Steel grades Q345A, Q345B, Q345C, Q345D, Q345E

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Q345 is a type of steel material. It is a low-alloy steel (C0.45%); preheating is required before welding, with a preheating temperature of T0=100–150°C, and an interpass temperature of Ti≤400°C. 6. Post-weld heat treatment parameters: To reduce welding residual stresses, lower the hydrogen content in the weld, and improve the metal structure and properties of the weld, heat treatment should be applied to the weld after welding. The heat treatment temperature is: 600–640°C, with a holding time of 2 hours (for a plate thickness of 40 mm). The heating and cooling rate is 125°C/h. On-site welding sequence: 1. Preheating before welding – Before welding the flange plates, they are first preheated; welding begins after maintaining this temperature for 30 minutes. The preheating, interpass temperature, and heat treatment during welding are automatically controlled by a heat treatment temperature control cabinet. Far-infrared crawler-type heating elements are used, with a microcomputer responsible for automatically setting and recording the temperature curves, while thermocouples are used to measure the temperature. During preheating, the measuring point of the thermocouple is 15 mm–20 mm away from the groove edge. 2. Welding 2.1 To prevent welding deformation, each column joint is welded symmetrically by two people, with the welding direction proceeding from the center toward the sides. When welding the inner face (the groove that is close to the web), small-scale welding procedures must be used for the first to third layers, as welding in these areas is the main cause of welding deformation. After welding layers 1 to 3 is completed, root cleaning is performed on the back side. After completing root cleaning using carbon arc gas gouging, the weld must be mechanically ground to remove any carburized material from its surface and to reveal a metallic luster, thereby preventing cracks caused by excessive carbonization in the surface layer. The outer side should be welded in one go, with the remaining part of the inner side being welded last. 2.2 When welding the second layer, the welding direction should be opposite to that of the first layer, and so on. The weld joints in each layer should be offset by 15-20 mm. 2.3 The welding current, welding speed, and number of welding passes used by the two welders should be kept consistent. 2.4 During welding, the process should start at the arc starting plate and end at the arc stopping plate. After welding, cut it off and polish it clean. 3. Post-weld heat treatment: Heat treatment should be carried out within 12 hours after the welding of the joint is completed. If heat treatment cannot be carried out promptly, insulation and slow cooling measures should be taken. During heat treatment, two thermocouples should be used for temperature measurement, with the thermocouple tips welded to the inner and outer sides of the weld joint. 4. Welding inspection: In accordance with the requirements of the Code for Construction and Acceptance of Steel Structure Projects, welds are inspected using ultrasonic testing, with a 100% inspection rate. On-site technical management 1. Prepare detailed welding operation instructions. 2. Full-process control of the welding process is key to ensuring quality. During welding of each column joint, a designated person must monitor the welding process; if a welder fails to follow the operating instructions, the welding must be immediately terminated. During the welding process, the heat treatment technician should monitor the interlayer temperature throughout the entire process; if it exceeds the specified limit, they must immediately inform the welder to pause work. 3. Enhancing the quality awareness of construction workers is crucial for implementing welding procedures. Before construction begins, a briefing should be conducted for all personnel, and construction procedure cards should be issued. The briefing explains in detail the characteristics of the welding process, as well as the necessity of strictly controlling the on-site welding process and the key points for such control. Conclusion: By following these welding process measures, a total of 102 welds were constructed on site, and the first-time pass rate for non-destructive testing reached 100%. Verified through actual construction, this welding process measure can not only guide the welding of Q345 steel on site, but also ensure the welding quality.