Weld Overlay Repair Technology for Medium-Speed Coal Mill Rollers
1. Definition and Technical Principles
Medium-speed coal mill rollers are critical wear components in pulverized coal preparation systems used extensively in coal-fired power plants, cement kilns, and industrial boilers. These rollers operate under severe conditions involving abrasive coal particles, thermal cycling, impact loading, and chemical erosion from sulfur and ash. Over time, the roller surface experiences progressive wear, typically ranging from 2 to 5 mm per operating season, leading to reduced grinding efficiency, increased coal fineness variability, and ultimately catastrophic failure if not restored.
The weld overlay repair technology for medium-speed coal mill rollers involves the systematic removal of worn material and the application of one or more layers of hardfacing or wear-resistant alloy deposits onto the roller surface using either Gas Tungsten Arc Welding (GTAW/TIG) or Gas Metal Arc Welding (GMAW/MIG) processes. The fundamental metallurgical principle relies on dilution control—the strategic selection of filler metal compositions and deposition strategies to ensure the final overlay achieves the required hardness (typically 50–65 HRC) while maintaining acceptable toughness and resistance to spalling under impact loading.
The repair process follows a multi-layer approach: a transition layer (typically 309L or 310L stainless steel) is deposited first to bridge the metallurgical compatibility gap between the base roller material (usually low-alloy steel such as 16Mn or 42CrMo) and the subsequent hardfacing layers. This transition layer prevents cracking at the fusion boundary and ensures adequate ductility in the heat-affected zone (HAZ). Subsequent hardfacing layers, composed of high-carbon martensitic steels (such as 20CrMnTi-based alloys), high-chromium cast irons (Cr20 or Cr26), or carbide-enhanced alloys (WC-Co or Cr3C2 composites), are deposited to achieve the target surface properties.
2. Category and Business Positioning
This technology falls squarely within the TIG/MIG Weld Overlay technology route of Cladding Technology Shanxi Co., Ltd. It represents a high-value-added repair and refurbishment service that directly addresses the operational continuity needs of thermal power generation and cement manufacturing customers.
The business positioning of this capability is threefold:
- Equipment Availability Optimization: By restoring worn rollers to original or improved specifications, the technology eliminates the need for full roller replacement, reducing customer downtime from weeks (procurement lead time) to days (on-site repair cycle).
- Performance Enhancement: Properly executed overlay repair can improve roller surface hardness by 30–50% compared to the original as-rolled condition, extending service life between maintenance intervals.
- Qualification Credibility: Successful execution of coal mill roller repair projects demonstrates the company's proficiency in complex industrial repair scenarios involving large-diameter cylindrical components, high-carbon filler metals, and demanding NDT acceptance criteria.
3. Technical Purpose and Value
The primary technical purpose of this weld overlay repair process is to restore and enhance the functional surface properties of medium-speed coal mill rollers while maintaining structural integrity of the roller body. The specific objectives include:
- Achieving surface hardness of 50–65 HRC in the top hardfacing layer
- Maintaining tensile strength of the base material with no reduction exceeding 10% in the HAZ
- Eliminating surface defects including porosity, cracking, and spalling
- Ensuring geometric accuracy with surface roughness Ra ≤ 25 μm after post-weld machining
- Achieving intergranular penetration of the transition layer into the base metal of 0.3–0.5 mm (optimal bonding zone)
The economic value is substantial. A single medium-speed coal mill roller (typical dimensions: 800–1200 mm diameter × 600–1000 mm length) costs 300,000–800,000 RMB for replacement. Weld overlay repair typically costs 40,000–120,000 RMB per roller, representing a 70–85% cost reduction while restoring service life to 80–100% of new condition.
4. Key Process and Implementation Points
4.1 Pre-Weld Preparation
Proper surface preparation is the foundation of successful overlay repair. The following sequence must be strictly followed:
- Worn material removal: Grinding or CNC machining to remove the full depth of worn material (typically 3–8 mm) plus 1–2 mm of additional stock to eliminate subsurface micro-cracking. The surface must be machined to reveal sound, defect-free base material.
- Surface profiling: A V-groove or U-groove profile (60° included angle for V-groove, radius 3–5 mm for U-groove) is machined to ensure adequate fusion of the transition layer with the base metal.
- Cleaning: Mechanical grinding to bare metal followed by acetone or solvent wiping to remove all oil, grease, and moisture contamination. Surface cleanliness must meet the requirements of NACE No. 2 Sa 2.5 (near-white metal blast) or equivalent.
- Preheating: For rollers with base material carbon equivalent (CE) ≥ 0.40%, preheating to 200–300°C is mandatory to reduce hydrogen-induced cracking susceptibility. For lower CE materials (CE < 0.40%), preheating to 100–150°C is recommended.
4.2 Welding Process Parameters
The following table summarizes the recommended welding parameters for the typical multi-layer overlay sequence:
| Layer | Filler Metal | Process | Current (A) | Voltage (V) | Travel Speed (mm/min) | Layer Thickness (mm) | Interpass Temp (°C) |
|---|---|---|---|---|---|---|---|
| Transition Layer | ER309L (GB/T 8110) | TIG (GTAW) | 120–180 | 12–16 | 40–80 | 2.0–3.0 | ≤ 150 |
| Build-up Layer | ER40432 (high-Cr) | MIG (GMAW) | 250–350 | 22–28 | 150–250 | 2.0–3.0 | ≤ 150 |
| Hardfacing Layer 1 | ER555D2 (Cr20) | MIG (GMAW) | 280–380 | 24–30 | 180–280 | 2.5–3.5 | ≤ 120 |
| Hardfacing Layer 2 | ER555D2 / WC-Co | MIG (GMAW) | 280–380 | 24–30 | 180–280 | 2.5–3.5 | ≤ 120 |
4.3 Critical Process Control Points
- Shielding gas purity: Argon gas purity must be ≥ 99.99% for TIG and ≥ 99.95% for MIG. Flow rates: TIG 8–12 L/min, MIG 15–25 L/min. Backing gas (argon) must be applied to the root side at 2–5 L/min during TIG welding to prevent oxidation.
- Weld sequence: For cylindrical rollers, a spiral or circumferential overlap pattern is used with a weave width of 1.5–2.0 times the electrode diameter. Each successive pass must overlap the previous pass by 50–60% to ensure full fusion.
- Electrode/wire preheating: High-carbon hardfacing electrodes (for TIG stick processes) must be preheated to 200–300°C and maintained at 100–150°C during welding to prevent moisture-induced porosity and hydrogen cracking.
- Post-weld cooling control: For rollers with CE ≥ 0.40%, post-weld cooling must be controlled by wrapping with insulating blankets or placing in a cooling furnace at 250–350°C and holding for 2–4 hours before air cooling. This prevents delayed hydride cracking.
- Post-weld stress relief: Stress relief annealing at 550–650°C for 2–4 hours (depending on roller thickness) is recommended to reduce residual stresses and prevent distortion. For high-hardness overlay layers, stress relief temperature must not exceed 600°C to avoid softening of the martensitic structure.
4.4 Post-Weld Machining and Finishing
After overlay deposition, the roller surface must be machined to restore the original geometric profile (typically a crowned cylindrical surface with 0.5–1.0 mm crown per 100 mm diameter). The machining sequence is:
- Crude turning to remove surface irregularities (leaving 2–3 mm stock)
- Semi-finishing to achieve dimensional accuracy within ±0.5 mm
- Finish turning to achieve Ra ≤ 25 μm surface roughness
- Final inspection for geometric accuracy: diameter tolerance ±0.3 mm, runout ≤ 0.1 mm, crown profile deviation ≤ 0.15 mm
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard Number | Title / Scope | Relevant Clause |
|---|---|---|
| GB/T 8110 | Welding consumables—Welding wire and rod for TIG and MIG | Filler metal classification and composition |
| GB/T 13916 | Welding consumables—Welding rod for hardfacing | Hardfacing electrode specifications |
| GB/T 3323 | Non-destructive testing—Radiographic testing of welds | RT acceptance for weld quality |
| GB/T 11345 | Non-destructive testing—Ultrasonic testing of welds | UT acceptance for volumetric defects |
| GB/T 11346 | Non-destructive testing—Magnetic particle testing | MT acceptance for surface defects |
| GB/T 26494 | Welding—Weld overlay—General requirements | Overlay process qualification |
| NB/T 47014 | Qualification test of welding procedure for pressure vessels | WPS/PQR qualification |
| ASME Section IX | Qualification Rules for Welding, Brazing, and Fusing | WPS qualification and performance qualification |
| ASTM A507 | Standard specification for carbon-manganese structural steel plates | Base material specification (roller body) |
| ISO 9001:2015 | Quality management systems | Process control and documentation |
| NACE No. 2 | Surface preparation of steel prior to the application of protective coatings | Surface cleanliness requirements |
5.2 Acceptance Criteria
The following acceptance criteria must be met for the repaired roller to be released for service:
- Radiographic testing (RT): 100% of weld volume inspected. Acceptance per GB/T 3323 Level II (or ASME Section V Article 2). No cracks, no slag inclusions exceeding 0.5 mm, porosity limited to single pores ≤ 1.5 mm or clustered porosity ≤ 2.0 mm equivalent diameter.
- Ultrasonic testing (UT): 100% of weld volume inspected per GB/T 11345. Acceptance Level 2 per GB/T 11345. No indications classified as Level B or higher.
- Magnetic particle testing (MT): 100% of weld surface and HAZ inspected per GB/T 11346. No linear indications (cracks, laps) permitted. Rounded indications limited to 3 mm length and 0.2 mm width.
- Hardness testing: Surface hardness of final hardfacing layer: 50–65 HRC (minimum 3 test points per roller, distributed circumferentially). Transition layer hardness: 25–40 HRC. Base material HAZ hardness: not exceeding base material hardness + 30 HV.
- Metallographic examination: Cross-section examination of transition layer showing no cracks at the fusion boundary, intergranular penetration of 0.3–0.5 mm, and proper microstructural gradient from austenite (transition layer) to martensite (hardfacing layer).
- Dimensional inspection: Roller diameter within ±0.3 mm of nominal, runout ≤ 0.1 mm TIR, crown profile deviation ≤ 0.15 mm, surface roughness Ra ≤ 25 μm.
6. Common Risks and Controls
| Risk | Cause | Consequence | Control Measure |
|---|---|---|---|
| Hydrogen-induced cracking (HIC) | Moisture in filler metal, inadequate preheat, excessive cooling rate | Catastrophic crack in HAZ or overlay, roller failure in service | Preheat to 200–300°C, use dry electrodes/wire, control interpass temperature ≤ 150°C, post-weld stress relief |
| Overlay spalling | Excessive hardness, poor dilution control, thermal fatigue cycling | Loss of overlay material during operation, exposure of base metal | Optimize layer sequence for hardness gradient, limit top layer hardness to ≤ 65 HRC, ensure adequate transition layer thickness |
| Weld porosity | Contaminated surface, insufficient shielding, moisture in flux | Reduced overlay integrity, potential crack initiation sites | Strict surface cleaning to NACE No. 2, verify gas purity ≥ 99.99%, preheat electrodes to 200–300°C |
| Excessive dilution | Too deep penetration, low current, large electrode diameter | Insufficient hardness in final layer, poor wear resistance | Control penetration depth, use appropriate current range, verify dilution rate ≤ 30% for hardfacing layers |
| Roller distortion | Excessive welding heat input, asymmetric weld sequence | Geometric inaccuracy, poor grinding efficiency, premature wear | Use symmetric weld sequence, limit heat input, machine 2–3 mm stock allowance for post-weld correction |
| Cracking at fusion boundary | Incompatible metallurgy between base and transition layer, high CE base material | Overlay delamination, catastrophic failure | Use ER309L/ER310L transition layer, control base material CE ≤ 0.45%, preheat and post-weld heat treat |
7. Application Across Technology Routes
7.1 TIG/MIG Weld Overlay (Primary Route)
The weld overlay repair of medium-speed coal mill rollers is the flagship application of the TIG/MIG weld overlay technology route. This route is characterized by:
- TIG (GTAW) for transition layer: The TIG process provides excellent control over heat input and penetration, making it ideal for the critical transition layer that must achieve controlled intergranular penetration without cracking. The use of tungsten electrodes (2.0–3.2 mm diameter, pure tungsten or lanthanated tungsten) with ER309L wire (1.6–2.4 mm diameter) enables precise deposition.
- MIG (GMAW) for build-up and hardfacing layers: The MIG process offers higher deposition rates (5–8 kg/h compared to TIG's 1–2 kg/h), making it economical for the bulk overlay layers. Self-shielded or gas-shielded flux-cored wires (FCAW) are also applicable for field repair conditions where shielding gas infrastructure is limited.
- Process flexibility: The TIG/MIG combination allows the company to perform repairs both in controlled workshop conditions (for high-quality overlays) and in field conditions at customer power plants (for emergency repairs), providing maximum service flexibility.
7.2 Hydraulic Explosive Bonding (Secondary Applicability)
While hydraulic explosive bonding (HEB) is primarily used for through-cladding of large flat plates and cylindrical components where a bonded clad layer is required, it has limited but relevant applicability to coal mill roller technology:
- Roller body cladding: For new roller fabrication, HEB can be used to bond a wear-resistant alloy layer (such as 17-4PH stainless steel or high-chromium alloy) to the roller body as an alternative to the full weld overlay approach. This provides a metallurgical bond with 100% integrity and eliminates the need for post-weld machining of the clad surface.
- Hybrid approach: HEB-clad rollers can subsequently receive a thin weld overlay hardfacing layer (1–2 mm) on top of the bonded clad, combining the integrity of explosive bonding with the surface hardness of weld overlay. This hybrid approach is particularly valuable for high-value roller applications where maximum service life is required.
7.3 Explosion Welding (Tertiary Applicability)
Explosion welding (EW) technology has the following relevance to coal mill roller applications:
- Large-diameter roller shells: For large-diameter rollers (≥ 1500 mm), explosion welding can be used to join a pre-fabricated clad shell to the roller body, creating a through-clad component. The explosive bonding process achieves bond strengths exceeding 100 MPa, far exceeding the requirements for roller service.
- Material combinations: EW enables bonding of material combinations that are not weldable by conventional fusion welding, such as dissimilar steel pairs with significant carbon equivalent differences. This expands the range of achievable surface properties for roller applications.
- Batch production: For OEM roller manufacturing programs, explosion welding provides a repeatable, high-integrity cladding process that can be integrated into the roller fabrication workflow before the roller is machined to final dimensions.
8. Qualification Building and Customer Value
8.1 WPS/PQR Qualification Framework
The company maintains a comprehensive WPS/PQR qualification framework for coal mill roller repair operations:
- WPS (Welding Procedure Specification): Each roller repair project is governed by a qualified WPS that defines all essential variables including filler metal type, welding process, current range, voltage, travel speed, preheat temperature, interpass temperature, and post-weld heat treatment parameters.
- PQR (Procedure Qualification Record): Qualification tests are performed per NB/T 47014 and ASME Section IX, including tensile testing (weld metal and HAZ), bend testing (face bend, side bend, root bend), hardness profiling, and metallographic examination of the weld cross-section.
- Welding Performance Qualification (WPQ): All welders assigned to roller repair projects must hold valid WPQ certificates demonstrating qualification for the specific process, filler metal, and joint configuration. WPQ is performed per NB/T 47014 and includes practical welding tests on representative specimens.
- Process validation: The company conducts periodic process validation studies, including wear testing of overlay samples under simulated coal mill conditions (abrasive wear rig testing per ASTM G65 or equivalent), to ensure continued conformance with performance requirements.
8.2 Quality Management and Documentation
Each roller repair project is documented in accordance with ISO 9001:2015 quality management system requirements, including:
- Material traceability records (base material certificates, filler metal batch certificates)
- Welding log sheets recording all parameters for each weld pass
- NDT reports with full-size radiographs and UT scan maps
- Hardness test reports with location maps
- Metallographic examination reports with micrographs
- Dimensional inspection reports with CMM or coordinate measurement data
- Final inspection and release certificate signed by authorized quality representative
8.3 Customer Value Proposition
The weld overlay repair technology for medium-speed coal mill rollers delivers measurable customer value through:
- Reduced downtime: On-site repair capability reduces mill outage from 2–3 weeks (replacement) to 3–5 days (repair), saving an estimated 500,000–2,000,000 RMB in lost generation revenue per event for a 600 MW power unit.
- Extended service life: Properly executed overlay repair extends roller life by 1.5–2.5 times compared to the original as-rolled condition, reducing the frequency of maintenance interventions.
- Improved coal fineness: Restored roller geometry ensures consistent coal grinding performance, maintaining boiler efficiency and reducing fuel consumption by 0.5–1.5%.
- Environmental benefit: Repair and reuse of rollers reduces steel consumption and associated carbon emissions by approximately 2–5 tonnes CO2 per roller repaired (compared to new roller manufacturing).
- Technical partnership: The company provides ongoing technical support including wear monitoring, predictive maintenance recommendations, and overlay optimization based on actual operating conditions, establishing a long-term value relationship with customers.
9. Conclusion
The weld overlay repair technology for medium-speed coal mill rollers represents a mature, well-qualified capability that demonstrates Cladding Technology Shanxi Co., Ltd's expertise in industrial wear repair applications. The technology combines rigorous process control, comprehensive NDT acceptance criteria, and robust qualification frameworks to deliver reliable, high-performance repair solutions. As the primary application of the TIG/MIG weld overlay technology route, this capability serves as a platform for further expansion into adjacent wear repair markets including ball mill liners, crusher jaws, and conveyor rollers. The integration of hybrid approaches combining explosive bonding with weld overlay hardfacing further differentiates the company's offerings in the competitive industrial repair market.