Research On Key Techniques Of Hot Stamping Of Turbine Hollow Static Blade With Large Size And Complex Line

As a very important component of steam turbine,the hollow static blade can not only guide the steam flow,but also accelerate it.Generally,the hollow static blade is manufactured by casting.With the development of stamping and welding technology in China,more and more hollow blades are manufactured by the method of sheet metal hot forming and re-welding,which reduces the amount of machining and the costs as well as enhancing the deformation efficiency and the quality of products.Thus,this method has a broad market prospect in the hollow blade manufacturing field.With the increasing demand of electricity,the need of large and high efficiency turbine generator sets is rising.Therefore,large size,low weight and complicated structure become the goals of the development of the hollow static blade.The die structure of the traditional hot forming for the hollow static blade only has upper die and lower die.So it is not capable for the high precision producing manufacturing of hollow blade with large size and complex lines,and it can not form the small fillet R of the steam inlet edge.Hence,improving the forming precision of large size complex line hollow blade and forming the small fillet R of the steam inlet edge are the key to the hollow blade sheet forming.This work offers a new forming process(hot drawing process)for the hollow blade based on the investigation of a last stage hollow blade with large size and complex lines which is installed in a nuclear steam turbine.The die structure of this process includes upper die,holder and lower die,which can make the sheet fully extend during forming and control the wrinkling,springback and warping of the blade.Overall,this work studied the hot drawing process of hollow blade comprehensively and systematically,and the main research contents and conclusions are listed as following:(1)The numerical simulation founded that the problems of the wrinkling,the large amount of springback distortion and the insufficient forming of the fillet R of the blade inlet edge appeared in traditional hot forming of hollow blade with large size and complex lines.The hot drawing process without water cooling on the back and inner arc surfaces was proposed.The stress-strain status of material was analyzed during this process.The result shows that during forming,this process can control the wrinkling of the blade,make the sheet fully extend,form the small fillet R of the blade inlet edge and ensure that the material is under tensile stress state.The surfaces of back arc and inner arc are smooth and no distortion,and the maximum springback amount of them are 2.5mm and 4.8mm,respectively.(2)In the design of the back arc and inner arc hot drawing dies,the modular frame die structure was put forward.In this die structure,the upper die base,lower die base and punch are made of HT300,and the working surfaces are made of 5Cr Ni Mo.This die structure can reduce the material cost of back arc and inner arc dies by 35.07% and 38.63% respectively.In addition,it can replace and repair the working surfaces of the die quickly,which can reduce the cost of mold repair.It can also increase the heat dissipation surface area of the die and accelerate the heat dissipation of the mould.(3)According to the design rules for drawing die surfaces,an open-type drawing was applied on the design of back and inner arc surfaces considering the effect of thickness of sheet.A half head-wrapped structure was designed to prevent the risk of wrinkling on the back arc big end faces during forming.(4)The effects of friction coefficient,holder force,temperature and holding time on the forming quality of the back and inner arc were studied by controlling variable method.Then the following optimized combination of process parameters was determined: friction coefficient of 0.3,holder force of 60 and 40 tons on the back and inner arc respectively,temperature of 880 ℃,holding time of 480 s and 360 s for the back and inner arc respectively.Through further numerical simulation analysis,the variation of temperature,thickness and springback distribution of the back arc and inner arc of the blade during forming were obtained,which offered the theoretical support for the following processing tests.(5)In order to solve the problem of large amount of springback compensation after forming the back arc and inner arc,a fast springback compensation method based on reverse engineering was proposed.After two springback compensations by this method,the deviations of back arc and inner arc size are both less than 1.5 mm,which meets the requirement of product dimension precision.This springback compensation method can greatly shorten the die test period and reduce the die debugging cost.Through the researching of two compensation process and results,the ‘one step’ springback compensation factor of hot stamping of back arc and inner arc is calculated,which provides the experimental data support for ‘one step’ springback compensation in the same kind of blade forming.(6)The observations for microstructure and fracture morphology and the detections for hardness and mechanical property of the products surface were conducted to understand the property of the formed products.It can be concluded that: The grain size of formed product varies little compared with the unformed,but its hardness was enhanced from 183.3 HV to 192.3 HV~205.3 HV because of the generation of a few fined grains on the surface;Compared with the unformed condition,the formed condition has higher yield strength and tensile strength but lower plasticity,also,they share a similar fracture morphology which has a structure of dimple fracture in the fibrous region and elongated dimple fracture in the radial region.