In-Situ Observations of Martensitic Transformation in Blast-Resistant Steel
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TO meet the rigorous requirements for U.S. Navy hull and deck application, a blast-resistant steel, BlastAlloy 160 (BA-160, QuesTek Innovations LLC, Evanston, IL), was developed by Northwestern University researchers.[1,2] This steel is based on a low-carbon martensitic matrix that is strengthened by nanometersized Cu-rich precipitates and M2C precipitates (where M = Cr, Mo, and V). The yield strength of BA-160 is 1104 MPa (160 ksi). The overarching goal of this development activity is to replace the currently certified high-strength low-alloy steels for use in surface ship structure. In order to meet this goal, the steel has to be weldable. Current research pertains to the on-going weldability research using small-scale samples. The activities include thermomechanical simulation, electron microscopy, and atom probe characterization, as well as thermodynamic and kinetic modeling of the microstructure evolution in the heat-affected zone (HAZ) region. The preceding data can be used to fine tune the base metal, as well as to design welding process parameters. Prior research samples were subjected to thermal cycles typical of that of HAZ regions, with controlled heating rate, peak temperature (TP), and cooling rates, XINGHUA YU, Graduate Student, SUDARSANAM SURESH BABU, Associate Professor, and JOHN C. LIPPOLD, Professor, are with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43221. Contact e-mail: babu.13@ osu.edu HIDENORI TERASAKI, Associate Professor, and YU-ICHI KOMIZO, Professor, are with JWRI, Osaka University, Osaka, Japan. Manuscript submitted February 27, 2011. Article published online June 8, 2011 1538—VOLUME 43A, MAY 2012
in a GLEEBLE* thermomechanical simulator. No *GLEEBLE is a trademark of Dynamic Systems Inc., Poestenkill, NY.
significant changes in microstructure and hardness were detected in the subcritical HAZ (TP < Ac1) samples. Some hardening was observed in the intercritical HAZ (ICHAZ, Ac1 < TP < Ac3) samples. Softening was observed in samples subjected to fine-grained HAZ (FGHAZ, TP > Ac3) and coarse-grained HAZ (CGHAZ, TP >> Ac3) thermal cycles. Atom-probe tomography characterization and strength models correlated the softening to the dissolution of Cu precipitates and carbides.[3] In the next step, methodology to restore the strength, without any postweld heat treatment, was considered. One of the innovative approaches is to leverage repeated thermal cycles that may ensue during multipass welding. In agreement with this hypothesis, Cu reprecipitation and recovery of strength were observed after a double thermal cycles heat treatment.[4] The preceding studies also demonstrated that martensite substructure plays an important role in strengthening of BA-160.[3] For example, fine martensite substructure contributed to the hardening in the samples subjected to the intercritical HAZ thermal cycle. This was attributed to the reduced martensite plate/lath/ packet size, which in turn is related to small austenite grain diameter. This observation is in agree
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