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48  April 2009 - IndustrialHeating.com

ecarburization (Fig. 1) is

defined as: “Loss of carbon

from the surface layer of

a carbon-containing alloy

due to reaction with one or more chemi-

cal 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 per-

formance. This is particularly true of

hardened components such as bearings,

where the performance of the surface lay-

ers is critical.

1. Do I really have decarburization?

It is often said that if it looks like some-

thing 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, caus-

ing carbonaceous layers that produce a

very thin high-carbon layer during hard-

ening. Typically, but not always, the layer

is discontinuous.

  The second problem that can be con-

fused 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 oxidiz-

ing 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.

 D

Strategies for EliminatingStrategies for EliminatingecarburizationDecarburization

FEATURE | IndustrialGases/Combustion

Paul Stratton – Linde Gas, Sheffield, U.K.

This article, which is in the form of questions and answers, should help heat

treaters with decarburization problems reach optimal solutions for the material

and processing plant they have available.

Fig. 1. Decarburized steel after hot rolling[2]

Fig. 5. A Carboflex® controsystem on a continuousannealing furnace

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  IndustrialHeating.com - April 2009 49

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 harden-

ing 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 (CFe) and oxygen or an oxi-

dizing species in the surrounding atmo-

sphere.[5]

2CFe + O2 => 2CO (1)

CFe + H2O => CO + H2  (2)

CFe + 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.

CFe + 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 activ-

ity in the steel and in the surrounding

atmosphere. The reaction rate is temper-

ature-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, en-

dothermically 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. Adventi-

tious air is discussed first.

5. How can leaks be reduced?

Malas[5] presents a reasonably comprehen-

sive list of causes of air ingress. Table 1 is

an edited version. He also sets out the pro-

cedure for carrying out a smoke test.

6. Can the atmosphere be changed

to get better results?

Often the answer is yes, but the details

depend on the furnace type and the com-

position of the atmosphere in use. Some

examples of this follow.

Semi-finished product annealing

Operators using bell, pit or top-hat fur-

naces 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 decarbur-

ization 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 addi-

tion will create a carbon potential without

making any carbon available to carburize

the load. If the furnace is electrically heat-

ed, 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

Table 1.  Examples of causes and solutions of decarburization

Cause Solution

1Insufficient atmosphere flow rate can allow air to

enter the furnace at the entry and/or exit.

Follow manufacturer’s data or get specialist

advice on the correct flow needed.

2

Products of combustion or air leaking from radi-

ant tubes and air leaking through muffles, seals,

covers, rollers, atmosphere fans, flanges, etc.

Periodic inspection (e.g., with smoke bomb)

3 Air infiltration in the atmosphere piping

Check pipeline integrity from the supply point

to control panel(s) and onward to furnace entry

points

4Drafts due to opening of doors or windows in the

building

Furnace should be positioned away from drafts

if possible. Effective furnace curtains could be

used to mitigate their effect.[6]

5 Exhaust stacks - up and down drafts Adjust with the damper

6 Furnace curtains not sufficiently gas tightCheck curtain integrity and/or install better gas

and solid curtains[7]

7 Workload contaminated with oil and waterDegrease and clean the components and dry

completely before charging the furnace

8

Incompatibility between atmosphere and refractory

lining. Reducible oxides can react with hydrogen

to form water.

Use appropriate refractory or muffleFig. 2. A carburized low-alloy steel etched

(top) and unetched (bottom) showing theeffect of internal oxidation[4]

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50  April 2009 - IndustrialHeating.com

FEATURE | IndustrialGases/Combustion

as above. This will usually allow a smaller

hydrocarbon addition, reducing the pro-

pensity to form soot.

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 reac-

tive 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 hydrocar-

bon (2-4%). The presence of the CO from

the endothermic-type gas buffers the reac-

tion 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?

Several approaches are possible depend-

ing 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% car-

bon 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 de-

carburization.[12]

Conclusions

Knowing what needs to be done and ap-

plying it to the furnace and atmosphere

system in use can always reduce and of-

ten 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

References (available online)

For more information:  Contact Dr. Paul

Stratton, CEng CSci FIMMM, heat-treatment

and electronic-packaging application devel-

opment, Linde AG BOC, Rother Valley Way,

Holbrook, Shef field, 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: decarbur-

ization, ferrite, retained austenite, internal

oxidation, endothermic

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

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

Without CARBOJET

With CARBOJET

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