Effects of P addition on creep of 1 Cr–Mo–V steels

In order to understand the variation of creep properties with the addition of P, 1 Cr–Mo–V steels with 0·006, 0·045, and 0·1 wt-%P were heat treated to obtain identical microstructure, matrix hardness, and prior austenite grain size. Creep tests were conducted at 546°C within the stress range 230–350 N mm⁻². The minimum creep rate was found to increase with the addition of P which was partly caused by accrued cavity formation. Examination of bulk precipitates by transmission and scanning electron microscopy, electrolysis, chemical analysis, and X-ray diffraction showed that the addition of P hindered the formation of VC and Mo₂C but favoured the precipitation of coarse (Fe, Cr)₃C. Coarse carbides were less effective than fine carbides in resisting creep deformation; thus the effect of P on the carbide composition and morphology also contributed to the low creep resistance of the P doped steels. The effect of P on carbide formation was also reflected in the decrease of the friction stress with P content, as measured by the stress drop test. In addition to the effect of P on cavity and carbide formation, the presence of P was found to increase the density of MnS particles which served as nucleation sites for creep and ductile voids. The effect of stress on the equilibrium segregation level of P was studied by Auger electron spectroscopy: application of stress up to 350 N mm⁻² did not have any significant influence. However, a much higher segregation level of P was found on cavity surfaces than on grain boundaries, which implies that a free surface type segregation of P to cavity surfaces occurred.