The tempering of iron-nitrogen martensite; Dilatometric and calorimetric analysis
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INTRODUCTION: STAGES OF TEMPERING
A large number of investigations have been devoted in the past to the tempering of iron-carbon martensite. Nevertheless, some basic problems, in particular regarding preprecipitation phenomena, have remained unsolved until now. [1,2'31 Much less attention was paid to the tempering of analogous iron-nitrogen martensite. This may reflect that no distinct practical application of iron-nitrogen martensite is known at present. However, study of precipitation reactions, in especially iron-nitrogen martensite, is of fundamental importance. In such a way, a general understanding of the precipitation phenomena of interstitial iron-base martensites may be achieved, because the precipitation reactions in iron-nitrogen martensite may be relatively simple, as compared with those of ironcarbon martensite. For example, the transition nitride, o~"-Fel6N2, is isostructural with the parent rnartensite: a mere ordering of nitrogen atoms suffices to bring about precipitation of ~" nitride. Further, knowledge of the tempering process in iron-nitrogen martensite is required to understand eventually the complicated tempering behavior of iron-carbon-nitrogen martensite, which is of practical importance, as this ternary martensite is produced during the well-known case-hardening carbonitriding process. According to the previous work, [4-9] the structural evolution of iron-nitrogen martensite on tempering may be divided as yet into the following sequence of processes:* *The bounding temperatures indicated here depend on the heating rates during the tempering (taken here as 20 ~
(1) a preprecipitation stage: ordering or clustering of nitrogen atoms in the martensite below about 100 ~ (2) the first stage of tempering: precipitation of a transition nitride, a"-Fe~6N2, below about 200 ~ leaving a ferrite matrix possibly containing some segregated nitrogen atoms; (3) the third stage of tempering: conversion of the transition nitride into 7 ' - F e a N nitride above about 200 ~ *In contrast with the formation of cementite on tempering ironcarbon martensite, the formation of 7' nitride on tempering iron-nitrogen rnartensite sets in amply before the decomposition of retained austenite occurs. To maintain a parallel designation of corresponding processes in both interstitial martensites, the formation of 3" nitride in iron-nitrogen martensite is still denoted here as the third stage of tempering; the decomposition of retained austenite is referred to as the second stage of tempering for both interstitial martensites.
(4) the second stage of tempering: decomposition of retained austenite into y' nitride and ferrite below about 350 ~ Until now, most research into the tempering process of Fe-N martensite was performed by applying X-ray diffraction,I4,6] electron microscopy and diffraction,t5,61 resistometry,tT] and M6ssbauer spectroscopy, tSj A comprehensive study using dilatometry and calorimetry has not been performed. As has been shown by us in a previous paper on the tempering of iron-carbon martensite, t31
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