Q1: How do I determine the preheat temperature when welding 12Cr1MoVG steel pipes?
The preheat temperature is typically controlled between 200°C and 250°C, depending on the pipe wall thickness. For pipes with a wall thickness ≤15mm, the preheat temperature should be no less than 200°C. For pipes with a wall thickness of 15-30mm, the preheat temperature should be increased to 220°C. Thick-walled pipes exceeding 30mm require a preheat of 250°C. The preheat range should be three times the wall thickness on each side of the weld, but no less than 100mm. Use electric heating tape or a flame for uniform heating, and monitor the temperature in real time with a temperature probe.
Q2: What welding consumables are recommended for welding 12Cr1MoVG steel pipes?
For manual arc welding, R317 (E5515-B2-V) low-hydrogen electrodes are recommended. For submerged arc welding, use H08CrMoVA wire with SJ101 flux. TIG-R40 wire is recommended for gas tungsten arc welding (GTAW) primer application. Welding consumables must be dried at 350-400°C for 1-2 hours before use. Welding procedure qualification tests are recommended for critical welds to verify consumable compatibility.
Q3: How can cold cracking in 12Cr1MoVG steel pipe welding be avoided?
Strictly control the diffusible hydrogen content of the welding consumables (≤5ml/100g). Ensure adequate preheat and interpass temperatures (200-300°C). Use a multi-pass welding process with low heat input (15-20kJ/cm). Perform post-heat treatment immediately after welding (250-300°C/2h). Perform 100% magnetic particle testing (MT) 24 hours after welding.
Q4: What are the requirements for post-weld heat treatment of 12Cr1MoVG steel pipe?
Post-weld stress relief heat treatment at 720-750°C is required. The holding time is calculated based on the wall thickness (2.5min/mm, minimum 1 hour). Control the heating rate below 150°C/h to prevent the generation of new stresses. The cooling rate should not exceed 200°C/h. Air cooling is permitted after furnace cooling to below 300°C. After heat treatment, hardness testing (HB ≤ 200) is required to ensure sufficient softening.
Q5: How is the quality of 12Cr1MoVG steel pipe welded joints inspected?
Visual inspection requires weld reinforcement ≤ 3mm, with no defects such as undercuts or cracks. RT testing complies with JB/T4730 standard, with a passing grade of Level II. UT testing focuses on detecting incomplete fusion and cracking defects. Hardness testing requires the weld zone to be ≤ 220HB and the heat-affected zone to be ≤ 240HB. For critical joints, samples should also be taken for tensile and bend testing.
