Alloy carbide precipitation and aging during high-temperature isothermal decomposition of an Fe-4Mo-0·2C alloy steel

Abstract

An electron metallographic study, coupled with X-ray diffraction, has been carried out on the decomposition and aging of an Fe-4Mo-0·2C low-alloy steel at 750%C in an attempt to characterize the nature of the alloy carbide dispersion and its stability. Intragranular precipitation of both the intermediate carbide Mo₂C and the equilibrium carbide M₆C was dispersed in sheetform at short aging times denoting interphase precipitation at the austenite/ferrite transformation interface. The orientation relationship of this interphase precipitate of M₆C with the ferrite matrix was identified as either Kurdjumov-Sachs or Nishiyama-Wasserman. Mo₂C also occurred with a fibrous morphology. At longer aging times further nucleation and growth of M₆C occurred and the Mo₂C dissolved, and the observation of new aligned dispersions of M₆C interleaved with rows of Mo₂C creates strong circumstantial evidencefor nucleation of M₆C on the ferrite/Mo₂C interface, although no unambiguous electron microscopy evidence of this phenomenon could be obtained.