Investigation On The Microstructures And Mechanical Properties Of Medium Carbon Carbide-free Bainitic Steels

In this dissertation,the object of research is medium-carbon bainitic steels.The effect of microstructutre evolution and chemical composition partitioning on the conventional mechanical properties,wear properties and fatigue properties has been explored.Heat-treatment processes to obtain carbide-free upper bainite,low bainite and low-temperature bainite in the 34 MnSiCrAlNiMo medium-carbon steel were explored.When transformation temperature,T>Ms + 75 °C,upper bainite consisted of catenary bainitic ferrite and blocky retained austenite is obtained in the steel.When Ms + 10 °C < T <Ms + 75 °C,lower bainite is the main morphology composed of lath-like bainitic ferrite and flake-like retained austenite.When T<Ms + 10 °C,the lower bainite,also known as low-temperature bainite,is obtained,which contains much thinner lath-like bainitic ferrite and film-like retained austenite.Mechanical testing results show that the lower the transformation temperature is,the better comprehensive performance is.The cyclic deformation behavior of low-temperature bainite,lower bainite and upper bainite obtained in the 34 MnSiCrAlNiMo carbide-free baintic steel has been examined through low-cycle fatigue testing.Low-temperature bainite exhibits a higher fatigue life under total strain amplitudes,which is attributed to its high strength and larger high-angle misorientation fraction.Under low-plastic strain amplitude,the high ability to coordinate deformation and uniform elongation leads to a prolonged fatigue life.Furthermore,phase components dominate the fatigue life under high-plastic strain amplitude.Higher stable film-like retained austenite is beneficial to arrest crack propagation.Blocky retained austenite easily transforms to martensite,which causes higher compatible deformation capability.The transformation kinetics of carbide-free bainitic steel containing Si+Al and the carbide-bearing bainitic steel without Si+Al elements were studied.The carbide-free bainitic steel requires a long incubation period because Si and Al hinder C diffusion,making the distribution of carbon atom more homogeneous.The carbide-bearing bainitic steel exhibits a long transformation time because of a large amount of transformation content and lower driving force.The distribution of carbon atom is extremely heterogeneous.The carbide-free upper and lower bainitic microstructures obtained in the steel with Si + Al mainly consist of bainitic ferrite and retained austenite.Carbide-bearing upper and lower bainitic microstructures obtained in the steel without Si + Al consist of bainitic ferrite,carbide and trace amounts of retained austenite.The carbide-free bainite exhibits higher strength and toughness than carbide-bearing bainite,especially the toughness.The solid solution strengthening caused by Si and Al is the important factor that leads to higher hardness and strength for carbide-free lower bainite with Si+Al.And the retained austenite plays an active role in improving tougthness.Phase transformation testing results show that the steel without Mn exhibits short transformation time and short incubation period.When Mn content increases from 1.8% to 3.2%,bainite transformation time is prolonged,especially incubation period.The microstructure of the steel without Mn is mainly grain boundary allotriomorphic ferrite,while the other three steels with Mn consist of bainitic ferrite plate and retained austenite.Mn is an essential element used to obtain lower bainite.The Coffin–Manson formula and damage hysteresis model are then used to evaluate the fatigue performances of the steels.Higher elongation of the steel without Mn is beneficial to improve fatigue life under plastic strain amplitudes.The steel with 2.3% Mn exhibits higher fatigue damage capacity,which attributes to the largest distribution of high-angle misorientation of in the bainitic steel with 2.3% Mn.The relationship between the wear and fatigue properties and microstructures with carbide or without carbide has been studied.Results show that under lower wear loading,carbide-bearing lower bainite(LB)exhibits higher wear resistance.Under higher wear loading,carbide-free LB exhibits higher wear resistance,which results from the improved surface hardness due to straininduced martensitic transformation from the retained austenite.Carbide-free lower bainite exhibits higher strength,plasticity,and toughness than carbide-bearing lower bainite,and the former also exhibits longer fatigue life under total strain amplitude,the latter exhibits longer fatigue life under plastic strain amplitude.The metastable retained austenite of large size in the carbide-free lower bainite obviously undergoes strain-induced martensitic transformation during deformation.Martensitic transformation delays crack propagation by absorbing the energy required for crack propagation,thus increasing the fatigue life of carbide-free lower bainite under total strain amplitude.The stable carbide and the coarse bainite ferrite plates play positive roles for the fatigue life under plastic strain amplitude,while the fine bainite ferrite plates and metastable retained austenite are negative.The stable secondary carbide phase strengthens carbide-bearing lower bainite under uniaxial tensile deformation but may shorten fatigue life under cyclic fatigue by acting as locations of stress concentration.The microstructure evolution of carbide-free bainitic steels in the process of tensile tested has been studied.It shows that the retained austenite in the microtructure is divided into three sizes.At the beginning of plastic deformation,massive blocky austenite takes TRIP firstly;successive the flake-like and the film-like retained austenite occurs martensitic transformation.The mechanism of retained austenite on ductility and toughness enhancement can be summarized as three effects during deformation: TRIP effect,blocks crack propagation(BCP)effect and dislocation absorption by RA(DARA)effect.Such a TRIP effect avoids the formation of cracks and BCP effect block further propagation of cracks.The DARA effect enhances the harmonious deformation ability of bainite /martenite /retained austenite.