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CONTROL of DECARBURIZATION of STEEL Paul Shefsiek

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CONTROL OF DECARBURIZATION OF STEEL Paul Shefsiek Introduction Historically, heating steel for forming, forging or rolling, was done in electrical resistance or natural gas heated furnaces. It was inevitable that these furnaces contained Oxygen and Decarburization of the steel surface occurred. This decarburization was either ignored or minimized by coating the steel with “ stopoff “. Also, this decarburization was minimized through the use nitrogen atmosphere furnaces. But, decarburization could not be reduced to acceptable limits until the Chemical Potential of the Carbon in the furnace atmosphere matched the dissolved Carbon in the steel. The Furnace Industry that provides Equipment for the processing of steel, because of the temperature ranges involved, divided itself into two categories, namely, Reheat and Heat Treating. Each has its own special Technology. But, because of this division, quite often one division does not know the Technology of the other. In some cases this has been unfortunate. If the Reheat side of the Industry had the Carburizing Technology of the Heat Treating side, this Technology could have been applied to applications where preventing Decarburizing was a requirement. Therefore, the purpose of the following discussion is to separate out that part of Carburizing Technology that is applicable to preventing Decarburization in Reheat applications. Fundamentals Control of the Decarburization of Steel has been standardized to the monitoring of - The Metal Temperature (Furnace Temperature) - The Atmosphere Concentration of Carbon Monoxide (CO) - The Atmosphere Concentration of Carbon Dioxide (CO2). Knowing the values of these three (3) parameters and the knowledge of - Saturated Austenite in Iron - The Alloy Content of the Steel - The Equilibrium Constant for the Controllable Chemical Reaction between the Steel and the Gas Atmosphere it can be determined if the Atmosphere will prevent Decarburization of the Steel. The Reversible Controllable Reactions that are present when Methane ( CH4 ) is the Hydrocarbon added to control the Carbon content of the Atmosphere are 2CO = ( C ) + CO2 --------------------------------- ( 1 ) CH4 = ( C ) + 2H2 --------------------------------- ( 2 ) CO + H2O = CO2 + H2 --------------------------------- ( 3 ) where ( C ) is the Carbon content of Saturated Austenite at Metal Temperature ( T ). Reactions ( 1 ) and ( 2 ) indicate the addition of Carbon to Steel from Carbon Monoxide and Methane to form a solution of Carbon in Austenite and also to form Carbon Dioxide and Hydrogen, respectively . Both are reversible and the Gas Concentrations required to maintain Equilibrium with a particular Surface Carbon Concentration at a definite Temperature can be calculated from thermochemical data. Reaction ( 3 ) results from the presence of H2 due to reaction ( 2 ). Equation ( 1 ) represents the reaction between the Atmosphere and the Steel and through the thermochemical equilibrium data and knowing the Concentrations of the reactants, the Carbon Concentration - “Carbon Potential” – can be determined. The Equilibrium Constant for reaction ( 1 ) is as follows: K = (Partial Pressure of CO) 2 -------------------- ( 4 ) (Partial Pressure of CO2) and Log10 K = - 15,966 + 9.060 ---------------------- ( 5 ) T ( R ) where T ( R ) = T ( in deg F + 460.) When using Equation ( 4 ) and ( 5 ) to determine the Carbon Potential of the Furnace Atmosphere, it is necessary to define an Activity Function [ A ] as the percentage of Saturated Austenite at Temperature T ( R ), that is, [ A ] = % Carbon in the Atmosphere ----------- ( 6 ) % Carbon in Saturated Austenite and [ A ] must be incorporated in Equations ( 4 ) and ( 5 ) for Carbon Potential values less than Saturated Austenite as shown in Equation ( 6 ). K = 1 ( P.P. of CO ) 2 -------------------------- ( 7 ) [ A ] (P.P. of CO2 ) This is a straight-line approximation of Carbon Activity, but this assumption does not produce any significant error in the evaluation of Carbon Potential. A & B Curve fitting of published data for Saturated Austenite in Iron at different Temperatures yields within 1 % accuracy the following relationship for Saturated Austenite : A Log 10 ( S.A.) = 1 - 1950 -------------------------- ( 8 ) T ( R ) And, the following Equation gives the relationship between the Carbon in the Alloy and the Carbon Potential of an Equilibrium Atmosphere based on the Iron – Carbon SystemC: FE Log 10 ( % C ) = + % Si (0.058) + % Al(0.0394) ( % C A ) + % Ni(0.012) - % W(0.014) - % Mn(0.018) - % Mo(0.0176) - % V(0.123) - % Ti(0.194) - % Cr(0.038) ------ ( 9 ) where % C A = Carbon in the Alloy % C FE = Carbon Potential of the Atmosphere based on the Iron-Carbon System Application The use of these Equations will be illustrated for M1 Steel having the composition as follows: C – 0.85 %; Si – 0.30 %; Mn – 0.40%; Cr – 4.50%; Mo – 9.00%; W –1.30%; V – 1.75% Applying this composition to Equation ( 9 ), yields, that the Carbon Potential of a atmosphere that would be in equilibrium with M1 steel based on the Iron - Carbon System, is 0.372 %. Knowing this value, Equation ( 6 ) & ( 8 ) will yield the Activity [A] as a function of Temperature. Equation ( 5 ) gives the Value of K as a function of Temperature. Combining the results of Equation ( 5 ) & ( 6 ) with Equation ( 7 ) yields a relationship 2 that gives a value for the [ (CO) / (CO2) ] Ratio as a function of Temperature for M1 steel. This Ratio is called PDR -- Prevent Decarb Ratio The result of this evaluation is shown in Figure 1. Figure 1. is a plot of the Prevent Decarb Ratio vs. Temperature for M1 steel. Also shown in Figure 1. is a plot of the required Carbon Monoxide Percentage( % CO ) vs. Temperature for M1 steel if the concentration of the Carbon Dioxide ( CO2 ) has a value of 0.01 %, a practical lower limit control value. Conclusion A method for the Prevention of Decarburization of steel during the Reheat process for Forging and Rolling has been presented. It incorporates the accepted procedure for controlling the Carbon Potential of the Furnace Atmosphere and the relationship between Carbon in Alloy Steel and the Iron – Carbon System. References A. Metals Handbook, Vol. 2 , 8th Edition, pp 113,114, (1964) American Society for Metals B. Shefsiek,et.al., U.S. Patent, No. 4,228, 062 (1981) C. Moiseev,et.al.,” Thermodynamic Activity of Carbon in Recarburizing” Central Scientific-Research Institute of Metallurgy, Translated from Metallovedenie i Termicheskaya Obrabotka, No.1, pp 21-26, Jan, 1974, UDC 536.777:669.784:699.14.018.298 .
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