The heat treater might be surprised to learn that in order to understand how various heat-treatment processes create a particular microstructure and its corresponding mechanical properties, it is necessary to have a fundamental understanding of both thermodynamics and kinetics (in this case involving solid-state phase transformations as a function of temperature and composition).^[1] Furthermore, when one wants to understand the fundamentals of processes such as gas carburizing or nitriding, we must also employ these principles. Let’s learn more.

Thermodynamics is that branch of science concerned with the relations between heat and all other forms of energy (e.g., mechanical, electrical or chemical). By contrast, kinetics is the study of reaction rates – the forces acting on various mechanisms (e.g., chemical, physical).

Thermodynamics is Related to Stability^[2]

Thermodynamics is all about how stable materials are in one state as opposed to another rather than about things moving or changing. In other words, thermodynamics has nothing to do with time. One must also be careful not to confuse thermodynamic quantities (e.g., Gibbs free energy of a chemical reaction) with kinetic ones (e.g., activation energy of a chemical reaction).

Thermodynamics can tell you only that a reaction should proceed to completion because the chemical products (i.e., what is formed in a chemical reaction) are more stable and thus have a lower free energy than the reactants (the substance that undergoes change in a chemical reaction). Another, albeit more scientific, way of saying this is that the reaction has a negative free energy.

The change in free energy (∆G) is negative. Therefore, the reaction is spontaneous. The formula ∆G = - RTlnK says all this at once, where R is the gas constant, T is absolute temperature in degree Kelvin and K is the equilibrium constant. Still another way of saying this is that the reaction has a large equilibrium constant, signifying that if nature could ever attain equilibrium, there would be many more products present than there are reactants; the products are at a lower free energy, or more stable, than the reactants.

Because of this, the reactants want to be converted into the products. For example, graphite and diamond are both forms of carbon, but graphite has a lower free energy. Therefore, diamond wants to convert into graphite. Another example is that your skin wants to dissolve in the soap when it is washed. In this case, the products of the dissolution reaction (namely, your skin dissolved in the soap) are more stable than the reactants (your undissolved skin and the soap separately).^[2]

The above examples were chosen specifically because even though the reaction should proceed (since the products are more stable than the reactant), it does not. Your hands are safe when you wash them as is the diamond on a woman’s finger. Even though the reaction is thermodynamically favorable, it is slow. It is far too difficult to get the diamond to break all of its bonds and re-form them to a different, more-stable graphite configuration. Even though the reaction should go thermodynamically, it does not because it is kinetically unfavorable.

Kinetics is Related to Reactivity^[2]

Kinetics is about how quickly or slowly species react. Kinetics can tell you how fast the reaction will go but, as seen above, doesn’t tell you anything about the final state of things once it gets there. Reactions occur at many different rates. For example, compare a geological change with that of combustion in a burner.

The rate constant, k, measures how fast a chemical reaction reaches equilibrium assuming the reactants were supplied with enough activation energy to enable the reaction to proceed in the forward direction – reactants to products. This requirement for input of energy symbolizes the fact that the reactants are unreactive under certain conditions. The reaction must have some form of energy input before it can proceed; otherwise, the reactants cannot cross the activation-energy threshold and convert to products. The reaction is activated by energy supplied to the reactants by different energy sources. The rate of reaction, the rate constant and the kinetic energy required for activation of reaction indicate how fast the reaction reaches equilibrium.  

One can compare the equilibrium constant (K) of a reaction with the rate constant (k), which tells you the rate of an elementary step in the reaction mechanism (Table 1).

Heat-Treat Example^[3]

Kinetics of mass transfer at the gas-steel interface for carbon diffusion in steel is an example of research in recent years that was dependent on a fundamental understanding of thermodynamics and kinetics. It has allowed us to develop a fundamental understanding of the mass transfer during the gas-carburizing process and included modeling of the thermodynamics of the carburizing atmosphere with various enriching gases. The models accurately predict: the atmosphere gas composition during the enriching stage of carburizing; the kinetics of carbon transfer at the gas-steel surfaces; and the carbon diffusion coefficient in steel for various process conditions and steel alloying.

From a thermodynamic standpoint, the generation of the carburizing atmosphere in gas-carburizing atmosphere reactions is a rather complex process involving the interaction of numerous gases. It has been estimated that nearly 180 chemical reactions occur simultaneously in the carburizing atmosphere, among which only the following three reactions are important and determine the rate of carbon transfer from the carburizing atmosphere to the steel surface:

(1) 2CO → C_(γ-Fe) + CO₂

(2) CH₄ → C_(γ-Fe) + 2H₂

(3) CO + H₂ = C_(γ-Fe) + H₂O

While carburizing most rapidly proceeds by CO-molecule decomposition, the by-products of the carburizing reactions (CO₂ and H₂O) act as decarburizing agents. The presence of CO₂, even in small quantities, requires a high CO concentration to balance this decarburizing action. Therefore, these decarburizing species must be reduced for the process to proceed further. Generally, the maximum amount of CO₂ that is tolerated at a particular carburizing temperature without causing decarburization can be determined based on thermodynamic calculations.         

Since carburizing with endothermic gas only is practically inefficient and requires large flow rates, the endothermic carrier gas is enriched by blending with an additional hydrocarbon gas. The purpose of the enriching gas is to react with CO₂ and H₂O, thus reducing their concentration and producing more CO and H₂ as reaction products by:

(4) CH₄ + CO₂ = 2CO + 2H₂

(5) CH₄ + H₂O = CO + 3H₂

Although enriching reactions 4 and 5 are slow and do not approach equilibrium, the effectiveness of the carburizing process is determined by the atmosphere carbon potential and controlled by the ratio of CO/CO₂ and H₂/H₂O components in the heterogeneous water-gas reaction:

(6) CO + H₂O = CO₂ + H₂

Summary

It is a mistake for all of us not to have a basic understanding of the role of thermodynamics and kinetics in heat treatment. They are two words that belong in every heat-treater’s lexicon.

References

1. Shama, Romesh C., Principles of Heat Treatment of Steel, New Age International (P) Limited, 1996
2. Chem 32, Stanford University (www.stanford.edu)
3. Karabelchtchikova, Olga, “Fundamentals of Mass Transfer in Gas Carburizing,” PhD dissertation, November 2007