In the previous article we examined the relationship between hardness and tensile strength for the AISI fatigue database iterations through 141. As the hardness increased the tensile strength also increased for a majority of the steels and the relationship was linear. However, with the high hardness, high carbon, carburized case steels there was a significant variation in strength in the 600-750 Brinell hardness range. The variation in strength was 881-2227 MPa, which is a difference of 2.5:1.
In this article we examine the relationship between hardness and fatigue strength for the same samples. The fatigue strength is defined as the stress level where one million cycles is achieved. The data is shown in Figure 1. The curve looks much like the previous hardness versus strength curve. There is a linear correlation between hardness and fatigue strength to 600 Brinell. However, there seems to be more scatter or variation in the hardness versus fatigue strength curve. At a hardness of 600-750 Brinell there is a large variation in fatigue strength primarily due to the carburized case samples. The fatigue strength range is 235-819 MPa, which is a difference of 3.5:1 between the high and low iterations.
The fatigue ratio is the fatigue strength divided by the ultimate strength. The fatigue ratio versus hardness data is shown in Figure 2. The fatigue strength does not appear to be a fixed percentage of the ultimate tensile strength throughout the entire hardness range. At 200 Brinell the fatigue ratio is approximately 0.45, while at 700 Brinell it is 0.35. As the hardness and strength increases the fatigue strength/ultimate strength becomes a smaller ratio.