Analysis On Surface Defects Of The Vehicle-loaded CNG Cylinder Fabricated By Rotary Pressing

In recent years,the oil resource of world is drying up,the environmental pollution is increasing by automobile exhaust.To the serious problems of fossil fuels and environmental pollution,the world automobile industry focus on developing energy-efficient cars,new sources of energy and looking for available alternative fuel.The compressed natural gas(CNG)can be combusted fully and has less carbon.It can greatly reduce the exhaust emissions of harmful substances,significantly reduce the oil consumption and thus reduce the car’s cost.Due to the advantages of low cost,high efficiency,no pollution,safe and convenient,CNG becomes the ideal vehicle alternative energy.CNG vehicles have increased year by year.However,with the increment of CNG vehicles,the explosion and fire accidents of auto frequently happened and the major reason is gas cylinders.Therefore,the quality and safety problem of gas cylinders becomes the key problems to be solved urgently.Traditional gas cylinders is made by welding the stamping-molded bottle shoulder and the bottle bottom together with the tube billet or the coil-welded bottle body.The disadvantages were more complicated manufacturing process,higher cost,and poor bearing capacity and sealing performance.With the increasing improvement and development of spinning forming technology,spinning forming is the best choice for the integral forming of CNG cylinders.It can fundamentally eliminate the defects of traditional welding production,such as low welding strength,easy to brittle fracture and tensile stress concentration.It can greatly enhance the air sealing performance and pressure resistance of the cylinder,decrease the weight of molding equipment,reduce the mold’s processing cost and improving product quality significantly.But if the process conditions is improper during the process of spinning cylinders,the cylinder will have some defects such as peeling,corrugated,bonding,beat,cracks and convergent instability.These defects will affect the quality of products seriously and should be eliminated.The vehicle CNG cylinder of Handan some car cylinder manufacturing company made by hot spinning forming process appeared some surface defects.The defects mainly concentrated in two aspects:(1)The cylinder of 30 CrMo seamless steel tube after hot spinning forming appeared some cracks at the fusion area of the cylinder head;(2)Orange peel phenomenon appeared in the surfaces of some cylinders after quenching and tempering treatments,influencing the cylinder’s quality seriously.The CNG cylinder surface defects are analyzed by metallography,scanning electron microscopy,energy spectrum analysis.It is found than the main reason of cylinder cracks at the bottom is the combination of coarse grains and the mixed grain structure.The oxide impurities do not appear as cluster,continuous network or string chain and have less effect on the crack.To solve the coarse grains and mixed grain structure,some measures were taken to increase the deformation rate of fusion at the bottom of the cylinder sealing head.The deformation time was shorten under the condition of sufficient fusion and the distance of the heating furnace of steel tube to the spinning machine clamping jaw was tuned to shorten the moving time of the steel pipe,which can solve the problem of coarse grains and the mixed grains and further prevent the crack phenomenon at the bottom of the cylinder.The analysis results show that the surface orange peel phenomenon of the cylinder after quenching and tempering is due to the following three reason,the corse grain,uneven stucture and the lower contents of the Mn,Cr of the steel tube raw material.These measurements were performed to increase the content of Mn,Cr(Mn is higher than 0.58,Cr is higher than 0.98)within the scope of the standard,and control the grain size and microstructure uniformity of steel tube raw material(level should be above 11.9 magnitude)strictly.The surface roughness was effectively reduced during quenching and tempering process and removed the orange peel phenomenon.