Precipitation Process and Age-Hardenability of Austenitic Fe-Co-Cr-Ni Base Heat-Resisting Alloys

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To obtain available informations on the precipitation strengthening the correlation between the age-hardening behavior and the process of precipitation was examined with austenitic age-hardenable Fe-Co-Cr-Ni base alloys, by observing mechanical properties, X-ray diffraction characteristics, and optical and electron microstructures. The results obtained are as follows : The age-hardenability and the behavior and dispersion characteristics of precipitates are markedly affected by minor added elements such as Mo, W, Cb and N. The precipitates detected by X-ray analysis are CbC or Cb (C, N), M_<23>C_6 M_6C, Laves phase and π phase detected by the present work, of which M_<23>C_6 and π phase play a main role in the age-hardening. There are two types of hardening, one-stepped and two-stepped. The former is observable in alloys only with one phase, either M_<23>C_6 or π, as precipitate. The degree of hardening in such alloys is principally affected by the volume fraction of the precipitates rather than by the properties of precipitates. On the other hand, the two-stepped hardening is remarkable in alloys containing Mo, W, C and N simultaneously, and related with the transition of precipitates from M_<23>C_6 to π. The mechanical properties of these alloys are superior to those of alloys of the former type. Such high precipitation strengthening seems to be induced by favorably combined dispersion of M_<23>C_6 and π phase.
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