PSI - Issue 42

Jan Klusák et al. / Procedia Structural Integrity 42 (2022) 1369–1375 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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4. Fatigue life of the studied steels The aim of the study was to determine fatigue life of both steels. S-N curves are presented in Fig. 5. The fatigue life of both steels is comparable, where the 1.4306 steel exhibits slightly higher numbers of cycles to fracture than 1.4307. It is probably caused by a higher content of nickel in the 1.4306 steel. The beneficial effect of high Ni contents in stainless steels is often explained by a combination of high austenite stability, high stacking fault energy and absence of other microstructural features promoting planar slip (Michler et al. 2014). A higher fatigue life of stainless steels with the increasing nickel content was also shown in (Michler et al. 2017). The main difference between both steels is the presence of two-step S-N curve in the case of 1.4306 steel. In the region of loading amplitude between 270 and 280 MPa, the life-time of 1.4306 steel occurs in two regions: below 10 6 cycles (high cycle fatigue region - HCF) and between 10 7 and 10 9 cycles (very high cycle fatigue region - VHCF). Further, in the case of 1.4307 steel, no failure was observed in VHCF region (beyond 10 7 cycles). The fatigue limit was determined from run out tests, which correspond to the samples capable to withstand 10 10 loading cycles without failure. The slope of the curve in the high cycle fatigue region (below 10 7 ) can be described by the Basquin’s law in the form (Basquin 1910): = . , (1) where the coefficients A and B of the relation (1) are in the Table 3. In Table 3, there are fatigue limits  f of both steels, where steel 1. 4306 exhibits a slightly higher value.

Table 3 . Coefficients of the Basquin’s law relation and the fatigue limit. Material A B

σ f [MPa]

1.4306 1.4307

750 553

-0.078 -0.055

255 245

Fig. 5. Results of fatigue testing of 1.4306 and 1.4307 stainless steels, empty points mean specimen run outs.

5. Fracture surfaces Fracture surfaces of all broken specimens were analyzed using scanning electron microscope, where only surface crack initiation was observed. The crack initiation mechanism was the same for all samples. The cracks have developed from extrusions and intrusions on the surface, (Polák 1991, Polák et al. 2017). Transcrystalline crack