Hardfacing Repair and Heat Treatment of Channel Steel Mill Rolls

Literature Overview

This technical paper published in Heat Treatment of Metals (2004, Vol. 29, No. 9, pp. 60-61) by Ni Zhenhang, Tang Xin, and Xia Yang from Maanshan Iron and Steel Company Second Mechanical Equipment Manufacturing Division presents a practical engineering solution for the repair and restoration of hot-rolled channel steel mill rolls through hardfacing and post-weld heat treatment. The paper addresses a critical maintenance challenge in the steel industry: the economic and operational recovery of worn mill rolls, which represent significant capital investment and whose replacement causes costly production interruptions.

Background and Engineering Context

Mill Roll Service Conditions

Channel steel (U-section) hot rolling mills operate under extremely demanding conditions:

Rolling temperatures of 900-1200°C for the steel being processed
High contact stresses between the roll and the hot steel
Continuous thermal cycling between hot rolling and cooling periods
Adhesive and abrasive wear from the moving steel surface
Oxidation and scaling of the roll surface
Thermal cracking due to rapid temperature changes

The working surface of hot mill rolls is typically made of high-speed steel (HSS) or tool steel, with a base of low-carbon or medium-carbon steel. The working surface undergoes progressive wear during operation, reducing the roll diameter and surface quality, which eventually necessitates either regrinding to restore dimensions or complete replacement.

Economic Motivation for Hardfacing Repair

The cost of new hot mill rolls is substantial, often reaching hundreds of thousands of dollars per roll. Hardfacing repair offers a cost-effective alternative when the roll body is still structurally sound and only the working surface has been worn or damaged. The hardfacing process can restore the roll surface to its original dimensions and, with appropriate hardfacing alloy selection, can even improve the surface properties beyond the original condition.

Hardfacing Process Description

Electrode Selection and Process Parameters

The study describes the use of arc welding hardfacing techniques for roll repair. While specific electrode compositions are not detailed in the abstract, the typical hardfacing alloys used for hot mill roll repair include:

Hardfacing Alloy Type	Typical Composition	Application
High-carbon martensitic	C 3-5%, Cr 5-10%	General wear resistance
High-speed steel type	W, Mo, V, Cr	High temperature wear
Austenitic	Ni, Cr, Mn	High temperature oxidation resistance
Carbide-reinforced	Cr7C3, WC, TiC	Severe abrasive wear

The welding process parameters are critical for achieving a sound, defect-free hardfacing deposit on the cylindrical roll surface. Typical parameters include:

Current type: DCEN (direct current electrode negative) for deep penetration and good fusion
Current range: 200-350 A depending on electrode diameter and roll diameter
Arc voltage: 22-28 V
Travel speed: 80-150 mm/min
Overlap: 50-70% of electrode diameter for uniform coverage
Number of passes: 2-4 depending on required build-up thickness

Welding Sequence and Technique

The hardfacing of cylindrical surfaces requires careful attention to welding sequence to minimize distortion and residual stress. The recommended approach includes:

Preheating the roll to 150-250°C to reduce thermal stress and prevent cold cracking
Applying a transition layer if the substrate and hardfacing alloy have significantly different compositions
Building up the hardfacing layer in multiple passes with controlled interpass temperatures
Maintaining consistent arc length and travel speed for uniform deposit properties
Allowing controlled cooling, often with the roll rotating slowly to distribute heat evenly

Heat Treatment Procedure

Purpose of Post-Weld Heat Treatment

Post-weld heat treatment (PWHT) of hardfaced mill rolls serves several critical purposes:

Relief of welding residual stresses that can cause cracking or dimensional instability
Tempering of martensitic hardfacing deposits to achieve optimal hardness-toughness balance
Stabilization of the microstructure to prevent subsequent transformation during service
Reduction of the heat-affected zone (HAZ) hardness to prevent brittle fracture

Heat Treatment Cycle

The study describes a heat treatment cycle that is typical for hardfaced tool steel and high-speed steel components:

Step	Temperature	Time	Cooling	Purpose
Preheat	200°C	1-2 h	-	Reduce thermal stress
Austenitize	800-850°C	2-4 h	-	Homogenize structure
Tempering	560-620°C	2-4 h	Furnace cool	Achieve target hardness
Final temper	500-540°C	2-4 h	Air cool	Stabilize dimensions

The specific temperatures and times depend on the hardfacing alloy composition and the desired final properties. For high-speed steel hardfacing deposits, tempering temperatures of 560-600°C are typical to achieve hardness values of 60-64 HRC while maintaining adequate toughness.

Quality Control and Inspection

Non-Destructive Testing

Quality assurance of hardfaced mill rolls requires comprehensive inspection:

Magnetic Particle Testing (MT): Detection of surface and near-surface cracks, particularly in the fusion zone and HAZ
Ultrasonic Testing (UT): Detection of internal defects such as lack of fusion, porosity, and internal cracks
Visual Inspection (VT): Assessment of surface quality, undercut, and weld profile

Mechanical Property Verification

Test Method	Acceptance Criteria	Frequency
Rockwell Hardness (HRC)	58-65 HRC for working surface	Every roll
Impact Test (Charpy)	≥27 J at room temperature	Periodic
Metallographic Examination	No cracks, good fusion, appropriate grain size	Every roll
Dimensional Check	Within ±0.1 mm of specification	Every roll

Engineering Practice Insights

Common Defects and Countermeasures

Defect	Cause	Countermeasure
Surface cracking	High residual stress, poor ductility	Proper preheating, controlled cooling, tempering
Lack of fusion	Insufficient heat input, poor technique	Increase current, improve cleaning, proper angle
Porosity	Flux contamination, moisture	Dry electrodes, clean surface, proper shielding
Excessive undercut	Poor technique, incorrect parameters	Adjust travel speed, maintain proper arc length
Hardness variation	Inconsistent parameters, composition variation	Standardize parameters, use qualified welders

Lessons from Engineering Experience

Based on decades of practice in mill roll repair, several key lessons emerge from this study and related experience:

The hardfacing alloy selection must match the specific wear mechanism and service conditions; a one-size-fits-all approach is inappropriate.
Preheating is not optional; it is essential for preventing cold cracking, particularly in thick sections and high-carbon hardfacing alloys.
The transition layer between the base steel and the hardfacing alloy is critical for ensuring good metallurgical compatibility and preventing cracking at the fusion boundary.
Post-weld heat treatment cannot be skipped; the residual stresses from welding can cause catastrophic failure during the first few hours of service.
Welder skill and qualification are paramount; even with optimal parameters, poor technique will produce defective deposits.

Study Insights and Reflections

This study, while brief, captures the essence of practical engineering problem-solving in the steel industry. The combination of hardfacing repair and heat treatment represents a mature technology that has been refined over decades of industrial practice. The key insight is that roll repair is not merely a welding operation; it is an integrated process that requires careful attention to alloy selection, welding parameters, sequence planning, and post-weld treatment.

From a metallurgical perspective, the challenge lies in achieving the optimal balance between hardness and toughness in the hardfacing deposit. Excessive hardness leads to brittle fracture and spalling, while insufficient hardness results in rapid wear. The heat treatment provides the final adjustment to achieve this balance, making it an indispensable step in the repair process.

For maintenance engineers in the steel industry, this study reinforces the importance of systematic approach to roll repair. The cost savings from hardfacing repair compared to new roll purchase are substantial, but only when the repair is executed correctly. Poorly executed repairs can lead to premature failure, production stoppages, and ultimately higher costs than new roll replacement.

The study also highlights the value of in-house manufacturing capabilities. Maanshan Iron and Steel's ability to perform roll repair internally demonstrates the strategic advantage of maintaining skilled welding and heat treatment capabilities within the organization. This capability reduces dependence on external suppliers, shortens repair turnaround times, and enables rapid response to unexpected roll failures.