Study of the effects produced by shot peening on the surface of quenched and tempered steels_ roughness, residual stresses and work hardening

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Applied Surface Science 356 (2015) 475–485
Contents lists available at ScienceDirect
Applied Surface Science
jou rn al h om ep age: www.elsev ier .com/ locate /apsusc
tudy of the effects produced by shot peening on the surface of
uenched and tempered steels: roughness, residual stresses and work
ardening
. Llaneza ∗, F.J. Belzunce
aterials Science Department, University of Oviedo, University Campus, 33203, Gijón, Spain
 r t i c l e i n f o
rticle history:
eceived 29 May 2015
eceived in revised form 29 July 2015
ccepted 13 August 2015
vailable online 15 August 2015
a b s t r a c t
Shot peening induces important effects on the surface of materials, both positive and negative, the correct
balance between them being the key to success.
Roughness, impact mark size, compressive residual stress and work hardening of six steel grades
obtained from an AISI 4340 steel were studied to explain their evolution according to the Almen intensity
and their mechanical properties. A linear relationship between the impact diameter, the kinetic energy
eywords:
hot peening
uenched and tempered steels
oughness
esidual stress profiles
ull width at half maximum
of the balls and the Almen intensity was found. Moreover, under full coverage, the surface and the max-
imum compressive stresses only depend on the mechanical properties of the steels, whereas the depth
subjected to high compressive residual stresses and the total depth subjected to compressive residual
stresses depend on the mechanical properties of the steel and the Almen intensity. Furthermore, several
mathematic expressions were formulated to predict the residual stress profiles using the Almen intensity
and the mechanical properties of the steels.
. Introduction
Conventional shot peening (SP) is a cheap surface treatment that
onsists in projecting very hard, tiny, spherical ceramic or metal-
ic balls (0.3 < Ø < 1.6 mm) at high speed onto the surface of the
omponent to treat. These impacts produce local surface plastic
eformation, the expansion of which is constrained by the adjacent
eeper material, giving rise to a uniform surface compressive resid-
al stress field (Fig. 1), along with other important effects. These
nclude modification of the roughness and appearance of the sur-
ace in addition to work hardening, which, if properly controlled,
an significantly improve the final properties of metallic compo-
ents [1–4]. The aforementioned effects provided by shot peening
reatments cannot be called merely positive or negative, as this role
epends on the purpose of each treatment.
Shot peening has many applications: for instance, it can be used
o improve the fatigue life of industrial components [5–8], obtain
 specific surface finishing [9], enhance the wear resistance [10] or
revent stress corrosion cracking [11,12]. Consequently, it is nec-
ssary to control the shot peening parameters, mainly the Almen
∗ Corresponding author. Tel.: +34 985182024.
E-mail addresses: llanezavictor@gmail.com (V. Llaneza), belzunce@uniovi.es
F.J. Belzunce).
ttp://dx.doi.org/10.1016/j.apsusc.2015.08.110
169-4332/© 2015 Elsevier B.V. All rights reserved.
© 2015 Elsevier B.V. All rights reserved.
intensity and the coverage degree, according to the mechanical
properties of the material treated, to obtain the best combination of
the aforementioned effects and, hence, maximize the performance
of the product. Coverage is the ratio of the area covered by the shot
impacts to the entire surface of the treated sample, expressed as a
percentage, whereas the Almen intensity is a measure of the energy
of the shot stream, which depends on the projection velocity and
also on the shot density, mass and size [13,14].
However, although shot peening is a relatively old technology,
even now, most companies are not able to employ it optimally, and
this means that they are not able to take full advantage of it. The
main reason is the complexity of the process, due to the different
parameters that must be simultaneously controlled to attain the
optimal balance among effects.
It is worth to remember here the existence of other surface
treatments which are based in similar concepts as conventional
shot peening, but they have some specific differentiating charac-
teristics. For instance, severe shot peening (SSP), which employs
more intense parameters, usually very high coverage degrees [15];
laser peening, which uses laser-generated shock waves to introduce
high level of surface compressive stresses deeper in the workpiece
[16]; roller burnishing, which rub the metal surface with a smooth
hard roller under a sufficient pressure [17] or surface mechanical
attrition treatment (SMAT), where shots are resonated by vibra-
tion using an ultrasonic transducer [18–20], as well as vibration
476 V. Llaneza, F.J. Belzunce / Applied Surfa
Fig. 1. Schematic illustration of the shot peening process.
Table 1
Chemical composition of AISI 4340 alloy steel.
Element C Mn Si P S Cr Ni Fe
p
s
t
m
c
t
o
b
o
e
T
H
wt% 0.410 0.710 0.260 0.013 0.024 0.870 1.920 Balance
Element Mo V Cu Al Sn Ti Nb
wt% 0.235 0.005 0.210 0.016 0.011 0.004 0.003
olishing, vibration peening or grinding [21]. In relation to these
urface treatments, shot peening is usually cheaper, versatile, effec-
ive enough and very easy to be implemented in most workshops.
Anyway, in order to attain the final goal on these surface treat-
ents and specifically in the case of shot peening, it would be
onvenient to have a tool able to foresee the main effects of any
reatment in order to select the most appropriate parameters for
ptimizing it. Numerous experimental and theoretical studies have
een performed along these lines to improve the state of knowledge
f shot peening and better understand its effects [9,22–28].
This paper focuses on the analysis of the evolution of the main
ffects induced by conventional shot peening treatments (surface
able 2
ardness and tensile properties of quenched and tempered AISI4340 steel (Vickers hardn
Steel Tempering temperaturea (◦C) HV (31,25 kg
Q + T200 200 552 
Q + T425 425 424 
Q + T540 540 350 
Q + T590 590 325 
Q + T650 650 255 
Q + T680 680 226 
a All tempering times were 150 min, except 10 h in Q + T680.
Fig. 2. Steel microstructures (nital etc
ce Science 356 (2015) 475–485
finish modification, surface work hardening and compressive resid-
ual stress fields), in different quenched and tempered steel grades
presenting a relatively broad range of mechanical properties sub-
mitted to different shot peening intensities. The main objective of
the experimental study was to understand the role played by the
mechanical properties of the treated steel and the applied Almen
intensity on the main effects induced by shot peening treatments.
Furthermore, several simple, practical expressions are proposed to
predict the impact diameter and some characteristic values of the
residual stress profiles. These expressions may be used in a practi-
cal way to predict the effects induced by shot peening treatments
on industrial components, being an effective tool to select the cor-
rect parameters to satisfy the requirements fixed by the final client
in an easy and fast way.
2. Materials and methods
2.1. Steel and mechanical properties
This study was carried out on samples of AISI 4340, a commercial
heat treatable low-alloy steel widely employed in the automotive
and aircraft industries for the manufacture of gears, shafts and other
structural components due to its favorable combination of strength,
toughness and ductility. The steel was supplied in the form of rolled
bars with a diameter of 16 mm, and its chemical composition is
given in Table 1.
This steel was subjected to different heat treatments in order
to obtain six different steel grades.The treatments consisted in
austenitizing at 850 ◦C for 45 min, water quenching (Q), plus differ-
ent tempering treatments (T), ranging from 200 ◦C to 680 ◦C, during
150 min. The use of different tempering temperatures allowed us
to obtain a wide range of mechanical properties, as can be seen
in Table 2, which shows a representative range of the mechani-
cal properties of typical martensitic steels employed in the metal
industry.
Fig. 2 shows the steel microstructure obtained after two of these
heat treatments (Q + T200 and Q + T680).
ess, HV, yield strength, �ys, ultimate tensile strength, �uts, and elongation, E).
) �ys (MPa) �uts (MPa) E (%)
1604 2057 10.5
1364 1426 10.6
1123 1201 13.7
983 1123 14.6
863 897 19.3
626 764 24.7
hed). (a) Q + T200; (b) Q + T680.
V. Llaneza, F.J. Belzunce / Applied Surface Science 356 (2015) 475–485 477
Table 3
Work parameters for the different shot peening treatments.
Almen intensity (deflection of
the Almen strip (A), in mm)
Shot size
(mm)
Pressure
(bar)
Shot Speed
(m/s)
Impact
angle (◦)
Stand-off
distance (mm)
Ø nozzle
(mm)
8A (0.2 mm) CW-0.3 2 –
10A (0.25 mm) CW-0.4 2 52.2
12A (0.3 mm) CW-0.5 2 53.4
55.1 90 240 5
43.9
49.9
–
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E
14A (0.35 mm) CW-0.5 3 
16A (0.4 mm) CW-0.7 1.5 
19A (0.475 mm) CW-0.7 3 
21A (0.52 mm) CW-0.7 4 
All the tests were carried out on small slices cut trans-
ersely from the bars, with an approximate thickness of 10 mm.
hese samples were ground in SiC papers of progressively
ower grit sizes and carefully polished with diamond paste
6 �m and, finally, 1 �m) to ensure a soft and homogeneous
nitial state (Ra ≤ 0.1 �m,Rmax ≤ 0.2 �m, residual stress in the
ear-surface region below 200 MPa, and depth affected by the
o-mentioned residual stresses lower than 20 �m), thus guaran-
eeing that all the evaluated effects were only induced by shot
eening.
.2. Shot peening treatments
Shot peening treatments were carried out by means of a direct
ompressed air machine (Guyson Euroblast 4 PF) using conditioned
ut wire shots with rounded off edges (CW, 670-730 HV). Seven
hot peening treatments were designed with Almen intensities ran-
ing between 8A and 21A (0.2–0.52 mm) following SAE J442 and
AE J443 specifications [29,30] employing ‘A’ type Almen strips.
n order to achieve this range of Almen intensities, it was neces-
ary to use shots with diameters ranging between 0.3 and 0.7 mm.
he combination of parameters selected in each treatment, includ-
ng the impact angle, the diameter of the nozzle and the distance
etween sample and nozzle, is shown in Table 3. It is important
o remark that both nozzle and samples remained fixed during the
hole treatment.
The last step in defining and performing the treatments is the
election of the exposure time to achieve the required degree of
overage. Residual stress profiles and surface work hardening were
lways evaluated in samples with full coverage (100%), but rough-
ess was also studied using different degrees of coverage (25%, 50%,
5%, 100% and 200%). The lower coverage degrees were used to
easure the impact marks.
.3. Surface finishing
.3.1. Impact diameters
The diameters of the impacts created by the shot peening
reatments were evaluated using a specific routine of an image
nalysis software, which allows the average diameter of each dim-
le to be estimated via images obtained using conventional optical
icroscopy (OM). In particular, more than 60 impact marks of
ach treatment and steel were assessed, thus obtaining a set of
ata which was subsequently analyzed to obtain the evolution of
he average equivalent diameter as a function of both the applied
able 4
xperimental parameters employed in the X-ray diffraction analysis.
Wavelength K� (Cr) 0.2291 nm 
Exposure time (s) 20 
Tilt (◦) 9 points between −45/+45 
Background Parabolic 
Measuring mode �-modified 
Miller indices (h k l) (2 1 1) 
Fig. 3. Shot peening impact mark (SEM). Q + T590-SP8A.
Almen intensity and the mechanical properties of the treated steel.
The typical geometry of one impact can be seen in the image taken
with a JEOL JSM5600 scanning electron microscope shown in Fig. 3.
2.3.2. Roughness
The surface roughness after shot peening was characterized on
a Diavite DH-6 roughness tester by means of the average roughness
Ra and Rmax parameters. The latter parameter is the largest of the
five Rimax within the assessment length of 4.8 mm, where Rimax is
the maximum peak-to-valley height of the profile in each of the
five aforementioned measurements [31]. Six different roughness
profiles were performed on each sample (three in the longitudinal
direction and another three in the transversal direction) and the
average results were reported.
2.4. Residual stresses
The shot peening residual stress profiles were determined by
X-ray diffraction (XRD) and incremental layer removal by elec-
tropolishing. Measurements were carried out on an X-Stress 3000
G3R device manufactured by Stresstech using the sin2 method
and the recommendations of NPL [32–34]. The experimental con-
ditions are shown in Table 4.
Diffraction data were determined in three different directions
on the specimen plane, −45, 0 and +45◦, subsequently calculat-
ing the average result. Electrochemical polishing was performed
Filter Vanadium
Ø collimator (mm) 2
Rotation angle, ϕ (◦) −45, 0 y 45
Fit Pseudo-Voigt
Diffraction angle 156.0◦
Elastic constant, E (1 + �)−1 (GPa) 168.9 ± 2.8
4 Surface Science 356 (2015) 475–485
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Table 5
Expressions to predict the compressive residual stress at the surface, �rcs .
�rcs (MPa)
Mechanical property Expression Error
78 V. Llaneza, F.J. Belzunce / Applied
y applying 45 V in an electrolyte composed of 94% acetic acid and
% perchloric acid. This process produces slight stress relaxation,
hich was corrected in accordance with Sikarskie [35], who, based
n the Moore and Evans procedure [36], has developed a special
ethodology to minimize the error. Furthemore, the values of the
xpanded uncertainties, which correspond to 95% confidence, vary
etween ±30 and ±50 MPa, growing as the mechanical properties
f the steel increase.
.5. Surface work hardening
This phenomenon was assessed by means of the Full Width
t Half Maximum (FWHM), a parameter that corresponds to the
idth of the diffraction peak at half of its height and which can
e calculated in the course of the X-ray diffraction tests at the
ame time as the residual stresses are estimated. This parame-
er can be considered an index of the distortion of the crystal
rain which takes into account the density of dislocations and
he so-called type II micro residual stresses present in the crys-
al lattice, although some instrumental broadening is always also
resent [33,37,38]. The FWHM parameter is widely used in shot
eening studies to quantify surface work hardening effects [15,38].
he expanded uncertainty related with these measurements is
bout ±0.1◦.
. Results and discussion
.1. Surface finishing
.1.1. Impact marks
As can be seen in Fig. 4, impact diameters depend on both the
pplied Almen intensity and the properties of the peened steel:
s impact size is a direct measure of the induced surface plastic
eformation, it increases with increasing Almen intensity and as
he mechanical strength of the steel decreases (higher tempering
emperature, see Table 2).
The results shown in Fig. 4 provide a clear linear relation-
hip between impact diameter and Almen intensity, the slopes of
hich are dependent on the mechanical properties of the steels.
qs. (1)–(3) were developed with a quite high degree of accuracy
around 4%), using hardness, yield strength or tensile strength as
he reference steel mechanical parameter. These expressions are
ble to predict the impact diameter on quenched and tempered
teels submitted to shot peening treatments (8A < AI < 21A) in an
asy and accurate way, as long as the shotpeening shot size is
etween 0.3 and 0.7 mm.
(HV) = (736 − 0.444 × HV) × AI Uncertainty < 4.1% (1)
Fig. 4. Evolution of impact diameter versus the applied Almen intensity.
�ys �rcs = −0.537 × �ys [Eq. (4)] 9.9%
�uts �rcs = −0.468 × �uts [Eq. (5)] 6.6%
HV �rcs = −1.654 × HV [Eq. (6)] 7.1%
D(�ys) =
(
747 − 0.154 × �ys
)
× AI Uncertainty < 4.1% (2)
D(�uts) = (720 − 0.114 × �uts) × AI Uncertainty < 4.3% (3)
In contrast to those proposed by other authors [11,39,40], these
expressions have been formulated without considering the influ-
ence of shot size. The influence of shot size on impact diameter was
found to be quite low. Using the same intensity (SP14A) but differ-
ent shot sizes (CW0.5 and CW0.6) non-significant differences were
observed between impact diameters. In line with this result and
the small reported error, this peening parameter has been ignored
in Eqs. (1)–(3).
3.1.2. Kinetic energy
The kinetic energy provided by the shot stream was measured
using an electronic device which uses two sensors separated by a
known distance. The time shots took to fly between these sensors
was measured, thus providing shot velocity and hence the average
kinetic energy of the shot stream (E = 0.5mv2). Shot geometry was
considered ideally spherical, average shot diameters were mea-
sured under a scanning electron microscope and a density of the
steel shot of 7.8 g cm−3 was also used, giving rise to the data shown
in Fig. 5. A linear plot of the shot kinetic energy versus the intensity
of the shot peening treatment was thus obtained. These results con-
firm that the Almen intensity is directly correlated with the kinetic
energy of the shot stream and this parameter is barely dependent
on shot size, as can also be seen by comparing the kinetic energy of
two 14A treatments produced using two different shot sizes (CW0.5
and CW0.6). The shot kinetic energy measured in these two treat-
ments was quite similar, and the respective values being situated
between 12A and 16A, as expected.
3.1.3. Roughness
As can be seen in the Q + T590 steel used as an example in
Fig. 6a, the Ra and Rmax parameters increase gradually with increas-
ing degree of coverage until reaching full coverage (100%). From
this point on, both roughness parameters remain constant, as the
surface work hardening induced by successive impacts finally lim-
its the depth and extension of surface impact marks. Fig. 6b shows
the evolution of Ra and Rmax versus Almen intensity in the case of
Fig. 5. Kinetic energy versus Almen intensity (AI). 14A intensity was provided using
CW0.5 and CW0.6 shots.
V. Llaneza, F.J. Belzunce / Applied Surface Science 356 (2015) 475–485 479
verage
s
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t
d
Fig. 6. Roughness. (a) Evolution of roughness versus the degree of co
amples submitted to full coverage (results also obtained with the
 + T590 steel). It can be seen that, in general, roughness increases
ith increasing Almen intensity (impact diameter has already been
een to increase with shot peening intensity). However, shot size
lso plays an important role, as a significant decrease in roughness
as always detected when increasing the shot size from CW0.5
14A) to CW0.7 (16A). Similar graphs were also found for the other
teel grades.
Moreover, Fig. 7 shows that, under the same shot peening condi-
ions (SP14A, CW0.5 and full coverage), the roughness parameters
ecrease linearly with increasing hardness of the treated steel (the
Fig. 7. Evolution of the roughness parameters, Ra and Rm
; (b) evolution of Ra and Rmax versus Almen intensity (full coverage).
steel initial hardness was used instead of the surface hardness after
shot peening, but as hardness increases were always below 10%,
results would not change significantly). The effect of steel hard-
ness on impact size was indirectly shown in Fig. 4, as tempering
temperature is inversely related to the hardness of the steel.
3.2. Residual stresses
Every compressive residual stress profile can be well charac-
terized using four parameters [39,41,42]: the compressive residual
stress at the surface, �rcs ; the maximum value of the compressive
ax, versus steel hardness. SP14A and full coverage.
480 V. Llaneza, F.J. Belzunce / Applied Surface Science 356 (2015) 475–485
r
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a
Table 6
Expressions to predict the maximum value of the compressive residual stress, �rcmax.
�rcmax (MPa)
Mechanical property Expression Error
�ys �rcmax = −0.67 × �ys [Eq. (7)] 9.7%
�uts �rcmax = −0.58 × �uts [Eq. (8)] 4.5%
HV �rcmax = −2 × HV [Eq. (9)] 6.1%
Table 7
Expressions to predict the total depth of the compressive residual stresses, Z0.
Z0 (mm)
Mechanical property Expression Error
�ys Z0 = (−0.0004 × �ys + 1.25) × AI [Eq. (10)] 6.6%
�uts Z0 = (−0.0003 × �uts + 1.19) × AI [Eq. (11)] 6.7%
HV Z0 = (−0.0011 × HV + 1.23) × AI [Eq. (12)] 5.8%
Table 8
Expressions to predict the depth subjected to high compressive residual stresses,
Zhc. (−�c > �ys/2).
Zhc (mm)
Mechanical property Expression Error
a useful and practical tool to evaluate the surface work hardening
Fig. 8. Typical residual stress profile and characteristic parameters.
esidual stress, �rcmax (usually located at a certain depth under
he surface); the total depth submitted to compressive residual
tresses, Z0; and the depth subjected to high compressive resid-
al stresses, Zhc. This last parameter was defined in this study as
he depth at which the compressive residual stress is at least half
he yield strength of the steel. These parameters are represented in
ig. 8 over a typical residual stress profile induced by shot peening.
Fig. 9 shows the residual stress profiles produced by two
iven shot peening treatments (10A and 16A) on the different
teel grades: surface and maximum compressive residual stresses
ecrease with decreasing strength of the steel (higher temper-
ng temperature) [22,39,40]. However, the total depth of the
ompressive residual stresses and the depth subjected to high com-
ressive residual stresses increase with decreasing strength of the
teel.
In addition, all the residual stress profiles obtained in our exper-
mental measurements onto the Q + T steels under the different
lmen intensities (full coverage) are shown in Fig. 10. According
o this last figure, compressive residual stresses (surface and max-
mum) barely depend on the applied Almen intensity. However,
he affected depths (total depth submitted to compressive stresses
nd depth subjected to high compressive residual stresses) increase
ith increasing Almen intensity, as previously reported by other
uthors [22,43,44].
As well as other authors [45–48], we have formulated differ-
nt simple and practical expressions to predict these parameters
aking use only of the applied Almen intensity (mmA) and one of
he main mechanical properties of the treated steel (yield strength,
ltimate tensile strength or hardness). The expressions shown in
ables 5–8, Eqs. (4)–(15), were obtained along with their aver-
ge error through lineal regressions and statistical analysis and
Fig. 9. Residual stress profiles following different SP treatments on divers
�ys Zhc = (0.91 − 0.0003 × �ys) × AI [Eq. (13)] 7.8%
�uts Zhc = (0.89 − 0.0002 × �uts) × AI [Eq. (14)] 10.9%
HV Zhc = (0.92 − 0.0009 × HV) × AI [Eq. (15)] 5.6%
combine precision (error < 10%; in the best cases around 5%) with
simplicity.
The best mechanical parameter for predicting residual com-
pressive stresses is seen to be tensile strength, though hardness
is the best for predicting affected depths. Fig. 11 compares the pre-
dicted surface and maximum compressive stresses produced by
shot peening with the experimental results, while Fig. 12 compares
the predicted depths with their experimentally measured values.
Good correlations have been found with the four parameters.
3.3. Work hardening. FWHM profiles
As previously stated, the shot peening work hardening study
was carried out employing the FWHM parameter, the profiles of
which were obtained by XRD at the same timeas those correspond-
ing to the residual stress. Moreover, this parameter was shown to be
induced by shot peening treatments.
Fig. 13 shows that the steel surface layer affected by shot peen-
ing becomes deeper as the applied Almen intensity increases, and
covers a similar depth to that subjected to the compressive residual
e steels. Two applied Almen intensities, 10A and 16A, full coverage.
V. Llaneza, F.J. Belzunce / Applied Surface Science 356 (2015) 475–485 481
Fig. 10. Residual stress profiles obtained by applying different SP treatments to the different Q + T steels (full coverage).
Fig. 11. Predicted compressive residual stress vs experimental results (full coverage) a) at the surface; b) maximum value.
482 V. Llaneza, F.J. Belzunce / Applied Surface Science 356 (2015) 475–485
Fig. 12. Predicted depths vs experimental results (full coverage) (a) total depth subjected to compressive residual stresses; (b) depth subjected to high compressive residual
stresses.
Fig. 13. FWHM profiles following different SP treatments applied to the studied steel grades (full coverage).
V. Llaneza, F.J. Belzunce / Applied Surface Science 356 (2015) 475–485 483
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Table 9
Predicted results from an average impact diameter of 179 �m (Q + T540 4340 steel).
�rcs = −562 MPa Z0 = 0.26 mm
�rcmax = −697 MPa Zhc = 0.18 mm
(
−�rc > �ys2 = 562 MPa
)
was derived. Subsequently, Eqs. (5), (8), (12) and (15) were used to
F
p
ig. 14. FWHM profiles obtained on different steel grades using a 14A SP (full cov-
rage). *Q corresponds to the quenched and non-tempered 4340 steel.
tress field. According to these results, it can be seen that any
hot peening treatment gives rise to significant surface hardening,
ut its final effects are also highly dependent on the strength of
he steel. The softer the steel, the greater the surface increase in
he FWHM parameter; that is to say, softer steels have a greater
ork-hardening capacity. However, if the hardness of the base
teel is high enough, shot peening treatments can also give rise
o a kind of local softening. This is clearly seen in the hardest steel
Q + T200), which was only submitted to stress relieving temper-
ng. The observed decrease in the FWHM parameter in this steel
s probably due to dislocation re-arrangement. The base steel has
 distorted structure with a high hardness and peening-induced
lastic deformation has resulted in a lower-energy dislocation
rrangement. This has, consequently, given rise to a reduction in the
WHM parameter in the surface and sub-surface regions, although
 slight increase in FWHM was observed in the first 0.05 mm. The
ehavior of the second hardest steel grade (Q + T425) is situated
etween the hardest grade and the other steels, confirming the
forementioned explanation. According to this same figure, it is
lso worth noting that the surface value of the FWHM parameter is
arely affected by the applied Almen intensity (no clear influence
f the shot peening intensity was observed).
Moreover, the initial FWHM parameter characteristic of each
teel (internal, base FWHM value) is linearly related to the hardness
f the steel, as can be seen in Fig. 14. This fact can be better appre-
iated in Fig. 15a, in which the base FWHM steel value has been
lotted versus the hardness of the steel (a last result obtained with
he quenched and non-tempered 4340 steel, 662 HV, has also been
ncluded in this figure). A quite good linear correlation between
hese two variables was obtained, confirming the possibility of
ig. 15. Evolution of the FWHM. The greatest hardness represented in the graphs (662 HV
arameter; (b) the surface FWHM minus the base FWHM versus steel hardness (full cove
Fig. 16. Experimentally measured residual stress profile and predicted values
(Q + T540 4340 steel, SP12A and full coverage).
using the FWHM parameter to detect changes in hardness. On the
other hand, surface hardening produced by shot peening treat-
ments is better represented as the difference between the surface
FWHM and the base FWHM parameters. Fig. 15b shows a linear
decrease in surface hardening with decreasing hardness of the steel.
It is also worth noting that the surface FWHM parameter of Q + T
steels whose hardness is above 470 HV does not increase through
conventional shot peening. In fact, the surface FWHM values of
these steels decrease below the base value characteristic of each
steel. Nonetheless, even in these cases, a certain degree of work
hardening can be appreciated in the first 0.05 mm (see Q + T200 in
Fig. 13).
3.4. Example of application
Eqs. (1), (5), (8), (12) and (15) were used to predict the residual
stress profile of a Q + T540 4340 steel using only the measurements
of the impact diameters produced in a shot peening treatment. The
average measurements of the impact marks were 179 �m. Using
Eq. (1), the application of an Almen intensity of 0.308 mm (12.3A)
determine the compressive residual stress at the surface, �rcs , the
maximum value of the compressive residual stress, �rcmax, the total
depth submitted to compressive residual stresses, Z0, and the depth
subjected to high compressive residual stresses, Zhc (−�rc > �ys/2).
) corresponds to the quenched and non-tempered 4340 steel. (a) The base FWHM
rage).
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84 V. Llaneza, F.J. Belzunce / Applied
able 9 shows the obtained results, which are also represented in
ig. 16 along with the residual stress profile experimentally mea-
ured on the shot peened sample.
. Conclusions
It is well-known that shot peening is a complex technology
hich produces different effects on the surface of the treated com-
onents, the most important being the modification of surface
ppearance, work hardening and residual stresses. The most rele-
ant results obtained using an AISI 4340 steel submitted to different
uenched and tempered heat treatments in order to obtain a wide
ange of mechanical properties are reported below.
Impact marks: as impact diameter is a direct measure of the
ntensity of any shot peening treatment, there is a linear relation-
hip between the diameter of the surface impact marks and the
pplied Almen intensity. A simple, accurate expression is proposed
o predict the impact diameter, which increases with increasing
lmen intensity and with decreasing steel hardness.
Kinetic energy: the kinetic energy of the projected balls is par-
ially transferred to the specimen surface and, consequently, Almen
ntensity and the kinetic energy of the shots are also linearly related.
Roughness: the analysis of surface roughness through Ra and
max confirmed that both parameters evolve in the same way. For
he same material, roughness increases with increasing applied
lmen intensity (larger impact marks). However, the size of the
hots also plays an important role: roughness decreases with
ncreasing shot size, even when a higher Almen intensity is pro-
uced. Moreover, roughness depends on the degree of coverage,
ncreasing until full coverage and subsequently remaining con-
tant, due to the saturation of surface work hardening.
Residual stress profiles: any compressive residual stress profile
s well defined using four parameters: the compressive residual
tress at the surface, �rcs ; the maximum compressive residual stress,
rc
max; the depth subjected to high compressive residual stresses,
hc; and the total depth subjected to compressive residual stresses,
0. It was confirmed that, under full coverage, �rcs and �
rc
max only
epend on the mechanical properties of the treated steel (they do
ot depend on the applied Almen intensity, as surface hardening
aturates after attaining full coverage), whereas, Z0 and Zhcdepend
n both the mechanical properties of the steel and the applied
lmen intensity. Several simple, accurate, practical expressions
ere formulatedto predict these four parameters in quenched and
empered 4340 steel grades which only require the applied Almen
ntensity (mmA) and one of the main mechanical properties of the
reated steel as input.
Work hardening: shot peening also induces an increase of the
ardness of the surface region which can be easily quantified by
eans of the FWHM parameter. It was seen that the base FWHM
f the steel, being a hardening parameter, is linearly related to its
ardness. On the other hand, from the study of the surface evolution
f this parameter, it can be stated that softer steels have a greater
ork-hardening capacity, although some kind of softening was also
bserved in the harder steels unduly associated with dislocation
e-arrangement.
cknowledgements
The authors are grateful for the financial support for this study
rovided by the European Union (FEDER funds) and Principado
e Asturias, through the Plan de Ciencia, Tecnología e Innovacion,
013–2017 (FC-15-GRUPIN14-001 Project). Víctor Llaneza grateful
cknowledges funding from the Principado de Asturias Govern-
ent, through the Severo Ochoa Programme (contract BP10-021).
[
[
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ce Science 356 (2015) 475–485
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