MLS Vertical Mill Roller Weld Overlay Repair Technology
1. Definition and Technical Principles
MLS Vertical Mill Roller Weld Overlay Repair Technology refers to the systematic restoration of worn or damaged grinding rollers in MLS (MPS/MLS) vertical roller mills through multi-pass weld overlay processes. Vertical mill rollers—critical rotating components in cement grinding, coal preparation, and mineral processing circuits—suffer progressive surface degradation due to abrasive contact with feed material, impact loading, and thermal cycling. This repair technology applies high-performance overlay alloys to reconstruct the working surface geometry, hardness profile, and wear resistance characteristics of the roller, extending service life by 200–400% compared to original equipment condition.
The fundamental principle relies on metallurgical bonding between the base steel substrate (typically low-carbon or medium-carbon forged steel, e.g., Q345, 42CrMo, or equivalent) and the overlay alloy layers. Through controlled heat input and precise thermal management, a diffusion-bonded interface is achieved without excessive dilution or hot cracking. The overlay material is selected to provide a hardness gradient—typically transitioning from ~200 HB base metal through a transition layer (HRC 35–45) to a wear-resistant surface layer (HRC 50–62), depending on the operating duty cycle.
2. Category and Business Positioning
This technology falls squarely within the TIG/MIG Weld Overlay technology route of Cladding Technology Shanxi Co., Ltd., representing a high-value repair and maintenance capability targeting the cement, coal, and power generation industries. Unlike new clad plate fabrication, roller repair is a condition-based service with significant urgency requirements—downtime costs in cement grinding circuits can exceed USD 50,000–150,000 per day, making rapid, reliable on-site or off-site repair a critical differentiator.
The business positioning encompasses:
- Emergency repair services — rapid response to sudden roller spalling, cracking, or excessive wear
- Planned maintenance contracts — scheduled overlay restoration at defined wear intervals
- Performance upgrading — converting standard rollers to enhanced wear-resistant configurations
- Technical consulting — failure analysis, root cause identification, and preventive maintenance recommendations
3. Technical Purpose and Value
3.1 Core Technical Objectives
- Restore roller surface geometry to manufacturer's original profile (typically a slightly convex or flat grinding surface with specific runout tolerances)
- Achieve surface hardness of HRC 50–62 for abrasive wear resistance
- Ensure metallurgical integrity with no cracks, porosity, or delamination
- Minimize residual stress and distortion to maintain roller balance and bearing life
- Extend roller service interval from typical 6–12 months to 18–36 months
3.2 Economic Value
Roller replacement requires procurement of forged blanks (USD 80,000–250,000 per roller depending on diameter), machining, and installation—often with 8–16 week lead times. Weld overlay repair typically costs 30–50% of new roller replacement while delivering comparable or superior surface performance. For a typical cement plant operating 6–10 vertical mills, annual savings from repair programs range from USD 500,000 to USD 2,000,000.
4. Key Process and Implementation Points
4.1 Surface Preparation
Proper surface preparation is the most critical prerequisite for successful roller repair. The worn surface must be prepared to expose sound base metal and provide mechanical anchoring for the overlay:
- Wear assessment: Measure remaining thickness, identify spalled areas, cracks (via magnetic particle inspection per ASTM E709), and hardness profile through cross-section
- Machining: Turn off all worn/spalled material using heavy-duty lathe or on-site boring; surface roughness Ra 6.3–12.5 μm
- Chamfering: Create 30°–45° chamfers at all edges and corners to prevent stress concentration during welding
- Cleaning: Solvent degrease (acetone or equivalent), remove all mill scale, rust, and contaminants; verify cleanliness per ASTM D5291 surface preparation standards
4.2 Weld Overlay Process Parameters
| Parameter | Transition Layer (Pass 1–2) | Wear Layer (Pass 3–6) |
|---|---|---|
| Process | TIG (GTAW) | MIG (GMAW) or TIG |
| Filler Metal | ER309L / ER312L (AISI 309L) | Hardfacing: ERNiCr-3, ERFe5, or proprietary carbide-composite |
| Wire Diameter | 1.6 mm / 2.4 mm | 1.2 mm / 1.6 mm |
| Current (A) | 120–180 | 100–160 |
| Voltage (V) | 16–22 | 18–24 |
| Travel Speed (mm/min) | 80–150 | 100–200 |
| Layer Thickness | 2–3 mm | 3–5 mm per pass |
| Shielding Gas | Argon (99.99%) | Argon or Ar/CO₂ (80/20) |
| Preheat Temperature | 150–250°C | Maintain 100–200°C interpass |
| Total Overlay Thickness | 12–25 mm (depending on wear pattern) | |
4.3 Thermal Management Strategy
Roller components present unique thermal challenges due to their large mass, high carbon equivalent base material, and the need to minimize distortion:
- Preheating: Induction heating or gas torch preheat to 150–250°C uniformly across the welding zone and 300 mm beyond
- Interpass temperature control: Maintain 100–200°C; do not exceed 250°C to avoid grain coarsening in base metal HAZ
- Weld sequence: Multi-directional, staggered pattern to distribute heat symmetrically around roller circumference
- Post-weld stress relief: Controlled furnace annealing at 550–650°C for 2–4 hours (depending on roller diameter), followed by furnace cooling to ≤100°C before removal
- Final machining: Post-stress-relief precision turning to final dimensional tolerances (concentricity ≤0.05 mm, surface roughness Ra 1.6–3.2 μm)
4.4 Overlay Material Selection Matrix
| Operating Condition | Recommended Overlay System | Achieved Hardness | Expected Life Extension |
|---|---|---|---|
| Coal grinding (abrasive + impact) | 309L transition + Ni-Cr-B-Si hardfacing | HRC 55–60 | 250–350% |
| Cement clinker grinding (high abrasion) | 309L transition + Fe-based with WC/TC carbides | HRC 60–65 | 300–400% |
| Wet grinding / corrosive | 310L transition + Ni-Cr-Mo austenitic | HRC 45–50 | 200–300% |
| High-temperature service (>200°C) | 312L transition + Co-Cr-W (Stellite-type) | HRC 45–55 | 200–280% |
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
- GB/T 12467 — Non-destructive testing of welds (ultrasonic examination)
- GB/T 3323 — Radiographic testing of welds
- GB/T 26517 — Magnetic particle testing of welds
- ASTM E709 — Magnetic particle test method
- ASTM E165 — Magnetic particle test method (general)
- ASTM E23 — Charpy V-notch impact testing
- ASTM A396 — Standard specification for cast iron (reference for hardness)
- ISO 17637 — Non-destructive testing of welds — Ultrasonic testing
- ISO 5817 — Welding — Fusion-welded joints in steel, nickel, titanium and their alloys — Quality levels
- NB/T 47013 — Non-destructive testing of pressure parts (where applicable)
- EN 12531 — Non-destructive testing of welds — Radiographic testing
5.2 Acceptance Criteria
| Inspection Item | Method | Acceptance Level |
|---|---|---|
| Weld porosity | Radiographic (RT) per GB/T 3323 | Level II per ISO 5817 (max 0.4 mm isolated pores) |
| Cracks (longitudinal/transverse) | Magnetic particle (MT) per ASTM E709 | No indication acceptable |
| Undercut | Visual + gauge | ≤0.5 mm depth, ≤10% of weld length |
| Surface hardness | Rockwell C (HR-C) per ASTM A396 | HRC 50–62 (±3 tolerance) |
| Transition zone hardness | Micro-Vickers on cross-section | Gradient from HRC 30 to HRC 50, no soft zone |
| Dimensional accuracy (post-machining) | CMM / precision gauging | Diameter tolerance ±0.10 mm; runout ≤0.05 mm |
| Surface roughness (final) | Profilometer | Ra 1.6–3.2 μm |
| Impact toughness (if required) | Charpy V-notch per ASTM E23 | ≥30 J at service temperature |
5.3 WPS/PQR Qualification
All roller repair welding procedures must be qualified per ASME Section IX (Part QW) or NB/T 47014 for pressure equipment applications. The WPS must specify:
- Base material P-number and filler metal F-number combinations
- Preheat and interpass temperature ranges
- Welding sequence and heat input limits (typically 0.8–2.5 kJ/mm)
- Post-weld heat treatment requirements
- NDT coverage (typically 100% MT on all overlay surfaces, RT on 10–20% or all critical areas)
6. Common Risks and Controls
6.1 Hydrogen-Induced Cracking (HIC)
Risk: High-carbon or high-carbon-equivalent base steels (CE > 0.4) are susceptible to cold cracking in the HAZ due to hydrogen diffusion from the weld pool.
Controls:
- Preheat to minimum 200°C (or per WPS qualification)
- Use low-hydrogen electrodes/wires (H₄ ≤ 5 mL/100g)
- Flux drying at 300–350°C for 1 hour if using covered electrodes
- Post-weld bake at 250–300°C for 2 hours to allow hydrogen diffusion
- Limit heat input to reduce cooling rate below 200°C/s in the critical temperature range
6.2 Overlay Spalling and Delamination
Risk: Poor metallurgical bonding at the interface between base metal and overlay, or between overlay layers, leading to premature spalling under impact loading.
Controls:
- Proper surface preparation (mechanical anchoring via machining roughness)
- Appropriate transition layer (309L or 312L) to bridge coefficient of thermal expansion mismatch
- Controlled layer thickness (no single pass exceeding 3 mm for hardfacing)
- Adequate interpass temperature to maintain plastic deformation of previous layer
- Post-weld stress relief to eliminate residual tensile stresses at interfaces
6.3 Excessive Distortion and Loss of Balance
Risk: Asymmetric heat input causes roller warpage, affecting grinding efficiency and accelerating bearing wear.
Controls:
- Symmetric welding sequence (opposite sides welded alternately)
- Staggered multi-directional pass pattern
- Closeout welds to balance thermal contraction
- Post-weld precision machining to restore geometry
- Dynamic balancing verification (G6.3 per ISO 21940-11) before reinstallation
6.4 Hardness Inhomogeneity and Soft Zones
Risk: Excessive dilution from base metal into overlay layers reduces hardness below required wear resistance threshold.
Controls:
- Optimize arc parameters to minimize base metal penetration (lower current, faster travel)
- Use multiple thin passes rather than few thick passes
- Hardness mapping (grid pattern, 10 mm spacing) on representative cross-sections
- Reject and rework any zone below HRC 45 (minimum for wear surface)
6.5 Residual Stress and Fatigue Initiation
Risk: High residual tensile stresses at overlay/base interface initiate fatigue cracks under cyclic roller loading.
Controls:
- Mandatory post-weld stress relief per WPS
- Peening (shot peening per SAE AMS 2750) on final overlay surface if fatigue-critical
- Ultrasonic residual stress measurement (ultrasonic grazing incidence) for critical repairs
7. Application Scenarios Across Company Technology Routes
7.1 TIG/MIG Weld Overlay (Primary Route for Roller Repair)
Vertical mill roller repair is the core application of the TIG/MIG weld overlay route. The technology leverages:
- TIG (GTAW): Used for transition layers, repair of cracks/spalls, and precision welding in tight geometries (roller neck, bearing seat areas)
- MIG (GMAW): Used for bulk overlay deposition where productivity is prioritized, particularly for large-diameter rollers (φ1.2–2.2 m) requiring 15–25 mm overlay thickness
- Submerged Arc (SAW): Used for very thick overlay builds (≥30 mm) in heavy-duty applications, though less common for roller repair due to heat input concerns
7.2 Hydraulic Explosive Bonding (Secondary/Complementary Application)
While hydraulic explosive bonding is primarily used for clad plate and pipe manufacturing, it has indirect relevance to roller technology:
- Roller housing/neck cladding: Hydraulic explosive bonding can produce clad roller housings where corrosion resistance is required at the bearing interface (e.g., 304/316L stainless over carbon steel housing)
- Roller trunnion replacement: Forged trunnion components with explosion-bonded corrosion-resistant outer layers for wet grinding environments
- Technical synergy: Understanding of metallurgical bonding mechanisms from explosive bonding informs overlay interface design
7.3 Explosion Welding (Reference Technology)
Explosion welding principles contribute to roller repair in the following ways:
- Material compatibility knowledge: Explosive bonding qualification data (per ASTM A498) provides validated material pairings that inform overlay system selection
- Interface metallurgy understanding: The jetting and wave formation mechanisms in explosion welding deepen understanding of interface bonding, applicable to optimizing overlay dilution control
- Alternative repair approach: For severely damaged rollers where extensive base metal removal is required, explosion-welded replacement sleeves can be applied to salvaged roller cores
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification and Certification Building
MLS vertical mill roller repair capability directly contributes to the company's qualification portfolio:
- WPS/PQR accumulation: Each roller repair project generates qualified procedures across multiple base materials (Q345, 42CrMo, C45, etc.) and overlay systems, building a comprehensive procedure database
- NDT certification: MT and RT inspection of roller repairs requires Level II/III personnel per NB/T 47013 or ASNT SNT-TC-1A, strengthening the company's inspection capabilities
- Industry-specific experience: Cement and coal industry roller repair builds domain-specific qualifications that support broader cladding product offerings
- ISO 9001 / ISO 3834 compliance: Documented repair procedures, traceability, and quality records strengthen the company's QMS credentials
8.2 Product Delivery Enhancement
- Cross-selling opportunity: Roller repair customers frequently require clad plates for mill internals, wear plates, and replacement components—creating a natural pipeline for the company's core clad plate business
- Service differentiation: Offering repair-as-a-service alongside new cladding products positions the company as a comprehensive metallurgical solutions provider
- On-site service capability: Mobile repair capability (transportable welding rigs, portable NDT equipment) demonstrates technical flexibility and responsiveness
- Technical documentation: Detailed repair reports with hardness maps, NDT results, and metallurgical analysis serve as marketing assets demonstrating technical depth
8.3 Customer Value Creation
- Reduced downtime: Off-site repair at company facilities (2–4 weeks) vs. new roller procurement (8–16 weeks) translates directly to production continuity
- Cost reduction: 50–70% cost savings vs. new roller replacement, with equivalent or superior performance
- Performance improvement: Overlay hardness exceeding original equipment specification extends service intervals beyond OEM design life
- Technical partnership: Root cause analysis and preventive maintenance recommendations reduce recurring failure rates and total cost of ownership
- Environmental benefit: Repair vs. replacement reduces material consumption, energy use, and waste disposal—aligning with customer ESG objectives
9. Summary and Technical Recommendations
MLS Vertical Mill Roller Weld Overlay Repair Technology represents a high-margin, high-technical-content service that leverages the company's core weld overlay expertise while creating strategic entry points into the cement and coal processing markets. Key recommendations for maximizing capability and market penetration include:
- Develop a standardized roller repair procedure library covering the top 10 roller designs (MLS 53/67/97/115/143/170 series, ZK/BRB series, etc.)
- Establish a dedicated roller repair laboratory with induction heating, stress relief furnace, precision turning, and full-spectrum NDT capabilities
- Qualify proprietary hardfacing consumables specifically optimized for roller duty cycles (reducing dilution sensitivity and improving spall resistance)
- Develop digital repair tracking systems linking each roller's repair history to performance data, enabling predictive maintenance recommendations
- Pursue OEM partnerships with vertical mill manufacturers (FLSmidth, Loesche, Bechtel) to become an authorized repair provider with direct supply chain integration
By systematically building roller repair capability within the TIG/MIG weld overlay technology route, Cladding Technology Shanxi Co., Ltd. positions itself as a full-service metallurgical solutions provider—bridging the gap between new cladding product delivery and in-service component restoration, thereby maximizing customer lifetime value and technical differentiation in the competitive cladding market.