Quantification of hydrogen diffusion and trapping in 2.25Cr-1Mo and 3Cr-1Mo-V steels with the electrochemical permeation
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25/3/04
11:36
Page 1449
Quantification of Hydrogen Diffusion and Trapping in 2.25Cr-1Mo and 3Cr-1Mo-V Steels with the Electrochemical Permeation Technique and Melt Extractions A.-M. BRASS, F. GUILLON, and S. VIVET The electrochemical permeation technique was used to investigate the effect of microstructure, hydrogen activity, and stationary dislocations generated by tensile straining on the permeation and degassing of hydrogen. A conventional 2.25Cr-1Mo steel, with which the existing hydrotreating reactors are made, and a 3Cr-1Mo-V steel, which is a candidate material for the future generation of reactors, were selected for this study. The effective diffusion coefficient of hydrogen derived from permeation and degassing transients shows a slower diffusivity in the V-containing steel at room temperature, regardless of the hydrogen activity. A large plastic deformation obtained by tensile straining in the homogeneous deformation domain only leads to a moderate decrease of the hydrogen diffusivity in both steels. The results are compared with the literature data on hydrogen permeation in iron and ferritic steels. On the other hand, the hydrogen content was measured with the melt extraction method after cathodic charging and subsequent aging at room temperature for different times to determine the diffusible (lattice reversibly trapped) hydrogen concentration. It was shown that the latter is larger in 3Cr-1Mo-V steel, which contains, in addition, a large fraction of “strong reversible” traps. A good concordance was found between the diffusible hydrogen concentration values computed from steady-state permeation measurements and from graphical integration of decay transients. The validity of the quantification, from permeation experiments, of the diffusible hydrogen concentration in materials with complex microstructures is discussed.
I. INTRODUCTION
THE analysis of hydrogen transport and distribution in the thick walls of hydrotreating pressure vessels and their welds used in the petro-chemical industry is a current concern with respect to the evaluation of the sensitivity to hydrogen damage, which can occur notably after shutdown.[1] In particular, the knowledge of the hydrogen behavior in the reactor walls is necessary for the choice of the minimum pressurization temperature during start-up operations, which can lead to excessive hydrogen concentrations at moderate temperatures, depending on the amount of hydrogen trapped during cooling. Assessment of the residual life of the old pressure vessels made of conventional 2.25Cr-1Mo steels is required; however, it is also necessary to manufacture new equipment for the better upgrading and desulfurizing of heavy oils. The future generation of hydrotreating reactors must meet industrial needs for larger sized equipment and more severe conditions of hydrogen environment, i.e., higher service temperatures and higher hydrogen pressures.[2] For this purpose, advanced steel grades with higher mechanical properties and a lower sensitivity to hydrogen damage must be develope
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