Influence of Microstructure in the Heat-Affected Zone on the Reheat Cracking Susceptibility of Cr-Mo steel

by Mitsuru Nakamura, Late Toshio Enjo and Yasushi Kikuchi Influence of microstructure in the Heat-Affected Zone on the reheat cracking susceptibility of Cr -Mo steels (11/4Cr-1/2Mo steel , 21/4Cr-lMo steel and 5Cr-1/2Mo steel) were studied. Reheat crackingg tests on 3 type steels were carried out by means of three point bending method, constant-strain rate test on heating and constant-load test on heating. Influence of microstructure of synthetic HAZ at high temperature during the reheat treatment process was investigated by atomic absorption spectrometry, X-ray diffraction and scanning electron microscopic observation. The relation between the concentration of alloying elements in the precipitated carbides and their concentration in the matrix of synthetic HAZ were investigated. The effect of this relationship on the stress relaxation behavior was discussed. The results are summarized as follows. 1) The reheat cracking susceptibility of Cr-Mo steels were very low. The reason why it is estimated that it had good stress relaxation behavior during reheat treatment process. 2) The reheat cracking susceptibility of Bainitic structure specimens (Air Cooled) was the higher than that of Lath-martensitic structure specimens (Water Quenched) of 11/4Cr-1/2Mo steel and 21/4 Cr-lMo steel. But, it was not occured to reheat cracking on both specimens A.C and W.Q of 5Cr-1/ 2Mo steel. 3) The difference of reheat cracking susceptibility could be explained by the stress relaxation behavior of specimens and change of the concentration of alloying elements in the matrix. Especially, change of Cr content during the reheat treatment process is effective.