Microstructure of a high boron 9-12% chromium steel
Konferensbidrag (offentliggjort, men ej förlagsutgivet), 2008
Additions of small amounts of boron (10-100 ppm) to 9-12% chromium steels are often made since they have been found to be beneficial for the creep strength up to and above 600°C. The effect of boron is to restrict the coarsening of M23C6 precipitates during service. It was found that increasing the boron content from 9 to 40 ppm gave a decrease in coarsening constant at 600°C by a factor of 2. The present understanding of boron solution, non-equilibrium grain boundary segregation, incorporation into M23C6, and diffusion is reviewed in the paper. A very high boron addition (300 ppm) was made in the trial TAF steel already in the 1950’ies. The microstructure of a similar trial steel, FT3B, has been studied detail. In this steel large Mo, Cr, Fe and V containing metal borides are formed rather than the expected BN, with the crystal structure M3B2. Nitrogen is therefore still available for the formation of VN. Due to tempering at a low temperature (690°C) to a high strength (830 MPa), this steel contained a dense distribution of very small VN precipitates, 5-15 nm in size. A similar VN distribution is probably the cause of the still unsurpassed creep strength of the TAF steel.