Development of resistance welding process. (5). Investigation test for the breakage of specimens in the internal creep rapture test

In the internal creep rapture test, the breakage was caused at the resistance welded area of ODS steel cladding tube. In order to investigate the cause of the breakage, the microstructure of the replica specimens extracted from the welded area was observed by transmission electron microscopy (TEM), and the residual stress near the welded area was measured by X-ray diffraction method (XRD). Following results were obtained in this test. (1) Presumption of the cause of the breakage in the internal creep rapture test for ferritic ODS steel. The breakage was supported to cause by the stress concentration of the micro crack in the folded area of welding boundary resistance welding, and the grain sliding along the elongated grain boundary. These can be improved by optimizing welding conditions as plunging length of cladding, angle melting face etc. (2) Presumption of the cause of the breakage in the internal creep rapture test for martensitic ODS steel. The breakage was supposed to cause by the declination of high temperature strength of ferritic steel for end plug, the micro crack caused at the folded area of melding and the strength declination of the melded boundary. These can be improved by use of ODS steel for end plug, stress release or re-annealing and re-tempering at about 780degC after melding in order to reduce coarsening of aggregation of carbide. (3) Measurement results of residual stress. The residual stress both of ferritic and martensitic ODS steel at the resistance-welded area was 500 MPa at a maximum. In heat treating applied after melding, the residual stress was 64 MPa and for ferritic ODS steel it was 150 MPa at the surface, and 170 MPa at the cross section of the resistance-welded area, and at a maximum. Therefore heat treatment conditions need to be adjusted as lengthening more heat treatment time etc. (author)