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Specification for welding and acceptance of 15CrMo heat-resistant steel. I. Welding characteristics of pearlitic heat-resistant steel. Pearlitic heat-resistant steel is a type of low-alloy steel; its main alloying elements are chromium and molybdenum, with small amounts of tungsten, vanadium, niobium, etc. Upon heating and subsequent air cooling, it exhibits a pronounced tendency toward hardening. During welding, hard and brittle martensitic structures tend to form in the weld zone and the heat-affected zone. This not only affects the mechanical properties of the welded joint but also generates significant internal stresses, often leading to cold cracks in both the weld and the heat-affected zone. The tendency to harden is also related to factors such as the carbon and chromium content in the steel, the thickness of the component, its stiffness, and the degree of restraint on the welded joint. Preheating during welding is an effective measure to prevent cold cracks. Failure to preheat the workpiece or using a too low preheating temperature, as well as an accelerated cooling rate of the workpiece, can all lead to increased hardening in the weld zone and the heat-affected area. To reduce or eliminate residual stresses in welded joints, prevent crack formation, and improve the microstructure and properties of the weld zone and heat-affected zone, preheating before welding and post-weld heat treatment should be carried out based on a comprehensive consideration of the steel’s hardenability, the thickness of the welded parts, and the ambient temperature conditions. II. Key process points during welding and selection of welding materials (1) Appropriate heat treatment measures must be carried out before, during, and after welding to complement the welding process. During the welding construction, it is necessary to follow the predetermined construction sequence and strictly adhere to the process quality management standards, implementing quality control throughout the entire process. This ensures that all factors—personnel, machinery, materials, and methods—remain under proper control, thereby guaranteeing the welding quality of 15CrMo pipes. (2) Before the assembly of the welded parts and before welding, impurities, dirt, burrs, oxides, etc. on the welding surface, at the groove, and within a range of 20 mm on its inner and outer surfaces must be removed completely; there shall be no defects such as cracks or delaminations. (3) Forced assembly of welded parts is not allowed. Due to the high tendency of cracking in chromium-molybdenum heat-resistant steel, welding procedures must be strictly followed; welds with large contraction should be welded first to minimize restraint. When aligning the butt welds of pipes or pipe fittings, the misalignment of the inner walls shall not exceed 10% of the thickness of the base material of the joint, nor shall it be greater than 2 mm. (4) Welding environment: The prefabrication of 15CrMo pipes should be carried out in a prefabrication yard. When welding on-site, rain and wind shelters must be erected to ensure that the welding environment meets the process requirements. Welding shall not be performed if any of the following conditions occur inside the shelter: a. For arc welding, when the wind speed is ≥8 m/s; for TIG welding, when the wind speed is ≥2 m/s ； b. Relative humidity greater than 90% ； c. Temperature below -10℃ ； d. It is raining or snowing. (5) TIG welding is used for the root pass; ER55-B2 (R30) wire is employed, and industrial-grade argon with a purity of not less than 99.98% is used as the shielding gas. Welding filler and surfacing are carried out using shielded metal arc welding, with E5515-B2 (Heat 307) electrodes being used; the welding materials must come with factory certification and quality certificates. Before use, the welding electrodes should be dried at a high temperature of 350°C for 1 hour. Maintain a constant temperature of 100–120°C in the insulated container, and use it as needed. After being baked, if the welded electrodes are not used for more than 4 hours, they should be baked again; however, this process should not be repeated more than twice. The coating on the surface of the electrodes must not come off. During welding, reverse DC current and short arc operation are used. (6) The weld groove angle is 60°±5°, the alignment gap is 2.5–3 mm, and the root bevel is 0.5–1.0 mm. A multi-layer, multi-pass welding method is employed. (7) Preheating is carried out by baking with an oxy-acetylene flame; temperature measurement is done using an infrared thermometer. The process is conducted simultaneously on both sides of the groove, ensuring uniform heating without any local overheating. Preheating is necessary for weld joints, whether it is spot welding or actual welding, with a preheating temperature of 150–200°C. The heating area prior to welding should be measured from the center of the weld; this area must be at least 3 times the thickness of the welded parts, and it should also be no less than 100 mm. (8) During welding, use the short arc welding method. Replace the electrode quickly and dexterously; minimize the number of times the arc is initiated and extinguished to prevent air from entering the weld pool and causing porosity. Carefully grind the end point of the weld. The weld bead in the intermediate layer is relatively wide, and a zigzag welding technique is generally used for continuous arc welding. When welding the cover, the crescent-shaped welding technique can be used, with a slow and steady movement to ensure a uniform and aesthetically pleasing weld bead. (9) The weld must be completed in a single, continuous pass; during welding, the temperature of the workpiece must be maintained at or above the preheating temperature (including the interpass temperature during multi-layer welding). If the welding process has to be interrupted for some reason, cooling and insulation measures must be taken in accordance with the process requirements to prevent cracks. Before resuming welding, the area must be preheated using the prescribed pre-welding heating procedure. III. Requirements for post-weld heat treatment process: (1) Post-weld heat treatment should be carried out immediately, heating the weld, heat-affected zone, and the adjacent base metal to 680–720°C. For pipes with a diameter of less than 100 mm, two thermocouples are installed symmetrically at the 12 o’clock and 6 o’clock positions; for diameters greater than 100 mm