The Shot-Peening

Introduction
The shot peening is a modern technique of metal parts surface work hardening, widely spread in the aircraft and the car industries. This surfaces mechanical treatment should not be mixed up with the shot-blasting other usual applications which are all directed towards the surfaces cleaning, preparation or finishing. The shot peening must be considered as a real machining operation which will improve the parts life.

Object of the operation
The shot peening is a hammering technique with a spherical shot which aim is to create a residual stresses on the surface of metal or hammer-hardenable parts. It allows thus to increase their mechanical qualities..

Principle
The technique consists in shooting at high speed small steel, glass or ceramic beads on the surface of the parts to be treated. Under the action of the beads hammering, the surface which exceeded the tensile yield strength is bent out of shape and encloses a subjacent coat of compressive metal that has not reached its tensile yield strength. The contained force which requires to be released is opposed to the permanent strain of the surface coat.

Result
The balance between these two forces generates surface tensions which improve thus:
- the resistance to fatigue,
- the resistance to corrosion,
- the friction coefficient.

Applications
There are numerous applications in the industry. The shot-peening applies to all the parts making subjected to repeated dynamic promptings such as:
- leaf springs or helical springs,
- pinions and propeller shafts,
- tappets rocker arms,
- torsion bars,
- aircraft elements, etc.
From the moment optimal mechanical characteristics will be required to a part, it will be judicious to choose shot peening.

Implementation
The result depends essentially on the shot-blasting intensity, the following factors must be controlled: (see heading parameters)
- the projectile efficiency, (granulometry, shape, hardness and cleanness of the beads),
- the projection velocity, (choice of the process),
- the angle of attack, (between 85° and 45°)
- the shot flow,
- the coverage degree,
- the exposure time.
It is absolutely essential to equip the installations with the equipments necessary to sort and eliminate the worn and broken grains as well as with a magnetic separator according to the kind of product used (glass or ceramic beads).

Projection
The wheel machine is used generally for the mass parts treatment. The compressed air machine is more reserved for the localized treatments. Whatever the process is, (see Stressonic process) the shot velocity must be sufficient to strain plastically the treated material.

The Peen-Forming

Principle
It is the same as shot-peening but it applies to small thickness metal parts. The bombardment of only one side of a panel or a thin steel sheet generates the elongation of the surface fibers and the deformation of the treated part.

Result
The result is a bending of the side concerned which is used for straightening out or forming operations. This is the peen-forming. This process is widely used in the aircraft industry for the planes wings forming. The machines and the projectiles used as well as the means of control are mentioned in the AFNOR NFL 06 831 (02-1990) standard amended by the directive BNAE L 06 832. This document was published for the "Aeronautical and Space industries".

Nota bene:
The shot-peening and peen-forming techniques are delicate operations to implement, that is why it is advisable to appeal to an experienced specialist. This will avoid the risks of damaging the treated surface (micro fissures) which is subjected to a too great fatigue.

Control

Parameters
As mentioned below, all the parameters are difficult to control. Therefore, we will rather take an interest in their result which is the shot-blasting intensity.

Means
The system now used for a long time and which was the subject of a standard is the "Almen" method whose name comes from its inventor J.O. ALMEN (U.S.A.).

Principle
A standardized plane test tube fixed on a standardized support is subjected to the same shot-blasting conditions as the part to be treated. After being unfixed of its support, the test tube presents a deformation in the form of arrow, which results from the introduced surface stresses.

Measurements
By means of a specific comparator called "Almen gauge", one measures the surface deflection. This deflection also depends on the covering rate which is the second parameter to be taken into account. It must always be equal or higher than 100% so that the whole surface be concerned with the treatment. Its measurement is done by optical examination, directly on the treated part.