Controlled shot peening of metal surfaces
Shot peening
Controlled shot peening, or shot peening, is a mechanical surface treatment designed to introduce residual compressive stresses. It is mostly applied in situations in which the component to be treated is subjected to loads that change over time, which in the long run can induce “fatigue” phenomena in the material constituting the component.
By “fatigue” we mean the reduction in the material’s ability to resist load values that in static conditions would not create any problems. Fatigue in metallic materials was studied in the second half of the nineteenth century by Eng. August Wohler, railway technician, who created the “Wohler curves” which allow us to estimate the number of cycles, depending on the load, that lead to the failure of a given material. He found that there is a load below which, regardless of the number of cycles, crack formation is not observed in the material.
This load is called the “fatigue limit”. Subsequent studies demonstrated that the fatigue limit is mainly influenced, in addition naturally by the state in which the material is found, by the presence on the surface of connecting radii due to section variation, by the roughness and by the presence of notches, even of small dimensions.
From here arose the need to use geometric measures to increase the fatigue resistance of the components, such as the widest possible connection radii, very low roughness, even reaching a mirror finish in some highly stressed details, and particular care in avoiding presence of carvings, including metallurgical ones (such as inclusions) on the surface. In other words, the importance of the surface state on the fatigue “life” of mechanical components was highlighted.
As the studies progressed, however, it was clear that with the same geometric and surface conditions, the resistance to fatigue was also influenced by the method used to obtain the geometry (milling, grinding, forging, casting), a phenomenon that only after the use of X-ray diffractometry to detect residual stresses, it was understood. It was understood that, all other conditions being the same, the residual stresses at the surface, and in the vicinity of it, considerably influence the resistance to fatigue. In particular, the presence of residual compressive stresses increases the ability to resist repeated loads over time, vice versa the residual traction on the surface significantly reduces this ability.
Around the 1940s J.O. Almen, an engineer at General Motors, developed the process called “Shot peening” and the method for controlling its repeatability. It essentially consists of bombarding the surface of the object (completely or only in certain areas) with tiny steel spheres (diameter between a few tenths of millimetres and a couple of millimetres, following SEM image)
Shot peening balls seen under SEM (97.5X)"
equipped with speeds of just under one hundred meters per second. The impact of the spheres with the surface of the object causes the formation of localised plastic deformations (next SEM image).
which increase the roughness but which induce residual compressive stresses in the order of approximately one third/half of the yield point of the treated material. Controlled shot peening differs significantly from sand blasting, producing a completely different surface morphology, as can be seen from the two SEM images shown below.
Plastic deformation from sphere impact seen by SEM (1710X)"
Shot peened surface (100X)
Sandblasted surface (100X)
Obviously the parameters used for the shot peening process (shot diameter, intensity and degree of coverage) must be chosen taking into consideration the type of material to be shot peened and the geometric characteristics of the component (for example connection radii and geometric tolerances).
All these parameters influence the trend of the tensions induced by the process, which for case-hardened surfaces (cogwheel tooth side) has the characteristic trend shown in the following graph.
Trend of residual stresses as a function of depth"
Obviously, the optimization of the controlled shot peening parameters requires analysis of the induced and destructive stresses, and analysis of the surface morphology, roughness and SEM investigation, as well as dimensional surveys if required by tight tolerances. The correct balance between the increase in surface roughness and the compressive stress state induced by controlled shot peening can increase the number of fatigue cycles on average by 15-30% up to even 200% in specific cases.
Nowadays, in addition to traditional carbon steel pellets, stainless steel, glass or ceramic pellets are used in order to improve the surface morphology or avoid contamination. 2Effe Engineering has recently developed a type of pellet that can be dissolved in solvent in order to avoid the presence of pellets, or residues, which could compromise the functionality of the component in operation (for example components with blind holes or finished parts of internal combustion engines ).
Controlled shot peening is beneficial for all mechanical components subjected to fatigue, in particular:
– gear wheels (2Effe Engineering processes approximately 1,500,000 gear wheels annually in its factories)
– springs (2Effe Engineering processes more than 3,000,000 springs annually)
– turbine blades (2Effe Engineering processes more than 10,000 turbine blades annually)
– connecting rods (2Effe Engineering processes more than 30,000 connecting rods annually)
– Aerospace components (2Effe Engineering deals with more than 1,000 aerospace components annually)
– Racing components (2Effe Engineering deals with around 500 F1 components annually)
– Gas turbine discs (2Effe Engineering processes around 50 gas turbine discs annually).
Being equipped with transportable shot peening systems, 2Effe Engineering also offers a controlled shot peening service in the field, for example on the following details:
– Pelton/Francis impellers directly in the power plant
– Large gear wheels and pinions (2Effe Engineering can handle masses of up to 20T in its factories)
– Large components of industrial plants
For specific requests on components of high technical/economic value, 2Effe Engineering offers the controlled shot peening service called Flap Peening, which allows obtaining high residual stress values with low roughness values, without the use of shot. This treatment can be carried out using CNC machines, 6-axis robots or if intervention is required directly on large-sized assembled components (for example aircraft) manually.
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