PSI - Issue 51

Milan Uhríčik et al. / Procedia Structural Integrity 51 (2023) 166 – 172 M. Uhrí č ik et al. / Structural Integrity Procedia 00 (2022) 000–000

168

3

a b Fig. 1. A fatigue experiment; (a) a display of fatigue test method; (b) the drawing of sample shape and dimensions.

After fatigue tests, broken samples were investigated in the SEM with the aim of determining the location and number of initiation places and investigating the micromechanism of fatigue crack initiation and evolution, which aims are also mentioned in the article by Konečná et al. (2020). Fracture surfaces of samples were evaluated using a TESCAN VEGA 2 LMU scanning electron microscope. For the identification of selected particles in the structure, a Bruker X-ray microprobe was used, which forms part of the scanning electron microscope. 3. Results and discussion The microstructure of both states of materials is shown in Fig. 2. Figs. 2a and 2b display the microstructure of both steels with austenitic polyhedral grains of different sizes, with a large number of inclusions in the microstructure and with a large number of annealing and deformation twins. Figs. 2b shows also a nitriding layer under which a diffusion layer is located on the surface of a material structure which has been formed by a nitriding process.

a b Fig. 2. Microstructure of austenitic steels AISI 304; (a) in the initial state; (b) after plasma nitriding.

One of the aims of the work was to determine the fatigue life of austenitic steels, in the initial state and after plasma nitriding. The S-N curves are shown in Fig. 3, on which they are compared. Austenitic steel after plasma nitriding