Low cycle fatigue behavior of quenched and tempered SAE 1045 steel

Monotonic stress-strain properties and axial fatigue results are reported for five high hardness levels of quenched and tempered SAE 1045 steel. Yield, ultimate and true fracture strength and the completely reversed fatigue strength are found to go through a maximum value as the hardness increases. The optimum hardness for maximum strength is found to be approximately 630 Brinell. Deformations during stress cycling are shown to change in a fashion dependent upon the initial hardness. At 630 BHN the same size and shape of hysteresis loop was obtained cycle after cycle. At lower hardnesses, extreme cyclic softening was observed. The as quenched or hardest condition cyclically hardened. The stress-life relation σa = σ_f^1(2Nf)b, is shown to hold over the hardness region investigated with the fatigue strength component, b, varying only slightly between -0.07 and -0.08. The fatigue strength coefficient, σ1f, is found to be approximately equal to the true fracture strength of the monotonic tension test. Attempts to relate the fatigue life to the plastic strain using the function, ∆ϵp/2 = ϵ1f (2Nf)c, were unsatisfactory because this plastic strain constantly increases when cyclic softening occurs under stress control making it difficult to select a representative value of ∆ϵp. Using terminal values of ∆ϵp, it is shown that the fatigue ductility exponent, c, of -0.5 suggested by Coffin does not fit the results. Instead, values of the exponent between -0.6 and -1 are obtained with the -1 value applying at high hardness.Caterpillar Tractor Company, LaSalle Steel Company, United States Steel Corporatio