Behavior of Precipitates in High Carbon Austenitic Heat-resisting Steels during Creep Rupture Test at 1 050°C

Abstract

A study has been made on the behavior of precipitates in niobium or titanium and niobium-containing centrifugal cast heat-resisting steels (HK40, HP and IN519) during creep rupture test at 1 050°C.
(1) In the as-cast condition, iron chromium carbide of M₇C₃ type precipitates, except for steel IN519 (0.33%C-24%Cr-24%Ni) where M₂₃C₆ type precipitates. Titanium niobium carbonitride (MX) precipitates in the steel containing titanium and niobium, and niobium carbonitride (NbX) precipitates in the steel containing only niobium. Chromium nitride (Cr₂N) precipitates except in the steel containing only niobium.
(2) During creep rupture test, M₇C₃ in the as-cast steel changes to (Cr_0.85Fe_0.15)₂₃C₆. As the results of nitriding by air, Cr₂N increases and complex nitride, (Cr, Fe)NbN of tetragonal lattice, precipitates in the niobium-containing steel. Moreover, substitution reaction proceeds between carbon and nitrogen or between titanium and niobium with change of lattice parameter.
(3) M₂₃C₆, Cr₂N and A1N are completely decomposed by hydrochloric acid (1+1) at 90°C. By the same acid M₇C₃, MX and NbX are decomposed gradually but (Cr, Fe) NbN is hardly decomposed.Based upon the differences in chemical stability of precipitates, the chemical composition and amount of each kind of precipitates can be successfully determined.