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

Influence of microstructure in the Heat-Affected-Zone on the reheat cracking susceptibility was studied.The reheat cracking tests on H.T. 80 steel were carried out by means of ring type specimens and three point bending test. Influence of microstructure of synthetic H.A.Z. at high temperature on the reheat treatment process were investigated by high temperature hardness measurements, X-ray analysis and scanning electron microscopic observations. The results are summarized as follows. 1) The reheat cracking's critical stress of Martensite structure specimens (Water Quenched) were generally double than that of Bainite and Ferrite structure specimens (Air Cooled). But, the reheat cracking susceptibility of A.C. specimens were little by the difference of forms in microstructure. 2) The reheat cracking susceptibility were estimated by the relation of the deformability of a material and the precipitate of alloying carbides. Accordingly, alloying carbides of A.C. specimens precipitated to M3C type carbides during cooling process of synthetic H.A.Z., but that of W.Q. specimens precipitated to M3C type carbides as well as A.C specimens and M2C type carbides during reheat treatment process. W.Q. specimens of the lower reheat' cracking susceptibility are affected mainly by the precipitate of M2C type carbides in the matrix. 3) It is seemed that the precipitation of M2C and M3C type carbides during reheat treatment process from W.Q. specimens decreased the alloying elements concentration in the matrix, and showed a good deformability of a material. As a consequence, the reheat cracking susceptibility of W.Q. specimens were lower than that of A.C. specimens.