Very High Cycle Fatigue Resistance of the Low Alloyed Steel 42CrMo4 in Medium- and High-Strength Quenched and Tempered Condition

Abstract

Low cyclical loadings can cause failure after a very high number of cycles in the so-called “Very High Cycle Fatigue” (VHCF) area. Thereby, failure initiates typically below the surface at defects like non-metallic inclusions. The appearance of VHCF failure depends on the microstructure of the material and the loading situation. E.g., a quenched and tempered steel with a given distribution of non-metallic inclusions may be insensitive to VHCF failure in a low strength condition but very sensitive in a high-strength condition. The present study investigates the influence of tempering temperature on the fatigue resistance of a low alloyed steel. Uniaxial tension-compression fatigue tests (50 Hz/1 kHz and R=-1) were performed on specimens made of 42CrMo4, which were tempered at six different temperatures to produce wide a range of ultimate strength. With the decrease in tempering temperature the sensitivity of subsurface crack initiation at inner defects increases. High tempered conditions with Rm < 1400 MPa show no failure between 106 and 109 cycles. Crack initiation almost occurs at the surface as a result of local plasticity and surface defects. The fatigue resistance at 109 cycles (Rw/9) matches to the fatigue resistance at 106 cycles (Rw/6). The low tempered conditions show a tendency of increasing life scatter and the threshold value for subsurface crack initiation increase with decreasing strength-level. The study indicated that for high-strength heat treatment conditions the difference between the fatigue strength at 106 and 109 increases with decreasing tempering temperature. A functional relationship between these two fatigue strength was found and verified experimentally. It seems that the stress intensity factor K which arises as a function of local loading conditions at inner stress-raisers depends on the yielding /hardening properties of the material around them.