Study On High Temperature Damage Criterion Of 30CrMo Steel In Hot Working Process

With the use of clean energy such as natural gas,the large-diameter seamless steel pipes for gas cylinders also have been widely used.It is well known that steel pipes can be made by cross rolling perforation,when cross rolling perforation,the surface of the hollow billet is easy to produce interlayer and hairline.This type of defects has a great impact especially on pressure steel pipes,which is directly related to the safety during application.In order to improve the quality of seamless pipes for gas cylinders,a thermal damage criterion is studied and established,which provides a theoretical basis for predicting the damage and failure in the production process of steel pipes.30CrMo is widely used in the production of seamless steel pipes for gas cylinders due to its good comprehensive performance.And the 30CrMo material will undergo phase transformation when deformation occurs at high temperature.This article takes 30CrMo as the research object,discusses the critical cavity expansion ratio parameter damage criterion of materials with phase change at high temperature,and further analyzes the effect of strain rate and deformation temperature on critical hole expansion ratio,and the calculation formula of the critical cavity expansion ratio parameter is derived.In addition,this article also established the thermal processing maps of 30CrMo material,which mainly used the function of the thermal processing map to identify defects for comparing the accuracy and acceptability of the critical cavity expansion ratio.The uniaxial thermal tensile experiment and thermal compression experiment of30CrMo were carried out on the Gleeble3800 test machine.The stress-strain data of the necked section of the tensile specimen need be corrected,so the accurate stress-strain curses at different temperatures and different strain rate of 30CrMo were obtained.Using the modified true stress-strain data of tension,the triaxial stress and the fracture strain of the material at tensile fracture by simulating,and the critical cavity expansion ratio parameter value under the corresponding conditions was calculated.In order to study the influence of strain rate and temperature on the critical cavity expansion ratio parameter,the Zener-Hollomon equation was established.The analysis result found that the critical cavity expansion ratio parameter V_GC has a linear relationship with ln Z.So the calculation formula of the critical hole expansion ratio parameter can be get.According to the true stress-strain curve obtained from the compression test,the thermal processing maps with strains of 0.2,0.3,0.4,and 0.5 were established,and it is found that the safe areas in the thermal processing diagrams are different at different strains.According to the comparison between the results of thermal processing map and the value of the critical cavity expansion ratio,it is found that the two methods are not completely consistent in the prediction of the damage that may occur during the processing,but the difference is very small.At the same time,it is also based on the results of the thermal processing map.It qualitatively verifies the rationality of the critical hole expansion ratio theory.