INFLUENCE OF MICROSTRUCTURE ON THE CREEP PROPERTIES OF GRADE P22 STEEL

The creep strength and ductility of Grade P22 steel (21/4 Cr) was measured at 600 degrees C under standard uniaxial tensile conditions at 150MPa. Test specimens were prepared by solution heat treatment at austenitisation temperatures ranging from 900 degrees C - 1200 degrees C followed by normalization at 900 degrees C before continuous air cooling to room temperature. In addition to specimens tested in the solution treated state, creep tests were also performed after tempering. The variable austenitisation temperatures gave rise to different prior austenite grain (PAG) sizes, which in turn influenced the crystallographic packet and block boundary misorientation angle distribution. The latter parameters were measured using electron backscattered diffraction which also allowed partial reconstruction of the PAG boundaries. The time to creep failure at 600 degrees C increased as function of PAG size up to approximately 70 m, but significantly decreased when the average prior austenite grain size measured approximately 108 m. However, the minimum creep rate decreased even up to the largest PAG size with corresponding decrease in creep ductility. The stability of the crystallographic packet and block boundaries influences the high strength-low ductility for the large PAGs in comparison to the dominant effect of PAG boundaries at the smallest grain size where extensive recovery and recrystallization reduces creep strength.