Study On Microstructure And Properties Of A New Nanoscale Precipitation Strengthening M&A Multphase Steel

Based on the principle of alloy design,Thermo-calc thermodynamic calculation software was used to design the composition of the test steel.A new type of nano-precipitated reinforced martensite/austenitic multi-phase steel with appropriate Mn content was developed to replace the material AISI4330.Scanning electron microscopy?SEM?,Physicochemical phase analysis,X-ray diffraction?XRD?,Electron backscatter diffraction?EBSD?and Transmission electron microscope?TEM?were used to observe the microstructure of test steels with different intercritical tempering temperature and the secondy phase precipitation was quantitatively calculated.Changes in material structure and mechanical properties are studied.Through studying the research of fatigue crack expansion behavior experiment and the stress corrosion slow strain rate stretch test on the mechanism of corrosion stress failure of the material,the mechanism of strengthening,toughening and stress corrosion of the Martensite/Austenite duplex phase steel.Solve the problem of quite short service life caused by ultra-high alternating internal pressure and fracture damage of Hydraulic end valve box.With intercritical tempering temperature increased,the yield strength of the test steel gradually decreased,and the tensile strength decreases and then increases,reaching the maximum at 600?.The tensile strength and yield strength were 1033MPa and 1006MPa,respectively.The plasticity and low temperature toughness of the reversed austenite increases and then decreases at room temperature with intercritical tempering temperature increased.When the tempering temperature was 620?,the plasticity and toughness were preferably,the elongation and shrinkage were 21.5%and 74%,and the impact energy at0?and-40?were 186J and 178J,respectively.When the tempering temperature is at 600?,in the range of intercritical temperature,the test steel shows good strength and toughness.With the increase of tempering temperature,the volume fraction of the reversed austenite increases and then decreases at room temperature.When the tempering temperature is below 620?,the microstructures are mainly thin-film,and the austenite grains generated during tempering were almost all retained to room temperature,showing good stability.When the tempering temperature is over 620?,during the cooling process,the bulk austenite content gradually increases,and part of the austenite turns into"fresh"martensite,which reduces the content of the stable elements C,Mn and Ni in the austenite,and resulting in a sharp decline in the stability of the reversed austenite.Test steel contains a lot of nano-sized precipitated,mainly the Cu and VC with FCC,and complex orthogonal lattice structure of the alloy cementite?Fe,Mn,Cr,Mo?₃C,mostly of which are oval shaped and long rod with size in 10-100 nm range.And with intercritical tempering temperature increased,VC and Cu precipitation phase increases gradually,and alloying cementite gradually dissolved.The results show that the precipitation strengthening tends to increase with the increase of tempering temperature.The strengthening quantity ranges from 300 MPa to 350MPa.By slow strain rate tensile in 3.5%NaCl solution and WOL stress corrosion experiments found 0.10CY-2 type complex phase steel surface shrinkage in the solution and extension rate is relatively high,stress corrosion sensitivity is low,stress corrosion crack growth rate is slow,the ability to resist crack is strong and stress corrosion critical stress intensity factor K_ISCC is high,stress corrosion resistance is the most excellent.From the morphology of crack tip,the corrosion mechanism is mainly anodic oxidation.Metastable restrained austenite is easy to induce strain strengthening due to its phase change induced plastic effect,weakens stress concentration,and hindering crack growth,improves the sensitivity of steel to stress corrosion.