Hydrogen-Trapping Mechanisms in Nanostructured Steels
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INTRODUCTION
NANOSTRUCTURED steels display excellent properties including strength and hardness,[2,3] which make them susceptible to hydrogen embrittlement. They contain several phases harmoniously engineered to maximize the mechanical response. Studies on the effect of hydrogen in steels show that the microstructure susceptibility to hydrogen’s deleterious influence decreases in the following order: martensite, spheroidized microstructures, perlite, quenched martensite, and lower bainite.[6] Unfortunately, retained austenite has not been included in earlier studies, but low hydrogen diffusivity and high solubility in austenite have been reported;[1] conversely, hydrogen diffusion in ferrite is high and its solubility is low. This paper describes the effect of hydrogen on superbainite, which combines nanoscaled carbide-free lath retained austenite and bainite.[12] Additionally, the hydrogen behavior in nanostructured martensitic steels is studied; 100Cr6 and a novel variant 100Cr6 + 0.5 V, which adds 0.5 wt pct V to increase resistance to hydrogen embrittlement, are examined in this work. When hydrogen is free to diffuse throughout the microstructure, damage is likely to result. Hydrogen is attracted to strain raisers such as microcrack tips and further accelerates crack growth by various mechanisms. These include
B.A. SZOST, Research Fellow, is with the Science and Applications Division (HSO-US), Directorate of Human Spaceflight and Operations, European Space Agency - ESTEC, 1 Keplerlaan Street, 2201 AZ Noordwijk, The Netherlands, and also with the Department of Materials Science and Metallurgy, SKF University Technology Centre, University of Cambridge, Pembroke Street, Cambridge, CB3 3QZ, U.K. R.H. VEGTER, Team Leader Fatigue Modelling, is with the SKF Engineering and Research Centre, Kelvinbaan 16, 3439 MT Nieuwegein, The Netherlands, and also with the P.O. Box 2350, 3430 DT Nieuwegein, The Netherlands. PEDRO E.J. RIVERA-DI´AZ-DELCASTILLO, Assistant Director of Research, is with the Department of Materials Science and Metallurgy, SKF University Technology Centre, University of Cambridge. Contact e-mail: [email protected] Manuscript submitted December 20, 2012. Article published online May 23, 2013 4542—VOLUME 44A, OCTOBER 2013
hydrogen-enhanced localized plasticity,[9,34,38] where hydrogen enhances dislocation mobility ahead of the crack tip, and hydrogen-enhanced debonding,[13,14,20,23] where the binding energy is decreased resulting in cleavage. Other forms of damage include hydrogen diffusing toward vacancies and nanovoids, increasing their size.[25,26,28,40,41] In every case, damage is minimized if hydrogen is immobilized. This can be achieved through microstructural traps. A hydrogen trap can be subscribed to a certain trapping category, having an irreversible or reversible trapping character, determined by its activation energy ðEA Þ. The value of EA is revealed from thermal desorption analysis and is closely related to the temperature at which the thermal desorption peak occurs (TTDA ).[4] If EA is above 50 kJ/
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