Thermodynamic Model Of Potential Factor And Its Validation For Spheroidizing Annealing Of Cold Heading Steelin Controlled Atmospheres

Avoiding decarburization in spheroidizing annealing is an important measure to ensure the fatigue strength of cold heading forming parts.An endothermic controlled atmospheres furnaces（STC furnace）are used for spheroidizing annealing of cold heading steel wire rod in a factory.During operation,the value ofx²_CO/x_CO2in the mixture of CO-H₂-N₂-CO₂in the furnace is usually controlled by dynamic regulation to avoid decarburization.The value ofx²_CO/x_CO2depends on the carbon potential factor（CPF）,which is a material characteristic parameter.CPF should be pre-determined and input into the computer of STC furnace before the furnace started by operator.Thus,it is of great significance to build a computational model in spheroidizing annealing to accurately calculate the CPF value based on the material and annealing process parameters.In this work,a CPF model has been established and validated based on the material thermodynamics theory and experiment data.Combining with the furnace atmosphere control principle and the theory of thermodynamics,the fundamental calculation of CPF can be defined as“CPF=100×K×a_C^Φ”,k is reaction’s balance constant,a_C^Φis carbon activity.The reaction balance constant Kwas obtained by two ways:the first one is from the empirical equations extracted from experimental measurement of theC（Φ）+CO₂（g）=2CO（g）at different temperatures,the second one is from the combination of empirical equations of the balanceconstantsoftworeactionsC（graphite）+CO₂（g）=2CO（g）and C（Φ）=C（graphite）in accordance withΓecc laws.Carbon activitya_C^Φwas calculated and verified by ferrite solid solution carbon and austenite solid solution carbon,respectively.The verification and validation of CPF model has been carried out with the consideration of two factors.Firstly,solid-dissolved carbon activity ofγin two-phase area ofα+γrange,that is,graphite as the standard state when the solid solution carbon activity value should not be greater than 1.Secondly,solid-dissolved carbon concentration of^γin two-phase area ofα+γis determined by the GS line of the Fe-Fe₃Cphase diagram.However,the GS line of cold heading steel in actually is different from the ideal GS line of theFe-Fe₃Cphase and varies with the chemical composition of steel,therefore each calculation of CPF shall be based on the reconstructed GS line in the phase diagram.In order to demonstrate the practicality of CPF model,the effects of four common alloy elements（Si,Mn,Cr,Mo）have been investigated.The value of CPF at different temperature for both ML35 and ML35CrMo cold heading steel was calculated using the established carbon potential factor model and the phase diagram reconstruction method.The comparison between the calculated values and those from the actual heat treatment process shows good agreement,indicating that the CPF model can be applied directly to engineering practice.