18 The Effect of Surface Ultrasonic Rolling Processing …
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increase of static load not only gradually improves the surface quality of the sample
and reduces the phenomenon of stress concentration on the contact surface, but also
the ability of the surface material to inhibit the initiation and development of fatigue
cracks can be increased accordingly. Therefore, increasing static load in a suitable
range can significantly enhance the fatigue resistance of the tappet material. At the
same time, grain refinement can improve the surface hardness of the material and
increase the fatigue life of rolling contact fatigue [39–41]. Obvious grain refinement
can be observed from Fig. 18.6. Compared with the untreated sample (Fig. 18.6a),
the surface microstructure of the treated sample is obviously refined. It can be seen
from Fig. 18.6b–e that the size of microstructure decreases with the increase of static
pressure. Therefore, the fatigue life of the specimens is gradually improved. Fine
grain can inhibit the crack growth and reduce the fatigue crack growth rate. The
slower the fatigue crack growth rate, the higher the fatigue life. This indicates that
the higher the fatigue resistance of the material [42, 43]. Therefore, the specimen has
the highest resistance to fatigue crack growth under 1000 N static load. Compared
with the 1000 N sample, although the 1200 N sample has higher surface hardness
and residual compressive stress, there are many cracks and peelings on the surface.
Through the comprehensive analysis of fatigue life and damage morphology, it is
found that the anti-fatigue performance of the 1000 N sample is better than that of the
1200 N sample, which shows that the quality of the contact surface has a remarkable
influence on the fatigue performance of the contact pair. Therefore, excessive static
load may result in defects on the surface of the sample, which has a negative effect
on the anti-fatigue performance and accelerate the process of fatigue failure.
18.4 Conclusion
This paper explored the mechanism of SURP on the surface quality, surface
microstructure, microhardness and residual compressive stress of the tappet material
GCr15 in an engine valve train at different static loads, and analyzed how the above
properties affect fatigue performance of the material, and then discussed the effect
of SURP on the fatigue behavior of this material. According to the results of this
study, the following conclusions can be drawn:
(1) SURP treatment can effectively improve the surface quality of this tappet material. When the static load is less than 1000 N, the surface quality can be iproved
with the increase of the static load; when the static load exceeds 1000 N, due to
fatigue spalling, the surface quality of the sample decreases instead.
(2) SURP treatment can effectively refine the surface microstructure of the tappet
material and improve the residual stress. The refinement of the surface
microstructure and the residual compressive stress show an upward trend with
the increase of static load.
(3) Compared with the untreated samples, the contact fatigue life of the SURP
samples are significantly improved, and the fatigue performance are more stable.
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increase of static load not only gradually improves the surface quality of the sample
and reduces the phenomenon of stress concentration on the contact surface, but also
the ability of the surface material to inhibit the initiation and development of fatigue
cracks can be increased accordingly. Therefore, increasing static load in a suitable
range can significantly enhance the fatigue resistance of the tappet material. At the
same time, grain refinement can improve the surface hardness of the material and
increase the fatigue life of rolling contact fatigue [39–41]. Obvious grain refinement
can be observed from Fig. 18.6. Compared with the untreated sample (Fig. 18.6a),
the surface microstructure of the treated sample is obviously refined. It can be seen
from Fig. 18.6b–e that the size of microstructure decreases with the increase of static
pressure. Therefore, the fatigue life of the specimens is gradually improved. Fine
grain can inhibit the crack growth and reduce the fatigue crack growth rate. The
slower the fatigue crack growth rate, the higher the fatigue life. This indicates that
the higher the fatigue resistance of the material [42, 43]. Therefore, the specimen has
the highest resistance to fatigue crack growth under 1000 N static load. Compared
with the 1000 N sample, although the 1200 N sample has higher surface hardness
and residual compressive stress, there are many cracks and peelings on the surface.
Through the comprehensive analysis of fatigue life and damage morphology, it is
found that the anti-fatigue performance of the 1000 N sample is better than that of the
1200 N sample, which shows that the quality of the contact surface has a remarkable
influence on the fatigue performance of the contact pair. Therefore, excessive static
load may result in defects on the surface of the sample, which has a negative effect
on the anti-fatigue performance and accelerate the process of fatigue failure.
18.4 Conclusion
This paper explored the mechanism of SURP on the surface quality, surface
microstructure, microhardness and residual compressive stress of the tappet material
GCr15 in an engine valve train at different static loads, and analyzed how the above
properties affect fatigue performance of the material, and then discussed the effect
of SURP on the fatigue behavior of this material. According to the results of this
study, the following conclusions can be drawn:
(1) SURP treatment can effectively improve the surface quality of this tappet material. When the static load is less than 1000 N, the surface quality can be iproved
with the increase of the static load; when the static load exceeds 1000 N, due to
fatigue spalling, the surface quality of the sample decreases instead.
(2) SURP treatment can effectively refine the surface microstructure of the tappet
material and improve the residual stress. The refinement of the surface
microstructure and the residual compressive stress show an upward trend with
the increase of static load.
(3) Compared with the untreated samples, the contact fatigue life of the SURP
samples are significantly improved, and the fatigue performance are more stable.