(I) Preparation before welding:
1. Defects on mechanical forgings should be removed before welding. Nondestructive testing for defects in important parts to determine the cleaning range. After cleaning, make sure the defect has been thoroughly cleaned by magnetic powder, coloring or pickling.
2. Defects can be removed by machining, air shovel, carbon arc air gouger, grinding wheel and other methods. Large defects in damaged forgings can be treated by gas cutting.
3. When removing defects by gas cutting or carbon arc gouging, carbon steel and various alloy steel forgings with a tendency to harden must be preheated at the same temperature as repair welding.
4. When cleaning defects with gas cutting method, in order to prevent crack growth, drill holes at both ends of the crack before cutting the crack. The diameter of the drill bit is determined by 10-40mm according to the thickness of the forging, and the drilling position is 10-15mm away from the top of the crack. The drilling depth should exceed the crack depth by 3 to 5mm.
5, in the selection of cleaning methods and the process of cleaning defects, not only to maintain thorough cleaning defects, but also to remove as little as possible the base metal, in order to reduce the repair welding workload, is beneficial to reduce welding stress.
6. According to the structure and defect characteristics of mechanical forgings, reasonable groove types are processed. Groove shall not interfere with welding operation.
7. If there is oil, rust, oxide skin and other dirt affecting welding quality on the matrix within 50mm around the defect, it should be cleaned up before welding.
8. Before formal welding, the groove should be checked by pickling and other methods to confirm that all defects have been eliminated before welding.
(II) Pre-welding and post-welding heat treatment:
1. Preheat the weldment before welding. The requirements for post-weld heat treatment depend on the following major factors: the material of the workpiece; The structural characteristics of the workpiece; Forging technical requirements; Heat treatment state of dressing welding parts; The working state of the workpiece.
2, welding preheating can be used as a whole preheating or local preheating. When preheating should be prevented from rapid heating or uneven heating, and take measures to prevent deformation, to protect the accuracy of the forging has reached the processing precision. When the local preheating method is used, the base metal temperature in the range of 80 ~ 100mm from the weld boundary shall not be lower than the specified preheating temperature.
3. When the groove depth of the repaired weldment exceeds 20% or 25mm of the wall thickness of the repaired weldment (whichever is smaller), stress relief heat treatment shall be carried out after the repaired welding. The holding time shall be calculated according to the holding time of each 25mm defect depth for 1H.
4. Post-welding heat treatment should be carried out in time after repair welding. In some cases, insulation and slow cooling measures should be taken at the repair weld site if heat treatment cannot be performed promptly after welding.
5. When the mechanical forgings are dressed before the end heat treatment, and the end heat treatment can be carried out in time after welding, the end heat treatment is allowed to replace the post-welding heat treatment. When the forgings are repaired after the end heat treatment, the tempering temperature after welding should be 30-50℃ lower than the tempering temperature specified in the end heat treatment process.