First stage of precipitation in iron-carbon-nitrogen martensites; diffraction analysis using synchrotron radiation

Abstract

A considerable amount of research has been done on the aging and tempering behavior of iron-carbon and iron-nitrogen martensites. These martensites can be described on the basis of an overall body-centered tetragonal (b.c.t.) lattice of iron atoms containing interstitial atoms which are randomly distributed over the c-type octahedral interstices. The interstitial martensites seem to be similar in many respects; they have, for instance, comparable lattice parameters and M{sub s} temperatures (the M{sub s} temperature is the temperature at which the martensite formation starts on quenching from the austenite-phase field). However, some distinct differences occur between the decomposition behavior of both martensites. On aging at room temperature for example, local enrichments of interstitials develop in both nitrogen and carbon martensites. In FeN martensites these enrichments are considered to possess a fully ordered arrangement of the nitrogen interstitials in c-type octahedral interstices according to the {alpha}{double prime}-Fe{sub 16}N{sub 2} structure. However, in FeC martensites no indications for such an ordering have been observed (yet) and the enrichments are denoted as clusters which are conceived as aggregates of iron and carbon atoms, containing different amounts of carbon interstitials distributed randomly over the c-type octahedral interstices. This paper reports that because of the abovemore » mentioned apparent difference between the aging of FeN and FeC martensites, it is interesting to study the (pre)precipitation behavior of iron-carbon-nitrogen martensitic alloys with different ratios of the amounts of carbon and nitrogen atoms. Only a few results on the tempering of FeCN have been reported until now. In these studies alloys were used which were very rich in nitrogen.« less