Thermoelectric Power Changes of Low Strength Steel Induced by Hydrogen Embrittlement Tests
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Mater. Res. Soc. Symp. Proc. Vol. 1243 © 2010 Materials Research Society
Thermoelectric Power Changes of Low Strength Steel Induced by Hydrogen Embrittlement Tests N. Mohamed-Noriega1, E. López Cuéllar1 and A. Martinez de la Cruz1 1 FIME, Universidad Autónoma de Nuevo León, San Nicolás de los Garza, México. ABSTRACT This work reports the thermoelectric characterization of a hydrogen embrittlement (HE) of low strength steel. Two sets of tests are performed in an electrochemical cell of H2SO4, with and without applied stress, lasting from 2 to 94 hours. Thermoelectric power (TEP) measurements are matched with ductility measurements (%RA and %EL) of samples tested in tension, as well as with microhardness measurements. Results indicate that TEP is sensitive to HE of low strength steels; the maximum variation of TEP is of ~80nV/°C for samples tested without stress. INTRODUCTION TEP is a material property which relates electrical and thermal properties. It is the sum of two components: the diffusive component and the lattice component. The diffusive component (Sd) represents the electron contribution and the lattice component or phonon drag (Sg) represents the interactions between electrons, phonons and lattice imperfections. The diffusive component is a linear function of temperature that depends on the evolution of the electrical resistivity (ρ); see equation 1. The lattice component depends on the probability that a phonon bounces with a lattice imperfection (Pi) or with an electron (Pe), see equation 2. S d = A[ρ (E )] T Pe Sg ∝ ∑ Pi
(1) (2)
For some materials the total TEP can be obtained simply by adding both components ( TEP = S d + S g ). However, for iron and its alloys (e.g. steel) the explanation of the total TEP by the two components has not been completely satisfactory and the deconvolution of the two components is uncertain [1]. From the definition of TEP, it can be deduced that any modification on the composition, the crystal structure or the microstructure of the material will alter its value. Kawaguchi et al. studied successfully the evolution of TEP as a function of Cr content in a cast duplex stainless steel [2]. López Cuéllar et al. have used TEP to analyze the growth of the different phases in an Inconel 718 superalloy [3]. Caballero et al. explored the variation in the TEP as a function of martensite content by quenching a stainless steel at different temperatures [4]. Steel ASTM A-572 grade 50 is frequently used in the manufacture of pipelines. In such a case, some of the hydrogen in the hydrocarbons flowing trough the pipelines is absorbed into the metal, producing hydrogen embrittlement. In the present study, TEP is used to follow the evolution of the ductility of the material as it is charged with hydrogen. In order to investigate
the variation of ductility and to correlate it with the TEP, Vickers microhardness, %RA and %EL are determined. EXPERIMENTAL PROCEDURE A commercial steel, ASTM A-572 grade 50 (API 5L), is used for hydrogen embrittlement tests. Hydrogen charging is carried out ele
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