Influence of titanium and carbon contents on the hydrogen trapping of microalloyed steels
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I.
INTRODUCTION
HYDROGEN permeation behavior plays an important role in steel embrittlement phenomena, such as hydrogeninduced cracking, delayed failure, sulfide stress corrosion cracking, etc. Several theoretical attempts of modeling the interactions of hydrogen atoms with lattice and interface heterogeneities have been developed.[1,2] Such interactions are denoted by the term ‘‘traps.’’ These traps either can be irreversible or reversible according to their binding energy for hydrogen atoms. Theories on traps for hydrogen in low-alloy steels are well known.[3,4,5] It has been observed that even small amounts of alloy elements can influence hydrogen diffusivity and solubility in a significant way.[4] In particular, the presence of Ti can alter the hydrogen permeation behavior considerably because it forms with carbon in TiC precipitates.[5,6] TiC has been considered an irreversible trap having a binding energy variable with the degree of coherency of the matrix-precipitate interface.[7,8,9] According to Pressouyre and Bernstein,[7] the incoherent precipitates form irreversible trapping sites of high binding energy (EB . 85 kJ/mol).[7] On the other hand, Takahashi et al.[8] have ob˚ ) are served that the fine coherent TiC particles (,100 A the most effective trapping sites. In addition, Stevens and Bernstein[9] have supposed that the binding energy of TiC decreases with a lessening degree of coherency. The way in which the boundary between the incoherent TiC precipitate and the matrix, or the strain field surrounding the coherent TiC, can interact with hydrogen atoms is still under active investigation. Ti substitutional atoms have been considered reversible traps for hydrogen with EB 5 227 kJ/mol.[7] Hydrogen diffusivity can be indirectly influenced by the presence of C, which affects the recrystallization rate of the austenitic R. VALENTINI, Scientist, and A. SOLINA, Associate Professor, are with the Department of Chemical Engineering, Industrial Chemistry and Materials Science, University of Pisa, 56100 Pisa, Italy. S. MATERA, Researcher, is with the Centro Sviluppo Materiali, 00129 Rome, Italy. P. DE GREGORIO, formerly Scientist, Centro Svilluppo Materiali, is retired. Manuscript submitted October 31, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
grains during hot rolling. The refinement of the austenitic grain determines a smaller ferritic grain size during the g → a phase transformation. Assuming that the trapping effect of the grain boundaries, which is generally considered reversible,[10] is proportional to the grain surface per unit volume, it follows that the smaller the grain size, the greater will be the grain capability to retain hydrogen. The aim of this preliminary work is to investigate the permeation behavior of hydrogen in hot-rolled steels having different Ti and C contents. The trapping capability for hydrogen atoms was evaluated by permeation measurements combined with microstructural analysis. II.
MATERIALS AND EXPERIMENTAL PROCEDURE
A. Materials The chemical compositions of the commer
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