Alloy Overlay Welding Technology for Roughing Mill Rolls

Literature Context and Background

This 1990 publication by Sun Guanquan and Ge Haitao from Ansteel Second Roughing Mill documents the development and implementation of alloy overlay welding technology for roughing mill rolls. Roughing mill rolls operate under extremely severe conditions: high contact stresses, abrasive wear from hot steel, thermal cycling, and impact loading. The base roll material (typically medium-carbon chromium steel or cast steel) provides adequate toughness but insufficient surface hardness for extended service life. Overlay welding with hardfacing alloys addresses this limitation by creating a wear-resistant surface layer while maintaining the tough core.

The work represents early Chinese industrial practice in applying weld overlay technology to heavy machinery components, and its findings remain relevant for roll refurbishment programs in modern steel mills.

Technical Approach and Process Design

Base Material and Surface Preparation

Roughing mill rolls typically have a core material of 40Cr or 45Cr steel, with a pre-existing case-hardened surface from induction hardening or flame hardening. Before overlay welding, the existing hardened layer must be removed by grinding or machining to a depth of 2–3 mm to ensure proper fusion of the overlay with the ductile base material. The surface is then cleaned to remove oil, rust, and scale.

Process Parameter	Specification
Base material	40Cr / 45Cr steel
Surface preparation	Grind to remove 2–3 mm hardened layer
Preheat temperature	200–250 °C
Interpass temperature	≤300 °C
Post-weld treatment	Tempering at 550–600 °C × 2 h
Overlay thickness	6–12 mm
Number of layers	3–5 passes

Selection of Overlay Alloys

The authors evaluated several hardfacing alloy systems:

High-speed steel type (HSS) – containing 4–6% C, 10–14% W, 4–5% Cr, 4–8% V; provides excellent wear resistance at elevated temperatures but is prone to cracking due to high hardenability.
Cobalt-base alloy – Co-Cr-W type; offers superior hot hardness and thermal fatigue resistance but at higher material cost.
High-carbon chromium iron – 2–3% C, 12–15% Cr; economical and provides good abrasive wear resistance but limited thermal shock resistance.

For roughing mill service, the authors recommended a multi-layer approach: a transition layer of medium-carbon austenitic stainless steel (such as E309L) followed by 2–3 layers of HSS-type or cobalt-base hardfacing alloy. This approach minimizes cracking while achieving the required surface hardness of HRC 55–65.

Welding Process Selection

The study compared submerged arc welding (SAW) and manual metal arc welding (SMAW) for roll overlay:

SAW was preferred for large flat or cylindrical surfaces due to higher deposition rates (up to 3–5 kg/h) and consistent quality.
SMAW was used for repair of localized damage and for areas inaccessible to automated equipment.

The welding sequence for cylindrical rolls followed a spiral pattern to distribute heat uniformly around the circumference, minimizing radial cracking and distortion.

Defect Analysis and Countermeasures

Common Defects Encountered

Defect Type	Cause	Countermeasure
Cracking in overlay	High hardenability, excessive cooling rate	Increase preheat, use preheated backing plate
Cracking at interface	Dilution, CTE mismatch	Use compositionally compatible transition layer
Porosity	Moisture in flux/electrode	Bake electrodes at 250 °C × 2 h
Incomplete fusion	Insufficient heat input	Increase current, optimize travel speed
Excessive dilution	Too low deposition rate	Use larger wire diameter, increase voltage

Quality Verification

The overlay quality was verified through:

Hardness testing at multiple depths to confirm hardness profile (HRC 55–65 in overlay, gradual transition to base hardness)
Metallographic examination for cracks, porosity, and unmelted inclusions
Impact testing of the base material beneath the overlay to confirm that tempering did not degrade toughness below acceptable limits
Visual inspection for surface defects and undercut

Engineering Practice and Service Performance

The Ansteel Second Roughing Mill reported that overlay-welded rolls achieved 2–3 times the service life of conventionally case-hardened rolls, with significant reduction in rolling force due to the smoother surface finish achievable on the overlay. The rolls were periodically re-ground to maintain surface profile, and the overlay thickness provided sufficient material for multiple regrinding cycles before replacement.

A key operational insight from the study is that the overlay must be tempered after welding to reduce residual stresses and prevent delayed cracking. The tempering temperature (550–600 °C) is selected to reduce overlay hardness slightly (from HRC 62–65 as-welded to HRC 55–58 after tempering) while significantly improving toughness and crack resistance.

Study Insights and Implications

This work demonstrates the practical application of hardfacing technology in heavy industrial environments where component availability and downtime costs are critical concerns. The emphasis on multi-layer welding with transition layers reflects a pragmatic approach to managing metallurgical incompatibility. The study also highlights an important economic consideration: overlay welding extends roll life at a fraction of the cost of replacement, making it a preferred maintenance strategy for large, expensive components.

The 1990 timeframe places this work at the beginning of systematic hardfacing application in Chinese steel mills. Subsequent developments in flux-cored arc welding (FCAW) and plasma transferred arc (PTA) welding have improved productivity and consistency, but the fundamental process design principles documented here remain valid. Engineers involved in roll refurbishment programs should reference this work for its practical process parameters, defect countermeasures, and service performance data, which represent hard-won industrial experience.