In-situ observation of phase evolution in fusion welding of hypereutectoid carbon steel

In order to understand the microstructure change during welding process, aiming to design high intensity and toughness weld structure, kinetic information about the phase transformation is essential. In our research group, in-situ observation system consisting of undulator beam and imaging plate have recently been used. The welding torch is driven by stepping-motor in the system. Those make possible that phase transformation can be identified in real-time under the condition of directional-oriented solidification and the spatial resolution of 100 μm width. The time-resolution is 0.3125 seconds. In the present work, combination of analyzing method: the in-situ observatrion system by X-ray diffraction technique using intense synchrotron radiation, morphological observation by high-temperature laser scanning confocal microscopy and observation of post-weld material by OM, SEM and micro diffraction-system, is suggested to analyze the phase transformation during welding process. Using the all results acquired by those analysis methods, Phase evolution of hypereutectoid carbon steel, during fusion welding was analyzed. The primary phase was directly identified as an austenite phase, during welding. Precipitation of pearlite phase was observed at 560 deg., during cooling followed by the martensitic transformation.