CDG Roller Press Squeeze Roller Fabrication and Hard Surfacing Weld Overlay Technology
1. Definition and Technical Principles
CDG roller press squeeze rollers are heavy-duty cylindrical components used in CDG (Coal Direct Gasification) roller press systems for crushing, grinding, and size reduction of coal and mineral feedstocks prior to gasification. These rollers operate under extreme conditions characterized by high compressive loads, abrasive particle impact, thermal cycling, and chemical attack from coal slurry. The technical entry encompasses two integrated disciplines: roller fabrication (involving forging, rolling, turning, welding of composite construction, and heat treatment) and hard surfacing weld overlay (applying wear-resistant alloy coatings to the roller working surface to extend service life).
The hard-facing process employs TIG (Tungsten Inert Gas) or MIG (Metal Inert Gas) arc welding to deposit multiple layers of wear-resistant alloys onto the roller surface. The metallurgical principles involve dilution control, microstructure refinement through rapid solidification, and the formation of hard phases (carbides, martensite, or intermetallic compounds) that provide superior abrasion and impact resistance compared to the base steel substrate.
1.1 Metallurgical Mechanisms of Hard Surfacing
Hard-facing deposits on CDG squeeze rollers rely on several reinforcing mechanisms:
- Carbide reinforcement: Chromium carbides (Cr₇C₃, Cr₃C₂), tungsten carbides (WC), and titanium carbides (TiC) provide micro-hardness exceeding 1200 HV, offering exceptional resistance to abrasive wear from coal and mineral particles.
- Martensitic transformation: High-carbon, high-chromium compositions (e.g., Cr20-Cu, Cr26) undergo martensitic transformation during rapid cooling, producing microstructures with hardness of 50-60 HRC in the as-welded condition.
- Composite layer design: A transition layer (typically 309L or 310 stainless steel) is applied between the base roller steel and the hard-facing layers to prevent cracking caused by thermal expansion mismatch and carbon segregation at the fusion boundary.
- Work hardening capacity: Some hard-facing alloys (e.g., high-manganese austenitic types) exhibit strain-induced martensitic transformation, maintaining surface hardness under impact loading conditions typical of roller press operation.
2. Category and Business Positioning
This capability falls squarely within the company's TIG/MIG weld overlay technology route, representing a high-value-added service in the power generation and coal chemical processing sector. The business positioning is threefold:
2.1 Strategic Sector Alignment
CDG roller press systems serve the coal-to-liquids (CTL) and coal-to-gas (CTG) industries, which are critical segments of China's energy security strategy. The company's involvement in hard-facing CDG squeeze rollers positions it as a qualified supplier within the national energy infrastructure supply chain, particularly for integrated coal chemical bases in Shanxi, Inner Mongolia, and Ningxia.
2.2 Value Chain Integration
- Upstream: Sourcing of forged steel billets (ASTM A48 Grade 50/55, or equivalent Chinese grades such as 45#, 50CrMo) for roller blank manufacturing
- Midstream: Machining, welding, heat treatment, and precision grinding of roller assemblies
- Downstream: Hard-facing overlay application, dimensional verification, and field installation support
- Service: Rebuild and refurbishment of worn rollers, extending equipment lifecycle and reducing capital expenditure for customers
2.3 Competitive Differentiation
The combination of roller fabrication expertise with hard-facing overlay capability creates a vertically integrated service offering. Unlike conventional machining shops that merely grind rollers, the company provides complete metallurgical solutions including material selection, WPS qualification, welder certification, and post-weld inspection — establishing a qualified supplier status that competitors cannot easily replicate.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
The hard-facing overlay on CDG squeeze rollers serves to:
- Extend service life by 3-8 times compared to unprotected base steel surfaces, reducing unplanned downtime for roller replacement
- Reduce total cost of ownership through fewer roller rebuilds and extended replacement intervals
- Maintain dimensional accuracy of the roller working surface throughout the wear cycle, ensuring consistent crush reduction ratios
- Resist adhesive and abrasive wear from coal particles containing silica, clay minerals, and moisture
- Tolerate thermal shock from gasification feed preparation processes where temperature fluctuations of 100-200°C may occur
3.2 Quantitative Value Metrics
| Metric | Unprotected Roller | Hard-Faced Roller | Improvement |
|---|---|---|---|
| Wear life (hours) | 2,000-4,000 | 12,000-30,000 | 3-8x |
| Surface hardness (HRC) | 25-35 | 50-62 | +20-27 HRC |
| Micro-hardness (HV) | 250-350 | 1,200-1,800 | 4-5x |
| Replacement cost per year | High (frequent rebuilds) | Low (extended intervals) | 40-60% reduction |
| Unplanned downtime | Frequent | Significantly reduced | 60-70% reduction |
4. Key Process and Implementation Points
4.1 Roller Fabrication Process Flow
The complete manufacturing sequence for CDG squeeze rollers involves the following critical steps:
- Material selection and inspection: Selection of appropriate base steel grade based on load requirements, thermal environment, and corrosion exposure
- Forging or rolling: Production of cylindrical blanks with required dimensions and internal quality
- Normalizing and tempering: Heat treatment to achieve target base hardness (typically 200-250 HB for weldability) and refine grain structure
- Machining: Rough and finish turning of roller body, boring of bearing bores, and machining of mounting features
- Pre-weld preparation: Surface cleaning (shot blasting to Sa 2.5), chamfering of weld prep, and preheating assessment
- Transition layer deposition: Application of compatible overlay layer (309L/310L) to prevent cracking
- Hard-facing layer deposition: Multi-pass application of wear-resistant alloy in specified layer configuration
- Post-weld heat treatment: Stress relief or tempering as required by WPS
- Final machining: Precision grinding to achieve dimensional tolerances (typically IT6-IT7 for diameter, Ra 1.6-3.2 μm for surface finish)
- NDT and dimensional verification: Complete inspection per applicable codes
4.2 Hard-Facing Process Parameters
| Parameter | Transition Layer (309L) | Hard-Facing Layer (Typical) | Notes |
|---|---|---|---|
| Welding process | TIG (GTAW) | TIG or MIG (GMAW) | TIG preferred for thin first layers |
| Electrode/wire type | ER309L / E309L-16 | ERCr20Cu / ERCr26 / ERNiCrSiB | Selected per wear mechanism |
| Layer thickness | 2-3 mm (1-2 passes) | 6-15 mm (3-8 passes) | Per WPS qualification |
| Preheat temperature | 150-250°C | 150-300°C | Depends on base steel carbon equivalent |
| Interpass temperature | ≤250°C | ≤250°C | Monitor with IR pyrometer |
| Shielding gas | Argon (99.99%) | Argon or Ar/CO₂ mix | Flow rate: 15-20 L/min |
| Travel speed | 50-80 mm/min | 60-120 mm/min | Adjusted for deposition rate |
| Current range | 150-250 A | 200-350 A | Depends on electrode diameter |
| Target dilution | ≤30% | ≤25% | Controlled by preheat and technique |
4.3 Layer Configuration Design
Optimal hard-facing layer configurations for CDG squeeze rollers follow a graded design principle:
| Layer Position | Material | Function | Typical Thickness |
|---|---|---|---|
| Base steel | ASTM A48 Gr.50 / 45# / 50CrMo | Structural load-bearing | Roller body |
| Transition layer | 309L / 310L (ER309L) | Crack prevention, dilution buffer | 2-3 mm |
| Build-up layer | ER309 / ER404 (high Ni-Cr) | Toughness buffer, residual stress relief | 2-4 mm |
| Working layer | ERCr20Cu / ERCr26 / ERNiCrSiB | Wear resistance, primary protection | 6-12 mm |
| Finish layer (optional) | Hard carbide-bearing alloy | Peak surface hardness, final machining | 1-3 mm |
4.4 Critical Implementation Controls
4.4.1 Dilution Management
Dilution is the single most critical factor in hard-facing success. Excessive dilution from the base steel reduces hardness and wear resistance of the deposit. Controls include:
- Proper preheating to reduce thermal gradient and base metal melting
- Use of "stringer bead" technique rather than "weaving" to minimize base metal inclusion
- Maintaining consistent travel speed and arc length
- Applying thin layers (1.5-2 mm per pass) for the first few passes
- Using a compatible transition layer to create a metallurgical buffer
- Monitoring dilution through hardness testing of each layer cross-section
4.4.2 Cracking Prevention
Hard-facing alloys, particularly high-carbon chromium types, are susceptible to hot cracking and cold cracking. Preventive measures:
- Preheating base steel to 150-300°C depending on carbon equivalent
- Controlling interpass temperature below 250°C
- Using low-hydrogen electrodes and dry storage conditions
- Applying transition layers with low carbon content (309L, not 309)
- Avoiding welding near high-stress geometric discontinuities
- Post-weld stress relief at 550-650°C for 1-2 hours per 25 mm thickness
- Proper weld sequence to minimize restraint and residual stress
4.4.3 Surface Preparation
Surface condition directly affects weld metal adhesion and defect formation:
- Shot blasting to Sa 2.5 minimum (ISO 8501-1)
- Removal of all paint, oil, rust, and scale from weld area
- Chamfer preparation: 30-45° bevel with 2-3 mm root face for full-penetration first pass
- Weld area cleaning within 24 hours before welding; re-clean if contaminated
- Surface roughness of 25-50 μm (Ra) provides optimal mechanical keying
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Domain | Standard | Applicability |
|---|---|---|
| Welding procedure qualification | ASME Section IX / AWS D1.1 / GB/T 9445 | WPS/PQR qualification for hard-facing processes |
| Welder qualification | ASME Section IX / AWS D1.1 / GB/T 9445 | Welder certification for overlay welding |
| Welding consumables | ASTM A5.4 / ASTM A5.15 / AWS A5.15 / AWS A5.22 | Electrode and wire specifications for overlay welding |
| NDT - Visual | AWS D1.1 / EN ISO 17637 / GB/T 3323 | Visual inspection of weld surface |
| NDT - Dye penetrant | ASTM E709 / EN ISO 3452 / GB/T 18851 | Surface defect detection on hard-facing layers |
| NDT - Magnetic particle | ASTM E1444 / EN ISO 9934 / GB/T 24511 | Surface/subsurface defect detection (ferromagnetic materials) |
| NDT - Ultrasonic | ASTM E164 / ASME Section V Article 4 | Internal defect detection in thick overlay deposits |
| Hardness testing | ASTM E18 (Rockwell) / ASTM E384 (Vickers) | Verification of layer hardness and hardness profile |
| Mechanical properties | ASTM A370 / GB/T 228 / GB/T 229 | Tensile and impact testing of weld metal |
| Surface preparation | ISO 8501-1 / Sa 2.5 | Pre-weld surface cleanliness |
| Quality management | ISO 9001:2015 | Quality management system compliance |
| Equipment qualification | NB/T 47014 (TSG Z0004) | Pressure equipment welding procedure qualification (if applicable) |
5.2 Acceptance Criteria
Hard-facing weld overlay on CDG squeeze rollers must meet the following acceptance criteria:
- Visual inspection (VT): No surface cracks, undercut exceeding 0.5 mm, porosity, or incomplete fusion visible to the naked eye. Weld surface should be uniform with consistent bead profile.
- Dye penetrant testing (PT): 100% coverage of all hard-facing surfaces. No indications of linear defects (cracks) permitted. Round indications (porosity) limited to 1-3 mm diameter, not more than 2% of total surface area.
- Magnetic particle testing (MT): Applied to all weld areas. No indications of cracks, lack of fusion, or incomplete penetration. Acceptance per AWS D1.1 Section 6.
- Hardness verification: Minimum 50 HRC (or 580 HV) for working layer. Hardness gradient from surface to base should be gradual without sharp discontinuities. Maximum hardness in base steel near fusion line should not exceed 35 HRC (to prevent cold cracking).
- Dimensional tolerance: Final roller diameter within ±0.5 mm of drawing specification. Surface roughness Ra 1.6-3.2 μm after grinding. Runout ≤0.1 mm TIR.
- Impact testing (if required): Charpy V-notch impact energy ≥47 J at -20°C for transition layer (per ASME Section IX qualification requirements).
6. Common Risks and Controls
6.1 Technical Risks
| Risk | Cause | Detection Method | Control Measure |
|---|---|---|---|
| Hot cracking in hard-facing layer | High sulfur/phosphorus in base steel; excessive travel speed; improper alloy composition | VT, PT, MT | Control base steel chemistry; use proper preheat; select appropriate alloy; maintain interpass temp |
| Cold cracking at fusion boundary | High carbon equivalent of base steel; hydrogen in weld metal; rapid cooling | MT, UT (delayed) | Preheat 200-300°C; use low-hydrogen consumables; apply post-weld heat treatment; control dilution |
| Excessive dilution | Poor technique; high current; weaving too wide; insufficient preheat | Hardness testing of cross-section | Use stringer beads; thin layers; proper preheat; transition layer; monitor hardness profile |
| Poor weld adhesion | Inadequate surface preparation; contamination; insufficient first-pass penetration | VT, MT,剥离测试 (adhesion test) | Sa 2.5 surface prep; thorough cleaning; full-penetration first pass; proper weld sequence |
| Porosity in deposit | Contaminated surface; inadequate shielding; wet electrodes | VT, PT, MT | Proper surface cleaning; adequate gas flow; electrode bake and storage control |
| Residual stress causing distortion | Excessive heat input; improper weld sequence; no stress relief | Strain gauge measurement; dimensional check | Control heat input; use balanced weld sequence; post-weld stress relief; back-step welding |
6.2 Quality Risks
- Welder skill variation: Hard-facing requires specialized technique different from structural welding. Control through formal training, written procedure qualification, and ongoing performance monitoring.
- Consumable traceability: Ensure all welding consumables are certified, properly stored, and traceable to heat numbers. Maintain electrode bake records and wire lot documentation.
- Equipment calibration: Regular calibration of preheat thermometers, IR pyrometers, hardness testers, and NDT equipment per ISO/IEC 17025 requirements.
- Documentation completeness: Maintain complete quality records including WPS/PQR, welder qualifications, NDT reports, hardness maps, dimensional reports, and material certificates.
7. Application Across Company Technology Routes
7.1 TIG/MIG Weld Overlay Route (Primary Application)
This capability is the core application of the company's TIG/MIG weld overlay technology route. The CDG squeeze roller hard-facing project demonstrates:
- Multi-layer overlay expertise: Ability to design and execute complex layer configurations with different alloy compositions for graded performance
- Large-diameter cylindrical welding: Technical capability for welding on large-diameter rollers requiring specialized position control and sequencing
- Integration of fabrication and overlay: End-to-end capability from roller forging through final hard-facing delivery
- Post-weld machining: Ability to grind hard-facing layers to precise dimensional tolerances without damaging the overlay
- WPS qualification for overlay: Development of qualified welding procedures specifically for hard-facing applications (distinct from structural welding procedures)
7.2 Hydraulic Explosive Bonding Route (Complementary Application)
While hard-facing is the primary technology for CDG squeeze rollers, hydraulic explosive bonding offers an alternative approach for certain roller applications:
- Full-surface cladding: For rollers requiring uniform wear protection over the entire cylindrical surface, hydraulic explosive bonding can achieve 100% metallurgical bond between base steel and wear-resistant cladding layer without dilution
- Thick cladding layers: Hydraulic explosive bonding can produce cladding layers of 5-25 mm thickness in a single operation, compared to 6-15 mm achievable through multi-pass welding
- Zero dilution: The explosive bonding process produces no dilution between base and cladding layers, ensuring full hardness and wear properties of the cladding material
- Hybrid approach: For CDG rollers, a hybrid approach may be employed: hydraulic explosive bonding for the main body cladding, followed by TIG hard-facing for localized high-wear zones or repair areas
- Material flexibility: Hydraulic explosive bonding enables bonding of dissimilar materials (e.g., austenitic manganese steel to carbon steel base) that would be difficult to achieve through conventional welding
7.3 Explosion Welding Route (Alternative Approach)
Explosion welding (explosive cladding) provides another alternative for CDG roller applications:
- Large-format cladding: For large-diameter rollers or roller assemblies, explosion welding can clad large areas in a single operation with excellent metallurgical bond quality
- High bond strength: Explosion welding produces bond strengths typically exceeding 90% of the weaker base material's tensile strength, ensuring reliable long-term performance
- Complex geometries: Explosion welding can accommodate rollers with complex surface geometries including grooves, ribs, and profiled working surfaces
- Material combinations: Enables bonding of materials with vastly different thermal expansion coefficients, melting points, and metallurgical properties — combinations that would be impossible through fusion welding
- Post-weld processing: Explosion-welded cladding can be machined, drilled, and formed post-bonding, allowing final dimensional accuracy to be achieved after the bonding process
7.4 Comparative Technology Selection Matrix
| Selection Criterion | TIG/MIG Hard-Facing | Hydraulic Explosive Bonding | Explosion Welding |
|---|---|---|---|
| Layer thickness | 6-15 mm | 5-25 mm | 3-15 mm |
| Dilution | 10-30% (controllable) | Zero | Zero |
| Surface coverage | Local or full | Full surface | Full surface |
| Material flexibility | High | Very high | Very high |
| Equipment requirement | Welding machines, gas supply | Hydraulic press, bonding apparatus | Explosives, safety infrastructure |
| Repair capability | Excellent (field repair) | Limited (factory only) | Limited (factory only) |
| Cost per unit area | Moderate | Low (large volumes) | High |
| Best for CDG rollers | ✓ Primary choice | Full-surface protection | Specialized applications |
8. Contribution to Qualification Building and Customer Value
8.1 Qualification Building
The CDG squeeze roller hard-facing capability contributes significantly to the company's qualification portfolio:
- WPS/PQR qualification: Development of qualified welding procedures for hard-facing applications on carbon and low-alloy steel substrates, including specific procedures for 309L transition layers and various hard-facing alloy compositions
- Welder certification: Training and qualification of welders in specialized overlay welding techniques, including multi-layer deposition, dilution control, and cylindrical surface welding
- Material qualification: Verification of specific hard-facing alloy consumables (ERCr20Cu, ERCr26, ERNiCrSiB, etc.) for CDG roller applications through destructive and non-destructive testing
- NDT procedure qualification: Development and qualification of NDT procedures specifically tailored for hard-facing weld inspection, accounting for the unique microstructure and hardness gradients of overlay welds
- Industry-specific qualification: Building track record in the coal chemical and power generation sectors, enabling participation in qualified supplier lists for major EPC contractors and end-users
8.2 Product Delivery Value
- Turnkey delivery: Ability to deliver complete roller assemblies (forged, machined, heat-treated, hard-faced, ground, and inspected) as a single integrated product
- Reduced project risk: Single-source supply eliminates interface risks between fabrication and overlay suppliers, ensuring consistent quality and accountability
- Accelerated schedules: Integrated manufacturing capability reduces project timelines by eliminating sequential handoffs between separate suppliers
- Warranty confidence: Comprehensive quality control from raw material through final inspection enables confident warranty commitments to customers
- Customization capability: Flexibility to tailor layer configurations, alloy selections, and dimensional specifications to specific customer operating conditions
8.3 Customer Value Proposition
The CDG squeeze roller hard-facing capability delivers measurable value to customers through:
- Extended equipment availability: 3-8x longer roller service life translates directly to increased plant throughput and reduced production losses
- Reduced maintenance costs: Fewer roller replacements and rebuilds reduce both capital and operating expenditures
- Predictable maintenance planning: Consistent wear rates with properly hard-faced rollers enable planned maintenance scheduling rather than reactive emergency repairs
- Technical support: Access to metallurgical expertise for wear analysis, failure investigation, and specification optimization
- Supply chain security: Qualified domestic supplier reduces dependence on imported rollers and associated logistics delays
9. Continuous Improvement and Future Development
9.1 Process Optimization Directions
- Robotized overlay welding: Implementation of robotic TIG/MIG welding for consistent bead placement, reduced operator fatigue, and improved productivity on large-diameter rollers
- Real-time monitoring: Integration of arc sensing, temperature monitoring, and deposition rate measurement for in-process quality control
- Advanced alloy development: Evaluation of new hard-facing consumables including nano-reinforced alloys, high-entropy alloys, and functionally graded materials
- Surface engineering integration: Combination of hard-facing with additional surface treatments (shot peening, induction hardening, PVD coating) for enhanced performance
9.2 Digital Transformation
- Digital twin development: Creating virtual models of roller wear behavior to predict service life and optimize replacement intervals
- IoT-enabled monitoring: Embedding sensors in roller assemblies for real-time condition monitoring and predictive maintenance
- Quality traceability systems: Implementing blockchain or digital record systems for complete material and process traceability throughout the manufacturing lifecycle
- AI-assisted NDT: Application of machine learning algorithms for automated defect recognition and classification in NDT imagery
10. Conclusion
The CDG roller press squeeze roller fabrication and hard-facing capability represents a mature, high-value technical competency within the company's TIG/MIG weld overlay technology portfolio. This capability enables the company to serve the critical coal chemical and power generation sectors with technically superior, cost-effective solutions that deliver measurable operational benefits to customers. Through rigorous adherence to international standards (ASME, AWS, ASTM, ISO), systematic qualification of procedures and personnel, and continuous process improvement, the company maintains a competitive position in the domestic and international markets for heavy-duty roller hard-facing services. The integration of fabrication and overlay capabilities within a single qualified organization provides unique value through reduced project risk, accelerated delivery schedules, and comprehensive technical support — establishing the company as a preferred supplier for CDG roller press systems in the coal-to-chemicals industry.