Fatigue Macrocrack Growth in Tempered Hy80, Hy130, and 4140 Steels:threshold and Mid-?K Range

Abstract

The rate of propagation of macrofatigue cracks down to near threshold was measured in air in three tempered martensitic steels; HY80, HY130 and 4140 (650°C temper). The value of A&,, was determined by the load-shedding technique in center notched panel specimens. Of the three steels, 4140 tempered at 65WC had the lowest AK,,, 3 3 MN/m3IZ, while HY80 had the highest, 4.2 MN/rn3". The 4140 (650°C temper) is intermediate in strength between HY80 and HY130. The results are discussed in terms of a recent theory of one of the authors.The fatigue crack propagation rates in the mid-AK range in HY80 and HY130 in argon were also studied by measuring, with foil strain gages, the cyclic plastic work to propagate a fatigue crack by a unit area, U . HY80 has a lower crack propqgation rate and correspondingly higher U. This was attributed in part to the higher yield strength of HY130 but the dislocation structure and carbide composition and morphology also play roles. Microstructural changes due to cyclic plastic deformation inside the plastic zone in HY80 and HY130 were observed by TEM of thin foils. SEM studies of the fracture surfaces at AK = 20 MN/m3" indicate a more ductile fracture mode for HYSO than for HY130. The fatigue crack propagation rate of HY130 is substantially higher in laboratory air (47% relative humidity) than in dry argon. This is not the case for HY80.

Correlation of Fatigue Crack Growth Rate at Different Stress Ratios for Quenched and Tempered Steels and Other Alloys

“…The steels QIN and HYI30 were examined by Jones [9] in dry air, laboratory air and seat water. The steels HY80 and HYI30 (HY80 is almost identical to QIN) were examined by Kwun and Fine (24] in laboratory air and dry argon. These investigations showed that the two steels have similar crack growth rates in dry air or dry argon.…”

“…The steels Q1N and HY130 were examined by Jones [9] in dry air, laboratory air and sea water. The steels HY80 and HY130 (HY80 is almost identical to Q1N) were examined by Kwun and Fine [24] in laboratory air and dry argon. These investigations showed that the two steels have similar crack growth rates in dry air or dry argon, but t i ( K n a x /oyI2 = 0," tIKmax the growth rate increases much more for HY130 when the environment is changed to laboratory air than it does for Q l N (the same trend does not apply for the seawater environment).…”

Fatigue and inelasticity of metals under nonuniform stressed state

“…The magnitude of inelastic strains can be judged from the data given in Fig. 5 which shows typical static and cyclic stress-strain curves for steels of various strengths [22]; the dash-and-dotted line represents the maximum value of inelastic strain per cycle, which was determined earlier [23] during the investigation of fatigue and inelasticity of various metals and alloys under high-cycle loading ( Table 1). The inelastic strain has even lower values at the stresses equal to fatigue limits of metals and alloys on a base of 10 7 cycles.…”