New Aspects of Sensitization Behavior in Recent 316 Type Austenitic Stainless SteelsAustenitic Stainless Steels(Materials, Metallurgy & Weldability)

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Intergranular precipitation behavior of 316 and 316L stainless steels after annealing at 600-900℃ for 5min-50hrs were examined using transmission electron microscopy of carbon extraction replicas and thin foil techniques. Precipitated particles were identified by electron diffraction analysis. Chemical compositions of precipitated particles were measured from EDX-spectra by a semiquantitative method. When 316 steel was annealed at 750-850℃ for 15 min or longer, only M_<23>C_6 carbide was identified at grain boundaries. For 316L steel, however, three kinds of particles, i.e. Laves phase, M_<23>C_6 and a quasicrystal, were precipitated at the grain boundaries when annealed at 700-800℃ for 10hrs or longer. Most of the precipitated particles at grain boundaries of annealed 316L steel were Laves phase. M_<23>C_6 precipitation caused Cr depletion at grain boundaries of the annealed 316 steel, but the formation of Laves phase did not induce the Cr depletion at grain boundaries of annealed 316L steel. Although no Cr depletion occurred, the grain boundaries of annealed 316L steel were attacked in Oxalic acid etch tests and Strauss tests, probably because of electrical potential difference between the Laves phases and matrix, and/or low Cr contents in Laves phases. After single pass welding with cooling rates higher than 0.07℃/s, the weld HAZs of both 316 and 316L seem to be free of sensitivity to intergranular attack.
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