Microstructure and Fatigue-Crack Growth of Quenched and Tempered 23crnimov Steel

The microstructure, mechanical properties and fatigue-crack growth of the heavy-forging steel 23CrNiMoV with quenching and tempering were systematically studied. Microstructural characterization involved an optical microscope, field-emission scanning electron microscope and a transmission electron microscope. The fatigue-crack growth (FCG) behavior of the steel tempered at 600 °C was investigated at various stress ratios. The microstructure of the 23CrNiMoV tempered at 600 °C was composed of tempered martensite and considerable M7C3-type acicular carbides and M23C6-type spheroidal carbides, which contributed to the excellent strengthening/toughness match of the steel. The high ultimate tensile strength (UTS), yield strength (YS), elongation and impact energy of the steel tempered at 600 °C were 1155 MPa, 1091 MPa, 15.4 % and 102.9 J, respectively. The FCG threshold ( Kth) of the steel decreased with the increasing R. The fatigue fracture was composed of tearing ridges and secondary cracks. Besides, the secondary cracks were easily caused by the spheroidal precipitates during cyclic deformation surrounding the fatigue crack tip, which may enhance the FCG rate.