The strength, fatigue and hardness data for several iterations with a high hardness level, around 60 HRC are shown in Table 1. The steels in these iterations are both plain carbon and low alloy grades with carbon contents ranging from 0.50% to 0.95%. The heat treatments used to obtain this hardness level were quench and tempering, carburizing and induction hardening. All of the samples during this time period were finish ground and polished after heat treatment, with the exception of iteration 41 which was not ground to preserve the intergranular oxidation (IGO) present at the surface. All of the samples were through hardened in the gage length.
A graphical representation of the data is shown in Figure 1. The graph shows the yield strength and ultimate strength decrease as the steel carbon content increases. Per SAE J413 the expected tensile strength at 60HRC is 2469 MPa. This is in good agreement with the SAE 6150 and SAE 9254 quench and tempered steels at the left end of the curve, which are 2343 MPa and 2450 MPa respectively. However, as the carbon content increases to 0.95% at the right end of the curve the ultimate strength decreases to about 1000-1500 MPa. All of the data follows this trend with the exception of the SAE 1552 induction hardened steel (iteration 92) which has the lowest strength at 933 MPa. An examination of the microstructure revealed the probable cause. The typical grain diameter of iteration 92 is 76 microns which is significantly larger than the other iterations at approximately 13 microns. In addition to the steel carbon content affecting the strength at this high hardness level it appears the grain size can also have a significant effect. Iteration 92 is approximately an ASTM E112 grain size 4.5 while the other samples are around grain size 10. A grain size of 4.5 is not unusually large for an induction hardened component, however a grain size 10 is unusually small. While the carbon content appears to have a significant effect on strength it does not appear to affect the fatigue life. The fatigue strength for one million cycles was relatively constant for all of the samples at approximately 500 MPa.