18 The Effect of Surface Ultrasonic Rolling Processing …
235
spalling. Because there were cracks in the rolling contact area of them, the lubricant was pressed into the existing cracks during the high-speed contact between the
bearing balls and the samples. With the impact of cyclic shear stress caused by highpressure lubricating oil, the material at both ends of the cracks fractured, causing the
crack to further expanded, and finally caused spalling failure [32]. It can be seen that
delamination occurs at the edge of the spalling pit of the 600 N sample, and there are
pretty obvious cracks distributed around the spalling pit, and the area of the spalling
pit is smaller than that of the untreated sample. Because there are still revolution
scratches and pores on the surface of the 600 N sample, it is easy to cause stress
concentration on the defects on the surface of the sample during the contact process,
which promotes the initiation and extension of cracks on the contact surface [33].
This is because during the process, the particles that has fallen off got into between
the ball and subsurface of the sample forming three-body abrasive grains system,
and continuously contacted with subsurface of the sample, causing scratches and
pitting corrosion. This damage mechanism has been verified by Zhang et al. [34]
when they studied the contact fatigue of 17Cr2Ni2MoVNb alloy steel. Therefore, it
can be inferred that the involvement of more abrasive particles due to surface defects
and higher surface roughness can affect the contact fatigue resistance of the material.
Secondly, a large number of studies show that the maximum residual compressive
stress occurs below the SURP [35–37]. There is no residual compressive stress in
untreated specimens. Therefore, the surface of untreated specimens will produce
contact fatigue cracks. After 600 N SURP, the surface residual compressive stress
is less than the subsurface residual compressive stress, and there are still rotary
scratches and pores on the surface of 600 N sample. In the contact process, it is
easier to produce stress concentration on the surface defects of the sample, so the
600 N sample is more likely to produce cracks on the surface. Garnham J.E. [38]
finds that under the combined action of residual stress and surface defects, it is easier
to crack on the specimen surface. This reason also applies to 800 N samples.
The fatigue failure morphology of the 1000 N sample is shown in Fig. 18.11d. The
failure mode of this sample is fatigue spalling. The spalling pit area is smaller and
shallower than other samples. The subsurface is smoother. There are fewer cracks
at the bottom of the spalling pit but more cracks around the spalling pit, indicating
that the location of the crack initiation and propagation were mainly on the contact
surface. As we can see, the spalling pit area and the depth of 1000 N sample is smallest
and shallowest. According to the previous analysis of the surface properties of the
material, we know that the 1000 N sample has smooth surface and high hardness,
which can obtain excellent anti fatigue crack initiation and propagation ability. It is
easy to draw the conclusion that the 1000 N static load has the most significant effect
on the material fatigue process.
Figure 18.11e is the fatigue morphology of the 1200 N sample. The damage
morphology is similar to that of the 800 N sample. The failure mechanism is also
fatigue spalling, and there are more pitting at the bottom of the spalling pit. Compared
with the 800 N sample, the 1200 N sample has a smaller spalling pit area. This is
because the surface quality of the 1200 N sample is better than that of the 800 N
sample, and the surface of the 800 N sample has more initial cracks and defects,
235
spalling. Because there were cracks in the rolling contact area of them, the lubricant was pressed into the existing cracks during the high-speed contact between the
bearing balls and the samples. With the impact of cyclic shear stress caused by highpressure lubricating oil, the material at both ends of the cracks fractured, causing the
crack to further expanded, and finally caused spalling failure [32]. It can be seen that
delamination occurs at the edge of the spalling pit of the 600 N sample, and there are
pretty obvious cracks distributed around the spalling pit, and the area of the spalling
pit is smaller than that of the untreated sample. Because there are still revolution
scratches and pores on the surface of the 600 N sample, it is easy to cause stress
concentration on the defects on the surface of the sample during the contact process,
which promotes the initiation and extension of cracks on the contact surface [33].
This is because during the process, the particles that has fallen off got into between
the ball and subsurface of the sample forming three-body abrasive grains system,
and continuously contacted with subsurface of the sample, causing scratches and
pitting corrosion. This damage mechanism has been verified by Zhang et al. [34]
when they studied the contact fatigue of 17Cr2Ni2MoVNb alloy steel. Therefore, it
can be inferred that the involvement of more abrasive particles due to surface defects
and higher surface roughness can affect the contact fatigue resistance of the material.
Secondly, a large number of studies show that the maximum residual compressive
stress occurs below the SURP [35–37]. There is no residual compressive stress in
untreated specimens. Therefore, the surface of untreated specimens will produce
contact fatigue cracks. After 600 N SURP, the surface residual compressive stress
is less than the subsurface residual compressive stress, and there are still rotary
scratches and pores on the surface of 600 N sample. In the contact process, it is
easier to produce stress concentration on the surface defects of the sample, so the
600 N sample is more likely to produce cracks on the surface. Garnham J.E. [38]
finds that under the combined action of residual stress and surface defects, it is easier
to crack on the specimen surface. This reason also applies to 800 N samples.
The fatigue failure morphology of the 1000 N sample is shown in Fig. 18.11d. The
failure mode of this sample is fatigue spalling. The spalling pit area is smaller and
shallower than other samples. The subsurface is smoother. There are fewer cracks
at the bottom of the spalling pit but more cracks around the spalling pit, indicating
that the location of the crack initiation and propagation were mainly on the contact
surface. As we can see, the spalling pit area and the depth of 1000 N sample is smallest
and shallowest. According to the previous analysis of the surface properties of the
material, we know that the 1000 N sample has smooth surface and high hardness,
which can obtain excellent anti fatigue crack initiation and propagation ability. It is
easy to draw the conclusion that the 1000 N static load has the most significant effect
on the material fatigue process.
Figure 18.11e is the fatigue morphology of the 1200 N sample. The damage
morphology is similar to that of the 800 N sample. The failure mechanism is also
fatigue spalling, and there are more pitting at the bottom of the spalling pit. Compared
with the 800 N sample, the 1200 N sample has a smaller spalling pit area. This is
because the surface quality of the 1200 N sample is better than that of the 800 N
sample, and the surface of the 800 N sample has more initial cracks and defects,