Research On Microstructure And Mechanical Properties Of Inertia Friction Welding Of DT300 Ultra-high Strength Steel

DT300 ultra-high strength steel is a new type of ultra-high strength steel with excellent comprehensive mechanical properties developed on the basis of 30CrMnSiNi2A in China.It has high absolute strength and good plastic toughness.It is widely used in aerospace,military industry and high load-bearing and impact-resistant parts.High strength is accompanied by the challenge to the weldability of materials.The carbon equivalent of DT300 ultra-high strength steel is about 1% with obvious hardening tendency and poor welding performance.The high heat input of fusion welding often leads to the microstructure embrittlement of the joints related to melting and solidification.After welding,the residual stress of the joint is large,which is easy to deform or even crack.Based on the problem that fusion welding easily causes defects in joints,these defects can be reduced and avoided by controlling the heat input,cooling rate and heat flow during the welding process.In this project,inertial friction welding(solid phase welding process with low heat input)of DT300 ultra-high strength steel was carried out.The effects of initial rotation speed and axial pressure on the joint morphology,microstructure and mechanical properties was analyzed.The low temperature tempering was carried out for the samples at the initial rotation speed,and the changes in the microstructure and mechanical properties of the joints were comparatively studied.The main research results of the paper show that:(1)The test of inertia friction welding process of DT300 ultra-high strength steel shows that: it is easy to cause unwelded defects that damage the mechanical properties of the joint under low speed due to insufficient heat input,and only a small amount of deformation occurs during the tensile process,the fracture shows typical brittle fracture characteristics.When the heat input is enough,the joint is well combined,and there is obvious necking phenomenon during tensile process.The fracture position is in the base material area,the joint and the base material have equal strength,the fracture surface has dimples,and the plasticity is good.There are no macro and micro defects in the welding samples under different axial pressure.With the increase of axial pressure,the grains are significantly refined,and the further increase of pressure increases the energy conversion efficiency and the heat input.The microstructure in the center of weld zone is fully austenitized at high temperature and then cooled to room temperature to obtain all the martensitic.(2)A temperature distribution curve for the microstructure transformation from the weld zone to the base metal at 2800 rpm is built.The microstructure transformation of each region of the joint observed under SEM is consistent with that derived from the temperature range of the temperature distribution curve,which verifies the correctness of the temperature distribution curve.High strength martensite is formed in the weld zone.The TMAZ in the peripheral area is divided into transformation area,partial recrystallization area and plastic deformation area,while the TMAZ in the central area only includes partial recrystallization area and pure plastic deformation area.(3)For the welding samples with rotational speeds of 2800 rpm and 3600 rpm,the surface of the weld zone remained mechanically polished after corrosion,and the corrosion resistance was significantly enhanced,and the microstructure at the weld zone was shown to be martensite after deep corrosion,while the corrosion resistance at the weld zone was reduced after low temperature tempering.The joint microstructure was all tempered martensite,but the grains at the weld zone are inherited,and the fine grains dynamically recrystallized before tempering are still maintained.(4)The elongation of the as-welded sample at high speed is lower than of the base metal(16.56%),and the elongation at high speed is slightly higher than of the base metal(10.48%)after low temperature tempering.The impact energy of the weld zone of the as-welded sample increases gradually with the increase of the rotating speed,reaching the maximum value at 3600 rpm,which is 15.54 J,and both of them are lower than the impact energy value of the base metal(24.72 J),while after low temperature tempering,the impact energy of the weld zone first increases and then decreases,reaching the maximum value 19.48 J at 2800 rpm,which is higher than the impact energy value of the base metal(17.36 J)of low temperature tempering.