TEM Characterization of a Titanium Nitride (TiN) Inclusion in a Fe-Ni-Co Maraging Steel
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m Alloys produces, in its steel plant of Imphy (France), an Fe-based maraging steel composed of 18 pct Ni, 9 pct Co, 5 pct Mo, and 0.45 pct Ti with high mechanical properties in fatigue. The production route involves a double remelting into a vacuum induction furnace (VIM) and a vacuum arc furnace (VAR). Traces of Nitrogen remain in the alloy. This element combines itself to the Titanium to form stoichiometric nitrides, which act as initiation sites to fatigue crack. Reducing their size and number density should therefore enhance the fatigue life limit of this alloy. According to Durand-Charre,[1] TiN particles are easily found in such grades due to the strong chemical affinity between titanium and nitrogen. The solubility product of TiN is very low, as determined by Wada and Pehlke,[2] and Morita et al.[3] Pervushin and Suito[4] and
VINCENT DESCOTES, formerly Ph.D. Student with the Institut Jean Lamour - UMR CNRS 7198, LabEx DAMAS, Universite´ de Lorraine, Parc de Saurupt, CS 50840, 54011, Nancy Cedex, France, is now R&D Engineer with the APERAM Alloys Imphy, BP1 Centre de recherches P. Chevenard, 58160, Imphy, France. Contact e-mail: [email protected] SYLVIE MIGOT, TEM Specialist, Research Engineer, JEAN-PIERRE BELLOT, Professor, Research Scientist, and ALAIN JARDY, Research Scientist, are with the Institut Jean Lamour - UMR CNRS 7198, LabEx DAMAS, Universite´ de Lorraine. FLORENCE ROBAUT, FIB Specialist, Research Engineer, is with the CMTC Grenoble INP, Baˆt. E PHELMA, 1260 rue de la piscine, BP 75-38402, Saint Martin d’He`res Cedex, France. VALE´RIE PERRIN-GUE´RIN, Metallurgist, R&D Engineer, and SYLVAIN WITZKE, Metallurgist, R&D Engineer, are with the APERAM Alloys Imphy, BP1 Centre de recherches P. Chevenard. Manuscript submitted October 24, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
Ohta and Suito[5] mention several oxides observed next to TiN particles, and established that the TiN inclusion size is influenced by the deoxidizer used in the steel. It is therefore thought that some oxides enable a larger nucleation of TiN, thus reducing their sizes. Experimental trials were made by these authors to identify the most effective particles for the TiN precipitation. MgO is often cited as the most efficient nucleus for TiN inclusions,[5] but other oxides like CaO, Al2O3, Y2O3, and ZrO are also potential nucleation sites. Titanium carbonitrides are reported by Zou and Kirkaldy,[6] and Shmulevitsh et al.[7] Thermodynamical calculations show that the TiN particles are not stable over the liquidus temperature for the grade produced by Aperam. However, industrial observations using optical and scanning electron microscopy (SEM) show that a germ is often detected near the TiN crystals, which suggests that their nucleation occurs on primary nuclei (oxides, sulfides) during the solidification thanks to the microsegregation in Nitrogen and Titanium. This hypothesis is also evoked by Kunze et al.,[8] Rocabois et al.[9] and Lehmann et al.[10] A previous study of this mechanism by Descotes et al.[11] emphasized
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