Shot peening process is one of the severe plastic deformation methods which is well-recognized for the surface layer integrity of the components. This process, according to its energy level, can be categorized into three different cases of conventional, severe, and over shot peening. The aforementioned types of shot peening have positive and/or negative effects on their related performing conditions. In this study, the boundaries between these processes were determined for the first time by presenting novel approaches. The foundations of the presented approaches are microstructural investigations. The microstructures were characterized using optical microscopy, scanning electron microscopy, field emission scanning electron microscopy, and high resolution transmission electron microscopy observations and X- ray diffraction analysis. In addition, micro-hardness, residual stress measurements, and fatigue tests were carried out to study the effects of those three different shot peening processes.