Metal deforming – By shot-blasting
Reexamination Certificate
2001-11-16
2003-01-14
Jones, David (Department: 3725)
Metal deforming
By shot-blasting
C029S090700, C219S121600
Reexamination Certificate
active
06505489
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method of surface treating and compressively prestressing by peening the wall of at least one oblong cavity formed in a part, said cavity opening to the outside via a lateral mouth and having two ends capable of having frontal openings.
In particular, it is necessary to compressively prestress the wall of an axial recess of a turbomachine rotor capable of containing a blade root, the lines of contact between the axial recess and the blade root of which are highly stressed. What happens in operation is that the turbine or fan blades, immobilized in the axial recesses by various means, are subjected to considerable centrifugal forces leading to significant frictional wear at these lines of contact. This frictional wear reduces the life of the parts in operation and leads to them having to be changed regularly.
2. Summary of Prior Art
In order to improve the fatigue strength of the turbine rotor and to harden the surface of the axial recesses in the region of the lines of contact, it is known practice for the axial recesses to be peened using beads ejected from a compressed air nozzle introduced into each recess. The beads cause permanent compression of the treated surface over a shallow thickness, opposing the onset and propagation of cracks at the surface of the part.
These nozzles are able to project only microbeads of a diameter smaller than 1 mm, and typically of between 0.3 mm and 0.5 mm.
This method is necessarily lengthy because the entire surface area of the cavity is treated only by a succession of local treatments which may, furthermore, introduce unwanted local deformation and cause the incrustation of bead residue. Furthermore, in this method, the distribution of the beads, both in terms of position and in terms of speed, follows a Gaussian law.
In addition, the peening performed is light in order not to generate excessive roughness in the region of the treated zone, and this leads to a limited hardening of the surface. The problem is that since the diameter of the projected beads is small, the more intense the peening, the greater the damage caused to the surface finish.
Finally, the method, which involves numerous parameters, is difficult to control and difficult to reproduce.
SUMMARY OF THE INVENTION
The object of the invention is to propose a method for peening oblong cavities, particularly axial recesses of turbine rotors, which makes it possible to create reproducible prestresses in all the cavities, in a relatively short period of time, while at the same time allowing relatively intense peening, that is to say a greater depth of compression at the surface. This is to be achieved safely, while at the same time limiting the damage caused to, or incrustation of, the treated surface.
According to the invention, there is provided a method of surface treating and compressively prestressing by peening a wall of at least one oblong cavity formed in a part, said cavity opening to the outside via a lateral mouth and having two ends presenting frontal openings, said method comprising the steps of providing a sonotrode arranged within a sleeve, said sonotrode having a vibratory surface and ultrasonic means for vibrating said vibratory surface, and said sleeve having opposed closing-off means capable of closing off the frontal openings of said cavity, placing a plurality of beads of a predetermined diameter on said vibratory surface of said sonotrode with said sonotrode arranged in an intermediate position in which said sleeve surrounds said vibratory surface, placing said lateral mouth of said cavity facing said sonotrode, moving said sonotrode, supporting said plurality of beads, together with said sleeve substantially vertically toward a raised peening position in which said closing-off means close off said frontal openings of said cavity and in which said sonotrode is disposed facing said lateral mouth to delimit, together with said sleeve, said closing-off means and said cavity, a chamber sealed to said beads, and operating said ultrasonic means to vibrate said vibratory surface of said sonotrode whereby said beads are mobilized in said chamber to conduct ultrasonic peening of said wall of said cavity.
The method makes it possible to obtain a uniform distribution of the beads within the chamber. As the beads within the chamber travel in random directions, they strike the walls of the cavities at varying angles, and this improves the surface finish by comparison with beads projected by a nozzle in one predominant direction.
Furthermore, the entirety of the surface is subjected at the same time to the impacts of beads, and this considerably diminishes the risk of deformation of the cavity, particularly of the recess containing the blade root.
To mobilize the plurality of beads, all that is required is for at least one bead to come into contact with the vibratory surface when the sonotrode is excited. Hence the sonotrode may be arranged obliquely with respect to the vertical.
According to a preferred embodiment of the invention, the method further includes the step of moving said sonotrode substantially vertically with respect to said sleeve so that said vibratory surface closes off said mouth after the step of moving said sonotrode and said sleeve together toward said raised peening position.
As the volume in which the plurality of beads is distributed is smaller than the volume obtained when the vibratory surface remains outside the mouth, the method requires a shorter peening treatment time.
In this case, having peened the wall of a cavity, the sonotrode is moved toward its intermediate position in the sleeve, the treated cavity is replaced with another cavity to be treated, the sonotrode is moved toward its peening position and the wall of the other cavity to be treated is ultrasonically peened.
If the vibratory surface is not introduced into the mouth, having peened the wall of a cavity, the treated cavity is replaced with another cavity to be treated, and the wall of the other cavity to be treated is ultrasonically peened.
When the part comprising at least one cavity to be treated is not circular, or alternatively when the method is applied for continuously treating a succession of parts which have just one cavity, the geometry of the part is not always suited to the apparatus and it is necessary to move both the vibratory surface and the closing-off means in order to clear the space available between the steps of respectively bringing in each piece and/or cavity.
When the sleeve has to move back in order to change from one cavity to another because of the geometry of the part, having peened the wall of a cavity, the sonotrode and the sleeve are moved together toward a lowered position, the treated cavity is replaced with another cavity to be treated, the sonotrode and the sleeve are moved together toward the raised peening position, and the wall of the other cavity to be treated is ultrasonically peened.
According to an alternative embodiment, the method further includes the steps of moving said sonotrode toward its intermediate position in said sleeve after peening the wall of said cavity, moving said sonotrode and said sleeve together toward a lowered position, replacing said treated cavity with another cavity to be treated, moving said sonotrode and said sleeve together toward said raised peening position, moving said sonotrode substantialy vertically with respect to said sleeve so that said vibratory surface closes off said mouth, and ultrasonically peening the wall of said other cavity to be treated.
When the method is used to treat a circular part, for example a rotor rim, comprising a number of axial cavities formed in the periphery of said rotor rim and having wall portions which diverge from each mouth, the rotor rim is turned stepwise about its axis of rotation arranged horizontally so as bring each axial cavity in turn to face the sonotrode after a cavity has been treated.
According to a further preferred embodiment of the invention, a deflector is placed in the cavity so
Berthelet Benoît Jean Henri
Chareyre Francis Lucien Guy
Fradin Willy Lionel
Hoffmann Hakim
Kerneis Stéphane Michel
Jones David
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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