Martensitic–austenitic 9–12% Cr steels—Alloy design, microstructural stability and mechanical properties

This work describes the alloy development of heat-resistant 9–12% chromium steels with a microduplex structure for application in advanced gas or steam turbines. Such alloys feature a two-phase microstructure, consisting of 30 vol.% of austenite in a matrix of tempered martensite with fine precipitates of vanadium nitride. This so-called microduplex structure is formed from a fully martensitic structure during tempering at 575–625 °C. The austenite is in lamellar arrangement with a lamella thickness of 100–500 nm and a lamella length of a few microns, parallel to the martensite plates. Within the scope of this work the effect of the individual alloying elements has been investigated both experimentally and by means of thermodynamic calculations using ThermoCalc®. The phenomena of austenite formation during tempering and the stability of the austenite against martensitic transformation are described in detail. The high Mn and Ni content is responsible for the formation of the microduplex structure. Very good mechanical properties are achieved, in particular an advantageous combination of strength and toughness, coupled with high creep strength at 550 °C. Microduplex steels are therefore promising candidates for the application in energy conversion systems, for example as compressor discs in high-efficiency stationary gas turbines.