Overlay Welding Repair Process for Medium-Speed Coal Mill Rollers

Literature Overview

This paper by Ju Zhenfu from the Electric Power Construction Research Institute (1999, Power Construction, Vol. 20, No. 2, pp. 50-51) addresses the overlay welding repair of medium-speed coal mill rollers—a critical component in pulverizing systems of thermal power plants. The author develops a specialized high-chromium, high-carbon flux-cored wire (FCAW) process for restoring worn rollers, demonstrating significant improvements in service life through field verification.

Technical Background and Failure Mechanism

Medium-speed coal mill rollers operate under severe conditions involving:

Operating Condition	Typical Value
Contact Pressure	3-8 MPa
Sliding Velocity	0.5-2.0 m/s
Coal Particle Size	0-10 mm
Ambient Temperature	80-150 °C
Wear Rate (original)	0.3-0.8 mm/month

The primary wear mechanisms include abrasive wear from coal particles, impact wear from coal lumps, and adhesive wear from high contact pressures. The original roller surface, typically quenched and tempered alloy steel (e.g., 42CrMo or 40CrNiMo), loses hardness progressively, leading to accelerated material loss and eventual replacement.

Repair Process Design

The proposed repair process employs a custom high-chromium, high-carbon flux-cored wire applied via FCAW (flux-cored arc welding) to the roller surface. The key process parameters are as follows:

Process Parameter	Recommended Range
Wire Diameter	1.2-1.6 mm
Current	250-350 A
Voltage	28-34 V
Travel Speed	200-400 mm/min
Shielding Gas	CO₂ or Ar+CO₂
Preheat Temperature	150-250 °C
Interpass Temperature	<250 °C
Overlay Thickness	4-6 mm
Post-Weld Treatment	Normalizing or tempering

The high-chromium (15-25% Cr) and high-carbon (1.0-1.5% C) composition promotes the formation of hard carbides (Cr₇C₃, Cr₂₃C₆) dispersed in a martensitic matrix, achieving hardness levels of HV 600-800 in the overlay layer. The flux-cored wire design provides inherent slag protection and deoxidation, reducing the need for external shielding in some configurations.

Process Control and Quality Assurance

Successful overlay welding repair of mill rollers requires strict attention to several quality-critical factors:

Surface preparation: Complete removal of the worn, work-hardened layer and any residual oxide is essential to ensure metallurgical bonding. Grinding to a clean, matte finish is recommended.
Heat input management: Excessive heat input causes carbide coarsening and softening of the overlay; insufficient heat input leads to poor fusion and cracking. The optimal window is narrow and must be maintained through continuous monitoring.
Geometric accuracy: The overlay must restore the roller to its original cylindrical profile within ±0.5 mm tolerance to maintain proper contact with the mill table.
Crack prevention: The high carbon and chromium content of the overlay material creates susceptibility to cold cracking. Controlled preheating, low hydrogen consumables, and post-weld stress relief are mandatory.

Field Performance Verification

The repaired rollers were subjected to field service evaluation in operational coal mills. The results demonstrated:

Metric	Before Repair	After Repair
Surface Hardness	HV 250-300	HV 600-750
Service Life	3-6 months	12-18 months
Replacement Cost	Full roller replacement	Overlay repair only
Downtime	48-72 hours	8-12 hours

The economic analysis reveals a cost reduction of 60-70% compared to complete roller replacement, with significantly reduced downtime. This makes the overlay welding repair approach highly attractive for power plant maintenance programs.

Study Insights and Engineering Recommendations

This paper represents an early but highly practical contribution to the field of component restoration through overlay welding. The approach of using FCAW with specialized hardfacing wire for large-diameter cylindrical components is now well-established in industry. Key lessons for current practice include:

The dilution rate between the high-alloy overlay and the low-alloy base metal must be carefully managed; multi-pass builds with progressive alloy content can optimize the transition zone.
Roller geometry restoration requires post-weld machining, which adds a critical finishing step to the repair process.
Long-term reliability depends on consistent process control—parameter drift during long welding sequences can lead to localized soft spots.
The approach is directly transferable to similar cylindrical wear components in mining, cement, and pulp-and-paper industries.

The study confirms that overlay welding repair is not merely a cost-saving measure but a technically sound engineering solution when properly designed and executed.