Copyright ©JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume 48.
NON-LINEAR 2θ-sin2ψ DIAGRAM BY SHOT PEENING ON SPRING STEEL
Takayuki Sakakibara and Yoshihiro Sato
Chuo Spring Co.,Ltd. and Osaka City University
ABSTRACT
Round and spiral-shaped coil springs enable various peening angles that correspond to the surface location,
and the directional shot angles may lead to a non-uniform residual stress on the coil spring surface. It is
commonly known that a material under directional deformation exhibits non-linear 2θ-sin2ψ diagram
(ψ-split ) due to the triaxial stress state. In this study, the residual stress distributions of spring materials
deformed by shot peening at different angles were measured, and the microstructure for carbide
precipitates was examined using a field emission scanning electron microscope (FE-SEM). The
non-linearity in the 2θ-sin2ψ diagram for the shot peening samples was revealed. The extent of the ψ-split
increased with increasing shot peening angles, and was dependent not only on the mass fraction of carbide
particles but also on the size distribution of the carbide particles.
INTRODUCTION
In recent years, the demand for high-performance automotive coil springs has increased, with especially
that for fatigue-resistant coils. Surface treatments like shot peening after coil formation have been
extremely effective for treating coil fatigue. Therefore, in order to realize improved fatigue resistance, it is
important to control residual stress. The residual stress imparted by shot peening is generally considered to
be uniform; however when shot peening was applied to a curved surface such as a coil, the collision angle
for the shot peening particles changes according to the location. Since residual stress depends upon
variable conditions such as the collision angle, therefore we predict that it may become uneven. However,
little research has been conducted on this type of variance in shot peening.
There is a large body of research on grinding and cutting with regard to the dependence of residual stress
on process direction. We performed all measurements and evaluations from the perspective of the
processing method and processed materials. Systematic data has been collected in the past [1,2]. The term
“directional processing” was coined by Wakabayashi to refer to processes that have a specific direction [2].
Test samples confirmed that the distribution of positive and negative sin2ψ points on the graph for annealed
iron and steel materials mainly formed ellipses (ψ-split). The basic method of calculating triaxial stress
conditions was discovered by Dölle [3]. Hanabusa et al. further deduced that microscopic residual stress
was a component in the formation of ψ-split [1].
In our experiment, we conducted shot peening while systematically changing the collision angles for the
shot particles and investigated the non-linearity of the 2θ-sin2ψ diagram while measuring residual stress.
EXPERIMENTAL
Samples
The materials used were an oil-tempered wire (SAE9254 + 0.2%V,Φ11) for the coil springs and SAE5160,
SAE6150(t= 15) for the leaf springs (SAE9254 + 0.2％V: C:0.57, Si:1.38, Mn:0.69, Cr:0.7, V:0.2,
SAE6150:C:0.52,Si:0.26,Mn:0.84,Cr:1.0,V:0.18, SAE5160:C:0.55, Si:0.25, Mn:0.85,Cr:0.83). The plate
spring was cut to a size of 10 ×15 ×55 mm after oil quenching at 1153K for 20 min. After tempering at
723K for 1 h,we polished the treated surface and further conducted electrolytic polishing. In order to avoid
the effects of decarburized layers and rolling processing we used a cross section of the treated surface.
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Copyright ©JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume 48.
DISCUSSION
Relationship with microstructure
Figure 8 shows a photograph of the structure of the test samples that were tempered at 873K for 1 h. We
concluded that the structure was one of cementite. Wakabayashi et al. state that the greater the amount of
cementite, the greater is the increase in microstress increases [2]. However, if a large amount of cementite
is distributed in the ferrite with a low density, there is a low probability that the cementite particles will
combine with the ferrite crystals, resulting in a low dislocation density accumulation in the particle as well
as low the microstress. With regard to the material that was tempered at 873K for 10 h, in which the area
of the carbide was smaller and since there was little interaction between the carbide and the matrix, the
ψ-split range decreased. This behavior is consistent with our results.
In order to quantitatively assess the relationship between the microstructure and the microstress, particle
distribution diameter and volume ratio portions of the estimate formula in the second phase distribution
strengthening Orowan mechanism was used. The relationship between σ31 and the structure parameters is
shown in Figure 10; both sides show a comparatively good correlation. The figure also shows that if the
carbide has a significant microscopic distribution, the microstress increases.
Relationship with texture
As shown in Figure 9, when multiple shot peening is performed, an oscillation is observed in the sin2ψ
diagram. Therefore we conducted our investigation from the view point of texture. The pole figures are
shown in Figure 12. Distribution with oscillation over 110 surfaces was observed [Figure 12(b)].
100
σ３１ （MPa）
80
60
40
〇:SAE6150
△:SUP5160
20
0
8
10
12
1/{（0.9ｆ-1/2-0.8）ｄ}
Figure 10. Relation between dispersion
strengthening factor and σ31
200 nm
Figure 11. TEM micrograph (Tempered
at 873k for 1h)
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Copyright ©JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume 48.
(a)
(b)
(c)
Figure 12. Pole figures[(a):second stage,0°,(b):second stage,-30°,(c):second stage,-60°]
CONCLUSIONS
Regarding the non-linearity of the 2θ-sin2ψ diagram, when measuring the residual stress after
systematically changing the shot particle collision angle for shot peening in spring steel, we arrived at the
following conclusions based on our investigation:
(1) A ψ-split was observed; this indicates the existence of a directional process layer for shot peening in
spring steel. The tendency of the ψ-split to increase with an increase in the shot angle was also observed.
(2) The correlation between the ψ-split and the microstructure was proved using micrographics.
(3) When multiple shots were used after changing the shot duration and shot angle, an oscillation
(possibly caused by texture) was observed in the sin2ψ diagram.
REFERENCES
[1] T.Hanabusa,H.Fujiwara,Zairyo,32,245(1983).
[2] M.Wakabayashi,M.Nakayama,K.Tamamura,Seimitsukikai,49,1202(1982).，
[3] Đölle,J.Appl.Cryst.,12,489(1979).
[4] T.Sakakibara,Y.Sato,Zairyo,53,746(2004).
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