Study On Thermal Behavior And Fatigue Performance Of Non - Quenched And Tempered Steel For Direct Cutting

As the increased application of the non-quenched and tempered steel by direct cutting and the improved requirement for the comprehensive performance of the component especially for the fatigue properties, it is of great significant to optimize the strength and toughness and the fatigue properties for the non-quenched and tempered steel. The main means to optimize mechanical properties and fatigue properties are microalloying and controlling rolling and cooling process to refine the grain size and microstructure. Therefore the dynamic recrystallization and static recrystallization behaviors and the influence of the microstructure on high cycle fatigue properties were investigated in this paper for two kinds of different vanadium content of non-quenched and tempered steels by direct cutting.The dynamic recrystallization and static recrystallization behaviors were studied on the Gleeble-3800 thermal simulated test machine for the two kinds of different vanadium content of non-quenched and tempered steels by direct cutting. First of all, the dynamic recrystallization took place for both V1 and V2 steels under different deformation for single pass compression test of thermal deformation. The higher the deformation temperature and the lower the strain rate, the easier the occurrence of dynamic recrystallization; the lower the deformation temperature and the higher the strain rate, the easier to get finer austenite grains in dynamic recrystallized. Based on the dynamic material model, the thermal processing maps of V1 and V2 steels were established, which were used to determine the best processing regions for V1 and V2 steels with the true strain of 0.4 roughly:deformation temperature is between 1050℃ and 1150℃, the strain rate is less than Is-1 for V1 steel; deformation temperature is between 1050℃ to 1150℃, the strain rate is between 0.1s-1 and 1s-1 for V2 steel. Secondly, in double pass compression test of thermal deformation, the deformation temperature and interval time were the major factors for the static recrystallization. The increased of deformation temperature and extended the interval time, The softening rate and volume fraction of static recrystallization for V1 and V2 steels increased gradually with increasing the deformation temperature and the interval time, and it was easier for the softening of static recrystallization. With the increasing of the degree of the static recrystallization, the austenite grain size in static recrystallization of V1 and V2 steels decreased gradually. Under the same deformation conditions, the austenite grain in static recrystallization was finer in V1 steel than that in V2 steel due to the precipitation of the second phase during hot deformation.The high cycle fatigue properties of V1 and V2 steels were studied on the rotating bending fatigue testing machine. The results showed that the fatigue strength and the ratio of fatigue strength to the tensile strength of V1 steel were much higher than those of V2 steel and commercial 42CrMo and the values were 552MPa and 0.50 respectively for V1 steel. With higher content of vanadium in V1 steel, a large number of small dispersed V(CN) was precipitated among vanadium and carbon and nitrogen, pinning the grain boundaries, which was good for both precipitation strengthening and grain refining. Thereby the resistances of fatigue crack initiation and propagation in V1 steel were improved and the high cycle fatigue properties were very excellent.