Influence of Weld Overlay Layer Composition and Microstructure on Hot Fatigue and Wear Resistance of Hot Rolled Rolls
1. Definition and Fundamental Principles
The weld overlay layer applied to hot rolled work rolls in steel rolling mills is subjected to a uniquely severe combination of thermal cycling, mechanical loading, and chemical attack from scale and oxidation products. The composition and microstructure of the overlay layer directly govern its resistance to two dominant failure modes: hot fatigue (thermally induced cracking from repeated heating and cooling cycles) and wear (material removal through abrasive, adhesive, and oxidative mechanisms). Understanding the relationship between overlay alloy design, solidification behavior, heat-affected zone (HAZ) metallurgy, and resulting tribological/thermal performance is essential for specifying and qualifying overlay systems for hot rolling applications.
The fundamental principle is that a well-designed weld overlay layer must simultaneously provide:
- High-temperature strength to resist plastic deformation and thermal shock cracking at operating temperatures of 800–1200 °C
- Thermal fatigue resistance through microstructural features that arrest crack initiation and propagation (e.g., fine grain size, controlled carbide morphology, retained austenite stabilization)
- Hardness retention at elevated temperatures to maintain wear resistance throughout the rolling cycle
- Low thermal expansion mismatch with the roll substrate to minimize residual stresses at the bond line
2. Category and Business Positioning
This technical knowledge area falls squarely within the Weld Overlay Technology business line of Cladding Technology Shanxi Co., Ltd., specifically addressing the metallurgical qualification and performance optimization of overlay systems for the metallurgical equipment refurbishment market. It bridges the gap between overlay welding process execution and the end-use performance requirements of rolling mill operations.
Within the company's three primary technology routes:
- TIG/MIG Weld Overlay: Primary route for applying precision-controlled overlay compositions to work roll surface and body
- Hydraulic Explosive Bonding: Complementary route for bonding overlay material to large-diameter roll bodies where thermal input must be minimized
- Explosion Welding: Alternative for rapid production of clad roll blanks with uniform overlay thickness
3. Technical Purpose and Value
3.1 Purpose
The systematic study of overlay layer composition-microstructure-performance relationships serves the following purposes:
- WPS Development and Qualification: Providing the metallurgical justification for selecting specific consumable alloys (e.g., Ni-Cr-Mo austenitic, Co-Cr alloy, or high-silicon cast iron) for particular rolling conditions
- Failure Analysis and Root Cause Determination: Enabling rapid diagnosis of premature overlay failure through microstructural examination (carbide morphology, intergranular vs. transgranular cracking, phase transformations)
- Customer Technical Support: Delivering evidence-based recommendations for overlay specification, thickness, and post-weld treatment to optimize roll life
- IP and Qualification Building: Generating proprietary performance data that differentiates the company's overlay solutions in competitive tendering
3.2 Value to Customer
By demonstrating quantitative understanding of how overlay metallurgy impacts roll life, the company can:
- Reduce unscheduled mill stoppages by predicting and preventing overlay-related roll failures
- Increase rolls-per-change intervals, directly reducing unit rolling costs
- Provide traceable metallurgical documentation supporting warranty claims and performance guarantees
4. Key Process and Implementation Points
4.1 Overlay Alloy Selection Matrix
| Overlay Type | Typical Composition (wt%) | Microstructure | Hot Fatigue Resistance | Wear Resistance | Typical Application |
|---|---|---|---|---|---|
| Ni-Cr-Mo Austenitic | Ni 35-40, Cr 25-30, Mo 2-4 | Single-phase austenite with fine M₇C₃ carbides | Excellent | Good | Hot strip finishing mills, slab caster rolls |
| Co-Cr Alloy | Co 60-65, Cr 25-30, W 3-5 | γ matrix with M₆C carbides | Outstanding | Excellent | High-temperature heavy plate mills |
| High-Si Cast Iron | Si 15-25, Cr 2-5, Mn 1-3 | Matrix + dispersed Si particles | Moderate | Very High (abrasive) | Hot slab mills, roughing stands |
| Martensitic Cr-Steel | Cr 8-12, C 0.5-1.0 | Tempered martensite + carbides | Poor-Moderate | High (cold) | Not recommended for hot rolling; cold work rolls only |
4.2 Critical Microstructural Features Affecting Performance
| Microstructural Feature | Effect on Hot Fatigue | Effect on Wear Resistance | Control Method |
|---|---|---|---|
| Grain size (overlay) | Fine grains delay crack initiation; large grains facilitate intergranular cracking | Fine grains improve hardness and toughness balance | Control heat input (q ≤ 15 kJ/mm for TIG); interpass temperature ≤ 150 °C |
| Carbide morphology | Continuous grain-boundary carbide networks act as crack paths | Discrete, rounded carbides improve abrasion resistance | Limit C and Cr content; avoid excessive cooling rates |
| Retained austenite fraction | Transformable austenite absorbs thermal strain energy, delaying cracking | Excessive austenite reduces hardness; optimal range 5-15% | Control Ni/Cr ratio; post-weld tempering at 650-750 °C |
| HAZ microstructure | Coarse HAZ grains reduce thermal fatigue life at bond line | Softened HAZ becomes preferential wear zone | Multi-pass welding with narrow HAZ; controlled preheat |
| Porosity and inclusions | Act as crack initiation sites under cyclic thermal loading | Reduce effective bearing area | Shielding gas purity ≥ 99.99%; dry consumable storage |
4.3 Welding Process Parameters for Optimal Microstructure
| Parameter | TIG (GTAW) Overlay | MIG (GMAW) Overlay | Target |
|---|---|---|---|
| Current | 80-150 A | 150-250 A | Deposition rate matched to thermal budget |
| Travel speed | 20-40 mm/min | 30-60 mm/min | Heat input 5-15 kJ/mm |
| Preheat | 100-200 °C | 150-250 °C | Reduce HAZ grain growth; minimize residual stress |
| Interpass temperature | ≤ 150 °C | ≤ 200 °C | Prevent grain coarsening in previously deposited layers |
| Post-weld treatment | Temper 650-750 °C × 2h | Temper 650-750 °C × 2h | Stabilize microstructure; relieve residual stresses |
| Shielding gas | Ar (99.99%) | Ar + 5% CO₂ or Ar + 2% O₂ | Minimize oxidation and porosity |
4.4 Layer Design Strategy
For hot rolled rolls, a multi-layer approach is typically required:
- Transition layer (1-2 passes): Ni-base or Ni-Cr alloy with high ductility to accommodate thermal expansion mismatch between substrate and overlay. Typical composition: Ni 50-60%, Cr 20-25%, C ≤ 0.3%.
- Functional overlay layer (2-4 passes): The performance-critical layer with the alloy selected for the specific rolling conditions (temperature, scale aggressiveness, load).
- Surface finishing: Precision grinding to achieve surface roughness Ra ≤ 0.8 μm for finishing mills or Ra 1.6-3.2 μm for roughing mills.
5. Applicable Standards and Acceptance Criteria
5.1 Welding Standards
- GB/T 985.1 — Welding procedure qualification test methods (visual, dimensional, mechanical tests)
- GB/T 19866 — Welding procedure qualification requirements for PTA and TIG overlay
- NB/T 47014 — Qualification test of welding procedures for pressure equipment (applicable by analogy for high-integrity roll overlays)
- ASTM A388/A388M — Standard specification for overlay welding consumables
- ASME Section IX, Part QW-451/QW-452 — Qualification of welding procedures for surfacing
- ISO 13919 — Welding — Qualification of welding procedures — Qualification requirements for TIG welding
5.2 Performance and Inspection Standards
- GB/T 13914 — Non-destructive testing methods for welds
- ASTM E165 — Standard practice for liquid penetrant examination
- ASTM E1444 — Magnetic particle examination
- ASTM E23 — Charpy V-notch impact testing (HAZ toughness verification)
- ASTM G65 — Dry sand/rubber abrasion testing (overlay wear qualification)
- ASTM E466 — Thermal fatigue testing procedures
- ISO 3079 — Metallographic examination of welds
5.3 Acceptance Criteria Summary
| Inspection Item | Acceptance Criteria | Standard Reference |
|---|---|---|
| Visual inspection | No surface cracks, undercut ≤ 0.5 mm, uniform bead profile | GB/T 985.1, ASME Section IX |
| Penetrant testing (PT) | No linear indications; round indications ≤ 3 mm | ASTM E165, Level II |
| Ultrasonic testing (UT) | No volumetric defects > 2 mm equivalent diameter within overlay | GB/T 13914, Level B |
| Hardness | Overlay: 35-55 HRC (Ni-Cr-Mo type); No soft zone > 1 mm below surface | ASTM E18 |
| Impact toughness (HAZ) | CVN ≥ 27 J at 25 °C (or per customer specification) | ASTM E23 |
| Chemical composition | Within ±1.0% of specified consumable composition (Ni, Cr, Mo) | ASTM E415, Spark OES |
| Microstructure | No continuous grain-boundary carbide networks; no untransformed retained austenite > 20% | ISO 3079 |
6. Common Risks and Controls
| Risk | Mechanism | Consequence | Control Measure |
|---|---|---|---|
| Hot cracking in overlay | Solidification cracking due to low melting eutectic phases (Ni-Si, Cr₂O₃ inclusions) | Overlay failure at first thermal cycle; scale penetration | Limit Si and S content; control cooling rate; use pure shielding gas; preheat substrate |
| Intergranular corrosion at overlay/substrate interface | Chromium depletion in HAZ; formation of Cr-poor zone susceptible to oxide attack | Delamination under thermal cycling; reduced bond strength | Use transition layer with high Ni content; minimize HAZ width; post-weld tempering |
| Thermal fatigue cracking | Cyclic tensile stress exceeding overlay's high-temperature tensile strength; crack initiation at microstructural defects | Surface crack propagation; roll body exposure; catastrophic roll failure | Optimize alloy for high-temperature ductility; control grain size; avoid porosity; ensure proper bond line metallurgy |
| Excessive dilution | Substrate iron diluting overlay composition beyond functional range | Loss of corrosion/wear resistance; formation of brittle Fe-Ni-C intermetallics | Use high Ni-base consumables with low dilution sensitivity; control heat input; multi-pass with low penetration |
| Residual stress-induced deformation | Thermal mismatch between overlay and substrate during cooling | Roll runout; overlay spallation; geometric deviation beyond tolerance | Controlled welding sequence (symmetric, spiral); post-weld stress relief at 600-650 °C; preheat |
| Microstructural instability during service | Precipitation of brittle phases (σ-phase, Laves phase) during prolonged high-temperature exposure | Gradual embrittlement; reduced thermal fatigue life | Avoid excessive Mo and Nb; limit Cr to < 30% for long-term service; select alloys with stable phase diagrams |
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay Route
The TIG/MIG weld overlay route is the primary method for applying performance-critical overlay layers to hot rolled work rolls. The knowledge of composition-microstructure-performance relationships directly informs:
- Consumable selection: Matching overlay alloy chemistry to the specific rolling temperature, material being rolled, and expected thermal cycling amplitude
- WPS parameter optimization: Setting heat input, travel speed, and interpass temperature to achieve target grain structure and avoid deleterious phases
- Post-weld heat treatment specification: Determining tempering temperature and duration based on the alloy system's precipitation behavior
- Quality assurance: Defining metallographic acceptance criteria specific to hot fatigue and wear performance requirements
Typical TIG overlay application on a hot strip finishing mill roll: Ni-Cr-Mo consumable (e.g., equivalent to Stellite 6 or Ni 60), 3-4 passes, total overlay thickness 3-5 mm, followed by tempering at 700 °C × 2h. Expected performance: 2-3× improvement in thermal fatigue life compared to unclad roll steel.
7.2 Hydraulic Explosive Bonding Route
Hydraulic explosive bonding (hydroforming-assisted) provides an alternative for applying overlay material to large-diameter roll bodies where excessive heat input from arc welding could compromise the substrate's metallurgical integrity. In this route, the composition-microstructure knowledge is applied to:
- Selecting compatible material pairs: Ensuring the overlay material (e.g., Ni-base plate) has sufficient plastic strain capacity to form a metallurgical bond with the roll steel substrate under hydrostatic pressure
- Determining optimal bond-line microstructure: The cold-worked bond interface must exhibit sufficient interfacial strength while maintaining toughness under thermal cycling
- Post-bonding treatment: Specifying annealing cycles to relieve cold-work-induced stresses in the bond zone without degrading overlay microstructure
7.3 Explosion Welding Route
Explosion welding produces clad roll blanks with uniform overlay thickness and a high-energy bond interface. The metallurgical considerations include:
- Explosive parameter optimization: Adjusting explosive charge, stand-off distance, and flyer velocity to achieve target plastic deformation in the overlay layer without melting or excessive intermetallic formation at the interface
- Interface microstructure control: The collision zone produces a characteristic wave pattern; the amplitude and frequency of this wave influence crack initiation resistance under thermal fatigue
- Subsequent machining allowance: Accounting for the deformed layer thickness when specifying the final overlay thickness after turning/grinding
For explosion-welded hot roll clad blanks, typical parameters include: Ni-base flyer plate (3-6 mm), steel roll body (backer plate), detonation velocity 1800-2200 m/s, collision angle 10-15°, interfacial wave amplitude 50-150 μm.
8. Qualification Building and Knowledge Transfer
8.1 Technical Qualification Documentation
The systematic study of overlay composition-microstructure-performance relationships generates the following qualification assets:
- WPS/PQR packages: Each overlay alloy variant is qualified with full mechanical testing (tensile, hardness, impact) and metallurgical evaluation, traceable to specific rolling mill conditions
- Performance databases: Accumulated field performance data correlating overlay specification with actual roll life in service, enabling data-driven recommendations
- Failure analysis reports: Documented case studies demonstrating the company's diagnostic capability and corrective action expertise
- Training materials: Standardized procedures for field metallurgists to evaluate overlay condition during roll inspection
8.2 Customer Value Proposition
This technical knowledge enables the company to:
- Provide tailored overlay solutions rather than generic consumable recommendations, increasing customer confidence and order value
- Offer predictive maintenance support by identifying microstructural degradation indicators before catastrophic failure occurs
- Differentiate in competitive tendering with documented metallurgical expertise and performance guarantees
- Build long-term technical partnerships with steel producers who value engineering support over commodity welding services
8.3 Continuous Improvement Cycle
The learning outcomes from this technical study feed directly into a PDCA cycle:
Plan: Select overlay composition based on rolling conditions and metallurgical requirements
Do: Execute qualified WPS with controlled parameters and NDT verification
Check: Monitor roll life in service; perform post-service metallurgical examination
Act: Refine alloy specification, welding parameters, and post-weld treatment based on field feedback
9. Conclusion
The influence of weld overlay layer composition and microstructure on hot fatigue and wear resistance represents a core competency that directly determines the success of overlay solutions in hot rolling applications. By maintaining rigorous metallurgical knowledge, systematic qualification documentation, and continuous improvement based on field performance data, Cladding Technology Shanxi Co., Ltd. delivers overlay solutions that extend roll life, reduce unplanned downtime, and provide measurable cost savings to steel production customers. This technical foundation underpins all three technology routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding—and ensures that every overlay solution is engineered for the specific thermal-mechanical demands of the customer's rolling mill.