In a recent posting, a comparison of the fatigue properties for normalized steels (163-259 BHN) showed that the strain-life curves fell within a rather narrow band, especially in the long-life regime. It was pointed out that, under a circumstance such as this, comparison of long-life fatigue performance as a function of hardness level is somewhat difficult, and it becomes advantageous to consider using strain-life data to obtain a value of fatigue strength at a particular life level for performance comparisons. In addition, it was noted that this has the added advantage of determining a maximum allowable stress level to achieve a given life.
AISI has also evaluated a number of quenched and tempered steels over a wide hardness range of 222-584 BHN. Included were carbon steels, low alloy steels, and spring steels. For these steels, the direct use of strain-life data offers a means of comparing long-life fatigue performance as a function of hardness. The table below shows the mechanical properties and hardness, taken from the AISI Bar Fatigue Database, for three quenched and tempered steels.
Steel Yield Str. Tensile Str. Red. in Area Hardness
Grade MPa Mpa % BHN
1045 509.5 746.6 62.0 222
4130 1284.5 1482.8 44.3 442
9254 2270.0 2950.0 3.99 584
Figure 1 shows the strain-life curves for these three steels. The strain life curve for Iteration No. 26 shows the fatigue properties for SAE 1045, the strain-life curve for Iteration No.29 shows the properties for SAE 4130, and the properties for SAE 9254 are given by the strain-life curve for Iteration No. 35. A comparison of the three curves in the long life regime shows that long-life fatigue performance can be easily distinguished as a function of hardness level. The highest fatigue performance is exhibited by SAE 9254 (584 BHN), and the lowest by SAE 1045 (222 BHN). It should be noted that this distinction is more discernable for quenched and tempered steels compared to normalized steels because of the wider range of hardness values.
While a determination of fatigue strength at a particular life level is not always needed to compare fatigue performance, it still offers a means of obtaining a maximum allowable stress level to achieve a given life. As was noted in the earlier posting for normalized steels, fatigue strength at a particular life level is determined by the following equation, which is a part of the strain-life equation:
Δσ/2 = σf‘(2Nf )b
Where Δσ/2 is the fatigue strength, 2Nf is life in reversals (Nf is life in cycles), σf‘ is the fatigue strength coefficient, and b is the fatigue strength exponent. For each of the quenched and tempered steels listed above, the fatigue strength coefficient and the fatigue strength exponent are determined from the fit of the strain-life equation to the fatigue data. For these steels the values of σf‘ and b are given in the AISI Bar Steel Fatigue Database.
At a life of one million cycles (two million reversals), the fatigue strengths for the three steels shown in Figure 1 are as follows:
Steel Grade Fatigue Strength @ 106 cycles, MPa
The differences in maximum allowable stress among the three steel grades are apparent.
As was noted in the recent posting, a study conducted by AISI showed that the fatigue strengths of normalized medium and high carbon steels can be related to hardness and chemical composition. A similar study on quenched and tempered steels also showed the same relationships. A simple linear regression equation to predict fatigue strength was developed was again developed from the data in the AISI Bar Steel Fatigue Database, and is shown below:
Δσ/2 @ 106 cycles = 1.52BHN+45.68%C+298.05%Mn-2300.97%S-25.63%Si
Figure 2 shows a plot of predicted versus measured fatigue strength. The agreement between predicted values and measured values is quite good.
It should be noted that use of the regression equation above should be restricted to quenched and tempered steels with microstructures, chemical compositions and hardness values within the range of those described in the AISI Bar Steel Fatigue Database. The equation should not be extrapolated to lower hardness steels with ferrite pearlite microstructures.
In summary, while comparative fatigue performance among various quenched and tempered steels can be established by strain-life curves, fatigue strength is a valuable tool to establish a maximum allowable stress level to achieve a given fatigue life.