Studies On Nano Bainitic Microstructure Transformation Mechanism And Properties Of Laser Cladded Fe-based Coatings

The new generation ultra-high strength steel with low alloy addition and low production price have been developed recently,nano bainitic steel is a typical one in this new generation steel.The novel steel has characterizations of low transformation temperature through high carbon addition,and the thickness of the bainite plate can be as fine as less than100 nm,the carbide formation can be suppressed through high silicon content,and the final microstructure consist of nano structured bainitic ferrite and film austenite between the ferrite are obtained through the isothermal heat treatment.The characterization of the microstructure make this steel have excellent mechanical properties,the tensile strength can be as high as 2.5GPa,the ductility of 30%,the hardness of 670HV and ductility of 30⁴0MPa m^1/2,and this steel can be applied to the high strength structural components,high strength wear-resisting gears,and other relevant components on industries of the mining machinery,coastal engineering,automobile industry,military science and technology,etc.However,the application of this excellent steel was limited by the poor weldability and low transformation rate accompanied by the high carbon concentration.And the characterization of low heat input,small heat effect zone,thin thickness of single layer,and low thermal stress of the laser cladding can be applied to fabricate the components with complex structure directly through additive manufacturing,make the cracks can be avoiding,on the other hand the small prior austenite grain size of the coatings can provide more areas for the bainite initiate nucleation,which can accelerate the bainitic transformation process.Therefore,the shortcoming of nano bainitic steels of the poor weldability and low transformation rate can have solved by addictive manufacturing of laser cladding.The laser cladded Fe-based coatings were fabricated by 3.5 kW Rofin DL-035Q diode laser system equipped with a powder delivery system and an argon shielding gas device.The chemical composition of the feeding powders for the laser cladding was optimized designed through the thermodynamic analysis,high carbon and silicon are added,and a small addition of Co and Al(to enhance the driving force of the transformation from austenite to bainite?^→),and the final chemical composition of Fe-0.81C-1.51Si-1.93Mn-1.59Co-1.06Al-1.08Cr-0.32Mo are chosen based on the preliminary exploring experiments,the CCS-A steel plates were chosen as the substrate material.To guarantee the chemical composition of the coatings not diluted by the substrate materials,and obtained high quality metallurgical bonding between coatings and substrate,the orthogonal experiment parameters of laser power,powder feeding rate,and the scanning velocity were optimized to obtained the optimal parameter under each preheated temperatures.These optimal parameters have been used to obtained the coatings with low dilution rate,no pores and cracks,and the microstructures homogeneously distributed,no macro segregations and inclusions are observed,and good surface appearance.The substrate was preheated to specific temperatures before the cladding process（20℃ higher than temperature of isothermal heat treatment）,and the shielding argon gas of the coaxial and tailing were both used to prevent the oxidation during the laser cladding process.Then the samples were quenched at room temperature or isothermal heat treated at 150℃,200℃,250℃,300℃,350℃ by using the electric heating furnace.The laser cladding and subsequent isothermal heat treatment were used to fabricate the Fe-based coatings with nano structured bainite rapidly,because bainitic transformation can be accelerated by the fine austenite grain in laser cladded coatings;the nano bainitic microstructure evolutions with the isothermal temperatures and times were analyzed,combined with the analysis of the thermodynamic and kinetics to establish the thermodynamic and kinetic models of the nano bainitic transformation in the laser cladded coatings,and which can be used to predict the nano bainitic transformation process in the laser cladded coatings;the mechanical properties of the nano bainitic coatings were tested,to figure out the relationships between the microstructures and the properties.The optical microscope（OM）,field emission scanning electron microscope（FE-SEM）,transmission electron microscope（TEM）,X-Ray diffraction are applied to characterized and analyzed the microstructures of the coatings.The results show that pores and cracks are not observed in the coatings,which have low dilution rate and good surface appearance.The microstructures in direct quenched coatings are mixture of martensite and retained austenite.The microstructures obtained under 150℃ isothermal treatment are needle like martensite with nano scale and retained austenite.The isothermal transformed microstructure obtained at the range from 200℃ to 300℃ consists of bainite and retained austenite,the microstructure transformed at 350℃ consists of carbide free bainite with scale of micrometer,small amount of martensite and retained austenite.The thickness of the low temperature isothermal transformed bainitic ferrite can be as thin as 30⁵0 nm,the retained austenite films are even thinner（10³0 nm）which are separated by the bainitic ferrite.The morphology,scale,distribution and volume fraction of the retained austenite and the bainitic ferrite,which are determinant factors for the mechanical properties of the coatings obtained at different heat treatment conditions.The overall transformation process of the nano bainite in the laser cladded coatings during the isothermal holding are accelerated by comparing with the tradition heat treatment processing of the nano bainitic steel fabrication;especially at relatively lower temperature,the acceleration is more obviously.Therefore,considering the fine grains and the micro segregations in the interdendritic regions of laser cladded coatings;the dendrite growth and the solute elements distribution in the dendrites of the Fe-based coatings are analyzed,the thermodynamic model of the under cooling Fe-based coating phases transformation are established,and the driving force of the austenite to bainite transformation under different isothermal temperatures are predicted.Based on the nucleation theory by shear mechanism and the bainite growth theory,the morphology evolution,transformation rate and ratio of the nano bainite transformation are studied,the kinetic model by shearing mechanism is employed for the bainitic transformation analysis combine with the thermodynamic calculated results;the effects of the fine grain size（the relatively larger initial nucleation surface）,the autocatalysis nucleation phenomenon of the bainitic transformation,the carbon diffusion during the transformation etc.are considering,and there factors are also employed to modified the kinetic model.The comparison of the experimental and calculated results shows that,the modified model can have predicted the accelerated nano banitic transformation process in the laser cladded coatings during the isothermal transformation accurately.The subunits grow as lenticular shape and repeated pile up to grows to be bainite plates,these phenomena are founded and analyzed based on the observation of ultrafine microstructures of the nano bainite by using the SEM and TEM with high magnification.Combine with the space structures of the bainite growth,the lattice relationship between the bainite ferrite and austenite are analyzed,and the shear way through the Shockley partial dislocations are employed to explained the bainite sheaves growth in the laser cladded coatings.The characterizations of mechanical properties show that he micro hardness of the coatings can be as high as 680HV,the tensile strength can be as high as 2.28GPa and the ductility can be as high as 7.3%,the tensile fracture analysis shows that the fracture mode is ductile fracture criterion,the austenite distribute at the interdendritic regions and film austenite between the bainitic ferrite lathes is the determinant factor to maintain the high ductility.The indentation tests are used to test the ductility of the nano bainitic coatings,and the results shows that the bainitic coatings obtained at different isothermal temperatures have high ductility.The friction and wear behavior testing results show that the crocking resistance ability of coatings with nanostructured bainite obtained at relative low temperature is better than the high temperature transformed coatings.The high micro hardness and high rigidity of the nano bainite and high ductility of the retained austenite are the determined factors for the high wear resistance.