Research On Residual Effects Of Cold Stamping On Elliptical Head And Its Characterization Method

With the growth of economy and national industrialization in recent years, the demand of austenitic stainless steel vessels increases. The austenitic stainless steel vessels are usually manufactured with heads made of austenitic stainless steel. The cold stamping ellipsoidal heads, which has features of good forming property, accurate machining precision, high surface qulity, etc, are applied widely.The technology of cold stamping will bring heads residual effects, which mainly behave in plastic deformation and altering the wall thickness. The phase change will take place in the austenitic stainless steel when plastic deformation occurs, the austenite will turn into martensite, as a result, the strength and hardness of material will be enhanced, whereas the plasticity and tenacity will decrease. Thus, the mechanical property of material may differ from one palce to another on heads significantly according to the location. The prediction formulae proposed by national and foreign standards are mainly used to calculate the maximum deformation of heads, but it’s not enough for us to acquire all the residual effects that cold stamping leads to.The change of mechanical property of material and wall thickness may bring some undesirable influence on the safety of vessels at service. In fact, the failure of heads did happen these years, therefore, the study on the residual effects of cold stamping will be of singnifiant importance on guaranteeing the safety of vessels. In addition, the residual effects should be considered into vessel analysis to make the design more reliable.On the support of Key Sscience and Technology Team of Zhejiang Province (Project No.2010R5001), Special Funds of Central University Basic Scientific Research Business Expenses (Project No.2014FZA4024) and National Postdoctoral Science Foundation (Project No.2014M551731), this paper took ellipsoidal heads made of austenitic stainless steel as research object to study cold stamping residual effects and its characterization method. The main contents are as fellows:(1) A Finite element method of stimulating the process of cold stamping was established by applying ABAQUS, and the distribution of plastic deformation and the thickness change on head of different specifications had been calculated. Moreover, the prediction formulae had been established to forecase the residual effects of cold stamping.(2) There’re some factors that may affect the residual effects of cold stamping, such as multi-stage forming, different mould chamfering, different blank diameter and different material properties. These effect factors had been analyzed by applying ABAQUS, and the simulation result showed that the material properties and multi-stage forming had no influence on the residual effects, whereas the palstic deformation will increase when mould chamfering decreased or blank diameter increased.(3) The cold stamping test had been carried out to verify the accuracy of the finite element models and the prediction formulae of cold stamping residual effects. The result showed that both method were accurate enough to be applied in the engineering.(4) Two finite element modeling methods of vessels, which take cold stamping residual effects into consideration, was established. One can reflected the result of cold stamping stimulation accurately by inputting the results of stimulation of cold stamping process, but can be only applied in vessels with simple construction, another one simplified the method of modeling with prediction formulae, and can be adapted to most suituations with sufficient accuracy.(5) Vessel test was carried out to examine the validity of the modeling method which take cold stamping residual effects into consideration. The experiment data matched the result of finite element calculation. Thus, the method was valid.(6) The change of wall thickness and the material mechanics properties were analysed seperately, the change of the material mechanics properties will decrease the deformation of vessels since the deformation brought by cold stamping will enhance the yield strength. Otherwise, the change of wall of heads will increase the deformation of vessels. When took cold stamping residual effects into consideration, except the top area, the new plastic deformation did not occur in the head under strengthen pressure.