Abstract
The present investigation aims at establishing the correlation between the age-hardening behaviour and the process of precipitation in austenitic age-hardenable Fe-Co-Cr-Ni base alloys, in order to obtain a useful information on the precipitation strengthening. In several stages of aging, measurements of mechanical properties and X-ray analysis of precipitates were made and microstructures were observed by means of the optical and electron microscopes. The results may be summarized as follows:
The age-hardenability and the behaviour and dispersion characteristic of precipitates are markedly affected by the minor additional elements such as Mo, W, Cb and N. The precipitates detected by X-ray analysis are CbC or Cb (C, N), M23C6, M6C, π and Laves phases, among which M23C6 and π phase play the main role in the age-hardening.
There are two types of hardening, that is, one-stepped and two-stepped hardening. The former is observed in alloys with only one phase of either M23C6 or π as precipitate, the degree of hardening being principally affected by the volume fraction of the precipitates rather than the properties of precipitates. On the other hand, the two-stepped hardening is remarkable in alloys containing simultaneously Mo, W and N, and it is related with the transition of precipitates from M23C6 to π. The mechanical properties of these alloys are superior to those of single phase precipitation hardening alloys. Such high precipitation strengthening seems to be induced by favorably combined dispersion of M23C6 and π phases.
