In earlier posts, limited data indicated that the fatigue properties of the cores of carburized components do not vary significantly with the composition of low alloy steels. There was evidence, however, that the fatigue properties of the cases of carburized components are influenced by steel composition. Better case fatigue performance was exhibited by more highly alloyed steels.
To more completely examine this issue, case and core fatigue data were analyzed for three low alloy carburizing steels. The three steel grades were SAE 4320, SAE 9310 and 20MnCr5. SAE 4320 is a Ni-Cr-Mo steel, SAE 9310 is a high-nickel (3%) steel, and 20MnCr5 is a Mn-Cr low alloy steel. Carburized cores were simulated by heat treating steel bars using a carburizing thermal cycle without the presence of carbon in the furnace atmosphere. Carburized cases were simulated by through-carburizing fatigue specimens using a carburizing thermal cycle in a carbon-bearing furnace atmosphere.
The mechanical properties for the case and core simulations for the three steel grades are shown below.
Some inconsistencies were observed in the tensile properties in the case simulations due to low ductility and early failure. The microstructures in the case simulations were martensite, and in the core simulations mixtures of martensite, bainite and ferrite were observed.
Since, in recent posts, it was noted that core fatigue properties varied with hardness resulting from changes in cooling rate following carburizing, core data for this analysis was obtained at constant hardness.
The strain controlled fatigue properties for the simulated carburized cores for all three steels are shown in Figure 1. The data for SAE 4320 is given by Iteration No. 122, for SAE 9310 by Iteration No. 125 and for 20MnCr5 by Iteration No. 128. It can be seen that the fatigue properties for the three steel grades all fall within a very narrow band. Thus, at constant hardness, equivalent fatigue properties were observed for the carburized cores of all three steels.
Figure 2 shows the strain controlled fatigue properties for the simulated carburized cases for all three steels. The data for SAE 4320 is given by Iteration No. 167, for SAE 9310 by Iteration No. 168 and for 20MnCr5 by Iteration No. 170. As can be seen the more highly alloyed SAE 9310 and SAE 4320 steels exhibit better fatigue performance than the 20MnCr5 steel.
Thus the data indicates that, while equivalent core fatigue properties can be obtained for carburized steels at various alloy levels and at constant hardness, applications requiring superior case performance require more highly alloyed steel grades.