High-Cycle Fatigue Properties of Ferritic-Pearlitic Medium-Carbon Forging Steels with Smooth and Notched Specimens

Abstract

High-cycle fatigue properties of two ferritic-pearlitic medium-carbon forging steels 30MnVS and 49MnVS with smooth and notched (stress concentration factor Kt=2.1 and 3.9) specimens were studied by using rotating bending fatigue and fatigue crack growth (FCG) rate testing. Structural steels 40Cr and 30MnVS with quenched & tempered microstructures (40Cr-QT, 30MnVS-QT) were used for comparison. It was found that both fatigue strength and fatigue strength ratio at each Kt of the as-forged 30MnVS steel (30MnVS-AF), which are comparable to those of 30MnVS-QT and 40Cr-QT, are notably higher than those of the as-forged 49MnVS steel (49MnVS-AF), and this difference tends to decrease with an increase in Kt. However, 30MnVS-AF exhibits slightly higher fatigue notch sensitivity and FCG rate than 49MnVS-AF, and the steels with ferritic-pearlitic microstructure also have higher fatigue notch sensitivity than the steels with QT microstructure. A simple and practical method which incorporates simple microstructural parameters, and hardness is proposed to estimate the smooth fatigue strength (σw) of ferritic-pearlitic forging steels, while notched fatigue strength can be expressed by a power function of Kt with its coefficient equals to σw, which can be further classified into two groups corresponding to forging steels with relatively lower and higher C contents, respectively.