Today’s low-pressure carburizing process, like many thermal-treating processes, demands an atmosphere that is highly reproducible and highly controllable. The right carbon source for this process has been available for use since well before the vacuum heat-treating process was developed.

Fig. 1. Acetylene's molecular structure


Acetylene was accidently produced in a lab in 1836 by English chemist Edmund Davy while looking for a way to isolate potassium metal. What Dr. Davy discovered along the way was a garlic-smelling, hydrocarbon gas with three distinctive properties:
  • The gas burned with a highly luminous flame
  • The gas burned at an extremely high temperature
  • The gas dissociated very quickly at atmospheric pressure
Acetylene became a commercialized product in the 1860s when the process of producing large quantities of acetylene through a reaction of calcium carbide with water – still in use today – was invented by German chemist Friedrich Wohler. The gas took another step toward widespread commercial use in 1896 when two French scientists found a way to safely transport this highly reactive gas by first dissolving it in acetone. Since then, acetylene has found its highly luminescent flame suitable as a light source for miner’s lamps and automobile headlights. Acetylene’s high flame temperature was put to use cutting and welding steel. However, the newest use of acetylene was surely not anticipated by Dr. Davy almost 175 years ago. Today, the third property of acetylene – its tendency to dissociate completely into atomic carbon and molecular hydrogen – makes it an excellent carbon source for low-pressure carburizing.

What is Acetylene?

At ambient temperatures and under pressure, acetylene is a hydrocarbon gas with the chemical formula C2H2. However, acetylene gas readily dissociates when it comes into contact with metal surfaces at low pressure and high temperature. A triple bond connecting the two carbon atoms is broken, leaving carbon and hydrogen as the products. Figure 2 illustrates the basic dissociation mechanism for acetylene on steel surfaces.

Acetylene is a lighter-than-air gas used both for its ability to produce heat and its tendency to dissociate into its component molecules of C and H2. Acetylene’s flame temperature of 5600°F is close to that of an electric arc. Acetylene is safely used for many applications in industry today through implementation of NFPA guidelines covering fuel gases.

Acetylene Dissociation Mechanism

The hydrocarbon dissociation reaction is very significant to the low-pressure carburizing process.

(Fe) + CxHy ® Fe (C) + y/2 H2

Although all hydrocarbons decompose rapidly in a furnace atmosphere above 800°C and in contact with a metal part, acetylene reactivity strongly favors the right side of the equation. As a result, there is little to no formation of carbon-chain intermediates that may cause sooting and/or tar formation as can be the case with more complex carbon sources such as methane (CH4), ethylene (C2H4), propane (C3H8) or cyclohexane (C6H12). The relatively small size of the acetylene molecule enables it to penetrate small spaces and holes more efficiently than other larger hydrocarbon molecules.

The acetylene dissociation process is catalyzed on contact with the metal surface. The reaction is virtually instantaneous, which allows for carbon to quickly begin diffusing into the steel without the formation of intermediates. The reaction produces hydrogen, which is available at the part surface to reduce metal oxides. Table 1 lists the average rate of carbon uptake per unit area for steel when utilizing various hydrocarbons for carburizing. Acetylene consistently provides a controllable and reproducible carburizing reaction, a plus for today’s data-driven thermal-treating industry.

Table 2 compares the properties of several hydrocarbons considered for the vacuum carburizing process.

Today's Acetylene

Although Wohler’s acetylene-generation process is still in commercial use today and produces acetylene for the industrial welding and cutting market, acetylene produced as a by-product of the ethylene production process is the preferred source of acetylene used for low-pressure carburizing.

Chemical acetylene, as it is commonly known, has a higher overall purity than industrial acetylene and, more importantly, is more consistent in its composition than industrial acetylene. An additional grade of acetylene available commercially is atomic-adsorption grade, which is used primarily in AA instruments for analyzing trace metals and also for glass mold-release coatings and chemical intermediates. Table 3 compares the typical composition (purity) of these three commercially available grades of acetylene.

Another important distinction between chemical acetylene and industrial acetylene is the transport solvent. Previous to the discovery of acetylene’s solubility in acetone by the two French scientists B. Claude and A. Hess, acetylene was generated using Wohler’s process, at the point where it was used. With the ability to transport acetylene over longer distances, its markets were expanded.

Fig. 3. Chemical structure of acetone and dimethyl formamide (DMF)

Acetone has drawbacks, however, in a metallurgical application like low-pressure carburization. Acetone is a volatile solvent that tends to “carry through” along with acetylene as it is withdrawn from a cylinder. Acetone vapors drawn into a vacuum furnace with acetylene gas have the potential to become a cause of sooting. Acetone can also react with certain rubber compounds used in control valves, diminishing the effectiveness of the control systems.

The solution, literally, to the carry-through problem is to use a less volatile solvent to transport acetylene. One such solvent is dimethyl formamide (DMF). The structure of acetone and DMF are illustrated in Figure 3. DMF has a boiling point about 100°C higher than acetone, and acetylene has a similar solubility in DMF as in acetone, as illustrated in Table 4. The difference in volatility and in acetylene solubility for two common transport solvents is shown. DMF is less likely to volatilize and enter the gas distribution system or the vacuum furnace.

Conclusion

In today’s demanding, data-driven environment, consistency plays a big role in the choice of atmosphere gases used for low-pressure carburizing. As a carbon source, acetylene breaks down into C and H2 quickly and efficiently. The furnace operator has less concern with the formation of other carbon species that create soot or tar on parts. The carbon molecules formed in acetylene’s dissociation reaction easily migrate through baskets of parts and are available for surface reactions on the most complex of shapes. Choosing chemically produced acetylene provides the heat treater with a more consistently pure carbon source. Chemically produced acetylene transported in a solvent such as DMF will also reduce the risk of introducing unwanted atmosphere constituents into the carburizing equilibrium. The choice of chemically produced acetylene for the low-pressure carburizing process is a key for achieving the highest level of reproduceability. IH

Special contributions to this article by Len Switzer, business development manager, combustion; John Byington, business manager, fuel gases; Lynn Mead, senior consultant; Patrick Diggins, business development manager, atmospheres – all of Praxair Inc.

For more information: Contact Robert Esper, marketing manager, aerospace and automotive, Praxair Inc., 900 Westpark Drive Suite 100, Peachtree City, GA 30269; tel: 205-980-3067; fax: 205-980-3068; e-mail: Bob_Esper@praxair.com; web: www.praxair.com

SIDEBAR: The Gases of Vacuum Carburizing

The vacuum carburizing process relies on several gases. In addition to providing a carbon source, gases help to circulate hydrocarbons, cool and quench after the carbon-transfer process, and provide a reduction reaction to oxygen that may be present on the surface of the parts or in the furnace atmosphere. Both nitrogen and argon are used during the cooling/quenching process. Nitrogen, due to its low relative cost and better heat-transfer capability, is the typical choice except in cases where nitrogen pickup in the steel is detrimental, such as in aerospace parts. For those situations, argon is totally inert and becomes the primary gas.

From a technical perspective, helium gas – because it is an inert element and has a very high thermal-transfer property – is ideal for the process. However, the high cost of helium means that it is likely to be used as a blend with nitrogen or reprocessed through a helium recovery system. Hydrogen gas is cheaper than helium, has better thermal-transfer properties and will react with oxygen in the furnace to clean parts, but it is a flammable gas. When using hydrogen, care must be taken to follow furnace manufacturer’s recommendations to avoid flammable atmospheres. Consult with your industrial-gas supplier for guidance on gas choices.