Abstract
Creep characteristics of 18-8 stainless steel at room temperature were studied. The specimens were fabricated from 20mm diameter bars, which were cold drawn and solution treated at 1050°C. Constant load creep tests were made under the stresses from 12.5kg/mm2 to 50.0kg/mm2. Durations of tests were about 2000 hours and the test under 20kg/mm2, which is still continued, were conducted over 17000 hours.
Under the stresses even lower than the yield strength, relatively large creep was observed and this is advancing even over 10000 hours. In the case of 20kg/mm2, instantial elongation was 0.24 per cent, and total elongation reached to 1.3 per cent at 15500th hour. The creep rate which showed 0.091 per cent per 1000 hours in the interval between 1000th and 2000th hour, was reduced to 0.098 per cent per 1000 hours between 10000th and 17000th hour.
For the test which were conducted under the higher stresses than 20kg/mm2, instantial elongation and initial creep rate was greatly increased with stress, but the creep rate at 1000th hour or after decreased to the similar order of the creep under 20kg/mm2.
Plottings of logarithm of permanent elongation vs. logarithm of time showed good liner relationship for all these tests and any tendency to deviate from the line to upper side at longer time range, was not detected. It may be concluded that the room temperature creep of this material consist mainly with transient creep, expressed by the formula of dε/dt=αt-m, and no steady creep is contained.
From practical point of view, these creep should be taken into account for the design of parts of installations, which should be confined to the small strain. For this purpose, the stresses which produce a definite strain at a certain time were determined from this results. The stress for 0.2 per cent at 1000th hour is about 75 per cent of the yield strength determined by routine test.
