Spatially resolved X-ray diffraction mapping of phase transformations in the heat-affected zone of carbon-manganese stee
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MICROSTRUCTURAL gradients are created in the heat-affected zone (HAZ) of fusion welds by solid-state transformations, such as grain growth, recrystallization, phase changes, annealing, and tempering.[1–4] In steel, these transformations result in the formation of different microstructural subregions that are referred to as the coarsegrained region, the fine-grained region, and the partiallytransformed region of the HAZ.[1–4] The presence of these different HAZ regions is known in a qualitative sense; however, their exact size and location is not well understood because they depend on both the heating cycle of the weld and the kinetics of the phase transformations. Although the heating cycle of the weld can be modeled and/or experimentally measured, the kinetics of each of the various phase transformations are difficult to determine and are rarely known under actual welding conditions. This lack of information has hindered both the efforts to develop comprehensive models for the prediction of the HAZ microstructure and our basic understanding of microstructural evolution in steel welds. Experimental measurements of phase transformation kinetics in welds are difficult to perform because of the highly nonisothermal and transient heating conditions that exist during welding. Complications are further caused by JOHN W. ELMER, Group Leader, and JOE WONG, Physical Chemist, are with the Chemistry and Materials Science Directorate, Lawrence Livermore National Laboratory, University of California, Livermore, CA 94551. THORSTEN RESSLER, Physical Chemist, is with the Department of Inorganic Chemistry, Fritz-Haber-Institut der MPG, D-14195 Berlin, Germany. Manuscript submitted September 29, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
the fact that small volume fractions of nonmetallic inclusions provide heterogeneous nucleation sites that have a large influence on the resulting microstructure.[5–8] In the absence of specific kinetic data under true welding conditions, isothermal time-temperature-transformation and continuouscooling-transformation (CCT) diagrams have been developed and are available for many ferrous alloys.[9,10] They are sometimes used to approximate the behavior of phase transformations that occur during cooling of the HAZ.[2] However, the prediction of weld microstructures from CCT diagrams requires many assumptions in order to deal with the nonisothermal and nonuniform cooling conditions of welds,[2] particularly under the high temperature gradients produced by intense laser beam welding.[11,12] Furthermore, these diagrams represent the cooling but not the heating portions of the HAZ, and there is no generally accepted method for verifying how well these diagrams predict actual HAZ behavior. Modeling the phase transformations that occur in the HAZ during welding requires both a good understanding of the temperature cycles that occur during welding and kinetics of the phase transformations. Numerical modeling of the weld temperatures has advanced considerably in the past few years and is now being used b
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