In the last recent posting, the case and core properties of carburized SAE 86B20 and SAE 4620 were compared. The data showed both grades to have comparable core fatigue properties; however the more highly alloyed SAE 4620 exhibited superior case fatigue properties. In this posting, the effect of alloy content on the case and core fatigue properties of alloy steels is explored further with the aim of confirming the earlier findings.
As has been noted in a number of earlier postings, 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 important in facilitating 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 4320. As was pointed out in the last posting, SAE 86B20 is a boron-enhanced variant of SAE 8620, which is a nickel-chromium-molybdenum steel. SAE 4320 is also a nickel-molybdenum grade, but contains a significantly higher amount of nickel (1.65-2.00% by weight). As in the earlier comparison, the case properties for each grade 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 tensile strength for SAE 86B20 at the case location was lower than expected due to a very low ductility; this resulted in failure during tensile testing before reaching the expected ultimate tensile strength. For the same reason, a reliable value of ultimate tensile strength could not be obtained for SAE 4320 at the case location.
The microstructures obtained for SAE 4320 consisted of martensite, bainite and ferrite in the core, and martensite in the case. For SAE 86B20, the martensite was observed at both the case and core locations.
Figure 1 shows the strain-life fatigue curves obtained for the core location of each of the two steel grades. The strain-life curve for SAE 86B20 is given by Iteration No. 74, and the strain-life curve for SAE 4320 is given by Iteration No. 49. The data and the calculated strain life curves show comparable fatigue performance for both steel grades.
Figure 2 gives the fatigue properties of the case locations for the two steel grades. Iteration No. 75 gives the strain-life curve for SAE 86B20, and Iteration No. 50 shows the strain-life curve for SAE 4320. In this location, the fatigue properties of SAE 4320 are significantly better than those of SAE 86B20.
These results are very similar to those reported in the earlier posting where SAE 4620 was compared with SAE 86B20. In that case, as well as for the data shown here, the more highly alloyed steel grade exhibited superior case fatigue performance. Thus in demanding applications, where case properties are critical, more highly alloyed steel grades would be preferred.