CLADDING TECHNOLOGY SHANXI CO., LTDStudy Note on Weld Overlay Repair of Medium-Speed Coal Mill Rolls
Literature Overview
This paper by Ju Zhenfu from the Power Construction Research Institute of State Power Corporation, published in Electric Power Construction in 1999, addresses the practical engineering challenge of restoring worn medium-speed coal mill rolls through weld overlay repair techniques. Medium-speed coal mills are critical components in coal-fired power plant boilers, where the mill rolls grind pulverized coal to the required fineness for combustion. The rolls are subjected to severe abrasive wear from coal particles and experience thermal cycling from hot flue gas, leading to progressive material loss and eventual replacement. This study provides valuable guidance for power plant maintenance engineers seeking to extend roll service life through economical and reliable overlay repair methods.
Core Technical Viewpoints
The research focuses on the following key aspects of coal mill roll overlay repair:
- Wear mechanism analysis: The dominant wear mechanism on medium-speed coal mill rolls is abrasive wear, with secondary contributions from thermal fatigue and impact loading. The wear pattern is typically non-uniform, with maximum material loss occurring at the roll circumference where coal contact is most intense.
- Repair strategy: Rather than complete roll replacement, weld overlay repair allows restoration of dimensional tolerance and surface hardness at a fraction of the cost and lead time of new roll fabrication.
- Alloy selection: The overlay alloy must provide adequate hardness (typically 500–650 HV) while maintaining sufficient toughness to resist spalling under impact loading from coal chunks.
- Process selection: The study evaluates flux-cored arc welding (FCAW) and submerged arc welding (SAW) as the primary processes for overlay repair, considering their productivity, dilution characteristics, and compatibility with the curved roll geometry.
Process Parameters for Coal Mill Roll Overlay
| Parameter | SAW Specification | FCAW Specification |
|---|---|---|
| Base material | Low-carbon steel roll (Q235/Q345) | Low-carbon steel roll (Q235/Q345) |
| Bond layer wire | ER50-6 or ER70S-6 | ER70S-6 |
| Wear layer wire | Hardfacing wire (Cr-Mo-C type) | Hardfacing wire (Cr-Mo-C type) |
| Current (A) | 500–700 | 250–400 |
| Voltage (V) | 28–35 | 28–36 |
| Travel speed (mm/s) | 8–15 | 5–10 |
| Pre-heat temperature | 150–200°C | 150–200°C |
| Interpass temperature | <250°C | <250°C |
| Total overlay thickness | 15–25 mm | 15–25 mm |
| Number of layers | 8–12 | 10–15 |
| Post-weld treatment | Stress-relief at 550°C for 2h | Stress-relief at 550°C for 2h |
Defect Analysis and Quality Control
The following table summarizes common defects encountered during coal mill roll overlay repair and their corresponding countermeasures:
| Defect | Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Roll surface cracking | Excessive residual stress; high carbon content in weld metal | Visual + MT | Reduce heat input; increase pre-heat; use lower-carbon alloy |
| Overlay spalling | Poor bonding; thermal expansion mismatch | UT/PT | Ensure substrate grinding; use bond layer; control cooling rate |
| Uneven thickness | Inconsistent travel speed; operator variability | UT thickness measurement | Use automated welding; establish thickness mapping |
| Hardness variation | Dilution variation; cooling rate differences | Hardness testing grid | Control interpass temperature; verify wire composition |
| Surface porosity | Flux moisture; inadequate shielding | PT/MT | Pre-dry flux at 300°C for 2h; improve gas shielding |
Engineering Practice and Maintenance Strategy
The practical implementation of coal mill roll overlay repair in power plant maintenance follows a structured approach:
- Roll removal and inspection: Remove the roll from the mill housing; measure current diameter and assess wear pattern distribution using ultrasonic thickness gauging at multiple circumferential and axial locations.
- Surface preparation: Grind the worn surface to remove cracked and decarburized material; ensure surface roughness of Ra 12.5–25 μm for mechanical interlocking of the overlay.
- Pre-heat: Apply induction heating or gas torch pre-heat to achieve uniform temperature of 150–200°C across the entire roll surface to reduce thermal gradient and minimize cracking risk.
- Bond layer application: Deposit 2–3 passes of low-carbon bond alloy to ensure metallurgical compatibility with the base roll steel and to reduce dilution effects on subsequent wear layers.
- Wear layer deposition: Apply multiple passes of hardfacing alloy using a systematic circumferential and axial pattern to ensure uniform thickness distribution; maintain interpass temperature below 250°C.
- Post-weld stress relief: Perform furnace stress-relief annealing at 550°C for 2 hours to reduce residual stresses and improve overlay toughness.
- Machining and finishing: Grind or machine the overlay surface to restore the original roll profile and achieve the specified surface finish (typically Ra 3.2–6.3 μm for coal grinding efficiency).
- Quality verification: Conduct hardness testing at a grid pattern across the overlay surface; perform magnetic particle inspection (MT) for surface cracking; measure final thickness by UT.
Study Insights and Implications
The most significant contribution of this paper is its practical orientation toward power plant maintenance engineering, where reliability and cost-effectiveness are equally important. The study demonstrates that weld overlay repair of coal mill rolls can extend service life by 3–5 times compared to unmodified rolls, with a repair cost representing only 20–30% of new roll procurement. A critical engineering consideration highlighted by the research is the importance of maintaining the roll's original geometric profile after overlay repair, as deviations from the specified profile can lead to uneven coal distribution, increased mill vibration, and accelerated wear of the mill table. Engineers should also note that the overlay repair process must be qualified through weld procedure qualification testing per the applicable standards (such as NB/T 47014 or equivalent), and that periodic in-service inspection of the overlay layer is essential to detect early signs of spalling or cracking that could lead to catastrophic roll failure during operation. The economic analysis presented in the paper provides a compelling business case for overlay repair programs in power plant maintenance budgets.