How austenite improves the fatigue behavior of high-speed steels

Optimizing the fatigue performance of tool materials, such as high-speed steels, is crucial for increasing the service life of parts and metalworking tools. An important property in this respect is a material’s resistance to the propagation of short cracks, evident in cyclic R-curve behavior. The potential to improve the fatigue crack propagation resistance by transformation-induced crack closure was studied for a high-speed steel grade in which significant fractions of metastable austenite were retained. The austenite’s resistance to martensitic transformation under cyclic thermal loads was evaluated. Transformation-induced plasticity was studied using tensile tests with in situ determination of austenite content by X-ray diffraction using synchrotron radiation. The cyclic R–curve behavior and the threshold for fatigue crack propagation were determined for stress ratios of R = 0.1, −1, and −5. Critical parameters regarding fatigue behavior, such as the slope of the cyclic R-curve and the long crack threshold of the stress intensity factor range, were significantly improved relative to comparable industry-relevant material states.