1. The α→γ transformation in steel N18K9M5T occurs on holding at 500–730°C or on heating at rates below 30–50 deg/sec by way of martensite decomposition in the two-phase α+γ region with redistribution of alloying elements between ferrite and austenite. 2. The main reason for the stable austenite formed on holding in the α→γ temperature range is the redistribution of alloying elements (enrichment of austenite in nickel). 3. The rate at which martensite decomposes is very high, and therefore during heating to quenching temperature or during welding there is always formed some stable austenite enriched in nickel, which leads to residual austenite in the structure. The amount of residual austenite can be reduced by homogenizing annealing for 30–60 min at 820°C.