Hot Cracking Prevention Measures in Weld Overlay of Continuous Casting Rolls
1. Definition and Fundamental Principles
Hot cracking (also termed solidification cracking) is a form of high-temperature weld metal fracture that occurs during solidification or in the near-solidus temperature range of the weld deposit. In the context of continuous casting roll (CCR) weld overlay, hot cracking arises from the synergistic interaction of three primary factors: (1) the composition and solidification behavior of the overlay alloy, (2) the thermal-mechanical constraints imposed by the roll substrate, and (3) the process parameters governing heat input and cooling rate.
Continuous casting rolls—used in slab, bloom, billet, and strip casting machines—undergo extreme thermal cycling, mechanical loading, and chemical attack from molten steel. The overlay layer, typically composed of high-chromium white iron, nickel-based alloys (e.g., Stellite), or austenitic stainless steels, must provide exceptional wear resistance, thermal shock tolerance, and spalling resistance. However, these high-performance alloys often exhibit wide solidification ranges or dendritic microstructures that are inherently susceptible to hot cracking when deposited on carbon steel or low-alloy steel roll shells.
The fundamental mechanism of hot cracking in CCR overlay involves the formation of liquid films along grain boundaries during solidification. As the weld metal cools through the mushy zone (approximately 1,150–1,350°C for most overlay alloys), the remaining inter-dendritic liquid becomes trapped and is subjected to tensile stresses from thermal contraction and substrate restraint. If the volume fraction of remaining liquid is insufficient to accommodate the strain imposed by solidification contraction, microvoids coalesce and propagate into macroscopic cracks. This phenomenon is particularly severe in:
- High-carbon equivalent substrates (CE > 0.45%), which increase the susceptibility of both the base metal and the heat-affected zone (HAZ) to cracking.
- Nickel-based and high-chromium overlay alloys, which exhibit wide solidification ranges (e.g., Stellite 6 solidifies over approximately 200°C range).
- Thick overlay deposits (single pass thickness > 3 mm) where thermal gradients are steep and residual stresses are elevated.
- Insufficient preheating, which increases cooling rates and promotes brittle phase formation in the HAZ.
2. Technical Purpose and Operational Value
The systematic implementation of hot cracking prevention measures in CCR weld overlay serves several critical purposes:
- Integrity assurance: Hot cracks are critical defects that compromise the structural integrity of the roll, potentially leading to catastrophic failure during casting operations with consequent molten steel leakage, production downtime, and safety hazards.
- Service life extension: Crack-free overlay layers deliver their full design life (typically 1.5–3× the life of uncoated rolls), directly reducing change-out frequency and associated production losses.
- Quality consistency: Standardized anti-cracking procedures ensure batch-to-batch reproducibility, which is essential for meeting customer specifications and passing non-destructive testing (NDT) acceptance criteria.
- Cost optimization: Each hot crack detected post-overlay requires local repair or full re-overlay, consuming additional consumables, labor hours, and machine time. Prevention is economically superior to remediation.
3. Key Process and Implementation Points for Hot Crack Prevention
3.1 Substrate Preparation and Preheating
The roll substrate condition is the first line of defense against hot cracking. The following measures must be rigorously applied:
| Parameter | Specification | Rationale |
|---|---|---|
| Surface cleaning | Grind to bare metal; remove scale, rust, oil, and previous weld spatter using power tools or shot blasting | Contaminants (S, P, O) promote intergranular liquid film formation |
| Preheat temperature (carbon steel shell) | 150–250°C (maintained throughout welding) | Reduces cooling rate below critical threshold; relieves residual stresses |
| Preheat temperature (low-alloy shell) | 200–350°C (maintained throughout welding) | Compensates for higher hardenability and lower ductility of alloyed steels |
| Interpass temperature | Minimum 150°C; maximum 350°C | Prevents cold cracking in HAZ while avoiding excessive grain growth |
| Roll straightness verification | Runout < 0.05 mm (per API 5L or customer spec) | Non-concentricity creates asymmetric thermal gradients promoting cracking |
3.2 Overlay Alloy Selection and Layer Design
Alloy selection is the most influential variable in hot crack susceptibility. The following design principles should be applied:
- Transition layer strategy: When overlaying nickel-based or high-chromium alloys onto carbon steel, a compatible transition layer (e.g., E309/E310 stainless steel or 309L cast iron) should be deposited first. This layer dilutes carbon and reduces the carbon equivalent of the subsequent weld metal, suppressing hot crack initiation at the interface.
- Low-sulfur and low-phosphorus consumables: Select welding wires and electrodes with S < 0.015% and P < 0.025% to minimize hot crack susceptibility. Consumables conforming to AWS A5.14 (SMAW), AWS A5.23 (GMAW), or AWS A5.18 (FCAW) should be used with verified chemical composition certificates.
- Multi-pass build-up: For overlay thicknesses exceeding 3 mm, employ multiple passes with each pass thickness controlled between 1.5–2.5 mm. This reduces peak thermal gradient and allows strain relief between passes.
- Crack-arresting layers: Intersperse ductile austenitic stainless steel layers (e.g., 309L) between hard overlay layers to arrest crack propagation across the overlay thickness.
3.3 Welding Process Parameters
Optimized process parameters minimize thermal strain and promote favorable solidification morphologies:
| Parameter | Recommended Range | Effect on Hot Cracking |
|---|---|---|
| Welding current (TIG) | 80–180 A (depending on wire diameter) | Moderate heat input prevents excessive dilution while ensuring full fusion |
| Welding speed | 150–350 mm/min | Higher speed reduces heat input per unit length, narrowing the mushy zone |
| Heat input | 0.6–1.5 kJ/mm | Controlled heat input balances HAZ hardness and solidification cracking risk |
| Travel angle | Push angle 5–15° (TIG with consumable insert) | Ensures uniform bead profile and minimizes porosity at bead root |
| Wire feed rate (MIG) | 3.0–6.0 m/min | Stable arc length and consistent bead geometry reduce cracking tendency |
| Shielding gas | 98% Ar + 2% O₂ (MIG) or pure Ar (TIG) | Oxygen addition improves wetting and reduces hot crack susceptibility in iron-based alloys |
| Back purging | Argon at 5–10 L/min (for through-thickness rolls) | Prevents oxidation and oxide-induced crack initiation on the opposite surface |
3.4 Post-Weld Heat Treatment (PWHT)
Post-weld heat treatment is a critical final measure for relieving residual stresses that may have contributed to crack initiation:
- Tempering for high-chromium overlays: Heat to 750–800°C for 1–2 hours, then air cool. This transforms brittle ledeburite to tempered martensite and carbides, reducing residual stress by 60–80%.
- Solution treatment for nickel-based overlays: Heat to 1,100–1,150°C, hold 1 hour, then water quench. Followed by aging at 870°C for 2 hours to precipitate strengthening carbides without re-introducing high stresses.
- Stress relief for carbon steel HAZ: If PWHT is not compatible with the overlay alloy, apply localized stress relief at 550–600°C targeting the HAZ region only.
4. Applicable Standards and Acceptance Criteria
4.1 Governing Standards
The following standards provide the normative framework for CCR weld overlay quality:
- GB/T 8165 — Welding consumables for weld overlaying (classification and specification of overlay electrodes and wires).
- GB/T 19866 — Welding procedure qualification and validation for weld overlay.
- GB/T 3375 — Welding terminology (definitions of hot cracking, solidification cracking, etc.).
- ASTM A743/A743M — Cast and wrought austenitic stainless steel castings (applicable to transition layers).
- ASTM A213 — Seamless austenitic stainless steel boiler, heat-exchanger, and heater-tube (reference for stainless steel consumable properties).
- ASME BPV Code Section IX — Qualification rules for welding procedures, welders, and welding operators (WPS/PQR qualification).
- API 5L — Specification for line pipe (reference for roll shell material properties and mechanical testing).
- ISO 9606-1 — Qualification testing of welders for fusion welding (visual and NDT examination of qualification welds).
- NACE MR0175/ISO 15156 — Materials for use in H₂S-containing environments (if CCR is used in sour service applications).
4.2 Acceptance Criteria for Hot Crack Freedom
| Examination Method | Standard | Acceptance Criteria |
|---|---|---|
| Visual inspection (VT) | GB/T 3323.1 / ISO 17637 | No cracks, undercut > 0.5 mm, or porosity clusters visible on overlay surface |
| Magnetic particle testing (MT) | GB/T 26952 / ASTM E1444 | No linear indications; no indications longer than 3 mm or > 3 mm in total length per 100 mm |
| Liquid penetrant testing (PT) | GB/T 18851 / ASTM E709 | No crack indications; pore indications < 1 mm and not more than 3 per 100 mm² |
| Ultrasonic testing (UT) | GB/T 11345 / ASTM E2312 | No indications exceeding level II per GB/T 11345 acceptance classification |
| Hardness testing | GB/T 231.1 / ASTM E18 | Overlay hardness within specified range (e.g., HRC 58–65 for high-chromium); HAZ hardness < 350 HV |
| Microstructural examination | GB/T 19540 | No intergranular cracking; HAZ martensite fraction < 30% for carbon steel substrate |
5. Common Risks and Control Measures
5.1 Risk Identification
- High dilution rate: Excessive base metal dilution (typically > 30%) raises the carbon equivalent of the weld metal and promotes hot cracking. Control: Use low-dilution techniques such as TIG with consumable insert (TIG-CCI) or submerged arc with backing strip.
- Insufficient preheat or interpass temperature control: Low preheat increases cooling rate, promoting columnar dendrite growth and hot crack formation. Control: Use infrared thermometers or thermocouples for continuous temperature monitoring; document preheat and interpass temperatures in the welding log.
- Improper bead sequencing: Welding in a single direction without a zigzag or step-back pattern concentrates thermal strain. Control: Implement multi-pass zigzag or step-back welding sequences to distribute heat input uniformly.
- Contamination from previous operations: Residual scale, oil, or moisture from grinding or handling introduces sulfur and phosphorus. Control: Perform final surface preparation immediately before welding; use acetone or solvent cleaning within 2 hours of welding start.
- Welding on cold or non-uniformly preheated surfaces: Thermal gradients across the roll circumference cause asymmetric contraction. Control: Preheat the entire roll circumference to uniform temperature; verify with at least 4 thermocouple points around the roll diameter.
5.2 Monitoring and Documentation
Every CCR overlay job must be accompanied by a comprehensive welding documentation package, including:
- Welding Procedure Specification (WPS) qualified per ASME Section IX or GB/T 19866.
- Welder qualification records per ISO 9606-1.
- Preheat and interpass temperature logs.
- Consumable batch certificates with chemical composition and mechanical property data.
- NDT reports with calibrated equipment records.
- Post-weld heat treatment thermocouple charts.
- Final dimensional and hardness verification reports.
6. Application Across Technology Routes
6.1 TIG/MIG Weld Overlay Route
The TIG (Gas Tungsten Arc) and MIG (Gas Metal Arc) weld overlay routes are the primary methods for applying functional overlays to continuous casting rolls. Hot crack prevention is most critical in this route due to the precise thermal control required. Key applications include:
- Slab caster rolls: Overlay with high-chromium white iron (e.g., E-Fe3MoSiCr or equivalent) using TIG-CCI for low dilution and controlled bead geometry. Preheat to 200°C; interpass < 250°C. Multi-pass build-up to 4–6 mm total overlay thickness.
- Bloom/billet caster rolls: Overlay with nickel-based alloys (Stellite 6 or equivalent) using MIG with 98% Ar + 2% O₂ shielding. A 309L transition layer (1.5 mm) is deposited first to mitigate hot cracking at the interface.
- Strip caster rolls: Overlay with austenitic stainless steel (310 or equivalent) using TIG with pure argon shielding. Preheat to 150°C; controlled travel speed of 250–300 mm/min to minimize heat input.
6.2 Hydraulic Explosive Bonding Route
While hydraulic explosive bonding is primarily used for clad plate and pipe fabrication, the principles of hot crack prevention inform the design of the subsequent weld overlay applied to the bonded surface. In CCR manufacturing, if a hydraulic explosive bonded substrate is used as the roll shell, the following considerations apply:
- The explosive bond interface must be free of voids and delamination, as these act as crack initiation sites during subsequent weld overlay.
- Weld overlay on explosively bonded substrates requires careful heat input control to avoid disturbing the metallurgical bond at the interface.
- Preheat levels should be moderate (150–200°C) to avoid exceeding the interface bonding temperature without disrupting the bond integrity.
6.3 Explosion Welding Route
Explosion welding (explosive cladding) is used for manufacturing clad roll shells where a high-purity overlay layer is required. Hot crack prevention is relevant in two contexts:
- Post-explosion weld repair: Any welding operations performed on explosion-welded CCR shells (e.g., hub welding, end-cap attachment) must follow hot crack prevention protocols, particularly preheat control and consumable selection compatible with the clad layer composition.
- Overlay on explosion-welded surface: If additional functional overlay is applied to the explosion-welded surface, the process parameters must be qualified through a WPS that accounts for the unique microstructure at the explosion weld interface.
7. Contribution to Qualification Building, Product Delivery, and Customer Value
7.1 Qualification Building
The systematic study and implementation of hot crack prevention measures directly contribute to the company's qualification portfolio:
- WPS qualification per ASME Section IX: Each hot crack prevention protocol is documented as part of a qualified welding procedure, enabling the company to demonstrate compliance to customers and certification bodies.
- Welder qualification per ISO 9606-1: Welders trained in hot crack prevention techniques are qualified through practical tests that include visual and NDT examination for crack freedom.
- ISO 9001 quality management system: The standardization of hot crack prevention procedures is a key element of the company's quality management system, demonstrating process control and continual improvement.
7.2 Product Delivery Reliability
By preventing hot cracks, the company achieves:
- Higher first-pass yield: Reduced NDT rejection rates and fewer rework cycles translate to faster delivery schedules and lower unit costs.
- Predictable overlay performance: Crack-free overlays deliver consistent wear life in service, enabling the company to provide customers with guaranteed service life commitments.
- Reduced warranty claims: Hot crack-related failures in service are among the most costly warranty events. Prevention eliminates this risk category entirely.
7.3 Customer Value
For steel mills and continuous casting operators, the value proposition of hot crack-free CCR overlay is quantifiable:
- Reduced unplanned downtime: A single roll failure due to hot crack propagation can cause 48–120 hours of production loss, with associated costs exceeding $500,000 per incident in large steel mills.
- Extended roll life: Crack-free overlays typically deliver 2–3× the service life of poorly executed overlays, reducing change-out frequency and associated labor costs.
- Improved steel quality: Rolls with intact overlay surfaces produce slabs and blooms with superior surface finish, reducing downstream processing costs and improving yield.
- Safety assurance: Elimination of hot cracks removes the risk of catastrophic roll failure during casting, protecting personnel and equipment.
8. Conclusion
Hot cracking prevention in continuous casting roll weld overlay is not merely a technical refinement—it is a fundamental requirement for delivering reliable, high-performance rolls that meet the demanding service conditions of modern steelmaking. The measures outlined in this analysis—substrate preparation, alloy selection, process parameter optimization, post-weld heat treatment, and rigorous NDT verification—form an integrated quality framework that directly supports the company's qualification building, product delivery reliability, and customer value proposition. By embedding these practices into every welding procedure specification and operator training program, Cladding Technology Shanxi Co., Ltd. positions itself as a leader in high-integrity weld overlay manufacturing for the global steel industry.