Low and high-cycle fatigue properties of an ultrahigh-strength TRIP bainitic steel

The scope of this study is to characterize the mechanical properties of a novel Transformation-Induced Plasticity bainitic steel grade TBC700Y980T. For this purpose, tensile tests are carried out with loading direction 0, 45 and 90° with respect to the L rolling direction. Yield stress is found to be higher than 700 MPa, ultimate tensile strength larger than 1050 MPa and total elongation higher than 15%. Low-cycle fatigue (LCF) tests are carried out under fully reverse axial strain exploring fatigue lives comprised between 102 and 105 fatigue cycles. The data are used to determine the parameters of the Coffin–Manson as well as the cyclic stress–strain curve. No significant stress-induced austenite transformation is detected. The high-cycle fatigue (HCF) behaviour is investigated through load controlled axial tests exploring fatigue tests up to 5 × 106 fatigue cycles at two loading ratios, namely R = 1 and R = 0. At fatigue lives longer than 2 × 105 cycles, the strain life curve determined from LCF tests tends to greatly underestimate the HCF resistance of the material. Apparently, the HCF behaviour of this material cannot be extrapolated from LCF tests, as different damage, cyclic hardening mechanisms and microstructural conditions are involved. In particular, in the HCF regime, the predominant damage mechanism is nucleation of fatigue cracks in the vicinity of oxide inclusions, whereby mean value and scatter in fatigue limit are directly correlated to the dimension of these inclusions.