| Standard elliptical heads made of metastable austenitic stainless steel have been widely used in industry.In order to meet the need of mass production, stamping process is usually used. However, for metastable austenitic stainless steel, large quantities of deformation-induced martensite phase transformation are induced during the cold stamping process, thus leading to an increase in strength and a shrinkage in plasticity of materials, which may count against the safety of head applications.Deformation-induced martensite phase transformation can be avoided by using hot stamping process, but its costs are much higher. Under this circumstance, warm stamping forming process would be more appropriate, as it can eliminate the deformation-induced martensite phase transformation and its costs are significantly lower than hot stamping.So far, few studies have been done on the warm stamping of heads that made of metastable austenitic stainless steels, there’s no enough theoretical support for the determination of the stamping temperature. It is difficult to confirm a reasonable stamping temperature on the basis of the volume of martensite examined in the head due to the lack of data, which led to the higher costs. Moreover, mechanical properties of materials after the process of warm stamping have not been studied either, which influences the safe use of head. Besides, temperature drop in the stamping process has not been taken into account yet, as well as no effective method has been put forward to estimate it. Therefore, it’s of significant importance for safety and economy of heads to analyze the repercussions of forming temperature on deformation-induced martensite phase transformation and mechanical properties of materials after stamping and to establish a calculation method on temperature drop during the stamping process.This subject is supported by Zhejiang Province key Science and Technology Innovation Team Project (Number:2010R50001) and Special Funds of Central University Basic Scientific Research Business Expenses (Pr oject No.:2014FZA4024) and National Postdoctoral Science Foundation (Project No.: 2014M551731). Studies on heads made of S30408 have been carried out. The main research work of this thesis are:(1) The influence of the temperature on the deformation-induced martensite phase transformation was investigated based on the tensile test results of different batches of 304 materials under different temperatures. The influence of material chemical composition was taken into consideration and an incidence formula interpreting the relation among deformation-induced martensite phase transformation, chemical composition and deformation temperature was established. The lower limit of stamping temperature is preliminarily given as 90℃.(2) Influence of pre-streching temperature on the mechanical properties of material was studied as a comlement to the study in Chapter 2, and the lower limit of stamping temperature given through this part of study is still 90℃.(3) Warm stamping and cold stamping tests were carried out respectively. It’s verified that the plastic deformation of heads made though warm and cold stamping is basically identical. By comparing the extent of martensite transformation and impact properties of the materials that the two heads were made of, the lower temperature limit mentioned above was confirmed reasonable.(4) A numerical analysis model was established to simulate the warm stamping forming process based on ABAQUS, and was validated though comparison with the experimental test results. Based on that, a numerical method for the calculation of the temperature of straight section of head after stamping process was established and the influence of different warm stamping parameters on the temperature drop was sdudied with the application of heat transfer theory. Finally, combined with the suggested lower limit of stamping temperature given in this paper, a stamping temperature determination method considering the temperature drop was established. |
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