Coal Mill Roller Open-Arc Weld Overlay Repair Technology

1. Definition and Technical Principles

Coal mill roller open-arc weld overlay repair technology refers to the restoration of worn or damaged coal mill grinding rollers through manual shielded metal arc welding (SMAW/MMA) deposit of specialized wear-resistant alloy layers onto the roller surface. Coal mill rollers are critical components in pulverized coal preparation systems in coal-fired power stations, subjected to extreme conditions including high-temperature flue gas, abrasive coal particles, and continuous mechanical loading. Over operational cycles, the roller surface experiences significant wear, developing grooves, pits, and dimensional degradation that compromise grinding efficiency and threaten mill integrity.

The open-arc (SMAW) method is selected for this application due to its inherent portability, adaptability to field conditions, and the ability of the operator to visually monitor weld bead profile and penetration in real time. Unlike mechanized processes, manual open-arc welding allows skilled welders to adjust travel speed, electrode angle, and deposition rate dynamically to accommodate irregular wear patterns, geometric constraints, and varying substrate conditions encountered during in-situ or shop repair operations.

The fundamental metallurgical principle involves the controlled deposition of high-carbon, high-chromium, or ceramic-reinforced alloy layers that provide superior abrasion resistance and hardness (typically 50–70 HRC after proper heat treatment) compared to the base roller steel. The weld metal dilution with the base material is managed through multi-pass strategies, electrode selection, and interpass temperature control to ensure the final overlay achieves target hardness and microstructural integrity.

2. Category and Business Positioning

Within Cladding Technology Shanxi Co., Ltd.'s technology portfolio, coal mill roller open-arc weld overlay repair falls under the Weld Overlay and Surface Engineering business line, specifically the Repair and Restoration segment. This technology serves as a critical bridge between new product fabrication and on-site service delivery, enabling the company to capture value across the entire asset lifecycle.

The business positioning of this capability is threefold:

• Revenue Diversification: Repair and restoration services generate recurring revenue streams from existing power plant customers who require periodic roller refurbishment, complementing new cladding product sales.
• Customer Lock-in: Demonstrated competence in roller repair builds trust and establishes the company as a preferred service provider, creating long-term contractual relationships.
• Technical Credibility: Mastery of field-repair techniques validates the company's metallurgical expertise and supports marketing of premium overlay products, as customers recognize the practical limitations and solutions.

While the company's primary technology routes include TIG/MIG weld overlay for new product manufacturing, hydraulic explosive bonding for clad plate/pipe production, and explosion welding for large-format cladding, the open-arc repair capability represents a specialized extension that addresses the maintenance and restoration market — a segment of significant volume in the power generation industry.

3. Technical Purpose and Value

3.1 Primary Technical Objectives

• Dimensional Restoration: Rebuild worn roller surfaces to original or specified diameters, ensuring proper mill gap and grinding performance.
• Tribological Enhancement: Apply overlay materials with superior hardness and wear resistance compared to the original base material, extending service life beyond the original design specification.
• Damage Remediation: Repair spalling, cracking, and localized material loss caused by thermal fatigue, impact loading, or inadequate maintenance.
• Cost Avoidance: Eliminate or significantly reduce the need for complete roller replacement, which typically involves 6–12 week lead times and substantial capital expenditure.

3.2 Quantifiable Value Metrics

Value Parameter	Typical Improvement	Measurement Method
Service Life Extension	2–5× original design life	Operating hours between overhauls
Cost Savings vs. Replacement	60–80% reduction	Comparative cost analysis
Hardness Achievement	50–70 HRC (target dependent on application)	Rockwell hardness testing per ASTM E18
Mill Availability	Reduced downtime by 70–90%	Unplanned outage hours
Overlay Integrity	<1% spalling rate	Post-service inspection

4. Key Process and Implementation Points

4.1 Substrate Preparation

Proper substrate preparation is the single most critical factor determining overlay adhesion and long-term service performance. The preparation sequence follows these mandatory steps:

1. Visual Inspection: Document all wear patterns, cracks, spalling areas, and dimensional deviations. Classify damage severity to determine repair strategy.
2. Mechanical Grinding: Remove all loose material, oxide scale, and contaminated layers using angle grinders with appropriate abrasive discs. The grinding pattern should be radial (along roller axis) to minimize stress concentration at the weld boundary.
3. Crack Detection and Treatment: Perform magnetic particle testing (MT) or dye penetrant testing (PT) per ASTM E709 to identify subsurface cracks. All detected cracks must be fully removed by grinding to a radius bottom (minimum 2 mm radius) or addressed by machined groove preparation.
4. Groove Preparation: For significant wear (exceeding 3–5 mm), machine a V-groove or U-groove to provide adequate root penetration and mechanical interlock. Groove geometry should follow AWS D10.0M recommendations for overlay welding.
5. Cleanliness Verification: Final surface must be free of oil, moisture, dust, and coatings. Use solvent cleaning followed by wire brush preparation within 2 hours of welding commencement.

4.2 Welding Procedure Parameters

Parameter	Specification	Rationale
Welding Process	SMAW (E70T/E80T or specialized overlay electrodes)	Field portability, visual control
Electrode Type	High-Cr, High-C, or ceramic-reinforced (e.g., D172, D212, or proprietary)	Abrasion resistance, hardness control
Electrode Diameter	3.2 mm (root/transition), 4.0–5.0 mm (fill/face)	Penetration control, deposition rate
Welding Current	80–140 A (3.2 mm), 140–220 A (4.0–5.0 mm)	Adequate penetration, minimal dilution
Polarity	DCEN (Direct Current Electrode Negative)	Deeper penetration, arc stability
Travel Speed	150–250 mm/min	Controlled bead width, dilution management
Interpass Temperature	≤ 150°C (measured by magnetic indicator)	Prevent base metal softening, control HAZ
Preheat Temperature	100–200°C (for carbon steels > 0.25% C)	Reduce hydrogen cracking susceptibility
Number of Passes	Minimum 2 (1 transition + 1–3 overlay)	Control dilution, achieve target hardness
Post-Weld Heat Treatment	Aging at 500–550°C for 2–4 hours (if specified)	Precipitate hardening, relieve residual stress

4.3 Multi-Pass Overlay Strategy

The multi-pass overlay strategy is essential for controlling dilution and achieving target microstructure:

1. Pass 1 – Transition Layer: A single pass of austenitic or high-alloy transition electrode (e.g., E309L equivalent) applied at low current and high travel speed to create a metallurgical buffer between base steel and overlay. This pass is ground flush or slightly below surface.
2. Pass 2 – Primary Overlay: First pass of wear-resistant overlay electrode deposited at moderate current with controlled travel speed. Bead profile should be slightly convex (1–2 mm crown) to promote self-hardening.
3. Pass 3 – Final Overlay (if required): Additional overlay pass(es) to achieve required thickness and hardness. Each subsequent pass should be deposited at slightly lower current to reduce dilution from the preceding pass.
4. Final Grinding: Post-weld grinding to specified profile (radial surface, 0.5–1.0 mm flatness tolerance) using progressive grit abrasives (80 → 120 → 240).

4.4 Welder Qualification and Skill Requirements

Open-arc overlay welding demands a higher skill level than conventional welding due to the need for precise dilution control and consistent bead geometry. Welder qualification should follow:

• Qualification per AWS D10.0M or GB/T 3485 for overlay welding
• Minimum 3 years' experience in weld overlay applications
• Demonstrated ability to achieve target hardness (verified by test coupon)
• Understanding of dilution control through electrode manipulation
• Competence in interpreting weld bead appearance for quality indicators

5. Applicable Standards and Acceptance Criteria

5.1 Governing Standards

Standard	Scope of Application	Key Requirements
AWS D10.0M	Welding procedures for overlay applications	Procedure qualification, welder performance qualification
GB/T 3485	Steel arc welding procedures for overlay	WPS parameters, dilution testing, hardness requirements
ASTM A213	Overlay welding consumables	Electrode classification and performance
ASTM E18	Rockwell hardness testing	Hardness measurement methodology
ASTM E709	Magnetic particle testing	Crack detection, acceptance of surface discontinuities
GB/T 3323	RT of welds (where applicable)	Volumetric defect detection
NACE SP0189	Corrosion protection (if applicable)	Post-repair coating and cathodic protection
ASME Section IX	Welder and WPS qualification	Qualification testing requirements

5.2 Acceptance Criteria

• Hardness: Minimum 50 HRC for the final overlay surface (unless otherwise specified by the mill manufacturer). Measured at 3 points per 100 mm of roller circumference, with no individual reading below 45 HRC.
• Visual: No undercut exceeding 0.5 mm, no surface porosity, no spatter, uniform bead profile. Per AWS D1.1 Section 6.
• MT/PT: No linear indications exceeding 1.5 mm length or 0.5 mm width in the overlay or HAZ region. Per ASTM E709.
• Dimensional: Final roller diameter within ±0.5 mm of specified dimension. Surface roughness Ra ≤ 12.5 μm after grinding.
• Spall Resistance: Pass drop test or bend test per AWS D10.0M (if applicable to the specific electrode system).
• Hardness Gradient: Transition zone hardness should decrease gradually from overlay to base metal without abrupt discontinuity exceeding 10 HRC over 1 mm depth.

6. Common Risks and Controls

6.1 Metallurgical Risks

Risk	Mechanism	Control Measures
Hydrogen-induced cracking	Diffusion hydrogen from electrode flux or moisture in base metal	Preheat to 150–200°C; use low-hydrogen electrodes; post-weld bake at 250°C for 2 hours
Hot cracking in overlay	Low-melting-point eutectics at grain boundaries during solidification	Control carbon equivalent; proper interpass temperature; avoid excessive restraint
Excessive dilution	High current or slow travel speed causing base metal melting into overlay	Low-current/high-speed technique; multiple thin passes; transition layer
Hardness non-uniformity	Inconsistent electrode deposition, variable dilution across roller	Welder qualification; systematic bead layout; hardness mapping at 50 mm intervals
Residual stress and distortion	Thermal gradients from sequential bead deposition	Staggered bead pattern; controlled interpass temperature; stress-relief PWHT

6.2 Operational Risks

• Roller rotation during welding: Ensure roller is securely locked in fixture or V-block. Use welding position indicators to maintain consistent deposition around circumference.
• Electrode contamination: Store electrodes in heated ovens (150–250°C) and issue in quantities sufficient for single shift. Reject electrodes exposed to moisture or exceeding storage time.
• Inadequate ventilation: SMAW generates significant fumes. Ensure adequate ventilation or use fume extraction in confined spaces. Monitor fume exposure per OSHA/NACE guidelines.
• Thermal damage to bearings: Maintain minimum 300 mm distance from roller bearing seats. Apply thermal barrier coating or water-cooled copper shields where necessary.

7. Application Across Company Technology Routes

7.1 TIG/MIG Weld Overlay (Primary Integration)

The open-arc repair technology integrates with the company's TIG/MIG weld overlay capabilities in several ways:

• Process Selection Logic: For new roller fabrication or shop repair with access to mechanized equipment, TIG/MIG overlay provides superior consistency, lower dilution, and higher deposition rates. Open-arc is reserved for field conditions where equipment portability is paramount.
• Hybrid Approach: In large-scale shop repairs, initial wear removal and transition layer may be applied by open-arc (leveraging welder skill for difficult geometry), followed by mechanized MIG overlay for the bulk deposit. This hybrid approach optimizes both quality and productivity.
• WPS Development: Experience gained through open-arc overlay directly informs TIG/MIG WPS development, particularly regarding dilution control strategies and multi-pass sequencing.

7.2 Hydraulic Explosive Bonding (Complementary Application)

While hydraulic explosive bonding is primarily applied to clad plate and pipe manufacturing, the roller repair experience contributes indirectly:

• Metallurgical Understanding: Knowledge of weld-overlay base metal interactions informs substrate preparation requirements for hydraulic bonding of roller components.
• Customer Portfolio Expansion: Power plant customers requiring roller repair are natural candidates for hydraulic bonded clad components (e.g., cladded mill liners, separator plates), creating cross-selling opportunities.
• Material Compatibility Data: Accumulated data on base metal/weld metal interactions supports hydraulic bonding process optimization for similar material combinations.

7.3 Explosion Welding (Strategic Synergy)

Explosion welding is applicable to large-format roller refurbishment where the entire roller surface requires cladding replacement:

• Full-Surface Cladding: For rollers with widespread wear exceeding 10 mm, explosion welding can deposit a complete overlay layer in a single operation, eliminating the time-intensive multi-pass welding approach.
• Material Selection: The wear mechanism understanding gained from repair welding guides selection of explosion-welded overlay materials (e.g., high-chromium cast iron, ceramic-reinforced composites) for new roller fabrication.
• Quality Benchmarking: Hardness profiles and spall resistance achieved through open-arc overlay serve as acceptance benchmarks for explosion-welded roller cladding.

8. Contribution to Qualification Building and Customer Value

8.1 Qualification and Certification Enhancement

• WPS Portfolio Expansion: Each successful roller repair generates qualified welding procedure specifications that expand the company's WPS database, supporting future bid submissions requiring demonstrated overlay capability.
• Welder Certification: Systematic welder qualification for overlay applications per AWS D10.0M and GB/T 3485 strengthens the workforce qualification matrix.
• Industry Certifications: Documented repair performance supports applications for power industry supplier qualifications (e.g., DL/T, GB/T 19001 quality management for repair services).
• Technical Database: Accumulated data on dilution rates, hardness profiles, and service performance by electrode type creates proprietary knowledge assets that differentiate the company in competitive bidding.

8.2 Customer Value Delivery

• Rapid Response Capability: Open-arc repair can be deployed within 24–48 hours of request, minimizing plant downtime compared to 4–8 week lead times for new roller procurement.
• Extended Asset Life: Overlay materials with superior wear resistance can extend roller service life by 2–5×, directly improving plant economics and reducing capital replacement cycles.
• Technical Consultancy: The repair service provides a platform for delivering value-added metallurgical consulting — wear analysis, failure investigation, and preventive maintenance recommendations.
• Integrated Lifecycle Service: Combining new product supply (explosion welding, hydraulic bonding) with repair services (open-arc overlay) positions the company as a single-source provider for the complete roller lifecycle.

8.3 Strategic Business Impact

"The open-arc weld overlay repair capability is not merely a service offering — it is a strategic enabler that deepens customer relationships, validates metallurgical expertise in real operating conditions, and creates data-driven insights that feed back into product development across all three technology routes."

9. Quality Assurance Framework

9.1 Pre-Weld Quality Gates

1. Substrate material verification (chemical analysis if unknown — confirm carbon equivalent ≤ 0.4% for crack sensitivity assessment)
2. NDE of substrate surface (MT/PT) to detect and document pre-existing defects
3. WPS review and approval by qualified welding engineer
4. Welder qualification verification (current certification, within scope)
5. Electrode receipt inspection, storage condition verification, and bake cycle documentation

9.2 In-Process Monitoring

1. Interpass temperature monitoring and documentation (magnetic indicator or infrared pyrometer)
2. Visual inspection of each pass for undercut, porosity, and bead profile
3. Periodic hardness spot checks on test coupons welded adjacent to production roller
4. Welding log completion (electrode lot numbers, current settings, welder ID, timestamps)

9.3 Final Inspection and Documentation

1. Full-surface hardness mapping (minimum 10 measurements per roller, documented on as-built drawing)
2. 100% visual inspection of overlay surface
3. MT or PT of overlay and HAZ (minimum 20% coverage, 100% for critical applications)
4. Dimensional verification (roller diameter, runout, surface finish)
5. Final report compilation including WPS reference, welder ID, NDE results, hardness data, and dimensional certificates

10. Conclusion

Coal mill roller open-arc weld overlay repair technology represents a high-value, technically demanding capability that positions Cladding Technology Shanxi Co., Ltd. as a comprehensive surface engineering provider. The technology demands rigorous process control, skilled personnel, and systematic quality assurance — attributes that directly translate to customer confidence and competitive differentiation. By integrating repair expertise with new product manufacturing across TIG/MIG, hydraulic explosive bonding, and explosion welding routes, the company delivers a unified value proposition spanning the complete lifecycle of wear-critical components in the power generation industry. This capability is not merely a revenue stream but a strategic asset that builds qualification credentials, generates proprietary metallurgical data, and deepens customer relationships through demonstrable technical competence in demanding field conditions.