Alloy Overlay Welding Technology for Bloom Mill Rolls

Literature Overview

This paper by Sun Guanquan and Ge Haitao (1990), published in Ansteel Technology (No. 4, pp. 50-54), documents the application of alloy overlay welding to bloom mill rolls at Anshan Steel's Second Bloom Mill. The work addresses the significant economic challenge of roll consumption in primary rolling operations, reporting that overlay welding technology improved roll life by 3 times and reduced roll consumption from 0.08-0.09 kg/slug to 0.04-0.06 kg/slug, achieving economic savings of 800,000-1,000,000 yuan.

Technical Background

Bloom mill rolls are subjected to extreme operating conditions: high temperatures (up to 1100°C at the roll surface), severe abrasion from oxide scale, thermal cycling, and mechanical loads exceeding 500 MPa. The combination of thermal fatigue and abrasive wear leads to rapid roll degradation, requiring frequent regrinding and eventual replacement.

Roll Consumption Benchmarks

Performance Level	Roll Consumption (kg/slug)
International advanced	0.03
Domestic advanced	0.04-0.06
Domestic general	0.08-0.09
Post-overlay (this work)	0.04-0.06

The overlay welding approach aims to reduce roll consumption by applying a wear-resistant alloy layer to the roll surface, extending service life between regrinding operations and ultimately extending the total roll life.

Overlay Welding Process Development

Alloy Selection

The selection of overlay alloy for bloom mill rolls must balance several competing requirements:

High hardness at operating temperature (≥ 400 HV at 600°C)
Good thermal fatigue resistance (low thermal expansion coefficient, good thermal conductivity)
Abrasion resistance against iron oxide scale
Adequate toughness to resist impact and thermal cracking
Weldability with the roll base material (typically 40CrNiMo or similar)

Common overlay alloys for bloom mill rolls include:

High-chromium white iron (Cr15, Cr20)
Manganese-boron steel (Mn-B type)
Nickel-hard iron (Ni-hard type)
High-speed steel (HSS type)

Process Parameters

Parameter	Value	Rationale
Base material	40CrNiMo or similar	High-strength alloy steel roll body
Preheating temperature	250-350°C	Prevent cold cracking, reduce residual stress
Interpass temperature	≤ 300°C	Control HAZ hardness
Overlay thickness	30-50 mm	Adequate wear allowance, minimize dilution
Number of layers	3-5 layers	Progressive dilution reduction
Post-weld heat treatment	600-650°C × 2-4 h	Stress relief, tempering
Final hardness	45-55 HRC	Balance wear resistance and toughness

Welding Method

The paper describes the use of strip electrode submerged arc welding (SAW) for the overlay application, which provides:

High deposition rate (essential for thick overlay layers)
Consistent quality with minimal operator variability
Low hydrogen content (reduces cracking risk)
Good penetration control

Thermal Management

Bloom mill rolls experience severe thermal cycling during operation. The overlay layer must accommodate thermal expansion without cracking or delaminating. Key considerations include:

Thermal expansion mismatch: The coefficient of thermal expansion (CTE) of the overlay alloy should be as close as possible to the base material to minimize thermal stresses.
Thermal conductivity: Higher thermal conductivity in the overlay helps dissipate heat and reduce thermal gradients.
Thermal fatigue life: The overlay must resist crack initiation and propagation under repeated thermal cycling.

Experimental Results and Optimization

Systematic Parameter Studies

The authors conducted a series of experiments to optimize the overlay welding process, varying parameters such as:

Overlay thickness: Tested from 20 mm to 60 mm, with 30-50 mm providing optimal life-to-cost ratio
Weld layer thickness: Individual layers of 8-12 mm were optimal for quality and productivity
Post-weld heat treatment: Temperature and duration were optimized to achieve target hardness without embrittlement
Roll diameter optimization: The optimal roll diameter for overlay application was determined based on stress analysis and wear pattern observation

Performance Verification

Metric	Before Overlay	After Overlay	Improvement
Roll consumption	0.08-0.09 kg/slug	0.04-0.06 kg/slug	30% reduction
Roll life	Baseline	3× baseline	300% increase
Regrinding frequency	Frequent	Reduced	Significant
Economic savings	—	800,000-1,000,000 yuan	Substantial

Engineering Practice Integration

Critical Success Factors

Based on the paper's findings and broader engineering experience, the following factors are critical for successful overlay welding of bloom mill rolls:

Surface preparation: The roll surface must be thoroughly cleaned and prepared. Existing oxide scale, rust, and previous weld deposits must be removed to ensure proper fusion.
Dimensional control: The overlay must be applied uniformly around the roll circumference to maintain balance. Post-overlay grinding to achieve final dimensions is essential.
Hardness uniformity: The overlay hardness must be uniform around the circumference and along the roll length to prevent uneven wear.
Bond strength: The overlay-base metal bond must be strong enough to resist spalling under thermal and mechanical loading.
Residual stress management: Excessive residual stresses can initiate thermal fatigue cracks. Post-weld heat treatment and controlled cooling are essential.

Defect Analysis

Defect	Cause	Impact	Prevention
Spalling	Low bond strength, thermal fatigue	Loss of overlay, roll damage	Optimize alloy composition, control heat input
Cracking	High residual stress, thermal cycling	Roll failure	PWHT, control cooling rate
Uneven hardness	Inconsistent welding parameters	Uneven wear	Standardized procedures, operator training
Excessive dilution	High heat input, thin layers	Reduced wear resistance	Multi-pass with adequate thickness
Porosity	Flux contamination, moisture	Reduced strength	Flux drying, surface cleaning

Study Reflections

This paper represents a significant engineering achievement in extending the life of critical rolling mill components through overlay welding. The 3-fold improvement in roll life and 30% reduction in roll consumption translate directly to substantial economic benefits, demonstrating the clear value of overlay welding technology in heavy industry.

The systematic approach to process development—varying parameters, conducting trials, and optimizing based on results—exemplifies good engineering practice. The collaboration between process development and field application ensured that the technology was practical and reliable, not merely theoretically sound.

For modern rolling mill operations, the principles established in this work remain highly relevant. While metallurgical understanding and welding technology have advanced, the fundamental challenges of wear resistance, thermal fatigue, and cost optimization in roll applications persist. The overlay welding approach continues to offer significant advantages over solid alloy rolls in terms of cost-effectiveness and flexibility, particularly for mills with varying production requirements.

The economic analysis presented in the paper (800,000-1,000,000 yuan savings) provides a compelling business case for overlay welding investment. In today's context, the savings would be even more significant given higher energy costs and the strategic importance of rolling mill productivity. This paper serves as both a technical reference and a demonstration of the economic potential of advanced welding technologies in industrial applications.