The effect of phosphorus on carbide composition in a CrMoV low alloy steel under creep conditions

A study has been made of the role of phosphorus in modifying the compositions of carbides in a 1Cr1Mo3/4V bolting steel during simulated service exposure. Creep tests were carried out at 565°C on two casts of material containing selected additions (wt%) of 0.08Ti and (0.08Ti+0.02P). The microstructures of the materials prior to testing consisted of tempered bainite, with one of two prior austenite grain sizes—either 8 or 150 μm. It was found that, under creep conditions at 565°C, the addition of 200 ppm of P enhanced the diffusion of Mo to coarse V-rich carbides. However ageing without stress reduces this diffusion. No new carbides were precipitated in fine grained material under either of these tests conditions. In coarse grained material, P has no apparent effect on the development of coarse carbides, in this case Ti-rich, under either ageing or creep at 565°C. For this material, V-rich carbides were found to develop under the influence of the creep stress, leading to the subsequent formation of Mo-rich carbides.