In this article we will examine Iterations #62, #63 and #70. These are carburized samples with a case/core composite to simulate an actual carburized part. Until now, all of the carburized samples we have looked at have been through carburized or nearly through carburized. The intent is to provide fatigue properties of the carburized case (through carburized) and for the noncarburized core region (via quench and temper heat treatment) to predict fatigue life of a case/core composite. This assumes the microstructure of the case is homogeneous as is the microstructure of the core.
The results for the case-core samples are shown in the table below, along with a summary of the previous through carburized data. Two of the case-core iterations were made from different heats of SAE 8620 steel (iterations #62 and #70), and one was made from SAE 4320 steel (iteration #63). The effective case depths were 0.024 inches (20%) for iteration #62, 0.011 inches (10%) for iteration #63 and 0.022 inches (20%) for iteration #70. The average ultimate strength is 1715 MPa, and the average yield strength is 1368 MPa. These results are high and they are consistent, compared to the through-carburized data we have previously viewed. In addition, the elongation, and reduction of area are also high, compared to the through-carburized samples. The fatigue strength (strength at 10^6 cycles) is also higher and more consistent than we have seen before. The fatigue ratio is about the same, however the consistency is better. The average surface hardness for the case-core samples is lower than either of the through-carburized groups. The case-core samples were ground after carburizing, which is the reason for the lower hardness. The carburized case was relatively thin, and the grinding process removed the hard outer layer. These samples are below the minimum surface hardness specification of 57 or 58 HRC, normally used with carburizing. The core hardness of the case-core samples is significantly lower than the through-carburized samples, which is expected due to the nature of the heat treatment. However, the core hardness is at the upper end of what is typical of a carburized component. It is possible the core hardness may have an effect on fatigue strength, as the sample with the highest core hardness has the greatest ultimate tensile strength. However, with only three samples this cannot be confirmed. The case depth, within the range used, does not appear to have any significant effect on strength. Again, more test results are necessary to make a firm conclusion.