SURFACE TREATMENT OF A METAL PART BY OBLIQUE SHOT PEENING

The invention relates to the field of the treatment of metal surfaces, in particular to the treatment by peening.

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 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 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 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. 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 Iron and Steel Institute of Japan, ISIJ 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.

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.

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 directions of incidence by a ball projection source in order to create deformations having any direction. A layer thickness of 10 μm is obtained with balls having a diameter of 300 μm and a layer thickness of 20 μm is obtained with balls having a diameter of 3 mm.

WO 02/10463 describes a process for generating nanostructures in which ball motion is triggered by the combination of a circular motion of a chamber containing the balls and a vibrating motion along a direction perpendicular to the plane of the circular motion of the chamber. A nanostructured layer thickness of 10 μm is obtained with balls having a diameter of 300 μm and a layer thickness of 20 μm is obtained with balls having a diameter of 3 mm.

EP1577401 describes vibrating rods which produce impacts on a material. The maximum impact speeds are equal to 3.6 m/s.

In a first embodiment, JP2003201549 teaches how to project a stream of particles onto a metal part along a normal incidence. In another embodiment, the document teaches how to generate a vibrating motion in order to produce projections, which involves relatively small projection speeds.

According to one embodiment, the invention provides a process for the surface 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 between 10° and 45°, until a surface layer of nanostructures, having in particular an average thickness of greater than 50 μm is obtained,

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 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 50 μm, the boundary of the surface layer of nanostructures being determined to be a region of 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. In other cases this threshold may be defined as a function of other parameters, especially the 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 mm and less than 1.4 mm.

According to one embodiment, the incidences of the particles are distributed substantially continuously in the cone or the conical film.

In one embodiment, the cone or the conical film has an outer half apex angle of between 10° and 30°.

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.

According to one embodiment, the inclination of the surface of the part with respect 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 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 50 μm, the boundary of the surface layer of nanostructures being determined to be a region of 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, the surface layer of nanostructures has an average thickness of greater than 100 μm.

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 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 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 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.

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 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 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:

FIG. 1 is a schematic representation of a process for nanostructuring a metal surface.

FIG. 2 is a schematic perspective view of a peening machine suitable for implementing the processes according to the embodiments of the invention.

FIG. 3 is a schematic representation of a particle jet produced by the machine from FIG. 2.

FIG. 4 is a diagram of the operation of the machine from FIG. 2.

FIG. 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.

FIG. 6 is a graph representing the change in the thickness of a nanostructured surface layer as a function of the degree of coverage for the peening conditions from FIG. 5.

FIG. 7 is a graph representing the change in the treatment time as a function of the degree of coverage for several shot sizes.

FIG. 8 is a graph 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 a peening condition.

FIG. 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.

FIG. 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 FIG. 2, for several peening conditions.

FIG. 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 surface.

FIG. 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.

FIGS. 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.

FIG. 15 is a graph 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 another peening condition.

FIG. 16 is a schematic cross-sectional representation of a part treated by a peening process representing the region of influence of an impact.

FIGS. 17 to 20 are optical micrographs of nanostructured surface layers.

FIG. 21 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 with another hardness measurement method.

FIG. 22 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.

FIG. 23 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.

Described below are embodiments of peening processes that make it possible to obtain a nanostructured surface layer on a metal part. Unless otherwise indicated, the experimental results presented below are obtained with flat metal samples.

With reference to FIG. 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 μm. In the right-hand view, after treatment, the grain size of the material at a surface layer 3 is reduced to a few tens of nm, for example around 20 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 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 fatigue resistance are critical properties.

With reference to FIG. 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 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 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 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 central projection axis of the nozzle 11. Positioned on the disk 13 is an inclinable 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 part between the clamps 15 and may be loosened in order to withdraw the part after treatment.

With reference to FIG. 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 β. The angle β may be measured, for example, as the ratio between 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 FIG. 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 a with respect to the local normal direction 26. The surface portion located around an edge of the jet receives the particles at an angle of incidence (α−β) 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 (α+β) with respect to the local normal direction 28.

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 α and broader towards the periphery of the jet, where it includes all the angles between (α−β) and (α+β). If β≧α, the conical film 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 β is equal to around 8° and the distance L to around 300 mm. Of course, it is not excluded for a small portion of the 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 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 of the shots according to the SAE J444 standard

Type of
Nominal

shot
diameter (D)
Distribution (fraction of the particles of larger size than)

S550
1.40
0

>85
>96

S330
0.85

0
<5

>85
>96

S280
0.71

0
<5

>85
>96

S170
0.425

0
<10

>85
>97

S070
0.18

0
<10

>80
>90

mm
2.00
1.70
1.40
1.18
1.00
0.85
0.71
0.60
0.50
.425
0.35
0.30
0.18
.125

Test 1

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 α=15°. Recorded in this table are the type of shot used in the test, the projection speed V, the degree of coverage R, the thickness of the nanostructured layer z_nobtained, 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.

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 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.

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 observations of the thickness of the visually amorphous layer corresponding to the 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 peak of hardness originating from the hardening by the effect of the nanometre-sized grains.

The method used for producing the hardness profile consists in making an indentation line with a step of 50 μm starting from the outermost surface with a micro Vickers hardness tester having a pyramidal tip with a load of 100 g (HV 0.1) 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 μm to 500 μm. 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 z_nmay be explained more precisely with the aid of FIG. 5. FIG. 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 appears, which corresponds to the nanostructured layer 3 and a second region 35 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 FIG. 5 where the thicknesses z_nobtained by 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 FIG. 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 50 μm, 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 μm.

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 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.

FIG. 6 represents the change in the thickness z_nobserved visually as a function of the peening treatment time, measured by the degree of coverage R, by the four types of shot. Curve 36 corresponds to type S170 shot. Curve 37 corresponds to type S280 shot. Curve 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 thickness no longer changes significantly beyond the R=3000% within the context of test 1.

FIG. 6 demonstrates that all the shots from test 1 make it possible to obtain a thickness z_nthat exceeds 100 μm, or even 140 μm. This figure also demonstrates two advantages of the type S280 and S330 shots (curves 37 and 38). On the one hand, the nanostructured layer 3 appears significantly at a lower degree of coverage R, around 300%, than with the larger particles (S550) or smaller particles (S170). On the other hand, the thickness z_nreaches its peak at a higher level than that obtained with larger particles (S550) or smaller particles (S170).

Not obtaining a maximum thickness z_nwith the largest particles (S550, curve 39) 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 involves a less even spatial distribution of the impacts.

This competition is illustrated schematically in FIG. 16, 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 z—that is relatively uniform over the entire treated surface, impacts that are relatively spaced apart give rise to edge regions where the material is deformed over a relatively small thickness z₀and central zones where the material is deformed over a relatively large thickness z₁. The thickness z_nthat can be observed lies between z₀and z₁.

Another property on which the size of the particles has an observable effect is the uniformity of the thickness z_nalong the treated surface. This property may be characterized by the standard deviation C of the thickness z_n. Table 3 recounts the values measured in the samples from test 1, micrographs of which are reproduced in FIGS. 17 to 19. For the chosen degree of coverage, it appears that the largest type S550 shot provides a mean thickness z_ncomparable to the thickness obtained with the type S330, but a doubling of the standard deviation C. FIGS. 17 to 19 also make it possible to observe nanocrystallization lobes.

TABLE 3

Standard deviation of the nanostructured thickness in test 1

Nano

Vickers
Nano
thickness

Degree of
hardness at
thickness
standard

Type
coverage R
the surface
z_n
deviation custom-character

FIG.
of shot
(%)
(HV)
(μm)
(μm)

17
S170
1000
263
72.05
11.1

18
S280
1000
290
119.7
12.5

19
S330
1000
290
159.76
19.6

20
S550
1000
292
175.5
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. FIG. 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 μm, 107 s are needed with type S550 versus 30 s with type S330 and 75 s with type S280. It is therefore seen 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.

FIG. 8 demonstrates the relationship between the nanostructured thickness z_nand the hardening observed at the surface of the treated sample. Curve 42 represents the thickness z_n(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 region 45 starting from the initial hardness 44 without however forming nanometre-size grains, and an effect of the nanostructuring of the material which causes a second increase in the hardness in a region 46.

Test 2

In order to evaluate the optional effect of clamping 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. FIG. 9 shows the change in the thickness z_nas a function of the 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.

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 α between 0° and 45° and the rotation of the support device 12. The nanostructured thicknesses obtained in this test 3 are recorded in Table 4.

TABLE 4

nanostructured thickness in test 3 for R = 3000%

Rotation
α (°)
0
15
30
45

Without
S170
0
97
92
72
z_n(μm)

With

0
130
102
105

Without
S280
131
156
134
153

With

133
189
171
160

Without
S330
111
168
134
144

With

116
236
183
125

It is observed for each type of shot that the thickness z_nvaries with the angle α in order to reach a peak very clearly at around α=15°, It is also observed that the rotation of the support does not produce any significant effect for α=0° but substantially increases the thickness z_nwhen 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 or a conical film which has an outer half apex angle of between around 10° and 45° within the context of this test.

FIG. 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 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 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 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

A comparative test was carried out with samples of 304L stainless steel and a 32CDV13 structural steel under conditions similar to test 1 with type S280 shot. FIG. 11 illustrates the results of these tests in terms of hardness profile for R=3000% in a representation analogous to FIG. 5. Curve 53 corresponds to the 304L stainless steel. Curve 54 corresponds to the 32CDV13 structural steel. The hardness profiles of these materials correspond to the trends observed in test 1. Regions 34 and 35 of FIG. 11 have the same meaning as in FIG. 5. Curve 31 from test 1 (E24 steel) is plotted by way of comparison. The thicknesses z_nobserved are visually 143 μm for E24, 176 μm for 32CDV13 structural steel and 155 μm for 304L stainless steel.

Once again, the validity of the empirical 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 μM. The choice of reference is explained 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-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 μm. 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, starting from a thickness of greater than 350 μm, the sample of 304L steel has its original hardness of the austenite and for a thickness of less than 300 μm 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 strain-induced martensite, which is here around 300 μm.

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 ferritic structure and its high SFE energy (around 200 mJ/m²). Curve 55 from FIG. 12 represents the thickness 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 the nanostructured layer at R=750%) than pure iron (appearance of the nanostructured layer at R=1000%) and has a thicker nanostructured layer (z_n=130 μm versus z_n=100 μm) at saturation. Test 5 shows that the process makes it possible to obtain nanostructured layers thicker than 100 μm for most of the materials that can be envisaged.

Test 6

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.

FIG. 13 illustrates the results obtained with a projection speed V=60 m/s in a representation similar to FIG. 8. Curve 60 represents the Vickers hardness at the surface and curve 61 the thickness z_nobserved visually. It is observed that the thickness z_nsaturates at a level close to 60 μm 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.

FIG. 14 illustrates the results obtained with a projection speed V=92 m/s in a representation similar to FIG. 8. Curve 62 represents the Vickers hardness at the surface and curve 63 the thickness Z_n. It is observed that the thickness saturates at a level close to 80 to 90 μm from R=3000% onwards.

These results should be compared with those presented in FIG. 4 of the International ISIJ 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 comparison of degrees of coverage requires a calibration due to different definitions 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.

Test 7

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.

FIG. 15 illustrates the results obtained with a projection speed V=57 m/s in a representation similar to FIG. 8. Curve 64 represents the Vickers hardness at the surface and curve 65 the thickness z_n. The numbers 44, 45 and 46 have the same meaning as in FIG. 8. It is observed that the thickness z_nsaturates at a level close to 100 μm.

Test 8

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 3 recounts the results of the second series of tests carried out 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 μm starting from 20 μm from the outermost surface to a depth of 100 μm. The indentation line is then continued with a step of 50 μm to a depth of 300 μm. The indentation line is made with a micro Vickers hardness tester having a pyramidal tip with a load of 25 g (HV 0.025) which possesses a lens. It is a Buehler Micromet 5104 microhardness tester comprising a motorized table having a step of 1 μm and Buehler Omnimet Mhtsa control and measurement software. The hardness profile is thus obtained from a depth of 20 μm to 300 μm. The values communicated are an average of three 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 A1 microscope, a Qimaging Micropublisher 5.0 RTV camera, a Zeiss EC EPIPLAN X10/0.2HD lens and Axiovision 4.8 software.

The first three columns of Table 3 correspond to the first three columns of Table 1 The fourth column mentions the thickness of the nanostructured layer, denoted by z_nhwith reference to the hardness. Indeed, in test 8, the thickness of the nanostructured layer z_nhwas obtained by a method solely based on the hardness 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 z_nhtherefore corresponds to the 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 depth of 20 μm. 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 few hundreds of nanometres. Furthermore, the hardness is measured with a larger load in test 1 than in test 1 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 3 mentions the uncertainty margin of the thickness measurement z_nhresulting 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 32CDV13 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 greater load is used for harder materials. Thus, a load of 50 g (HV 0.050) is used for the 32CDV13 steel and for the 304L steel.

FIG. 21 represents the hardness profiles obtained by the method explained above for the samples corresponding to the samples from test 1 with R=3000%. Curve 70 corresponds to type S170 shot. Curve 72 corresponds to type S330 shot. Curve 73 corresponds to type S550 shot. On all the curves 70, 71 and 73, 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 FIG. 21, the hardness value 74 measured in the deep layer and the maximum hardness value 75 measured on the surface layer of the sample associated with curve 70 are respectively equal to 142 and 300 Vickers. The corresponding threshold 71 has a value of 221 Vickers, which corresponds to the median value between the hardness value 74 measured in the deep layer of the sample and the maximum hardness value 75 measured on the surface layer of the sample.

This threshold makes it possible to determine a thickness z_nhof the nanostructured layer having a value approximately equal to 81.5 μm for the test corresponding to the S170 shot.

An uncertainty range of the thickness z_nhof the nanostructured layer is therefore determined from the hardness threshold and from the uncertainty range of the hardness. By way of example, for the threshold 71 of 221 Vickers presented previously, the boundary values of the thickness of the nanostructured layer are plotted for hardness values 85 and 86 respectively of 231 Vickers and 211 Vickers. Thus, the thickness of the nanostructured layer lies within a range of around 69 to 92 μm. The uncertainty ranges of the thickness of the nanostructured layer are presented in Table 3. 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 hardness agrees satisfactorily with the visual determination method: FIG. 22 schematically represents the regions observed on the optical micrographs of the sample corresponding to curve 70 from FIG. 21 (S170 peening at R=3000%). The hardness profile 70 as a function of the depth z from the surface of the sample is plotted on the schematic representation of these regions.

Observed in FIG. 22 is a nanostructured surface layer 77 corresponding to a region in which the material is substantially amorphous and homogeneous. Layer 77 corresponds to the darker zone observed in FIGS. 17 to 19. Layer 77 extends from the surface 76 of the part to a second layer 78, This second layer 78 corresponds to the region in which grain boundaries are observed and in which the size of the grains delimited by the grain boundaries increases with the depth. On the optical micrographs, layer 78 corresponds to the region which extends from a sudden change in contrast starting from layer 77. This second layer 78 corresponds to the strain-hardening region of the material. A third layer 79 comprises a region where the size of the grains remains constant. The hardness threshold 71 agrees substantially with the boundary 84 observed visually between the nanostructured surface layer 77 and the layer 78.

The difference between the thickness values z_nobserved visually listed in Table 2 and the thickness values z_nhlisted in Table 3 originate essentially from the relatively high uncertainty margin of the measurements mentioned in Table 2 typically of the order of ±30 μm. In reality, the visual observations listed in Table 2 encompass a portion of the transition layer 78, 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 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 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 FIG. 23, curve 80 represents the thickness of the nanostructured layer as a function of the coverage and curve 81 represents the surface hardness of the sample as a function of the coverage for the S170 peening test. A minimum detectable thickness 82 of the nanostructured layer appears for a coverage of 150%. However, the surface hardness 83 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 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 3, the hardness thresholds were determined with this other method for coverage values of less than 750%. In Table 3, the values determined with this other method have an asterisk.

These results should be compared with those presented in FIGS. 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 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 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 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 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 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 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 FIG. 2. However, it is also possible to conceive a peening machine with several projection nozzles. These projection nozzles may especially be arranged so 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 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.

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 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 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 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.

In the claims, any reference sign between parentheses should not be interpreted as a limitation of the claim.

TABLE 2

test 1, E24 steel, Rockwell hardness of the shots = 48HRC

Hardness
Hardness

Type
V
R
z_n
of treated
of untreated
Hardness

of shot
(m/s)
(%)
(μm)
face (HV)
face (HV)
gain (%)

S170
57
100
0
198
108
83%

150
0
211
114
85%

200
0
200
113
77%

300
0
212
111
91%

500
0
241
112
115%

750
69
256
108
137%

1000
72
263
111
137%

1500
91
274
116
136%

3000
129
308
113
173%

6000
138
309
113
173%

10000
140
302
116
160%

S280
52
100
0
215
130
65%

150
0
224
132
70%

200
0
224
138
62%

300
67
247
139
78%

500
91
262
137
91%

750
101
278
138
101%

1000
120
290
113
157%

1500
134
295
116
154%

3000
143
298
114
161%

6000
178
301
113
166%

10000
172
315
114
176%

S330
60
100
0
213
114
87%

150
0
233
116
101%

200
0
234
110
113%

300
111
264
111
138%

500
112
253
108
134%

750
142
282
114
147%

1000
160
290
114
154%

1500
175
298
112
166%

3000
192
310
123
152%

6000
193
300
131
129%

10000
186
304
142
114%

S550
49
100
0
206
129
60%

150
0
216
144
50%

200
0
223
131
70%

300
0
227
135
68%

500
0
243
145
68%

750
104
278
148
88%

1000
176
292
147
99%

1500
168
279
153
82%

3000
164
292
159
84%

6000
175
295
157
88%

10000
173
308
167
84%

TABLE 3

Test 8, samples corresponding to test 1, E24

steel, Rockwell hardness of the shots = 48HRC

Hardness
Hardness

z_nh
z_nh

Type
V
R
z_n
of treated
of untreated
Hardness
min.
max.

of shot
(m/s)
(%)
(μm)
face (HV)
face (HV)
gain (%)
(μm)
(μm)

S170
57
100
0.00
225
142
58%

150
0.00
226
133
70%

200
27.46*
234
140
67%

300
30.28*
252
143
76%

500
46.47*
276
132
109%

750
50.00*
281
135
108%

1000
54.22
288
140
106%
49.29
62.67

1500
59.15
290
140
107%
57.74
74.64

3000
81.69
292
131
123%
69.01
92.25

6000
94.36
323
135
139%
90.84
96.47

10000
87.32
327
127
157%
73.23
95.77

S330
60
100
0.00
240
136
76%

150
35.21*
244
136
79%

200
34.50*
253
139
82%

300
39.43*
260
135
93%

500
67.60*
267
129
107%

750
69.71
284
128
122%
61.26
90.14

1000
76.05
297
129
130%
69.01
96.47

1500
111.26
299
126
137%
102.11
121.83

3000
111.97
309
128
141%
97.88
123.23

6000
123.94
310
157
97%
109.50
139.43

10000
97.14
310
126
146%
90.00
113.57

S550
49
100
0.00
222
135
64%

150
0.00
225
139
62%

200
0.00
227
141
61%

300
29.57*
240
144
67%

500
44.36*
248
128
94%

750
57.74*
261
141
85%

1000
98.59
271
134
102%
76.76
161.97

1500
108.45
289
148
95%
81.69
133.09

3000
97.18
295
132
123%
83.80
146.47

6000
115.00
309
142
118%
85.71
140.00

10000
119.28
325
144
126%
98.57
150.00

SURFACE TREATMENT OF A METAL PART BY OBLIQUE SHOT PEENING

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