FEATURE | Heat Treating
Comparison of Carburizing-Furnace
Technology for Long-Cycle Processes
in Wind-Energy Applications
John W. Gottschalk – Surface Combustion, Inc.; Maumee, Ohio
With the push for alternative energy technologies, the wind-energy field has seen
significant growth over the last 10 years. Projections for the next 10 years indicate
an even larger requirement for wind turbines to reduce dependence on fossil fuels.
W
i this increased need
ith
iin the wind-turbine
market, there has been
m
a significant increase in
component sizes, i.e. gears, bearings, pinions, etc. Existing gearbox manufacturers
and gear heat-treating facilities do not
have the required equipment sizes or production capacity to accommodate these
larger parts and higher volumes associated
with the deep case-depth requirements
of these components. Due to the large
load volumes and weights, the primary
installed base expansion in furnace technology has been in pit-furnace systems.
Furnaces sized to process gears 2-3 meters
in diameter are not uncommon with load
weight requirements of 15-20 tons.
As these large components are under
severe duty cycles and not easily replaced,
the quality of the heat-treatment process
is critical to final part and gearbox function. Of primary concern are quality of
the carburized case and intergranular oxidation (IGO). All of these attributes are
functions of the heat-treating process and
the process equipment designs.
Two leading furnace technologies are
used for this process. One technology
utilizes an atmosphere-tight retort, while
the other utilizes conventional refractorylined furnace technology. The differences
in furnace technology will be discussed
with emphasis placed on how system design affects the carburized case and IGO.
In addition, a comparison of processing atmospheres is provided between generated
endothermic gas and nitrogen/methanol
atmosphere. A description of both furnace
designs is provided.
Large-Component Furnace
Technologies
Retort-Muffle Design
By definition, a muffle-furnace design utilizes an atmosphere-tight, metallic muffle
to isolate the work from both the heating
system and furnace refractory. In practice,
the design most commonly used in large
pit carburizing furnaces is a hybrid design
with furnace top and bottom refractory
exposed to the workload.
The primary component of the system
is the muffle (Fig. 1). In the muffle-system
design, there is a seal at both the top and
bottom. Typically, the top seal is a water-
cooled metallic flange arrangement utilizing an o-ring seal for atmosphere integrity. The bottom seal can be of multiple
designs, including sand, ceramic fiber or
oil. With the furnace sealed at both ends,
expansion of the retort is critical. In most
applications, an alloy bellows is added to
the lower portion of the retort above the
lower seal to allow for expansion. Materials of construction are typically wrought
330 SS. As the muffle is the primary atmosphere seal in the system, muffle maintenance is critical to process performance.
Through-carburizing of the muffle within
the hot zone as well as metal dusting of the
muffle near the top and bottom seals are of
concern.
A benefit of the muffle design is incorporation of reduced-cost electric heating
systems. As the heating elements are not
exposed to the carburizing environment,
metallic rod overbend elements can be
used without risk of short circuits from
carbon deposit or carburization of element lead-in connections. These heating
systems are generally lower in cost than
comparable radiant-tube furnace designs.
However, this cost saving is often offset by
IndustrialHeating.com - September 2010 43
FEATURE | Heat Treating
the higher operating costs associated with
electric-furnace designs.
One area of concern with the muffle
concept is temperature and atmosphere
uniformity within the working chamber.
In larger furnace designs, heat losses from
the furnace cover and floor are not compensated for within the furnace design
because the muffle allows for heating only
from the side walls of the furnace.
One improvement to the design is the
addition of an internal baffle inside of the
muffle to direct atmosphere flow between
the baffle and muffle, ensuring that atmosphere is circulated through the full depth
of the furnace. The disadvantages of this
approach are additional alloy content
within the system and the possibility of
reduced heat transfer from the shielding
effect of the second retort because the
primary form of heat transfer is re-radiation from the heating elements through
the retort.
Internal Baffle Design
This furnace design is substantially the
same as the muffle-furnace design with
several key differences. An atmosphere recirculation baffle replaces the muffle and
both the heating elements, and all of the
refractory is exposed to the furnace atmosphere. With this design, there is only one
atmosphere seal provided at the top of the
furnace. It is typically made of ceramic
fiber and without the use of cooling water. The internal baffle is constructed of
wrought 330 SS material in a similar fashion as the muffle design.
The baffle resides entirely within the
hot zone of the furnace, making it less
susceptible to metal dusting. Additionally, any holes that may develop in the
baffle are not critical to atmosphere containment in the process.
The major area of concern for atmosphere integrity is the radiant-tube heating system. Both gas-fired and electrified
radiant tubes can be utilized in this design. Risk of short circuit from carbon
buildup on feed throughs is eliminated by
the use of special alloy heating elements.
As this is a mature furnace technology
used in carburizing processes, maintenance programs are well established.
Unlike the muffle-furnace design, atmosphere is continuously recirculated
from the top to the bottom of the furnace
based on the design of the internal baffle.
This includes wiping of the radiant tubes
and is accomplished without the addition
of a secondary baffle. The major benefit of
this baffle is improved temperature uniformity throughout the work envelope.
A typical retort system is shown in
Figure 2.
Temperature Uniformity
Two of the major factors affecting part
quality are system temperature uniformity
and atmosphere composition. The system
temperature uniformity is a function of
the heating and atmosphere recirculation
system designs. The localized temperature
in the furnace chamber has a direct effect on the achieved case depth within
the parts. This includes both inconsistent
case of multiple parts in a load and within
the same large part. For a typical 1750°F
boost/diffuse carburizing cycle operating
on a 12- and 24-hour cycle time at temperature respectively, the predicted effective case depths are indicated in Table 1.
Table 1. Predicted case depths for
different times and temperatures
Localized part
temperature
Effective
case depth
(inch) at
12 hours
Effective
case depth
(inch) at 24
hours
1740°F
0.0789
0.1115
1745°F
0.0801
0.1133
1750°F (Target
case depth)
0.0814
0.1151
1755°F
0.0827
0.1170
1760°F
0.0840
0.1188
These variations in case depth are
strictly temperature-dependent and do
not account for additional variations that
arise from part geometry or atmosphere
variations within the furnace. Temperature spread within the working envelope
of the two furnace designs was measured
by a typical nine-point temperature survey. Temperature spread during soak conditions at 1750°F is displayed in Table 2
along with the expected case-depth variations for the two furnace types.
9
4
5
1
2
3
11
10
8
7
6
Fig. 1. Muffle furnace
44 September 2010 - IndustrialHeating.com
Fig. 2. Typical retort system
1. Furnace external
casting
2. Furnace refractory
lining
3. Furnace muffle
4. Furnace cover
5. Furnace top seal
6. Furnace bottom seal
7. Muffle expansion
bellow
8. Furnace heating
eystem (electric)
9. Recirculation fan
10. Load support
11. Secondary
recirculation baffle
Table 2. Temperature spread and casedepth variation during soak conditions
at 1750°F
Baffle furnace Muffle furnace
Furnace effective
diameter
Heating system
Temperature
spread
90 inches
90 inches
Gas-fired
radiant tube
Electric nickel/
chrome
elements
<5°F
<15°F
Expected casedepth variation,
12-hour cycle
1.6% of case
(0.0013 inch)
Expected casedepth variation,
24-hour cycle
1.6% of case 4.6% of case
(0.0018 inch) (0.0055 inch)
4.6% of case
(0.0038 inch)
By improvement of the temperature
uniformity within the furnace chamber,
measurable improvement in the casedepth uniformity can be achieved. This
improvement can lead to more precise
case-depth range specifications, allowing
for shorter carburizing times and improved
cycle-to-cycle repeatability.
The second factor in the case-depth
uniformity within the furnace is the atmosphere consistency throughout the
Table 3. Microstructural characterization of various furnace cycles
Material grade
Furnace type
Processing atmosphere
Figure 3
Figure 4
Figure 5
17CrNiMo6
4320
18 CrNiMo7-6
Baffle
Baffle
Endothermic with natural Endothermic with natural
gas enriching
gas enriching
Baffle
Nitrogen/methanol with
natural gas enriching
Total process time
235 Hours
87 Hours
39 Hours
Carburizing carbon
potential
0.90
1.00
1.00
Diffusion carbon potential
0.75
0.80
0.80
Carburizing temperature
1700°F
1700°F
1750°F
Equalize temperature
prior to quench
1525°F
1525°F
1550°F
47 microns
27 microns
Total case depth
IGO depth
working chamber. Assuming that both
furnace systems utilize the same carbonpotential control system, including oxygen probe and infrared gas-analyzing
equipment, the ability to control the atmosphere within the furnace chambers
is equivalent. Variations in atmosphere
composition between endothermic and
nitrogen/methanol-based
atmospheres
are essentially eliminated by addition of
an enriching agent (typically natural gas).
The enriching gas is controlled based on
the measured oxygen content and/or CO/
CO2 ratio in the system.
Microstructure analysis (SEM) of various furnace cycles is provided in figures
3 - 5 as defined by Table 3.
The additional factor controlling the atmosphere consistency in the system is the
recirculation system provided. In both furnace designs, large recirculation fans are
typically located in the furnace cover. At-
IndustrialHeating.com - September 2010 45
FEATURE | Heat Treating
60µm
30µm
10µm
Fig. 3. 17CrNiMo6 in a baffle furnace
(see Table 3)
Fig. 4. 4320 in a baffle furnace
(see Table 3)
Fig. 5. 18CrNiMo7-6 in a baffle furnace
(see Table 3)
mosphere circulation in the baffle furnace
and muffle with internal baffle are equivalent. In muffle designs without internal
baffle, atmosphere variance is greater from
top to bottom within the furnace because
there is no assurance that the atmosphere
flow reaches the bottom of the furnaces.
ture, as would be expected for endothermic-based atmospheres.
As both furnace technologies are capable of producing parts meeting the required
specifications, other factors should be used
in determining the appropriate technology.
Total cost of ownership, including utility costs, should be strongly considered.
Gas-fired designs will typically operate
at 50–60% total utility cost of an equivalently sized electric design. This fact coupled with a lower initial investment cost
for baffle-furnace designs leads to a significantly reduced cost of ownership over the
lifetime of the equipment. IH
Conclusions
As can be seen by the SEM photos, all
samples show varying amounts of IGO
independent of furnace type or processing atmosphere. Depth of IGO seen is
strictly dependent on time and tempera-
46 September 2010 - IndustrialHeating.com
For more information: John W. Gottschalk,
director, special products, Surface Combustion, Inc., 1700 Indian Wood Cir., Maumee, OH
43537; tel: 419-891-7145; fax: 419-891-7151;
e-mail: [email protected]; web:
www.surfacecombustion.com
Additional related information may be
found by searching for these (and other)
key words/terms via BNP Media SEARCH
at www.industrialheating.com: intergranular oxidation, endothermic gas, nitrogen/methanol, pit carburizing furnace,
muffle furnace, temperature uniformity,
case depth, SEM
IndustrialHeating.com - September 2010 47