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Z. Wang et al.
parts are also increased. This results in an significant increase to the mechanical
load, thermal load, friction and wear, vibration and noise of the valve train system
[1]. In the valve train system, the tappet and the camshaft consist a contact pair in
the form of point or line generally. When the camshaft rotates at high speed, a large
alternating stress is generated between the tappet and the camshaft, which lead to the
fatigue wear of the tappet easily [2]. As one of the core components in the valve train
system, the performance of the tappet has a direct impact on the power performance,
reliability and service life of the engine [3].
Contact fatigue failure is a typical failure mode. Its mechanism has been widely
used in key components of the machine manufacturing industry, including the automotive industry, to improve its performance and service life. Based on this actual
demand of production, the research on the contact fatigue behavior of the tappet and
its strengthening mechanism has become an urgent problem to be solved [4]. There
are many factors influencing fatigue life, such as lubrication conditions, surface hardness of contact materials, mechanical properties, etc. [5–8]. Surface quality is one
of the most significant factors affecting the fatigue behavior of the tappet. Domestic
and foreign scholars have carried out extensive research on the surface strengthening
process of tappet materials, such as surface ultrasonic rolling processing (SURP),
shot peening, surface coating, etc. [9–11]. As a mature surface strengthening process,
SURP is widely used to improve the surface properties of metal parts [12, 13].
Zhang Fei, etc. [14] discussed the influence mechanism of various SURP parameters
including static load, amplitude, feed rate, etc. on the surface properties of 45 steel,
and compared the friction and wear behavior of materials at different SURP parameters. Liu Yu et al. [15] applied SURP to the surface strengthening of 40Cr alloy steel,
and measured the elastic modulus, nano hardness and residual stress of the surface
layer. It was found that after ultrasonic surface rolling, the elastic modulus and hardness of the 40Cr surface layer significantly improved. The maximum value of elastic
modulus and hardness were found to be located on the surface, and increased with the
increase of processing times. At the same time, a certain value of residual compressive stress was obtained on the surface. A study by Liu et al. [16] indicates that SURP
can refine the surface layer, improve the microhardness and residual stress. These
qualities result in the significant improvement of the alloy’s (Ti-6Al-4 V) fatigue
properties.
GCr15 alloy steel has good mechanical properties and is the most widely applied as
high chromium bearing steel. After quenching and tempering, it has high and uniform
surface hardness, good wear resistance and contact fatigue properties. The components made of GCr15 alloy steel are usually used in high load, multi cycle contact
and sliding friction environment. The main failure modes are contact fatigue, friction
wear and corrosion. The surface hardness of GCr15 after heat treatment is slightly
higher than that of medium carbon steel used for camshaft, which ensures good
matching with cam material. Due to the traditional tappet without special surface
treatment, the fatigue resistance is insufficient under high strength alternating stress,
which greatly affects the service life of tappet and even the whole engine. Therefore,
it is necessary to further strengthen the surface of the tappet to meet the increasing
demand of automobile industry. In this paper, the influence mechanism of SURP on
Z. Wang et al.
parts are also increased. This results in an significant increase to the mechanical
load, thermal load, friction and wear, vibration and noise of the valve train system
[1]. In the valve train system, the tappet and the camshaft consist a contact pair in
the form of point or line generally. When the camshaft rotates at high speed, a large
alternating stress is generated between the tappet and the camshaft, which lead to the
fatigue wear of the tappet easily [2]. As one of the core components in the valve train
system, the performance of the tappet has a direct impact on the power performance,
reliability and service life of the engine [3].
Contact fatigue failure is a typical failure mode. Its mechanism has been widely
used in key components of the machine manufacturing industry, including the automotive industry, to improve its performance and service life. Based on this actual
demand of production, the research on the contact fatigue behavior of the tappet and
its strengthening mechanism has become an urgent problem to be solved [4]. There
are many factors influencing fatigue life, such as lubrication conditions, surface hardness of contact materials, mechanical properties, etc. [5–8]. Surface quality is one
of the most significant factors affecting the fatigue behavior of the tappet. Domestic
and foreign scholars have carried out extensive research on the surface strengthening
process of tappet materials, such as surface ultrasonic rolling processing (SURP),
shot peening, surface coating, etc. [9–11]. As a mature surface strengthening process,
SURP is widely used to improve the surface properties of metal parts [12, 13].
Zhang Fei, etc. [14] discussed the influence mechanism of various SURP parameters
including static load, amplitude, feed rate, etc. on the surface properties of 45 steel,
and compared the friction and wear behavior of materials at different SURP parameters. Liu Yu et al. [15] applied SURP to the surface strengthening of 40Cr alloy steel,
and measured the elastic modulus, nano hardness and residual stress of the surface
layer. It was found that after ultrasonic surface rolling, the elastic modulus and hardness of the 40Cr surface layer significantly improved. The maximum value of elastic
modulus and hardness were found to be located on the surface, and increased with the
increase of processing times. At the same time, a certain value of residual compressive stress was obtained on the surface. A study by Liu et al. [16] indicates that SURP
can refine the surface layer, improve the microhardness and residual stress. These
qualities result in the significant improvement of the alloy’s (Ti-6Al-4 V) fatigue
properties.
GCr15 alloy steel has good mechanical properties and is the most widely applied as
high chromium bearing steel. After quenching and tempering, it has high and uniform
surface hardness, good wear resistance and contact fatigue properties. The components made of GCr15 alloy steel are usually used in high load, multi cycle contact
and sliding friction environment. The main failure modes are contact fatigue, friction
wear and corrosion. The surface hardness of GCr15 after heat treatment is slightly
higher than that of medium carbon steel used for camshaft, which ensures good
matching with cam material. Due to the traditional tappet without special surface
treatment, the fatigue resistance is insufficient under high strength alternating stress,
which greatly affects the service life of tappet and even the whole engine. Therefore,
it is necessary to further strengthen the surface of the tappet to meet the increasing
demand of automobile industry. In this paper, the influence mechanism of SURP on