Steel MLX 17 (X1CrNiMoAlTi 12-11-2) belongs to a family of martensitic precipitation-hardening steels for use in demanding applications in aviation and offshore oil and gas industry. The unique combination of high strength, toughness, fatigue strength and corrosion resistance is obtained in this steel based on 11 % Ni, 12 % Cr and precipitation hardened by Mo, Al and Ti after ageing of low carbon martensite above 500 °C. Due to its chromium content the steel MLX 17 is a material threatened by embrittlement at 475 °C induced by decomposition of the microstructure into two arranged solid solutions α+α´ and/or embrittlement caused by precipitation of chromium-rich brittle σ-phase. Although this steel is not typically exploited at higher temperatures, it is interesting to know the effect of long-term ageing on the material properties, microstructure and substructure. Therefore, the ageing at 475 °C for 1,000, 2,000 and 3,000 hours was performed and material properties, microstructure and substructure were evaluated. Analysis of substructure confirmed the additional precipitation of Lave phase, the main source of precipitation hardening in MLX 17 steel, as well as the decomposition of the solid solution α into the chromium-rich particles of nanometric size. Ageing at 475 °C was also accompanied by increasing of the reverse austenite content, which overcame the possible effect of the additional precipitation hardening of martensite.Keywords: MLX 17, reverse austenite, mechanical properties, substructure, 475 °C embrittlement, long-term ageing
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