Surface hardening of shot peened H13 steel by enhanced nitrogen diffusion

A study was performed to investigate the surface hardening mechanism of H13 steel after shot peening and ion nitriding. The shot peening process was used as a pre-treatment causing severe plastic deformation to the specimen's surface. The ion nitriding process was then performed to a point where almost no compound layer on the top surface formed. The nitrided specimen after shot peening had a surface hardness of 1250. HV. This is approximately 300. HV higher than that of an ion nitrided specimen. The severe plastic deformation induced by the shot peening process resulted in nano-sized grains and numerous defects on the surface region. Nitrogen concentrations of the shot peened and nitrided specimen were still higher than that of a single nitrided specimen. In addition, chromium dissolved in the matrix regardless of pre-shot-peening treatment enhanced the bulk diffusion of nitrogen in the surface area of both nitrided specimens. This was shown via results from elemental mapping using transmission electron microscopy with electron energy loss spectroscopy (TEM-EELs). It was deduced that refined grains and higher nitrogen concentration acted as main factors for the surface hardening of shot peened and ion-nitrided specimen. Besides, lattice distortion caused by higher nitrogen concentration and abundant defects could be considered as the other factor for the surface hardening.