Study On Welding Process Of 800MPa High Strength Structural Steel In Hydropower Station

The hydropower Station studied in this paper is one of the largest hydropower station approved by China.In order to ensure the normal operation of such a large water flow,new type domestic 800MPa high-strength structural steel is used as its pressure pipe.In this paper,a series of research are carried out on common welding method process of these domestic 800MPa low alloy high-strength structural steels,such as Shield metal arc welding(SMAW),metal active gas welding(MAG)and submerged arc welding(SAW).Firstly,the structure and properties of 780CF and 800CF were compared.The joint weld zone is a kind of acicular ferrite and granular bainite structure with good toughness.The heat affected zone is granular bainite structure,in which the coarse grain zone and the partial phase change zone are weak regions of the joint.The difference between the two steel joints' performance is slight.Combined with all aspects of consideration,780CF steel is used.Then,the performance of 780CF steel joints in different welding methods,different welding materials and different thermal input is tested.Due to flux coverage,submerged arc welding's cooling rate is slower in the same thermal input,it's grain is slightly thicker than that of the electrode arc welding and argon-rich gas protection welding,but the atomic are more fully diffused,the strengthen particle and the second phase distribution more uniform.Under the conditions it can meet the requirements of the standard,submerged arc welding is more efficient,it's welding process is more stable and it has higher automation.So SAW could be used as the main welding method of pressure steel pipe.The welding performance of different welding materials is compared.Their chemical composition and joints' metallographic structure are analyzed and compared.The welding materials with better performance are selected.In this paper,two kinds of thermal input are designed for ench welding methods.Both types of thermal input welding joints can satisfied the demand of the standard GB/T 50766-2012.The thermal input is higher,the grain size is larger.The atomic diffusion ability is enhanced,which is conductive to form and evenly distributed the strenthen particles.The joint's strength and toughness could be improved.Therefore,the recommended electrode arc welding thermal input in the range of 25-35KJ/cm;argon-rich gas shielded thermal input range between 25?35KJ/cm;submerged arc welding energy range between 28-35KJ/cm.The performance of the two successive welds in the multi-layer multi-pass welding is analyzed,and the performance of the unmodified reheat coarse zone and subcritical reheat coarse grain zone did not change much,which is similar to that of the primary coarse zone.The grain refinement of the overcritical reheat coarse grain zone makes the strength and toughness increase.The critical reheat coarse grain zone forms a tissue with uneven grain size and composition,resulting in toughness's decreasing,but its performance is still good compared to primary partial phase transition zone.Therefore,the interaction between the welds makes areas that the welding temperature field coincides have better performation,and the weak region of the joint is still the primary coarse grain area and the primary partial phase change zone.