Correlation of microstructural
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Correlation of Microstructural Parameters and Hydrogen Permeation in Carbon Steel D. L. JOHNSON, G. KRAUSS, J. K. WU, and K. P. TANG Coarse iron carbide and low carbon content correlate with a low corrosion rate of steel in 1 N H 2 5 0 4 and high permeD.L. JOHNSON is Professor of Mechanical Engineering, Metallurgy Program, University of Nebraska-Lincoln, Lincoln, NE 68588. G. KRAUSS is AMAX Foundation Professor and Director, Steel Research Center, Colorado School of Mines, Golden, CO 80401. J. K. WU is Associate Professor of Materials Engineering, Tatung Institute of Technology, 22 Chungshan North Road, 3rd Sec., Taipei, Republic of China. K.P. TANG is Metallurgical Engineer, National Hand Tool Co., 12827 Valley Branch Lane, Dallas, TX 75234. Manuscript submitted April 28, 1986. METALLURGICALTRANSACTIONS A
ability to hydrogen at constant potential. Fine iron carbide and high carbon content correlate with a relatively high corrosion rate and lower permeability. ~-4A higher corrosion rate in quenched and tempered high carbon steel is consistent with the fact that carbide is a depolarizer for hydrogen. 5 Lower permeability and diffusivity in fine iron carbide is more difficult to explain. One possibility is that carbides act as obstacles or repellers to the diffusion of hydrogen; another suggests that fine coherent carbides act as reversible traps. 6 Since the former explanation depends upon carbide spacing or mean free path and the latter depends upon carbide size or surface area, quantitative metallographic techniques which measure carbide size and spacing would be expected to correlate with permeation data. The application of quantitative metallographic measurements in hydrogen permeation studies has received little attention although Robertson and Thompson, 3 Hong and Lee, 7 Hirth, 8 Pressouyre and Bernstein, 6 and Pressouyre9 made trapping calculations based on unsteady state data for specific microstructures. Pressouyre ~° recently introduced the concept of obstacles and repellers and related this concept to hydrogen diffusion, solubility, and embrittlement. Steady state permeation is influenced by obstacles/repellers whereas it is normally understood that only unsteady state transients used to measure effective diffusivity are influenced by traps. Pressouyre discusses the consequences of hydrogen solubility on microstructure and concludes that solubility decreases with increase in repellers whereas solubility increases with increase in trapping. The purpose of the present communication is to extend correlation between microstructure and hydrogen behavior in carbon steels. As a base for this discussion, quantitative metallographic measurements were made on the same specimens used by Wu and Johnson 2 for aqueous phase hydrogen permeation and diffusion studies of several carbon steels. Specifically, microstructures of AISI 1035, 1050, and 1090 carbon steel in spheroidized, annealed, normalized, quenched and tempered, and quench only conditions were evaluated. The results of recent measurements on additional spher
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