EATON TN05 520-1004 MTL Gas Analysers and Systems Instructions
- June 12, 2024
- EATON
Table of Contents
CROUSE-HINDS SERIES
Technical note
MTL gas analysers & systems
TN05 520-1004 MTL Gas Analysers and Systems
Oxygen Potential Measurements in Metal Heat Treatment Processes
For any particular metal or other oxidisable element a value of oxygen
potential called the “free energy of formation” of the oxide exists. Above
this value the metal will be oxidised but below no oxidation will take place.
This value is very much dependent upon temperature and usually increases with
increasing temperature. The diagram on this page illustrates this for several
elements. Nickel, for example, will undergo oxidation to nickel oxide (NiO) in
an atmosphere with an oxygen potential greater than -60 kilocalories/mole at
1000°C, or -81 kilocalories/mole at 500°C. Chromium has a greater affinity for
oxygen than nickel as shown by its lower line on the diagram. It would require
an oxygen potential of less than -130 kilocalories/mole at 1000°C to prevent
oxidation. Thus an atmosphere which is just adequate to prevent nickel from
oxidising would not protect chromium from oxidation. Note that the lines on
the diagram are for the lowest oxidation state of element.
Diagrams such as this enable the theoretical level of oxygen potential to be
determined for a variety of conditions. For example if steel is to be
decarburised by oxidising some of the carbon in the metal, and yet
simultaneous oxidation of the metal is to be prevented, then at 1000°C this
could be done at an oxygen potential of -100 kilocalories/mole. Below about
750°C however, it will not be possible to decarburise without simultaneously
oxidising the iron in the steel, since the carbon line crosses the iron line
on the free energy diagram at this temperature.
In the past the measurement of the oxidising/reducing power of a protective
atmosphere, or the carbon potential of a carburising atmosphere, was achieved
inferentially; either by the measurement of the CO2 content, the CO/CO2 ratio
or the dew point (effectively the moisture content), depending upon the nature
of the atmosphere. These measurements can however be directly related to the
oxygen potential or the oxygen partial pressure. For example a rich exothermic
gas having a CO/CO2 ratio of 10 has an oxygen potential of -93
kilocalories/mole or an oxygen partial pressure of 10-16 atmospheres at
1000°C. Similarly a lean exothermic gas with a CO/CO2 ratio of 0.1 has an
oxygen potential of -70 kilocalories/mole or an oxygen partial pressure of
10-12 atmospheres at 1000°C. Cracked ammonia, which is frequently used when
bright annealing stainless steel, is usually monitored by dew point
measurement. This is related to the water content in the gas from which the H2
/H2 O ratio can be calculated – assuming that the H2 content is known and
constant – which is in turn related to the oxygen potential.
This data only provides information on whether or not a particular metal etc.
will oxidise or not under certain conditions; it says nothing about factors
such as rate of oxidation etc. Factors such as these will sometimes mean that
higher oxygen potentials can be permitted than theoretical considerations
alone would indicate. Furthermore, a certain amount of surface oxidation of
the metal may be quite acceptable. Factors such as these should be taken into
account when determining the most economic way to operate a process.
A zirconia oxygen sensor/analyser can provide very valuable information for
analysing the effect of various conditions, as well as for controlling at the
optimum condition once this has been established.
Atmospheres
Exothermic atmospheres are produced by burning a gaseous fuel, typically
natural gas or methane, in varying amounts of air. These atmospheres can vary
widely in composition depending on the air/gas ratio. They are frequently
referred to as either rich or lean. Rich atmospheres contain excess ‘fuel’
which can sometimes mean they are flammable themselves, whereas lean
atmospheres do not. The atmosphere is used to protect metals from oxidation
during heat treatment processes such as annealing. Rich atmospheres tend to be
used for ferrous metals, which require a lower oxygen content than non-ferrous
(copper bearing) metals, which tend to use lean atmospheres to protect them.
Sometimes exothermic atmospheres are treated to remove water and/or carbon
dioxide. Again these vary greatly in composition depending on the starting gas
and the treatment it undergoes. They often contain high amounts of hydrogen
and this can be used as a measure of the gas quality along with oxygen
potential.
Endothermic atmospheres are also produced by reacting gas with air,
except that these are so gas rich they need to be reacted in the presence of a
heated catalyst. Their composition is mainly carbon monoxide, hydrogen and
nitrogen. Because the mixture is so rich, sooting up of the reactor and
catalyst can occur if the process is not controlled. Monitoring hydrogen and
oxygen potential provides the means to do this.
Cracked ammonia is produced by catalytically decomposing ammonia (NH3)
into hydrogen and nitrogen. It is a convenient way of producing hydrogen and
the process may be controlled by measuring the hydrogen content. It is
sometimes required to provide a “percentage dissociation” figure and it is
possible for a MTL K1550 analyser to be programmed to display this.
Burned ammonia is produced by doing just that, usually in the presence of
a catalyst. An atmosphere consisting of hydrogen, nitrogen and water is
generated, and this is frequently dried to some extent by using the
evaporation of the ammonia to cool the gas. The monitoring of the hydrogen
content is again important.
The processes that use these gases are mainly of three types.
Annealing where a metal is heated to soften it prior to some other
process such a rolling or drawing. Depending on the atmosphere oxygen,
hydrogen or both are usefully monitored.
Carburising where the surface of ferrous metals is reacted with carbon to
form a “case” of harder or tougher metal. It is usual these days to use insitu
zirconia oxygen probes for this application. The disadvantage of using an ex-
situ approach would be the sooting that occurs with the very rich atmospheres
employed. There is scope for the lower cost ex-situ type if the process is a
so called “long cycle” type, where the atmospheres are not so sooty.
Cosmetic surface treatment such as the ‘blueing’ and bright annealing of
steel. ‘Blueing’ is often done by atmospheres that are wet (steam is sometimes
used ,in fact), so it often necessary to close-couple the sensor to the
process to avoid condensation. The MTL Z1900 type of sensor is ideally suited
to this.
Conversion factors
- kiloJoule (kJ) = kilocalorie (kcal) x 4.187
-
- kJ = (17.6 x log % oxygen) – 34.72
- Only applies to MTL zirconia oxygen sensors operating at 45mV per decade (634°C)
Note: The MTL Z1110 analyser is used in this type of application.
The given data is only intended as a product description and should not be regarded as a legal warranty of properties or guarantee. In the interest of further technical developments, we reserve the right to make design changes.
Eaton Electric Limited,
Great Marlings, Butterfield, Luton
Beds, LU2 8DL, UK.
Tel: + 44 (0)1582 435600
Fax: + 44 (0)1582 422283
www.mtl-inst.com
E-mail: mtlgas@eaton.com
EUROPE (EMEA):
+44 (0)1582 723633
mtlenquiry@eaton.com
THE AMERICAS:
+1 800 835 7075
mtl-us-info@eaton.com
ASIA-PACIFIC:
+65 6 645 9888
sales.mtlsing@eaton.com
© 2016 Eaton
All Rights Reserved
Publication No. TN05 520-1004 Rev 3 191016
October 2016