In-situ phase mapping and direct observations of phase transformations during arc welding of 1045 steel
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zed base metal properties of steels are altered during welding by the severe thermal cycles imparted on the workpiece by the localized welding heat source. As a result, nonequilibrium microstructures are created in the fusion zone (FZ) and heat-affected zone (HAZ) of the weld. These microstructures differ significantly in appearance and properties from those found in the base metal.[1,2,3] Transformation from the body-centered cubic (bcc) form of iron, referred to as ferrite, and the face-centered cubic (fcc) form of iron, referred to as austenite, is principally responsible for these changes in microstructure. This transformation during the weld heating cycles has a distinct effect on the final microstructure that forms during cooling by controlling the extent of the transformation and the resulting carbon content in the austenite.[4] It is well known that carbon plays an important role in these transformations.[1,2,3] Increases in the carbon content are known to intensify the microstructural changes that occur and to increase the amount of nonequilibrium phases that are created during cooling. In order to better understand the microstructural evolution in steel welds, in-situ X-ray diffraction (XRD) studies are being performed to directly observe the phase transformations that occur during welding of low and medium carbon steels. Time-resolved X-ray diffraction (TRXRD) has recently been used to investigate phase transformations that occurred during stationary gas tungsten arc (GTA) spot welding of both medium carbon (AISI 1045)[5] and low-carbon (AISI 1005) steels.[6] These real-time synchrotron-based experiments tracked the phases present at discreet time resolutions, J.W. ELMER, Deputy Program Element Leader, and T.A. PALMER, Metallurgist, are with the Lawrence Livermore National Laboratory, Livermore, CA 94550. Contact e-mail: [email protected] Manuscript submitted September 22, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
on the order of 100 ms, in the HAZ of welds under rapid heating and cooling rates, typically in excess of 100 °C/s. The relative fractions of a and g phases present at each time increment were then measured during both weld heating and cooling. Modeling of the TRXRD spot welds was performed using a transient three-dimensional (3-D) coupled thermal fluids code to determine the time-temperature profile of the welds.[7] Differences in austenitization rates of the low and high-carbon steels were observed and were attributed to differences in amounts and distributions of the pearlite and allotriomorphic ferrite phases in the base metal microstructures.[5] Spatially resolved X-ray diffraction (SRXRD) has also been used to investigate phase transformations in the lowcarbon 1005 C-Mn steel.[8–13] From these data, a map showing the locations of the g and a phases present across the width of the HAZ in a typical arc weld was created.[8,9] Kinetic modeling of the data presented in the a / g phase transformation map was performed using a Johnson–Mehl– Avrami (JMA) approach in order to determine JMA kinet
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