Research On Heat Treatment Process Of 34CrMo4 Large Diameter Seamless Steel Gas Cylinders

In recent years,with the rapid development of the world economy,energy consumption is growing,the natural environment is getting worse and worse,the energy structure is gradually transforming,and the new energy is developing more and more rapidly.Natural gas,with clean and efficient advantages,has become the optimum selection of improving the environment and promoting sustainable economic development for the world.The development of the gas industry depends on natural gas transmission.The steel seamless gas cylinder is one of the main methods of gas transmission,and its market share is higher and higher with the advantage of short production cycle and no welding seam.And it has become an integral part of gas equipment.With the rapid growth of the market demand,it is becoming the key of researching and developing to improve the storage capacity and safety of gas cylinders in the gas cylinder industry.It is an important manufacturing process to ensure the mechanical properties of the seamless cylinder to meet the standard requirements.If the mechanical property is not enough,its security cannot be guaranteed.The paper investigates the influence of quenching and tempering temperature and time on the microstructure and properties of 34CrMo4 after complete quenching and high temperature tempering,by using metallographic observation,scanning electron microscopy(SEM),energy dispersive spectrum(EDS)and mechanical properties test.The conclusions of this paper are as follows:(1)Firstly,the critical temperature of 34CrMo4 was determined by differential thermal analysis: Ac1=756.4℃,Ac3=880.4℃.Then according to the critical temperature,the heat treatment scheme was made.(2)After complete quenching,the ferrite of 34CrMo4 alloy steel is completely dissolved in austenite,then after tempering treatment,the microstructure is tempered sorbite.After different quenching and the same tempering heat treatment,34CrMo4 is experimented with grain size test,metallographic observation and mechanical properties,by observing the microstructure and mechanical properties of materials,to determine that the optimum process is quenching at 930℃,for 60 min,for water cooling.(3)After quenching and tempering heat treatment,the microstructure of34CrMo4 is tempered sorbite which is composed of ferrite matrix and fine granular cementite scattered on it,and the organization is relatively uniform.After the same quenching and different tempering heat treatment,34CrMo4 is experimented with metallographic observation,mechanical properties test,scanning electron microscopy(SEM)and energy dispersive spectrum(EDS).The result shows that the optimum process is tempering at 560℃,for 120 min,for air cooling.(4)The impact fracture morphology of 34CrMo4 is typical ductile fracture.In the fracture surface,there are dimples in different sizes of the axial and a small amount of torn grains.There exist the second phase particles and inclusions S and Mn in the center of the torn grain.(5)Comprehensive analysis shows that the sample heat treated at an optimum process quenching at 930℃,for 60 min,tempering at 560℃ for 120 min can obtain the best comprehensive properties.It’s beneficial to the application and safety of34CrMo4 large diameter seamless steel gas cylinders.