The tempering of FeNiN martensite
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I.
INTRODUCTION
K N O W L E D G E of the tempering of iron-nitrogen martensite is desired from a fundamental point of view, because it contributes to the understanding of the behavior of interstitials in iron-based martensites, as well as from a technological point of view, in recognition of the recent development of high nitrogen steels.t1] A number of investigations have been carried out on the tempering of "pure" iron-nitrogen martensite. ~2-9] However, the aginginduced structural changes in freshly prepared martensite have hardly been studied. Only recently has it been recognized that tempering of iron-nitrogen martensite occurs via a preprecipitation process involving the development of local enrichments of interstitial atoms, wherein the interstitials are ordered. Migration of only interstitial atoms suffices to accomplish this enrichment and ordering. Therefore, it is possible that part of this process takes place already during the production of a martensitic specimen, namely, during the quench from the austenitic phase field into the martensitic phase field. In the study of preprecipitation processes it is important to avoid this so-called "autotempering." The autotempering effect can be suppressed by reducing the Ms temperature (i.e., the temperature at which the martensite starts to form on quenching). Addition of a sufficient amount of appropriate alloying elements, such as Ni, to FeN reduces the Ms temperature to below room temperature, where the mobility of the interstitial atoms can be ignored. For a similar reason, Ni has been added to FeC alloys in order to study the tempering processes of freshly quenched FeC martensites.tl~ However, the presence of Ni may affect the tempering sequence significantly, and the precipitation processes in the Ni-alloyed martensites may not be representative of those in "pure" FeN and FeC martensites. In this article, the tempering processes of FeNiN martensite will be analyzed and compared to those of FeN LIU CHENG, Graduate Student at the time of this work, A. B O T T G E R , Scientist, and E.J. MITTEMEIJER, Professor, are with the Laboratory of Metallurgy, Delft University of Technology, 2628 A L Delft, The Netherlands. Manuscript submitted August 6, 1990. METALLURGICAL TRANSACTIONS A
martensite. It will be shown that the presence of Ni changes drastically the tempering-induced phase transformations. In what follows, reference will be made frequently to the tempering scheme of FeN martensite. According to previous work, [2-9] the structural evolution of FeN martensite on tempering is divided into the following sequence of processes: (1) preprecipitation stage below about 370 K: segregation and transfer from a-, b-, to c-type octahedral interstices of small amounts of interstitials, and formation of ordered arrangements (local enrichments) of nitrogen atoms in the martensite; (2) the first stage of tempering below about 470 K: precipitation of a transition nitride, o t " - F e l 6 N 2 ; (3) the third stage of tempering above about 470 K: conversion of the transiti
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