strategies for eliminating decarburization
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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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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: [email protected]; 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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