Enlarge this picture Fig. 1. Decarburized steel after hot rolling[2]

Decarburization (Fig. 1) is defined as: “Loss of carbon from the surface layer of a carbon-containing alloy due to reaction with one or more chemical substances in a medium that contacts the surface.”[1] It occurs in many steel heat-treatment processes and, except for a few processes where it is deliberately induced, is considered to prejudice performance. This is particularly true of hardened components such as bearings, where the performance of the surface layers is critical.

1. Do I really have decarburization?

Enlarge this picture Fig. 2. A carburized low-alloy steel etched (top) and unetched (bottom) showing the effect of internal oxidation[4]

It is often said that if it looks like something and feels like something, then it is that thing. Unfortunately, in the case of decarburization this is not true. In hardened high-carbon steels, it is fairly common that a thin white-etching layer formed on the surface is assumed to be decarburization. It looks like ferrite and is soft like ferrite, but it is actually retained austenite and the result of the opposite problem – excessive surface carbon. The problem is often traced back to oil-based lubricants baked onto the surface, causing carbonaceous layers that produce a very thin high-carbon layer during hardening. Typically, but not always, the layer is discontinuous.

The second problem that can be confused with decarburization in hardened parts is internal oxidation. The results can be exactly the same, an under-hardened surface layer – bainitic or even ferritic. In decarburization, this layer is caused by lack of carbon, but internal oxidation is caused by lack of hardening elements in solution, like chromium, that have been converted to oxides. The giveaway is the presence of the small oxides easily visible in an unetched cross section (Fig. 2).

The only way to eliminate internal oxidation in carburizing is to keep oxidizing species out of the process. In practical terms this means either using low-pressure gas carburizing followed by high-pressure gas quenching[3] or vacuum hardening for high-carbon parts.

2. Did the last process cause the decarburization?

It is always a good idea to check that the process under investigation is indeed the one that has caused the problem. If, for example, the steel going into a hardening process was decarburized by the prior process, say annealing, then elimination of decarburization in the hardening step is not the problem. Either take a step back and look at the annealing process, or ask a different question: How do I recarburize my parts?

3. What are the causes of decarburization?

Decarburization is usually caused by a reaction between the carbon dissolved in the steel (C_Fe) and oxygen or an oxidizing species in the surrounding atmosphere.[5]

2C_Fe + O₂ => 2CO (1)

C_Fe + H₂O => CO + H₂ (2)

C_Fe + CO₂ => 2CO (3)

Decarburization can also be caused by hydrogen, as in Equation 4, but this is rarely the case and the reaction is slow compared to those of the oxidizing species.

C_Fe + 2H₂ => CH₄ (4)

More carbon diffuses down the carbon gradient to create a layer that gets thicker with time. The carbon gradient, in these layers, is determined by the carbon activity in the steel and in the surrounding atmosphere. The reaction rate is temperature-dependent.

4. Where do the oxidizing species come from?

There are two answers here. Either they are deliberately introduced in atmospheres such as exothermically generated gas, endothermically generated gas or nitrogen/cracked methanol, or they come from air that gets in by accident and reacts with other atmosphere species. The case where the oxidants are introduced deliberately will be addressed in Question 7. Adventitious air is discussed first.

5. How can leaks be reduced?

Enlarge this picture

Malas[5] presents a reasonably comprehensive list of causes of air ingress. Table 1 is an edited version. He also sets out the procedure for carrying out a smoke test.

6. Can the atmosphere be changed to get better results?

Enlarge this picture Fig. 3. An Ebner HICON/H2 bell annealing installation (Courtesy of Ebner Industrieofenbau)

Often the answer is yes, but the details depend on the furnace type and the composition of the atmosphere in use. Some examples of this follow.

Semi-finished product annealing
Operators using bell, pit or top-hat furnaces with nitrogen/hydrogen or nitrogen/hydrocarbon atmospheres should consider changing to 100% hydrogen annealing in specialist bell furnaces like those in Fig. 3. Not only will this eliminate decarburization if the product is clean, but it will reduce costs as well.[8]

Component hardening in continuous furnaces
If the furnace is almost leak free and using nitrogen/hydrogen, it will be beneficial to create a carbon potential by adding a little hydrocarbon. A small hydrocarbon addition will create a carbon potential without making any carbon available to carburize the load. If the furnace is electrically heated, care must be taken to ensure that the addition is small enough not to crack on the heating elements with the potential for arcing. Typically, perhaps ¼% propane or 1% natural gas could be added to 2-4% hydrogen to achieve the desired result.

If the furnace is almost leak free and using nitrogen/hydrocarbon, hydrogen can be added to achieve the same results as above. This will usually allow a smaller hydrocarbon addition, reducing the propensity to form soot.

Enlarge this picture Fig. 4. A model showing the effect of CARBOJET® on the gas velocity profile in a roller-hearth furnace in the plane of the nozzles

Annealing in continuous furnaces
It is difficult, particularly in the roller-hearth furnaces used for semi-finished product annealing, to achieve the low leak rates that allow the use of the less reactive atmospheres detailed above, and it is necessary to use a hybrid technology. An example of this might be to use a mixture of nitrogen with a small addition (5-10%) of either endo-type gas sourced from a Linde Carbocat® in-situ generator[9] or from cracked methanol, depending on the annealing temperature, and a hydrocarbon (2-4%). The presence of the CO from the endothermic-type gas buffers the reaction with the adventitious oxygen. Fitting a Carbojet® to stir the mixture greatly improves consistency (Fig. 4).

7. How can decarburization be minimized when oxidizing species are present?

Enlarge this picture Fig. 5. A Carboflex® control system on a continuous annealing furnace

Several approaches are possible depending upon the atmosphere system currently in use. If exothermically generated gas is being employed, nothing can be done economically to eliminate decarburization with this atmosphere. If the furnace has a low leak rate, it is recommended to change the atmosphere to nitrogen with a small hydrocarbon addition (e.g., 4% natural gas).[11] This atmosphere will eliminate decarburization if the furnace is leak free. If not, return to Question 4.

If an endo-type atmosphere is being employed, two strategies are possible. The first is to use an atmosphere of the type described in Question 6 and rely on low carbon availability to reduce or eliminate decarburization. The second is to use an atmosphere containing at least 10% carbon monoxide and use carbon control. A control system, such as the Carboflex® system shown in Figure 5, can balance the carbon potential of the atmosphere with the carbon in the steel and eliminate decarburization.[12]

Conclusions

Knowing what needs to be done and applying it to the furnace and atmosphere system in use can always reduce and often completely eliminate decarburization. Sometimes these changes will be minor, but more intractable cases can entail a complete change of the atmosphere system. It is usually best to consult the experts. IH

For more information: Contact Dr. Paul Stratton, CEng CSci FIMMM, heat-treatment and electronic-packaging application development, Linde AG BOC, Rother Valley Way, Holbrook, Sheffield, S20 3RP, UK; tel: +44 1484 328736; e-mail: paul.stratton@boc.com; web: www.boc-gases.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: decarburization, ferrite, retained austenite, internal oxidation, endothermic