Precipitate phases in type 321 steel

Abstract

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

Evolution of sigma phase in 321 grade austenitic stainless steel parent and weld metal with duplex microstructure

“…The predominant intemetallic phase to form during ageing in a 321 grade austenitic stainless steel is sigma s phase. 2 Pre-service, the material is given a heat treatment with the aim of producing fully austenitic steel. The addition of titanium is primarily to stabilise the steel as it is known to have the effect of reducing the precipitation of chromium rich carbides, this reduces the sensitisation effect as well as decreasing the time taken to form s. 3 These titanium carbo-nitrides remain after solution heat treatments and affect the subsequent precipitation sequence.…”