In past postings, the case and core properties of case hardened steels have been discussed. These discussions have included the fatigue properties of both carburized and surface induction hardened steels.
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. Surface induction hardening is utilized on higher carbon steels (~0.7 wt. %). During heat treatment, the surface of a part is rapidly heated by induction and then cooled to create a high hardness case coupled with a lower hardness core. Of interest are direct comparisons of the fatigue properties that can be obtained with either case hardening method, since such a comparison would help in the selection of a given combination of steel grade and processing for a particular application.
In this posting, the fatigue properties of carburized SAE 4620 are compared with surface induction hardened SAE 1070. For the SAE 4620 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. For the SAE 1070 the properties of as-hot rolled bar stock were used to simulate the core, and the case was simulated by through induction hardening companion bar stock.
The table below summarizes the mechanical properties and hardness values obtained for the two steel grades and processes.
It should be noted that equivalent hardness values were observed for the core location of each grade as well as for the case location. The microstructures obtained for SAE 4620 were a mix of martensite, bainite and ferrite in the core, and martensite in the case. For SAE 1070, since as-hot rolled bar was used to simulate the core, the microstructure was pearlite with some ferrite. The case, simulated by through induction hardening, was a mix of bainite, pearlite and a small amount of martensite.
Figure 1 shows the strain-life fatigue curves obtained for the core of each steel grade. The strain-life curve for the simulated core of surface induction hardened SAE 1070 is given by Iteration No. 36, and the strain-life curve for the simulated core of carburized SAE 4620 is given by Iteration No. 47. As can be seen, the fatigue properties for both test iterations are the same.
Figure 2 gives the fatigue properties of the simulated cases for both surface induction hardened SAE 1070 and carburized SAE 4620. Iteration No. 37 gives the strain-life curve for surface induction hardened SAE1070, and Iteration No. 48 shows the strain-life curve for carburized SAE 4620.
In this instance, the fatigue properties of carburized SAE 4620 are superior to those of the surface induction hardened SAE 1070. This suggests that, where case fatigue properties are an important consideration, the carburized low alloy SAE 4620 should be selected as opposed to the surface induction hardened SAE 1070.
Future postings will provide further comparisons of these two case hardening methods, and will also examine the effects of alloy content on the fatigue properties of case hardened steels.