Effects of Retained Austenite on the Tensile Properties and Toughness of Ultra-High Strength Martensitic Precipitation Hardened Stainless Steels

摘要

The purpose of this work was to assess the effects of the amount of retained austenite content on the ductile-to-brittle transition temperature of martensitic precipitation strengthened stainless steels for four different precipitation strengthening systems, one utilizing NiTi strengthening and three utilizing R-phase strengthening. The retained austenite contents in the four systems were varied by varying composition. The austenite content in the NiTi strengthened system was varied by varying the chromium content and the austenite content in the R-phase strengthened Systems was varied by varying the nickel content. The room temperature toughness levels of the NiTi strengthened system were quite low and it was decided not to pursue this system further. The three R-phase strengthened systems had sufficient room temperature toughness and strength to be of further interest. Of these three systems the primary focus was on the 12Cr/12Co/5Mo system. In this system four alloys, identical except for variations in nickel content, were the primary focus of the work. These alloys achieved, on tempering at 5250 C for 3.16 hours, yield strengths on the order of 210 ksi and ultimate tensile strengths of 235 ksi. The effect of test temperature on the Charpy impact energy was investigated for two tempering temperatures for these four alloys. It was found for both tempering conditions that lower ductile-to-brittle transition temperatures were favored by increasing amounts of austenite in the structure. In fact, the ductile-to-brittle transition temperature was quite low, about -750 C, for the tempered at 5250 C for 3.16 hours microstructure of the alloy in this series which contained the highest nickel and the highest amount of retained austenite after quenching. At this point it is believed the austenite content is an important contributor to the low ductile-to-brittle transition temperature of this microstructure.