Fatigue Property of Nickel-Free High Nitrogen Austenitic Stainless Steels in Simulated Body Fluid

Two kinds of high nitrogen-containing Ni-free austenitic stainless steels for medical applications were used for evaluation of fatigue behavior. One was a Fe-23%Cr-2%Mo-1%N stainless steel heat-treated in a N2 gas atmosphere (NA) and the other was a Fe 23%Cr-1%Mo-1%N stainless steel fabricated with an electro-slag remelting method in a pressurized N2 gas atmosphere (P-ESR). Fatigue tests were carried out both in air and in the phosphate-buffered saline solution, PBS (−). Cyclic stress with a sine waveform was applied to the specimen in a tension-to-tension mode with a stress ratio of 0.1 at a frequency of 20 Hz in air and 2 Hz in PBS (−). During testing, the saline solution was kept at 310 K and at the pH of 7.5. A nitrogen-4% oxygen gas mixture was bubbled into the PBS (−). The results obtained are as follows. There was no difference between S-N (stress-number of cycles to failure) curves in air and in PBS (−) for each stainless steel. The fatigue strength at 107 cycles for NA was 245 MPa. No difference was found in the fatigue strength between the NA specimens heat- treated for 259.2 ks and 129.6 ks. The fatigue strength at 107 cycles for P-ESR was 320 MPa. The fatigue life at various stress amplitudes depended on preparation of the test specimens for P-ESR. The fatigue life was decreased by the plastic deformation layer and/or cracks on the specimen surface caused by machining and increased by elimination of the deformation layer and/or cracks. The fracture surface exhibited a cleavage-like appearance at the fatigue crack initiation site for both the stainless steels.