Research On Microstructure And Properties Optimization Of Welded Joints Of Q420High Strength Low Allov UHV Tower Steel

In recentyears, as the electric demandincreases, ultra-high volt(UHV) towers started to be applied in the national major transmission line. And Q420high strength low alloy(HSLA) steels’ application in UHV towers can increase the towers strength, which helps to improve the carrying capacity and economic benefit. But because of its material chemical composition, the performance welded joints is comparatively poor which mainly manifested in that the trend of generating weld crack in welded joints is comparatively big and the toughness of welded joints couldn’t meet the requirements. All these flaws obstruct the HSLA steels to be widely used in the UHV towers.Currently, articles about welding of HSLA steels in UHV tower are very few. There is not appropriate theory and practice to instruct the actual production. In order to solve these problems, first, the weldability theory and its evaluations are discussed in this paper and finish the weldability analysis of Q420steels. Then, the optimization program that includes the formulation of effective welding process and rare earth element welding alloying technique, which can reduce the trend of weld cracks and improve the toughness of welded joints, is proposed. On the end of this paper, the influence of rare earth element Ce on the welded joints of HSLA steels is studied by method of welding experiment, chemical composition analysis, mechanical properties experiment and microstructure observation.Main conclusions are as follows:the situation of welding cracks in welded joints of Q420HSLA steels are severe and the toughness of the welded joints is comparatively poor. In order to solve these welding flaws, effective welding process is formulated:the preheat temperature is set at90℃and restrict heat input between10kJ/cm and20kJ/cm. At the same time, the key measure—Ce welding alloying technique is applied. Ce element is added to weld metal through deoxygenation and desulfurization reaction which reduce the harmful elements in weld metal. And appropriate content of Ce element added to weld metal makes the inclusions refining and spheroidizing. These complex inclusions act as inert surfaces leading to a reduction of activation energy. So acicular ferrite nucleation is promoted while the growth of bulk primary ferrite and side ferrite are restrained. Thus pure and refining weld metal structure improve crack resistance, reduce the trend of generating weld crack, increase the toughness of welded joints. Therefore Ce can effectively optimize Q420steels welded joints performance. This research also provides theoretical guidance and practice on developing the special welding material for HSLA in UHV towers by adding rare earth element.