| As developmrnt of electric power industry, the demand of bearing capacity of the transmission tower, especially the ultra-high volt (UHV) and long span tower, is higher and higher. The application of low alloy high strength steel (ASTM A572 GR.65 steel) in the UHV tower manufacturing conduces to increase the yield strength and bearing capacity of UHV tower. However, the charater of GR.65 steel has not been realized, for example the trend of weld crack, especially cold crack, after welding in heat affected zone is greater than that of other steels, which is a threat to the quality of UHV tower and its application.The study on heat affected zone of GR.65 steel, which is applied to UHV tower, have not been reported until now.The current investigation attempts to research the problems during the welding process of GR.65 steel. The welding characteristics of GR.65 steel has been expounded and the correlation among heat input, microstructure and properties in heat affected zone of ASTM A572 GR.65 steel was investigated by methods of weld experiment, hardness testing, crystallite size testing, and impact toughness experiment, chemical composition analysis experiment and metallographic observation. Meanwhile, another work includes two dimensional temperature field of the welding jiont by numerical simulation and the microstructure prediction of coarse grain heat affected zone had been carried out. Thus, the conclusions are includes two parts, shown as follows:The results of the welding process:1) The average grain size in CGHAZ at high input is 151μm, which is two times larger than that at low heat input. This indicates that the GR.65 steel cannot inhibit the austenite grain growth effectively at high heat input, but the solute drag effect would play a significant role in controlling the grain size at low heat input.2) Weld preheating in the low heat input situation (10kJ/cm) is propitious to the toughness in CGHAZ. However, the formation of upper bainite at high heat input (40kJ/cm), due to the longer cool time t8/5 caused by preheating, would seriously weaken the toughness of HAZ.The results of welding joint temperature field by numerical simulation:1) Due to the groove shape of plate butt joint, the temperature field on the right side of the weld pool presents a considerable inhomogeneity in vertical direction at the beginning time and it tends to uniform as time goes on. However, the temperature field on the left side always appears the smaller difference.2) The temperature distribution in the transverse section shows that when at time 0.51s, the temperature in CGHAZ (x<2mm) experience a rapid cooling process, while at time 5.25s, all the temperature in different depth decrease mildly with distance. Besides, the temperature along transverse direction in the depth of 6 mm at time 0.51s is much higher than that in other depth.3) The temperature near the top surface is always higher than that near the bottom, but the temperature difference is smaller as time goes on.4) The thermal cycling curve obtained by finite element analysis is contributed to predict the microstructure of CGHAZ, and the predicted microstructure is the same as that observed by butt joint welding. Thus, the accurate of FEM model has been proved by welding experiment.
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