CLADDING TECHNOLOGY SHANXI CO., LTDCause Analysis of TIG Welded Joint Cracking in 12Cr1MoVG Steel
Literature Overview
This study note addresses the cracking phenomenon observed in TIG (gas tungsten arc welding) welded joints of 12Cr1MoVG steel, a low-alloy martensitic steel widely used in power plant boiler tubes, pressure vessels, and high-temperature structural components. 12Cr1MoVG steel contains approximately 0.9-1.1% chromium, 0.25-0.35% molybdenum, and 0.4-0.6% vanadium, providing excellent creep strength and oxidation resistance at elevated temperatures. However, the high hardenability and susceptibility to hydrogen-induced cracking (HIC) and low-temperature cracking make welding a challenging process requiring careful control of welding parameters, preheat, and post-weld heat treatment (PWHT).
Material Characteristics and Welding Challenges
The 12Cr1MoVG steel belongs to the Cr-Mo-V alloy steel family, which exhibits high hardenability due to the alloying elements that promote martensite formation during cooling. The carbon equivalent (CE) of 12Cr1MoVG is typically in the range of 0.45-0.55%, indicating a moderate to high susceptibility to cold cracking. The carbon equivalent can be calculated using the formula CE = C + Mn/6 + (Cr + Mo + V)/5, and a CE above 0.4% generally requires preheat and controlled cooling to prevent cracking.
The welding challenges associated with 12Cr1MoVG steel include the following:
| Challenge | Description | Mitigation Measure |
|---|---|---|
| High hardenability | Martensite formation in HAZ | Preheat 200-300 °C, controlled cooling |
| Hydrogen-induced cracking | Diffusion of hydrogen into weld metal | Low-hydrogen electrodes, bake electrodes |
| Low-temperature cracking | Stress concentration at weld toes | Proper joint design, post-weld stress relief |
| Sensitivity to interpass temperature | Excessive interpass temperature causes grain growth | Maintain interpass below 250 °C |
| PWHT requirements | Stress relief and microstructure transformation | PWHT at 720-760 °C for 1-2 hours per 25 mm |
The susceptibility to cracking in 12Cr1MoVG welded joints is influenced by three main factors: the material composition (carbon equivalent and alloy content), the welding process parameters (heat input, cooling rate, and interpass temperature), and the residual stress state (caused by thermal contraction during welding). The interaction of these factors determines the likelihood of crack initiation and propagation.
Crack Types and Root Cause Analysis
Cracking in TIG welded joints of 12Cr1MoVG steel can be classified into several types based on their location, morphology, and formation mechanism. The most common crack types include hydrogen-induced cracks (HIC), low-temperature cracks (LTC), and reheat cracks (RHC).
Hydrogen-induced cracks typically occur in the heat-affected zone (HAZ) or in the weld metal near the fusion line. They form when hydrogen atoms, generated during welding from moisture in the atmosphere or in the electrode coating, diffuse into the weld metal and accumulate at microstructural defects such as grain boundaries, inclusions, or dislocation clusters. The high hardenability of 12Cr1MoVG promotes the formation of martensite in the HAZ, which provides favorable sites for hydrogen trapping. The combination of high hydrogen concentration and high hardness leads to crack initiation and propagation.
Low-temperature cracks, also known as delayed cracks, form at temperatures below 200 °C after welding. They are caused by the combined effect of hydrogen embrittlement, high residual stresses, and the presence of hard, brittle microstructures in the HAZ. The cracking process is time-dependent, with cracks often appearing hours or even days after welding. The susceptibility to low-temperature cracking increases with increasing carbon equivalent, cooling rate, and restraint stress.
Reheat cracks occur during post-weld heat treatment (PWHT) or during service at elevated temperatures. They form in the HAZ or in the weld metal when the material is subjected to high temperatures in the presence of residual stresses. The cracking mechanism involves the precipitation of carbides and the formation of brittle phases at grain boundaries, which reduces the ductility and promotes intergranular cracking.
Welding Procedure Optimization
To prevent cracking in TIG welded joints of 12Cr1MoVG steel, a comprehensive welding procedure optimization strategy must be implemented. The following key measures are recommended:
- Preheat temperature: Maintain a preheat temperature of 200-300 °C, depending on the thickness of the material and the level of restraint. Higher preheat temperatures reduce the cooling rate and minimize martensite formation in the HAZ.
- Interpass temperature: Control the interpass temperature below 250 °C to prevent excessive grain growth and to maintain a controlled cooling rate. Excessive interpass temperatures can lead to softening of the HAZ and reduced creep strength.
- Heat input: Optimize the heat input to balance the competing requirements of minimizing martensite formation and avoiding excessive grain growth. A heat input range of 0.8-1.5 kJ/mm is typically recommended for TIG welding of 12Cr1MoVG steel.
- Electrode selection: Use low-hydrogen electrodes or filler metals with hydrogen content below 5 mL/100g. Bake the electrodes at 300-400 °C for 1-2 hours before use to remove moisture.
- Post-weld heat treatment: Perform PWHT at 720-760 °C for a duration of 1-2 hours per 25 mm of thickness. The PWHT relieves residual stresses, transforms martensite to tempered martensite or bainite, and improves the ductility and toughness of the weld joint.
- Joint design: Use joint designs that minimize restraint stress, such as groove welds with adequate root clearance and reduced weld volume. Avoid T-joints and fillet welds in high-stress regions.
Engineering Practice and Case Study
In a recent engineering case, cracking was observed in TIG welded joints of 12Cr1MoVG boiler tubes during hydrostatic testing. The root cause analysis revealed that the cracking was primarily due to hydrogen-induced cracking, exacerbated by inadequate preheat and excessive cooling rates. The welding procedure specified a preheat temperature of only 150 °C, which was insufficient to prevent martensite formation in the HAZ. Additionally, the interpass temperature was not monitored, leading to excessive cooling rates between passes.
The corrective measures included increasing the preheat temperature to 250 °C, implementing strict interpass temperature control below 250 °C, and baking the filler metal at 350 °C for 2 hours before use. The welding procedure was revised to include a post-weld heat treatment cycle at 740 °C for 1.5 hours per 25 mm of thickness. After implementing these measures, no further cracking was observed, and the welded joints passed all required non-destructive testing (NDT) inspections, including radiographic testing (RT) and ultrasonic testing (UT).
Study Insights and Recommendations
The study of TIG welded joint cracking in 12Cr1MoVG steel underscores the critical importance of welding procedure qualification and strict adherence to welding specifications. The high hardenability and susceptibility to hydrogen-induced cracking of 12Cr1MoVG steel require careful control of welding parameters, including preheat, interpass temperature, heat input, and post-weld heat treatment. Engineers must adopt a systematic approach to welding procedure development, incorporating metallographic analysis of the weld joint microstructure, hardness testing of the HAZ, and hydrogen content measurement of the weld metal. The carbon equivalent serves as a useful screening tool for assessing cracking susceptibility, but it should not be relied upon exclusively, as it does not account for all metallurgical and process factors that influence cracking behavior. A comprehensive understanding of the interaction between material composition, welding process parameters, and residual stress state is essential for preventing cracking failures in 12Cr1MoVG welded joints.