In this article we will compare two through-carburized boron steels which were both atmosphere and vacuum heat treated. As in previous axial fatigue tests; approximately 30 test samples were prepared to ensure there were three replicate tests at a minimum of 6 strain levels to develop a complete fatigue curve. These test bars were finished prior to the carburizing heat treatment with no subsequent grinding. The through-carburizing cycle of 26 hours at 927 °C does not completely through carburize the 0.2 inch gage length diameter, but typically leaves the core hardness at or above 55 HRC. The two steel grades are SAE 41B17 and SAE 86B20. The chemical composition of each is shown in Table 1. The primary difference between the two steels is the nickel content, which is significantly lower in the SAE 41B17 grade as it is a residual element.
The mechanical properties, fatigue strength and hardness are shown in Table 2. Iterations #73 and #75 were atmosphere carburized while iterations #79 and #83 were vacuum carburized. The elongation and reduction of area is 1% or less for all iterations with the exception of iteration #79. This iteration has the lowest surface (case) hardness at 653 Brinell or 60 HRC. The remaining iterations are 665-710 Brinell or 61-63 HRC. All of these hardness values, including iteration #79, would be acceptable for a typical carburized component.
A graphical representation of the yield and ultimate strength, as well as the fatigue strength, is shown in Figure 1. The fatigue strength is the stress level which provides one million cycles during strain controlled axial fatigue testing. Iteration #79 has the lowest hardness, however, the yield and ultimate strength is approximately 40% greater than the others. This indicates a lower hardness may be beneficial in improving fatigue properties. While the fatigue strength does vary from 235-536 MPa for these tests there is no clear cause (chemistry or carburizing method) for the variation and is considered to be within normal fatigue result variation.
Plots of surface hardness versus ultimate strength and hardness versus fatigue strength are shown in Figure 2. The plot shows no correlation between hardness and fatigue strength. This was also observed in the prior blog article, “Strength and Fatigue Life versus Carbon Content at High Hardness (60 HRC).” Obviously, there is a relationship between hardness and fatigue strength (see previous blog article, “Hardness versus Fatigue Strength”) but it has considerable variability and is not evident here. Figure 2 shows a significant increase in ultimate strength when the hardness decreases to 653 Brinell. This is in agreement with the previous blog article, “Hardness versus Strength,” where strength increased linearly with hardness to just below 600 Brinell and then decreased rapidly with any further increase in hardness. SAE J413 teaches us strength should increase with hardness beyond 600 Brinell. However, this does not appear to be true with high carbon carburized steel.