A critical-strain criterion for hydrogen embrittlement of cold-drawn, ultrafine pearlitic steel
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I. INTRODUCTION AND BACKGROUND A. Metallurgy of Isothermally Transformed Prestressing Steel
EUTECTOID steels with a fine pearlitic microstructure are used extensively in prestressed and post-tensioned concrete structures due to their high yield strength. When utilized in such applications, these steels are candidates for cathodic protection to reduce corrosion-induced losses in the cross section, especially in marine bridge piles. Cathodic protection introduces the potential for hydrogen embrittlement of the prestressing tendon.[1,2,3] The prestressing steel investigated in this study is similar, compositionally, to AISI/ SAE 1080 carbon steel, with a yield strength of 1696 MPa. The steel of interest is austenitized and isothermally transformed to a fully pearlitic condition, with an average pearlite interlamellar spacing of 95 nm. Processing of the prestressing steel involves, first, isothermally transforming the previously austenitized steel to achieve a fully pearlitic microstructure, after which it is cold drawn to an 85 pct reduction in area and stress relieved. The microstructure produced by the cold drawing of the prestressing strand is highly anisotropic, with pearlite lamellae preferentially aligned parallel to the long axis of the strand.[4,5] These wires have a 具110典 wire texture (具110典 direction within ferrite D.G. ENOS, Scientist, is with the 3M Austin Center, Austin, TX 787269006. J.R. SCULLY, Professor of Materials Science and Engineering, is with the Center for Electrochemical Science and Engineering, Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903-2442. Contact e-mail: [email protected] Manuscript submitted November 9, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
lamellae aligned roughly parallel to the drawing axis), such that {011} planes in the ferrite are oriented at 0, 60, and 90 deg from the wire axis. Moreover, the 具112典 directions in these planes are oriented at angles of 30, 54.7, 73.2, and 90 deg from 具110典. The deformation associated with the drawing process also results in the formation of an elongated dislocation cell structure, with the cell size determined by the combination of the interlamellar spacing of the pearlite coupled with the degree of cold work.[4] The interlamellar spacing within the pearlite and fine dislocation cell structure controls the yield strength of both isotropic and anisotropic eutectoid steels. Ductility is controlled by the lamellar spacing, pearlite colony size, carbide thickness, and prior-austenite grain size. The role of hydrogen in each of the controlling processes is not well understood. Typical fracture-initiating inclusions in such steel include MnS, Ti(C,S), and Al-Ca-S.[6,7,8] B. Fracture Toughness of Isothermally Transformed Prestressing Steel Due to the highly anisotropic nature of the pearlite, the limited ferrite slip distance, more homogeneous slip, and the ability of fine cementite to deform in response to tensile deformation,[6,9] prestressing steels possess a high transverse mode I (i.e
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