The Research And Application On NG-SAW Technology Of Supercritical Pressure Boiler High Temperature Header

With continuous development of parameters for thermal power generating units,and more and more mature for supercritical and ultra-supercritical,the subcritical boiler with 300~330 MW for thermal generator set in the world has been gradually transmitted to be the supercritical and the ultra-supercritical with 600 MW and 1000 MW.However,as those high temperature headers like the superheat header and reheat header are chronically operated under the highest state of the working temperature and pressure,as well as the function of the corrosive medium,the issue of fatigue defects become more and more common,wherein the main defective body is nearby welding.According to calculation of strength,in order to ensure usability and thermal efficiency,the high temperature header is usually made of the material of P91.With the higher level of material,the difficulty of producing header also increases.Especially,the difficulty of welding level for the high temperature header increases,as well as the cost increases.Meanwhile,the relevant manufacturing issues gradually brings out.The traditional circumferential seam welding technology generally possess a large the groove angel of submerged welding,more welding filter metal and long production cycle.Besides,the position of welding gun should be adjusted manually so as to achieve a good fusion quality between the welding wires and the groove sidewall.However,with the workload of welding increasing automatic circumferential seam welding technology is needed for many domestic boiler manufactures.Furthermore,it is a hot research task to improve grove of circumferential seam welding for header.The narrow gap submerged arc welding(NG-SAW)is a kind of boiler header narrow groove can be implemented,the automatic welding technology.According to the operating defects and the main materials for the high temperature header,this article focus on the NG-SAW for the high temperature header and thereof technical analysis,economical analysis,and technical comparison.The article further explores constitute of welding system,principle of system,key technology,welding characteristic for NG-SAW.The welding technology design for NG-SAW for the high temperature header includes choice of welding material,design of groove,and selection of welding parameter.With experiment of evaluation for welding technology for P91 material for the high temperature header,to research the technology of NG-SAW for the high temperature header.The experiment of evaluation includes metallographic microstructure,welding hardness and mechanical property.The reasonable parameter of welding technology of NG-SAW for the header with P91 material is obtained from the result of the experiment.Specifically,the welding current is 350-420 A,and the welding speed is 25~28m/h.The NG-SAW technology disclosed by this article has been applied onto the high temperature header in a domestic manufacture of boiler and the percent of pass is high,which testify the practicability of this technology.Additionally,factors which may affect the quality of welding are complex.Through researching the technology of repairing defects of NG-SAW in the practical manufacturing process and the temperature of heat treatment affecting hardness of welding,this article discloses other factors in addition to the parameters of welding technology.It is further testified that the technology disclosed by this article is applicability for headers with thick wall for the supercritical boiler high temperature header and find a key factor for controlling the quality of welding for the high temperature header.