Precise determination of the activation energy for desorption of hydrogen in two Ti-added steels by a single thermal-des
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4/27/04
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Precise Determination of the Activation Energy for Desorption of Hydrogen in Two Ti-Added Steels by a Single Thermal-Desorption Spectrum F.G. WEI, T. HARA, and K. TSUZAKI Critical assessment of the existing models for the desorption rate of hydrogen trapped in steel indicated that the desorption rate can be described by the kinetic formula dX/dt ⫽ A(1 ⫺ X) exp (⫺Ed/RT). Good fit of the formula has been found to the hydrogen released during thermal-desorption spectrometry (TDS) analysis from the coherent and incoherent TiC particles in 0.05C-0.22Ti-2.0Ni and 0.42C-0.30Ti steels. The activation energy (Ed) and the constant parameter A can be determined uniquely with high accuracy by a single spectrum simulation. The activation energy for hydrogen desorption from the incoherent TiC particle in the well-tempered 0.05C-0.22Ti-2.0Ni steel is 85.7 kJ/mol. In the 0.42C-0.30Ti steel, a higher activation energy of 116 kJ/mol was obtained for the coarse incoherent TiC when tempered at 650 °C and 700 °C. The activation energy decreased from 116 kJ/mol at 650 °C to 68 kJ/mol at 500 °C. The nanosized TiC coherent precipitates in the 0.42C-0.30Ti steel were found to have an activation energy ranging from 46 to 59 kJ/mol, depending on the tempering temperature. A low value of much less than 104 s⫺1 was obtained for the constant parameter A for most cases, which suggested that the retrapping of the released hydrogen is not important in the thermal-desorption analysis.
I.
INTRODUCTION
HIGH-STRENGTH steel is susceptible to hydrogen embrittlement.[1] To reduce the susceptibility, one of the promising approaches is to introduce hydrogen traps, in particular, carbides, in the steel to fix the hydrogen and prevent it from moving to the tip of a crack, where a local stress concentration occurs. Fine titanium carbide[2] and vanadium carbide[3] were reported to have the hydrogen-trapping property and can increase the resistance to hydrogen embrittlement. The hydrogen-trapping effect of carbide depends on the intensity of its interaction with hydrogen, which can be described by any two of the three energies: the binding energy between the hydrogen and the trap (Eb), the detrapping activation energy required for hydrogen to escape from the trap (Ed), and the trapping activation energy for hydrogen to jump into the trap (Et), as schematically illustrated in Figure 1. Of the three parameters, the detrapping activation energy is the most important one for characterizing the trap. If detrapping is the rate-controlling process during desorption, Ed will be designated as the desorption activation energy. Desorption activation energy can be deduced from the Arrhenius relation of the hydrogen diffusion coefficient with temperature. Activation energy, as well as the diffusivity of hydrogen, in pure iron and steel has been measured extensively by electrochemical-permeation,[4–8] gas-permeation,[9–12] and isothermal-desorption[13,14] experiments at a certain temperature. Activation energies for diffusion that were estimated
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