1998 Volume 84 Issue 10 Pages 755-761
780 MPa high strength steel with low cold cracking sensitivity was characterized by lowering carbon equivalent (Ceq) and weld cracking sensitivity (PCM), and made a strong thermo-mechanical control process (TMCP) treatment. Therefore, these new 780 MPa steels have a good weldability. This paper has investigated the evaluation of reheat cracking susceptibility during post-weld heat treatment (PWHT), especially the relation between the stress relaxation behaivor and high temperature deformability. Simulated-weld heat treatment was carried out a maximum temperature of 1623K. PWHT was done at a heating rate of 111K/ks. The results are summarized as follows : 1) From the results of stress relaxation test, reheat cracking occurred all HAZ microstructures. Especially, the reheat cracking susceptibility of Bainitic types structure were higher compared with Lath-Martensite structure. Threshold retained stress of reheat cracking was differed about 200 MPa values on both structures during PWHT. 2) From the results of the high temperature deformability obtained by constant-load test on heating, all HAZ microstructures shows the minimum embrittle value (about 5%) from 800K to 900K during PWHT. The reason of these embrittlement causes the carbide precipitation behavior during PWHT process, and exerted the effect on deformability in the matrix. 3) When the reheat cracking susceptibility and the brittleness of material were evaluated, the grasp of the deformability of HAZ microstructure in PWHT was necessary. It was important that the brittleness evaluation chart of the material was shown from the relation to a necessary the amount of displacement with the deformability of the microstructure during PWHT.
