Thermal Desorption Spectroscopy Evaluation of the Hydrogen-Trapping Capacity of NbC and NbN Precipitates
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NTRODUCTION
ALTHOUGH the first detrimental effects of hydrogen on the mechanical properties of iron and steels were described by Johnson in 1875, many features of the involved mechanisms are still poorly understood and are the subject of debate. As a result of the rising demand for high-strength steels, since these materials are known to be sensitive to the harmful consequences of hydrogen, an increasing interest for hydrogen embrittlement research arose. The introduction of nanocarbides or nanonitrides as hydrogen-trapping sites in steels is often discussed to play a crucial role in hydrogen embrittlement[1,2]. Carbides, such as VC, TiC, and NbC, are most often considered as hydrogen-trapping sites. Asahi et al.[3] investigated hydrogen interaction with VC precipitates using thermal desorption spectroscopy (TDS) and estimated the activation energy for the de-trapping to be in the range of 33-35 kJ/mol. The trapping of hydrogen was also observed for this type of precipitates using small-angle neutron scattering (SANS) by Malard et al.[4] They concluded that the intensity increase was consistent with hydrogen being homogeneously distributed in the VC precipitate rather than at the precipitate/matrix interface. By cathodically charging austenitic Fe-Mn-C TWIP steel, 5 ppm wt pct H was mentioned to be trapped inside the VC precipitates. ELIEN WALLAERT and TOM DEPOVER, Ph.D. Researchers, and KIM VERBEKEN, Professor, are with the Department of Materials Science and Engineering, Ghent University (UGent), Technologiepark 903, 9052 Ghent, Belgium. Contact e-mail: [email protected] MUHAMMAD ARAFIN, Research Engineer, is with the ArcelorMittal Global R&D Gent, OCAS NV, Technologiepark 935, 9052 Zwijnaarde, Belgium. Manuscript submitted September 6, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
TiC precipitates were investigated by Asaoka et al.[5] in a Fe-0.5 wt pct Ti alloy charged with tritium using an auto-radiographic method for observation. The binding energy was found to be above 61 kJ/mol. Pressouyre and Bernstein[6], however, found a trapping activation energy of 95 kJ/mol by means of the hydrogen permeation technique.[7] They also concluded that coherent TiC precipitates were not as effective traps as incoherent precipitates and that the activation energy increased with precipitate size. Lee and Lee[8] used TDS to determine the hydrogen interaction with the matrix–particle interface. They obtained an activation energy of 86.9 kJ/mol and a binding energy of 28.1 kJ/mol. With the same technique, Wei et al.[9] determined an activation energy of 85.7 kJ/mol for H desorption from incoherent TiC particles formed in a 0.05C-0.22Ti-2.0Ni alloy. While for another steel (0.42C-0.30Ti) in which larger incoherent Ti precipitates were formed, the activation energy was found to be 116 kJ/mol, the coherent particles had an activation energy of 46 to 59 kJ/mol. Pe´rez Escobar et al.[10] investigated TiC precipitates in an experimental steel (0.025 wt pct C-0.09 wt pct Ti), which was annealed in a hydrogen environment and electr
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