IntroductionThe precipitate phases in seven casts of AISI type 321 steel aged at 600°,Chave been studied. Apart from titanium carbides, nitrides, carbosulphides, and a-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-21 Ti-12Si (wt-%). Theformation 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 Ti4C 2 S 2 • When there is no 'excess' titanium, the remaining carbon is precipitated as M23C 6 • 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