PRECIPITATE PHASES IN TYPE 321 STEEL.

The precipitate phases in seven cases of AISI type 321 steel aged at 600 degree C have been studied. Apart from titanium carbides, nitrides, carbosulfides, and sigma -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 compositon 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. When there is no 'excess' titanium, the remaining carbon is precipitated as M//2//3C//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.