Reducing CO2 Emissions by Using Carburizing Gas Regenerator
Toshikazu Yoshii, Hiroyuki Nakatsu and Tomohiko Fujino
Heat Treatment Furnace Div, Chugai Ro Co., LTD., Sakai, 592-8331, Japan
Gas carburized quenching process exhausts much CO2 because fossil fuels is burned. For this reason, reducing CO2 is one of the urgent tasks for thisprocess. “carburizing gas regenerator” is an equipment, including polyimide hollow-fiber membrane filter, which allows H2 to permeate therethrough. AsH2 is generated during carburizing process, by circulating carrier gas using this equipment, H2 is eliminated by filter, and H2 eliminated carrier gas is able
to reuse to the furnace.
Using carburizing gas regenerator enables to reduce total amount of carrier gas and CO2 emissions up to 50%, and get less variation of hardness and carbon
concentration among carburized quenching specimens.
(Received September 26, 2023; Accepted December 27, 2023)
Keywords: gas carburized quenching, reusing carrier gas, H2 permeating filter, carburizing gas regenerator, CO2-reduction

1. Introduction

following equation 3).
C3H8+3CO2→6CO+4H2C3H8+3H2O→3CO+7H2
C3H8→3(C)+4H2

In the conventional gas carburizing process, carrier gas isused at least 6 times of furnace volume per hour 1).
However, only small amount is utilized for carburizing in
the introduced carrier gas, and most of it burned anddisposed of outside the furnace, resulting in CO2 emissions.
Carburizing reaction produces especially H2 during process
and affects atmosphere compositions. The reason why
introducing large amount of carrier gas into furnace is tokeep atmosphere compositions stable 2). If the amount ofcarrier gas is not enough, it is difficult to control carbon
potential (CP) precisely and results in variation of carbon
concentration and hardness in the carburized quenchingtest.
In this study, we conducted gas carburized quenching
tests with three conditions: conventional amount of carriergas, 50% reduction of carrier gas using carburizing gas
regenerator and 50% reduction of carrier gas without using
carburizing gas regenerator. After that, we comparedatmosphere compositions, carbon concentration, hardness
and metallographic structure among three conditions.

Reaction of enrich gas and CO2 generate H2.
Fig. 1 shows generated H2 volume during carburizing at
1203K-CP1.15. Total surface area of specimen is 10.8m2.
According to Fig. 1, large amount of H2 generates at thebeginning of carburizing period and decreases gradually.
Fig. 2 shows calculated value of the change of COconcentration in the furnace according to the generated H2shown in Fig. 1 when volume of carrier gas is introduced
into the furnace 20m3/h. The carrier gas of 20m3/h is ageneral volume of gas carburized quenching process. As
the carrier gas is based on C4H10, initial CO concentration
is 23.5%. It can be confirmed that CO concentration
decreases due to H2 generation by carburizing and does not
return to 23.5% even after 100 minutes.
0.7
Amount of H2 generation, Q H /m3/h

2. Method of carburizing gas regenerating
2.1 Gas reactions of during carburizing
During gas carburizing, carrier gas (RX gas) containing
CO, N2 and H2 as main components is used and addedenrich gas to control CP. Carrier gas is generated by adding
a certain percentage of air to C3H8 or C4H10 reacting attemperature around 1323-1373K. The enrich gas is C3H8 orC4H10 which is added to carrier gas at small amount.
Carburizing to steel is caused by the reaction of gases
containing carbon, mainly CO. Carburizing reaction on thesurface of steel is mainly expressed by the following
equation.
2CO→(C)+CO2
CO+H2→(C)+H2O
CH4→(C)+2H2

(4)
(5)
(6)

0.6
0.5
0.4
0.3
0.2
0.1
00

20

4060
Carburizing time, t /min

80100Fig. 1 Amount of H2 generation during carburizing.

(1)
(2)
(3)

Above (C) represents carbon, carburized into steel. CO2and H2O are generated by carburizing reaction which affect
CP. To maintain CP, enrich gas is added.
If C3H8 is used as enrich gas, reaction is expressed by the©2024 JSHT

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NETSU SHORI, Vol.64, Extra edition (2024)

it is possible to control the amount of H2 exhausting bycontrolling CO concentration stable.

CO concentration, C CO /(%)

24

23.5

large flow

carriergas

CO analyzer

23

small flow

controller

N2 gasfurnace22.5

22membranefilter

0204060
Carburizing time, t /min

80

cooler100

exhausteductor pumpCarburizing Gas
Regenerator
circulation pump

Fig. 2 Calculated change of CO concentration affectedby H2 generation.

signalFig. 4 Constitution of gas regeneration equipment.

In the conventional gas carburizing, a large amount ofcarrier gas is introduced to mitigate the effect of increased
H2 during carburizing. If carrier gas is reduced to reduce
exhausted CO2, it is difficult to control CP because H2
increases. Therefore, if the increased H2 can be removed,
carrier gas in the furnace can be reusable (see Fig. 3).

reduce carrier gas
Difficult to CP
controlcarrier gas

H2(a) Carburizing gas regenerator (b) Control panel
Fig. 5 Appearance of “carburizing gas regenerator”.

enrich gas

reduce carrier gas& remove H2Carrier gas canbe reusable

Fig. 3 Concept of reusing carrier gas.
2.2 Constitution of gas regeneration equipment
To regenerate carrier gas, the gas inside furnace iscirculated by a circulation pump and H2 is removed byusing “carburizing gas regenerator” at the path ofcirculation gas, as shown in Fig. 4.
“Carburizing gas regenerator” as shown in Fig. 5 canselectively permeate H2 through polyimide hollow-fiber
membrane filter (see Fig. 6) by using educator pump andexhaust it. Polyimide has excellent thermal stability andmechanical strength, and has high gas separation properties4), 5). Since the atmosphere in the furnace is disturbed whencharging the specimen, conventional amount of carrier gas(large flow) is introduced during heating period andholding period to ensure sufficient replacement ofatmosphere. Since carburizing period of using carburizing
gas regenerator, small amount of carrier gas (small flow) isintroduced. To change amount of carrier gas, twoswitchable flow paths is necessary.
In addition, in order to prevent excessive H2 exhausting,

NETSU SHORI, Vol.64, Extra edition (2024)

polyimide hollow-fiber membraneregeneration gas

circulation gas

eductor pumpP

exhaust gas (H2)

Fig. 6 Polyimide hollow-fiber membrane filter.

228©2024 JSHT

3. Experiment

3.3 Heat treatment condition
Heat cycle of carburized quenching is shown in Fig. 9.
Three heat cycle conditions were conducted as below.
(a) Base condition:
Volume of carrier gas is conventional.
(b) Reduce carrier gas:
Not using carburizing gas regenerator.
(c) Reduce carrier gas:
Using carburizing gas regenerator.

3.1 Specimens
The specimens used for experiment were cylindricalshape (φ18×50mm) and made of chromium steel (JISSCr420). Chemical compositions of chromium steel (JISSCr420) is listed in table1.
To investigate carburizing variation, we set corner andcenter position of loading position. In order to increase thesurface area of specimens, we loaded dummy steel plates tomake the surface area about 11m2 as shown Fig. 7.

C
0.22

Table 1 Chemical compositions ofchromium steel (JIS SCr420)/ (mass%).
SiMnPSCuNi
0.25 0.68 0.022 0.016 0.12 0.071203K

Cr
1.04

heatingholding

carburizing
CP1.15

diffusion
CP0.85

60min

30min

100min50min
5.6m3/h(a)(b)(c)

5.6m3/hcool down/holding
CP0.8560min

oil quenching
393K

1.6m3/hCarburizing Gas Regenrator

Fig. 9 Heat cycle of carburized quenching.
For (a), the volume of carrier gas was 5.6m3/h
throughout the cycle, for (b) and (c), heating period andholding period were 5.6m3/h and since carburizing periods
were 1.6m3/h. For carburizing gas regenerator, circulating
gas volume was 6m3/h and the valve of educator pump
was controlled to maintain a constant CO concentration of
23.5%.
Total amount of carrier gas was 28m3 for (a), and 14m3for (b) and (c), which were as half as (a).
3.4 Characterization
After carburized quenching test, we measured
distribution of hardness with micro vickers hardness tester,
metallographic structure with digital microscope anddistribution of carbon concentration with EPMA ofspecimens. Besides, during carburizing, we measured H2and CO concentrations of furnace atmosphere with H2sensor and infrared analyzer.

Fig. 7 Loading method of carburized quenching test.
3.2 Test furnace
A batch-type test furnace shown in Fig. 8 was used for
carburized quenching test. Effective dimensions were
600mm in width, 600mm in length, 600mm in height, andthe maximum load capacity was 200kg. The atmosphere
during carburizing was controlled by CP using O2 sensor.
The carrier gas was based on C4H10, and CP was calculatedwith a constant CO concentration of 23.5%. Aboutquenching, hot type oil of Daphne Hi Temp Oil A(IDEMITSU) was used as a quenching media.

4. Results

Fig. 8 Batch-type test furnace.

1100

0.8

90

0.6

80

0.4

70
Exhausting gas volume

0.2

60

Hydrogen concentration

0H2 concentration, Ch /%Exhausting gas volume, Vd /m3/h

4.1 Verification of H2 emissions
Before carburized quenching test, we checked theamount of exhausting gas volume in circulating gas andconcentration of H2 contained in the exhausting gas. Theresults are shown in Fig. 10. As the volume of circulating
gas increased, the volume of exhausting gas also increased.
Concentration of H2 in exhausting gas was over 95%.

500123456789Circulating gas volume, Vc /m3/hFig. 10 Exhausting gas volume and H2 concentration.

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NETSU SHORI, Vol.64, Extra edition (2024)

4.2 Behavior of the atmosphere
Fig. 11 shows the behavior of CO and H2 concentrationsfor (a), (b) and (c) during carburized quenching test.
CO concentration was constant at 23.5% since
carburizing period under condition of using carburizing gas
regenerator (c). On the other hand, under condition of notusing carburizing gas regenerator (b), CO concentration
decreased to a minimum of about 21%, and was as low as
22% or less even immediately before quenching. Inaddition, a slight decrease in CO concentration from 23.5%
was observed even with the conventional carrier gas
volume (a). Therefore, even in conventional gas volume
condition of (a), controlled CP was considered to be slightdifference from calculated CP.
Concerning H2, concentration of H2 was increased at thesame time as carburizing period starts, and was decreased
accordingly for (a) and (b). In (c), the increase of H2
concentration was suppressed and the CO concentration
was stable.
25
heating

CO concentration, C co/(%)

24

holding

carburizing

diffuisioncase depth and less variation. This is probably due to thestable CO concentration in the atmosphere.
900

Hardness, H /(HV 0.3)

700
600
500
400
300
200
00.1

0.2

0.3

0.4
0.5
0.6
0.7
0.8
Distance from surface, d /mm

0.9

1

1.1

1.2(a) Base condition:
Volume of carrier gas is conventional.
(b) Reduce carrier gas:
Not using carburizing gas regenerator.
(c) Reduce carrier gas:
Using carburizing gas regenerator.

cool down/holding

2322Fig. 12 Distribution of hardness of loading position.

2120
19

Table 2 Effective case depth of loading position.
Effective case depth, de/mm
CornerCenter
(a)
0.57
0.57
(b)
0.56
0.48
(c)
0.63
0.62

(a)
(b)

18(c)

17424038
H2 concentration, Ch/ (%)(a): corner(a): center(b): corner(b): center(c): corner(c): center800(a) Base condition:
Volume of carrier gas is conventional.
(b) Reduce carrier gas:
Not using carburizing gas regenerator.
(c) Reduce carrier gas:
Using carburizing gas regenerator.

363432302826(a)

24(b)

22(c)

20020

40

6080

4.4 Metallographic structure
Fig. 13 shows metallographic structure of (a), (b) and(c). All conditions show martensite structures and nodifferences.

100 120 140 160 180 200 220 240 260 280 300
Time, t /min(a) Base condition:
Volume of carrier gas is conventional.
(b) Reduce carrier gas:
Not using carburizing gas regenerator.
(c) Reduce carrier gas:
Using carburizing gas regenerator.
Fig. 11 Behavior of CO and H2 concentrations.
4.3 Distribution of hardness
The distribution of hardness is shown in Fig. 12, and theeffective case depth is shown in Table 2. (b) showed
shallower effective case depth than (a) and (c), andvariation between center and corner was large. The reason
why center position of (b) shallower than corner position islow carbon concentration (see Fig. 14) and low
hardenability due to slow flow rate of quenching oil.
Compared to (a) and (c), (c) was a slightly deeper effective

NETSU SHORI, Vol.64, Extra edition (2024)(a) Condition (a)230©2024 JSHT0.8(a): corner

Carbon concentration, C /(%)

0.7(a): center(b): corner0.6(b): center(c): corner0.5(c): center

0.4
0.3
0.2
0.1

0

0.1

0.2

0.3

0.4
0.5
0.6
0.7
0.8
Distance from surface, d /mm

0.9

1

1.1

1.2

(a) Base condition:
Volume of carrier gas is conventional.
(b) Reduce carrier gas:
Not using carburizing gas regenerator.
(c) Reduce carrier gas:Using carburizing gas regenerator.

(b) Condition (b)Fig. 14 Distribution of carbon concentration.
Table 3 0.35wt%C depth of loading position.
0.35wt%C depth, dc/mm
CornerCenter
(a)
0.57
0.61
(b)
0.52
0.51
(c)
0.60
0.57
(a) Base condition:
Volume of carrier gas is conventional.
(b) Reduce carrier gas:
Not using carburizing gas regenerator.
(c) Reduce carrier gas:
Using carburizing gas regenerator.

(c) Condition (c)(a) Base condition:
Volume of carrier gas is conventional.
(b) Reduce carrier gas:
Not using carburizing gas regenerator.
(c) Reduce carrier gas:
Using carburizing gas regenerator.

4.6 The amount of CO2 emissions
The amount of CO2 emissions generated by exhausted
carrier gas at this test, using a formula of Japanese Ministryof the Environment was open to the public 6), were 14kg in(a), 7kg in (b) and (c), which were reduced 50% to (a).

Fig. 13 Metallographic structures.
4.5 Distribution of carbon concentration
Fig. 14 shows the distribution of carbon concentration,
and Table 3 shows 0.35wt% carbon concentration depth. As
with the distribution of hardness, (b) had a shallower
distribution of carbon concentration than (a) and (c), andshowed variation near surface.
From results of Fig. 14 and Table 3, the reason why
effective case depth of (b) in Fig. 12 and Table 2 wasshallow was caused by shallower carbon concentration.
Shallow carbon concentration was due to inaccurate CP
control because amount of carrier gas was not enough.

©2024 JSHT

5. Discussion
Fig. 15 shows minimum volume of carrier gas whenusing carburizing gas regenerator. The minimum volume ofcarrier gas represents the limit at which the furnacepressure and atmosphere can be maintained when using
carburizing gas regenerator. According to Fig. 15, thesmaller total surface area of specimens and lower CP, thesmaller minimum volume of carrier gas, this results in morecarrier gas can be reduced.
Table 4 shows the calculated results of CO2 emissions in
actual production process. In this simulation, carrier gas
volume of conventional process is assumed 20m3/h. In caseof using carburizing gas regenerator, carrier gas volume isused of Fig. 9 at total surface of specimen 10, 15 and 20m2.
Annual operating time is assumed as 24 hours*300 days.
As a result, by using carburizing gas regenerator,

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NETSU SHORI, Vol.64, Extra edition (2024)

gas regenerator, distribution of both hardness andcarbon concentration were shallow, and showedvariation.

annual CO2 emission can be reduced by more than 50%.

Minimam required carrier gas volume, V /m3/h

6.5
CP1.05

6.0

References

CP1.15CP1.255.51) The Iron and Steel Institute of Japan: Hagane noNetsusyori (Heat treatment of steel) (5th edition),pp. 334.
2) T. Naito: Shintanyakiire no Jissai (Practical carburizingtechnology) (2nd edition): Nikkan kogyo shimbunsya,
pp.9-10.
3) M. Ichihara: Jitsumu Hyoumen Gijutsu Vol.19, No.9(1972) 442-449.
4) Y. Kusuki and S. Nakanishi: SEN-I GAKKAISHI
Vol.51, No.2 (1995) 21-27.
5) A. Nakamura, K. Ninomiya and M. Hotta: Journal ofthe Fuel Society Vol.67, No.12 (1988) 1038-1051.
6) Japanese Ministry of the Enviroment: Calculation
method and list of emssion factors.
https://ghg-santeikohyo.env.go.jp/files/calc/itiran_20
20_rev.pdf.

5.0
4.5
4.03.53.012141618
Total surface area, A /m2202224Fig. 15 Minimum volume of carrier gas.
Table 4 Annual CO2 emissions.

Totalsurfacearea, A/m2(a)(b)(c)

101520

Amount oftotal carriergas, Q/(m3/ch)

Annual CO2emissions,
E /t

CO2 emission
ratio

73.6
30.9
31.1
35.8

1.0
0.42
0.42
0.49

100
41.90
42.25
48.55

(a) Base condition:
Volume of carrier gas is conventional.
(b) Reduce carrier gas:
Not using carburizing gas regenerator.
(c) Reduce carrier gas:
Using carburizing gas regenerator.
6. Assignment
For gas carburizing process, it is possible to reuse carriergas by using carburizing gas regenerator at batch-typefurnace. But following assignments still remain.
(1) As polyimide hollow-fiber membrane filter does nothave NH3 resistance, carburizing gas regenerator
cannot use at atmosphere including NH3, such ascarbonitriding process.
(2) For continuous gas carburizing furnaces, circulation
carrier gas of carburizing gas regenerator may affectoriginal gas flow in the furnace and result in disturbing
atmosphere in the furnace. Therefore, some attention isnecessary when using carburizing gas regenerator forcontinuous gas carburizing furnaces.
7. Conclusions
The carburized quenching test using carburizing gas
regenerator was conducted and got following results.
(1)
Compared to conventional carburized quenching,total amount of carrier gas could be reduced by 50%,and distribution of both hardness and carbon
concentration were deeper and less variation thanconventional results.
(2) In case of reducing carrier gas not using carburizing

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