Weld Overlay Repair of Slab Continuous Casting Machine Rollway Rolls

Literature Overview

Slab continuous casting machine rollway rolls are critical components in steelmaking operations that support and guide the movement of continuously cast slabs from the caster to downstream processing areas. These rolls are subjected to severe wear conditions due to contact with hot slabs, mechanical loading, and thermal cycling, necessitating periodic repair through weld overlay techniques. This study examines the challenges, methods, and outcomes of weld overlay repair for these rolls, with emphasis on process optimization and economic considerations.

Service Conditions and Wear Mechanisms

Rollway rolls in slab continuous casting operations operate under uniquely demanding conditions. The rolls must withstand contact with slabs at temperatures ranging from 800°C to 1200°C, depending on the steel grade being cast and the distance from the caster. The combination of high temperature, mechanical loading, and thermal cycling creates a complex wear environment that involves multiple simultaneous wear mechanisms.

The primary wear mechanism for rollway rolls is typically a combination of abrasive and oxidative wear, with thermal fatigue contributing to surface cracking and spalling. Understanding these mechanisms is essential for selecting appropriate overlay materials and process parameters.

Overlay Material Selection

The selection of overlay material for rollway roll repair requires balancing multiple competing requirements: wear resistance, thermal stability, bond strength to the base material, and resistance to thermal shock. Common overlay materials include high-chromium white cast iron (HCCI), cobalt-based alloys, and martensitic stainless steels.

For most rollway roll applications, high-chromium white cast iron overlays provide an excellent balance of wear resistance, cost, and processability. However, for applications involving very high slab temperatures or where thermal fatigue is a dominant failure mode, cobalt-based alloys may be preferred despite their higher cost.

Welding Process Considerations

The welding process used for rollway roll overlay must be carefully selected to ensure adequate bond strength, minimize residual stresses, and achieve the desired overlay composition and microstructure. Commonly used processes include submerged arc welding (SAW), flux-cored arc welding (FCAW), and gas metal arc welding (GMAW).

Process Parameters and Defect Prevention

The most common defects encountered during rollway roll overlay include lack of fusion at the base-metal-overlay interface, porosity within the overlay layer, and cracking due to excessive residual stresses. Each defect type requires specific countermeasures to be addressed effectively.

Surface Preparation and Preheating

Proper surface preparation is critical for achieving a sound bond between the overlay and the base roll material. The worn surface must be ground or machined to remove all worn material, oxidation, and contamination, exposing fresh, clean base metal. A preparation depth of 2-5 mm is typical, depending on the extent of wear and the condition of the underlying material.

Preheating is recommended for most rollway roll repairs to reduce the risk of cracking and to minimize residual stresses. Preheat temperatures of 200-300°C are typical for carbon steel rolls, while higher temperatures of 300-400°C may be required for alloy steel rolls. The preheat temperature should be maintained throughout the welding operation, with interpass temperature control being particularly important.

Economic Analysis and Repair Feasibility

The economic viability of weld overlay repair versus roll replacement is a critical consideration in continuous casting operations. The decision depends on factors including the extent of wear, the cost of overlay materials and labor, the availability of replacement rolls, and the production schedule constraints.

The economic analysis clearly demonstrates that weld overlay repair is significantly more cost-effective than roll replacement for most wear conditions, particularly when production downtime is considered. However, the economic advantage diminishes as the extent of wear increases, as multiple overlay passes are required to restore the roll to specification, increasing both material and labor costs.

Study Insights and Reflections

The study of weld overlay repair for slab continuous casting rollway rolls highlights the importance of integrating metallurgical understanding with practical process knowledge and economic considerations. The key insight is that successful roll repair requires not only selecting the appropriate overlay material but also optimizing the welding process parameters, surface preparation, and preheat treatment to achieve a durable, wear-resistant overlay that performs reliably in service.

A particularly valuable finding is the recognition that the thermal cycling experienced by rollway rolls during service creates unique challenges for overlay performance that are not fully captured by laboratory wear testing. Engineers must account for the cumulative effects of thermal fatigue, oxidative wear, and mechanical loading when evaluating overlay performance and planning repair intervals. This holistic approach to roll repair management can significantly extend component life and reduce overall production costs.