Abstract
The sensitivity of reheat cracking (SR cracking) of 720 N/mm2 class high strength steel was discussed from the viewpoint of the chemical composition. The content of each impurity or alloying elements was shifted in a certain range from the original chemical composition of this steel (2.8%Ni-1.5%Cr-0.5%Mo, 0.10%C, 0.005%P, 0.001%S, 0.006%N), and the change of the cracking sensitivity was examined. The following results were obtained by the implant type cracking test.
(1) This steel is originally less sensitive to reheat cracking ; the low contents of phosphorus and sulfur contribute to minimize the cracking sensitivity.
(2) The harmful effects of phosphorus and sulfur appear when their contents exceed certain critical values. The critical phosphorus content (Pcrit) is 0.011%. The critical sulfur content (Scrit) is about 0.006%. Reducing the nitrogen content increases the Pcrit and Scrit.
(3) The critical carbon content (Ccrit) is 0.15%.
(4) Small quantities of vanadium and niobium increase the cracking sensitivity. The critical con-tents, Vcrit and Nbcrit are 0.02% and 0.03%, respectively.
(5) Those results agree well with the results of fundamental researches on Cr-Mo type heat-resisting steels.
The Y-type joint cracking test was also carried out on all the specimens mentioned above, but no cracking was detected.
