4.9
Heat Treatment
P. Mayr, H. Klümper-Westkamp, Stiftung Institut für Werkstofftechnik IWT, Bremen,
Germany
4.9.1
Introduction
After mechanical manufacturing, heat treatment is mainly applied in one of the
final steps of production, to adjust the workpiece properties to the later mechani-
cal, tribological, and corrosive load. The lifetime of these components is defined
through the raw material used, the construction geometries, and the quality of
heat treatment.
The great demands for quality control and the documentation of process param-
eters have led to the increasing importance of sensor applications in heat treat-
ment. Furthermore, increasing automation combined with the idea of integrating
the heat treatment into the production line needs many different sensors to make
the whole process as reliable as possible.
4.9.2
Temperature Monitoring
The primary process variable to be measured and controlled in heat treatment is
the temperature, which directly influences component properties. There are a very
large number of different temperature-dependent physical properties that can
serve, at least in principle, as the basis for thermal sensors. Only a few of them
are used in heat treatment plants to measure the temperature.
Thermocouples are the most commonly used temperature sensors in heat treat-
ment. They consist of two conductors of different metallurgical composition,
which are made up into an electrical circuit with two junctions. If the two junc-
tions are brought to a different temperature, a thermoelectric potential is created.
The magnitude depends on the composition of the conductors chosen and the
temperature difference [1].
Their simple construction allows sensors to be made which are able to provide
reliable and accurate data under extreme measurement conditions. With thermo-

10
4
Wcm
±2
l
m
±1
sr
±1
Spectral radiance L
k S
10
3
10
2
10
1
10
0
10
±1
10
±2
10
±3
10
±4
10
±5
6000 K

mometers. They are used in industrial applications up to 450 8C when encased in
ceramic, glass, or other protective materials. Atmospheres containing carbon or
hydrogen are poisonous for platinum and lead to unstable resistance. Resistance
thermometers have the advantage of high accuracy measurement. Typically, a tem-
perature resolution of ± 10
–3
8C can be achieved up to 500 8C whereas thermocou-
ples are limited to ± 0.5 8C. Special geometries are available for surface tempera-
ture measurement.
In special applications other metals such as copper, nickel, and iridium are
used [2].
A number of physical effects have been checked to realize fiber-optic temperature
sensors [4]. The main fiber-optic thermometers today use the temperature depen-
dence of the fluorescence decay time. This effect has been utilized in commercial
instruments by Luxtron and Sensycon. The main advantage of this temperature
measurement method is the capability to allow measurements in severely hostile
environments. They can be made in the presence of intense radiofrequency and mi-
crowave fields as well as very high voltages and strong magnetic fields. In the range
from –200 to 450 8C, an accuracy of ± 0.1 8C can be reached at the calibration point.
In continuously working furnaces, temperature measurements can be made by
furnace tracker systems. A data logger for thermocouples is positioned in a ther-
mal protection unit made of inert and maintenance-free material. During the fur-
nace travel the data logger stores the measured temperatures, which can be read
later. Such a furnace tracker works up to temperatures of 1200 8C. Such systems
have been developed and are sold by DATAPRO and Stoppenbrink.
4 Sensors for Process Monitoring328
4.9.3
Control of Atmospheres
Heat treatment is done, depending on the aim of the treatment, in different fur-
naces with different pressures, atmospheres, and plasma application. The scale in

lized ZrO
2
. The measured potential E depends on the difference in oxygen partial
pressure P
O
2
at the two electrodes. P
0
O
2
is the known reference oxygen pressure,
normally of air. In Figure 4.9-3 a schematic diagram of such a sensor is shown.
The temperature T is measured in kelvin [7].
E mV0:0496 T logP
O
2
=P
0
O
2
4:9-1
In oxygen-containing carburizing atmospheres, the following chemical reaction is
the main indicator for the carburizing reaction:
CO C
1
2
O
2
4:9-2
4.9 Heat Treatment 329

the degree of sealing of the furnace. Because of the mass production of this lamb-
da probe it is much cheaper but in many applications not as reliable as the stan-
dard in situ oxygen probe.
A direct method for measuring the carbon uptake in almost every atmosphere
is the so-called wire sensor [8]. This sensor is based on the resistivity change of a
thin, pure iron wire with increasing amount of dissolved carbon, as shown in Fig-
4 Sensors for Process Monitoring330
Fig. 4.9-3
Mode of action of an oxygen probe [7]
ure 4.9-4. Periodically the wire is decarburized with hydrogen so that it can be
used again. This in situ sensor is mostly applied for periodic checking of the abso-
lute carbon potential in furnaces. Continuous control of the carbon potential in a
furnace is not practicable because of problems with oxidation, formation of car-
bides, and changing surface of the wire. This in situ sensor is sold by Process-
Electronic.
4.9.5
Nitriding
Gas nitriding is usually carried out in ammonia-containing atmospheres. Since
Lehrer’s paper [9], it is known that the nitriding potential K
N
is the driving force
of nitriding and defines the phase composition of the white layer. The nitriding
potential K
N
is defined by the well-known ratio of the partial pressures of ammo-
nia, P
NH
3
, and hydrogen, P
H

Fig. 4.9-4
Probe tip of the wire sensor (Werkfoto PE-Essen)