Special Issue; Products for Industrial Machinery - NTN

Atmospheric Control Method for JIS-SUJ2 Carbonitriding Processes
Table 1 Equipment used for atmospheric measurement
Equipment Measurement Measurement technique
IR gas concentration analyzer
Mirror-surface cooling dew-point recorder
Zirconia sensor
Thermal conductivity gas analyzer
Thermal conductivity detector
CO/CO2 concentration
H2O concentration
O2 concentration
H2 concentration
Undecomposed NH3 concentration
Non-dispersion IR absorption
Optical dew point
Zirconia oxygen partial pressure analyzer
Thermal conductivity
Gas chromatograph (carrier: helium)
Vestibule
Heating
chamber
Cooling blast air
chamber
Heating
chamber
A
B
Oil
quenching
tank
C
Fig. 1 Heat treatment furnace
pieses were determined by electron beam analysis (2
m spot diameter, 2m intervals between measuring
points) with an Electron Probe Micro Analyzer
(EPMA). The test pieces were placed in the center of
the experimental furnace for carbonitriding prior to this
measurement. A sample piece was cut, as illustrated
in Fig. 3, from each test piece that underwent
carbonitriding (after quenching). Then, the nitrogen
concentration distribution on each sample piece was
measured on the cut surface, beginning at the
midpoint at the outer diameter and proceeding toward
the bore.
To determine the amount of nitrogen penetration of
each test piece, the nitrogen concentration distribution
obtained from analysis were integrated in the direction
to where the nitrogen concentration approached zero
(Fig. 4). The total amount of nitrogen penetration was
converted into a weight, assuming that the specific
gravity of the original material was 7.85 x 10 -3 g/mm 3 .
2.3 Definition of carbon activity
One very important factor that can influence the
atmosphere for carbonitriding is the carbon activity,
which determines carbon penetration. The author
employed the Boudouard reaction for calculating
carbon activity. The equilibrium reaction is defined by
expressions (1), (2) and (3) below:
Fig. 2 Base gas feeder
(A: Supply from pure gas tank, B: mass flow controller,
C: gas mixer)
Table 2 Chemical composition of JIS-SUJ2 used (wt%)
C Si Mn P S Ni Cr Mo Cu
0.99 0.26 0.44 0.012 0.006 0.08 1.46 0.03 0.15
38
30
10mm
5mm
Area analyzed by EPMA
Cross-section
of test piece
Outside
diameter
Inside
diameter
Fig. 3 Test specimen dimensions and scanning position
1.2
1.1
1
0.9
0.8
0.78wt%
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
-0.1
0 0.2 0.4
Depth from surface mm
Amount of nitrogen penetrationg/mm 2 =7.8510 -3 g/mm 3 0.002mm0.78wt%100
Fig. 4 Definition of nitrogen penetration content
0.6
-45-