Heat Treatment Process Optimization And Properties Of AISI 4140 Steel For Abysmal Sea Forging

Subsea wellhead and oil production equipment in offshore engineering are important unit equipment in offshore oil and gas field development.At present,such forging materials need to be imported,so the research and application of high-strength special steel has become the key to the development of offshore engineering in China.In order to ensure the quality and high safety of subsea equipment,an improved AISI4140 steel is designed and manufactured according to the actual working condition of subsea wellhead in China.The main research contents and results are as follows:Vacuum induction furnace is used for alloy smelting.The hot compression behavior of cast steel was studied by Gleeble-3500 testing machine.The hot compression rheological behavior of the alloy at different temperature(850?1200?)and different strain rates(0.1 s^-1,1 s^-1,10 s^-1)was analyzed.The high temperature rheological stress constitutive equation and hot working diagram were established.The optimal hot working deformation condition was 950?1100?,and the strain rate was0.1?1 s^-1.A three-factor four-level orthogonal test was designed to study the influence of heat treatment process parameters on the forging microstructure and mechanical properties.The results of the range analysis method show that the tempering temperature has the greatest influence on the hardness and toughness of the material.After tempering at high temperature,a typical tempered martensite structure can be obtained.High-resolution transmission electron microscope observation shows that spherical M₂₃C₆ and short rod-shaped carbide Fe₃C are distributed on it.When the tempering temperature increases,the width of the martensitic slab gradually increases,and at the same time,the dislocation density in the martensite decreases,causing the carbides to grow up and reduce the number,the ability of pinning dislocations to decrease,the hardness and strength to decrease,and the impact Increased toughness.The optimal heat treatment process was determined as follows:880?normalizing heat preservation for 2 h,850?quenching heat preservation for 1 h,650?tempering heat preservation for 4 h.A good combination of strength,toughness and plasticity is achieved.The electrochemical behavior and immersion corrosion of the material in 3.5%Na Cl solution under different heat treatment processes were studied,and the results showed that the corrosion resistance was stable under different heat treatment processes.Based on this,the sensitivity of H₂S stress corrosion under the best process was tested,and the material meets the H₂S stress corrosion standard.