Special Issue; Products for Industrial Machinery - NTN

Atmospheric Control Method for JIS-SUJ2 Carbonitriding Processes
4. Nitrogen Penetration Mechanism,
and Prediction of Nitrogen
Concentration Distribution
From these findings, it is apparent that regarding
nitrogen penetration during carbonitriding of JIS-SUJ2
high-carbon chromium bearing steel, the following
three factors are critical: 1) partial pressure of
undecomposed NH3 in the furnace, 2) ac* in the
atmosphere (carbon activity), and 3) H2 partial
pressure in the furnace. Based on these findings, the
nitrogen penetration mechanism and the method for
predicting nitrogen concentration are discussed
below.
4.1 Hypothesis of the nitrogen penetration
mechanism
Let us assume that the nitrogen penetration in
carbonitriding of JIS-SUJ2 proceeds according to the
equilibrium reaction defined in expression (5).
Expression (6) represents the equilibrium constant KX
for expression (5).
N3/2H2NH3
P NH3
a N
K X P H2
3
2
5
6
where, aN* is “an unknown quantity equivalent to
the nitrogen activity” in equilibrium between
atmospheric gas and steel, and the unit of partial
pressure is atm. For convenience, the equilibrium
constant K X in expression (6) is allowed to remain an
unknown quantity.
In a carbonitriding process, even if the carbon
potential, carburizing duration and process
temperature remain unchanged, the carburized depth
can vary depending on variation in the atmospheric
components according to Neumann et al. 7) They
expressed the cause of this phenomenon with a
carbon penetration velocity formula (expression (7))
that uses a carbon transfer coefficient .
qC G C s 7
where, q = carbon penetration velocity (g/mm 2 s);
= carbon transfer coefficient (g/mm 2 s); C G = final
carbon concentration (wt%); C S =carbon concentration
on the material’s outermost surface (wt%).
Being equivalent to the carbon potential, C G can be
expressed as a product of the carbon activity ac and a
carbon solubility limit concentration A S . Therefore,
expression (7) can be rewritten as expression (7’)
below.
qacAsCs7’
If the manner of the nitrogen penetration
mechanism is assumed to be identical to that of the
carbon penetration mechanism, then, by referring to
expression (7), the nitrogen penetration velocity can
be described by expression (8) below.
q N N N G N s 8
where, q N = nitrogen penetration velocity (g/mm 2 s);
N = nitrogen transfer coefficient (g/mm 2 s); N G = final
nitrogen concentration (wt%); N S = nitrogen
concentration on the material’s outermost surface
(wt%).
Then, expression (8) is rewritten as expression (8’)
below by replacing N G in expression (8) with a N -A SN .
q N N a N A SN N s 8'
where, A SN is the saturation value of nitrogen
concentration (wt%).
When expression (6) is substituted in expression
(8’), then the nitrogen penetration velocity is defined
by expression (9).
P
NH3
3
K X P H2
2
q N N A SN N s
9
In expression (9), the variables that can be
investigated by measuring the atmosphere for
carbonitriding processes are NH3 partial pressure and
H2 partial pressure. Since NS is determined based on
the actual measurements of nitrogen concentration
distribution, there are three unknown quantities in
expression (9): N , K X , and A SN . ( N S can also be
determined through calculation involving N , K X , A SN
and the diffusion coefficient D.)
4.2 Hypothesis about nitrogen concentration
distribution in steel
Suppose that the nitrogen concentration distribution
profile is compatible with the Gaussian error function
as in the case of the carbon concentration profile, then
the nitrogen concentration distribution profile can be
expressed by the expression (10).
X
NN s 1erf
2Dt t
10
where, N = nitrogen concentration at a position X
(wt%); N S = nitrogen concentration on the material’s
outermost surface (wt%); X = distance from the
surface of the material being processed (mm); D =
diffusion coefficient (mm 2 /s); and t t = process duration
(s). The unknown quantity in expression (10) is the
diffusion coefficient D because the diffusion coefficient
is dependent on nitrogen concentration. Next, let us
assume that the diffusion coefficient is approximated
-49-