Thermal Desorption Analysis of Hydrogen in High Strength Martensitic Steels
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INTRODUCTION
HIGH strength steels are widely used for PC bar (steel bar for prestrained concrete), spring steels, high strength bolts, etc., and environmental degradation caused by a very small amount of hydrogen, usually less than 1 ppm, has been a subject of keen interest for many years.[1] These steels are strengthened by the high density of dislocations, vacancies, lath boundaries introduced during quenching to martensite as well as prior austenite, and twin boundaries. It is believed that hydrogen absorbed in steels during service is accumulated in these defects and causes the formation of microcracks, which lead to the delayed fracture. High strength steels contain a large amount of carbon, and thus, it is also believed that carbon has a marked influence on the trapping of hydrogen. Au and Birnbaum[2] first reported the interaction between carbon and hydrogen in iron. Indeed, Hagi and Hayashi[3] reported that the potency of dislocation for hydrogen trapping was lessened considerably presumably by segregation of carbon to dislocations. Moreover, retained austenite, transition carbides, and alloy carbides, which form during tempering, may serve as potential and often effective trapping sites of hydrogen. It is thus very important to understand the characteristics and mechanisms of hydrogen trapping M. ENOMOTO, Professor, is with the Department of Materials Science and Engineering, Ibaraki University, Hitachi 316-8511, Japan. Contact e-mail: [email protected] D. HIRAKAMI, Senior Researcher, and T. TARUI, Chief Researcher, are with the Steel Research Laboratories, Nippon Steel Corporation, Chiba 293-8511, Japan. Manuscript submitted February 25, 2011. Article published online October 8, 2011 572—VOLUME 43A, FEBRUARY 2012
to increase the reliability and durability of high strength steels. In this report, hydrogen trapping during quenching and aging at room temperature was studied in four high strength steels containing 0.33 to 1.01 mass pct carbon with due attention to the influence of carbon segregation on trapping. Specimens were charged with hydrogen at high temperatures in the austenite region under hydrogen atmosphere, whereas aged specimens were charged electrolytically at room temperature. Thermal desorption analyses (TDA) were conducted to identify the trapping sites, and the results are discussed with comparison to the results of computer simulation based upon the McNabb–Foster theory[4] and theories of strain aging.[5,6]
II.
EXPERIMENTAL PROCEDURE
Four alloys containing 0.33 to 1.01 mass pct C were vacuum-induction melted. The 20-kg ingots were hotforged into rods of 15-mm diameter. The chemical compositions of alloys are shown in Table I. The martensite-start (Ms) temperatures were measured by a hot deformation simulator and are shown in Table II together with the amount of retained austenite, measured by X-ray diffractometry. The measured Ms temperature agreed with those calculated from the formula in the literature[7] within 20 K except steel B. A small amount of austenite was retained even in
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