Weldability Evaluation of a Cu-Bearing High-Strength Blast-Resistant Steel
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TRODUCTION
BLASTALLOY 160 (BA-160) steel was developed to meet the property requirements for blast-resistant naval material applications.[1,2] The nominal chemical composition of BA-160 is Fe-0.05C-3.65Cu-6.5Ni-1.84Cr0.6Mo-0.1V (wt pct). The minimum yield strength of 1103 MPa is provided by fine (3 to 5 nm) Cu-rich precipitates and M2C carbides (where M represents the elements Cr, Mo, and V) in a mixed martensitic/bainitic matrix. An impact toughness level of 176 J at an ambient temperature of 293 K (20 C) is attributed to dispersed phase transformation toughening that is provided by Ni-stabilized austenite precipitates, which form during multistep tempering. The microstructure evolution during welding has been established from autogenous welding experiments and heat-affected zone (HAZ) simulations.[3] Advanced characterization of simulated HAZ microstructures was completed to understand the observed HAZ hardness profile.[4]
JEREMY L. CARON, formerly Graduate Research Associate, with the Department of Materials Science and Engineering, Welding Engineering Program, The Ohio State University, Columbus, OH 43221, is now Welding Metallurgist, with the Research and Technology Group, Haynes International, Inc., Kokomo, IN 46904. Contact e-mail: [email protected] SUDARSANAM SURESH BABU, Associate Professor, and JOHN C. LIPPOLD, Professor, are with the Department of Materials Science and Engineering, Welding Engineering Program, The Ohio State University. Manuscript submitted April 3, 2011. Article published online October 19, 2011 4032—VOLUME 42A, DECEMBER 2011
Atom-probe tomography (APT) results revealing Cu segregation in the single-pass HAZ regions of BA-160 led to concerns of this alloy being susceptible to HAZ liquation cracking.[4] The metallurgical basis for HAZ liquation cracking involves the simultaneous presence of continuous liquid films at grain boundaries and a critical level of restraint.[5] These liquid films cannot accommodate the strains that accompany thermal and mechanical restraints during cooling, leading to cracking along the grain boundaries in the partially melted zone (PMZ). The hot ductility test is a simple and accurate test method to evaluate the HAZ liquation cracking susceptibility of an alloy.[6–8] These tests simulate the metallurgical degradation that occurs during a weld thermal cycle and provides a measure of the ductility of the alloy that would be experienced at high temperatures in the HAZ. Ductility is a key component of overall weldability because weld cracking is often associated with an exhaustion of available ductility. The hot ductility test is ideally suited for an initial assessment of weldability, such as in the alloy development phase, because of its requirement of a relatively small amount of material. Both on-heating (OH) and on-cooling (OC) ductility tests are performed to obtain a complete ductility ‘‘signature’’ of the alloy. The nil-ductility temperature (NDT) is defined as the OH temperature where ductility is reduced to zero.[7,8] Essentially, this can be viewed as t
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