Effect of initial microstructure on long term creep strength of a low alloy ferritic steel

The effect of initial microstructure, martensite, tempered martensite, bain ite and ferrite + pearlite, on long term creep strength property up to abou t 80000 h at 848K has been investigated on a 0.5Cr-0.5Mo low alloy ferritic steel. No significant effect of tempering after quenching on the long term creep strength has been observed. However, strong effect of transformation process during cooling from austenite has been observed. Creep strength of bainite microstructure is higher than those of martensitic microstructure, independent of tempering after quenching. In the low stress condition in w hich applied stress is lower than proof stress. creep strength of full anne aled microstructure which consists of ferrite and pearlite has been found t o be higher than the others. The smallest creep rate of a full annealed mic rostructure in the long term creep region should be caused by both very low dislocation density of the initial microstructure and small amount of disl ocations generated under the low stress condition.