Mechanical properties of austenitic stainless steel single crystals: Influence of nitrogen and hydrogen content
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I. INTRODUCTION AUSTENITIC stainless steels are alloys which exhibit high corrosion resistance combined with high mechanical properties, especially ductility. It is, however, well established that when they are exposed to cathodic charging or gas environments of low or high hydrogen pressures, a possible degradation of mechanical properties with less or more pronounced brittleness can occur.[1,2] Alloying these alloys with nitrogen enhances both mechanical and corrosion resistance.[3,4] To explain the role of nitrogen from a physical point of view, a study of single crystals alloyed with nitrogen was found to be very useful.[5] A strong effect of orientation on plasticity has been demonstrated on these single crystals. Since problems of hydrogen embrittlement are usually analyzed through mechanical tests, it seems that employing single crystals with different nitrogen contents should be a promising way in the determination of the role of hydrogen on the mechanical properties of austenitic stainless steels. II. MATERIALS INVESTIGATED The material used for this study is a Fe-18 Cr-14 Ni-2 Mo austenitic stainless steel single crystals with or without nitrogen addition. The single crystals were producted at Tomsk University using the Bridgman method. Large ingots were first cast in MgO crucibles and oriented using X-ray diffraction. Tensile test samples with ^111& orientation were then prepared by spark cutting. Alloying with nitrogen (0.4 wt pct) was carried out using a nitrogen atmosphere of 2.5 atm pressure at 1453 K for 1800 seconds. The chemical composition of both alloys is listed in Table I. III. EXPERIMENTAL PROCEDURE A. Hydrogen Cathodic Charging The influence of nitrogen on the electrochemical behavior of stainless steels has been characterized in a 1 N sulfuric Y. RIGUAL SUCRE, Postdoctoral Student, and J.B. VOGT, Professor, are with the Laboratory of Physical Metallurgy and Material Engineering, Lille University, CNRS UMR 8517, 59655 Villeneuve d’Ascq Cedex, France. A. IOST, Professor, and D. NAJJAR, Associate Professor, are with the Laboratory of Physical Metallurgy and Material Engineering, Lille ´ University, and Lille Research Centre of Ecole Nationale Superieure d’Arts ´ et Metiers, 59046 Lille Cedex, France. Y.I. CHUMLYAKOV, Professor, is with the Siberian Physical Technical Institute, Tomsk 634050, Russia. Manuscript submitted December 17, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
acid solution without the addition of any hydrogen poison. The electrochemical tests have been carried out using specimens with tensile test geometry. Specimens were polished with different grades of silicon carbide paper up to 1200 grit 2400 grade (8 mm grain size). To ensure an undeformed surface, the specimens were finally electropolished in a 80 pct acetic acid, 10 pct perchloric acid, and 10 pct deionized water solution at 14 V during 60 seconds at room temperature. Before electrochemical tests, the specimen surfaces were degreased with acetone and then rinsed with deionized water. The free-working area was the
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