Dimensional measurement	Runout and taper	Within acceptable limits

Repair Strategy Development

Repair Approach Comparison

Approach	Advantages	Disadvantages	Applicability
Complete replacement	New roller, full life	High cost, long lead time	Core damaged
Machining + overlay	Cost-effective, fast	Limited by remaining core	Surface damage only
Induction hardening	Surface hardness, no overlay	Limited depth, no alloying	Minor wear
Overlay + induction hardening	Maximum surface properties	Complex process, higher cost	Severe wear

The selected strategy was machining to remove damaged material followed by multi-pass overlay welding with a wear-resistant alloy, followed by post-weld stress relief and final machining to dimensional specifications.

Repair Procedure

Machining: Removal of 20-25 mm of worn and cracked surface material to expose sound core material
Surface preparation: Grinding to remove residual scale and contaminants
Preheat: Heating to 200-250 °C to prevent cracking
Overlay welding: Multi-pass SAW overlay with high-carbon martensitic alloy
Post-weld stress relief: 550-600 °C for 2 hours per 25 mm thickness
Final machining: Grinding to final diameter and surface finish
Final inspection: UT, MT, hardness testing, dimensional verification

Overlay Material and Process Selection

Overlay Material Selection

For backup roller applications, the overlay material must provide:

High hardness (HRC 55-62) for wear resistance
Good toughness to resist impact from scale and strip
Thermal stability at 400-600 °C operating temperatures
Resistance to thermal cracking

The selected overlay material was a high-carbon martensitic alloy with the following composition:

Component	Range (wt%)
C	1.0-1.5
Cr	10-14
Mo	2-4
V	1-3
Mn	0.5-1.5
Ni	0.5-2.0

This composition produces a microstructure of martensite with dispersed M7C3 and M23C6 carbides, providing excellent wear resistance and adequate toughness.

Overlay Process Parameters

Parameter	Value	Notes
Process	SAW (submerged arc)	High deposition rate
Current	600-800 A	DC
Voltage	32-40 V
Travel speed	250-400 mm/min
Wire diameter	4.0 mm
Preheat	200-250 °C
Interpass temperature	250-350 °C
Number of passes	3-4	Build up to required thickness
Overlay thickness	15-20 mm
Post-weld treatment	580 °C, 4 h	Stress relief

Performance Validation

Post-Repair Testing Results

Test	Specification	Result	Status
Hardness (overlay)	HRC 55-62	HRC 58-60	Pass
Hardness (core)	HRC 28-35	HRC 30-32	Pass
UT bond strength	No separation	No separation	Pass
MT surface	No cracks	No cracks	Pass
Diameter tolerance	±0.5 mm	±0.3 mm	Pass
Runout	<0.05 mm	<0.03 mm	Pass

Field Performance

After reinstallation, the repaired roller achieved a service life of 14 months before the next maintenance interval, compared to the original 12-18 months for a new roller. The wear rate was measured at 0.12 mm/month, with the overlay maintaining hardness above HRC 55 throughout the service period. No cracking or spalling was observed, confirming the adequacy of the overlay material and process selection.

Study Insights and Reflections

The cladding repair and reuse of the 1800×2740 mm backup roller demonstrates that major capital equipment can be economically refurbished through advanced overlay welding techniques. The study reinforces several key principles: first, thorough condition assessment is essential before committing to a repair strategy; second, the overlay material must be selected based on the specific failure mode of the original roller, not merely on hardness values; and third, the post-weld stress relief treatment is critical for preventing delayed cracking in high-carbon martensitic overlay welds. The economic analysis showed that the repair cost was approximately 35% of a new roller, with a comparable service life, representing a significant cost saving. However, the study also highlights that the repair approach is only viable when the core material remains sound; if internal defects or excessive core wear are present, complete replacement is the only safe option. The successful outcome of this repair validates the technical approach and provides a template for similar roller refurbishment projects across the industry.