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

摘要

Effects of the initial microstructure and its changeson creep deformation behaviour of a lCr-0.5Mo steel have beeninvestigated in order to understand a mechanism of complex creepdeformation behaviour which shows several local minima in creeprate. Sigmoidal inflections the observed on stress vs. time torupture curves and stress vs. minimum creep rate curves at 823and 873K. Under the stress conditions lower than 100 MPa, creeprate vs time curves indicate inflection at the same time of a tertiarycreep stage.Significant decrease in creep rupture strength due to prior ageingfor 500 h at 873K has been observed at stress conditions higherthan 100 MPa However no difference in creep rupture strengthbetween pre-aged and un-aged steels has been observed in thestress conditions lower than 100 MPa. The inflection of the creeprate vs. time curves has disappeared by ageing prior to creep test.High density of dislocations and many fine carbide particles areobserved within ferrite grain of the un-aged steel. On the otherhand, coarse needle-like carbides and very low density ofdislocations are observed within ferrite grain of the pre-aged steel.Because of the similar decrease in the number of dislocations andprecipitate occurring in the un-aged steel during creep deformation,no difference in microstructural morphology is observed for theun-aged and pre-aged steels after creep for 200 h at 873K-88 MPa.It has been considered that the effect of precipitation strengtheninghave disappeared during ageing for 500 h at 873K prior to creeptest and creep deformation for 200 h at 873K-88MPa.It has been concluded that complex creep deformation behaviourand sigmoidal inflection of stress vs. time to rupture curve arecaused by decrease in creep strength due to microstructural changefollowed by advent of inherent creep strength.