This book explores the long-term ageing effects at 475°C on the microstructure and properties of Custom 465® alloy. It covers the theoretical aspects of precipitation-hardened stainless steels, including the formation of reverted austenite and its impact on mechanical properties, along with the 475°C embrittlement process in Fe-Cr steels. Experimental findings using TEM and XRD show that proper heat treatment leads to the precipitation of η-Ni3Ti particles and a small amount of reverted austenite in tempered lath martensite. Extended ageing at 475°C for 1000 and 2000 hours results in further precipitation and an increase in reverted austenite, which helps mitigate the hardening effect of the α´-Cr rich phase. The study demonstrates that while strength decreases with prolonged ageing, elongation and impact energy remain largely unaffected.