Surface shot peening can reduce stress corrosion in fasteners

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Surface shot peening can reduce stress corrosion in fasteners. Among the failure modes of fasteners, fatigue failure accounts for approximately 60% to 90% of all cases; therefore, over the years various methods such as quenching and tempering, carburizing, and surface treatment have been widely used to improve fatigue properties – including stress corrosion resistance – by altering the material’s microstructure. Today, the surface shot peening treatment is already used on the shanks of bolts and screws; it has a wide range of applications, strong adaptability, and low costs. Shot blasting is a process in which a stream of pellets continuously impacts the surface material of fasteners, thereby causing cyclic plastic deformation in the surface layer (with a depth of about 0.05–0.20 mm). The cyclic plastic deformation induced by shot peening creates a residual compressive stress field in the surface layer of the material. When this compressive stress combines with the tensile stress from the applied alternating stress in the same cross-section, the maximum tensile stress experienced by the material is shifted from the surface to a sub-surface region. In the specimens without surface strengthening by shot peening, fatigue cracks initiate on the outer surface, whereas in those with surface deformation strengthening via shot peening, fatigue cracks initiate in the subsurface layer. Theoretical analysis confirms that deformation residual stress causes fatigue cracks to initiate beneath the surface of the material, thereby yielding an internal fatigue limit that is 1.05~1.35 times higher than the surface fatigue limit. Surface shot peening is an effective surface strengthening process for improving the resistance of fasteners to stress corrosion and hydrogen embrittlement fractures caused by fatigue. Shot materials include cast steel shot, glass shot, ceramic shot, etc. They can increase the surface roughness of reinforced fasteners from 0.65–2 μm to 0.63–2.5 pm, thereby significantly improving the reliability and durability of the workpieces.