Overlay Repair and Heat Treatment of 60CrMnMo Steel Rolling Mill Rolls

Literature Overview

This study by Zhao Hui and colleagues from Shenyang University of Technology and Benxi Iron and Steel Co., Ltd. Rolling Mill Roll Repair Plant, published in Metal Heat Treatment (2009, Vol. 34, No. 4, pp. 82-84), documents a complete engineering solution for the overlay repair of 60CrMnMo steel rolling mill rolls. The case study involves the successful repair of a roll with diameter of 1150 mm using Stellite multipass 224 flux-cored wire and HJ107 flux in a submerged arc welding process. The reported result is a 50% increase in roll service life, demonstrating the economic and technical viability of overlay repair as an alternative to roll replacement.

Substrate Material Analysis

60CrMnMo is a medium-carbon alloy steel widely used for rolling mill rolls due to its excellent combination of:

High hardness after heat treatment (HRC 45-55)
Good toughness and impact resistance
Excellent wear resistance under rolling conditions
Good hardenability for large section sizes
Resistance to thermal fatigue

The typical composition includes 0.55-0.65% C, 1.0-1.3% Cr, 0.8-1.1% Mn, and 0.15-0.25% Mo, providing a balanced matrix with sufficient carbide-forming elements for wear resistance.

Repair Process Design

Process Sequence

The repair process follows a systematic approach:

Preparation: Surface cleaning, crack detection (MT), and groove preparation for damaged areas
Preheating: Controlled preheating to reduce thermal gradients and minimize residual stress
Local repair welding: Filling of localized damage (pitting, spalling) with compatible filler
Full-surface overlay: Submerged arc overlay of the entire roll working surface
Post-weld heat treatment: Controlled tempering to optimize properties and relieve stress
Final inspection: Dimensional check, hardness verification, and surface quality assessment

Welding Material Selection

Component	Material	Key Properties	Function
Overlay wire	Stellite multipass 224 (FCAW)	High Cr, Mo, C content	Wear-resistant surface
Flux	HJ107 (rutile-type)	Good wettability, low hydrogen	Stable arc, clean weld
Repair filler	Matching 60CrMnMo composition	Similar thermal expansion	Damage restoration

Critical Process Parameters

Parameter	Specification	Rationale
Preheat temperature	250-350°C	Reduce cooling rate, prevent cracking
Interpass temperature	200-300°C	Control HAZ transformation
Arc current	500-650 A	Adequate penetration, controlled dilution
Travel speed	300-450 mm/min	Balance deposition rate and cooling rate
Number of overlay passes	3-5	Achieve required thickness (3-5 mm)
Post-weld tempering	600-650°C × 2-4 h	Stress relief, hardness optimization

Heat Treatment Optimization

The post-weld heat treatment is critical for the long-term performance of the overlay-repaired roll:

Purpose of tempering:

Relieve welding residual stresses (typically 200-400 MPa in overlay welds)
Transform any retained austenite or untempered martensite in the overlay
Optimize the hardness of the overlay layer to the target range (HRC 50-58)
Reduce the risk of thermal fatigue cracking during rolling service

Temperature selection rationale:

Below 550°C: Insufficient stress relief, retained stress promotes fatigue failure
600-650°C: Optimal balance of stress relief and hardness retention
Above 700°C: Excessive softening, carbide coarsening, reduced wear life

Engineering Practice and Results

The practical outcome of this repair approach is significant for rolling mill operations:

Service life improvement: 50% increase over standard new rolls
Cost reduction: Overlay repair costs approximately 30-40% of new roll procurement
Downtime reduction: Repair turnaround time is 3-5 days versus 4-8 weeks for new roll supply
Surface quality: Post-grinding surface finish achieves Ra 0.8-1.6 μm, meeting rolling quality requirements

Common Defects and Countermeasures

Defect	Cause	Countermeasure
Overlay spalling	Excessive residual stress, poor adhesion	Proper preheat, controlled cooling, post-weld tempering
Cracking at fusion line	High carbon dilution, HAZ embrittlement	Use of compatible transition, low hydrogen flux
Uneven hardness	Variable dilution, inconsistent process	Process monitoring, consistent travel speed
Surface porosity	Flux moisture, inadequate shielding	Flux drying, gas flow verification
Thermal fatigue cracks	Insufficient toughness, high residual stress	Adequate tempering, stress relief annealing

Study Insights and Implications

This case study exemplifies the practical application of overlay welding technology in heavy industry maintenance. The 50% life extension achieved through overlay repair represents a significant economic benefit—particularly when considering that rolling mill rolls are expensive components with long lead times for replacement.

From a metallurgical perspective, the success of this approach depends on three critical factors: (1) proper welding material selection that matches or exceeds the substrate properties, (2) controlled thermal input to minimize distortion and residual stress, and (3) appropriate post-weld heat treatment to optimize the microstructure and relieve internal stresses.

For rolling mill engineers, this work demonstrates that overlay repair should be considered as a routine maintenance strategy rather than an emergency measure. Establishing a systematic overlay repair program—with standardized procedures, trained personnel, and quality control protocols—can significantly reduce operating costs and improve production reliability. The key is to implement the full process package including proper heat treatment, not merely the welding operation itself.