The energy optimization of thermoprocessing equipment is of great ecological and economical importance. Thermoprocessing equipment consumes up to 40% of the energy used in industrial applications in Germany. Therefore it is necessary to increase the energy efficiency of thermoprocessing equipment in order to meet the EU’s targets to reduce greenhouse gas emissions. In order to exploit the potential for energy savings, it is essential to analyze and optimize processes and plants as well as operating methods of electrically heated vacuum plants used in large scale production.

1. by Volker Heuer, Klaus Löser
The energy optimization of thermoprocessing equipment is of great ecological and economical importance.
Thermoprocessing equipment consumes up to 40 % of the energy used in industrial applications in Germany.
Therefore it is necessary to increase the energy efficiency of thermoprocessing equipment in order to meet the
EU’s targets to reduce greenhouse gas emissions. In order to exploit the potential for energy savings, it is essential
to analyze and optimize processes and plants as well as operating methods of electrically heated vacuum plants
used in large scale production. For processes, the accelerated heating of charges through convection and
higher process temperatures in diffusion-controlled thermochemical processes are a possibility. Modular
vacuum systems prove to be very energy-efficient because they adapt to the changing production requirements
step-by-step. An optimized insulation structure considerably reduces thermal losses. Energy management
systems installed in the plant-control optimally manage the energy used for start-up and shutdown of the
plants while preventing energy peak loads. The use of new CFC-fixtures also contributes to reduce the energy
demand.
E
nhancing the energy efficiency of thermoprocess-
ing plants is of great importance from an ecological
as well as from an economical point of view. The
operators of industrial furnaces in Germany pay approx.
30 billion Euros [1] for energy costs per year.
The EU has released the following goals under the so-
called “Energy and climate-change package”:
■ the increase of energy efficiency by 20 %,
■ the reduction of greenhouse gas emissions by 20 %
■ the increase of electricity from renewable energy
sources by 20 %.
These goals can only be reached if the energy consumption
of thermoprocessing plants is significantly reduced. More
than 40 % of the total energy used in the german industry
is consumed by thermoprocessing plants [1]. For example,
approx. 270 TWh were consumed in 2005 which is equiva-
lent to the energy consumption of 14 million households.
Enhancing energy efficiency contributes significantly
to the reduction of carbon dioxide in both gas and elec-
trically heated industrial furnaces. Saving one kilowatt
hour of electricity avoids the emission of 520 g carbon
dioxide (status 2005).
THERMOCHEMICAL PROCESSES
IN VACUUM-SYSTEMS
Many processes in the thermoprocessing technology are
carried out in vacuum which is used to prevent undesired
surface reactions of the material during the thermal process,
for example the oxidation of metallic parts. The vacuum
process technology offers simple and economic means to
create a protective gas atmosphere. The atmospheric qual-
ity in vacuum of 10-2 to 10-3 mbar can be compared to that
of industrial gases such as nitrogen (quality 4.6) [2]. Many
different processes are carried out in vacuum such as:
Enhancing energy
efficiency of thermochemical
vacuum-processes and systems
01 I 2012
Special reproduction
11-2012 heat processing

2. ■ Annealing
■ Quench and temper (=neutral hardening)
■ Thermochemical processes (e.g. low-pressure carburiz-
ing, plasma carburizing, low-pressure nitriding, plasma
nitriding, low-pressure carbonitriding)
■ Brazing
■ Bainitizing (=austempering).
One example of a thermochemical process performed in
vacuum is the process of low pressure carburizing (LPC)
with subsequent high pressure gas quenching (HPGQ).
LPC is a carburizing process where the exposure to any
traces of oxygen is prevented during the whole process.
It is performed under pressures between 5 and 15 mbar
and temperatures ranging between 870 and 1,050 °C. In
most cases the carburizing temperatures range between
920 and 980 °C.
Fig. 1 shows a schematic diagram of the process. First,
the charge is placed into the furnace under vacuum,
this is followed by convective heating under 1.2 bar
nitrogen. Convective heating is used to heat up the
parts rapidly and homogeneously. This is followed by an
additional heating phase under vacuum. After all parts
have reached the specified carburizing temperature, the
actual carburizing and diffusion process begins. After an
optimized process the case is free of carbides and con-
tains a low amount of retained austenite. Since the used
gases and the furnace atmosphere are free of oxygen,
any surface oxidation of the parts is
safely prevented.
In most cases the LPC-process
is followed by a high pressure gas
quenching (HPGQ) -process. During
HPGQ, the components are exposed
to an inert gas-stream with pres-
sures up to 20 bar. The gas-veloci-
ties range from 1 to 20 m/s. Com-
pared to liquid quenching such as
oil-, polymer- or water-quenching,
HPGQ is considered as an environ-
mentally friendly and low-distor-
tion quenching process. Normally,
technical gases such as nitrogen,
helium and argon and mixtures
of these gases are used in HPGQ.
Fig. 2 shows a vacuum system
for LPC with subsequent HPGQ, in
which gear components are case-
hardened for large-scale production.
ENERGY FLUXES IN THE
FURNACE CHAMBER
Vacuum heat treatment systems are
generally electrically heated using
indirect resistance heating with graph-
ite or metal heating-conductors. These conductors are
shaped as rods or bands. The heating- conductors transfer
the energy to the parts by radiation and convection.
Fig. 3 illustrates the energy fluxes in a steady-state
vacuum heat treatment plant. This means it is assumed
that the plant has already reached the required tem-
perature before the charge enters the plant. This example
shows a charge with a gross weight of 800 kg which was
carburized to a case depth of 1.5 mm at 930 °C with sub-
sequent quenching using 12 bar helium. The electrical
energy which is brought into the system consists 1st of
the heating power (which is brought into the treatment-
chamber) and consists 2nd of the power for the fan and
the pumps (which is required in the quench-chamber
during gas-quenching). Fig. 3 shows the distribution of
the energy into 1st heating of the charge, 2nd quenching
of the charge and 3rd wall-losses. “Wall-losses” are the so-
called idle losses which are released through the wall to
the outside during the process.
However not only the steady-state condition of the
plant but also the non-steady state, which includes the
heating and the cooling of the treatment chambers,
must be considered. It is distinguished between single-
chamber systems and multi-chamber systems. In single-
chamber systems the thermochemical process and the
quench process are performed in the same chamber. In
multi-chamber plants the thermochemical process and
Fig. 1: Schematic diagram of the low pressure carburizing and high pressure gas
quenching-process
REPORTS Vacuum Technology
2 heat processing 1-2012

3. the quench process are performed in separate chambers
(“treatment-chamber” and “quench-chamber”, see Fig. 2.)
The energy advantage of the multi-chamber technol-
ogy lies in the fact that the temperature in the treatment
chambers is always maintained. Therefore no additional
energy is consumed to heat and cool the inner compo-
nents of the chamber such as insulation, heating-con-
ductors, charge-support, etc., see Fig. 4.
OPTIMIZED THERMAL INSULATION
The inner walls of vacuum heat treatment plants are
usually fitted with plates of hard-felt graphite. In order to
reduce the energy loss through the wall (so called “wall-
losses”), additional insulation with ceramic fibre modules
is recommended. These modules cost less and have a
better insulation quality compared to hard-felt plates.
However, ceramic fibre modules are hygroscopic, i.e. they
absorb moisture in the air relatively fast when coming in
contact with ambient atmosphere. This can be a disad-
vantage in terms of evacuation-time and surface-quality
of the treated parts. Therefore they are suited perfectly
to reduce wall-losses and increase energy efficiency in
vacuum multi-chamber systems, since both tempera-
ture and vacuum are continuously maintained in the
treatment-chambers. With the installation of additional
ceramic fibre modules made of material Al2O3 + SiO2
-material, the wall-losses at 950 °C and 5 mbar nitrogen
have been reduced by 44 %, as shown in Fig. 5.
OPTIMIZED FIXTURES FOR
LOAD-CHARGING
For many applications, heat-resistant cast steel with high
nickel-content is used as material for fixturing in thermo-
chemical processes. However the required energy for
heating of the fixtures can be reduced significantly, when
using carbon fibre reinforced carbon (CFC-material).
Fig. 6 compares CFC-material and cast steel. For high
temperatures CFC clearly has a better strength than heat-
resistant cast steel. Combined with the lower density, this
results in a significantly reduced weight of the fixtures, as
the example in figure 6 clearly demonstrates. Although
CFC has a higher specific thermal capacity compared to
cast steel, it is possible to reduce the required energy for
heating of the fixture significantly. The example in Fig. 6
shows that the energy needed to heat up to 1,000 °C was
reduced from 8,232 kJ to 1,764 kJ by switching from cast
steel to CFC. This equates to a saving of 79 %.
ENERGY EFFICIENCY FOR PARTIAL
UTILIZATION OF PRODUCTION-
CAPACITY
When analysing the energy consumption of thermo-
processing plants, typically the scenario of full 100 %
production-capacity utilization is reviewed. But in order
Fig. 3: Energy fluxes in the furnace chamber (furnace in steady-state
condition)
Fig. 4: Comparison of the temperature profiles in single-chamber
and multi-chamber systems for vacuum process technology
Fig. 2: Modular vacuum furnace system ModulTherm® for case-
hardening in large scale production
Vacuum Technology REPORTS
31-2012 heat processing

4. to review a more realistic production-scenario, it is nec-
essary to consider as well the partial utilization of the
production-capacity.
In the automobile industry for example, during the
ramp-up phase of a new transmission, the number of
parts is increased step by step. Fig. 7 shows a simplified
diagram of the equipment utilization after the SOP of a
new transmission. If a continuous furnace (e.g. a pusher
furnace) is installed for the heat treatment of the parts,
then this continuous furnace is utilized partially for a
longer period of time. This leads to an excessively higher
energy consumption. In comparison, the use of a modu-
lar expandable system (such as a modular multi-cham-
ber vacuum furnace) offers the possibility to customize
capacity to the actual production requirements, see Fig.
7. When using a flexible multi-chamber plant of the type
ModulTherm®, an additional treatment chamber can be
installed within two to three days.
Fig. 8 shows the typical annual production numbers
of an automotive transmission. The production-rate is
usually reduced in the summer months. When using a
continuous furnace, the energy consumption stays at an
unnecessary high level. However, the use of a modular
system allows the shut-off of individual treatment cham-
bers, as required by the production numbers. This leads
to significant reductions in energy consumption. The
Fig. 5:
Optimized thermal
insulation in vacuum
furnaces
energy consumption of the plants shown in Fig. 8 was
reduced from 6,400 MWh to 4,800 MWh when using a
modular multi-chamber system. This is a reduction of the
annual energy consumption of 25 %.
ENERGY MANAGEMENT
SYSTEMS FOR REDUCING THE PEAK
DEMAND
When reviewing the cost of electrical energy, the actual
energy consumption is only one aspect of the calcula-
tion. The costs for the operation of an industrial furnace
are also based on the maximum peak demand, which the
energy-provider needs to keep available for the thermo-
processing plant. Modern thermoprocessing plants are
equipped with an energy management system for the
automated start and shutdown of the plant. In such a sys-
tem the starting time and the sequence of the different
units are set, thus lowering peak demand and avoiding
unnecessary heating. Fig. 9 shows the example of a visu-
alization’s entry mask for automated start and shutdown
of a thermoprocessing plant.
ENHANCING ENERGY EFFICIENCY WITH
ACCELERATED PROCESSES
As shown in Fig. 3, the wall-losses contribute consider-
ably to the energy consumption of a vacuum system.
REPORTS Vacuum Technology
4 heat processing 1-2012

5. Fig. 6:
Comparison between
fixtures made of CFC
and cast steel [3]
These wall-losses can be reduced significantly when the
processes are shortened. Here are two possible solutions
to accelerate the thermochemical process and increase
energy efficiency.
Convective heating
Since the heat transfer by radiation is very low in a tem-
perature range below 700 °C, it is recommended to use
an additional so called “convective heating” step. For this
purpose an internal gas fan is installed in the treatment
chambers. When starting the process, the treatment
chamber is first evacuated and then flooded with nitro-
gen to a pressure of 1.2 to 2 bar abs. The nitrogen is circu-
lated by the internal gas fan to improve the heat transfer.
Thus in addition to a shorter heating time, the charge is
also heated more homogeneously.
Fig. 10 shows that convective heating reduces the
time for heating up a densely packed load of metal bolts
to 890 °C from 130 min to 90 min [5] (in a pre-heated
treatment-chamber). The wall-losses of this treatment-
chamber are 28,5 kW under vacuum and 37 kW under
convective heating with 1.2 bar. Although the wall-losses
are higher, the total energy consumption with convec-
tive heating was reduced from 62 to 56 kWh because the
heating time was reduced by 1.5 h. This equals a reduc-
tion of wall-losses during the process step “heating” of
approx. 10 %.
High temperature case-hardening
Low pressure carburizing is a diffusion-controlled pro-
cess. The diffusion rate increases sharply with increasing
temperature and the corresponding carburizing time is
significantly reduced, see Fig. 11. Additionally the limits
of carbide precipitation shift to higher values. The precip-
itation limit of unalloyed steels (e.g. C15) rises according
to the iron-carbon-diagram from approx. 1.3 % at 930 °C
to approx. 1.65 % C at 1,030 °C. Thus higher surface car-
bon contents can be targeted in each carburizing pulse.
The higher concentration gradient results in an additional
reduction of treatment times which is not even reflected
in Fig. 11.
When parts made of 18CrNiMo7-6 are carburized to a
carburizing depth of 1,5 mm, the total process time can
be reduced by 40 %, if the carburizing temperature is
increased from 930 to 1,030 °C. Fig. 12 shows the result-
ing effect on energy consumption. These values refer to
1 kg charge weight when treating a charge with 800 kg
gross weight. After carburizing the parts are quenched
with 12 bar Helium. In this application the energy con-
sumption was reduced from 0,61 kWh per kg to 0,483
kWh per kg after increasing the carburizing temperature
from 930 to 1,030 °C. This equals a reduction of 21 %.
For applications with deeper case depths, the potential
for improvement is even bigger. E.g. for 15CrNi6-material
Vacuum Technology REPORTS
51-2012 heat processing

6. with a case depth of 3 mm, a 55 %
reduction of the total process time was
verified after raising carburizing tem-
perature from 950 to 1,050 °C. How-
ever, it should be noted that carburizing
temperatures above 980 °C may have
a negative impact on the microstruc-
ture of the components. With tempera-
tures above 980 °C large grains may be
formed. This grain growth may deterio-
rate the fatigue properties of the com-
ponents. In order to counter this effect,
micro-alloyed materials were developed
which can be carburized at tempera-
tures above 1,050 °C without significant
grain growth.
FURTHER STEPS TO
ENHANCE ENERGY EFFI-
CIENCY
In addition to the topics described
above, there are further possibilities to
enhance the energy efficiency of ther-
mochemical processes. For the LPC-pro-
cess, it is recommended to review care-
fully the heat treatment specification. If
a smaller case depth can be specified,
then the process cycle can be shortened
which leads ultimately to a reduction of
the wall-losses.
Furthermore, energy can also be
saved through low-distortion heat treat-
ment. Low distortion heat treatment
requires less grinding tolerance in the
green manufacturing of transmission
parts. Therefore a smaller case depth can
be specified leading to shorter process
times. If it is possible to reduce distor-
tion in such a way that subsequent hard
machining is completely eliminated,
then the additional energy consumption
for the hard machining process is also
eliminated.
When selecting auxiliary systems such
as drives, vacuum pumps and compres-
sors, their energy efficiency should be
taken into account. It is advisable to
select components with a minimum IE2-
efficiency rating. Furthermore, the gas
fans need to be designed specifically for
the given application.
It is anticipated for the future, that
more so-called “combined processes”
will be used in industry. One example
Fig. 8: Equipment utilization in the gear production over a period of one year
Fig. 9:
Automated start-
up and shut-off to
avoid peak demands
(ModulTherm®-
system)
Fig. 7: Equipment utilization during the ramp-up phase for a new transmission
REPORTS Vacuum Technology
6 heat processing 1-2012

7. for such a “combined process” is the
sinter-hardening of powder-metallurgi-
cal parts. During sinter-hardening both
sintering and hardening are performed
“in one heat”. This leads to a significant
reduction of energy consumption.
Practical implementation of an
energy efficient case-hardening
process in large-scale production
A Japanese automotive company
expanded its production capacity for
case-hardening of transmission compo-
nents. A modular vacuum heat treat-
ment system of the type ModulTherm®
with oil-quenching was installed. After
the successful start of production, the
vacuum furnace technology was com-
pared with the existing conventional
technology, i.e. atmospheric gas car-
burizing with oil quenching in a con-
tinuous furnace. The result of this com-
parison is shown in Fig. 13. The new
vacuum furnace technology led to a
significant energy conservation of 39 %
which equals a CO2-reduction of 50 %,
according to calculations by the NREA
(New & Renewable Energy Authority).
The manufacturer of the vacuum system
was awarded the “Agency for Natural
Resources and Energy“- prize in 2011 for
this achievement [7].
CONCLUSION
Enhancing the energy efficiency of
thermoprocessing systems is of great
ecological and economical impor-
tance. Vacuum heat treatment plants
are generally heated electrically. There
are many possibilities to increase the
energy efficiency of these plants. The
use of optimized thermal insulation
of the furnace chamber and the use
of fixtures for load-charging made of
carbon-fibre reinforced carbon (CFC)
reduces energy consumption signifi-
cantly. Compared to continuous fur-
naces, modular systems offer clear
advantages when the production-
capacity is only partly needed. The use
of a modular system allows the user to
tailor the number of individual treat-
ment chambers depending on the
respective production requirements.
Fig. 10: Comparison of heating rates with vacuum heating and convective heating
Fig. 11: Carburizing depth as a function of carburizing temperature and
carburizing time (without heating up) [4]
Vacuum Technology REPORTS
71-2012 heat processing

8. avoiding unnecessary heating. Accelerated processes
such as high temperature case hardening enhance the
energy efficiency significantly. An application was pre-
sented where the energy consumption per kg material
was reduced from 0.61 kWh to 0.483 kWh after increas-
ing the process temperature from 930 to 1,030 °C. This is
equivalent to a saving of 21 %. When selecting auxiliary
units such as drives, vacuum pumps and compressors
their energy efficiency must be taken into account.
LITERATURE
[1] Beneke, F. et al: VDMA-Leitfaden „Energieeffizienz von Ther-
moprozessanlagen“; VDMA Thermoprozesstechnik, 2011
[2] Heuer, V. und Löser, K.: Kapitel 8.2 „Grundlagen der Vakuum-
wärmebehandlung“ in Praxishandbuch Thermoprozesstech-
nik, Vulkan-Verlag 2010; ISBN 978-3-8027-2947-8
[3] GTD Graphit Technologie GmbH
[4] Autorenkollektiv AWT-FA 5; AK4: Die Prozessregelung beim
Gasaufkohlen und Einsatzhärten; Expert Verlag 1997
[5] Beneke, F., S. Schalm: Prozesswärme – Energieeffizienz in der
industriellen Thermoprozesstechnik. Essen: Vulkan-Verlag
2011, S.402-410, ISBN 978-3-8027-2962-1
[6] Koch, A., Steinke, H., Brinkbäumer, F., Schmitt, G.: Hoch-
temperatur-Vakuumaufkohlung für große Aufkohlungstiefen
an hoch belasteten Rundstahlketten. In: Der Wärmebehand-
lungsmarkt 4/2008, S. 5-7
[7] Information Daido Steel Co. Ltd., to be published in 2012
AUTHORS
Dr. Volker Heuer
ALD Vacuum Technologies GmbH
Hanau, Germany
Tel.: +49 (0)6181/ 307-3372
dr.volker.heuer@ald-vt.de
Dr. Klaus Löser
ALD Vacuum Technologies GmbH
Hanau, Germany
Tel.: +49 (0)6181/ 307-3366
dr.klaus.loeser@ald-vt.de
An example from the gear industry showed a reduc-
tion of the annual energy consumption of 25 %. Modern
thermoprocessing plants are equipped with an energy
management system with automated start and shut
down of the plant. The starting time and sequence of the
different units are set, thus reducing peak demand and
Fig. 13: CO2-reduction during case-hardening of transmis-
sions through the use of modular vacuum-systems [7]
Fig. 12: Energy consumption in kWh corresponding to 1 kg
load weight (800 kg gross weight, carburizing depth
1.5 mm and 12 bar He-quenching)
8 heat processing 1-2012
REPORTS Vacuum Technology