Kiyotsugu Ohji

Creep of 13-Cr Steel under Varying Stresses

Oil-quenched and tempered 13-Cr steel SUS-2 was tested at a temperature of 500°C under constant stresses, stepwise changing stresses and periodic stresses of rectangular wave. Behaviours of the material under changing stresses were compared with those under constant stresses.It was found that, strictly speaking, the law of“the mechanical equation of state in solid” was not valid for the material tested. The strain rate after the abrupt increase in stress was higher than that of the reference test under the constant stress equal to that after the stress change, and the strain rate was lower after the abrupt decrease in stress. This tendency was accentuated when the first stress change was delayed from primary creep to secondary creep, and further to tertiary creep. The behaviour of this material may be estimated with a small error by the use of the law of“the mechanical equation of state in solid, ”when the stress change is given during earlier stage of creep. The earlier the time of stress change, the smaller the error induced in the estimation.This is quite the same in the tests under periodic stress. The deformation in primary and secondary creep under periodic stress could be estimated from the data of constant stress creep tests with sufficient accuracy. This is applicable for the prediction of creep limit under varying stresses from the data of constant stress creep tests.