Effect of microstructural change on creep deformation behaviour and long-term creep strength of 1Cr-0.5Mo steel

Effects of the initial microstructure and its changes on creep deformation behaviour of a 1Cr-0.5Mo steel have been investigated, in order to understa nd a mechanism of complex creep deformation behaviour which shows several l ocal minima in creep rate. Sigmoidal inflections are observed on stress vs. time to rupture curves and stress vs. minimum creep rate curves at 823 and 873K. Under the stress conditions lower than 100 MPa, creep rate vs, time curves indicate inflection at the same time of a tertiary creep stage. Significant decrease in creep rupture strength due to prior ageing for 500 h at 873K has been observed at stress conditions higher than 100 MPa. Howev er, no difference in creep rupture strength between pre-aged and un-aged st eels has been observed in the stress conditions lower than 100 MPa. The inf lection of the creep rate vs. time curves has disappeared by ageing prior t o creep test. High density of dislocations and many fine carbide particles are observed w ithin ferrite grain of the un-aged steel. On the other hand coarse needle-l ike carbides and very low density of dislocations are observed within ferri te grain of the pre-aged steel. Because of the similar decrease in the numb er of dislocations and precipitate occurring in the un-aged steel during cr eep deformation, no difference in microstructural morphology is observed fo r the un-aged and pre-aged steels after creep for 200 h at 873K-88 MPa. It has been considered that the effect of precipitation Strengthening have dis appeared during ageing for 500 h at 873K prior to creep test and creep defo rmation for 200 h at 873K-88 MPa. It has been concluded that complex creep deformation behaviour and sigmoida l inflection of stress vs. time to rupture curve are caused by decrease in creep strength due to microstructural change followed by advent of inherent creep strength.