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 an Fe-23%Cr-1%N stainless steel heat-treated in a N 2 gas atmosphere (NA alloy) and the other was an Fe 24%Cr-2%Mo-1 %N stainless steel fabricated with an electro-slag remelting method in a pressurized N 2 gas atmosphere (P-ESR alloy). Fatigue tests were carried out both in air and in the phosphate-buffered saline solution, PBS(-). Cyclic stress with a sinusoidal 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 value 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 10 7 cycles for the NA alloy 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 10 7 cycles for the P-ESR alloy was 320 MPa. The fatigue life at various stress amplitudes depended on preparation of the test specimens for the P-ESR alloy. 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 alloys.