In the most recent posting, the case and core properties of case hardened steels were compared for carburized and surface induction hardened steel grades. In this posting, the case and core fatigue properties for two different carburized alloy steel grades are compared.
As was noted earlier, carburizing is usually employed on carbon or low alloy steels containing approximately 0.2-0.25 wt. % carbon. Carbon is diffused into the surface of a part during heat treatment resulting in a high carbon (0.8 wt. %), high hardness case and a lower carbon, softer core. Comparisons of case and core properties for various alloy steels are of interest in terms of helping in the selection of a particular steel grade for a given application.
In this posting, the fatigue properties of carburized SAE 86B20 are compared with carburized SAE 4620. SAE 86B20 is a boron-enhanced variant of SAE 8620, which is a nickel-chromium-molybdenum steel. SAE 4620 is a nickel-molybdenum grade containing a high amount of nickel (1.7% by weight). For each grade, the properties of the case were developed through simulation by diffusing carbon completely through fatigue specimen blanks. The properties of the core were simulated by subjecting specimens to the carburizing thermal cycle absent the presence of carbon in the atmosphere.
The table below summarizes the mechanical properties and hardness values obtained for the two steel grades.
The lower than expected tensile strength for 86B20 at the case location was due to a very low ductility; this resulted in failure during tensile testing before reaching the expected ultimate tensile strength. The microstructures obtained for SAE 4620 were a mix of martensite, bainite and ferrite in the core, and martensite in the case. For SAE 86B20, the microstructure at both the case and core locations was martensite.
Figure 1 shows the strain-life fatigue curves obtained for the core location of each steel grade. The strain-life curve for SAE 86B20 is given by Iteration No. 74, and the strain-life curve for SAE 4620 is given by Iteration No. 47. The calculated strain life curves show somewhat better fatigue performance for the SAE 86B20 at long life. The data points however show approximately comparable behavior. It should be noted that the core hardness of SAE 86B20 exceeded that of SAE 4620.
Figure 2 gives the fatigue properties of the case locations for each steel grade. Iteration No. 75 gives the strain-life curve for SAE 86B20, and Iteration No. 48 shows the strain-life curve for SAE 4620.
In this location, the fatigue properties of SAE 4620 are clearly superior to those of the SAE 86B20. This suggests that, under conditions of demanding fatigue service, SAE 4620 should be selected over SAE 86B20. This would be especially true where superior case properties are required.