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Reactions of Carburization

The document discusses gas carburization, which involves using a carrier gas enriched with hydrocarbons like propane or methane to control the carbon availability in the atmosphere. The carrier gas typically contains 15-25% CO, 35-45% H2 and nitrogen, and hydrocarbons are added to enrich it. Hydrocarbons increase carbon availability by reducing water and carbon dioxide through reactions, and allow direct carburizing via reaction of methane with iron. The main carburizing reactions involve carbon monoxide, hydrogen, and methane reacting with iron, but these reactions do not reach equilibrium quickly in furnaces. Unreacted methane can also dilute the carbon monoxide level and affect the real carbon potential if not accounted for

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Ahsiur Nirjhar
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42 views2 pages

Reactions of Carburization

The document discusses gas carburization, which involves using a carrier gas enriched with hydrocarbons like propane or methane to control the carbon availability in the atmosphere. The carrier gas typically contains 15-25% CO, 35-45% H2 and nitrogen, and hydrocarbons are added to enrich it. Hydrocarbons increase carbon availability by reducing water and carbon dioxide through reactions, and allow direct carburizing via reaction of methane with iron. The main carburizing reactions involve carbon monoxide, hydrogen, and methane reacting with iron, but these reactions do not reach equilibrium quickly in furnaces. Unreacted methane can also dilute the carbon monoxide level and affect the real carbon potential if not accounted for

Uploaded by

Ahsiur Nirjhar
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Reactions during the process: Gas carburization is described here as it is vastly used.

In the
gas carburizing process, a low carbon bearing carrier gas is used, which is enriched with a
hydrocarbon gas, such as propane or methane (natural gas), to increase and control the carbon
availability of the atmosphere. The carrier gas is usually of the Endothermic gas type
produced from a sub stoichiometric mixture of a hydrocarbon and air at elevated temperature
in the presence of a catalyst. The production of Endothermic gas is usually carried out in an
external gas generator. Alternatively, a nitrogen – Methanol mixture, injected into the
furnace can be used to produce a synthetic Endothermic gas. Dependent upon the type of
hydrocarbon used and the mixture ratio, the typical composition of the carrier gas is:

15-25% CO, 35-45% H2, Balance N2,

plus, small quantities of CO2, H20, CH4

The gases CO and CH4 are carburizing, whilst H2, H2O and CO2 are decarburizing. In order
to control the carbon availability of the atmosphere – the carbon potential, a hydrocarbon gas
is used to enrich the carrier gas, by reducing the H2O (Dewpoint) according to the reaction:

CH4 + H2O = CO + 3 H2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (2)


And by reducing the CO2 according to the reaction:

CH4 + CO2 = 2 CO + 2 H2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. (3)

as well as allowing the following carburizing reaction to take place:

CH4 = CFe + 2 H2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (4)

in addition to reaction (4) the other main carburizing reactions in a CO – CO2 – H2 – H2O –
CH4 atmosphere are:

CO + H2 = CFe + H2O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... (5)

2 CO = CFe + CO2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (6)


(In the gas reactions described CFe is the available carbon, from the atmosphere, for diffusion
into the steel surface)
Reactions (5) and (6) are considered the main equilibrium reactions for the basic method of
carbon transfer into the steel surface. The assumption is that the carburizing is taking place as
a direct result of the CO content in the atmosphere. However, the Oxygen probe has no way
of measuring the %CO. A fixed value has to be introduced into the Carbon potential
controller in the form of a Process Factor or CO factor (see below). Furthermore, equilibrium
conditions do not exist in a furnace until several hours have passed – the actual %CO will be
considerably lower than expected.
It has been shown that reaction (5) is 10-100 times faster than reactions (4) and (6), and it is
therefore this that is rate determining.
In most systems, the addition of a hydrocarbon gas such as methane is used for the control of
carbon potential. If all the methane was “cracked” using reactions (2) and (3), the atmosphere
would remain in equilibrium and predicting carbon potential would be straight-forward.
Unfortunately, these reactions occur at very slow rates and only near catalytic surfaces. They
are never close to equilibrium.

In an atmosphere with a significant level of free methane, some carburizing will be taking place
according to reaction (4), but increasing levels unreacted methane will result in the dilution of the
%CO. Without measuring and including the effects of free methane in the carbon calculation, the
real potential of the atmosphere is not known.

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