Isothermal formation of quasicrystalline precipitates and their effect on strength in a 12Cr-9Ni-4Mo maraging stainless steel

Abstract

A new type of maraging steel, Sandvik 1RK91, intended for use in surgical applications has recently been developed at Sandvik Steel, Sandviken, Sweden. In the present article, a comparison was made between this steel and a traditional maraging steel with respect to mechanical and physical properties after tempering treatments in the temperature range 375 °C to 580 °C. The tempering behavior was studied by analyzing hardness, resistivity, and volume fraction of magnetic phase. Sandvik 1RK91 showed a remarkably high microstructural stability against overaging, softening being observed only at 580 °C. In comparison with the reference steel, the hardness of Sandvik 1RK91 was found to be considerably higher in the whole range of temperatures, with the maximum values corresponding to a strength of 3000 MPa for the smallest dimensions of wire. A detailed microstructural investigation of precipitation reactions in Sandvik 1RK91 revealed a new type of precipitate at 475 °C that was found to be of a quasicrystalline nature. These precipitates, which were found to give the major contribution to particle strengthening after tempering at 475 °C, showed a close resemblance to intermetallic trigonalR phase with a chemical composition of about 50 pct molybdenum, 15 pct chromium, 30 pct iron, and 5 pct silicon. The similarity between the quasicrystalline precipitates andR phase was reflected as a second-order transition occurring at about 525 °C. The extremely high strength in 1RK91 can be ascribed to the quasicrystalline nature of the precipitates because of impeded particle shearing.