Hydrogen absorption of incoherent TiC particles in iron from environment at high temperatures
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I. INTRODUCTION
HYDROGEN diffusing in an iron lattice is likely absorbed by certain kinds of microstructural inhomogeneities that act as hydrogen traps, such as vacancies; impurity elements; dislocations; grain-boundary, precipitate/ferrite, and inclusion/ferrite interfaces; etc.[1] Many data on the hydrogen/trap interaction energy have been documented in the literature.[1,2,3] Incoherent TiC particles in steel are known to be a strong hydrogen trap with a high interaction energy of 95 kJ/mol, as reported by Pressouyre and Bernstein[4] by means of the electrochemical permeation method, or of 87 kJ/mol, as reported by Lee and Lee[5] by means of thermal-desorption spectrometry (TDS) analysis. The interaction energy is so high that once hydrogen is trapped, it cannot be released unless it is heated to high temperatures. In spite of a number of data on the hydrogen-trapping properties of a variety of traps, little is known about the effect of heat treatment on the trapping property. Pressouyre[6] suggested that the increase in coherency of TiC precipitates with the ferrite matrix makes it more reversible in trapping nature. Lee and Lee[7] found that annealing temperature and time significantly influence the amount of hydrogen trapped by incoherent TiC and the temperature of the maximum hydrogen desorption rate. Recently, the present authors[8] found that incoherent TiC particles in 0.42C-0.30Ti steel demonstrate a variable hydrogen-trapping capability; they trapped the most hydrogen when tempered at 500 °C, and the hydrogen amount decreased with increasing tempering temperature above 500 °C with a concomitant increase in the temperature of the desorption peak. Moreover, it was further found by the pre-
F.G. WEI, Postdoctoral Fellow, 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 11, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
sent authors that, accompanying the change in the absorbedhydrogen amount and peak temperature, the activation energy for desorption of hydrogen from incoherent TiC particles increases with increasing tempering temperature, presumably due to the chemical change at the particle/ferrite interface.[9] Besides the tempering temperature and time, the heat-treatment atmosphere may influence the hydrogen-trapping property. However, almost no attention has been paid, to date, to the effect of heat-treatment-atmosphere environment on the hydrogen trapping in iron and steel. Huang et al.[10] found that oxygen from the atmosphere segregates to Pd/MgO and Pd/Al2O3 interfaces in Pd-based alloys when annealed in air, and the excess oxygen traps hydrogen in the subsequent hydrogencharging process. Annealing in vacuum or in metal vapor removed the excess oxygen from the interface, and no hydrogen trapping was observed. Oxygen segregation has been found in other noble metal/oxide interfaces.[11,12] These results indicate that atmosphere
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