Special Issue; Products for Industrial Machinery - NTN

NTN TECHNICAL REVIEW No.742006
Technical Paper
Atmospheric Control Method for JIS-SUJ2
Carbonitriding Processes
Chikara OHKI
NTN has been investigating the mechanism of nitrogen
penetration into JIS-SUJ2 bearing steel during carbonitriding
processes. In the course of carbonitriding experiments under
various conditions and Electron Probe Micro Analysis (EPMA)
analysis, the influences of carbon activity and base gas
components on nitrogen penetration behavior were examined. As
a result, the nitrogen penetration content of steel increased with
an increase in undecomposed NH3 partial pressure and with an
decrease in carbon activity, and H2 partial pressure.
Assuming that the penetration mechanism of nitrogen is similar
to that of carbon, a calculation method was developed in order to
estimate nitrogen concentration distribution profiles. It was
confirmed that the nitrogen concentration distribution profiles
predicted by NTN's method agreed well with the experiment
results under various conditions. Hence, a development of a new
atmosphere control method for JIS-SUJ2 carbonitriding
processes was achieved.
1. Introduction
Carbonitriding processes are heat treatment
techniques that are often used to improve the fatigue
strength of steel materials. There are many
applications for using carbonitriding techniques on
bearing steels. In particular, it has been verified that
carbonitriding bearing steel materials contribute to
longer bearing life under contaminated lubrication
conditions. 1)
The carbon penetration mechanism that takes place
during carburizing is understood. In contrast, although
there has been active research on the nitrogen
penetration mechanism in carbonitriding processes, 2)
there has been little research on nitrogen penetration
in carbonitriding processes for high carbon steels
(including bearing steel). To address this, NTN has
systematically performed a series of carbonitriding
experiments for JIS-SUJ2 bearing steel in an attempt
to clarify the nitrogen penetration mechanism.
3) 4)
2. Test Methods
2.1 Experimental furnace and furnace
atmosphere measuring method
For the series of experiments performed, a batchtype
heat treatment furnace (capacity 120 L) was
used. During carbonitriding processes, the
atmosphere was analyzed by the equipment
summarized in Table 1. The furnace itself is
schematically illustrated in Fig. 1 and the base gas
feeder is shown in Fig. 2.
2.2 Material tested and method for measuring
nitrogen concentration
The material used for the test was a JIS-SUJ2 high
carbon chromium bearing steel. The chemical
components of the material are summarized in Table 2.
The test pieces were ring-shaped with dimensions of 30
ID x 38 OD x 10 T (mm).
Nitrogen concentration distributions of the test
Elemental Technological R&D Center
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