The document discusses advanced carburizing processes and issues with vacuum carburizing. It then summarizes the SOLO ECOCARB process as an alternative that does not require vacuum. The key points are: 1) Vacuum carburizing had limitations in terms of productivity, flexibility of hardening media, and mechanical properties achieved. 2) SOLO developed the ECOCARB process that inhibits surface oxidation without vacuum through tight furnace control and gas mixtures. 3) ECOCARB allows carburizing in normal furnaces, with no loading restrictions and the ability to quench in various media for optimized properties.

1. Advanced Carburizing in Muffle-type Furnaces
The so-called "advanced carburizing processes" were developed in the early 90's in Europe and in the
mid-90's, the automobile industry has invested in production line furnaces based on vacuum carburizing
associated with high pressure gas quenching for gears and shafts. As the vacuum carburizing process
was not protected by international patents, many furnaces manufacturers worldwide have proposed this
technique on the market. At this period of time, most of users had the feeling that this new process would
replace gas carburizing in the very near future.
After almost 10 years of experience, the market for vacuum carburizing furnaces undergoes a serious
drop since the recent 4 years and the investment is now limited to specific applications and special parts.
Three main reasons explain the lack of interest for the vacuum carburizing industrial applications:
I. Costs and productivity
Although the overall carburizing duration is reduced by vacuum carburizing since the saturation and
carbon enrichment phases are processed at the maximum theoretical speeds, the productivity in terms
of tons/hour with respect to the furnace e volume is limited. This is mainly due to required spacing
between the parts for the carburizing but more specifically to the fact that vacuum carburizing is most of
the time associated with high pressure gas quenching which limits the loading density.
As the investment cost is much higher than that of a gas carburizing furnace it is obvious that the limited
productivity is major disadvantage.
II. Flexibility and hardening media
Despite intense technical development, the application of the vacuum carburizing is mainly restricted to
high pressure gas quenching, up to 15-20 bars. Industrial oil quenching applications are rare and hot oil
(> 130 °C) or salt quench applications (> 200 °C) have not been successfully applied for industrial
production. Although the steel manufacturers have increase their efforts in developing new high alloyed
steels to optimize the use of gas quenching at reasonable pressures (5 to 8 bars) [1], the scattering of
the results (dispersion) is still very important and the expected reduced distortion with respect to oil is still
not reliable.
III. Mechanical properties
The most expected benefit of the vacuum carburizing was a significant increase of the mechanical
properties, namely the fatigue resistance and the resilience resistance. This increase was expected
since the vacuum carburizing inhibits the formation of the intergranular oxidation at the surface of the
parts compared to conventional gas carburizing. Recently published reports [2], [3] have shown such
was not the case, and that in most of the cases, the measured mechanical resistance properties were
lower (20 - 30%) for the vacuum carburized and gas quenched parts that for the conventional
carbonitrided and oil quenched parts [2] for 16MnCr5 and 27MnCr5 type of steels. The main reason for
this lack of mechanical properties is explained by the evaporation and the migration of alloy element
(namely Mn) at the surface of the steel in vacuum. Furthermore, the results clearly show that the
measured properties (fatigue, hardness) had a much broader dispersion for the vacuum carburized
parts, leading to poor statistical CAM/CPK results.
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2. Alternatives to vacuum carburizing
Since the vacuum carburizing did not process did not meet the expected advantages in terms of quality,
productivity and mechanical properties, furnaces manufacturers and steel producers are developing new
techniques to overcome both the problems related to conventional gas carburizing and vacuum
carburizing.
One of the current trends is to develop new steels for high temperature carburizing application, in order
to reduce the carburizing and diffusion duration [4]. Such future steels would have a stabilized grain
growth at elevated temperatures, but their use in vacuum carburizing would be limited since
measurements [4] have shown an important drop in nitrogen content (> 50%) in the steel close to the
surface (0 - 0,2 mm); elevated temperatures will also increase the evaporation and migration of alloy
elements mentioned before.
Another trend is to develop new processes and furnaces design to overcome the difficulties faced with
vacuum carburizing furnaces. The goal is to obtain a carburizing process which inhibits the formation of
intergranular oxides network without modifying the alloy elements dispersion (no Mn evaporation) even
at high temperatures, and a furnace design which allows to harden (quench) in any media to get the best
possible quality, productivity and mechanical properties.
Figure 1 :
Schematic representation of a gas-solid reaction with respective kinetics resistances
In the recent years, SOLO Switzerland has developed and patented a process to meet such
requirements:
- inhibit intergranular oxidation at the surface
- use of normal hot wall furnaces (no vacuum, inhibit Mn evaporation)
- no loading density limitation
- enable maximum flexibility for mechanical requirements (controlled rest-austenite from 1-2 % to over
30%)
- quench and harden in any media including high gas pressure, hot oil or hot baths
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3. SOLO ECOCARB process description
The general flow rate of carbon in a conventional carburizing process is given by the combination of
three flows (figure 1):
Φ = Φ1 = Φ2 = Φ3 (1)
Whereas, Φ1 is the flow density into the gas towards the surface of the part, Φ2 is the flow density which
goes along with the chemical reaction of oxygen desorbtion at the surface of the steel (O adsorbed + H2 = H2O)
and Φ3 is the flow density which transport the carbon into the steel by diffusion.
The equation (1) can also be written:
Φ = Φ1 = Φ2 = Φ3 = (Pc - Pci)/ R1 = (Pci - Cs)/R2 = (Cs - Co)/R3 (2)
Whereas :
Pc : carbon potential in the gas in equilibrium
Pci : carbon potential at the interface
Cs : carbon concentration at the surface in the part
Co : initial carbon content of the part (core carbon content)
R1,2,3 : individual resistance for the respective 3 mechanisms
In the case of carburizing with CO/H2 gas mixtures, the resistance R3 is much smaller the resistances R1
or R2, so the reaction normally writes:
Φ= (Pc - Cs)/R = h (Pc - Cs) whereas R= R1 + R2 (3)
Therefore, should R2 << R1 is the kinetics controlled contorted by the transport in the gas phase, on the
other hand if R1 << R2 the kinetics is controlled by the kinetics of the chemical reaction on the interface.
In practice, such a situation is unfortunately never true for the following reasons (see figure 4):
- the Cs value varies slowly during the carburizing process, never reaching the set value of Pc
- the transport coefficient h is not constant during the cycle
- the formation of an oxide layer at the metal/gas interface modifies the conditions and makes it more
difficult for the carbon to diffuse into the steel. It shall be notified that this oxide layer takes place
even during the early stages of the carburizing process when the carbon content is still very low
close to Co.
In other words, the carbon transfer coefficient does not take place at constant concentrations and
speeds, leading to a difficult accurate control of the real situation.
To get rid of these perturbation effects, the process shall meet following requirements:
- the Cs concentration shall be fixed and controlled in order to have a clear picture of the flow density
- in the carbon enrichment phase, the surface concentration Cs shall reach the value of the saturated
austenite in order to enables the maximal carbon flow theoretical speed.
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4. The ECOCARB process takes place in a tight retort or bell-type furnace, equipped with a metallic muffle
to inhibit soaking effects and ensure a perfect inertia of the furnace with regard to the atmosphere. The
furnace has to be equipped with a very efficient convection system (designed turbine to enable constant
flow with variable resistance (∆P), defectors, etc. (figure 2). This design enables a temperature accuracy
of =/- 2,5 C, together with perfect gas convection and agitation (radial design）to allow fast purging and
very homogeneous distribution of the treatment gas.
Figure 2 :
Schematic representation of a SOLO bell-type
carburizing furnace showing the radial design
The principle of the process is basically identical to the so called "vacuum carburizing" but the major
difference is that it does not require vacuum. It can be decomposed into 4 major steps (figure 3).
Figure 3 :
Representation of the ECOCARB
process according to 4 major steps
(phases I, II, III and IV).
I. The heating up phase
The heating up takes place under pure nitrogen up to the enrichment temperature. This presents the
advantage to have a fast and homogeneous heating up duration and to avoid any oxidation of the
surface which could influence the process.
During this step, Φ =0, and Cs = Co.
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5. II. The enrichment phase
When the enrichment temperature T is reached, the conditions are set to get Cs = C saturation, so Φ = max.
This very rapid saturation of the austenite at the surface is obtained by injection of an hydrocarbon in the
furnace. Once the saturation is obtained (typically within a few minutes Cs = C saturation) the required
amount of carbon is provided according to different methods: additional hydrocarbon injection pulses
separated by nitrogen purging phases, b) controlled hydrocarbon flow rate with time (see figure 4) or c)
by setting a conventional carbon potential Pc = C saturation which presents the advantage to have a perfect
control of the flow conditions over an oxygen probe and/or a CO/CO2 infrared equipment. For this last
case, no intergranular will issue since the change in atmosphere from hydrocarbon to controlled high
carbon potentials is very fast due to the metallic muffle and Cs remains almost at Cs = C saturation, so no
oxidation may occur.
Figure 4 :
a) Calculated and measured weight
increase using a controlled hydro-
carbon flow rate adapted to the
weight increase (enrichment b-type
at 950 C);
b) Carbon profiles after the enrichment
phase and after the diffusion phase
[5] /
III. The diffusion phase
The diffusion takes place once all the carbon has been put into the parts. The diffusion takes place at
Φ =0, and Cs = variable, until the required carburizing depth and final carbon surface Cs final is obtained.
During the diffusion phase, it is possible to add a nitrogen profile to the carbon in order for instance to
control the amount of the rest-austenite in the superficial structure. This can be achieved by adapting an
accurate NH3 flow rate and allows to set up the austenite content from a few percent to some 35% on the
surface without increasing the carbon concentration at the surface to high values (see figure 5).
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6. IV. The final phase
During the final phase, the temperature is reduced in a controlled manner using heat exchangers to
reach the quenching temperature.
Then, the parts may be immerged in any possible quenching media according to the specifications
(distortion, hardness, etc.).
Note that in the SOLO bell-type furnaces, the transfer duration is reduced to zero since the parts are
directly transferred from the furnace into the quench tank(s) with no vestibules (see figure …). As a
result, the microstructure shows perfect carburizing profile, with no carbides and no superficial or
intergranular oxidation (figure 6)
Figure 5 :
a) Weight evolution using a final
nitrogen enrichment after diffusion
and
b) Respective carbon and nitrogen
profiles in the steel [5]
a) b)
Figure 6 : c)
a) Macrostructure of an ECOCARB carburized gear
b) Surface of the parts carburized with ECOCARB process without acid attack showing no
intergranular oxides at the surface and c) typical structure without oxidation
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7. Practical examples: gear parts
The ECOCARB process does not require specific equipment with respect to conventional gas
carburizing process, provided the furnace has a metallic muffle to change rapidly the atmosphere, so all
ECOCARB equipped furnaces can also run:
- gas carburizing or carbonitriding
- austenitisation under controlled or neutral atmosphere
- annealing
- tempering
The modular design of the SOLO bell-type furnaces (figure 7) allows any quenching media so the
quenching occurs in hot oil (>130 C) in high pressure gases or in hot baths > 220°C.
a) b)
Figure 7 :
a) Schematic example of a SOLO Profitherm bell-type installation with different quenching media;
b) The modular design allows any combinations for direct quenching with no vestibule.
Figure 8 :
a) Typical data record for a computer controlled ECOCARB carburizing
b) Hardness and carbon profiles
Figure 8 shows a typical recorded data file for an ECOCARB process with enrichment technique
according to IIc. It can be seen that the advantage of this process is to provide a reliable file for the
process control in accordance with ISO 9000 requirements to ensure reproducibility. The results are in
full accordance to the calculated values.
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8. Major obtained results
The overall treatment time at 940 C (type low carbon MnCr or NiCr steels) for a carburizing depth of 1,2
mm including heating up is approx 6 hours depending on the load weight and on the specific surface of
the parts.
The surface hardness are all located at 62 +/- 1 HRC after oil quenching within one load (6 samples) and
perfectly reproducible.
The very accurate control of the carbon at the surface, together with controlled additional nitrogen profile
enables to adapt the level of residual austenite at the surface to the required value (figure 9).
a) b) c)
Figure 9 :
Microstructures showing different rest-austenite at the surface of the steels obtained by setting the
nitrogen enrichment to the required values a) 25%, b) 35% and c) > 50%.
Figure 10:
Example of gear loads treated with Ecocarb
Figure 10 show typical gear parts treated with the SOLO ECOCARB process. The gross weight varies
from 350 Kg to 700 Kg and the typical requirements are carburizing depths 0,8 - 1,0 mm; 1,0 - 1,2 mm.
The microstructures show no formation of oxide layer and no intergranular oxidation network and the
micro-probe tests do not point out any significant variation of the Mn and Cr contents at the surface of
the steel.
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9. As a result, the expected benefits concerning the improved mechanical properties can be verified by
measuring the residual stressed in the carburized layer:
It can be seen on figure 11 that the ECOCARB process creates the expected compressive stresses at
the surface although conventional gas carburizing generates tensile stresses on approx 20 microns.
Such compressive stresses will inhibit surface micro-cracks to propagate under fatigue solicitations.
Figure 11 :
Measured residual stresses at the surface of gas
carburizing and ECOCARB carburizing gears
(treatment at T 940 C, depth 1,1 mm)
The fatigue resistance has been significantly increased compared to classical gas carburizing process
formerly used in pusher type furnaces. The effective useful torque could be raised by more than 35%
with increased fatigue and resilience resistances.
The amount of rest-austenite will transform under high stresses in use, increasing the residual
compressive stresses at the surface.
The perspective of special alloys for high temperature carburizing applications (above 1000 C) will also
increase the interest for the ECOCARB process, since no deterioration of the surface quality can be
expected.
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Bibliography:
[1] B. Maisant, D. Forest, C. Pichard, Ascometal, SVW/ASTT, 3 - 4 April 2003 Zurich, Conference Proceedings,
p.47
[2] Fernand Da Costa, Renault, European Congress, ATTT/AWT/ASTT-SVW/VWT, 18 - 19 March 2004,
Strasbourg, France, Conference Proceedings
[3] B. Clausen, F. Hoffmann, P. Mayr, SVW/ASTT, 3 - 4 April 2003 Zurich, Conference Proceedings, p.159
[4] Frank Hippenstiel, Walter Grimm, SVW/ASTT, 3 - 4 April 2003 Zurich, Conference Proceedings, p.59
[5] D. Zimmermann, Haerterei Technische Mitteilungen, 47, 1992/1, p.3
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