Due to the presence of a large number of non-metallic inclusions in the forging billet for manufacturing flanges, during the forging process, the volume-type inclusions are repeatedly forged to become area-type inclusions, resulting in forging cracks along the tip of the inclusions inside the material. Larger inclusions become larger cracks after forging; smaller inclusions become smaller microcracks after forging. Most of these cracks are buried cracks.
After the forging billet is machined, some of the buried cracks become surface cracks, which is the reason why there are many micro-cracks on the inner and outer surfaces of the flange and in the bolt holes in the macro inspection. For larger cracks on the surface of the flange, in order to allow the product to leave the factory, the manufacturer has adopted means such as grinding → eliminating surface cracks → repairing welding → re-machining, etc., so that the appearance of the flange does not appear to have obvious defects.
The presence of a large number of non-metallic inclusions will reduce the plasticity and toughness of the material. The slag inclusions inside the crack cause stress concentration at the tip of the crack, and the flange cannot withstand the external load, so that the test pressure has just reached 0.6MPa during the pressure test. , the flange will crack and leak.
Second, the flange manufacturer tells you the solution is as follows:
1. For the newly replaced Hebei flange, in addition to re-examination of its chemical composition and mechanical properties, a macro inspection should be carried out to evaluate its geometric dimensions and surface finish, and to check whether there are abnormal geometric discontinuities.
2. Replace all defective Hebei flanges.
3. Continue to search for other pipe fittings of the same manufacturer and batch used in this device, and use corresponding inspection methods to confirm whether they are qualified.
Second, the flange manufacturer tells you the solution is as follows:
1. For the newly replaced Hebei flange, in addition to re-examination of its chemical composition and mechanical properties, a macro inspection should be carried out to evaluate its geometric dimensions and surface finish, and to check whether there are abnormal geometric discontinuities.
2. Replace all defective Hebei flanges.
3. Continue to search for other pipe fittings of the same manufacturer and batch used in this device, and use corresponding inspection methods to confirm whether they are qualified.