Nitride Formation and Excess Nitrogen Uptake After Nitriding Ferritic Fe-Ti-Cr Alloys
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NITRIDING is a widely used thermochemical surface treatment for, in particular, ferritic steels.[1] The formation of inner nitrides can cause a large improvement of the mechanical properties. Because of the possibility of precise control of the nitriding atmosphere, i.e., the chemical potential of nitrogen can be tuned,[2] gaseous nitriding of metallic alloys, by applying a NH3/H2 gas mixture at atmospheric pressure, is often applied to introduce nitrogen in the ferrite matrix at the surface of a specimen. Note that the application of specific NH3/H2 gas mixtures allows the adjustment of the chemical potential of nitrogen corresponding hypothetically to thousands of atmospheres of pure N2 gas.[3] During internal nitriding of iron-based alloys containing alloying elements (Me) with a strong affinity for nitrogen, as Cr, Al, V, and Ti, fine alloying element nitride precipitates can develop in the nitrided zone KYUNG SUB JUNG, formerly PhD Student, with the Max Planck Institute for Intelligent Systems (formerly known as Max Planck Institute for Metals Research), Heisenbergstraße 3, 70569 Stuttgart, Germany, is now Senior Research Engineer, with the Samsung Corning Precision Materials, Myeongam-ri, Tangjeong-myeon, Chungnam 336-725, South Korea. Contact e-mails: k.s.jung@is. mpg.de and [email protected] SAI RAMUDU MEKA, PhD Student, and EWALD BISCHOFF, Scientist, are with the Max Planck Institute for Intelligent Systems (formerly known as Max Planck Institute for Metals Research). RALF E. SCHACHERL, Scientist, is with the Institute for Materials Science, University of Stuttgart, Stuttgart 70569, Germany. ERIC J. MITTEMEIJER, Professor, is with the Max Planck Institute for Intelligent Systems (formerly known as Max Planck Institute for Metals Research), and is also with the Institute for Materials Science, University of Stuttgart. Manuscript submitted November 15, 2010. Article published online October 27, 2011 934—VOLUME 43A, MARCH 2012
adjacent to the surface (called ‘‘diffusion zone’’), which leads to a pronounced increase of the hardness of the nitrided component. The increase of hardness and related (mechanical) properties depends strongly on the amount of alloying elements, the chemical composition of the nitride precipitates, the degree of coherency of the nitride precipitates with the matrix, and the precipitate size and morphology.[4,5] Until now, most studies concerning internal nitriding have focused on binary Fe-Me alloy systems, i.e., Fe-Cr, Fe-Al, Fe-V, and Fe-Ti.[6–23] However, commercial nitriding steels often contain more than one alloying element with affinity for nitrogen. Only a few investigations were performed until now on ternary Fe-Me1-Me2 alloy systems. Recently, Ti-based ternary nitrides such as (Ti,Al)N, (Ti,Zr)N, and (Ti,Cr)N have gained much attention as second-phase particles in steels because of their contribution to the enhanced performance for cutting tools and machinery components, e.g., regarding wear/corrosion protection.[24–28] In the current work, Ti and Cr were selected as
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