necessary, as well as increasing the hardness in the
nitrided layer, because fish-eye fractures occur both in
and beneath the nitrided layer. Thus, the fatigue
strengths of 45C and 45C-P were distinctly improved
by nitriding; i.e., 45C and 45C-P successfully main-
tained high hardness beneath the nitrided layer,
because grain growth was suppressed during nitriding.
In contrast, it was not possible to suppress grain
growth in 15C and 15C-P, leading to loss of hardness
beneath the nitrided layer, so the improvement in
fatigue stren gth was small.
The results ending in surface fracture of the nitrided
specimens are distributed in a lower fatigue strength
region than those of the as-rolled specimens. This means
that, although the fatigue strength of the nitrided
specimens is improved by the increased hardness at the
surface, as seen in Figures 8 and 9, that improvement is
not entirely attributable to the hardness at the surface,
even in the case of surface fracture. One possible reason
for this is that the hardness distribut ions seen in
Figure 5 show no plateau region in the nitrided layer.
The hardness falls steeply with increasing depth below
the surface, so once initiated, a fatigue crack propagates
easily, resulting in a loss of fatigue strength. The other
possible reason is that the fatigue strength of the
nitrided layer itself is lower than that of the as-rolled
specimens. In terms of the effects of residual stress,
[11,12]
plasma-nitrided specimens might initially appear to
harbor compressive residual stress in an internal nitrided
layer. However, compressive residual stress improves
fatigue strength, so the trend of the results ending in
surface fracture would indicate the opposite to be the
case.
In summary, the most important finding of this
research is that suppressing grain growth is a necessary
condition for achieving high fatigue strength in nitrided
ultrafine ferrite-cementite steel. This grain growth must
be suppressed beneath, as well as in, the nitrided layer.
Using the pinn ing effect of precipitated particles is an
effective way of achieving this. On the other hand, for
greater improvement of fatigue strength, refinement of
inclusion sizes is a necessary process, because fish-eye
fractures originating from an inclusion occurred even
in the nitrided 45C-P specimens in which grain growth
had been successfully suppressed. An effective way to
avoid fish-eye fracture is to reduce the inclusion size
appearing at the fracture origin. The other item is
modification of the nitrided layer, which improves the
surface fracture properties. The surface fracture prop-
erty of the nitrided specimens is not as good as might
be expected from their surface hardness. This modifi-
cation of the nitrided layer would require an investi-
gation of appropriate alloying elements in addition to
Mn, in order to yield a plateau region of hardness in
the nitrided layer and thus improve the fatigue strength
of the nitrided layer itself.
V. CONCLUSIONS
Fatigue tests under rotating bending were conducted
for nitrided ultrafine ferrite-cementite steels. The ultrafine
ferrite-cementite steels included carbon-increased and
phosphorus-added versions to investigate the effects of
grain growth suppression during nitriding. The main
conclusions obtaine d in this study are as follows.
1. Ferrite grains near the surface of the nitrided layer
remained ultrafine, regardless of the version of the
ultrafine ferrite-cementite steel, due to the pinning
effect of fine nitrides precipitated during the nitrid-
ing process.
2. Although the ferrite grains in the core region were
enlarged in the low-carbon versions, grain growth
during nitriding was successfully suppressed in the
carbon-increased versions due to the pinning effect
of dense cementite particles. As a result of this sup-
pression of grain growth, hardness degradation in
the core region during the nitriding was smaller in
the carbon -increased versions than in the low-
carbon versions.
3. In the fatigue tests, many of the nitrided specimens
revealed fish-eye fractures originating from inclu-
sions located in or beneath the nitrided layer. In
spite of the occurrence of fish-eye fractures, the fati-
gue strength of the carbon-increased versions was
markedly improved by nitriding, but only slightly
improved in the low-carbon versions.
4. The fatigue strength of the nitrided specimens was
closely related to hardness at the fracture origin,
even though fish-eye fractures occurred. This was
why nitriding markedly improved the fatigue
strength of the carbon-increased versions in which
suppression of grain growth had been successful
and where high hardness had been maintained
beneath the nitrided layer.
5. These results indicate that suppression of grain
growth is a necessary condition, even in the core
region, to achieve high fatigue strength in nitrided
ultrafine ferrite-cementite steels, and to this end,
using the pinning effect of precipitated particles is
effective.
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METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 39A, SEPTEMBER 2008—2075