I. Welding process of high-pressure boiler tubes: Key Practice Guidelines for 12Cr1MoVG alloy
High-pressure boiler tubes are core components in power generation and industrial systems that withstand extreme temperatures and pressures. The quality of their welding directly determines the safety and lifespan of the equipment.
12Cr1MoVG, as a pearlitic low alloy heat-resistant steel, is widely used in the manufacturing of high-pressure boiler tubes due to its high thermal strength and fatigue resistance at 500 to 550℃.
This article comprehensively analyzes the welding process of 12Cr1MoVG high-pressure boiler tubes, covering method selection, material matching, and post-treatment, to enhance the quality and efficiency of welding.
II. Selection of welding methods for high-pressure boiler pipes
The root welds of high-pressure boiler pipes must meet strict requirements such as full penetration and a flat back free of slag. The following combined processes are recommended:
(1) Base layer: Manual tungsten inert gas welding (GTAW), achieving an oxidization-free molten pool through argon gas protection, precisely controlling the depth of penetration and forming.
(2) Filling and covering layer: Manual arc welding (SMAW), taking into account both efficiency and weld strength.
This method not only ensures the reliability of the root welding of high-pressure boiler tubes, but also optimizes the stress resistance capacity of the overall structure through layer-layer welding.
III. Matching of welding materials for high-pressure boiler pipes
The welding materials need to be highly compatible with the chemical composition and mechanical properties of 12Cr1MoVG high-pressure boiler tubes:
(1) Argon arc welding wire: ER50-6 (φ2.5mm), its low silicon and high manganese characteristics can suppress porosity and enhance the fluidity of the molten pool.
(2) Welding rod: R307 (E5515-B2, φ3.2/4.0mm), matching the Cr-Mo-V alloy system of the base material to ensure the high-temperature performance of the weld seam.
(3) Key note: Welding materials must be strictly dried (R electrodes at 350℃ for 1 hour) to avoid the risk of hydrogen-induced cracking.
IV. Preheating control of high-pressure boiler tubes before welding
Preheating is the core measure to prevent cold cracks in the welding of 12Cr1MoVG high-pressure boiler tubes:
(1) Preheating temperature: 200-300℃. Use electric heating tapes or flames for uniform heating, covering an area of ≥100mm on both sides of the weld seam.
(2) Interlayer temperature: Maintained at ≥200℃ throughout the process, with real-time monitoring by an infrared thermometer.
Precautions: Thick-walled tubes (>20mm) need to be preheated in sections to avoid local overheating that may cause grain coarsening.
V. Key Points of welding operation for high-pressure boiler tubes
(1) The bottom layer is welded by argon arc welding
Full-position fixed welding, the welding torch is perpendicular to the pipe wall, the argon gas flow rate is 8-12L/min, and argon protection is passed through the back (purity ≥99.99%).
Pulsed current (base value/peak value =70%/130%) is adopted to reduce heat input and avoid burden-through.
The joint needs to be ground to remove the arc pit defect, and the overlap and lap joint should be ≥5mm.
(2) Arc welding filler layer
Short arc operation (arc length ≤3mm), crescent-shaped transport bar, stay on both sides of the bevel for 0.5 to 1 second to ensure fusion.
The heat input should be controlled at 15 to 25kJ/cm ³. When multi-layer and multi-pass welding is carried out, the slag between layers should be thoroughly cleaned, and the staggered joints should be ≥30mm.
(3) Forming of the cover surface layer
Reduce the current by 10% to 15%, move the bar in a straight line, control the excess height at 1 to 3mm, and stop at the edge to prevent edge biting.
Immediately after the final welding, use asbestos cloth for slow cooling to avoid the formation of hard and brittle structure due to rapid cooling.
VI. Post-weld heat treatment of high-pressure boiler tubes
To eliminate residual stress and enhance the toughness of the joint, the following must be carried out:
(1) Stress relief annealing: Hold at 720-760℃ for 1-2 hours (increase by 1 hour for every 25mm of wall thickness), cool in the furnace to 300℃ and then air cool.
(2) Hardness verification: The hardness of the weld seam after heat treatment should be ≤250HB. The area exceeding the standard needs to be locally re-tempered.
Warning: Do not cool slowly in the range of 370 to 550℃ to prevent tempering brittleness!
VII. Quality Inspection of high-pressure boiler tube welding
(1) NDT testing: 100% radiographic testing (RT) combined with ultrasonic testing (UT), in compliance with the ASME BPVC IX standard.
(2) Mechanical tests: Samples were taken for high-temperature tensile tests (550℃) and impact tests (KV2≥41J).
(3) Metallographic analysis: Check that there is no martensite zone in the heat-affected zone (HAZ) and that the weld metal has a uniform sorbite structure.
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