# Surface Engineering of Steels: Understanding Carburizing

Carburizing is a widely used, effective technique to increase surface hardness of steel used in gears, and achieve a compressive residual stress. There are several methods, and hotter isn’t always better.

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Carburizing is the addition of carbon to the surface of low carbon steels. It is generally accomplished at temperatures between 850-1,000°C. Once quenched, the high-carbon surface layer yields a high hardness martensitic case with excellent wear and fatigue resistance [1]. This carburized case surrounds the tough, low carbon steel core. The case hardness is primarily a function of the carbon content. There is little advantage increasing the carbon content beyond 0.65% to increase hardness. Higher carbon content can increase microstructural properties such as wear, sliding contact fatigue, and rolling contact fatigue [2]. Too high carbon content can result in excessive carbide networks or massive carbides.

The case depth is a function of time temperature and chemistry, of the process, and the available carbon (carbon potential) at the surface of the steel. This follows Fick’s Second Law of Diffusion for the concentration of a diffusing species as a function of time, t, and position C(x,t):

Equation 1

The solution to this differential equation is:

Equation 2

Where C(x,t) is the concentration of carbon (in the case of carburizing) at position x at time t;C0 is the initial carbon content in the steel, Cs is the carbon potential of the atmosphere at the surface of the part, and D is the diffusion of carbon in austenite. The function erf(z) is called the error function and is often encountered in diffusion and heat transfer calculations. The diffusion coefficient of carbon in austenite is a function of carbon content and temperature [3]:

Equation 3

Where D is in cm2/s, C is the weight percent carbon, T is °K and R is the gas constant (1.99 cal/mol•K). Using this method, the depth of carburizing can be predicted (Figure 1, Figure 2 and Figure 3).

The diffusion coefficient is also dependent on the alloy content, with increased alloy content tending to decrease the diffusion of carbon. As can be seen, increasing the temperature offers significant savings in time. However, increasing the temperature also costs more in alloy and fixture usage, as well as increased cost in furnace operation.

Presently, the most common method is gas carburizing. In this method, the parts are surrounded by a carbon-rich atmosphere, which supplies the carbon to the part. In most cases, this is an endothermic atmosphere, which contains approximately 40% hydrogen, 20% carbon monoxide, 0.5% carbon dioxide and the balance nitrogen. Carbon is supplied to the part surface by the reaction:

Equation 4

The carbon potential of the atmosphere is related to the relative partial pressures of CO and CO2 in the atmosphere:

Equation 5

where K is the equilibrium constant.

Carburizing is performed industrially by heating parts up to the carburizing temperature, at the desired carbon potential (typically 0.8% C). The part is then quenched and tempered. In some cases, for very deep case depths, the part is carburized at a very high temperature (927°C and above) to shorten process times. The part is then cooled slowly to ambient temperatures, and then reheated to an intermediate temperature (815-871°C) to allow the carbon to diffuse to obtain the desired carbon profile (Figure 4).

After soaking, the part is then quenched and tempered. This approach has the advantages of a final austenite grain size that is smaller, improving ductility. The low reheat temperature places an upper limit on the amount of carbon dissolved in the austenite, and reduces the amount of retained austenite in the case. The primary disadvantage of this method is the potential for increased distortion. Often, parts processed by this method are quenched in a press to reduce distortion.

For lighter case depths found in automotive applications (<0.6 mm), an alternative process was developed called the boost-diffuse technique. In this method, the part is introduced to the furnace at the carburizing temperature, with the carbon potential set near the saturation point of carbon in austenite for a short period of time. This brings the surface carbon well above the desired surface carbon of 0.8%. This is called the boost phase. The carbon potential is then dropped to the desired surface carbon potential of 0.8%. The carbon is then allowed to diffuse to the desired case depth. This method can reduce the carburization time by 20%. After carburizing, the part is then quenched and tempered. However, one common assumption is “more is better.” Often, heat treaters will use carbon potentials greater than the saturation point of carbon in austenite in the mistaken perception that the carbon will be driven into the part faster. However, the steel cannot absorb carbon into the steel above the saturation point. The excess carbon then generates soot. This soot is carried into the oil, and contaminates the furnace. At this point, the furnace must be burned out to eliminate the soot. A comparison of the different methods is shown in Figure 5.

Carbonitriding introduces both carbon and nitrogen into the austenite during processing. The presence of nitrogen in the case improves hardenability, and increases the hardness of the case. This is often used in low-carbon steel to achieve surface hardnesses equivalent to alloy grades without resorting to severe quenching, resulting in lower distortion. Case hardnesses of HRC 65+ are often achieved.

Nitrogen is an austenite stabilizer and can result in increased retained austenite formation. Dissociated ammonia is used as a nitrogen source, and is added to the furnace atmosphere. Increased furnace maintenance can be a disadvantage of carbonitriding.

Vacuum carburizing is a relatively new method that is similar to normal carburizing, but uses a vacuum and a hydrocarbon as the carburizing medium. In this method, the part is heated to the carburizing temperature under a vacuum. Once the parts are at temperature, a hydrocarbon is injected into the chamber. The hydrocarbon used varies by furnace manufacturer, and is typically methane (CH4), propane (C3H8), acetylene (C2H2), or cyclohexane (C6H12). Mixtures of these gases are also used.

Methane was a first choice for vacuum carburizing, since it is readily available, and used in gas carburizing. However, methane is stable at elevated temperatures, and requires the furnace to operate at pressures of 250-400 torr. Sooting is a problem at these higher pressures.

Propane has also been used successfully. It is less stable than methane in a vacuum, and readily breaks apart at temperature, producing carbon for carburizing, and hydrogen. There is a lower tendency of sooting using propane than methane. Typical pressures of 20-30 torr are often used.

Unsaturated hydrocarbons such as acetylene have weak bonds and are reactive at room temperature. This high reactivity results in a gas that carburizes easily; however, it also can produce soot readily. To overcome this issue, very low pressures, similar to propane, are used.

Napthene hydrocarbons such as cyclohexane are saturated with hydrogen. This means that each of the carbon atoms is bonded to two hydrogen atoms and two carbon atoms. The carbon atoms form a cyclic ring of six carbon atoms. Cyclohexane is a liquid at room temperature, and readily available. Once heated, the parts act as catalysts and the carbon is released at the part surface. Since the vapor pressure of cyclohexane is on the order of 80-170 torr, operating pressures below this pressure ensures that the liquid cyclohexane vaporizes inside the furnace and remains a vapor until contacting the surface of the part.

Operating temperatures for vacuum carburizing can be much higher. This requires quality steels to reduce grain growth. The higher temperatures enable much faster carburizing times than atmosphere carburizing to achieve similar depths. Often, better carbon profiles in the roots of gear teeth are obtained with vacuum carburizing. Since there is no oxygen present, intergranular oxidation does not occur.

Carburizing is a widely used, effective technique to increase surface hardness of steel used in gears, and achieve a compressive residual stress. This contributes to long life, and cost-effective production.

### References

1. ASM International, “Introduction to Steel Heat Treatment,” in Steel Heat Treating Fundamentals and Processes, vol. 4A, J. Dossett and G. E. Totten, Eds., Materials Park, OH: ASM International, 2013, pp. 3-25.
2. Metals Handbook, Metals Park, OH: ASM, 1981.
3. G. L. Tibbetts, J. Appl. Phys, vol. 51, pp. 4813-4816, 1980.
4. J. I. Goldstein and A. E. Moren, “Diffusion Modeling of the Carburization Process,” Met. Trans. A, vol. 9A, no. 11, pp. 1515-1525, 1978.

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is a senior research scientist of metallurgy at Houghton International, Inc., a global metalworking fluids supplier based in Valley Forge, Pennsylvania. Go online to www.houghtonintl.com.