An austenitic steel used as a structural material of a superconducting magnet shows creep deformation and stress relaxation even at medium and low temperatures. Small plastic and creep strains in eight austenitic steels and a low-alloy steel were measured in a temperature range from 4 to 573K. Every steel including the low-alloy steel showed logarithmic creep strain at temperatures when stress was high enough to produce plastic strain. Although the creep strain rate in SUS316L at 293K was proportional to about the 7th power of stress when it was around 0.2% plastic-offset stress, the order of power decreased to 1 under lower stresses. The ratio of creep strain at 10
5s and plastic strain was 1-3 at 293K and 0.5-2 at 77K for the steels, though precipitate-hardened steel SUH660 showed a lower value. When creep strain at 10
5s in the steels was 0.02% at 77K, the stress levels were from 0.7 to 0.85 of 0.2% plastic-offset stress. Because creep strains were small compared to plastic strains at lower and higher temperatures, the ratios of 0.02% proof stresses for 10
5s creep and plastic strains were as high as 1.5 both at 4 and 573K. They were steadily lower values, 0.9-1.0, between 77 and 450K. Creep deformation in a component can be prevented by pre-straining with a plastic pre-strain larger than an estimated inelastic (plastic+creep) strain during operation. The pre-straining effect holds even when operation and pre-straining temperatures are different.
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