Thermodynamic Discuss Of Divorced Eutectoid And Rapid Spheroidization Treatment Of GCr15 Steel

Bearing steel is widely used in industry,in which GCrl5,as the representative,occupies a large proportion.The spheroidizing treatment of bearing steel in the production process is to facilitate subsequent production and increase service life.However,the GCrl5 bearing steel traditional spheroidizing annealing process is time-consuming and laborious.Based on solid phase transformation theory and thermodynamic model,the thermodynamic theory of divorced eutectoid transformation is analyzed,and subsequent rapid spheroidization experiments are carried out.The main conclusions of this paper are as follows:(1)Calculations by Thermo-Calc software show that the carbide types of GCr15 steel are mainly(Fe.Cr)3C between 805? and 600?.Under slow cooling conditions,the carbide types of GCr15 steel are M7C3 at 600 to 543?and M3C2 below 543?.(2)The driving force of austenite decomposition transformation of bearing steel is calculated by using traditional thermodynamic model and super-component model.It is calculated that the driving force of direct spheroidization is 6.25 times that of flake pearlite spheroidization.It is theoretically possible to realize rapid spheroidization by means of divorced eutectoid transformation.Using the basic principle of solid phase transformation,two different thermodynamic equations of nucleation and growth of pearlite transformation were constructed.(3)Complete austenite + undissolved carbide structure can be obtained under austenitizing condition at 805? for 30 minutes.The microstructure obtained after quenching is martensite and residual carbide with dispersed distribution.The average Vickers hardness of the sample is 827.8HV,indicating that complete martensite and undissolved carbide can be obtained by austenitizing at 805 and there is no undissolved ferrite.According to the SEM photos,the volume fraction of residual carbides is 5%after processing by professional metallographic analysis and processing software.(4)Rapid spheroidization test:After austenitizing at 805? for 30 minutes,it is cooled to 745? and 715 ? respectively and quenched after holding for 90 minutes.The average Vickers hardness of carbides is 202 HV.It takes 5.1 hours from the beginning of heating up with furnace to obtaining fully spheroidized structure.