Shot peening is a cold working process in which the surface of a part is bombarded with small spherical media called shot. Each piece of shot striking the metal acts as a tiny peening hammer imparting a small indentation or dimple on the surface. In order for the dimple to be created, the surface layer of the metal must yield in tension . Below the surface, the compressed grains try to restore the surface to its original shape producing a hemisphere of cold-worked metal highly stressed in compression. Overlapping dimples develop a uniform layer of residual compressive stress.
It is well known that cracks will not initiate nor propagate in a compressively stressed zone. Since nearly all fatigue and stress corrosion failures originate at or near the surface of a part, compressive stresses induced by shot peening provide significant increases in part life. The magnitude of residual compressive stress produced by shot peening is at least as great as half the tensile strength of the material being peened.
In most modes of long term failure the common denominator is tensile stress. These stresses can result from externally applied loads or be residual stresses from manufacturing processes such as welding, grinding or machining. Tensile stresses attempt to stretch or pull the surface apart and may eventually lead to crack initiation . Compressive stress squeezes the surface grain boundaries together and will significantly delay the initiation of fatigue cracking. Since crack growth is slowed significantly in a compressive layer, increasing the depth of this layer increases crack resistance. Shot peening is the most economical and practical method of ensuring surface residual compressive stresses.
Glass beads are used as iron contamination (steel shots) is of concern. They are generally smaller and lighter than other media and can be used to peen into sharp radii of threads and delicate parts where very low intensities are required.
Peening media must be of uniform diameter. The impact energy imparted by the media is a function of its mass and velocity. Thus, heavier media such as steel shots could effect the metal parts producing dents.
Hence, Glass beads are the right choice.