Surfacing Technology Summary for 750 Slabbing Mill Rolls

Literature Overview

The 1990 paper published in Shandong Metallurgy (Vol. 12, Issue 3) presents a technical summary of the surfacing technology applied to 750 mm slabbing mill rolls. While the paper is relatively brief (pages 51-54), it represents early Chinese industrial practice in roll refurbishment technology. The classification codes TG333.17 (metal forming equipment) and TG455 (welding process applications) indicate the intersection of metallurgical equipment and welding technology that defines roll refurbishment.

Technical Context and Service Requirements

Slabbing mill rolls operate under extreme conditions: high contact pressure (up to 200-400 MPa), severe thermal cycling (surface temperatures of 800-1200°C during rolling), abrasive contact with scale and oxide, and cyclic bending loads from the rolling force. The 750 mm designation refers to the roll diameter, which is characteristic of medium-to-large slabbing mills producing wide slab for subsequent rolling operations.

The surfacing of slabbing mill rolls serves multiple purposes:

• Restoration of worn roll dimensions to specification
• Application of wear-resistant overlay to extend service life
• Repair of surface defects (pitting, scoring, thermal cracks)
• Improvement of roll surface properties for product quality

Surfacing Process Selection

For 750 mm slabbing mill rolls, submerged arc welding (SAW) or electroslag surfacing would be the most practical primary methods, given the large surface area and the need for high deposition rates. SMAW would be used for touch-up and repair of localized defects.

Surfacing Material Selection

The selection of surfacing alloy for slabbing mill rolls depends on the specific service conditions:

For slabbing mill service, high-chromium cast iron or high-speed steel type surfacing alloys are most commonly specified. The high-chromium alloys provide excellent wear resistance through Cr₇C₃ carbide formation, while the high-speed steel types offer superior performance at elevated temperatures.

Process Parameters and Quality Control

Submerged Arc Surfacing Parameters (Typical)

Quality Control Requirements

• Visual inspection of each pass for porosity, undercut, and excessive spatter
• Magnetic particle testing (MT) of each layer for cracks and lack of fusion
• Hardness verification at multiple locations on the finished surface
• Dimensional inspection after machining to verify roundness and taper
• Penetrant testing (PT) of final machined surface for surface-breaking defects

Distortion and Residual Stress Control

Surfacing large-diameter rolls introduces significant challenges related to distortion and residual stress:

1. Circumferential distortion: Non-uniform heat input around the roll circumference can cause ovality. This is controlled by maintaining consistent travel speed and ensuring the roll is properly supported.
2. Axial distortion: Differential expansion along the roll length can cause bowing. Symmetric welding sequence from both ends toward the center, or from center outward, helps minimize this.
3. Residual stress management: The compressive residual stresses in the surfacing layer are beneficial for fatigue life, but excessive tensile stresses in the roll body can be detrimental. Post-weld stress relief at 550-650°C is typically applied, though this must be compatible with the surfacing alloy's temper stability.

Engineering Practice Integration

The 1990 publication date places this work in an era when Chinese metallurgical equipment manufacturing was rapidly developing. The 750 mm slabbing mill represents a significant capacity investment, and the surfacing technology described would have been critical for maintaining production availability through roll refurbishment cycles.

In modern practice, the following advances have been incorporated into slabbing mill roll surfacing:

• Automated welding systems with closed-loop parameter control
• Real-time monitoring of welding parameters and deposition rate
• Advanced surfacing alloys with improved hot hardness and thermal fatigue resistance
• Computational simulation of residual stress and distortion for process optimization
• Non-destructive testing with phased array ultrasonics for subsurface defect detection

Key Reflections

The relatively brief nature of this paper suggests it was written as a practical technical summary rather than a research publication. This format is actually quite valuable for practicing engineers who need actionable information rather than theoretical discussion. The focus on practical process parameters and quality control measures reflects the applied nature of roll refurbishment technology.

A significant challenge in roll surfacing that the paper likely addresses is the transition zone between the surfacing layer and the base metal. The metallurgical compatibility between the base roll steel (typically 45# or 50# steel, or alloy grades like 5CrMo) and the surfacing alloy determines the bond strength and the resistance to spalling. Proper dilution control is essential—too much dilution reduces the hardness and wear resistance of the overlay, while too little dilution can create a brittle interface prone to delamination.

The economics of roll refurbishment versus replacement is a critical consideration. Surfacing typically costs 30-50% of new roll cost while extending service life by 2-3 times. However, the quality of the refurbishment directly impacts this economic advantage—poor surfacing quality leads to premature failure and negates the cost savings.

Summary

This paper documents the practical application of surfacing technology to 750 mm slabbing mill rolls, representing an important contribution to the Chinese metallurgical equipment industry. The technical content, while presented concisely, covers the essential aspects of roll refurbishment: process selection, material selection, parameter optimization, and quality control. For modern practitioners, the fundamental principles remain applicable while the specific techniques have been enhanced by automation, advanced materials, and improved NDE capabilities. The economic and operational importance of roll refurbishment technology in maintaining continuous steel production makes this a perennially relevant subject.