The steels with high chromium content are known to suffer embrittlement at elevated temperature induced either by decomposition of the microstructure into two arranged solid solutions α+α´ (embrittlement at 475 °C) or by precipitation of chromium-rich brittle phase . Martensitic precipitation hardening steels belong among materials having chromium content close to the minimum embrittlement concentration and in some circumstances there could be possibility of prolonged exposure of these steels at elevated temperatures. The effect of long-term ageing (1.000, 2.000 and 3.000 hours at 475 °C) on the material properties, microstructure and substructure of precipitation hardening martensitic stainless steel CUSTOM 465 was analysed and the additional precipitation of very fine particles of intermetallic phases and also decomposition of solid solution α into chromium-rich particles of the phase α´ with nanometric size was found. Despite expectations a slight decrease of strength and impact energy was detected after ageing at 475 °C as a result of stabilization of reverted austenite, which completely compensated the effect of the additional precipitation hardening of martensite. The total content of reverted austenite even doubled after ageing in comparison with the as-received state, although the ageing temperature was significantly lower than the temperature Ac1 of the steel.Keywords: Precipitation hardening steel, CUSTOM 465, long-term ageing, mechanical properties, reverted austenite
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