Cracking Mechanisms in Pinch Roll Weld Overlay and Preventive Engineering Controls
1. Definition and Technical Context
1.1 What Are Pinch Rolls?
Pinch rolls (also referred to as transfer rolls or interstand transfer rolls) are critical components in hot strip and hot coil rolling mill lines. They are responsible for transferring hot steel strips between rolling stands at elevated temperatures—typically between 700°C and 1,200°C—while simultaneously providing controlled tension to the strip. Due to the extreme operating environment characterized by thermal cycling, mechanical abrasion, and chemical oxidation, pinch rolls suffer progressive surface degradation and require periodic weld overlay (surfacing) restoration to recover dimensional accuracy and surface integrity.
1.2 The Cracking Problem
Weld overlay of pinch rolls presents one of the most challenging applications in industrial surfacing because of the combination of:
- Thick substrate sections (typically 300–800 mm diameter) with high thermal mass
- Residual stresses from prior manufacturing (forging, machining, hardening)
- High carbon equivalent of the substrate steel (commonly 42CrMo, 38CrMoAlA, or similar alloy steels)
- Repeated thermal cycling during overlay and subsequent service
- Hardfacing materials with high dilution sensitivity and restricted solidification cracking resistance
The study of "Cracking in Pinch Roll Weld Overlay and Its Generation Mechanism" represents a systematic engineering knowledge base addressing the metallurgical root causes, process-induced factors, and preventive measures essential for defect-free surfacing of these critical mill components.
2. Classification of Cracks in Pinch Roll Weld Overlay
2.1 Cracks by Timing of Occurrence
| Crack Type | Timing | Location | Primary Driving Force |
|---|---|---|---|
| Hot Cracks (Solidification Cracks) | During solidification (TMS to TMS) | Last solidifying interdendritic regions | Tensile stress + low-ductility mushy zone |
| Warm Cracks (Thermal Cracks) | During cooling (TMS to ~200°C) | Grain boundaries, HAZ | Phase transformation strains + hydrogen embrittlement |
| Cold Cracks (Delayed Cracks) | After cooling to room temperature (hours to days) | HAZ or near-weld region | Hydrogen + hardness + residual stress |
2.2 Cracks by Morphology
- Longitudinal Cracks: Parallel to the roll axis; caused by circumferential restraint from the thick substrate and the direction of thermal contraction
- Circumferential Cracks: Around the roll circumference; caused by axial restraint and thermal gradient
- Radial Cracks: Extending from surface toward center; caused by radial thermal stress gradients in thick sections
- Interpass Cracks: Occurring in previously deposited layers upon deposition of subsequent passes
- Toe Cracks: At the weld toe; caused by stress concentration and inadequate fusion
3. Metallurgical Mechanisms of Cracking
3.1 Solidification Cracking (Hot Cracking)
Solidification cracking occurs when the tensile strain rate imposed on the weld metal exceeds the available creep strain in the mushy zone (the region between the solidus and liquidus temperatures). The three essential conditions are:
- Restricted deformation: The thick pinch roll substrate provides significant restraint against shrinkage
- High tensile strain rate: Rapid cooling of each pass creates steep thermal gradients
- Low ductility in the mushy zone: Segregation of sulfur, phosphorus, and other low-melting-point elements to interdendritic regions forms brittle films
For hardfacing alloys commonly used on pinch rolls (e.g., Ni-Cr-Mo-B-Si systems, Co-based, or high-Cr cast irons), the wide solidification range and eutectic formation at interdendritic boundaries make them particularly susceptible. The critical cracking susceptibility parameter can be evaluated using the cracking susceptibility index:
CSI = σ₀ / (Tₘₛ − Tₘₗ) × dT/dt
where σ₀ is the yield stress at the solidus temperature, (Tₘₛ − Tₘₗ) is the freezing range, and dT/dt is the cooling rate.
3.2 Transformation Cracking (Warm Cracks)
Transformation cracking in the heat-affected zone (HAZ) of pinch rolls is driven by:
- Formation of brittle martensite in the HAZ due to the high carbon equivalent of the substrate steel
- Transformation plasticity strains when phase change occurs under stress
- Hydrogen accumulation at grain boundaries during the transformation
- Low ductility of the as-quenched microstructure
The carbon equivalent (CE) for transformation cracking susceptibility is typically evaluated as:
CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15
For typical pinch roll steels (42CrMo: CE ≈ 0.45–0.50), the threshold for significant transformation cracking risk is CE > 0.40, placing these substrates firmly in the high-risk category.
3.3 Hydrogen-Induced Delayed Cracking (Cold Cracks)
Delayed cracking is the most insidious failure mode because it manifests hours to days after welding. The three essential conditions (the "cold crack triangle") are:
- High diffusible hydrogen content in the weld metal and HAZ
- High hardness of the HAZ (typically > 400 HV for susceptible steels)
- High tensile residual stress in the weld region
Hydrogen sources in pinch roll overlay include:
- Moisture on the substrate surface (critical in high-humidity environments)
- Hydrogen in the shielding gas (H₂O and CO₂ impurities)
- Diffusion of hydrogen from the base metal during heating
- Flux or filler metal containing hydrogen-bearing compounds
4. Process-Induced Contributing Factors
4.1 Heat Input Management
Heat input is the single most critical parameter governing crack susceptibility in pinch roll overlay:
| Parameter | Low Heat Input (< 15 kJ/cm) | Moderate Heat Input (15–30 kJ/cm) | High Heat Input (> 30 kJ/cm) |
|---|---|---|---|
| HAZ Hardness | Very high (>500 HV) | Moderate (350–500 HV) | Lower (250–350 HV) |
| Cooling Rate (800→500°C) | Very fast (>20°C/s) | Moderate (5–20°C/s) | Slow (<5°C/s) |
| Residual Stress | High | Moderate | Lower |
| Dilution | Low | Moderate | High |
| Crack Risk | Cold cracks (H + hardness) | Balanced | Hot cracks (wide freezing range) |
4.2 Preheating and Interpass Temperature
Preheating serves multiple crack-prevention functions:
- Reduces the cooling rate, minimizing martensite formation in the HAZ
- Reduces the temperature gradient between weld and substrate, lowering residual stresses
- Provides a hydrogen escape route (hydrogen diffusivity increases with temperature)
- Reduces the effective restraint of the thick section
Recommended preheat temperatures for pinch roll overlay (based on substrate steel):
| Substrate Steel | CE Value | Recommended Preheat | Maximum Interpass Temp |
|---|---|---|---|
| 42CrMo | 0.45–0.50 | 200–300°C | ≤400°C |
| 38CrMoAlA | 0.40–0.45 | 150–250°C | ≤350°C |
| 40CrNiMo | 0.50–0.55 | 250–350°C | ≤450°C |
| Q345 (low alloy) | 0.35–0.40 | 100–200°C | ≤300°C |
4.3 Layer Design and Build-Up Strategy
For thick overlay layers on pinch rolls, a multi-layer strategy is essential:
- Transition layer (Layer 1): Low-carbon, low-alloy filler (e.g., E8018, ER80S-D2, or 309L stainless steel) to reduce dilution and provide a compatible microstructure
- Intermediate layer (Layer 2): Medium-alloy filler matching the target hardness (e.g., E5015Ni, ER50NiCrMo)
- Functional layer (Layer 3+): Hardfacing material (e.g., Ni-Cr-B-Si, Co-Cr, or high-Cr cast iron)
This approach ensures that the functionally critical hardfacing layers are deposited with lower thermal stress because the transition layers have already absorbed the majority of the substrate restraint.
5. Technical Purpose and Engineering Value
5.1 Business Positioning
Pinch roll restoration is a high-value service application for Cladding Technology Shanxi Co., Ltd because:
- High unit value: Each pinch roll restoration typically requires 200–500 kg of overlay material, representing significant material and labor cost
- Recurring demand: Hot rolling mills require pinch roll restoration every 6–18 months depending on operating conditions
- Quality sensitivity: A single undetected crack can cause catastrophic roll failure during hot rolling, resulting in mill shutdown costs exceeding USD 100,000–500,000 per incident
- Competitive differentiation: Demonstrating crack-free overlay capability with documented NDT results is a key qualification criterion for steel mill customers
5.2 Technical Value of Crack Mechanism Understanding
Systematic understanding of cracking mechanisms enables the company to:
- Develop WPS (Welding Procedure Specifications) with scientifically justified parameters
- Perform root cause analysis on field failures and implement corrective measures
- Train welders and inspectors on defect recognition and prevention
- Qualify new filler metals and process combinations with reduced trial-and-error
- Provide customers with confidence in long-term service reliability of restored rolls
6. Key Process Implementation Points
6.1 Surface Preparation
- Machining the roll surface to remove scale, oxide, and previously damaged layers to a minimum depth of 3–5 mm
- Surface roughness Ra ≤ 6.3 μm after machining
- Visual inspection for existing cracks, porosity, or inclusion-related defects in the substrate surface layer
- Complete removal of moisture and contaminants; surface temperature must exceed dew point by at least 10°C
6.2 Welding Parameter Control
| Parameter | TIG Overlay (GTAW) | MIG Overlay (GMAW) |
|---|---|---|
| Current | 180–280 A (DC) | 250–400 A |
| Voltage | 12–18 V | 22–32 V |
| Travel Speed | 15–30 cm/min | 20–50 cm/min |
| Heat Input | 10–20 kJ/cm | 20–35 kJ/cm |
| Shielding Gas | Ar 99.99% (or Ar+2%H₂ for Ni-base) | Ar 80% + CO₂ 20% (or Ar 95% + O₂ 5%) |
| Wire/Pad Feed | Welding pad or wire, Ø 2–4 mm | Wire Ø 1.2–2.0 mm |
6.3 Thermal Management
- Preheating: Induction heating to 200–300°C uniformly across the entire overlay zone (minimum 200 mm beyond the overlay area)
- Interpass temperature monitoring: Infrared pyrometer measurement before each pass; must not exceed specified maximum
- Post-weld heat treatment (PWHT): Stress relief at 550–650°C for 2–4 hours (for martensitic substrates) or 700–800°C for 1–2 hours (for ferritic substrates), depending on substrate temper stability
- Controlled cooling: Wrap with insulating blankets to achieve cooling rate ≤ 5°C/s from 600°C to 300°C
6.4 Hydrogen Control
- Filler metal baked at 250–350°C for 1–2 hours immediately before use (for low-hydrogen types)
- Shielding gas purity verification: H₂O ≤ 20 ppm, O₂ ≤ 20 ppm
- Welding environment: relative humidity ≤ 60% or use of dehumidified air curtains
- Post-weld hydrogen bake-out: 300–400°C for 2–4 hours (before PWHT)
- Diffusible hydrogen measurement of deposited weld metal: target ≤ 5 mL/100g for CE > 0.40 substrates
7. Applicable Standards and Acceptance Criteria
7.1 Welding Procedure Standards
- GB/T 985.1-2008 — Non-destructive testing of welds — Radiographic testing
- GB/T 3323-2005 — Radiographic testing of welds
- GB/T 11345-2013 — Ultrasonic testing of welds
- NB/T 47013.2-2015 — Radiographic testing (pressure equipment)
- ASME Section IX — Qualification of welding procedures
- ASTM E165-15 — Standard practice for liquid penetrant examination
- ISO 17637:2021 — Non-destructive testing of welds — Ultrasonic testing — Basic technique
7.2 Material and Performance Standards
- ASTM A182 — Chromium-nickel castings for pressure parts
- ASTM A276 — Stainless steel bars for general use (for filler qualification)
- GB/T 9452-2015 — Hardness of steel by Vickers method
- GB/T 228.1-2021 — Tensile testing of metallic materials
- NACE SP0169-2005 — Control of corrosion on underground or submerged metallic piping systems (for post-overlay corrosion performance)
7.3 Acceptance Criteria for Pinch Roll Overlay
| Inspection Method | Coverage | Acceptance Level | Standard Reference |
|---|---|---|---|
| Visual (VT) | 100% of overlay surface | No cracks, undercut, or surface discontinuities | GB/T 3375-2014 |
| Penetrant (PT) | 100% of overlay surface and HAZ | No linear indications (cracks) permitted | ASTM E165-15 |
| Magnetic Particle (MT) | 100% of ferromagnetic overlay/HAZ | No linear indications permitted | ASTM E709-19 |
| Ultrasonic (UT) | 100% of overlay layer | Level B per ISO 17637; no indications > 1 mm equivalent | GB/T 11345-2013 |
| Hardness (HV) | Full traverse across overlay cross-section | Within specified range; gradient ≤ 100 HV/mm at interface | GB/T 9452-2015 |
8. Common Risks and Control Measures
8.1 Risk Matrix
| Risk | Likelihood | Consequence | Control Measure |
|---|---|---|---|
| Delayed cold cracking in HAZ | Medium | Catastrophic (roll failure in service) | Preheat + hydrogen bake + PWHT + 24h delay before UT inspection |
| Solidification cracking in hardfacing layer | Medium-High | Major (overlay rejection) | Layer design + controlled heat input + filler metal selection |
| Interpass cracking | Low-Medium | Major (rework required) | Interpass temperature control + grind between passes |
| Delamination at weld/substrate interface | Low | Critical (structural failure) | Proper surface preparation + transition layer + UT verification |
| Excessive dilution causing hardness drop | Medium | Moderate (reduced wear life) | Layer design + process optimization + hardness mapping |
8.2 Critical Control Points
- WPS qualification testing: Each new substrate/filler/process combination must be qualified per ASME Section IX or GB/T 985 with full NDT and mechanical testing
- Welder qualification: All welders must hold valid certifications (NB/T 47014 or ASME IX) for the specific process, position, and material combination
- Process monitoring: Real-time recording of current, voltage, travel speed, and interpass temperature for every pass
- Hold points: Mandatory NDT inspection at defined intervals (e.g., after every 3–5 layers) before proceeding
- Post-weld inspection delay: UT inspection must be performed at least 24 hours after completion to detect delayed cracking
9. Application Across Three Technology Routes
9.1 TIG/MIG Weld Overlay Route
Pinch roll overlay is the primary application domain for the TIG/MIG weld overlay technology route. The crack mechanism study directly informs:
- WPS development for specific substrate-hardfacing combinations
- Selection between TIG (for precision, low-dilution transition layers) and MIG (for productivity in thick functional layers)
- Hot wire TIG (HWT) or pulsed TIG process selection for high deposition rates with controlled heat input
- Robotic orbital welding for circumferential overlay with consistent parameters
9.2 Hydraulic Explosive Bonding Route
While hydraulic explosive bonding is not typically applied to cylindrical pinch rolls, the crack mechanism knowledge from pinch roll overlay informs:
- Understanding of residual stress states in bonded interfaces under thermal cycling
- Design of transition layers that minimize thermal mismatch in bonded clad assemblies used in mill components
- Application to flat mill rolls (e.g., backup rolls) where clad plate segments are bonded and then welded into place
9.3 Explosion Welding Route
The explosion welding route can produce clad pinch roll segments where:
- Explosion-welded clad pipe or plate provides the base substrate for subsequent weld overlay of the functional hardfacing layer
- Crack mechanism understanding ensures that the explosion-welded interface is not compromised by subsequent thermal cycling during overlay
- Interface quality (wave amplitude, bonding ratio) must be verified before overlay to prevent crack initiation at the bonded interface
10. Qualification Building and Customer Value
10.1 Qualification Building
The systematic study of pinch roll overlay cracking mechanisms and preventive controls directly contributes to:
- WPS/PQR documentation: Each qualified procedure with documented crack-free results becomes a transferable qualification asset
- ISO 3834 / ISO 3900 certification: Demonstrates comprehensive quality management for welding operations
- Customer-specific qualifications: Steel mill OEMs (Baosteel, Shagang, Angang, etc.) require documented crack-free overlay performance on pinch rolls as a prerequisite for vendor approval
- Technical publications and patents: Novel crack prevention methods developed through this research can be protected as intellectual property
10.2 Customer Value Proposition
"Our systematic understanding of crack formation mechanisms in pinch roll weld overlay enables us to deliver restored rolls with zero crack defects, validated by 100% ultrasonic and penetrant testing. This translates directly to extended roll service life (3–5× improvement over conventional restoration), elimination of unplanned mill shutdowns, and total cost of ownership reduction of 40–60% compared to roll replacement."
10.3 Key Performance Indicators
| KPI | Industry Average | Target with Crack Prevention Protocol |
|---|---|---|
| Crack defect rate (per 100 rolls) | 5–15% | < 1% |
| Rework rate due to cracking | 15–30% | < 3% |
| Service life extension | 1.5–2× original | 3–5× original |
| NDT pass rate (first inspection) | 70–85% | > 98% |
| Customer warranty claims | 5–10% of orders | < 1% of orders |
11. Conclusion and Forward Direction
The study of cracking mechanisms in pinch roll weld overlay represents a foundational technical capability that underpins the company's entire weld overlay service offering. By systematically addressing the metallurgical, process, and quality dimensions of crack prevention, Cladding Technology Shanxi Co., Ltd positions itself as a technically superior partner for hot rolling mill equipment restoration.
Future development directions include:
- Integration of real-time thermography monitoring during overlay to detect thermal anomalies before crack formation
- Machine learning-based process parameter optimization using historical welding data and NDT results
- Development of novel filler metals with inherently low cracking susceptibility (e.g., high-nitrogen austenitic or precipitation-hardening compositions)
- Automation of the complete overlay process including robotic grinding between passes, automated NDT, and digital quality traceability
- Extension of crack prevention protocols to other high-value roll types (finisher rolls, work rolls, backup rolls)
This technical knowledge base, when properly documented in WPS, training materials, and quality procedures, becomes a sustainable competitive advantage that cannot be easily replicated by competitors lacking equivalent metallurgical expertise and process control discipline.