Quantitative analysis on hydrogen trapping of TiC particles in steel
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I. INTRODUCTION
TITANIUM carbide (TiC) with a NaCl-type crystal structure has long been the model particle for studying the hydrogen-trapping mechanism in steels and alloys in both academic and practical fields.[1–6] However, only the incoherent TiC particles have received attention in the decades, and their trapping behavior has been studied by the electrochemical permeation method,[1,6–8] thermal-desorption spectrometry (TDS),[4,5] and the potentiostatic-pulse technique.[9,10] Incoherent TiC particles interact strongly with hydrogen and are characterized by a high desorption activation energy of 95 kJ/ mol[1] or 87 kJ/mol,[4] which is needed for hydrogen to leave the particles. Asaoka et al.[11] identified via tritium autoradiography that the hydrogen that was trapped by coarse TiC particles in an Fe-1.5 mass pct Ti alloy containing 15 ppm carbon (weight unit) was present at the particle/ferrite interface. No hydrogen-trapping data had been reported on coherent and semicoherent TiC particles until the present authors[12,13] recently investigated the hydrogen-trapping behavior of the 0.42C-0.30Ti steel containing both fine TiC precipitates and coarse incoherent TiC particles. The fine TiC precipitate was found to trap hydrogen in a distinctly different way from that of the incoherent TiC particles. The hydrogen that is trapped by the fine TiC precipitate can be desorbed at about 150 °C F.G. WEI, Researcher, and K. TSUZAKI, Deputy Director-General, are with the Steel Research Center, National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan, Contact e-mail: [email protected] Manuscript submitted December 13, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
in the TDS analysis,[12] corresponding to a desorption activation energy of approximately 58 kJ/mol.[13] Meanwhile, new findings have been obtained for incoherent TiC particles. The hydrogen that was trapped by incoherent TiC particles was found to decrease with increasing tempering temperature, while the activation energy for hydrogen desorption increases.[13] Furthermore, incoherent TiC particles do not trap hydrogen through cathodic charging at room temperature, because the energy barrier for hydrogen trapping is too high for hydrogen to jump into the particle at room temperature.[14] Instead, the incoherent TiC particles absorb hydrogen via water-vapor dissociation during heat treatment at high temperatures. This situation raises a question of whether the particle/ ferrite interface that was outlined by tritium radiography in Asaoka et al.’s experiment is really the occupation site for hydrogen in incoherent TiC particles. A further investigation on the identification of the trap sites associated with incoherent TiC particles becomes necessary. Pressouyre and Bernstein[1,15] mentioned earlier that the TiC particle becomes more reversible when its coherency with the matrix is increased. The lower activation energy of the (semi)coherent TiC precipitate compared to that of the incoherent TiC particle[13] supported Pressouyre and Bernstein’s
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