Strength Recovery in a High-Strength Steel During Multiple Weld Thermal Simulations
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INTRODUCTION
TO meet the rigorous requirements for the United States Navy hull and deck application, a blast resistant steel, BlastAlloy 160 (BA160), was developed at Northwestern University.[1,2] BA160 is a low carbon martensitic steel additionally strengthened primarily by nanometersized Cu-rich precipitates and M2C precipitates (where M = Cr, Mo, and V). The yield strength of BA160 is 160 ksi (1104 MPa). In addition to high strength, very good room-temperature Charpy impact toughness [176 J (130 ft-lb)] was achieved through precipitation of Ni-stabilized austenite within a martensitic matrix. The aim of BA160 development is to replace currently certified high-strength low-alloy (HSLA) steels used in surface ship structure. Typical shipyard welding procedures include gas metal arc welding (GMAW), submerged arc welding (SAW), shielded metal arc welding (SMAW), and flux cored arc welding (FCAW).[3] These welding procedures will also be used for deploying BA160. To employ BA160 in ship building applications, it is important to understand its weldability, which is related to, among other factors, the complex solid-to-solid phase transformations that occur in heat-affected zone (HAZ) regions as a function of initial microstructure, heating rate, peak temperature (TP), and cooling rate. XINGHUA YU, Graduate Student, S.S. 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: [email protected] JEREMY L. CARON, formerly Graduate Student, with the Department of Materials Science and Engineering, The Ohio State University, is now a Welding Metallurgist, with the Research and Technology Group, Haynes International, Inc., 1020 West Park Avenue, Kokomo, IN 46904. DIETER ISHEIM, Research Assistant Professor, and DAVID N. SEIDMAN, Professor, are with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208. Manuscript submitted November 22, 2010. Article published online April 27, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
There are many testing methodologies available to determine the weldability of structural materials and selection of appropriate tests depends on the material.[4] Although weldability tests are not often routinely used when optimizing mechanical properties such as tensile strength and fracture toughness during alloy development, they are of great importance during weld procedure development to ensure proper deployment in service. Weldability testing techniques, including the HAZ thermal simulations, hot ductility testing, and reheat cracking testing, were employed to evaluate the weldability of BA160.[5] For steels, there are four distinct HAZ regions depending upon the peak temperature in a given weld thermal cycle. (1) In subcritical HAZ regions (SCHAZ; TP < Ac1), no detectable transformation of ferrite to austenite occurs. (2) In the intercritical HAZ (ICHAZ), partial transformation of ferrite to austenite occurs because the peak temper
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