Aging and tempering behavior of iron-nickel-carbon and iron-carbon martensite
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INTRODUCTION
IRON-BASED martensites (a') with carbon and/or nitrogen interstitial atoms are of great practical importance. Recognizing that martensitic steels are often used in the tempered condition, it is important to understand the aging and tempering behavior of these martensites.* The carbon*The terms aging and tempering will be used such that aging refers to storage of the martensitic specimen at room temperature (RT) and that tempering refers to heating the specimen for some time above RT.
containing martensites FeC and FeNiC have already been subject of many studies (References 1 through 7) and, in recent years in particular, the FeN and FeNiN martensitic systems have drawn considerable attention also,[8–12] whereas only a few studies have been devoted to FeCN martensites.[13–16] Tempering of most martensites starts with a pre-precipitation process in which enrichments of the interstitials (which initially are randomly distributed) are formed. In the enrichments formed in FeN martensite, the nitrogen atoms occupy specific ‘‘order’’ sites,[11,17] whereas in FeC martensite the carbon atoms in the enrichments appear to be distributed randomly.[7,17] For the development of these enrichments, diffusion of only the interstitial atoms is sufficient and therefore can take place at relatively low temperatures. If the Ms temperature (i.e., the temperature at which the martensite starts to form on quenching from the
M.J. VAN GENDEREN, Researcher, formerly with the Laboratory for Materials Science, Delft University of Technology, is with Hoogovens Research Laboratory, 1970 CA IJmuiden, The Netherlands. M. ISAC, Researcher, formerly with the Laboratory of Materials Science, Delft University of Technology, is with McGill University, Montreal, PQ, ¨ TTGER, Researcher, and E.J. MITTEMEIJER, Canada H3A 2A7. A. BO Professor, are with the Laboratory for Materials Science, Delft University of Technology, 2628 AL Delft, The Netherlands. Manuscript submitted December 1, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
austenite-phase field) is well above RT, as is the case for most binary iron martensites, it is possible that part of this pre-precipitation process already takes place during the quench because of the high mobility of the interstitial atoms; so-called auto-tempering occurs. Thus, to study the very early stages of aging of iron-based martensites, nickel is often used as an alloying element to lower the Ms temperature (cf. Reference 18). However, the introduction of nickel can influence the aging and tempering behavior of the martensite. When the nitrogen-containing martensites FeN and FeNiN are compared, a striking difference in the aging/tempering behavior is observed. In the FeN martensite, before actual precipitation of the equilibrium nitride g'-Fe4N takes place, a so-called transition nitride (a"-Fe16N2) is formed.[8,10] In FeNiN martensite, occurrence of an a"-like structure has not been detected.[12] The nitrogen enrichments formed in FeNiN martensite are thought not to exhibit an ordered arrangement o
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