Low-cycle fatigue behaviour of boron-added 9% Cr martensitic steel: Effects of temperature, strain rate, and strain amplitude

The Low-Cycle Fatigue (LCF) behaviour of a boron-added 9% Cr martensitic stainless steel, COST FB2, was investigated at temperatures of 300 °C, 600 °C, and 650 °C. Mechanical strain amplitudes were prescribed between 3×10−3 and 8×10−3, with four different strain rates ranging from 3.3¯×10−5/s to 1×10−2/s applied at the higher test temperatures. Cyclic softening was observed under all the investigated loading conditions, driven by a reduction in dislocation density and lath coarsening, which became more pronounced at higher strain amplitudes and temperatures. Investigations into the damage mechanisms revealed transgranular cracking as the predominant failure mode, with the frequency of secondary cracks increasing with strain amplitude. At 600 °C and 650 °C, environmentally assisted cracking and oxidation effects became more significant, especially at higher strain amplitudes and lower loading frequencies. Fatigue lifetime decreased with an increasing temperature and strain amplitude, and a decreasing strain rate. To account for the environmentally assisted cracking effects, a frequency-modified Manson–Coffin model was proposed. This modified model accurately captured the reduction in lifetime at lower mechanical strain rates.