The steel specimens of nominal composition 0.3C-1.0Cr-1.0Mn-2.0Si-1.0Ni-0.04 Nb were quenched and tempered or isothermally quenched from various temperatures,It is found that the steel quenched and tempered with a tensile strength of 1500-1600MPa has a KISCC(critical stress ibntensity factor)Value below 15.0MPa.m^1/2,The stel isothermally quenched with a tensile strength of 1350-1750 MPa has a KISCC value about 20.0MPa.m^1/2,greatly and KISCC value does not pronouncedly change.The microstructure of isothermally quenched specimens is composed of bainite and retained austenite.The delayed fracture resistance is dependent on the stability of austenite,which is in turn related to the retained austenite volume fraction and carbon content in austenite.
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