Creep deformation of austenitic steels at medium and low temperatures

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An austenitic steel used as a structural material of a superconducting magnet undergoes 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 the temperature range from 4 to 573 K. Every steel including the low-alloy steel showed logarithmic creep strain at these temperatures when stress was high enough to produce plastic strain. Although the creep strain rate in specimens, JIS SUS316L, at 293 K was proportional to about the 7th power of stress when the stress was around 0.2%-plastic-offset stress, the order of the power decreased to 1 as stress decreased. The ratio of creep strain at 10 5 s to plastic strain was in the range 1–3 at 293 K and 0.5–2 at 77 K, though the precipitate-hardened steel JIS SUH660 had a lower ratio. When creep strain at 10 5 s was 0.02% at 77 K, the stresses ranged 0.7–0.85 of 0.2%-plastic-offset stress. As creep strains were smaller than plastic strains at lower and higher temperatures, the ratios of 0.02%-creep-offset stress at 10 5 s creep to 0.02%-plastic-offset stress were as high as 1.5 both at 4 and 573 K. These ratios were lower (0.9–1.0) between 77 and 450 K. Creep deformation in a component can be prevented by pre-straining with a plastic pre-strain larger than the estimated inelastic (plastic plus creep) strain during operation. The pre-straining effect is also effective even when operation and pre-straining temperatures are different.