Different from forged steel parts, steel castings for hull structure have low carbon content and good welding performance. Defective parts are allowed to be repaired by welding. The control of the welding repair process is mainly related to the carbon equivalent of the casting material, the area of the welding repair area, the notch depth, the welding rod used, and more importantly, the repair ability of the production enterprise. For the inspection and control of defect repair quality of steel castings, ZHY casting believes that the following aspects should be started:
1) The steel casting manufacturer shall approve the defect repair process of steel castings. This process approval shall be regarded as one of the necessary conditions for factory approval of steel casting manufacturers and inspection of their castings, otherwise it shall be regarded as insufficient capacity of the enterprise. As mentioned above, steel castings inevitably have more or less casting defects. If the repair quality of steel castings cannot be guaranteed, it shall be deemed that the quality of steel castings cannot be guaranteed.
2) The defects shall be classified into minor defects and serious defects, and the corresponding repair process shall be reported to the Classification Society for approval.
3) The welding repair process of important parts shall be controlled, and its important links shall be witnessed.
① Groove quality: defects must be removed, the surface is metallic luster, and the groove size is convenient for welding repair.
② Preheating: when using gas flame, the flame shall not be concentrated. It shall be evenly heated from the periphery to the central area of the welding repair, and the preheating area shall not be less than twice the area of the welding repair area. The temperature gradient in the preheating zone shall be gentle to avoid cracks caused by local rapid heating (as shown in the figure).
③ Interpass temperature and Interpass weld repair quality. In particular, CO2 gas shielded welding shall be adopted to prevent the temperature between continuous welding repair layers from being too high and cracks caused by excessive current. During the inspection, it was found that during the repair process of CO2 gas shielded welding, the welding seam had hairy cracks due to excessive current.
4) Re flaw detection and heat treatment of repaired castings: for minor defects, magnetic particle flaw detection can be carried out again after repairing and cooling for 24 hours. If the carbon equivalent of qualified castings does not exceed 0.41, heat treatment can not be carried out; For serious defects, magnetic particle inspection shall be carried out again after 48 hours of repair, insulation and cooling. After passing the inspection, stress relief tempering or complete annealing shall be carried out (depending on the severity of the defect). Whether ultrasonic flaw detection should be carried out for steel castings for hull structure after heat treatment depends on the control of welding repair process and the state of post welding heat treatment.