Low-Temperature Cold Crack Risk Assessment and Preheat Temperature Correction
1. Definition and Fundamental Principles
Cold cracking (also referred to as delayed cracking or hydrogen-induced cracking) is one of the most critical and insidious failure modes in welded joints involving low-alloy steels, high-strength steels, and thick-section components. Unlike hot cracking, which occurs during solidification, cold cracking develops post-weld during cooling or even days to weeks after welding, typically in the Heat-Affected Zone (HAZ) or weld metal. The phenomenon is governed by the classic "triangle of cold cracking," which requires the simultaneous presence of three factors:
- Diffusible Hydrogen: Introduced from moisture in the base material, flux, shielding gas, or ambient atmosphere. Hydrogen diffuses into the weld metal and HAZ, accumulating at grain boundaries and microstructural discontinuities.
- High Hardness/Hard Microstructure: Martensitic or bainitic microstructures formed during rapid cooling in the HAZ create high-hardness regions susceptible to hydrogen embrittlement. The hardness threshold is generally considered to be above 400 HV for most low-alloy steels.
- Residual Stress: Thermal stresses generated during welding create tensile stresses that, combined with hydrogen concentration and high hardness, exceed the material's yield strength, initiating crack nucleation and propagation.
The fundamental principle underlying preheat temperature correction is that an elevated preheat temperature slows the cooling rate of the weld and HAZ, thereby:
- Reducing the formation of hard martensitic phases by allowing more time for diffusional transformations (bainite, pearlite, ferrite)
- Providing sufficient time for diffusible hydrogen to escape from the weld metal and HAZ before the material reaches critical low temperatures (typically below 100°C)
- Reducing thermal gradients and residual stresses in the weldment
The Carbon Equivalent (CE) and Carbon Equivalent for Toughness (CET) are the primary indices used to quantify the cold cracking susceptibility of a given steel grade. Higher CE/CET values indicate greater hardenability and thus higher cold crack susceptibility, necessitating higher preheat temperatures.
2. Category and Business Positioning
Within the capability framework of Cladding Technology Shanxi Co., Ltd., this technology entry falls under the category of Welding Quality Assurance and Environmental Control. It represents a proactive, engineering-driven quality control methodology rather than a reactive inspection approach. Its positioning within the company's value chain is as follows:
- Pre-qualification Gate: Before any Welding Procedure Specification (WPS) is qualified, cold crack risk assessment must be performed to establish appropriate preheat parameters. This ensures that qualified procedures are inherently crack-resistant.
- Production Control: During actual fabrication, ambient temperature monitoring and preheat temperature verification are mandatory hold points, ensuring that production conditions match qualified WPS parameters.
- Customer Assurance: A documented cold crack risk assessment and preheat correction methodology demonstrates engineering rigor to end customers, particularly in high-integrity applications such as pressure vessels, pipelines, and structural components.
- Cost Avoidance: Preventing cold cracks avoids the enormous costs associated with crack repair, rework, component scrapping, and project delays. A single cold crack in a thick-section pressure vessel can result in six-figure repair costs.
3. Technical Purpose and Value
The primary technical purpose of this capability is to prevent cold cracking in low-alloy steel and thick-section welded components by systematically assessing cold crack susceptibility and applying scientifically justified preheat temperature corrections, particularly under low ambient temperature conditions.
The value delivered includes:
- Quantitative Risk Assessment: Moving beyond empirical "rules of thumb" to a calculation-based methodology using CE/CET, plate thickness, and ambient temperature as input variables.
- Dynamic Preheat Correction: Ability to adjust preheat temperatures in real-time based on seasonal variations, workshop conditions, and field welding environments where ambient temperatures may drop significantly.
- Standard-Compliant Documentation: Producing traceable, auditable records of cold crack risk assessments that satisfy customer requirements, regulatory inspections, and certification body audits.
- Cross-Route Applicability: The methodology applies universally across all welding-based fabrication routes, ensuring consistent quality regardless of the specific cladding technology employed.
4. Key Process and Implementation Points
4.1 Carbon Equivalent Calculation Methods
The first step in cold crack risk assessment is the determination of the carbon equivalent value for the base material. Multiple calculation formulas exist, each with different weightings for alloying elements:
| Formula | Expression | Typical Application |
|---|---|---|
| AWS CE (C_eq) | C + Mn/6 + (Cr + Mo + V)/5 + Ni/15 + Cu/5 | General welding suitability assessment |
| IIW CE | C + Si/30 + Mn/20 + Cr/20 + Ni/60 + Mo/15 + V/10 + 5B | Weldability assessment per ISO standards |
| Pcm (Cold Crack) | C + Si/30 + Mn/20 + Cr/20 + Ni/60 + Mo/15 + V/10 + 5B | Cold crack susceptibility (Pcm method) |
| CE_T (Toughness) | C + Si/30 + Mn/20 + Cr/20 + Ni/60 + Mo/15 + V/10 + 5B | Hardenability and toughness prediction |
| CEV (Welding) | C + Mn/6 + Cu/15 + Ni/30 + Cr/20 + Mo/20 + V/5 + 5B | Welding preheat determination |
Implementation Note: For low-alloy steels commonly used in cladding applications (e.g., 16Mn, Q345R, 15CrMo, 12Cr1MoV, ASTM A516 Gr.70, ASTM A387 Gr.11), the CE values typically range from 0.35 to 0.75. Values above 0.60 are generally classified as "difficult to weld" and require mandatory preheat.
4.2 Preheat Temperature Determination Methodology
The preheat temperature is determined through a two-step process: (1) establishing the baseline minimum preheat temperature from standard lookup tables, and (2) applying a correction factor for low ambient temperatures.
Step 1: Baseline Preheat Temperature (Standard Lookup)
| CE Value Range | Plate Thickness ≤ 20 mm | Plate Thickness 21–50 mm | Plate Thickness 51–100 mm | Plate Thickness > 100 mm |
|---|---|---|---|---|
| CE ≤ 0.40 | 0°C (no preheat required) | 0°C | 50°C | 100°C |
| 0.40 < CE ≤ 0.50 | 0°C | 50°C | 100°C | 150°C |
| 0.50 < CE ≤ 0.60 | 50°C | 100°C | 150°C | 200°C |
| 0.60 < CE ≤ 0.70 | 100°C | 150°C | 200°C | 250°C |
| CE > 0.70 | 150°C | 200°C | 250°C | 300°C |
Step 2: Low Ambient Temperature Correction
When the ambient (workshop or field) temperature drops below a defined threshold, the preheat temperature must be increased. The correction methodology is as follows:
| Ambient Temperature | Correction Factor | Rationale |
|---|---|---|
| ≥ 20°C | 0°C (no correction) | Normal conditions; standard preheat sufficient |
| 10°C ≤ T < 20°C | +25°C to +50°C | Moderate reduction in heat input; increased cooling rate |
| 0°C ≤ T < 10°C | +50°C to +75°C | Significant thermal loss; hydrogen re-trapping risk increases |
| -10°C ≤ T < 0°C | +75°C to +100°C | Severe thermal gradient; mandatory elevated preheat |
| T < -10°C | +100°C to +150°C | Extreme conditions; consider supplemental heating or process modification |
Example Calculation: For a 60 mm thick plate of 15CrMo steel (CE ≈ 0.68), welded in a workshop at 5°C ambient temperature:
- Baseline preheat from table: CE 0.60–0.70, thickness 51–100 mm → 200°C
- Ambient correction: 0°C ≤ 5°C < 10°C → +50°C to +75°C
- Corrected preheat temperature: 250°C to 275°C
4.3 Interpass Temperature Control
In addition to initial preheat, interpass temperature must be maintained within specified limits to prevent cold cracking in multi-pass welds:
- Maximum interpass temperature: Typically limited to 250°C–300°C for low-alloy steels to avoid excessive grain growth and softening of the HAZ
- Minimum interpass temperature: Must not fall below 100°C (or the specified minimum preheat temperature) between passes, as cooling below this threshold allows hydrogen re-concentration
- Monitoring method: Infrared thermometers or thermocouple-attached surface sensors; readings taken within 50 mm of the previous weld pass
4.4 Hydrogen Control Measures
Preheat temperature alone is insufficient; a comprehensive cold crack prevention strategy must include hydrogen control:
- Electrode/flux drying: Low-hydrogen electrodes (E7018, E8018) stored at 100°C–150°C in drying ovens; maximum 4 hours exposure to ambient atmosphere before use
- Flux conditioning: Fluxes dried per manufacturer specifications (typically 250°C–300°C for 1–2 hours)
- Shielding gas purity: Argon ≥ 99.99% purity; oxygen content < 0.05% for TIG welding; avoid moisture ingress in gas lines
- Base material preparation: Remove rust, scale, oil, and moisture from the welding area (minimum 25 mm on either side of the weld line); use dry abrasives or mechanical cleaning only
- Post-weld heat treatment (PWHT):strong> Where applicable, PWHT at 580°C–650°C for 2 hours per 25 mm thickness to facilitate hydrogen diffusion and residual stress relief
4.5 Preheat Application Method
| Method | Applicable Thickness | Advantages | Limitations |
|---|---|---|---|
| Gas torch (oxy-fuel) | ≤ 30 mm | Portable, quick application | Non-uniform heating; risk of local overheating |
| Induction heating | ≤ 50 mm | Uniform, controllable, fast | Equipment cost; limited to conductive materials |
| Resistance heating | ≤ 20 mm | Precise control; compact | Requires electrical access; limited geometry |
| Electrical resistance plates | 10–100 mm | Uniform; suitable for large areas | Requires power supply; bulky |
| Gas-fired blanket/blanket heater | 50–200 mm | Uniform over large areas; field-applicable | Slow ramp-up; requires monitoring |
5. Applicable Standards and Acceptance Criteria
5.1 International Standards
- AWS D1.1/D1.1M: Structural Welding Code – Steel. Provides preheat temperature tables based on CE value and plate thickness. Section 6.3 specifies minimum preheat temperatures for carbon and low-alloy steels. The code mandates preheat for CE > 0.43 in all thicknesses.
- ISO 15614-1: Qualification testing of welding procedures for metallic materials – Welding of steels. Provides CE calculation methods and preheat guidance for procedure qualification.
- ISO 2553: Welding of ferrous metals – General recommendations for welding of steels. Specifies preheat requirements as a function of CE and thickness.
- EN 1011-2: Recommendations for welding of structural steels. Provides preheat temperature tables and interpass temperature limits.
- ASME BPVC Section IX: Qualification of welding procedures. Requires consideration of preheat temperature as a variable in WPS qualification and performance qualification.
- ASME BPVC Section VIII Div. 1: Requires preheat for P-No. 1 through P-No. 5 materials based on thickness and carbon equivalent.
- API 510/API 570: Inspection codes that reference cold crack prevention requirements for pressure vessel and piping repairs.
- NACE MR0175/ISO 15156: While primarily a sour service standard, it specifies hardness limits (≤ 250 HV) that indirectly constrain preheat and PWHT requirements.
5.2 Chinese National and Industry Standards
- GB/T 985.1: Welding procedure specification – General rules. Provides CE calculation and preheat guidance.
- GB 50661: Code for welding of steel structures. Specifies preheat temperatures for low-alloy steels based on CE and thickness.
- NB/T 47014: Qualification rules for welding procedures of pressure vessels. Requires preheat temperature as a mandatory variable.
- TSG 21-2016: Supervision regulation for safety technology of stationary pressure vessels. Mandates cold crack prevention measures for low-alloy steel pressure vessels.
- GB/T 19418: Welding of steels – Recommendations for welding procedures.
5.3 Acceptance Criteria for Cold Crack Prevention
- Visual inspection: No visible cracks, particularly transverse or longitudinal cracks in the HAZ or weld metal, detected by 100% visual examination per AWS D1.1 Section 7
- Magnetic Particle Testing (MT): 100% MT of all welds and HAZ per AWS D1.1 Section 7.7; no indications of linear discontinuities exceeding acceptance limits
- Ultrasonic Testing (UT): 100% UT for full-penetration welds per AWS D1.1 Section 7.5 or ISO 17640; no indications classified as B or higher
- Hardness testing: Maximum hardness of ≤ 400 HV (or ≤ 350 HV for sour service per NACE MR0175) in the weld metal and HAZ, measured per ASTM E18
- Diffusible hydrogen measurement: Where required, hydrogen content in weld metal ≤ 5 mL/100 g (or ≤ 2 mL/100 g for critical applications) per AWS D1.1 or ISO 3690
- Post-weld delay cracking test: For critical applications, a 24-hour or 72-hour delay after welding before NDT to detect any delayed cold cracks
6. Common Risks and Controls
| Risk | Consequence | Control Measure |
|---|---|---|
| Incorrect CE value used in calculation | Insufficient preheat; cold crack initiation | Verify CE from certified material test reports; use conservative (higher) CE if data uncertain |
| Preheat temperature not verified at weld location | Actual preheat below specified minimum | Measure temperature within 50 mm of weld line using calibrated IR thermometer; document reading |
| Ambient temperature drops during welding | Cooling rate exceeds design basis; hydrogen re-trapping | Monitor ambient temperature continuously; apply additional preheat if T drops below qualified value |
| Interpass temperature exceeds maximum limit | Excessive grain growth; reduced toughness; potential for reheat cracking | Use IR thermometer between passes; stop welding if interpass > 300°C; allow cooling |
| Interpass temperature drops below minimum | Hydrogen re-concentration; cold crack in previous pass | Re-heat to minimum preheat temperature before next pass; use thermal blankets to maintain temperature |
| Moisture contamination of electrodes/flux | Excessive hydrogen input; cold crack | Mandatory electrode drying; use electrode ovens; limit electrode exposure time; inspect electrode surface for moisture |
| Wind/airflow disruption of shielding gas | Oxygen and moisture ingress; hydrogen pickup | Use wind shields; reduce gas flow rate to prevent turbulence; monitor gas purity with inline analyzer |
| Welding on cold, unpreheated base material | Excessive thermal gradient; high residual stress; cold crack | Enforce preheat as a mandatory hold point; do not proceed without documented preheat verification |
| Restriction of welding sequence causing high restraint | Elevated residual stress; cold crack initiation | Plan welding sequence to minimize restraint; use back-step or skip welding; consider stress-relief cuts |
7. Application Across Company Technology Routes
7.1 TIG/MIG Weld Overlay Cladding
In TIG (GTAW) and MIG (GMAW) weld overlay applications, cold crack risk assessment and preheat temperature correction are directly applicable to the base material being clad. The overlay process itself introduces a multi-layer weld structure where cold cracks can initiate in:
- Base material HAZ: The fusion boundary between the base material and the first overlay layer is the most susceptible region. Preheat temperature must be sufficient to prevent cold cracking in this critical zone.
- Overlay weld metal: When using low-alloy overlay consumables (e.g., E8018, ER80S-D2), the overlay weld metal itself can be susceptible to cold cracking if preheat is inadequate.
- Inter-layer boundaries: In multi-layer overlay builds, cold cracks can form between layers if interpass temperature drops too low.
Specific considerations for TIG overlay:
- TIG welding provides excellent shielding gas coverage, minimizing hydrogen pickup from the atmosphere. However, the lower heat input compared to MIG means the base material cools faster, potentially requiring higher preheat temperatures.
- For thick-section cladding (e.g., 50–200 mm base material), the thermal mass of the base material acts as a heat sink, accelerating cooling. Preheat temperature must be increased accordingly.
- When overlaying high-CE base materials (e.g., 12Cr1MoV with CE ≈ 0.68) with austenitic stainless steel (e.g., 309L), the cold crack risk is primarily in the base material HAZ. The preheat temperature is determined by the base material CE, not the overlay material.
Specific considerations for MIG overlay:
- MIG welding provides higher heat input, which partially compensates for lower preheat temperatures. However, the higher travel speed and shorter arc time mean hydrogen pickup per unit length can be lower.
- Wire feed rate and voltage settings affect the dilution ratio and cooling rate. Higher heat input settings reduce cold crack risk but may increase dilution of the overlay layer.
- For automated MIG overlay (e.g., robotic cladding), preheat temperature must be maintained uniformly across the entire cladding area to ensure consistent results.
7.2 Hydraulic Explosive Bonding
In hydraulic explosive bonding (also known as hydraulic shock bonding or hydraulic explosion welding), the bonding process itself does not involve welding fusion and therefore is not directly susceptible to cold cracking. However, cold crack risk assessment and preheat temperature correction remain relevant in the following contexts:
- Pre-bonding preparation: If the base material requires machining, beveling, or welding of preparation joints prior to explosive bonding, cold crack prevention measures must be applied to these operations.
- Post-bonding repair welding: After explosive bonding, any defects in the bond interface may require repair by welding. Cold crack risk assessment and appropriate preheat are mandatory for these repair welds.
- Subsequent cladding welds: In hybrid processes where explosive bonding is followed by weld overlay (e.g., explosive bonding of a transition layer followed by TIG overlay of the final cladding layer), the weld overlay steps require full cold crack risk assessment.
- Base material pre-treatment: If the base material has been previously welded (e.g., a pre-existing weld joint in the base plate), the pre-existing HAZ may have elevated hardness and hydrogen content, increasing cold crack susceptibility for any subsequent welding operations.
7.3 Explosion Welding
Similar to hydraulic explosive bonding, the explosion welding process itself is a solid-state bonding technique that does not involve melting and is therefore not susceptible to cold cracking during the bonding event. However, cold crack risk assessment applies to the following associated operations:
- Pre-explosion welding preparation: Edge preparation, beveling, and any welding of positioning or clamping fixtures require cold crack risk assessment if low-alloy steels are involved.
- Post-explosion welding machining and repair: After explosion welding, the clad plate/pipe is machined to final dimensions. Any welding operations performed after machining (e.g., welding of nozzles, reinforcing rings, or repair welds) must follow cold crack prevention protocols.
- Explosion-welded pipe joint welding: When explosion-welded clad pipes are joined to form a pipeline, the circumferential and longitudinal welds must be assessed for cold crack risk based on the base material CE and thickness.
- Field installation welding: Explosion-welded components installed in the field (e.g., clad piping in offshore platforms or cryogenic facilities) may be welded in low ambient temperatures. Preheat temperature correction for low ambient conditions is critical in these scenarios.
8. Integration with Qualification Building and Customer Value
8.1 Welding Procedure Qualification (WPQ)
Cold crack risk assessment is an integral part of the Welding Procedure Qualification (WPQ) process. The following elements must be documented in the Welding Procedure Specification (WPS):
- Base material CE value: Calculated from certified material composition and recorded in the WPS
- Minimum preheat temperature: Determined from standard lookup tables based on CE and plate thickness
- Ambient temperature correction: Documented correction factors for low ambient conditions, with explicit instructions for field welders
- Maximum interpass temperature: Specified in the WPS and enforced during production
- Hydrogen control measures: Electrode drying specifications, shielding gas purity requirements, and base material cleaning procedures
- Post-weld heat treatment (if applicable): PWHT temperature, time, and ramp rate specified in the WPS
During the Performance Qualification Test (PQT), the test coupon must be welded under conditions that match the qualified WPS parameters, including preheat temperature. The qualified WPS is then valid for production welding within the qualified variable ranges.
8.2 Certification and Audit Readiness
A documented cold crack risk assessment methodology supports the company's certification objectives:
- ISO 3834 (Quality requirements for fusion welding of metallic materials): Requires documented procedures for cold crack prevention, including preheat temperature determination and verification.
- ISO 3834-2 (Comprehensive quality requirements): Requires full documentation of welding procedures, including cold crack risk assessments, preheat records, and interpass temperature monitoring logs.
- ASME Section IX: Requires preheat temperature as a qualifying variable; changes in preheat temperature require requalification.
- NB/T 47014 (China): Requires preheat temperature determination based on CE and thickness for pressure vessel WPS qualification.
- API 510/570: Requires documented cold crack prevention procedures for pressure vessel and piping repair welding.
8.3 Customer Value Delivery
The cold crack risk assessment and preheat temperature correction capability delivers direct value to customers through:
- Reduced rework and scrap rates: Systematic cold crack prevention reduces the incidence of cold cracks, minimizing costly rework and component rejection. Industry data indicates that cold crack-related rework can account for 15–30% of total welding rework costs in low-alloy steel fabrication.
- Improved first-pass yield: Higher first-pass NDT yield rates translate to shorter project schedules and lower costs for the customer.
- Enhanced product reliability: Cold crack-free welds ensure the long-term structural integrity of cladded components, particularly in critical applications such as pressure vessels, pipelines, and nuclear components.
- Field welding capability: The ability to perform cold crack risk assessment and apply preheat corrections in field conditions (including low ambient temperatures) enables the company to undertake on-site cladding and repair work in diverse geographic and climatic conditions.
- Regulatory compliance: Documented cold crack prevention procedures satisfy regulatory and customer requirements, facilitating project approval and regulatory inspections.
9. Implementation Checklist for Production
To ensure consistent implementation of cold crack risk assessment and preheat temperature correction across all production activities, the following checklist should be enforced as a mandatory quality gate:
- Material Verification: Obtain certified material test reports for all base materials; calculate CE/CET values; record in the Welding Job Ticket (WJT).
- WPS Selection: Select the appropriate qualified WPS based on base material, overlay material, joint geometry, and thickness. Verify that the WPS includes preheat temperature requirements.
- Ambient Temperature Recording: Record the ambient temperature at the start of welding and at regular intervals (every 2 hours) throughout the welding operation. If the temperature drops below the qualified value, apply the correction factor.
- Preheat Application: Apply preheat using the method specified in the WPS. Measure and record the preheat temperature at multiple points (minimum 3 points within 50 mm of the weld line) using a calibrated infrared thermometer or thermocouple.
- Preheat Verification: Confirm that the measured preheat temperature meets or exceeds the specified minimum (including any ambient temperature correction). Do not proceed with welding until preheat is verified.
- Interpass Temperature Monitoring: Monitor and record interpass temperature before each subsequent pass. Enforce both maximum and minimum interpass temperature limits.
- Hydrogen Control: Verify electrode/flux drying status; check shielding gas purity; inspect base material surface for moisture and contamination.
- Post-Weld Hold Time: For critical applications, maintain a 24-hour hold time before NDT to allow for detection of any delayed cold cracks.
- NDT and Acceptance: Perform NDT (VT, MT, UT) per the WPS and applicable standards. Verify hardness if required. Document all results.
- Documentation and Traceability: Complete the Welding Job Ticket with all preheat and interpass temperature readings, ambient temperature logs, and NDT results. File for traceability and audit purposes.
10. Conclusion
Low-temperature cold crack risk assessment and preheat temperature correction is a foundational engineering discipline that underpins the quality and reliability of all welded cladding products manufactured by Cladding Technology Shanxi Co., Ltd. By systematically applying CE-based preheat determination, ambient temperature corrections, hydrogen control measures, and interpass temperature monitoring, the company ensures that cold cracks are prevented at the process design stage rather than detected and repaired after the fact.
This capability is directly applicable across all three technology routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding—ensuring consistent quality regardless of the fabrication method employed. It is a critical enabler for qualification building, certification maintenance, and customer value delivery, particularly in demanding applications involving low-alloy steels and thick-section components where cold crack susceptibility is highest.
The integration of this capability into the company's quality management system, supported by documented procedures, trained personnel, calibrated instrumentation, and rigorous audit practices, provides a robust framework for delivering crack-free, high-integrity cladded products that meet the most stringent international and national standards.