In the previous post, the effects of a cooling rate change after carburizing, due to a change in steel bar diameter, on core fatigue properties was examined. The data was developed for a Ni-Cr-Mo low alloy steel (SAE 4320), and was aimed at demonstrating the effects of varying section size on the core fatigue properties of carburized components. It was shown that smaller bar diameters resulted in higher percentages of martensite, higher hardness and improved fatigue properties compared to larger bar diameters.
Additional data have been developed for a Mn-Cr low alloy steel, 20MnCr5. Two bar diameters were again evaluated: 15.2 mm (0.6 in.) and 60.9 mm (2.4 in). Both bar diameters were heat treated using a carburizing thermal cycle, again without the presence of carbon in the furnace atmosphere. As in the previous post, the aim was to simulate carburized cores at different cooling rates. The mechanical properties and hardness values obtained for the two bar diameters are shown in the table below.
The 15.2 mm diameter bar developed a microstructure estimated at 90% martensite, 5% bainite and 5% ferrite. The 60.9 mm diameter bar exhibited a microstructure estimated at 50% martensite, 10% bainite and 40% ferrite.
Comparative strain-controlled fatigue properties are shown in Figure 1. The 15.2 mm diameter bar is represented by Iteration No. 130, and the 60.9 mm diameter bar by Iteration No. 128.
It can be seen that the smaller diameter, higher hardness bar exhibits improved fatigue properties compared to the larger diameter bar, especially as run-out is approached at lower strain amplitudes.
These results correlate with the earlier results in the previous post, and again indicate that variations in the section size of a carburized component can affect cooling rate after carburizing. This in turn can result in changes in core fatigue properties.