Abstract
In order to determine high fatigue strength of 18-8 stainless steel at room temperature (to be hereunder abridged as R. T.), the authors performed the two following series of experiments.
Experiment A:
(1) Measurement of the amount of ferrite and micro-Vickers hardness at R. T. of the specimens to which various extents of stretching were given at -77°C.
(2) Measurements of the same at R. T. of the specimens heat-treated at several elevated temperatures after sub-zero working.
Experiments B: Rotary bending fatigue test at R. T. of the four followiug kinds of test specimens:
(1) sub-zero worked specimens, 20% stretched at -77°C. (2) cold worked specimens, 20% stretched at R. T. (3) specimens 20% sub-zero stretched at -77°C, and later heat-treated at 550°C for 100 minutes. (4) specimens heat-treated in solution at 1050°C for 60 minutes (S. H. T.), followed by water quenching.
The results of experiments can be summarized as follows;
Experiment A: An extraordinary amount of ferrite was recognized in the specimens heat-treated at 100°C for 30 minutes.
This phenomenon disappeared at 300°C.
Experiment B: The combination of sub-zero working and subsequent heat-treatment at elevated temperature was found to increase fatigue strength effectively at R. T. The specimens (3) showed the highest fatigue strength 53.2kg/mm2, i.e. 90% greater than that of the specimen (4).
These phenomena may be explained as due to the decomposition of metastable austenite and to the solubility of micro carbides.
