Precipitation reactions in a martensitic precipitation-strengthened stainless steel and the effects of nickel and silicon on precipitation reactions and toughness

Nickel additions to a 12Cr/12Co/5Mo/4.5Ni martensitic precipitation-strengthened stainless resulted in a marked improvement in toughness. It was also observed that the ductile-to-brittle transition temperature, DBTT, of the highest nickel alloy is much lower than the other alloys for both tempering at 500°C and 525°C. In addition, for the highest nickel alloy, the DBTT obtained after tempering at 525°C is much lower than that obtained after tempering at 500°C. It is known that nickel decreases the DBTT. Also, considering the fact that the austenite reversion starts at 525°C, it is suggested that much lower DBTT obtained at 525°C is resulted from higher volumes of reverted austenite.; Two types of precipitates were observed. It was found that the monoclinic phase is the majority phase after tempering at 500°C. R-phase precipitation starts after tempering at 525°C and it becomes the majority phase after tempering at 550°C. It is believed that at low temperatures, strengthening is provided by the monoclinic phase whereas R-phase is the strengthening phase at and above the peak aging temperature. Moreover, it is found that nickel additions cause an increase in yield strength. Comparison of the difference between yield strength values of the alloys suggests that the increase resulted from the nickel additions cannot be explained by solid solution hardening effect of nickel alone. Since nickel contents of the precipitates are found to be quite low, less than 10 wt.%, and almost constant, it is believed that nickel play an indirect role in the precipitation of the precipitates.; Effect of silicon content on the strength of 12Cr/12Co/5Mo/4.5Ni was investigated. The results showed that 1.7wt.% silicon addition to the base alloy increased the peak yield strength by about 300MPa. Volume fractions measurement of the precipitates showed that the improvement in the peak yield strength is due to the increase in the volume fraction of R-phase. Charpy impact energies at and around peak tempering temperature were found to be very low. Austenite volume fractions were measured and found to be about 16%. One would expect an increase in Charpy impact energies due to the increase in the austenite content; however, this was not observed. It is believed that the improvement in the Charpy impact energies was not observed due to the molybdenum rich precipitate, which forms at about 1050°C. It was found that this precipitate dissolves after austenitizing at 1150°C. For this reason, austenitizing temperatures higher that 1100°C were used to investigate whether removing the molybdenum precipitates would improve the Charpy impact energies. Nevertheless, no improvement was observed after the new austenitizing treatments. It was also observed that because of austenitizing treatment at higher temperatures the fracture mode becomes intergranular, which was ductile after austenitizing at 1050°C.