Cristalografia

Crystallography and Tempering Behavior of Iron-Nitrogen Martensite
A. VAN GENT, E C. VAN DOORN, and E.J. MITTEMEIJER Iron-nitrogen martensite was prepared by the gaseous nitrogenization of iron inthe austenite-phase field, followed by quenching. Low- and high-nitrogen contents (lath vs plate morphology) were investigated. Optical microscopy, transmission electron microscopy and diffraction, anddifferential thermal analysis were employed to study the crystallography and the tempering behavior of the martensites. The orientation relation between austenite and martensite, the habit plane andtypes of (post-) transformation twins were determined. Tempering-induced changes in hardness and release of heat were related to structural changes as revealed by electron diffraction. Differenceswith analogous iron-carbon martensites were discussed. Aging at room temperature leads to nitrogen diffusioncontrolled precipitation of coherent ~"-Fel6N2 platelets, which process is completed afterabout one day. Aging at higher temperatures (up to about 475 K) results in incoherent a" particles, which, at temperatures above 460 K, exhibit small deviations (= t8 deg) from the usual {001}~,j~ habitplane.

I.

INTRODUCTION

A large amount of established knowledge on the crystallography and the tempering behavior of iron-carbon martensites exists (cf. reviews presented in References 1through 3). This contrasts with the few corresponding data available for iron-nitrogen martensites (cf. References 4 through 8). This may reflect the great technological importance of iron-carbonmartensites, whereas no distinct technological application of iron-nitrogen martensites is apparent at present. However, the case-hardening process of carbonitriding implies the development of ternaryironcarbon-nitrogen martensites. Even aging at room temperature of carbonitrided workpieces leads to significant changes in microstructure and mechanical properties. The microstructure and the tempering...