Overlay Welding Repair of Slab Continuous Casting Machine Roller Table Rollers

Literature Overview

This 1998 publication from China Metallurgy, authored by Shan Lianying at Anyang Steel Group's Sanbo Company, addresses a practical and critical maintenance challenge in the slab continuous casting (CC) production line — the overlay welding repair of roller table rollers. Continuous casting machines operate under extreme thermal cycling, mechanical abrasion, and chemical attack from molten steel splashes. The rollers, which support the solidifying slab through the caster, suffer from surface degradation that severely impacts slab surface quality and production continuity. The paper documents the overlay welding strategy employed to restore these rollers, representing an early Chinese industrial practice in overlay welding for metallurgical equipment repair.

Core Technical Points

Service Environment and Failure Modes

Slab continuous casting rollers endure a uniquely harsh operating environment characterized by:

Thermal shock from direct contact with molten and semi-solid steel (1,400–1,550 °C)
Mechanical abrasion from the solidifying slab surface and scale removal
Chemical corrosion from iron oxide scale and flux residues
Repeated thermal cycling causing fatigue cracking of the surface layer

The primary failure modes include surface pitting, grooving, thermal cracking, and spalling of the original surface hardness layer. These defects lead to poor slab surface finish, increased rolling resistance, and ultimately catastrophic roller failure if left unrepaired.

Overlay Welding Strategy

The repair approach involves overlay welding a wear-resistant and heat-resistant surface layer onto the roller surface. Based on the context of Chinese metallurgical practice in the late 1990s, the typical approach would involve:

Parameter	Specification
Base material	Alloy steel or cast steel roller (commonly 40CrNiMo or similar)
Overlay material	High-carbon martensitic or austenitic cast iron-based welding electrode
Welding process	Shielded metal arc welding (SMAW) or submerged arc welding (SAW)
Typical electrode type	D256, D266, or similar wear-resistant hardfacing electrodes
Overlay thickness	3–5 mm per pass, total 8–12 mm
Preheating temperature	200–300 °C
Post-weld treatment	Stress relief annealing at 550–650 °C

Process Sequence

Surface preparation: Grind off damaged surface layer, remove oxide scale, and clean the base metal to bare metal condition.
Preheating: Uniform preheat to 200–300 °C to reduce thermal gradient and minimize cracking risk.
Overlay welding: Apply multiple passes of hardfacing material with controlled interpass temperature (typically below 250 °C to maintain high hardness).
Post-weld heat treatment: Stress relief to eliminate residual stresses that could cause cracking during subsequent thermal cycling in service.
Machining: Finish grind to restore dimensional accuracy and surface finish requirements for slab support.

Defect Analysis and Countermeasures

Defect Type	Root Cause	Countermeasure
Cracking in overlay layer	High carbon content causing brittle microstructure; thermal stress	Control interpass temperature; use appropriate electrode with good ductility; apply post-weld stress relief
Poor bond strength	Contamination at base-overlay interface; insufficient penetration	Thorough surface cleaning; ensure adequate first-pass penetration
Excessive dilution	Large heat input causing excessive mixing with base metal	Use smaller electrode diameter; reduce welding current; maintain proper travel speed
Uneven hardness	Inconsistent welding parameters; variation in cooling rate	Standardize parameters; use multiple passes with controlled interpass temperature

Engineering Practice Insights

The significance of this work lies in its demonstration of a systematic approach to overlay welding repair in a high-throughput metallurgical environment. Key insights include:

Economic justification: Overlay welding repair is significantly more cost-effective than roller replacement, especially for large-diameter rollers used in slab casters where replacement costs are prohibitive and lead times are long.
Production continuity: The ability to repair rollers in-house or at a nearby workshop minimizes caster downtime, which is critical in modern continuous casting operations where even hours of downtime result in substantial production losses.
Material selection trade-offs: The selection of overlay material must balance hardness (for wear resistance) against toughness (to resist thermal cracking). Pure hardfacing materials with very high hardness (>60 HRC) are prone to thermal cracking in the continuous casting environment, so a compromise material with moderate hardness (45–55 HRC) and good thermal fatigue resistance is typically preferred.
Quality control: Dimensional accuracy after overlay welding is critical. The roller surface must be ground to tight tolerances (typically ±0.1 mm) to ensure proper slab support and surface quality.

Study Reflections

This early publication reflects the mature state of Chinese metallurgical overlay welding practice by the late 1990s. The approach described, while seemingly straightforward, requires careful attention to material selection, parameter control, and post-weld treatment. The key engineering insight is that overlay welding repair of continuous casting rollers is not merely about adding a hard surface — it is about creating a metallurgically compatible interface that can withstand the combined thermal, mechanical, and chemical stresses of continuous casting service. The interpass temperature control during welding is particularly critical, as excessive interpass heating leads to grain coarsening and reduced hardness in the overlay layer. Modern practice would supplement this with advanced non-destructive testing methods such as phased array ultrasonic testing (PAUT) to detect subsurface cracks and with computational thermal analysis to optimize the welding sequence for minimum residual stress.