Special Issue; Products for Industrial Machinery - NTN

NTN TECHNICAL REVIEW No.742006
CCO22COBoudouard reaction
a c
K 1 P CO2
K exp
P CO
2
G
T
RT
G T 172500175.6T
where ac = carbon activity; K1 = equilibrium
constant; R = gas constant (8.31441 J/(mol-K)); T =
absolute temperature (˚K); G** = standard free
energy of formation; 5) PCO = CO partial pressure and
PCO2 = CO2 partial pressure (atm).
Because of the definition of carbon activity, ac1 is
not possible. However, as shown by expression (1), a
calculated value of ac can be greater than 1 in certain
situations, and a carbonitriding process can take
place. 6) To investigate the effect of such an
atmosphere, expression (1) was redefined as (1’), and
this expression was used as a parameter for the
experiment.
P CO
2
1
2
3
a c 1'
K 1 P CO2
where, when ac*1, ac* = ac (carbon activity), and
when ac*1, ac* is a variant expressed by expression
(1’).
2.4 Experimental Parameters
2.4.1 Effects of partial pressure of
undecomposed NH3 and carbon activity
on carbonitriding processes
Within every set of parameters in these experiments,
temperature was maintained at 850˚C and furnace
pressure was held at 1.03 atm (1 atm = 1.013 x 10 5
Pa). At the same time, the total surface area of the
test pieces was the same. Four types of test
parameters were varied: the carbon activity ac*,
carbonitriding process duration, base gas flow rates
and partial pressure of undecomposed NH3. The ac*
was held at one of six levels ranging between 0.78
and 1.08 and the carbonitriding process duration
varied from 1,800 to 18,000 seconds. The base gas
flow rate ranged from 6 to 20 L/min and the
undecomposed NH3 varied from 0.36 to 3.4 x 10 -3 atm.
Incidentally, the base gas feeder shown in Fig. 2 was
not used for these experiments. Instead, the base gas
used was an ordinary endothermic modified gas
derived from propane.
temperature was maintained at 850˚C, the process
duration was set at 9,000 seconds, and the furnace
pressure was kept at 1.03 atm. At the same time, the
total surface area of the respective test pieces was the
same.
The flow rates of each component in the base gas
and the partial pressure of undecomposed NH3 are
listed in Table 3. The H2 flow rate was set to a range
from 1.15 to 6.9 L/min, N2 flow rate from 0 to 9.2 L/min
and the CO flow rate from 1.15 to 4.6 L/min. The flow
rates of these gases were adjusted so that the total
flow rate of the mixture of gases was 11.5 L/min in
each set of conditions. This adjustment was achieved
with the base gas feeder shown in Fig. 2. Two levels
of partial pressure of undecomposed NH3 were used:
0.0010 atm and 0.0021 atm.
Table 3 Conditions for experiments to investigate the
influence of base gas components
Undecomposed
NH3 volumetric
partial pressure
H2 flow rate
(L/min) 1
N2 flow rate
(L/min) 1
CO flow rate
(L/min) 1
Total
(L/min) 1
0.0010 6.9 3.45 1.15 11.5
0.0010 6.9 2.30 2.3 11.5
0.0021 6.9 2.30 2.3 11.5
0.0010 6.9 0 4.6 11.5
0.0021 6.9 0 4.6 11.5
0.0010 1.15 9.2 1.15 11.5
0.0021 1.15 9.2 1.15 11.5
0.0010 1.15 8.05 2.3 11.5
0.0021 1.15 8.05 2.3 11.5
0.0010 1.15 5.75 4.6 11.5
0.0021 1.15 5.75 4.6 11.5
0.0010 3.45 5.75 2.3 11.5
0.0021 3.45 5.75 2.3 11.5
0.0010 3.45 6.9 1.15 11.5
0.0021 3.45 6.9 1.15 11.5
0.0010 3.45 3.45 4.6 11.5
0.0021 3.45 3.45 4.6 11.5
1) Converted to values under ordinary temperature and atmospheric pressure
2.4.2 Effects of base gas component partial
pressures on carbonitriding processes
For this series of experiments, the heating
-46-