A Study of Submicron Grain Boundary Precipitates in Ultralow Carbon 316LN Steels
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316LN stainless steel is widely used as a conduit steel for Nb3Sn superconductor coils in high field magnet systems. During the fabrication of the magnets, the material is exposed to a reaction heat treatment temperature up to 973 K (700 C) for 100 hours to create superconducting Nb3Sn within the coils. Under these processing conditions, the primary issue in 316-type steels is the sensitization that results in a deterioration of the mechanical properties. The deterioration is due to the formation of the grain boundary carbide precipitates. Early work and some recent literature stated that the grain boundary carbide in austenite stainless steels is ‘‘always’’ (Cr,Fe)23C6.[1–6] Sensitization can be minimized by solution annealing at elevated temperatures. However, the annealing results in a distortion in conduit alloys and the damage of the conductors in the conduits. Minor changes to the chemistry of the material may significantly affect the precipitation behavior. Significant efforts have been made to reduce the carbon content for avoidance of the sensitizations. Previous work stated that ‘‘complete immunity’’[6] to sensitizations in the austenite S. DOWNEY, II, Materials Engineer, formerly with the FAMUFSU College of Engineering, Department of Mechanical Engineering, Tallahassee, FL, 32310-6046, is with the U.S. Nuclear Regulatory Commission, Washington, DC 20555. K. HAN, Scholar Scientist, is with the National High Magnetic Field Laboratory, Magnetic Science & Technology, Tallahassee, FL 32310. Contact e-mail: han@magnet. fsu.edu P.N. KALU, Professor, is with the FAMU-FSU College of Engineering, Department of Mechanical Engineering, Tallahassee, FL 32310-6046. K. YANG, Professor, is with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China. Z.M. DU, Professor, is with the Department of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, P.R. China. Manuscript submitted January 16, 2009. Article published online February 4, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
stainless steels requires carbon content below 0.15 wt pct.[7] To compensate for the strength reduction due to low carbon content, nitrogen is added. Nitrogenstrengthened austenitic stainless steels also provide good cryogenic properties and corrosion resistance, low magnetic permeability, and higher elevated-temperature strengths.[8] Recently, we found examples that sensitizations remain in the ultralow carbon 316LN steel with carbon content below 0.15 wt pct and the sensitizations in this material may be related to grain boundary precipitates due to nitrogen additions.[9] Therefore, identifying the precipitates and studying the microstructure in ultralow carbon 316LN-type stainless steels remains the subject of continuing research. Because the precipitates in the heat-treated 316LN steel are generally fine, the traditional method of identification involves either transmission electron microscopy (TEM) or X-ray diffraction of extracted residues[10–15] that involve
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