AbstractThe precipitate phases in seven casts of AISI type 321 steel aged at 600°C have been studied. Apart from titanium carbides, nitrides, carbosulphides, andσ-phase, the intermetallic G-phase was also detected in samples having a small grain size. The G-phase, which has not previously been reported in type 321 steel, is a fcc nickel-titanium-silicon phase with a lattice parameter of 1·12 nm. The chemical composition was found by energy-dispersive X-ray spectroscopy in a scanning transmission electron microscope to be 63Ni–21Ti–12Si (wt-%). The formation of G-phase is inhibited by large grain size and low‘excess’titanium content, i.e. the amount of titanium remaining after, deducting the amounts that are combined in TiC, TiN, and Ti4C2S2. When there is no‘excess’titanium, the remaining carbon is precipitated as M23C6. The formation of G-phase causes a further reduction in the amount of‘excess’titanium in solution, and this may lead to a reduction in creep strength in long-term creep tests.MST/53