Quantitative Design Of Deep Drawing Process For CNG Cylinders

With the increasing depletion of oil resources, as a kind of alternative energy,natural gas has been got much more people’s attention. Its weight in industrialproduction is becoming more and more highly. And the application of natural gas isbecoming more and more widely. As a kind of commonly used equipment for naturalgas storage and transportation, the application of compressed natural gas (CNG) isdeveloping in the direction of the large-scale, severe condition and lightweight. Theformability of CNG has been put forward much more stringent requirements. At thepresent time, study on the forming of large CNG cylinders still does not see more in ourcountry. Also there is no complete large domestic CNG cylinder production line.Therefore, it is very important for the industrial development of our country toinvestigate the molding process of large CNG cylinders and used to guide the relevantindustrial production.The experimental materials to research the formability of CNG cylinders in thepaper is34CrMo4alloy steel. The original billet is a large thick plate with size of Φ1200×10.5mm, eventually forming height is more than1000mm. Because of the largesize of cylinders, it is more prone to appear forming instability phenomenon, such asuneven deformation, microstructure defects, etc. And it is more difficult to guarantee theprecision shape size. Based on the analysis, the investigation of forming process forCNG cylinders has been carried out in this paper and the main research contents are asfollows:①Properties of34CrMo4steel has been analyzed. The flow stress of34CrMo4steel was obtained through hot compression simulation tests Due to the deformationenergy of materials cannot be send out in the process of compression test, there will bethermal effect in materials, which leads to the measured flow stress data is not accurate.Therefore, the flow stress data obtained by compression test must be modified withconsideration of thermal effect.②To identify the flow instability and the ideal plastic working region of34CrMo4steel, processing map based on DMM (dynamic material model) bas beenestablished by the modified flow stress. There are two ideal plastic working zones: Oneis at the temperature from273K to380K with strain rate below0.03s-1; the other one isat the temperature from530K to593K with strain rate from below0.368s-1. But in the practical production of material forming, strain rate and temperature of materials is noteasy to control to keep in the workability zones. Therefore, an optimized deformationpath was obtained by linking the greatest efficiency of power dissipation at everytemperature based on the processing map. When the deformation of material is along tothe optimized deformation path, it can be sure that forming process is in a relativelystable condition.③In the deep drawing process of CNG cylinders, strain rate and temperaturedistributions of the deforming material are related to the velocity and stroke of punch.By means of series of FE simulations and curve fitting, the relationship between thepunch velocity and stroke was obtained, which should be controlled close to theoptimized deformation path as much as possible. In this way, the availability ofprocessing map is guaranteed.④In order to guarantee the requirements of forming seize and precision of CNGcylinders, the mould parameters about the deep drawing of CNG cylinders by FEsimulation. The three passes of deep drawing were determined by relevant processanalysis. Considering the similarity of three passes, only the first pass was analyzed inthis paper. The minimum thickness and thickness uniformity of the cylinders bottomwere considered as evaluation indexes. After a serial of simulation analysis, it can bededuced that the reasonable radius of bottom die is60-100mm; reasonable spacerheight is12.6-13.1mm.⑤The effect of the interface friction and punch geometry in deep drawing processalso were analyzed in this paper. It is found that the cambered punch center couldguarantee the relation of billet and bottom of punch keeps contacted. And it isadvantageous for the formability of CNG cylinders.The forming process of CNG cylinder are analyzed in a comprehensively way inthis paper, both the process parameters and mould parameters are analyzed andoptimized. This investigation enriches the forming theory of CNG cylinders and alsocan provide an effective guideline and reference to the practical production of CNGcylinders.