High-Temperature Wear Behavior of Hard Surfacing Layers on 42CrMo Continuous Casting Rolls
1. Definition and Fundamental Principles
Continuous casting rolls are critical rotary components in steelmaking that guide and shape molten steel during solidification. The 42CrMo alloy steel substrate—characterized by its high tensile strength (≥1080 MPa), excellent toughness, and good hardenability—serves as the base material for high-performance casting rolls. Hard surfacing weld overlay is applied to the working surface of these rolls to create a thermally and mechanically resistant layer capable of withstanding the extreme conditions of the casting process, including temperatures exceeding 1400°C at the roll surface, intense thermal cycling, and abrasive contact with solidifying steel.
The high-temperature wear behavior of hard surfacing layers encompasses a complex interaction of multiple wear mechanisms operating simultaneously:
- Abrasive wear: Caused by hard inclusions (Al₂O₃, MnS) in the solidifying steel shearing through the overlay surface at elevated temperatures.
- Adhesive wear: Occurs when localized welding of the overlay to the steel surface happens at contact points under high pressure and temperature.
- Oxidative wear: Accelerated oxidation of the overlay surface in the presence of moisture from the secondary cooling water spray, forming brittle oxide scales that spall.
- Thermal fatigue cracking: Repeated thermal cycling induces residual stresses that cause microcracking in the overlay and at the overlay-substrate interface.
- Hot hardness degradation: At sustained temperatures above 600°C, carbide phases may soften or dissolve, reducing the overlay's resistance to deformation.
The governing principle is that the hard surfacing alloy must maintain sufficient hardness and microstructural stability at the operating temperature while retaining adequate fracture toughness to resist thermal shock and impact loading from the casting process. The carbide system (WC, Cr₇C₃, Cr₃C, Mo₂C) and matrix composition are the primary levers for achieving the desired high-temperature wear resistance.
2. Category and Business Positioning
This technical capability falls squarely within the TIG/MIG Weld Overlay technology route of Cladding Technology Shanxi Co., Ltd. The continuous casting roll hard surfacing application represents a high-value, technically demanding segment of the weld overlay business that requires:
- Precision control of overlay thickness and uniformity across large cylindrical surfaces
- Deep understanding of high-temperature metallurgy and wear mechanisms
- Ability to deliver qualified WPS/PQR packages for critical safety components
- On-site service capability for emergency roll repair and scheduled maintenance
Within the company's portfolio, this entry positions the organization as a specialist in hot metal processing equipment surface protection, targeting the steel industry's continuous casting operations where roll downtime directly impacts production throughput and profitability.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
- Extend roll service life by 2–5× compared to uncoated or conventionally hardened rolls
- Maintain dimensional accuracy of the casting mold profile throughout the roll's operational life
- Reduce frequency of roll removal, grinding, and replacement operations
- Minimize surface defects (dents, scratches, scale buildup) on the cast slab/strand surface
3.2 Customer Value
For steel producers, roll wear directly translates to production losses. Each hour of unplanned roll change represents approximately $50,000–$150,000 in lost production value depending on the mill capacity. Hard surfacing solutions that double or triple roll life provide immediate ROI. Furthermore, improved roll surface quality reduces downstream finishing costs and improves slab yield quality.
4. Key Process and Implementation Points
4.1 Overlay Alloy Selection for High-Temperature Wear
| Overlay System | Key Hardness Phases | HV Hardness (Room Temp) | Effective Temp Range | Primary Wear Mechanism Resistance |
|---|---|---|---|---|
| WC-Co (e.g., Stellite 6 + WC) | WC, Co solid solution | 1200–1500 | Up to 900°C | Abrasive, adhesive |
| Cr-Cr₇C₃ (e.g., Stellite 12) | Cr₇C₃, Cr₂3C₆ | 400–550 | Up to 1100°C | Oxidative, abrasive |
| Cr₃C₃-Ni (e.g., Stellite 6) | Cr₂3C₆, Ni solid solution | 400–450 | Up to 1000°C | Thermal fatigue, adhesive |
| Mn-Cr-Ni austenitic (e.g., Stellite 21) | Matrix toughening | 200–300 | Up to 800°C | Thermal shock, impact |
| Cr-C-Mo martensitic | Fe₃C, Mo₂C, tempered martensite | 550–700 | Up to 650°C | Abrasive (lower temp) |
4.2 Multi-Layer Design Strategy
For continuous casting rolls operating at the mold section, a multi-layer approach is recommended:
- Transition layer: 1–2 mm of 309L or 310 stainless steel deposited by TIG to ensure metallurgical compatibility between the 42CrMo substrate and the hard surfacing alloy, reducing residual stresses and preventing cracking at the interface.
- Intermediate layer: 1–3 mm of austenitic alloy (Stellite 21 or similar) to absorb thermal cycling stresses and provide a tough buffer zone.
- Working layer: 2–5 mm of the selected hard surfacing alloy (Stellite 6, Stellite 12, or WC-Co composite) deposited by MIG or TIG to provide the primary wear resistance.
4.3 Critical Process Parameters
| Parameter | Recommended Range | Rationale |
|---|---|---|
| Preheat temperature (42CrMo substrate) | 200–300°C | Reduce hydrogen cracking risk; 42CrMo is susceptible to cold cracking |
| Interpass temperature | ≤250°C | Prevent grain coarsening and maintain hardness in previous passes |
| Heat input per pass | 0.8–1.5 kJ/mm | Minimize dilution while ensuring adequate fusion |
| Post-weld heat treatment | 600–650°C × 2h, furnace cool | Relieve residual stresses; avoid temper brittleness range (425–525°C) |
| Final surface finish | Ground to Ra ≤ 0.8 μm | Ensure smooth contact surface for casting mold function |
| Dilution control | ≤20% substrate in first overlay pass | Maintain overlay alloy properties |
4.4 High-Temperature Wear Behavior Analysis
The wear performance of the hard surfacing layer at operating temperature is governed by the following factors:
4.4.1 Carbide Stability at Elevated Temperatures
- WC particles in Co-matrix alloys remain stable up to 900°C but may oxidize to W₂C (lower hardness) beyond 700°C in oxidizing environments
- Cr₇C₃ carbides in Fe-Cr-C alloys are thermodynamically stable up to 1100°C, making them ideal for the hottest zones of the casting roll
- Cr₂₃C₆ carbides dissolve above 800°C, leading to softening of Cr₃C₃-based overlays in the mold section
4.4.2 Matrix Softening Behavior
At temperatures exceeding 0.5Tm (homologous temperature), the metallic matrix undergoes accelerated creep and dislocation recovery. For cobalt-base alloys (Tm ≈ 1495°C), this threshold is approximately 750°C. Nickel-base alloys (Tm ≈ 1455°C) show similar behavior. Iron-base martensitic overlays soften significantly above 500°C due to carbide coarsening and tempering of the matrix.
4.4.3 Oxidation-Induced Wear Acceleration
In the secondary cooling zone, the roll surface experiences alternating wet-dry conditions. The formation of Fe₂O₃ and Cr₂O₃ scales on the overlay surface, combined with thermal cycling, leads to:
- Scale spallation exposing fresh overlay material to accelerated wear
- Sub-scale void formation at the overlay-oxide interface
- Reduction of effective overlay thickness over time
5. Applicable Standards and Acceptance Criteria
5.1 Material Standards
- GB/T 1299-2014: Alloy tool steels — 42CrMo substrate specification
- ASTM A29/A29M: Standard specification for wrought and cast alloy steel bars and shapes
- GB/T 3403-2017: Welding consumables for cast iron and steel hard surfacing
- ASTM A567/A567M: Standard specification for castings, iron, for general engineering purposes (reference for roll casting)
5.2 Welding Procedure Standards
- GB/T 985-2008: TIG welding qualification test requirements
- ASME Section IX: Qualification of welding procedures and personnel
- NB/T 47014-2011: Qualification test of welding procedure for pressure vessels
- ISO 15614-1:2017: Qualification testing of welding procedures for metallic materials — General rules
- ISO 15614-6:2016: Qualification testing of welding procedures for metallic materials — Gas tungsten arc welding (GTAW)
5.3 Non-Destructive Testing Standards
- GB/T 11345-2013: Ultrasonic testing of welds
- GB/T 1805-2019: Magnetic particle testing methods
- ASTM E1417/E1417M: Standard practice for magnetic particle testing
- ASTM E709/E709M: Standard practice for electromagnetic (eddy current) testing of metal products
- GB/T 1955-2018: Liquid penetrant testing method
5.4 Acceptance Criteria for Cast Roll Overlay
| Test Method | Acceptance Standard | Inspection Frequency |
|---|---|---|
| Magnetic Particle Testing (MT) | No linear indications ≥ 2 mm; no cluster of indications exceeding 10 mm in any direction | 100% of overlay surface |
| Ultrasonic Testing (UT) | No indications of delamination or lack of fusion at overlay-substrate interface; per GB/T 11345 Level 2 | 100% at weld toes; spot check on overlay |
| Hardness Testing | Overlay: ≥ specified value (e.g., HV 400-500 for Stellite 6); Transition layer: HV 200-350 | 5 points per roll per shift |
| Dimensional Check | Overlay thickness uniformity within ±0.5 mm; surface profile Ra ≤ 0.8 μm after grinding | 100% dimensional survey |
| Dye Penetrant Testing (PT) | No surface-breaking indications ≥ 0.5 mm | 100% of overlay surface |
6. Common Risks and Controls
6.1 Metallurgical Risks
| Risk | Cause | Control Measure |
|---|---|---|
| Cold cracking in 42CrMo substrate | High carbon equivalent (CE ≈ 0.35); hydrogen from arc; rapid cooling | Preheat to 250°C; low hydrogen consumables; controlled cooling rate; post-weld bake at 300°C for 4h | Overlay cracking | High residual stress; insufficient ductility of overlay; thermal mismatch | Multi-pass with interpass temperature control; include ductile transition layer; PWHT |
| Delamination at interface | Insufficient fusion; contamination; thermal shock | Thorough surface preparation (grind to bare metal); ensure adequate heat input for fusion; visual + UT verification |
| Hot cracking in overlay | Sulfur/phosphor segregation; columnar grain structure | Use low-S, low-P consumables; proper travel speed to promote equiaxed grains |
| Temper embrittlement of substrate | PWHT in 425-525°C range | Avoid temper embrittlement range; use 600-650°C PWHT or 250°C stress relief |
6.2 Process Risks
- Excessive dilution: Results in softer overlay with reduced wear resistance. Control by limiting first pass depth, using proper travel speed, and employing low-heat-input techniques.
- Porosity: Caused by moisture contamination or improper shielding gas flow. Control by using dry consumables, ensuring adequate gas coverage, and maintaining clean work surfaces.
- Uneven thickness: Results in uneven wear life and potential premature failure at thin spots. Control by using automated or semi-automated deposition with thickness monitoring.
- Surface defects after grinding: Grinding burns can create localized hardening or cracking. Control by using appropriate grinding wheel Grit size progression (60→120→240) and coolant application.
6.3 Service Risks
- Spalling under thermal shock: Sudden water contact on hot roll surface can cause overlay spalling. Control by selecting overlay with adequate fracture toughness and thermal fatigue resistance.
- Progressive wear exceeding design limits: Wear rate varies with steel grade, casting speed, and cooling conditions. Control by periodic thickness monitoring and establishing wear allowance limits.
- Galvanic corrosion at overlay edges: Exposure of bare substrate at overlay boundaries in the cooling water environment. Control by ensuring complete surface coverage and proper edge treatment.
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay (Primary Route for Cast Roll Application)
This is the dominant technology route for continuous casting roll hard surfacing. The specific implementation includes:
- TIG (GTAW) for transition layers: Precise heat input control ensures minimal substrate dilution while achieving sound metallurgical bonding. Used for the 309L/310 transition layer on 42CrMo.
- MIG (GMAW) for build-up layers: Higher deposition rate (3–5 kg/h vs. 0.5–1 kg/h for TIG) makes it economical for depositing multiple layers of hard surfacing alloy on large roll surfaces.
- Automated orbital welding: For cylindrical roll geometry, automated TIG systems with tracking capability ensure consistent bead quality and uniform thickness around the roll circumference.
- Flux-cored arc welding (FCAW): For field repair applications where shielding gas is impractical, FCAW with self-shielded flux-cored wire provides adequate performance for emergency roll restoration.
7.2 Hydraulic Explosive Bonding (HEB) — Limited Direct Application
While hydraulic explosive bonding is primarily used for large-format clad plate and pipe manufacturing, the knowledge gained from high-temperature wear behavior studies contributes to:
- Design of clad plate products where a 42CrMo or similar alloy substrate is bonded to a hard surfacing alloy for use in hot metal processing equipment
- Development of explosion-welded cladding for guide plates, wear strips, and structural components in steel mill equipment that experience similar thermal and abrasive conditions
- Understanding of interface metallurgy (diffusion bonding, intermetallic formation) that parallels weld overlay interface behavior
7.3 Explosion Welding — Indirect Application
Explosion welding technology finds application in manufacturing clad components for steel mill infrastructure where:
- Large-area cladding of 42CrMo or Cr-Mo steel substrates with corrosion and wear-resistant overlays is required for structural components (e.g., ladle walls, furnace linings, transfer car components)
- The high-velocity impact bonding creates a metallurgical bond without melting, preserving the base metal's mechanical properties — advantageous for components requiring both toughness and surface protection
- Post-explosion grinding and machining can achieve similar surface finish requirements as weld overlay grinding
8. Contribution to Qualification Building and Customer Value
8.1 Qualification and Certification Value
Developing deep technical understanding of high-temperature wear behavior on 42CrMo cast rolls directly contributes to:
- WPS/PQR qualification packages: Qualified welding procedure specifications for 42CrMo substrate with specific overlay alloys, validated for the thermal and mechanical conditions of continuous casting applications.
- ISO 3834-2 certification: Demonstrating systematic understanding of process variables and their effects on weld quality supports quality management system certification.
- ASME Section IX qualification: Procedure qualification records that demonstrate capability to weld overlay systems on alloy steel substrates.
- Industry-specific qualifications: Steel plant suppliers often require qualification testing at their facilities; technical expertise accelerates this process and builds credibility.
8.2 Product Delivery Value
- Reduced warranty claims: Understanding wear mechanisms enables proper alloy selection and process specification, reducing premature failure and associated warranty costs.
- Technical consultation capability: Ability to advise customers on overlay selection, maintenance schedules, and wear monitoring based on metallurgical understanding.
- Custom solution development: For specific steel grades or casting conditions, ability to recommend tailored overlay systems based on wear mechanism analysis.
- Faster qualification cycles: Knowledge of failure modes and their prevention reduces the number of iterations needed to qualify new procedures.
8.3 Customer Value Enhancement
"The value of hard surfacing on continuous casting rolls is not merely in extending roll life, but in providing predictable, consistent performance that enables steel producers to optimize their casting campaign planning, reduce unplanned downtime, and maintain consistent slab surface quality throughout the roll's service life."
Specific customer benefits include:
- Quantifiable ROI: Typical payback period of 3–6 months based on extended roll life and reduced grinding/replacement frequency
- Reduced environmental impact: Less frequent roll replacement means reduced energy consumption in roll manufacturing and less scrap generation
- Improved product quality: Smoother, more consistent roll surface produces higher-quality slab surfaces, reducing downstream processing requirements
- Safety improvement: Predictable roll performance reduces the risk of emergency roll changes during hot operations, which are inherently dangerous
9. Recommended Implementation Protocol
- Pre-inspection: Verify roll condition (cracks, dents, corrosion) via MT and visual inspection. Repair any substrate defects before overlay application.
- Surface preparation: Grind existing coating or hardened layer to bare metal with 60-grit wheel; clean with solvent to remove all contaminants.
- Preheat: Apply uniform preheat of 250°C using induction heating or torch; verify with infrared thermometer at multiple points.
- Transition layer deposition: Apply 1–2 mm of 309L or 310 by TIG with low hydrogen electrode or solid wire; maintain interpass temperature ≤ 250°C.
- Hard surfacing deposition: Apply 2–5 mm of selected overlay alloy by MIG in 2–3 passes; maintain interpass temperature ≤ 200°C.
- Post-weld heat treatment: Furnace cool to 600°C, hold 2 hours, furnace cool to below 200°C before air cooling.
- Machining/grinding: Grind to final profile and surface finish; use progressive grit sizes; apply coolant.
- Final NDT: Perform MT and PT on 100% of overlay surface; perform UT on weld toes and interface; record all results.
- Hardness verification: Test at 5+ locations per roll; confirm values within specification.
- Documentation: Compile complete inspection report with all test results, WPS reference, operator qualification, and material certifications.
10. Conclusion
Mastery of high-temperature wear behavior for hard surfacing layers on 42CrMo continuous casting rolls represents a critical technical competency that differentiates a qualified overlay service provider from a commodity welding contractor. This knowledge enables proper alloy selection, optimized process design, effective quality control, and ultimately, superior service life in the demanding environment of continuous steel casting. By integrating this understanding across the company's TIG/MIG weld overlay operations and informing the broader clad plate and pipe product lines, Cladding Technology Shanxi Co., Ltd. delivers measurable value to steel industry customers while building a robust qualification portfolio that supports long-term market positioning.