Time-resolved in-situ analysis of phase evolution for the directional solidification of carbon steel weld metal
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formation about the phase transformation, during solidification process and solid-state transformation, is essential to materials processing. The material in the fusion welding process, for example, is heated and continuously cooled thorough liquid phase and hot cracking due to segregation of minor elements might happen during solidification. Depending on phase transformation kinetics of primary phase, the residual liquid rate during solidification is affected and this is an important factor for the defect such as hot cracking. In order to acquire the kinetic information about the phase transformations occurring in the solidification process of welding, a quenching method using liquid-tin has been used.[1–4] This method freezes the welding bead in the liquid-tin bath, during welding, and the microstructure change due to diffusion is stopped. Analyzing the quenched weld, the phase transformation from liquid phase can be identified. Athermal transformation, however, cannot be stopped. Furthermore, the experiments become time-consuming if high spatial resolution is used to identify the phase, for quenched specimen. In the late 1990s, more developed analytical methods for phase transformations during welding, using synchrotron radiation, were developed, as presented by Elmer et al.[5] In their experiments, the welding torch is stationary and the workpiece is rotated under the fixed torch. The X-ray beam scans the surface of the rotating workpiece and the phase mapping of pure titanium welding has been made in the spatial resolution of 250 3 500 mm. In their research, bcc phase near the solidification front was directly identified. The phase mapping for lowcarbon steel[6] and duplex stainless steel[7] also has been presented and analyzed. The measuring time of phase iden-
tification for one region was 10 seconds with their experimental setup. So, the welding torch and workpiece were translated with respect to the fixed X-ray beam, in order to probe discrete regions around the weld. This proves time-consuming if high spatial resolution observation for the continuous cooling process is used to identify the phase. Elmer et al. also presented the high time resolved X-ray diffraction investigation using synchrotron radiation,[8,9] with spatial resolution of 730 mmf and static welding, in which directional solidification did not occur. In this work, phase selection phenomena of primary phase due to the difference of cooling rate have been clearly shown.[9] In our research group, an in-situ phase identification system consisting of an undulator beam and imaging plate has recently been used.[10] The welding torch is driven by the stepping motor in the system. This makes is possible to identify the phase transformation in real time under the condition of directional solidification of the weld metal, and the spatial resolution is 100 mm perpendicular to the welding direction 3 500 mm parallel to the welding direction. The time resolution is 0.3125 seconds. Furthermore, a combination of analyses, the in-situ phase identification system, morpholo
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