抄録
Hydrogen embrittlement of ultra-high strength steels is a problem of great importance in view of the increased use of these steels in airframe application. In order to estimate the influence of carburization or decarburization during austenitizing treatment in an endothermic gas atmosphere of controlled carbon potential on the susceptibility for hydrogen embrittlement of 4340 steel heat-treated to a tensile strength of 190 kg/mm2, tensile tests and stress rupture tests were performed with cadmium plated notched specimens (Kt=3.9). The results obtained are as follows:
(1) The susceptibility for delayed failure or hydrogen embrittlement is increased by carburization and decreased by decarburization. It seems that this behavior is caused by the essential characteristics of martensite that the toughness decreases largely as the carbon content increases, and depends on the carbon content of the surface layer rather than the carburized or decarburized depth.
(2) Notch tensile strength and reduction in area are decreased by carburization and increased slightly by decarburization.
(3) The fracture surface of delayed failure consists of the hydrogen embrittled cracking area and the final fracture area. Hydrogen embrittled crackings of delayed failure appear intergranular with respect to pre-austenite grain boundaries, but crackings during cathodic charging in the absence of applied stress occur most likely around inclusions and show a transgranular mode.
