Shot peening is a very efficient treatment in order to increase fatigue resistance and prolong the life of metal components.
The efficacy of the treatment is widely demonstrated by studies carried out by organizations such as NASA, Ford Motor Company, Fiat Research Center and many universities in Italy and around the world.
Shot peening is a refined technological process that has to be carried out under severe control and rigorous conditions. On field theoretical considerations and experiences have shown that many factors influence the result of shot peening. Every case has to be treated and solved specifically.
How does Shot Peening work?
Shot peening, otherwise known as “controlled shot peening” is a cold working mechanical process that consists in “shooting” the surface of a particular metal with a controlled spray of micro-bead shots at very high speed. The impact of the shots plastically deforms the component’s superficial fibers which tend to elongate whereas the inner ones tend to bring them back to the original position. From the balance that we get, we produce a residual stress of compression of the surface layer that increases fatigue resistance of the treated part.
28 April 1988, a miraculous landing, 90 passengers and 4 crew members were safe. The only victim was a flight attendant. The landing of an airplane with such a loss of structural integrity was and will remain an unprecedented event. The official bulletin of United States National Transportation Safety Board reached the conclusion that the accident was caused by mechanical stress due to fatigue due to repeated compressions and decompressions of the cabin. It is calculated that over 90% of mechanical component breakages are due to fatigue and often with catastrophic accidents. Fatigue is a mechanism of accumulation and propagation that damages the mechanical component which undergoes variable strains in time. It is always a very complex phenomenon and may not easily be evaluated whilst being designed.
As general rule we may say that shot peening increases fatigue resistance from a minimum of 10-15% to reach, by means of treatments optimized through experimentation, surprising increases of more than 50% . The residual stress of compression, together with levels of lattice distortion, enable to increase the fatigue resistance of components that undergo uniaxial or multi-axial loads even in cases of strong tension concentration. It can successfully be used also in contact fatigue such as pitting and fretting. The superficial micro-roughness generated by special treatments can also help solve problems connected to noise or insufficient lubrication.
The shot peening plants must be totally automatic and are a fundamental part of the process. Shot peening belongs to the category of “special processes”. Consequently the only way to control the process is to make sure that the operational set up parameters are constantly kept within strict tolerances during all the treatment. Plants with compressed air technology are universally considered to be the best. The more modern and evolved plants that guarantee the best repeatability are those that are completely robotized. The media projection is controlled by computers which elaborate feedback signals of the various sensors in order to guarantee the most accurate shooting.
The Principle and the Effects
When the bead hits the surface of the component a micro-dimple is formed with a little rim around. The dimple and the rim are the result of two events that take place simultaneously. During the shot, the bead plasticizes the superficial layers of the material making them longer and compresses the sub-surface ones according to Hertz’ well known theory. The superimposition of the two effects produces a profile of residual stress with a high compression value and a sub-surface peak that depend on the condition of the treatment. The residual compression stress generated by shot peening are the cause of increased resistance to fatigue of the treated parts.
Shot peening’s technological history is quite recent and still in evolution. The process is described by three fundamental parameters: The Shot, Intensity and Coverage. The shot is the “tool” through which residual stress are introduced. Its shape must rigorously be round. Edgy shots can trigger breakages. Intensity is a parameter that is measured through the “saturation curve” and the corresponding average kinetic energy of the shots. The coverage is expressed as the percentage ratio between the sum of the dimple areas of the shots and the surface area that needs treatment. The three fundamental parameters are both control and design parameters. In order to get the best performance we need to work on two fronts, a true shot peening “planning” and a strict control of the process.
Know-How and Masking
To get the best benefits from the treatment we need to place the utmost attention on all the process phases and it is absolutely necessary to carefully define the areas of the component that need to be treated. Very often the surfaces of mechanical parts have very good surfaces. It is therefore very important to consider the variations of roughness that the treatment can cause; thus carefully evaluate the process parameters and adjust them according to specific needs and finally identify the areas on which the treatment needs to be done and those that instead need to be protected. Notwithstanding the apparent simplicity, the hold tools and the protection of the components are of fundamental importance for the treatment to be successful. Designing the tools is a real “craft in the craft”.
The know-how of the process and the experience of the cases treated are fundamental requisites for planning an excellent shot peening treatment. It is a multi-disciplinary approach that involves metallurgic competences, design competences for the load conditions and obviously specific competences of the process itself. From the dimension, from the geometry, from the material, from the external loads which the component undergoes, by means of careful analysis of the project and technological constraints we can define the main characteristics of an optimized shot peening treatment. Notwithstanding the numerical simulations are always more elaborate, experimental research, residual stress measurement and analytical capacities remain to date the fundamental elements for high-level projects.