Carbon and nitrogen in iron-based austenite and martensite : An attempt at comparative analysis

Abstract
Similarities and differences in the structure of carbon and nitrogen austenites and martensites are reviewed. It is shown that different atomic distribution of interstitial and substitutional solute atoms (tendencies towards clustering in carbon austenite and short range atomic ordering in nitrogen austenite) stems from the preferential localisation of electrons on the atomic sites by carbon and increase in the concentration of free électrons by nitrogen. Increased stability of nitrogen austenitic steels in relation to sensitisation treatments and martensitic transformation is attributed to the nitrogen-assisted short range atomic ordering. A more homogeneous distribution of interstitial and substitutional solutes is inherited by martensite, which results in delayed precipitation during tempering, the lower rate of coarsening in nitrides as compared to carbides and improved mechanical properties of nitrogen martensitic steels.