Special Issue; Products for Industrial Machinery - NTN

Atmospheric Control Method for JIS-SUJ2 Carbonitriding Processes
expression (13) may be used in several calculations to
determine the surface nitrogen concentration and
nitrogen concentration distribution.
Note, however, that prediction of nitrogen
concentration distribution with expression (13) is
possible only until the time that the surface nitrogen
concentration on the steel material being processed
reaches saturation. In reality, once having reached
saturation, surface nitrogen concentration will increase
little, if any.
Therefore, after the nitrogen concentration on the
outermost surface of the steel has reached saturation,
it is necessary to maintain the surface nitrogen
concentration at a constant level. Then, solve the
diffusion formula through numerical calculations in
order to predict the nitrogen concentration distribution.
4.4 Verification by comparison of predicted
values of nitrogen concentration distribution
and values obtained through experiments
By comparing the actual measurements from
experiments with predicted nitrogen concentration
distribution values, NTN has proved the validity of the
nitrogen penetration mechanism and nitrogen
concentration distribution prediction method described
above. For this verification, the results of experiments
with the test conditions summarized in Table 5 were
adopted.
The actual measurements and predicted values for
nitrogen concentration distribution are graphically
plotted in Fig. 12. NTN has learned that the predicted
nitrogen concentration distribution values match actual
measurements well with various partial pressures of
undecomposed NH3, carbon activity ac* values, H2
partial pressure values and process duration values.
Thus, it is believed that the assumed values and
determined unknown quantities related to nitrogen
penetration in this paper are valid.
Test
conditions
Table 5 Experimental conditions
Gas component partial
pressure (atm) 1)
Undecomposed NH3
Process
duration
(s)
1 0.0016 0.354 1.03 9,000
2 0.0020 0.622 0.97 9,000
3 0.0016 0.356 0.94 9,000
4 0.0010 0.332 0.88 4,500
1) Converted to values under ordinary temperature and atmospheric
pressure
H2
a c
Nitrogen concentration (wt%)
Nitrogen concentration (wt%)
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1
Test condition 1) 0.9
Test condition 2)
Actual measurements
0.8
0.7
Actual measurements
Predicted value
0.6
Predicted value
0.5
0.4
0.3
0.2
0.1
0
0.2 0.4 0.6
Depth from surface (mm)
Nitrogen concentration (wt%)
1
Test condition 3)
0.9
0.8
Test condition 4)
Actual measurements
0.7
Actual measurements
Predicted value
0.6
Predicted value
0.5
0.4
0.3
0.2
0.1
0
0 0.2 0.4 0.6
Depth from surface (mm)
Nitrogen concentration (wt%)
0.2 0.4 0.6
Depth from surface (mm)
0 0.2 0.4 0.6
Depth from surface (mm)
Fig. 12 Nitrogen concentration distribution profile of experimental numbers 1)4)
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