The main benefit of the shot peening process is high compressive stress state in the surface or just below the surface. This increases the resistance of the material to fatigue failures, corrosion fatigue, stress corrosion, hydrogen assisted cracking, etc. Other properties, which are changing and which are affecting the performance of the part in the shot peening, are roughness and hardness of the surface. Residual stresses can be divided roughly into macro and micro stresses. When shot peening is examined, attention is typically paid only to macro stresses but micro stresses (hardness) have also important affect on fatigue strength.

X-ray diffraction method is the conventional way to measure residual stresses. Penetration depth of the X-ray is about 5-10 µm. Measuring subsurface stresses with the X-ray diffraction method requires successive removal of material by electropolishing and repeated X-ray measurements. Typically, maximum residual stresses can be found just below the surface at a depth of about 0.1–0.3 mm. To find out the maximum stresses, the depth profile of residual stresses needs to be measured. Another important value resulting from measurements is full-width half-maximum (FWHM, also sometimes referred as HW) of the X-ray diffraction peak. FWHM correlates very well with the hardness so that the harder the material, the wider the peak and the higher the FWHM value.

In the shot peening process, the hardness of the material can increase or decrease depending on the combination of the hardness of the material before shot peening. When the hardness of the material is over about 50 HRc, the material work softens, and if it is under about 50 HRc, the material work hardens. The shot peening process can both soften and harden the steel, which is intermediate hardness range. Figure 1 shows microhardness and FWHM distribution of the shot peened layers. Those graphs show that microhardness measurements of the shot peened surface give higher hardness than what could be expected based on the FWHM values. This can be explained with the high hydrostatic compression state of the shot peened surface, which decreases the indentation size. Therefore, microhardness measurements show higher hardness compared to the surface without high compressive stress. Work softening is mainly related to the rearrangement of dislocations into a lower energy level in the same way as in fatigue softening. Rising surface temperature caused by shot peening affects in the same direction, helping dislocations to move.

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