Machining, fabrication, welding, casting and additive manufacturing all create within the material an imbalance of stresses. These are known as anisotropic stresses in that the magnitude of these stresses are directional and can introduce distortion into the part and also cause premature failure. Wherever possible it is good practice to heat treat the part post manufacture. As this is not always possible shot peening can be beneficial by introducing surface and sub surface compressive stresses. The majority of fatigue failures are by surface and near surface cracking, the introduction of compressive stresses by shot peening significantly reduces this risk and acts as a component life enhancing process.
As can be seen from the graph, welding can produce both positive and negative stress, with significant tensile stress in the heat affected zone. Shot peening can introduce compressive stress into the weld area to the extent that all of the fabrication has negative stress within this area.
lida, J. Ito, Meiji University Department of Mechanical Engineering, Higashi-mita, Tama-ku, Kawasaki, 214-Japan on the ‘Peening Effect on Machining of Steel’
As can be seen, the work done by K. lida, J. Ito, Meiji University Department of Mechanical Engineering, Higashi-mita, Tama-ku, Kawasaki, 214-Japan on the ‘Peening Effect on Machining of Steel’ shows the anisotropic stresses induced in this instance by turning of a 0.45%C Steel with different nose radius tips, and the subsequent balancing of stresses achieved using a very modest shot peening technique.