Abstract
This study investigates the role of shots on shot peening of austenitic steel AISI 304 as a function of shot peening cycles when the conventional steel shots are compared with the ceramic ones. The surface state is analysed in terms of dislocation density, stress state, and the phase transformation of the metastable paramagnetic austenite into the ferromagnetic strain-induced martensite. The sharp transition in magnetic state is monitored via Barkhausen noise emission. It was found that the height of the surface irregularities is unaffected by the number of shot peening cycles and by the different shots. On the other hand, an increasing number of shot peening cycles leads to greater surface damage, as evidenced by the strain-induced martensite fraction and dislocation density. The use of steel shots results in greater microstructural alterations at greater penetration depths due to their higher mass and lower shot velocity. In contrast, ceramic shots produce a higher amplitude of surface stress. Barkhausen noise emission from the steel shots is stronger, which is linked to the higher fraction of strain-induced martensite, especially in the deeper regions. The application of steel shots initiate formation of the ferromagnetic as well as non-ferromagnetic oxides on the surface, in contrast to the non-ferromagnetic oxides only for ceramic shots.