Surfacing Repair and Heat Treatment of Channel Steel Rolls

Literature Overview

This technical paper by Ni Zhenhang, Tang Xin, and Xia Yang from Maanshan Iron and Steel Co., Ltd. Second Machinery Equipment Manufacturing Company, published in Heat Treatment of Metals (2004, Vol. 29, Issue 9), addresses the practical problem of extending the service life of channel steel hot rolling rolls through surfacing repair and subsequent heat treatment. Channel steel rolls are critical components in the hot rolling of structural steel sections, and their working surfaces are subjected to severe thermal and mechanical loads that cause wear, cracking, and deformation over time. The paper documents the engineering approach to restoring these rolls to serviceable condition through a combination of hardfacing and controlled heat treatment.

Repair Process Description

The repair methodology involves several sequential steps that must be carefully coordinated to achieve satisfactory results. The process begins with thorough preparation of the damaged roll surface, which typically includes grinding away the worn or damaged layer, cleaning to remove scale and contamination, and preheating to reduce thermal stresses during welding. The surfacing is then applied using appropriate hardfacing electrodes or wires, followed by controlled heat treatment to relieve residual stresses and optimize the microstructure.

Process Step	Key Parameters	Purpose
Surface preparation	Grinding, cleaning, preheating	Remove defects, ensure bonding
Surfacing welding	Electrode selection, current, travel speed	Build up wear-resistant layer
Post-weld heat treatment	Temperature, duration, cooling rate	Relieve stress, refine microstructure
Final inspection	Dimensional check, hardness, NDT	Verify quality

The selection of surfacing material is critical and depends on the specific wear mechanism experienced by the roll. For channel steel rolling, where the primary damage mechanisms include abrasive wear from scale, thermal fatigue from cyclic heating and cooling, and plastic deformation under high contact pressure, a surfacing alloy with high hardness, good thermal stability, and adequate toughness is required. Common choices include high-chromium cast iron alloys, cobalt-based alloys, or martensitic stainless steel alloys, depending on the severity of service conditions.

Heat Treatment Considerations

The post-weld heat treatment is perhaps the most critical aspect of this repair process. Surfacing welding introduces significant residual stresses due to the differential thermal expansion between the weld metal and the base roll material. These stresses, if not properly relieved, can lead to premature cracking and spalling of the surfacing layer during service. The heat treatment must be carefully designed to balance stress relief with microstructural optimization.

Typical heat treatment parameters for rolled roll repairs include:

Stress relief annealing at 550–650°C for 2–4 hours, followed by controlled furnace cooling
Tempering of martensitic surfacing alloys at 500–600°C for 1–2 hours to achieve the desired hardness-toughness balance
Avoidance of excessive temperatures that could cause grain coarsening or softening of the base material

The cooling rate after heat treatment must also be controlled to prevent re-introduction of thermal stresses. Air cooling from stress relief temperatures is generally acceptable for most repair situations, but furnace cooling may be necessary for heavily loaded components or when the surfacing alloy is susceptible to cracking on rapid cooling.

Quality Control and Defect Analysis

Common defects encountered during roll surfacing repair include:

Defect Type	Root Cause	Countermeasure
Cracking	Excessive residual stress, poor weldability	Preheating, stress relief, electrode selection
Spalling	Poor interfacial bonding, thermal mismatch	Surface preparation, filler metal matching
Porosity	Flux contamination, inadequate shielding	Cleaning, parameter control
Hardness variation	Inconsistent heat input	Parameter standardization, multi-pass welding
Excessive dilution	Too high heat input, poor fit-up	Reduce current, improve joint geometry

The use of a systematic FMEA approach is recommended for identifying and mitigating potential failure modes in the repair process. Each step of the procedure should be evaluated for its potential to introduce defects, and appropriate controls should be implemented.

Engineering Practice Integration

For rolling mill maintenance operations, this paper provides a practical framework for roll repair that can significantly extend component life and reduce downtime. The key success factors are:

Proper selection of surfacing material matched to the specific wear mechanism
Careful control of welding parameters to minimize dilution and residual stress
Appropriate post-weld heat treatment to optimize the microstructure and relieve stresses
Thorough quality inspection before returning the roll to service

The economic benefit of roll repair over replacement is substantial, particularly for large-diameter rolls where replacement costs are high and lead times are long. However, the quality of the repair must be maintained to avoid unexpected failures that could cause production stoppages and safety hazards.

This paper serves as a valuable reference for maintenance engineers in the steel industry, demonstrating that a disciplined approach to surfacing repair and heat treatment can effectively restore critical rolling equipment to serviceable condition.