Microstructure And Mesothermal Thermal Stability Of Moderate Temperature Austempered Phase Transformation Products In 9SiCr Steel

As a new type of high strength steel,the bundle-like bainite steel has excellent strength and toughness coordination which has broad prospects in the field of bridge cables.The higher the proportion of bundle-like bainite in steel,the better the mechanical properties of steel.How to increase the proportion of bunched bainite structure in steel has become the focus of research.The phase transition mode of the bundle-like bainite is like martensitic phase transition,which makes it worth considering whether the bundle-like bainite steel should be tempered before being used in engineering.Especially the bridge cable steel needs to be galvanized,and this process is equivalent to tempering the steel,this means that the tempering of the bundle-like bainitic steel is very necessary.This paper studied the conditions for promoting the transformation of bundle-like bainite and the tempering stability of bundle-like bainite at medium temperature,systematically analyzed the influence of austenitization conditions on the bundle-like bainite structure in 9SiCr steel and the change rule of the bundle-like bainite properties during tempering,and identified the influencing factors of microstructure and properties.The microstructure analysis results of one-step austenitized austempering of9 Si Cr steel show that with the increase of austenitizing temperature,the proportion of the bundle-like bainite is higher,and the amount of the bundle-like bainite is the most at 950 ℃,the tensile strength is 1938 MPa.In order to further increase the proportion of the bundle-like bainite in 9SiCr steel,holding at 850 ℃ for different time and austempering after austenitizing at 950 ℃ was carried out.The experimental results show that with the extension of the second step of austenitizing holding time,the proportion of the bundle-like bainite increased first and then decreased,and the hardness and tensile strength increased first and then decreased.When the holding time of the second step austenitizing was extended to 15 min,the bundle-like bainite occupied the highest proportion in the steel,the strength and tensile strength are up to66.7 HR and 2150 MPa.After tempering the material with the most bundle-like bainite after two-step austenization,it was found that the microstructure of the bundle-like bainite changed slightly.The oversaturated carbon atoms in the bainite strip were diffused to the residual austenite film and each subunit,the bainite strip was slightly coarsened,the residual austenite was obviously rich in carbon,and the hardness and tensile strength of the experimental steel decreased to a certain extent.With the increase of tempering temperature and the extension of tempering time,the change of microstructure was more and more obvious,and the decrease of mechanical properties was obviously increased.Tempered at 300 ℃ for 30 min,hardness decreased by 18.3%,tensile strength decreased by 11.6%,the smallest decline.The hardness and tensile strength decreased by 29.7% and 16.7% for 1 h tempering at 300 ℃,which significantly increased compared with 30 min tempering.The hardness and tensile strength decreased by 25.8% and 15.8% respectively at 400 ℃ for 30 min,which is greater than that at 300 ℃ for 30 min.