Carbon potential may be defined as that carbon content which a specimen of a carbon-steel foil acquires when equilibrium conditions have been achieved between the carbon potential of the carburising medium and the carbon content of the foil. Fig. 8.4 illustrates that surface carbon content may be less than the carbon potential of the atmosphere.
The activity of carbon in steel depends on its carbon content as well as the alloying elements in steel:
ac = (wt % C) Г …(8.1)
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where, Г is the activity coefficient, and ac is the activity of carbon and wt% C is the carbon present. The activity coefficient is decreased by the carbide forming elements like Cr, W, Mo, etc., but is increased by non-carbide forming elements like Ni, Si, etc. Thus, chromium containing steel foil, when in equilibrium with specific furnace atmosphere will take on more carbon than pure iron, i.e., the effective carbon potential of the atmosphere is increased i.e. the surface carbon content may even exceed the solubility limit of carbon in austenite at the given temperature.
Ni steels dissolve less carbon. The total effect is small in low alloy steels, but can be significant in high alloy steels, if proper balance between carbide forming and non-carbide forming elements is not there.
The effect on carbon potential can be calculated as:
log (correction factor) = 0.005 (% Si) + 0.014 (%Ni) -0.013 (%Mn) -0.04 (%Cr) -0.013 (Mo%) …(8.2)
Carbon Potential of a Carburising Atmosphere:
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The carbon potential of a carburising atmosphere at a given temperature is defined as the carbon content of pure iron that is in thermodynamic equilibrium with the atmosphere The carbon potential of the carburising atmosphere must be greater than the carbon potential of the surface of the work pieces as it is the difference in the carbon potentials that provides the driving force for the carburising.
Gas carburising is truly a non-equilibrium process, i.e., neither the gaseous constituents of the atmosphere are fully in equilibrium with one another, nor the atmosphere is with steel being carburised.
However, reactions discussed below approach equilibrium rapidly enough to permit predictions of the rate of carburising from the atmosphere composition. Thus, assuming all the gases present in the carburising atmosphere to be in equilibrium, consider the reaction-
where, C(Fe) is carbon dissolved at the surface of the steel. The equilibrium constant, K1, for the reaction (8.30) can be written as-
where, Pco and Pco2 are partial pressures of CO and CO2 respectively in the gas atmosphere; ac is the activity of carbon at the surface of steel. The constant K1 is related with Gibbs free energy for the reaction 8.30, Δ G° as-
where, R is the gas constant and T is the specified temperature. From the thermodynamic data, the constant K1 comes out to be a function of temperature only, and is given by-
Thus, the activity of carbon, ac can be obtained from equation (8.31) as-
Thus, at a given carburising temperature (which is used to get K1 from equation 8.33), if PCO and Pco2 in the carburising atmosphere are known, then the activity of carbon in the atmosphere or at the steel surface, ac can be calculated. Such carburising atmospheres contain almost constant amount of CO = 20%, and thus requires at a time the Pco2to get the value of ac, the carbon potential of the atmosphere.
The carburising atmosphere normally also contains H2 and H2O.
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Let us consider the following reaction, also called water-gas reaction:
CO + H2O Û CO2 + H2 …(8.35)
From the thermodynamic data, the equilibrium constant, K2, of this reaction as a function of temperature only is given as:
Thus, by measuring the partial pressure of oxygen in the atmosphere, the Pco/Pco2 or PH2/PH2o can be calculated, which gives the carbon potential of the carburising atmosphere.
Measurement of Carbon Potential:
Some of the common methods used to measure the carbon potential of the atmosphere are:
1. CO2 Measurement by Infrared Spectroscopy:
The equations 8.34 is rewritten;
The activity of carbon in atmosphere, or the carbon potential of the atmosphere can be obtained by knowing both Pco and Pco2 at a temperature. However, as said earlier, the carburising atmospheres contain about 20% CO, but the CO2 amount varies from 0 to 2%. Thus, any variation in CO2 content affects the carbon potential much more than the variation in CO content. Thus, the measurement of Pco2 sufficient to, estimate the carbon potential of the atmosphere.
Normally, Pco + Pco2 first estimated for a given carburising atmosphere. Then the carbon potential is measured or controlled by measurement of CO2 content of the atmosphere as and when required.
Infrared spectroscopy is a simple test to know CO2 content of the atmosphere. Fig. 8.13 illustrates the effect of CO2 content and the temperature on the carbon potential of the atmosphere for a given (Pco + Pco2). Fig- 8.14 illustrates at different temperatures, the effect of variation of the CO2 and the CO content on the carbon potential of the atmosphere.
The activity of carbon has been related with carbon content of austenite as:
In ac = In Yc + (9167 Yc + 5093)/T – 1.867 …(8.42)
where Yc = (4.65 w)/(100 – w), in which w in the weight percent carbon in austenite and Yc is the atom ratio of carbon in iron. Equation 8.31 and equation 8.42 can be combined to give a relationship between the carbon potential (that is, equilibrium carbon content in austenite) and the CO2 and CO contents. Actually, measuring CO2 content is sufficient to known the carbon potential. Fig. 8.15 relates the CO2 content and carbon potential for endothermic gas atmosphere derived from methane.
2. Dew Point Measurement:
Dew point is defined as the exact temperature, at a given pressure, at which a mixture of gases will begin to precipitate its moisture content. Dew point analyzer measures the partial pressure of water vapour in a furnace atmosphere. There is a relationship between water content, vol% and the dew point as illustrated in Fig. 8.16.
Consider reaction 8.27 which relates the water vapour content and the carbon potential of the atmosphere. The activity of carbon is thus,
The hydrogen content of furnace atmosphere remains almost constant (≈ 40%). The water content is obtained by determining its dew point. The equation relating the dew point in °C and PH2O atmosphere is:
The equations 8.42, 8.43 and 8.44 result in Fig. 8.17 for atmosphere based on methane.
3. Oxygen Probe:
The use of oxygen probe to measure carbon potential of the furnace atmosphere has become most common as it is fitted in the furnace through a hole with almost instant response, though ceramic probe element must be replaced after a year of its use. The carbon potential of an atmosphere can be calculated from the oxygen partial pressure measurements. The oxygen partial pressure in carburising atmosphere is approximately 10-14 to 10-20 Pa (10-19 to 10-25 atm).
The voltage output of a zirconia oxygen probe with air as reference gas is related with oxygen partial pressure (Po2) and absolute temperature (T) as:
where, emf is in volts. Equations 8.39, 8.42, 8.45 are combined to give the graph in Fig. 8.18 which relates carbon potential and emf. Solid electrolyte oxygen probe can be used to measure Po2.
4. Hot Wire Test:
In this test, a thin wire of pure iron is kept in (he carburising atmosphere in the furnace for a time sufficient to homogeneously carburise it. The analysis of this wire for carbon content gives the carbon potential of the atmosphere. This method could also be used to estimate the carbon potential of liquid carburising baths. Hot wire test, thus, is the most direct method for estimating the carbon potential of the atmosphere.
In the modified but almost instant method, an analyser continuously measures the carbon potential by measuring the electrical resistance of (his wire inserted in work-chamber of the furnace.
The resistance of (he wire varies directly with its carbon content, i.e., a continuous record is obtained directly in carbon percent as carburising progresses. Analyzer is suitable for 0.15 to 1.15% carbon over a temperature range of 790 to 1015°C.
In another simple method “analysis of Shim stock” can be used effectively to estimate carbon potential of an atmosphere. A strip of annealed 1008 or 1010 steel, 10-30 mm wide, 75-160 mm long and 0.10-0.25 mm thick is cleaned in solvent and weighed before and after through carburising.
Analytical balance is used to obtain gain in weight, and thus, the carbon of the atmosphere:
where, c is carbon potential; CO is original carbon % in shim stock; Wf is the final weight of shim stock and Wi is the initial weight. This method determines the maximum carbon available in the atmosphere for batch or continuous furnace.
Generally, the atmosphere carbon so determined is slightly higher than the surface carbon of parts of most steels; exception is with steels which contain carbide forming elements. As alloying elements present form surface carbides, thus, steel has higher surface carbon than that obtained by shim stock test.