An Investigation of Carbide Precipitation in Iron

Abstract

A study has been made of the carbides precipitated by quench-ageing in iron-carbon alloys containing various amounts of nitrogen in solution, or 1% nickel, by eletron microscopy of carbon replicas after ageing at 150–300°C for up to 600 h. The precipitation processes were followed by means of internal-friction measurements; the precipitates consisted of ε-carbide and cementite, and the conditions of time and temperature for the occurrence of each type were determined. It has been established that the precipitation of carbon is a two-stage process; furthermore, carbon and nitrogen appear to precipitate independently. The extra internal-friction peaks observed in a 1.5% Al-containing alloy are regarded as being due to interaction between carbon atoms and single Al atoms or Al pairs. In a 0.37% Mn-containing alloy the presence of extra internal-friction peaks caused by N/Mn interaction has been confirmed. No extra damping peaks were displayed by a 1% Ni-containing alloy. The rate-determining process for the change from ε-carbide to cementite is either the solution of e or the precipitation of cementite in alloys containing carbon and nitrogen, while in Ni-containing alloys it is the diffusion of carbon through the ferrite matrix.