ZGM95G Coal Mill Roller and Raceway Plate Weld Overlay Repair Technology
1. Definition and Technical Principles
The ZGM95G coal mill is a medium-speed bowl-type coal mill widely employed in thermal power stations across China and globally. The critical wear components—grinding rollers (磨辊) and grinding raceway plates (磨盘衬瓦)—are subjected to extreme abrasion from coal particles, moisture, and continuous mechanical loading. Over extended operating cycles, these components experience progressive material loss, geometric deviation, and surface degradation that compromise grinding efficiency and threaten unplanned outages.
Weld overlay repair of ZGM95G rollers and raceway plates involves the restoration of worn surfaces through the controlled deposition of hardfacing alloys using arc welding processes. The fundamental principle is to build up a wear-resistant overlay layer over the base material, restoring dimensional accuracy while providing superior abrasion resistance compared to the original material. The overlay process leverages dilution control, interlayer bonding metallurgy, and post-weld heat treatment to achieve a microstructure optimized for impact-abrasion resistance.
The metallurgical mechanism relies on the formation of a graded transition zone between the base steel and the overlay layer. Hardfacing alloys—typically containing chromium, molybdenum, tungsten, and carbon in specific proportions—form carbide networks (Cr₇C₃, WC, Mo₂C) within a martensitic or austenitic matrix. The resulting hardness, typically in the range of HRC 58–66, provides exceptional resistance to the slurry-abrasion regime encountered in coal mill service.
2. Category and Business Positioning
This repair technology falls within the TIG/MIG Weld Overlay technology route of Cladding Technology Shanxi Co., Ltd. It represents a high-value aftermarket service offering that bridges the gap between preventive maintenance and catastrophic component failure. Within the company's business portfolio, this capability serves multiple strategic functions:
- Aftermarket Component Restoration: Providing cost-effective alternatives to full roller or raceway replacement, reducing customer capital expenditure by 60–75% compared to new component procurement.
- Outage Optimization: Enabling repairs during scheduled maintenance windows (typically 15–30 days), preventing extended unplanned shutdowns that carry penalties of ¥500,000–2,000,000 per day in large-capacity power stations.
- Technical Qualification Building: Demonstrating deep domain expertise in power generation equipment, which serves as a reference project for subsequent qualification submissions to major power group companies (State Grid, Huaneng, Datang, Huadian, etc.).
In the competitive landscape of coal mill repair in China, this capability positions the company as a specialist rather than a generalist, differentiating through documented process control, metallurgical analysis, and traceable quality records.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
- Restore worn grinding surfaces to original geometric dimensions (roller curvature tolerance ±0.5 mm, raceway flatness ≤0.3 mm/m)
- Extend component service life to 80–100% of new component equivalent
- Achieve overlay hardness ≥HRC 58 with controlled dilution ≤15%
- Eliminate internal defects (porosity, cracks, lack of fusion) to ASME Section IX standards
- Ensure metallurgical compatibility between base material and overlay to prevent intergranular cracking under thermal cycling
3.2 Economic Value to Customer
| Value Metric | Repair Solution | New Component | Savings |
|---|---|---|---|
| Unit Cost (per roller) | ¥80,000–150,000 | ¥500,000–800,000 | 70–80% |
| Delivery Time | 10–20 days | 45–90 days | 60–75% reduction |
| Logistics Complexity | On-site or workshop | International import | Eliminated |
| Service Life Restoration | 80–100% equivalent | 100% | Minimal |
3.3 Environmental and Sustainability Value
Each repaired roller or raceway plate avoids the carbon footprint associated with forging, machining, heat treatment, and international shipping of a new component. Industry estimates indicate that repair solutions reduce CO₂ emissions by approximately 3.2–5.8 tons per component compared to manufacturing new replacements, aligning with China's "dual carbon" (carbon peak and carbon neutrality) national strategy.
4. Key Process and Implementation Points
4.1 Pre-Repair Assessment and Preparation
Successful repair begins with comprehensive condition assessment of the worn component. The following evaluation protocol must be followed:
- Visual and Dimensional Inspection: Measure wear depth, curvature deviation, and surface integrity using coordinate measuring machines (CMM) or laser scanning systems.
- NDT Screening: Apply magnetic particle testing (MT) per GB/T 26052.1 and ultrasonic testing (UT) per GB/T 11345 to detect subsurface cracks, laminations, or fatigue damage in the base material.
- Material Verification: Confirm base material composition (typically ZG270 or equivalent high-manganese cast steel) through optical emission spectrometry (OES) per GB/T 223.
- Repair Feasibility Decision: Components with wear exceeding 25% of original thickness, extensive internal cracking, or core material degradation must be rejected for repair and replaced.
4.2 Surface Preparation and Preheating
- Weld Preparation: Grind away all worn, contaminated, and heat-affected zones to sound metal. For raceway plates, prepare a V-groove or J-groove with 60° included angle. For rollers, prepare a U-groove or cove preparation.
- Preheating: Apply preheat at 200–300°C (maintained throughout the operation) to reduce residual stress and prevent cold cracking. Use induction heating or gas torch with pyrometer monitoring.
- Interpass Temperature: Maintain interpass temperature between 200–350°C to control cooling rate and avoid excessive hardness in the HAZ.
4.3 Weld Overlay Process Parameters
| Parameter | TIG (GTAW) Overlay | MIG (GMAW) Overlay | Submerged Arc (SAW) |
|---|---|---|---|
| Filler Material | Cr-Mo-B hardfacing wire (e.g., 6013, 6104 equivalent) | Cr-Mo-B hardfacing wire (e.g., 6013, 6104 equivalent) | Cr-Mo-B hardfacing flux-cored wire |
| Shielding Gas | Ar 99.99% + 0.5% O₂ | Ar 80% + CO₂ 20% or Ar 95% + CO₂ 5% | Flux-covered (no external gas) |
| Current Range | 80–150 A | 180–350 A | 350–600 A |
| Travel Speed | 25–50 mm/min | 200–400 mm/min | 200–350 mm/min |
| Deposition Rate | 0.8–1.5 kg/h | 4–8 kg/h | 10–20 kg/h |
| Weld Pass Thickness | 2–4 mm | 3–5 mm | 5–8 mm |
| Typical Number of Passes | 3–5 passes | 2–4 passes | 1–2 passes |
| Final Overlay Hardness | HRC 58–66 | HRC 58–64 | HRC 56–62 |
4.4 Layer Strategy and Dilution Control
The overlay build-up follows a three-layer strategy to manage dilution and ensure metallurgical compatibility:
- Transition Layer (1st Pass): Deposit a compatible alloy (e.g., 309L or austenitic stainless steel equivalent) to bridge the composition gap between the high-manganese base and the hardfacing overlay. This layer absorbs dilution without compromising final hardness.
- Build-up Layer (2nd Pass): Apply the primary hardfacing alloy at controlled dilution levels (target ≤15%). Monitor dilution through micro-hardness profiling across the transition zone.
- Surface Finish Layer (3rd Pass): Apply a final thin pass to ensure uniform hardness distribution and surface quality. This pass may use a slightly modified composition to optimize surface properties.
4.5 Post-Weld Heat Treatment
- Stress Relief: Furnace cool from 600–650°C at a controlled rate of ≤50°C/h to relieve residual stresses and prevent delayed cracking.
- Tempering (if applicable): For martensitic overlays, temper at 400–450°C for 2 hours to reduce brittleness while maintaining hardness above HRC 55.
- Post-heat: Maintain component at 200–300°C for a minimum of 4 hours after final welding to prevent moisture-induced cold cracking.
4.6 Post-Repair Machining
After overlay and heat treatment, the surface must be machined to restore precise geometry:
- Roller surface: CNC turning to achieve original curvature profile with Ra ≤6.3 μm surface finish
- Raceway plate: CNC planing or milling to achieve flatness ≤0.1 mm/m
- Final dimensional verification using laser scanning or CMM with deviation tolerance ±0.3 mm
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard | Application | Key Requirements |
|---|---|---|
| GB/T 13814 | Welding consumables - hardfacing electrodes and wires | Composition, hardness, dilution limits |
| GB/T 985 | Weld preparation of steel for welding | Groove geometry, edge preparation |
| GB/T 26052.1 | Magnetic particle testing of welds | Surface crack detection sensitivity |
| GB/T 11345 | Ultrasonic testing of welds | Internal defect detection |
| GB/T 16493 | Acceptance criteria for welds | Defect classification and limits |
| ASME Section IX | Welding procedures and qualification | PQR/WPS documentation, essential variables |
| ASTM A396 | Welding overlay procedures | Overlay procedure qualification |
| NACE SP0169 | Corrosion control in buried/underground piping | Environmental considerations for coal mill components |
| DL/T 5044 | Coal mill maintenance and repair (Chinese power industry) | Industry-specific repair acceptance |
| JB/T 8738 | Medium-speed coal mill technical specifications | Roller and raceway dimensional tolerances |
5.2 Acceptance Criteria
- Visual Inspection (VT): No surface cracks, undercut >1 mm, or spatter. Overlay surface must be uniform without excessive reinforcement. Acceptance per GB/T 19418.
- Magnetic Particle Testing (MT): 100% coverage of overlay surface. No indications classified as Acceptable (A) or better per GB/T 26052.1.
- Ultrasonic Testing (UT): 100% of overlay area. No internal defects exceeding 2 mm equivalent flat-bottom hole. Acceptance per GB/T 11345.
- Hardness Testing: Minimum 5 test points per weld length per meter. Average hardness ≥HRC 58; individual readings ≥HRC 55. Dilution zone hardness gradient must be continuous.
- Dimensional Verification: Roller profile deviation ≤±0.5 mm from original drawing; raceway flatness ≤0.3 mm/m; overlay thickness uniformity ±0.5 mm.
- Macrograph Examination: Cross-section verification showing sound fusion, no lack of fusion, and acceptable dilution zone width.
6. Common Risks and Controls
| Risk | Root Cause | Control Measure |
|---|---|---|
| Cold cracking in HAZ | High carbon equivalent of base + rapid cooling + hydrogen | Preheat 250°C; use low-hydrogen consumables; post-heat immediately; limit interpass temperature |
| Overlay cracking (hot cracking) | Low melting point eutectics at grain boundaries in overlay | Control Si and S content in filler; use multiple thin passes; avoid excessive heat input |
| Excessive dilution | High heat input, poor arc control, thick first pass | Use transition layer; reduce heat input; maintain thin first pass (≤3 mm); monitor with micro-hardness |
| Porosity | Moisture in flux/ consumables; contaminated base; inadequate shielding | Dry consumables per GB/T 13814; clean base surface; verify gas flow rate; use back-purge for TIG |
| Geometric deviation after welding | Welding distortion from thermal gradients | Use balanced welding sequence; apply clamping/restraint; post-weld machining; monitor distortion with dial indicators |
| Insufficient bonding strength | Poor surface preparation; contamination; incorrect current | Mechanically clean to bare metal; verify gas shielding; use appropriate current/voltage combination per WPS |
| Hardness non-uniformity | Inconsistent travel speed; overlapping patterns; varying preheat | Use mechanized welding where possible; maintain consistent parameters; perform hardness mapping after each layer |
6.1 Special Considerations for ZGM95G Components
- High-Manganese Base Material: The ZG270 base steel has high carbon equivalent (CE ≈ 0.55–0.65), requiring elevated preheat and strict hydrogen control. Avoid using high-carbon electrodes that could form brittle carbide networks at the fusion boundary.
- Roller Geometry Constraints: Cylindrical surfaces create variable root gap and heat dissipation. Implement multi-directional welding sequences (clockwise and counter-clockwise alternation) to minimize distortion.
- Raceway Plate Flatness: Large flat surfaces are susceptible to warping. Use step-welding technique with maximum step width of 300 mm and weld from center outward.
- Thermal Cycling in Service: Coal mill components experience repeated thermal cycling (ambient to 150–200°C). Overlay must maintain integrity under 10,000+ thermal cycles. Verify through thermal cycling simulation testing.
7. Application Across the Three Technology Routes
7.1 TIG/MIG Weld Overlay (Primary Route for This Application)
For ZGM95G roller and raceway repair, TIG and MIG welding are the primary and most versatile processes:
- TIG (GTAW): Preferred for the transition layer and final surface pass due to superior arc control, clean weld appearance, and precise heat input management. Ideal for curved roller surfaces where precision is critical.
- MIG (GMAW): Preferred for build-up passes due to higher deposition rates (4–8 kg/h vs. 0.8–1.5 kg/h for TIG). Reduces total welding time by 50–60% for large raceway plate surfaces.
- Hybrid Approach: Transition layer (TIG) → Build-up (MIG) → Surface finish (TIG) achieves optimal balance of quality and productivity.
7.2 Hydraulic Explosive Bonding (Complementary Route)
While hydraulic explosive bonding is not directly applicable to roller/raceway repair (due to component geometry and thickness constraints), it contributes to the overall technology ecosystem in the following ways:
- Base Material Supply: Hydraulic explosive bonding can produce clad base plates (e.g., 16Mn/304L or 16Mn/6Mo-1Ti) for raceway plate fabrication, providing the substrate that subsequently receives hardfacing overlay.
- Technology Synergy: Understanding of impact bonding metallurgy from hydraulic explosive bonding informs overlay metallurgy optimization—particularly regarding dilution control and interface bonding mechanisms.
- Prototype Development: Small-scale hydraulic explosive bonded test coupons can be used to qualify overlay alloys against specific base material combinations before full-scale repair deployment.
7.3 Explosion Welding (Extended Application)
Explosion welding technology contributes to the broader capability set in the following manner:
- Replacement Component Manufacturing: When repair is not feasible (excessive wear, structural damage), explosion welding can produce new clad rollers or raceway plates with integrated wear-resistant surfaces, eliminating the need for post-fabrication overlay.
- Material Development: Explosion welding enables the creation of multi-layer composite structures (e.g., base steel/explosive-clad hardfacing alloy) that can be machined to final dimensions, providing an alternative to traditional welding overlay for extreme wear conditions.
- Qualification Portfolio: Demonstrating explosion welding capability alongside weld overlay repair positions the company as a full-spectrum solution provider—from preventive cladding through to reactive repair.
8. Contribution to Qualification Building and Customer Value
8.1 Qualification Building
- WPS/PQR Development: Each ZGM95G repair project generates qualified welding procedure specifications (WPS) and performance qualification records (PQR) per ASME Section IX, building a growing library of qualified procedures for diverse base materials and overlay alloys.
- Industry Certification: Successful repair documentation supports applications for power industry supplier qualification (DL/T standards), establishing the company as an approved maintenance partner for major coal mill OEMs (Babcock & Wilcox, Fichtner, Loesche, etc.).
- Technical Knowledge Base: Accumulated experience with ZGM95G repairs creates institutional knowledge that reduces risk and improves consistency for subsequent projects, particularly for similar mill models (ZGM113G, ZGM123G, ZGM973G).
8.2 Product Delivery Enhancement
- Standardized Repair Packages: Develop standardized repair offerings for common ZGM95G component sizes, enabling rapid quoting and delivery scheduling.
- Mobile Repair Capability: Develop field-deployable welding equipment packages that allow on-site repair during scheduled outages, reducing component logistics and turnaround time.
- Performance Guarantee: Back repair services with quantifiable performance guarantees (e.g., minimum 8,000 hours service life or HRC 58 minimum hardness) to build customer confidence.
8.3 Customer Value Proposition
The ZGM95G coal mill repair capability delivers a compelling value proposition to thermal power customers: 70–80% cost reduction versus new component procurement, 60–75% faster turnaround versus international supply chains, full traceability through ASME/GB compliant documentation, and proven metallurgical performance validated through hardness mapping, NDT, and macrograph examination. This positions Cladding Technology Shanxi Co., Ltd. as a strategic partner in optimizing power plant availability and reducing lifecycle costs for critical coal preparation equipment.
8.4 Strategic Business Impact
- Revenue Diversification: Aftermarket repair services generate recurring revenue streams independent of new construction cycles, providing business stability during market downturns.
- Customer Lock-in: Once qualified as an approved repair vendor, switching costs are high due to documentation requirements, performance history, and OEM approval processes.
- Technology Platform: The ZGM95G repair capability serves as a technology platform that can be extended to similar applications: ball mill liners, crusher hammers, kiln wear plates, and wind turbine gearbox components.
- Brand Authority: Published repair case studies and technical papers based on ZGM95G projects establish thought leadership in the coal mill maintenance market, attracting inbound business from power plant maintenance departments.
9. Conclusion and Recommendations
The ZGM95G coal mill roller and raceway plate weld overlay repair technology represents a mature, high-value capability that directly addresses a critical pain point in thermal power generation. By maintaining rigorous adherence to GB and ASME standards, implementing systematic dilution control, and delivering quantifiable performance guarantees, Cladding Technology Shanxi Co., Ltd. can position this capability as a cornerstone of its aftermarket services portfolio.
Recommended next steps include: formalizing WPS/PQR documentation for all common ZGM95G configurations, developing a mobile field repair package, establishing strategic partnerships with major power group MRO departments, and conducting periodic metallurgical audits to continuously improve overlay performance and extend service life guarantees.