There is a very good relationship between hardness and strength for steel. If the hardness is known the tensile strength can be estimated, and the reverse is also true. This relationship is valid for tensile strength but not for yield strength. This relationship is true for most any kind of steel, whether it is as rolled, forged, heat treated or cold worked. A graph of this relationship is provided in SAE J413.
The relationship of hardness versus strength for most of the iterations through 141 is shown in Figure 1. Three different classes of steels are shown, through-hardened, induction-hardened case and carburized case. The through-hardened steels are primarily made up of as rolled, forged and cold worked low to medium carbon steels which may or may not have been quenched and tempered. With these steels there is a linear relationship between hardness (from 130 to 650 Brinell) and tensile strength, which is in good agreement with SAE J413. With the induction-hardened case steels there are only three data points. These three steels were SAE 1050M, SAE 1552 and SAE 1070. Two of the data points seem to follow the same curve as the through-hardened steels while one, the SAE 1552, is significantly lower in strength. In a previous blog article, “Strength and Fatigue Life versus Carbon Content at High Hardness” it was determined the reduced properties were likely caused by the large grain size present in these samples. With the carburized-case steels there is a tremendous variation in strength at the same hardness. At 600-750 Brinell there is a difference of 2.5:1 between the maximum and minimum strength of the test iterations. There are about 4 or 5 data points that fall within the through-hardened curve while the majority is much lower. It is typically assumed the carburized case has high strength because of the high hardness and the core is lower in strength because it is softer. However, the data indicates in many cases the carburized case is equal in strength to the softer core. At the very low end it may only be equal to a core with a hardness of about 250 Brinell.
What is the reason for this behavior of the carburized case? Again, from the previous article referenced above we know that both a large grain size and a high carbon content can decrease strength at a hardness of 60 HRC. Certainly, many of the carburized case samples have both of these conditions as some were through-carburized with very long cycle times. However, others were case hardened to produce a case/core composite at more typical cycle times. In future articles we will take a closer look at the carburized case samples to try to determine what other factors may be affecting the strength level, as well as what the relationship between hardness and fatigue looks like.