CLADDING TECHNOLOGY SHANXI CO., LTDCracking Analysis of 12Cr1MoVG Steel TIG Weld Joints
Literature Overview
This investigation by researchers from Shanxi Vocational and Technical College and Taiyuan University of Technology examines the root causes of cracking in 12Cr1MoVG steel TIG weld joints, published in the Physical Testing journal. The 12Cr1MoVG steel is a critical material used in power plant applications, particularly for high-temperature boiler tubes, headers, and pressure vessel components operating at temperatures up to 580 °C. Cracking in these weld joints poses a severe threat to component integrity and operational safety.
Material Characteristics and Weldability Assessment
12Cr1MoVG is a low-alloy martensitic steel containing 1.0–1.2% carbon equivalent contributions from chromium, molybdenum, and vanadium. The material's excellent high-temperature strength and creep resistance make it indispensable for power generation applications, but its weldability is constrained by several metallurgical factors:
| Property | Typical Value | Welding Implication |
|---|---|---|
| Carbon content (wt%) | 0.08–0.12 | Moderate cold cracking risk |
| Carbon equivalent (CE) | 0.38–0.45 | Requires preheat control |
| Yield strength (MPa) | 290–390 | Residual stress management needed |
| Hardness (HV) | 200–260 (as-received) | Potential for HAZ hardening |
| CTE (10⁻⁶/K) | 12.0–12.5 | Thermal stress during cooling |
| Preheat temperature required | 200–250 °C | Minimum for crack prevention |
The carbon equivalent calculated using the IIW formula (CE_IIW = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15) for 12Cr1MoVG typically falls in the range of 0.38–0.45, which places it in the category requiring controlled preheating and post-weld heat treatment (PWHT) to prevent hydrogen-induced cracking.
Cracking Mechanism Analysis
The research identifies multiple cracking mechanisms that can operate simultaneously or sequentially in 12Cr1MoVG TIG weld joints:
Cold Cracking (Hydrogen-Induced Delayed Cracking)
This is the most common cracking mode in 12Cr1MoVG welds and typically occurs within 1–72 hours after welding. The mechanism involves the triple combination of:
- Diffusible hydrogen: Introduced from welding flux, moisture in base metal, or atmospheric absorption during welding
- High-hardness microstructure: Martensitic transformation in the HAZ due to rapid cooling, producing hardness values exceeding 400 HV
- Tensile residual stress: Generated by the differential thermal contraction between weld metal and base metal
| Cracking Parameter | Critical Threshold | Typical Weld Condition |
|---|---|---|
| Diffusible hydrogen (mL/100g) | <5 for crack-free | 8–20 (uncontrolled) |
| Maximum hardness (HV) | <350 for crack-free | 400–550 (rapid cooling) |
| Residual stress (MPa) | <500 for crack-free | 400–650 (as-welded) |
| Preheat temperature (°C) | ≥200 recommended | 100–150 (insufficient) |
| Interpass temperature (°C) | 250–300 maintained | 150–200 (dropping) |
Reheat Cracking (Temper Embrittlement Cracking)
This delayed cracking mode occurs during PWHT or subsequent service at temperatures of 400–600 °C. It is associated with the precipitation of sulfides and phosphides at prior austenite grain boundaries, particularly in the intercritical HAZ where partial grain growth has occurred. The research highlights that reheat cracking is more prevalent in thick-section welds where the cooling rate through the austenite transformation range is slow enough to promote sulfide stringer formation.
Stress Rupture Cracking
In service conditions at elevated temperatures, the weld joint is susceptible to stress rupture along the grain boundaries of the HAZ, particularly in the coarse-grained region. This is exacerbated by the segregation of sulfur and phosphorus to grain boundaries during prolonged exposure at operating temperatures.
Root Cause Investigation Methodology
The research employs a systematic approach to cracking diagnosis, combining macroscopic examination, metallographic analysis, fractography, and chemical analysis:
- Macroscopic examination: Identification of crack location (weld metal, HAZ, or base metal), crack orientation relative to the weld axis, and crack propagation direction
- Metallographic analysis: Characterization of HAZ microstructure, identification of martensitic and bainitic phases, assessment of grain size in the intercritical zone
- Fractographic analysis (SEM): Distinction between transgranular and intergranular fracture modes, identification of microvoid coalescence or cleavage features
- Hardness mapping: Tracer bar hardness measurements across the weld cross-section to identify high-hardness zones exceeding 350 HV
- Hydrogen analysis: Measurement of diffusible hydrogen content in the weld metal using the electrolytic extraction method
Preventive Measures and Process Recommendations
Based on the cracking analysis, the following comprehensive preventive measures are recommended for 12Cr1MoVG TIG welding:
| Preventive Measure | Implementation Detail | Effectiveness |
|---|---|---|
| Preheating | 200–250 °C using induction or gas flame | High - reduces cooling rate |
| Interpass temperature | Maintain 250–300 °C between passes | High - prevents hydrogen embrittlement |
| Post-weld heat treatment | 760–790 °C × 2h per 25mm thickness | Essential - relieves residual stress |
| Shielding gas purity | Ar ≥99.99%, flow rate 12–18 L/min | Moderate - reduces hydrogen ingress |
| Electrode preparation | Low-hydrogen electrodes, oven-dried at 300 °C × 2h | High - reduces hydrogen source |
| Base metal preparation | Clean, dry, free of oil and moisture | High - eliminates hydrogen source |
| Weld sequence optimization | Symmetric welding to balance residual stress | Moderate - reduces stress concentration |
| PWHT cooling rate | ≤100 °C/h below 600 °C | High - prevents reheat cracking |
Engineering Practice and Quality Control
For pressure vessel and power plant applications governed by ASME Section VIII or NB/T 47002, the qualification of 12Cr1MoVG welding procedures requires demonstration of crack-free performance under the most severe conditions anticipated in service. The research findings emphasize that the cracking susceptibility of 12Cr1MoVG is not solely a function of welding parameters but is also influenced by the restraint condition of the weldment, the geometry of the joint, and the sequence of welding operations.
In practice, the most effective approach to preventing cracking in 12Cr1MoVG welds is a combination of adequate preheating, controlled interpass temperatures, proper PWHT, and attention to joint design to minimize restraint. The TIG process, while offering excellent control over heat input, requires particular attention to hydrogen control through rigorous cleaning procedures and high-purity shielding gas.
Study Insights and Reflections
This research underscores a fundamental truth in welding engineering: material-specific cracking susceptibility must be understood at the metallurgical level to develop effective preventive strategies. The 12Cr1MoVG steel, despite being a well-established material in power plant applications, continues to present cracking challenges that require careful process control and quality management. The systematic approach to cracking diagnosis demonstrated in this research provides a template for investigating similar failures in other low-alloy steel weldments. For engineers responsible for the fabrication and inspection of power plant components, the key takeaway is that cracking prevention is not a single-parameter optimization but requires a holistic approach that addresses hydrogen control, thermal management, residual stress mitigation, and microstructural refinement simultaneously.