What's the differece between blasting and peening?

In blasting,the impacting medium cuts the steel surface, removing small amounts of metal from the surface. The resulting surface finish and hence surface corrosion resistance of the treated stainless steel is partly dependant on the blasting medium. Very hard media, such as aluminium oxide can leave a jagged or torn faces to metal peaks on the surface, whereas the softer media, such as silicon carbide give a smoother finish. The aluminium oxide finish can result in reduced corrosion resistance and cleansibility compared to silicon carbide of a similar grit size.

In contrast, peening a surface does not remove metal. The impact of the rounded bead blasting medium results in small craters in the surface, giving a dimpled appearance, at a suitable magnification, as metal is pushed aside. The edges of these small craters are not however sharp and so the corrosion resistance and cleansibility of peened surfaces compares well with blasted surfaces, for a given steel type and 'roughness' of the surface.

It is important that only clean round beads are used in the peening process. such as stainless steel shot and cut wire shot.

A build up of broken beads in the peening medium can result in an undesirable uneven surface finish being produced and the excessively rough surfaces produced could be prone to staining or micro-pitting in service. The surface of the steel being treated should be cleaned before and after blasting.