Explaining the results obtained because it causes a “hardening” or eliminates surface tensions should not be considered.

Similarly, the increase in surface hardness it causes (the lower the higher the original hardness) is only applicable when related to frictional resistance, which is undoubtedly an important solution, but does not explain its other consequences.

The most characteristic effects of shot peenig, which result and explain most of its applications are:

1 – Hammering creates a uniform (+) compression stress across the blasted surface. Treating a thin plate on only one side, paradoxically, it deforms, with the convexity to the blasted side. The compression stress created by the impact of the particles, and balanced by the deformation of the blade, results in a movement graph in its cross section. Note that, at the top, the positive voltage is permanent and reaches small depths. The other values ​​are elastic tensions resulting from deformation. Blasting the opposite surface with the same intensity (analogously as the previous one) the two surface tensions oppose each other, the deformation disappears and the moment graph changes.

2- Hammering causes compaction and sealing of the surface microporisity of metals.

3- The roughness caused by the impact of spherical surfaces has interesting characteristics regarding the slip and retention of lubricants.

Peening intensity concept:

The concept of intensity is intuitive. The heavier the hammer, the hammering operation will be more intense. The faster the speed of a particle at impact, the more violent its action will be. Mathematically, the intensity “I” is related to the mass “M” and the velocity of “V”.

I = MV2 / 2

Assessing intensity from the formula is virtually impossible. In general, steel or glass balls are supplied within grit ranges and their diameter may vary by wear. The average speed is not easy to determine either using long air or turbocharged units. On the other hand, specifying a particular intensity to be reproduced indicating the various variables involved such as material (steel or glass), grain size, turbine pressure or rotation, angle of attack, distance, operating time, among others, does not provide accuracy and accuracy. reliability required in shot peening operations.

Almen Method

It is the process internationally adopted to specify the intensity of the blasting. It assumes that equal deformations in standard thin sheets correspond to applications of equal intensities. Almen standardized three types of steel plates equal in dimensions, hardness, flatness, finish etc. varying only the thickness.

It also standardized a platelet support block and a comparator micrometer device for measuring the deformation arrow. The “N” nameplate is used for small intensities such as those obtained, usually with glass spheres. The “C” ones are the most used and the “A” type ones for high intensities, usually accelerated steel grit with pressure nozzles or turbines.

The process is simple. The selected nameplate is attached to the device with micrometer and this zeroed, taking advantage to inspect the flatness. The plate is fixed with the first measurement side down. The support block is positioned in jigs so that the plate attached to it is in a position that coincides with the exact surface of the part to be treated. When the device is blasted, the jet and / or parts are moved, under the same conditions to be reproduced in the future in the real parts, the plate is removed and taken to the micrometer and measures the arrow at the same point, on the non-blasted side.

This reading in inches or millimeters is what is called Almen Number, for example:

0.010A to 0.015A (inches) or 0.25A to 0.38A (mm)

This means that the intensity to be applied to the workpiece must be within the limits that cause deformation of the “A” plate between 0.010 and 0.015 ”(0.25 to 0.38 mm). Sometimes a number is given as 0.010A2, where “2” is nothing more than a specification that the second Almen test should be adopted. As the first one has been discarded many years ago, this reference is now becoming unnecessary and less and less used.