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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 1 500-1 600 MPa has a K_(ISCC)(critical stress intensity factor) value below 15.0 MPa · m~(1/2).The steel isothermally quenched with a tensile strength of 1 350-1 750 MPa has a K_(ISCC) value about 20.0 MPa·m~(1/2).In addition,with increase of isothermal quenching temperature,the tensile strength decreases greatly and K_(ISCC) 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.
The steel specimens of nominal composition 0.3C-1.0Cr-1.0Mn-2.0Si-1.0Ni-0.04Nb were quenched and tempered or isothermally quenched from various temperatures. It is found that the steel quenched and tempered with a tensile strength of 1 500 The steel isothermally quenched with a tensile strength of 1 350-1 750 MPa has a K_ (ISCC) -1 600 MPa with a critical stress intensity factor value about 20.0 MPa · m ~ (1/2) .In addition, with increase of isothermal quenching temperature, the tensile strength reduced greatly and K_ (ISCC) value does not pronounced change the microstructure of isothermally quenched specimen 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.