SURFACE TREATMENT OF A METAL PART
The invention relates to the field of the treatment of metal surfaces, in particular to
the treatment by peening, optionally combined with thermochemical treatments.
5 Shot peening is a technique that is widely used for improving certain properties of
metal surfaces, such as the fatigue life. A shot-peening treatment is typically
characterized by a degree of coverage that describes the proportion of the surface
impacted by the peening and an intensity that describes the amount of kinetic
energy applied per unit area. The literature in the field of shot peening prescribes
10 limits for the degree of coverage and intensity parameters, beyond which the
peened material incurs degradation such as cracking and reduction of the fatigue
life. The conditions giving rise to these degradations are commonly denoted by the
term "overpeening".
Subsequent research into peening treatments has highlighted the possibility of
15 producing a nanostructuring of the material at a surface layer by pushing the
peening treatment beyond the limits normally prescribed. The term
"nanostructuring" denotes the obtaining -of a stable phase, the grain size of which is
of the order of a few tens of nanometres. Under certain conditions, it is assumed
that the nanostructuring of the material prevents the propagation of microcracks, so
20 that the aforementioned degradations do not occur.
The nanostructuring of the material produces advantageous effects such as the
increase of the fatigue life, of the hardness, of the corrosion resistance, of the atomic
diffusivity, of the biocompatibility, the improvement of the tribological properties,
etc.
25 Among the processes known for producing a nanostructured surface layer, note may
essentially be taken of:
• techniques for projecting fine or very fine particles at high or very high speed (Fine
Particle Bombarding or Air Blast Shot Peening) at normal incidence onto the sample
to be treated. These techniques are especially presented in the publication of the
30 Iron and Steel Institute of Japan, ISU International, Vol, 47 (2007), No. 1, pp 157-162;
• techniques for vibrating larger particles at lower speeds, known as UltraSonic Shot
Peening or Surface Mechanical Attrition Treatment. These techniques are especially
presented in the publication of the Japan Institute of Metals Materials Transactions,
Vol. 45, No. 2 (2004), pp 376-379.
5 WO02/10461 describes a process for generating nanostructures at the surface of a
metal'part in which perfectly spherical balls similar to ball bearing balls are projected
onto a point of impact of the part under variable incidences. In order to obtain a
thickness of nanostructures of a few tens to a few hundreds of microns, it is taught
to mechanically and/or thermally stress the surface of the metal part to be treated.
10 Document CN101580940A describes a tyre mould treatment method. The
method comprises a step of treatment via nanocrystallization. The
nanocrystallization step is carried out using a continually oscillating peening tool.
WO 02/10462 describes a process for generating nanostructures in which
balls are projected onto a point of impact of a part along different and varied
15 directions of incidence by a ball projection source in order to create deformations
having any direction. The process also comprises a step of chemical treatment of the
nanostructured layer at temperatures of 550°C and 350°C.
According to one embodiment, the invention provides a process for the surface
20 treatment of a metal part, comprising:
exposing a surface of the metal part to a stream of substantially spherical particles,
so that any portion of said surface receives said particles along several primary
incidences, the primary incidences of the particles on a portion of the surface being
essentially distributed in a cone or a conical film which has an outer half apex angle
25 between 10° and 45°, until a surface layer of nanostructures is obtained, for example
a layer having an average thickness of greater than 40 or 50 nm,
the particles having a diameter of less than 2 mm and greater than 0.1 mm and
being projected at a speed between 40 m/s and 100 m/s.
One idea at the heart of the invention is to create one or more streams of particles
30 capable of hitting a surface to be treated along varied and controlled incidences in
order to stress a large number of atomic slip planes of the material. According to
one embodiment, the surface layer of nanostructures has an average thickness of
greater than 40 or 50 \}rr\, the boundary of the surface layer of nanostructures being
determined to be a region of the metal part where the hardness is greater thaji a
5 threshold that is dependent on the metal material from which the part is made.
According to one embodiment, said hardness threshold is defined by a hardening of
the material with respect to a prior art upon surface treatment which is equal to 50%
of the hardening obtained at the treated surface of the metal part. In other cases
this threshold may be defined as a function of other parameters, especially the
10 position of a crystalline phase transition in the material when such a transition takes
place.
According to other advantageous embodiments, such a process may have one: or
more of the following features.
According to one embodiment, the particles have a diameter of greater than 0.3 (nm
15 and less than 1.4 mm.
According to one embodiment, the incidences of the particles are distribijted
substantially continuously in the cone or the conical film.
In one embodiment, the cone or the conical film has an outer half apex angh; of
between 10° and 30°.
20 According to one embodiment, the stream of particles comprises a jet of particles
projected along a central direction, the metal part being fixed to a support so as to
present said surface oriented obliquely with respect to said central direction, the
support being rotated about an axis coaxial with the central direction of the jet of
particles.
25 According to one embodiment, the inclination of the surface of the part with resj^ect
to the central direction is between 10° and 30°, preferably close to 15°.
According to one embodiment, the particles are projected at a speed of between 50
and 80 m/s.
According to one embodiment, the particles have a hardness greater than the
30 hardness of the surface of the part before treatment.
According to one embodiment, the invention thus provides a metal part comprising
a surface treated by the aforementioned process, said surface comprising a surface
layer of nanostructures having an average thickness of greater than 40 or 50 pm, the
boundary of the surface layer of nanostructures being determined to be a region of
5 the metal part where the hardness is greater than a threshold that is dependent on
the metal material from which the part is made.
According to one embodiment, said hardness threshold is defined by a hardening of
the material with respect to a prior art upon surface treatment which is equal to 50%
of the hardening obtained at the treated surface of the metal part.
10 According to one embodiment, the surface layer of nanostructures has an average
thickness of greater than 100 pm.
According to one embodiment, the invention also provides a surface treatment
device for a metal part, comprising:
a projection means capable of producing a stream of substantially spherical particles
15 having a diameter of less than 2 mm and greater than 0.1 mm and thus are
projected at a speed of between 40 m/s and 100 m/s,
a support capable of holding a metal part, the support comprising a surface exposed
to the stream of particles, and
an actuator capable of modifying an orientation of the support with respect to the
20 stream of particles so that the primary incidences of the particles on a surface of the
support are essentially distributed in a cone or a conical film that has an outer half
apex angle of between 10° and 45°.
According to one embodiment, the projection means is capable of producing a jet
of particles projected along a central direction, the surface of the support being
25 oriented obliquely with respect to said central direction, the actuator being capable
of pivoting the support about an axis that is coaxial with the central direction of the
jet of particles.
The above peening treatments may be followed by thermochemical processes.
According to one embodiment, the surface of the metal part is exposed to
30 thermochemical conditions that lead to the diffusion of a substance in the metal
structure of the part in order to modify the chemical composition of the metal part
over at least one part of its thickness starting from the surface.
It was observed that in the above peening processes, it was possible to obtain a high
density of grain boundaries emerging at the surface of the part. Owing to the
5 nanostructuring of the material previously obtained, diffusion treatments are thus
capable of enriching the metal structure with elements that are diffused more
rapidly and/or over a greater depth and/or at a lower temperature than in a part
without surface nanostructuring. Such a thermochemical treatment may firstly, but
not solely, affect the nanostructured layer. Indeed, the nanostructured region is
10 capable of providing particularly effective inlet channels for also affecting the
underlying metal.
According to one embodiment, in which the part is made of steel, the
thermochemical conditions are nitriding conditions at a temperature between 300°C
and 590°C that lead to a diffusion of nitrogen into the grain boundaries of the steel
15 of the surface layer of nanostructures, the process resulting in the formation of fine
precipitates dispersed in this layer, for example in the form of nitride or carbonitride
particles finely dispersed in the surface layer of nanostructures.
According to one embodiment, the metal part is composed of an austenitic stainless
steel or of a structural steel, and the surface is exposed to the stream of particles
20 with a degree of coverage of 1000 to 2000%.
According to one embodiment, the metal part is composed of a tool steel and the
surface is exposed to the stream of particles until a nanostructured layer of at least
40 pm thick is obtained, for example by carrying out the peening with a degree of
coverage of greater than 3000%.
25 According to one embodiment, the thermochemical conditions are low-pressure
carbonitriding conditions at a temperature between 750°C and 1100°C that lead to a
recrystallization of the surface layer of nanostructures and a diffusion of nitrogen
into the grain boundaries of the steel of the recrystallized surface layer, the process
producing or favouring the formation of carbonitride particles finely dispersed in the
30 recrystallized surface layer.
According to one embodiment, the step of exposing the metal part to the
thermochemical conditions comprises:
subjecting the metal part to a gradual temperature rise up to said carbonitriding
temperature and
5 holding the temperature at said carbonitriding temperature, the combined duration
of the temperature rise and of the temperature hold being less than three hours.
According to one embodiment, the combined duration is between 0.5 and 1.5 hours.
According to one embodiment, the surface is exposed to the stream of particles with
a degree of coverage of between 400% and 1000%.
10 Similarly, other thermochemical treatments lead to the formation of precipitates
having a different chemical nature, as a function of the diffused elements. In any
case, the prior nanostructuring of the material favours the structuring of these
precipitates in a fine and dispersed form and inhibits the formation of precipitates in
a coarse form or in the form of a weakening continuous network.
15 Certain aspects of the invention are based on the idea of designing a process for
nanostructuring the material which has a high productivity in order to produce
relatively thick nanostructured surface layers in a relatively short time. Certain
aspects of the invention are based on the idea of producing relatively homogeneous
nanostructured surface layers. Certain aspects of the invention are based on the idea
20 of designing a process for nanostructuring the material which can be applied to
varied geometries, in particular concave shapes. Certain aspects of the invention are
based on the idea of designing a process for nanostructuring the material which is
relatively easy and economical to implement.
The invention will be better understood, and other objectives, details, features and
25 advantages thereof will become more clearly apparent in the course of the following
description of several particular embodiments of the invention, given solely by way
of illustration and nonlimitingly, with reference to the appended drawings.
In these drawings:
• Figure 1 is a schematic representation of a process for nanostructuring a
30 metal surface.
• Figure 2 is a schematic perspective view of a peening machine suitable
for implementing the processes according to the embodiments of the invention.
• Figure 3 is a schematic representation of a particle jet produced by the
machine from Figure 2.
5 • Figure 4 is a diagram of the operation of the machine from Figure 2.
• Figure 5 is a graph representing the change in the hardness of a metal
part as a function of the depth below/ the treated surface, for several peening
conditions.
• Figure 6 is a graph representing the change in the thickness of a
10 nanostructured surface layer as a function of the degree of coverage for the peening
conditions from Figure 5.
• Figure 7 is a graph representing the change in the treatment time as a
function of the degree of coverage for several shot sizes.
• Figure 8 is a graph representing the change in the surface hardness and
15 in the thickness of a nanostructured surface layer as a function of the degree of
coverage for a peening condition.
• Figure 9 is a graph representing the change in the thickness of a
nanostructured surface layer as a function of the degree of coverage for various
modes of attachment of the treated part.
20 • Figure 10 is a graph representing the change in the thickness of a
nanostructured surface layer as a function of the inclination of a support in the
machine from Figure 2, for several peening conditions.
• Figure 11 is a graph representing the change in the hardness of parts
made of various metal materials as a function of the depth below the treated
25 surface.
• Figure 12 is a graph representing the change in the thickness of a
nanostructured surface layer as a function of the degree of coverage for various
metal materials.
• Figures 13 and 14 are graphs representing the change in the surface
hardness and in the thickness of a nanostructured surface layer as a function of the
degree of coverage for two different rates of projection.
• Figure 15 is a graph representing the change in the surface hardness and
5 in the thickness of a nanostructured surface layer as a function of the degree of
coverage for another peening condition.
• Figure 16 is a schematic cross-sectional representation of a part treated
by a peening process representing the region of influence of an impact.
• Figures 17 to 20 are optical micrographs of nanostructured surface
10 layers.
• Figure 21 is a graph representing the diffusion profile of carbon and of
nitrogen in a part treated by a nitriding process without prior peening.
• Figure 22 is a graph representing the diffusion profile of carbon and of
nitrogen in a part treated successively by a peening process and the nitriding
15 process from Figure 21.
• Figure 23 is a graph representing the profile of the residual stresses
respectively in a part treated successively by nanostructuring, nitriding process and
shot-peening process and in a part treated only by the nitriding process and shotpeening
process.
20 • Figure 24 is a graph representing the profile of the Vickers hardness
respectively in a part treated successively by a peening process and a low-pressure
carbonitriding process and in a part treated only by the low-pressure carbonitriding
process.
• Figures 25 and 26 are electron microscopy images showing the cross
25 section of a sample of structural steel 32CrMoV13 in the vicinity of its surface, for a
gaseous nitriding treatment at 520°C without and with prior nanostructuring.
• Figures 27 and 28 are optical microscopy images showing the cross
section of a sample of tool steel X38CrMoV5 in the vicinity of its surface, for a
gaseous nitriding treatment at 520°C without and with prior nanostructuring.
^
• Figures 29 and 30 are optical microscopy images showing the cross
section of a sample of steel of 23MnCrMo5 in the vicinity of its surface, for a lowpressure
carbonitriding treatment without and with prior nanostructuring.
• Figure 31 is a graph representing the change in the hardness of a metal
5 part as a function of the depth below the treated surface, for several peening
conditions with another hardness measurement method.
• Figure 32 is a schematic cross-sectional representation of a metal part
having a nanostructured surface layer as a function of the depth below the treated
surface on which the measured hardness curve is superposed.
10 • Figure 33 is a graph representing the change in the surface hardness of a
part treated by peening and the change in the thickness of a nanostructured surface
layer as a function of the degree of coverage.
• Figure 34 is a graph representing the change in temperature as a
function of time during a simulation of a temperature rise over 30 minutes up to the
15 high level of a carbonitriding treatment (around 900°C).
• Figure 35 is a graph representing the change in temperature as a
function of time during a simulation of a temperature rise of the same type as
Figure 34 but with a duration of 1 h 30 min.
• Figure 36 is a graph representing the change in temperature as a
20 function of time during a simulation of a temperature rise of the same type as
Figure 34 but with a duration of three hours.
Described below are embodiments of peening processes that make it possible to
obtain a nanostructured surface layer on a metal part. Unless otherwise indicated,
25 the experimental results presented below are obtained with fiat metal samples.
With reference to Figure 1, a process for nanostructuring a metal surface 1 is
schematically represented. In the left-hand view, before treatment, the size of the
grains 2 of the material all the way to the surface 1 is typically a few tens to a few
hundreds of |am. In the right-hand view, after treatment, the grain size of the
30 material at a surface layer 3 is reduced to a few tens of nm, for example around 20
10
nm, whilst grains of larger size continue to exist more deeply in the material.
Subsequently, an axis z perpendicular to the surface 1 and oriented towards the
inside of the material starting from the surface is defined. The surface serves as a
reference of the dimensions. The transition of the size of the grains between the
5 surface layer 3 and the unmodified deep material is in reality more gradual than in
the drawing.
The nanostructuring of the material in the layer 3 is stable up to a temperature of at
least 600°C. A metal part coated with such a nanostructured layer may be used in
various industries, for example in applications where the wear resistance and the
10 fatigue resistance are critical properties.
With reference to Figure 2, a peening machine 10 which may be used to produce
such a nanostructured layer is now described.
The machine 10 comprises a projection nozzle 11 supplied from a shot reservoir and
from an air compressor (which are not represented) in order to produce a jet of shot
15 projected at a speed V which may vary depending on the size of the shot particles.
As a variant, the projection of the shot particles may also be carried out using a vane
turbine, according to the known art. Common peening equipment makes it possible
to obtain speeds ranging from 20 m/s to around 120 m/s.
The shot used preferably consists of particles obtained by atomization. Such
20 particles may be produced in a large amount at a relatively advantageous cost and
have quite good sphericity, for example greater than or equal to 85%. Their cost is
substantially lower than that of ball bearing balls, the process for the manufacture of
which is virtually unitary in order to achieve a sphericity of greater than 99%.
Alternatively, other conventional peening media can be used, such as conditioned
25 cut wire, glass beads or ceramic beads.
The projection nozzle 11 is fixed facing a mobile support device 12 constructed in
the following manner: a metal disk 13 is mounted on the shaft of a rotary motor that
is not represented, for example an electric motor, in order to be able to pivot with
respect to a fixed frame 19. The central pivoting axis of the disk 13 is coaxial with a
30 central projection axis of the nozzle 11. Positioned on the disk 13 is an inclinable
11
support 14, the angle of inclination of which with respect to the disk 13 can be
adjusted by means of a screw. Fastened around a central portion of the inclinable
support 14 are fastening clamps 15 provided with screws 16 parallel to the support
14. The screws 16 may be tightened onto a part to be treated in order to fasten the
5 part between the clamps 15 and may be loosened in order to withdraw the part after
treatment.
With reference to Figure 3, a jet of particles 20 produced by the projection nozzle 11
is schematically represented. The jet 20 has an approximately conical shape with a
half apex angle p. The angle p may be measured, for example, as the ratio between
10 the radius p of an impacted region 21 and the distance L from the region 21 to the
orifice 22 of the nozzle 11.
With reference to Figure 4, the operating principle of the peening machine 10 is now
described. For a flat sample parallel to the support 14, the surface portion located
around the central axis 25 of the jet 20 receives the particles at an angle of incidence
15 a with respect to the local normal direction 26. The surface portion located around
an edge of the jet 20 receives the particles at an angle of incidence (a-P) with
respect to the local normal direction 27. The' surface portion located around the
opposite edge of the jet 20 receives the particles at an angle of incidence (a+P) with
respect to the local normal direction 28.
20 When the support device 12 turns during the projection of the particles, any portion
of the sample located in the jet 20 is hit at incidences located in a more or less wide
conical film. This conical film is thin towards the centre of the jet where it coincides
exactly with the angle a and broader towards the periphery of the jet, where it
includes all the angles between (a-P) and (a+P). If p>a, the conical film
25 degenerates into a cone. During the rotation of the support device 12, a treated
surface region may be hit at all the angle of elevation values located in the conical
film. This property of the machine 10 makes it possible to produce nanostructured
layers on different metals with a relatively high productivity, as will be recounted in
the tests below. In the tests below, the angle p is equal to around 8° and the
30 distance L to around 300 mm. Of course, it is not excluded for a small portion of the
12
particles to be projected along atypical trajectories outside of the main directions of
the jet 20.
The tests which will be described below were carried out with various types of shot,
the main properties of which are mentioned in Table 1, according to the SAE J444
5 standard. The nominal diameter of a type of shot is defined as the median diameter
of the distribution: 50% by weight of the particles of the type of shot considered
have a diameter of less than the nominal diameter, and 50% have a larger diameter.
Table 1: properties
Type of
shot
S550
S330
S280
S170
S070
Nominal
diameter
(D)
1.40
0.85
0.71
0.425
0.18
mm
of the shots according to the SAE J444 standard
Distribution (fraction of the particles of larger size than)
0
2.00 1.70
>85
0
1.40
>96
<5
0
1.18
<5
1.00
>85
0
0.85
>96
>85
<10
0.71
>96
0.60 0.50
>85
0
.425
>97
<10
0.35 0.30
>80
0.18
>90
.125
10
Testl
Table 2 recounts the results of a first test carried out with the machine 10 on flat
samples of E24 steel (low-alloy steel: 0.2% C, 1.5%Mn, 98.2% Fe) fastened by
clamping to the support 14 with an inclination a = 15°. Recorded in this table are
15 the type of shot used in the test, the projection speed V, the degree of coverage R,
the thickness of the nanostructured layer Zn obtained, the Vickers hardness of the
sample on its face exposed to the peening, the Vickers hardness of the sample on its
opposite face, and the ratio between the two hardnesses, known as the hardness
gain.
20 The degree of coverage R is a measurement of the proportion of the surface
impacted by the peening. In the present description, it is defined as follows: the
reference 100% indicates that an amount of shot which is statistically sufficient to
impact 98% of the exposed surface was projected. Beyond 100%, a linear law is
13
applied with respect to this reference amount. A degree of coverage of 1000%
therefore indicates that ten times the reference amount has been projected. At
constant flow rate, the degree of coverage is therefore also a measurement of the
treatment time of the sample.
5 The thickness of the nanostructured layer zn was obtained by two methods:
observation by optical microscopy and observation of the hardness profile of the
material as a function of the depth z.
Via optical microscopy, the thickness measured is an arithmetic mean of nine
obsen/ations of the thickness of the visually amorphous layer corresponding to the
10 nanostructured region 3. The width of the sample treated is scanned over three
regions and three measurements are taken per region, which ensures the
reproducibility of the measurement method.
The microscope observations are then correlated to hardness profiles, in order to
confirm that the visually amorphous region observed indeed corresponds to the
15 peak of hardness originating from the hardening by the effect of the nanometresized
grains.
The method used for producing the hardness profile consists in making an
indentation line with a step of 50 pm starting from the outermost surface with a
micro Vickers hardness tester having a pyramidal tip with a load of 100 g (HV 0.1)
20 which possesses a lens. The surface of the sample and the nanostructured layer are
visualized as in optical microscopy. The hardness profile is thus obtained from a
depth of 50 pm to 500 pm. The values communicated are an average of three
indentation lines in order to have a reliable and reproducible measurement.
The connection between the hardness profile and the thickness Zn may be explained
25 more precisely with the aid of Figure 5. Figure 5 represents the hardness profiles
obtained by the method explained above in test 1 samples with R=3000%. The curve
30 corresponds to the type S170 shot. The curve 31 corresponds to the type S280
shot. The curve 32 corresponds to the type S330 shot. The curve 33 corresponds to
the type S550 shot. On all the curves 30 to 33, a region of very high hardness 34
30 appears, which corresponds to the nanostructured layer 3 and a second region 35
14
appears where the hardness decreases more gradually with the depth and which
corresponds to the strain hardening of the material. The boundary of the
nanostructured layer 3 must therefore correspond to a steep change of slope of the
hardness. This point is verified in Figure 5 where the thicknesses Zn obtained by
5 visual observation have been plotted as a dot-and-dash line for each type of shot.
More specifically, for the four types of shot tested in Figure 5, the boundary of the
nanostructured layer 3 observed visually corresponds substantially to the region in
which the hardness is equal to the median value between the hardness value at the
surface, which is here represented by the first measurement point at a depth of
10 50 |jm, and the hardness value far from the surface, where the material has not been
substantially affected by the peening, which is represented by the last measurement
point at 500 urn.
A quantitative definition of the nanostructured layer 3 may therefore be provided as
a function of the hardness curve: the nanostructured layer 3 is the region in which
15 the hardening of the material produced by the peening treatment is greater than or
equal to 50% of the maximum hardening obtained at the surface of the sample. This
empirical definition has been verified experimentally for the degrees of coverage of
greater than 750%, as will be explained below.
Figure 6 represents the change in the thickness Zn observed visually as a function of
20 the peening treatment time, measured by the degree of coverage R, by the four
types of shot. Curve 36 corresponds to type S170 shot. Cun/e 37 corresponds to type
S280 shot. Cun/e 38 corresponds to type S330 shot. Curve 39 corresponds to type
S550 shot. Curves 36 to 39 demonstrate a detection threshold of the nanostructured
layer 3 and a saturation threshold of its thickness. In particular, it is seen that the
25 thickness no longer changes significantly beyond the R=3000% within the context of
test 1.
Figure 6 demonstrates that all the shots from test 1 make it possible to obtain a
thickness Zp that exceeds 100 \^m, or even 140 pm. This figure also demonstrates two
advantages of the type S280 and S330 shots (curves 37 and 38). On the one hand,
30 the nanostructured layer 3 appears significantly at a lower degree of coverage R,
15
around 300%, than with the larger particles (S550) or smaller particles (S170). On the
other hand, the thickness z^ reaches its peak at a higher level than that obtained
with larger particles (S550) or smaller particles {S170).
Not obtaining a maximum thickness Zn with the largest particles (S550, curve 39)
5 may be considered surprising. This observation can however be explained by the
competition effect that exists, when the size of the projectiles increases, between on
the one hand the increase in the kinetic energy per particle, which involves a deeper
and more intense plastic deformation of the material at each particle impact, and on
the other hand the increase of the mean spacing between the impacts, which
10 involves a less even spatial distribution of the impacts.
This competition is illustrated schematically in Figure 15, where the region of
influence of an impact, also referred to as the nanocrystallization lobe, is
represented by a semisphere. Whereas close impacts produce a thickness Zp that is
relatively uniform over the entire treated surface, impacts that are relatively spaced
15 apart give rise to edge regions where the material is deformed over a relatively small
thickness Zo and central zones where the material is deformed over a relatively large
thickness Zi. The thickness Zn that can be observed lies between ZQ and Zi.
Another property on which the size of the particles has an observable effect is the
uniformity of the thickness Zn along the treated surface. This property may be
20 characterized by the standard deviation t of the thickness Zn. Table 3 recounts the
values measured in the samples from test 1, micrographs of which are reproduced in
Figures 17 to 19. For the chosen degree of coverage, it appears that the largest type
S550 shot provides a mean thickness Zn comparable to the thickness obtained with
the type S330, but a doubling of the standard deviation L Figures 17 to 19 also make
25 it possible to observe nanocrystallization lobes.
16
Table 3: Standard deviation of the nanostructured thickness in test 1
Fig.
17
18
19
20
Type of shot
S170
S280
S330
S550
Degree of
coverage R
(%)
1000
1000
1000
1000
Vickers
hardness at
the surface
(HV)
263
290
290
292
Nano
thickness z„
(nm)
72.05
119.7
159.76
175.5
Nano
thickness
standard
deviation 6
(urn)
11.1
12.5
19.6
40
Moreover, depending on the nature of the projection nozzle 11, the time needed to
obtain a given degree of coverage may increase with the size of the particles.
5 Figure 7 represents, for a conventional peening nozzle model, the change in the
degree of coverage R with the projection time t for two different particle sizes, all
conditions being otherwise equal. Curve 40 relates to type S550 and curve 41 to
type S280. In test 1, in order to form a thickness of 100 jjm, 107 s are needed with
type S550 versus 30 s with type S330 and 75 s with type S280. It is therefore seen
10 that the optimal type of shot in terms of productivity, that is to say that produces
the greatest nanostructured thickness per unit time, lies below the S550 particle size.
Test 1 therefore shows that counter-productive effects of the large particles begin to
arise with the type S550 shot and that it is not advantageous to use even larger
sizes.
15 Figure 8 demonstrates the relationship between the nanostructured thickness Zn and
the hardening observed at the surface of the treated sample. Curve 42 represents
the thickness Zn (left-hand axis) and the curve 43 the Vickers hardness at the surface
(right-hand axis) as a function of the coverage R for type S280 in test 1. Curve 43
demonstrates a strain-hardening effect which causes a first increase in hardness in a
20 region 45 starting from the initial hardness 44 without however forming nanometresize
grains, and an effect of the nanostructuring of the material which causes a
second increase in the hardness in a region 46.
Test 2
»
17
In order to evaluate the optional effect of dannping the part by the clamps 15 in test
1, a test 2 was carried out with the type S280 shot under conditions similar to test 1
by adhesively bonding the sample to the support 14 without applying any clamping
stress thereto. Figure 9 shows the change in the thickness Zn as a function of the
5 coverage R in test 2 (square symbols) superposed on curve 42 from test 1. No
significant difference emerges between the results of the two tests, neither in the
thickness measurements, nor in the hardness measurements, which means that the
fastening of the part by clamping in test 1 has no causal relationship with the
nanostructuring effects observed.
10 Test 3
In order to evaluate the effect of orienting the part to be treated with respect to the
jet of shot, a test 3 was carried out with the type S170, S280 and S330 shots under
conditions similar to test 1 by varying the angle a between 0° and 45° and the
rotation of the support device 12. The nanostructured thicknesses obtained in this
15 test 3 are recorded in Table 4.
Table 4: nanostructured thickness in test 3 for R = 3000%
Rotation
Without
With
Without
With
Without
With
an
S170
S280
S330
0
0
0
131
133
111
116
15
97
130
156
189
168
236
30
92
102
134
171
134
183
45
72
105
153
160
144
125
Zn (Mm)
It is observed for each type of shot that the thickness z„ varies with the angle a in
order to reach a peak very clearly at around a = 15°. It is also observed that the
20 rotation of the support does not produce any significant effect for a = 0° but
substantially increases the thickness Zn when the support is inclined. These
observations show that the production of impacts of the particles at incidences
varied at any point of the treated surface substantially increases the productivity of
the nanostructuring process. In particular, these instances are distributed in a cone
18
or a conical film which has an outer half apex angle of between around 10° and 45°
within the context of this test.
Figure 10 graphically represents the results from Table 4 with rotation of the
support. Curve 50 corresponds to type S170 shot. Curve 51 corresponds to type
5 S280 shot. Curve 52 corresponds to type S330 shot.
In order to evaluate the effect of the nature of the treated material on the
nanostructuring process, other tests were carried out with different materials. In
theory, different materials have a different receptivity to severe plastic deformation
and therefore to nanocrystallization mechanisms. The grain refinement procedures
10 under severe plastic deformation depend on many intrinsic and extrinsic factors,
such as the structure and the stacking fault energy (SFE) of the material. The higher
the SFE energy of the material, for example such as pure iron, the more difficult the
activation of the various slip planes and the generation of dislocations necessary for
the grain requirement procedures are made. The crystallographic structure of the
15 metal and the optional presence of other elements such as carbon or other alloy
elements, especially in the form of precipitates that favour the formation of
dislocations, therefore have an influence on the productivity of the nanostructuring
process.
Test 4
20 A comparative test was carried out with samples of 304L stainless steel and a
32CrMoV13 structural steel under conditions similar to test 1 with type S280 shot.
Figure 11 illustrates the results of these tests in terms of hardness profile for
R=3000% in a representation analogous to Figure 5. Curve 53 corresponds to the
304L stainless steel. Curve 54 corresponds to the 32CrMoV13 structural steel. The
25 hardness profiles of these materials correspond to the trends obsen/ed in test 1.
Regions 34 and 35 of Figure 11 have the same meaning as in Figure 5. Curve 31
from test 1 (E24 steel) is plotted by way of comparison. The thicknesses Zr, observed
are visually 143 jjm for E24, 176 \irr\ for 32CrMoV13 structural steel and 155 |jm for
304L stainless steel.
19
Once again, the validity of the ennpirical quantitative definition given above for
curves 54 and 31 is observed.
This definition clearly corresponds to curve 53 (304L steel) when the reference for
the hardening is chosen at a depth of 300 pm. The choice of reference is explained
5 by the change of microstructure specific to the 304L steel, during the peening of the
material, and more particularly during a first step of the peening corresponding to a
step of strain-hardening of the material.
During the first step of the peening of the material, a certain amount of austenite of
the 304L steel is converted to strain-induced martensite. This conversion to strain-
10 induced martensite gives rise to a significant increase in the hardness. On curve 53, a
significant reduction in the hardness is visible between 300 and 350 pm. This
reduction in the hardness corresponds on the whole to the austenitic phase
transition zone and the phase having a high content of strain-induced martensite. In
a second step, the nanostructured layer 3 appears in the martensitic phase. Thus,
15 starting from a thickness of greater than 350 nm, the sample of 304L steel has its
original hardness of the austenite and for a thickness of less than 300 jjm the
hardness of the material is increased both by the nanostructured layer and by the
presence of strain-induced martensite. Thus, the reference hardness used for
determining the nanostructured layer is the hardness at the deepest layers of the
20 strain-induced martensite, which is here around 300 |jm.
Test 5
A comparative test was carried out with samples of pure iron containing 0.03C
(99.8% Fe) under conditions similar to test 1 with type S170 shot. The pure iron is
assumed to be one of the least favourable materials for grain refinement due to its
25 ferritic structure and its high SFE energy (around 200 mJ/m^). Curve 55 from
Figure 12 represents the thickness Zn resulting from this test, observed visually, as a
function of the degree of coverage R. Curve 36 from test 1 (E24 steel) is plotted by
way of comparison.
It is thus confirmed that the E24 steel nanocrystallizes more rapidly (appearance of
30 the nanostructured layer at R=750%) than pure iron (appearance of the
t
20
nanostructured lay«r at R=1000%) and has a thicker nanostructured layer
(Zn=130 ym versus Zn=100 pm) at saturation. Test 5 shows that the process makes it
possible to obtain nanostructured layers thicker than 100 pm for most of the
materials that can be envisaged.
5 Teste
In order to evaluate the effect of smaller particles, tests were carried out with
samples of pure iron containing 0.03C (99.8% Fe) and type S070 shots. The other
conditions are similar to test 1.
Figure 13 illustrates the results obtained with a projection speed V = 60 m/s in a
10 representation similar to Figure 8. Curve 60 represents the Vickers hardness at the
surface and curve 61 the thickness Zn observed visually. It is observed that the
thickness Zn saturates at a level close to 60 |jm from R= 3000% onwards. With small
particles such as type S070, this degree of coverage may be rapidly achieved, for
example in less than 300 s with a common peening material.
15 Figure 14 illustrates the results obtained with a projection speed V= 92 m/s in a
representation similar to Figure 8. Curve 62 represents the Vickers hardness at the
surface and curve 63 the thickness Zn. It is observed that the thickness Zn saturates at
a level close to 80 to 90 Mm from R= 3000% onwards.
Test 8
20 A second series of tests will now be described. In this second series of tests,
the hardness profiles of samples were measured with a more precise method in
order to provide a definition of the nanostructured layer based solely on the
hardness curve of the material.
Table 10 recounts the results of the second series of tests carried out
25 according to the same conditions as test 1 presented in Table 2.
The method used for producing the hardness profile during this second
series of tests consists in making an indentation line with a step of 10 pm starting
from 20 pm from the outermost surface to a depth of 100 ijm. The indentation line
is then continued with a step of 50 [im to a depth of 300 pm. The indentation line is
30 made with a micro Vickers hardness tester having a pyramidal tip with a load of 25 g
#
21
(HV 0.025) which possesses a lens. It is a Buehler Micromet 5104 microhardness
tester comprising a motorized table having a step of 1 [im and Buehler Omnimet
Mhtsa control and measurement software. The hardness profile is thus obtained
from a depth of 20 |jm to 300 |jm. The values communicated are an average of three
5 indentation lines in order to have a reliable and reproducible measurement. In the
same way as in the preceding tests, the surface of the samples and the
nanostructured layer are visualized by optical microscopy. The observation of the
samples is carried out using a Zeiss axio scope Al microscope, a Qimaging
Micropublisher 5.0 RTV camera, a Zeiss EC EPIPLAN X10/0.2HD lens and Axiovision
10 4.8 software.
The first three columns of Table 10 correspond to the first three columns of
Table 2. The fourth column mentions the thickness of the nanostructured layer,
denoted by Znh with reference to the hardness. Indeed, in test 8, the thickness of the
nanostructured layer z^h was obtained by a method solely based on the hardness
15 profile as a function of the depth z. For this, a hardness threshold is determined by
calculating the median value of the hardness between the hardness measured on
the surface layer and the hardness of the sample in the deep layer in which the
material is not substantially modified by the peening.
The thickness of the nanostructured layer Znh therefore corresponds to the
20 depth at which the increase in the hardness is equal to half of the increase in
hardness observed at the surface of the sample after treating this surface.
The fifth and sixth columns mention the hardness at the surface of the
sample on the treated face and on the untreated face. These values correspond to
the first measurement points of the measured hardness curve, that is to say to a
25 depth of 20 pm. On the whole, the hardness is measured closer to the surface than
in test 1, so that the hardness value is higher than in Table 2. Indeed, the size of the
grains in the vicinity of the surface varies according to a gradient. Thus, in one
outermost surface region, the size of the grains varies between 10 and 50 nm, and in
a deeper region, the size of the grains varies between a few tens of nanometres to a
30 few hundreds of nanometres. Furthermore, the hardness is measured with a larger
22
load in test 1 than in the second series of tests. The impression made in the material
therefore has larger dimensions in test 1 and therefore generates a less precise
measurement.
The last column from Table 10 mentions the uncertainty margin of the
5 thickness measurement Znh resulting from the uncertainty margin of the
microhardness tester. Indeed, the hardness measurements have an uncertainty of
around ±10 Vickers for the E24 steel, ±9.5 Vickers for the 32CrMoV13 steel and ±13.5
Vickers for the 304L steel. For better accuracy of the hardness measurement, the
hardness tester load is adapted as a function of the hardness of the material: a
10 greater load is used for harder materials. Thus, a load of 50 g (HV 0.050) is used for
the 32CrMoV13 steel and for the 304L steel.
Figure 31 represents the hardness profiles obtained by the method
explained above for the samples corresponding to the samples from test 1 with
R=3000%. Curve 170 corresponds to type S170 shot. Curve 172 corresponds to type
15 S330 shot. Curve 173 corresponds to type S550 shot. On all the curves 170,171 and
173, a zone of very high hardness appears which corresponds to the nanostructured
layer 3 and a second zone appears where the hardness decreases more gradually
with the depth and which corresponds to the strain hardening of the material.
By way of example, in Figure 31, the hardness value 174 measured in the
20 deep layer and the maximum hardness value 175 measured on the surface layer of
the sample associated with curve 170 are respectively equal to 142 and 300 Vickers.
The corresponding threshold 171 has a value of 221 Vickers, which corresponds to
the median value between the hardness value 174 measured in the deep layer of the
sample and the maximum hardness value 175 measured on the surface layer of the
25 sample.
This threshold makes it possible to determine a thickness Znh of the
nanostructured layer having a value approximately equal to 81.5 jjm for the test
corresponding to the S170 shot.
An uncertainty range of the thickness Znh of the nanostructured layer is
30 therefore determined from the hardness threshold and from the uncertainty range
23
of the hardness. By way of example, for the threshold 171 of 221 Vickers presented
previously, the boundary values of the thickness of the nanostructured layer are
plotted for hardness values 185 and 186 respectively of 231 Vickers and 211 Vickers.
Thus, the thickness of the nanostructured layer lies within a range of around 69 to
5 92 |jm. The uncertainty ranges of the thickness of the nanostructured layer are
presented in Table 10. Thus, due to the uncertainty of the hardness, the thickness of
the nanostructured layer measured graphically itself also has a measurable
uncertainty.
As indicated previously, the second measurement method based on the
10 hardness agrees satisfactorily with the visual determination method: Figure 32
schematically represents the regions observed on the optical micrographs of the
sample corresponding to curve 170 from Figure 31 (S170 peening at R=3000%). The
hardness profile 170 as a function of the depth z from the surface of the sample is
plotted on the schematic representation of these regions.
15 Observed in Figure 32 is a nanostructured surface layer 177 corresponding
to a region in which the material is substantially amorphous and homogeneous.
Layer 177 corresponds to the darker zone observed in Figures 17 to 19. Layer 177
extends from the surface 176 of the part to a second layer 178. This second layer 178
corresponds to the region in which grain boundaries are observed and in which the
20 size of the grains delimited by the grain boundaries increases with the depth. On the
optical micrographs, layer 178 corresponds to the region which extends from a
sudden change in contrast starting from layer 177. This second layer 178
corresponds to the strain-hardening region of the material. A third layer 179
comprises a region where the size of the grains remains constant. The hardness
25 threshold 171 agrees substantially with the boundary 184 observed visually between
the nanostructured surface layer 177 and the layer 178.
The difference between the thickness values Zn observed visually listed in
Table 2 and the thickness values z^h listed in Table 10 originate essentially from the
relatively high uncertainty margin of the measurements mentioned in Table 2
30 typically of the order of ±30 pm. In reality, the visual observations listed in Table 2
24
encompass a portion of the transition layer 178, which explains the higher thickness
values.
The method of measuring thickness based on the hardness described above may
display a difference with the optical observation when the thickness of the
5 nanostructured layer is thin, which corresponds to the case of the samples from test
8 at a degree of coverage of less than 750%. Another method for determining the
thickness of the nanostructured layer may then be used. This other method is also
based on the principle of determining the thickness of the nanostructured layer from
a hardness threshold. This method starts from the observation that, when it appears
10 on the sample and therefore when it has a very thin thickness, the nanostructured
layer 3 has a hardness value at the surface which corresponds to this threshold. By
way of illustration, with reference to Figure 33, curve 180 represents the thickness of
the nanostructured layer as a function of the coverage and curve 181 represents the
surface hardness of the sample as a function of the coverage for the S170 peening
15 test. A minimum detectable thickness 182 of the nanostructured layer appears for a
coverage of 150%. However, the surface hardness 183 measured during this
appearance of the nanostructured layer is 226 Vickers. This hardness threshold of
226 constitutes a realistic value of the hardness threshold for determining the
thickness of the nanostructured layer after treatment with a coverage of less than
20 750%. This alternative value has a value close to the hardness threshold determined
with the aid of the median value at R=3000% (221 Vickers). In test 8 listed in Table
10, the hardness thresholds were determined with this other method for coverage
values of less than 750%. In Table 10, the values determined with this other method
have an asterisk.
25 These results should be compared with those presented in Figure 4 of the
International ISU publication cited above, where the Fe-3.3Si alloy used has a ferritic
crystalline structure comparable to Fe-0.03C. Test 6 demonstrates the obtaining of a
greater nanostructured thickness with a degree of coverage, a particle size and a
projection speed that are all lower than in this publication. It is noted that the
30 comparison of degrees of coverage requires a calibration due to different definitions
25
in the two cases. The use of a lower projection speed may prove advantageous for
reducing the roughness of the treated sample or protecting a material more
vulnerable to microcracks.
Testy
5 Test 7 was carried out with samples of pure iron containing 0.03C (99.8% Fe) and
type S170 shots. The other conditions are similar to test 1.
Figure 15 illustrates the results obtained with a projection speed V= 57 m/s in a
representation similar to Figure 8. Curve 64 represents the Vickers hardness at the
surface and curve 65 the thickness Zn. The numbers 44, 45 and 46 have the same
10 meaning as in Figure 8. It is observed that the thickness Zn saturates at a level close
to 100 pm.
These results should be compared with those presented in Figures 3(a) and 4(b) of
the Materials Transactions publication cited above. In particular, much greater
thicknesses are obtained in a much shorter time and with a much more flexible
15 process than ultrasonic shot peening (USSP) for the same shot size.
Although the results presented above are obtained with flat metal samples, the
processes used are applicable to metal parts of any shape. In particular, in order to
treat a non-planar surface, it is possible to successively treat limited portions of the
non-planar surface, by each time orienting the treated surface portion so that the
20 angle conditions described previously with reference to the flat surface are
approximately respected for each successive portion of the non-planar surface. The
expression "successive portion" is understood here to mean a surface portion that is
relatively small with respect to the local radius of curvature, so that an average
orientation of the surface portion can be defined, and relatively large with respect to
25 the size of the shots projected, so that a large number of impacts can statistically be
envisaged.
Certain non-planar geometries are capable of producing multiple impacts by one
and the same particle on the part, that is to say rebounds. However, given that the
rebounds lead to very high energy losses, it is assumed that it is the primary
26
incidence of the particle, that is to say the incidence before the first impact on the
part which is the most significant.
If it is not desired nor even possible to carry out the aforementioned orientation
conditions for each surface portion of the part to be treated, it is preferable to
5 identify the portions of the metal part intended to be the most stressed in its final
use, that will be referred to as the working surfaces of the part. For example, the
working surfaces of a gear pinion are generally the bases of the teeth. The
nanostructuring treatment of a pinion can therefore be carried out, in one particular
embodiment, by successively orienting the tooth base surfaces facing the particle
10 jet, so as to carry out the particular orientation of the primary incidences of the
particles on the tooth base surface.
A single projection nozzle has been presented in the embodiment of the machine
from Figure 2. However, it is also possible to conceive a peening machine with
several projection nozzles. These projection nozzles may especially be arranged so
15 as to target the same surface of the part along several different incidences.
Projection nozzles may also be arranged so as to target various surfaces of the part
to be treated.
Other relative arrangements of the projection nozzles and of the support of the part
can be envisaged in order to produce primary incidences of the particles which are
20 distributed in a cone or a conical film having an outer half apex angle between 10°
and 45°. In particular, a displacement may be carried out at the projection nozzles.
Combination of peening with thermochemical processes
The nanostructuring processes described previously may be combined with
thermochemical treatments in order to modify, by diffusion, the chemical
25 composition of the surface layers of the metallic materials and give them particular
mechanical, physical and chemical properties, for example so as to improve the wear
resistance, the fatigue resistance, the high-temperature or low-temperature
oxidation resistance, or the corrosion resistance. In this case, the surface chemical
modification does not impair the chemical composition of the core of the metal. The
30 surface chemical modification may be combined with a heat treatment. The choice
t
27
of the element diffused and of the heat treatment depends on the properties
desired at the surface and in the core.
The main thermochemical treatments that can be used for this purpose are mentioned in
Table 9 below.
5 Tab
CHEMICAL SYSTEM
Fe-N
Fe-C
Fe - C - N
Fe-S
Fe - S - N
Fe - C - N - S
Fe-B
Fe-AI
Fe-Cr
Fe - Cr - Al
Fe-Si
e 9: Main thermochemical treatments
NAME
Nitriding
Carburizing
Carbonitriding
Sulphurizing
Sulphonitriding
Sulphocarbonitriding
Bonding
Aluminizing
Chromizing
Chromoaluminizing
Siliconizing
DESIRED RESISTANCES
Friction (wear) - corrosion
fatigue
Friction (wear) - contact
fatigue under high loads
Friction (wear) - contact
fatigue under moderate
load
Friction (seizing)
Friction (seizing - wear) -
corrosion-fatigue
Friction (seizing - wear) -
corrosion-fatigue
Friction (wear)
High-temperature
oxidation
Abrasion - oxidation
Abrasion - hightemperature
oxidation
Acid resistance
Due to the prior application of the nanostructuring treatment, it is useful to
distinguish between thermochemical treatments that may be carried out without a
substantial recrystallization taking place in the nanostructured phase, that is to say
10 typically the treatments at relatively low temperatures, and thermochemical
treatments which cause a substantial recrystallization of the nanostructured phase,
that is to say typically the treatments at relatively high temperatures.
28
Lov\/-temperature treatments
In the thermochemical treatments in which the temperature does not exceed around
590°C for steel, the grain size in the nanostructured layer does not change
5 substantially during the diffusion process. The nanostructuring of the material has
the effect of multiplying the grain boundaries which emerge at the outer surface of
the part and which thus favour the diffusion process. It is thus observed that a
diffusion process carried out after a nanostructuring peening has a higher
effectiveness in terms of kinetics and diffusion depth than in the absence of
10 nanostructures. Furthermore, the nanostructuring limits the size and the connectivity
of the grain boundaries, so that precipitation phenomena, in particular precipitation
of carbonitrides, are inhibited in the nanostructured layer.
Nitriding tests recounted below in connection with Table 6 illustrate these points.
Test 8.1
15 A flat sample of 304L stainless steel is used. The nitriding is carried out in a plasma
furnace over a hold of 100 h at 350°C.
On the part nitrided without peening, a layer of nitrogen austenite having a
thickness of 3 to 5 microns is formed. On the part nitrided with peening, a nitrogenenriched
nanostructured dark layer of mixed martensite/austenite structure is
20 observed between a depth of 4 and 10 microns.
The Vickers hardness was measured at the surface of the part under a load of 25 and
50 g. It is increased by around 30% by the prior peening treatment and may reach
around 1000 HV.
Figures 21 and 22 represent the concentration (mass fraction) of nitrogen and of
25 carbon as a function of the depth respectively for a part nitrided without prior
peening (curves 70 and 71) and for a part successively peened and nitrided (curves
73 and 74).
Without peening treatment, the diffusion stops around 5 microns with a nitrogen
content of 0.06 %. The nitrogen content at the surface is around 11 %. The presence
29
of a carbon-enriched sublayer 72 is also observed at the nitrogen diffusion front, at
around a depth of 5 microns.
With the prior peening treatment, the nitrogen diffusion is deeper. The degree of
enrichment at the surface is around 9%. The carbon-enriched sublayer is attenuated
5 by the peening treatment, hence a more continuous carbon concentration profile
results.
Test 8.2
A flat sample of 304L stainless steel is used. The nitriding is carried out in a plasma
furnace over a hold of 100 h at 400°C.
10 The increase in temperature favours the diffusion of nitrogen to greater depths. On
the part without peening, an even white layer mainly consisting of nitrogen austenite
is formed at a depth of 8 to 10 microns. The peened part is characterized by a
nanostructured dark layer having a depth of 8 to 15 microns. The enrichment depth
increase may range up to 50% owing to the prior peening.
15 The gain in hardening effect is at least 15% with respect to the nitriding treatment
without prior peening: the Vickers hardness measured at the surface tends towards
1300 HV, versus around 1150 HV without prior peening.
Furthermore, the same phenomena as in test 8.1 are observed, that is to say a more
continuous carbon profNe with the peening treatment.
20
For type 304L, 304, 316 or 316L austenitic stainless steels, the hardening occurs in
particular by the reduction of grain size and by the partial or total conversion of the
austenite to martensite known as strain-induced martensite. However, the formation
of strain-induced martensite tends to greatly reduce the corrosion resistance
25 properties of these steels. Thus, a peening with too high a coverage on such a
material reduces the corrosion resistance properties of the material. The inventors
have therefore observed that a coverage between 1000 and 2000% for steels of this
type presents the best compromise in order to benefit from the hardening effect
without greatly degrading the anti-corrosion properties.
30
30
Test 9.1
A flat sample of 32CrMoV13 structural steel quenched-tempered at around 600°C is
used.
On the part nitrided at 480°C without peening treatment, the presence of a thin
5 homogeneous white layer of iron nitride having a thickness of 1 to 2 microns is
observed. Many carbonitride network precipitates are present in the diffusion layer.
When the concentration of nitrogen increases, these precipitates tend to form a
relatively continuous layer which constitutes a region of brittleness of the part and
which favours spalling of the surface.
10 On the part nitrided at 480°C with nanostructuring, there is a start of growth of a
white layer having a thickness of 1 to 3 microns. In the nanostructured layer, the
diffusion of nitrogen is not accompanied by the visible appearance of networks of
nitrides or carbonitrides.
This hardness at the surface is increased by around 15% owing to the prior peening
15 treatment with respect to nitriding alone.
Test 9.2
A flat sample of 32CrMoV13 structural steel quenched-tempered at around 600°C is
used.
20 Figure 25 is an electron microscopy image showing the cross section of the sample
in the vicinity of its surface, for a gaseous nitriding treatment at 520°C without prior
nanostructuring. Without prior peening, the presence of a sublayer is observed in
the lower part of the white layer 80 at the interface with the diffusion layer which is
seriously enriched in carbon (0.5%). This favours the development of connected, and
25 therefore embrittling, carbonitride networks visible at the number 79.
Figure 26 is an electron microscopy image showing the cross section of the sample
in the vicinity of its surface, for a gaseous nitriding treatment at 520°C after prior
nanostructuring. With the prior nanostructuring, the gaseous nitriding treatment at
520°C maintains a layer of nanostructured material in which the carbonitride
30 precipitates are finely dispersed in the grain boundaries without forming networks of
31
carbonitrides in the layers affected by the nanostructuring treatment. The nitrogen
diffusion depth, around 260 microns, is not negatively affected by the
nanostructuring treatment. The enrichment content of nitrogen at the outermost
surface in the white layer exceeds 14% in order to tend towards 18% with a
5 nanostructuring treatment.
An additional hardening, estimated at around 10%, is obtained by the effect of the
nanostructuring treatment not only at the surface but also on the first 100 microns.
The surface hardness tends towards 1000 HV.
The thickness of the white layer 80 is very similar between the two treatments and is
10 around 10 microns. At a depth of 50 microns, the nitrogen contents are close: 1.5%
with nanostructuring and 1.3% without nanostructuring.
Figure 23 represents the residual compressive stress as a function of the depth for
various treatments:
Squares: nitriding treatment followed by shot peening
15 - Triangles: nanostructuring treatment followed successively by a
nitriding treatment and shot peening.
The two stress profiles are substantially identical in terms of compressive stress
values to a depth of 50 [im. Beyond a depth of 50 |jm, the sample treated by
nanostructuring, nitriding and shot peening shows a significant increase in the stress
20 values and depths with respect to a nitriding treatment and shot peening alone. The
compressive depth increase is close to 200 microns. This may favour the fatigue
behaviour if the shear stresses are deep and are located in this region, especially in
the case of rolling stress.
25 Generally, it is observed that a peening having a coverage between 1000% and
2000% makes it possible to obtain the best compromise between the mechanical
properties of hardening resulting from the relatively deep diffusion of the nitrogen
and the surface finish of the steel on a structural steel, especially a 32CrMoV13 steel.
Indeed, above 2000%, the deformations caused by the peening generate a surface
30 finish which may prove unsuitable for the typical uses of such a steel, in particular in
32
precision mechanics and gearing.
Test 9.3
5 A flat sample of X38CrMoV5 tool steel, quenched and double tempered to 40-43
HRC, subjected to a gaseous nitriding treatment at 520°C, is used.
Figure 27 is an optical microscopy image showing the cross section of the sample
subjected to the nitriding treatment without prior nanostructuring. Figure 28 is an
optical microscopy image showing the cross section of the sample subjected to the
10 nitriding treatment after prior nanostructuring. The region close to the surface of the
sample is shown each time.
The nanostructuring treatment made it possible to retain a nanostructure with a
high attenuation of the carbonitride network precipitates, visible at number 79 in
Figure 27. The nitriding treatment at 520°C also made it possible to obtain a nitrided
15 layer that was as deep with or without the nanostructuring treatment. The
conventional depths of enrichment in nitrogen are respectively 145 microns without
nanostructuring treatment and 170 microns with nanostructuring treatment. The
surface Vickers hardnesses are of the same level and are around 1300 HV.
In this test too, the presence of a nanostructure therefore limits the precipitation of
20 networks of carbonitrides under the white layer 80. The expected gain is therefore a
better impact and spalling resistance under the combination layer. For tools such as
those used in a forge or in a foundry, this results in a longer service life.
For tool steels, especially the X38CrMoV5 steel, it was observed that a
25 nanostructured layer having a thickness of at least 40 |jm enabled a significant
improvement in the nitriding. Such a thickness may especially be obtained by a
coverage of at least 3000% using type S170 shots. The use of shots of larger size,
typically S280 or S330, would make it possible to obtain the same thickness with a
reduced coverage but would significantly degrade the surface finish of the treated
30 part.
33
High-temperature treatments
Low-temperature diffusion processes limit the atomic size of the elements that it is
possible to make diffuse into the metal. On the other hand, in high-temperature
5 processes, above around 750°C for steel, a recrystallization occurs which may
remove the surface nanostructuring. A high temperature is especially necessary in a
low-pressure carbonitriding process, which consists in injecting hot nitrogen into a
vacuum chamber and which makes it possible to limit the oxidation of the material
with respect to conventional carbonitriding processes.
10 However, the inventors have discovered that it is also possible to obtain a fine-grain
recrystallization when a previously peened part is subjected to a high-temperature
process such as low-pressure carbonitriding. The recrystallization size depends on
the degree of strain hardening obtained by peening, on the temperature and on the
duration of the thermochemical treatment.
15 Owing to the fineness of the grains, for example greater than or equal to 10,
preferably greater than or equal to 12 according to the NF A 04-102 standard, it is
possible to obtain a high density of grain boundaries emerging at the surface, which
favours the diffusion and makes it possible to increase the nitrogen concentration
and optionally carbon concentration of the carbonitrided layers and the diffusion
20 rates. In addition, it is also obsen/ed that the prior nanostructuring treatment makes
it possible to inhibit the precipitation of dense carbonitride networks, which are a
cause of brittleness of the surface of the part.
Low-pressure carbonitriding tests recounted below in relation to Table 7 illustrate
these points.
25 Test 10
The treated sample is a pinion tooth. The inventors observed, for excessively intense
nanostructuring conditions, a coarse-grain recrystallization. Thus a coverage of 400
to 1000% was identified as being the best peening condition.
34
In order to evaluate the recrystallization as a function of the time, the sample was
subjected to tests of temperature rise following various modes in order to achieve a
final hold at 880°C. Three total treatment times T were tested: 0.5 h, 1.5 h and 3 h.
Each time, the sample was cooled in open air. It is furthermore known that the
5 temperature rise kinetics of the furnace have an influence on the nature of the
recrystallization and that a gradual heating favours a fine-grain recrystallization. This
is why relatively gradual temperature rise curves were chosen in the examples below.
In order to identify the best peening conditions, tests were carried out for different
degrees of coverage and different types of shot. The grain index according to the NF
10 A 04-102 standard was measured at the surface of each of the samples
corresponding to a duration of the treatment and a corresponding degree of
coverage.
15
The results of the tests have been listed in Table 11 below:
Table 11: Grain size measured on the sample as a function of the surface treatment
carried out and the treatment time T
Total time
of the heat
treatment
0.5 h
1.5 h
3h
Without
peening
(R=0%)
9
9
9
Prior
peening
S170,
R=500%
>12
12
9
Prior
peening
S170,
R=750%
>12
12
9
Prior
peening
S170,
R=1000%
>12
11
9 to 10
Prior
peening
S280,
R=1000%
>12
8 to 9
8 to 9
On the samples with nanostructuring, the following structures are observed at the
20 end of the temperature rise and cooling:
- T= 0.5 h, corresponding to a gradual rise from 200 to 800°C: very
fine ferritic grains (index > 12) can be observed at the surface. Just
below, a start of recrystallization is witnessed, but very quickly the
i
35
structure without nanostructuring is encountered.
The temperature curve during test T = 0.5 h is illustrated in Figure
34. Firstly, the temperature rises gradually before reaching, at
around T = 0.3 h, a hold 200 at SSOX, the temperature hold 200
5 being maintained until T = 0.5 h.
Although this short heat treatment appears highly advantageous
owing to the fineness of the grains obtained at the surface, it cannot
be adapted to all situations due to the thermal inertia of the
materials, which may prevent a uniform temperature being obtained
10 in the treated part in as short a time once the size of this part is
large.
- T = 1.5 h corresponding to a rise from 200 to 800°C with two
intermediate holds: the very fine grains are also present at the
surface for coverages of less than or equal to 1000% depending on
15 the size of the shot. It is clearly observed that the recrystallization at
the triple boundaries becomes increasingly visible and affects a
greater depth when the coverage R increases. For R = 1000%, an
enlargement phenomena of the ferritic grains at the surface of this
steel is noted, which results in a grain size index of less than 12.
20
The temperature curve during test T = 1.5 h is illustrated in Figure
35. The temperature rises up to a hold 201 at 760°C maintained until
around T = 0.75 h. The temperature is then increased up to a hold
202 having a temperature of around 850°C maintained up to T =
25 1.25 h, then the temperature is brought to a final hold 203 at 8B0°C
until the end of the test.
- T = 3 h corresponding to a rise from 200 to 800°C with three
intermediate holds: all of the nanostructurings tested result in an
enlargement phenomena of the ferritic grains at the surface with
30 indices of less than or equal to 9 and just under the surface. Then,
36
underneath, a fine-grain recrystallization is perfectly identified.
The temperature curve during test T = 3 h is illustrated in Figure 36.
During the test, the temperature is gradually brought to a
temperature hold 204 of 880°C by means of three temperature
5 holds: a first hold 205 at a temperature of around 533°C followed at
T = 1 h by a second hold 206 at a temperature of around 630°C,
itself followed at T = 2 h by a third hold 207 at a temperature of
around 740°C. The hold 204 is achieved at around T = 2.75 h.
10 It is therefore observed that a coarse-grain recrystallization (index < 10) took place if
the heat treatment time is too long, whereas it is possible to observe fine grains
(index > 10) for heat treatments having a duration of less than three hours. The 1.5 h
high-temperature treatment therefore appears to be a good compromise for
obtaining a fine-grain recrystallization on standard mechanical parts. The grain size
15 obtained depends however both on the heat treatment parameters and on the prior
peening parameters.
Thus, it may be noted that too large a coverage, that is to say a coverage of greater
than or equal to 1000%, favours the crystallization with coarser grains, in particular
in the 1.5 h treatment. Furthermore, the inventors have observed that when the
20 degree of coverage is too large, the deformations of the part that are produced by
the peening become too large, which degrades its surface finish. This is especially
the case for samples subjected to degrees of coverage of 1000%. Thus, the inventors
have observed that a degree of coverage of less than or equal to 1000% made it
possible to obtain the best compromise for the surface finish while avoiding the
25 enlargement of the grains. In particular, in the case where the surface finish is an
essential property of the part to be treated, it may be necessary to reduce the
degree of coverage. In this case, a coverage of 400 to 500% may be chosen.
Similar quantitative and qualitative results have also been observed with samples
30 consisting of 20MnCr5 and 27MnCr5 gearing-type steel. The above observations
37
apply to other alloyed steels, especially gearing-type steels, for example the steels
20CrMo4, 27CrMo4,18MnCrB5, 29MnCr5,15MnCrMo5,18NiCrMo5, or 20NiCrMo7.
For R = 500% and T = 1.5 h, the average grain size is around 3.3 \xm, i.e. twelve
5 emerging boundaries counted at the surface over 40 |im.
Figure 29 is an optical microscopy image showing the cross section of the sample
subjected to the low-pressure carbonitriding treatment without prior
nanostructuring. Figure 30 is an optical microscopy image showing the cross section
of the sample subjected to the low-pressure carbonitriding treatment after prior
10 nanostructuring. A region of the sample located between a depth of 0.4 and 0.5 mm
is shown each time. For the low-pressure carbonitriding tests, the time T = 1.5 h was
retained. The tests led to it being observed that the nanostructuring makes the
carbonitride networks disappear. Indeed, the structure observed at the end of the
carbonitriding treatment is composed of martensite and a start of carbonitride
15 network at the grain boundary is visible at number 79 on Figure 29 for a sample
without prior peening, whereas the surface with nanostructuring comprises only
martensite, which is difficult to attack, in which globules of carbonitrides are
distributed. The grain size index measured according to NF A 04-102 at the surface
of the sample is equal to 9 for the sample from Figure 29 and equal to 12 for the
20 sample from Figure 30. It may be assumed that the grain boundaries in which the
globules of carbonitrides are located have a size proportional to the grain size, so
that the fineness of the grains inhibits the percolation of the carbonitride
precipitates in the form of connected networks.
25 Figure 24 represents the hardness of the sample as a function of the depth after
low-pressure carbonitriding, with prior peening (curve 75) and without peening
(curve 76). A distinct increase in the depth hardened owing to the prior
nanostructuring is observed, the depth at which a hardness of 650 is observed
moving from 0.41 mm without nanostructuring to 0.54 mm with nanostructuring.
38
On the carbon concentration profile, no change is obsen/ed between the simply
carbonitrided sample and the sample that has undergone both treatments. On the
other hand, the nanostructuring improves the nitrogen concentration profile over
the first two tenths of mm below the surface, which explains the higher hardness
5 plateau. This gain is measured in Table 8 below.
Table 8: mass fraction of nitroger
pressure carbonitriding
Depth z (mm)
0.06
0.10
0.15
0.20
0.25
Sample without
nanostructuring
0.13
0.12
0.10
0.09
0.08
{%) as a function of the depth after low-
Sample with
nanostructuring
0.21
0.16
0.14
0.11
0.08
Gain (%)
62
33
40
22
0
10 Although the invention has been described in connection with several particular
embodiments, it is clearly obvious that it is in no way limited thereto and that it
includes all the technical equivalents of the means described and also combinations
thereof if the latter come under the scope of the invention.
In particular, the embodiments described in the examples relate to initially
15 homogeneous materials on which the peening processes described make it possible
to form relatively thick nanostructured surface layers. It is possible to characterize
the degree of coverage applied to a given material by the thickness of the
nanostructured layer that this coverage made it possible to obtain. Hence, the
application of a similar degree of coverage to a material having undergone other
20 prior treatments is also capable of effectively producing nanostructured surface
layers, even if this pretreated material does not correspond to the examples
described, for example a heterogeneous material.
The use of the verb "to have", "to comprise" or "to include" and its conjugated forms
does not exclude the presence of elements or steps other than those mentioned in a
39
claim. The use of the indefinite article "a" or "an" for an element or a step does not
exclude, unless otherwise mentioned, the presence of a plurality of such elements or
steps. Several means or modules may be represented by one and the same material
element.
5 In the claims, any reference sign between parentheses should not be interpreted as a
limitation of the claim.
40
Table 2: test 1, E24 steel, Rockwell hardness of the shots = 48HRC
Type of
shot
S170
S280
S330
S550
V (m/s)
57
52
60
49
R (%)
100
150
200
300
500
750
1000
1500
3000
6000
10000
100
150
200
300
500
750
1000
1500
3000
6000
10000
100
150
200
300
500
750
1000
1500
3000
6000
10000
100
150
200
300
500
750
1000
1500
3000
6000
10000
Zn (urn)
0
0
0
0
0
69
72
91
129
138
140
0
0
0
67
91
101
120
134
143
178
172
0
0
0
111
112
142
160
175
192
193
186
0
0
0
0
0
104
176
168
164
175
173
Hardness
of treated
face (HV)
198
211
200
212
241
256
263
274
308
309
302
215
224
224
247
262
278
290
295
298
301
315
213
233
234
264
253
282
290
298
310
300
304
206
216
223
227
243
278
292
279
292
295
308
Hardness of
untreated face
(HV)
108
114
113
111
112
108
111
116
113
113
116
130
132
138
139
137
138
113
116
114
113
114
114
116
110
111
108
114
114
112
123
131
142
129
144
131
135
145
148
147
153
159
157
167
Hardness
gain (%)
83%
85%
77%
91%
115%
137%
137%
136%
173%
173%
160%
65%
70%
62%
78%
91%
101%
157%
154%
161%
166%
176%
87%
101%
113%
138%
134%
147%
154%
166%
152%
129%
114%
60%
50%
70%
68%
68%
88%
99%
82%
84%
88%
84%
w
41
Table 6: nitriding treatments after peening
Definition
of the
test
Test 8.1
Test 8.2
Test 9.1
Test 9.2
Test 9.3
Sample
304L
stainless
steel
32CrMoV13
steel (for
gearing)
X38CrMoV5
steel (for
forge tool)
Peening
conditions
Size =
S170
Har.=
48HRC
V=57
m/s
R= 1125%
Zn= 80 |jm
Size =
S170
Har.=
48HRC
V=57
m/s
R= 1875%
Zn= 90 |jm
Size -'
S170
Har.=
58HRC
V=57
m/s
R= 3000%
Zn= 40 |jm
Nitriding
conditions
Plasma at
350°C for
100 h
Plasma at
400°C for
100 h
Plasma at
480°C for
50 h
Gaseous
nitriding
at 520°C
for 50 h
Gaseous
nitriding
at 520''C
for 50 h
Thickness
of layer
having
high
nitrogen
content
(Mm)
4 to 10
8 to 15
l t o 3
10
14
Idem
without
peening
(|jm)
3 to 5
8 to 10
l t o 2
10
10
Nitrogen
content
at the
surface
(%)
9
12
14
18
12
Idem
without
peening
(%)
11
12
14
14
10
Table 7: low-pressure carbonitriding treatments after peening
Definition
of the test
Test 10
Sample
23MnCrMo5
steel (for
gearing)
Peening conditions
Size = S170
Har.= 48 HRC
V= 57 m/s
R= 400%
Zn= 40 |jm
Carbonitriding conditions
Low-pressure
carbonitriding at 880°C
V
42
Table 10: Test 8, samples corresponding to test 1, E24 steel, Rockwell hardness of the shots
48HRC
Type of
shot
S170
S330
S550
V
(m/s)
57
60
49
R (%)
100
150
200
300
500
750
1000
1500
3000
6000
10000
100
150
200
300
500
750
1000
1500
3000
6000
10000
100
150
200
300
500
750
1000
1500
3000
6000
10000
Zn (urn)
0.00
0.00
27.46*
30.28*
46.47*
50.00*
54.22
59.15
81.69
94.36
87.32
0.00
35.21*
34.50*
39.43*
67.60*
69.71
76.05
111.26
111.97
123.94
97.14
0.00
0.00
0.00
29.57*
44.36*
57.74*
98.59
108.45
97.18
115.00
119.28
Hardness
of treated
face (HV)
225
226
234
252
276
281
288
290
292
323
327
240
244
253
260
267
284
297
299
309
310
310
222
225
227
240
248
261
271
289
295
309
325
Hardness of
untreated
face (HV)
142
133
140
143
132
135
140
140
131
135
127
136
136
139
135
129
128
129
126
128
157
126
135
139
141
144
128
141
134
148
132
142
144
Hardness
gain (%)
58%
70%
67%
76%
109%
108%
106%
107%
123%
139%
157%
76%
79%
82%
93%
r 107%
122%
130%
137%
141%
97%
146%
64%
62%
61%
67%
94%
85%
102%
95%
123%
118%
126%
Znh min.
(nm)
49.29
57.74
69.01
90.84
73.23
61.26
69.01
102.11
97.88
109.50
90.00
76.76
81.69
83.80
85.71
98.57
Znh max.
(urn)
62.67
74.64
92.25
96.47
95.77
90.14
96.47
121.83
123.23
139.43
113.57
161.97
133.09
146.47
140.00
150.00

0 5 m "m
CLAIMS
1. Process for the surface treatment of a metal part made of steel, comprising:
exposing a surface (1) of the metal part to a stream (20) of substantially spherical
5 particles, so that any portion of said surface receives said particles along several
primary incidences, the primary incidences of the particles on a portion of the
surface being essentially distributed in a cone or a conical film which has an outer
half apex angle (a, a+p, a-|3) between 10° and 45°, until a surface layer (3) of
nanostructures is obtained,
10 the particles having a diameter of less than 2 mm and greater than 0.1 mm and
being projected at a speed between 40 m/s and 100 m/s, and
exposing the surface (1) of the metal part to thermochemical conditions that lead to
the diffusion of a substance into the metal structure of the part in order to modify
the chemical composition of the metal part over at least one part of its thickness
15 starting from the surface (1), the thermochemical conditions being low-pressure
carbonitriding conditions at a temperature between 750°C and 1100°C that lead to a
recrystallization of the surface layer (3) of nanostructures and a diffusion of nitrogen
into the grain boundaries of the steel of the recrystallized surface layer, the process
producing the formation of carbonitride particles finely dispersed in the
20 recrystallized surface layer.
2. Process according to Claim 1, in which the step of exposing the metal part
to the thermochemical conditions comprises:
subjecting the metal part to a gradual temperature rise up to said carbonitriding
temperature and
25 holding the temperature at said carbonitriding temperature, the combined duration
of the temperature rise and of the temperature hold being less than three hours.
3. Process according to Claim 2, in which the combined duration is between
0.5 and 1.5 hours.
44
4. Process according to one of Claims 1 to 3, in which the surface (1) is
exposed to the stream of particles with a degree of coverage of between 400% and
1000%.
5. Process according to one of Claims 1 to 4, in which the grains of the steel in
5 the vicinity of the surface (1) of the nitrided or carbonitrided part have a size index
greater than or equal to 10 according to NF A 04-102.
6. Process according to one of Claims 1 to 5, in which the substantially
spherical particles have a diameter of greater than 0.3 mm and of less than 1.4 mm.
7. Process according to one of Claims 1 to 6, in which the incidences of the
10 substantially spherical particles are distributed substantially continuously.
8. Process according to one of Claims 1 to 7, in which the cone or the conical
film has an outer half apex angle of between 10° and 30°.
9. Process according to one of Claims 1 to 8, in which the stream of particles
comprises a jet of particles (20) projected along a central direction (25), the metal
15 part being fixed to a support (14) so as to present said surface oriented obliquely
with respect to said central direction, the support being rotated about an axis coaxial
with the central direction of the jet of particles.
10. Process according to Claim 9, in which the inclination of the surface of the
part (a) with respect to the central direction is between 10° and 30°, preferably close
20 to 15°.
11. Process according to one of Claims 1 to 10, in which the substantially
spherical particles are projected at a speed of between 50 and 80 m/s.
12. Process according to one of Claims 1 to 11, in which the substantially
spherical particles have a hardness greater than the hardness of the surface of tP
25 part before treatment.