Typically, when parts are blasted following heat treatment, it’s done for the purpose of cleaning the part, in preparation either for painting or coating or to simply to remove heat treatment scale that may have built up on the part during heat treating.
Another type of blasting that’s not done for the purpose of cleaning a part, though, is known as shot peening. Shot peening entails blasting a part with steel shot to produce a compressive residual stress layer on a part, which will allow the material to withstand greater loads before fatigue becomes an issue. Essentially, this procedure entails intentionally inducing stress to a part with the intention of work hardening it and increasing its resistance to application tensile stress.
The two main types of blasting used for cleaning parts are glass bead blasting and steel shot blasting.
Glass bead blasting uses the same type of beads that are added to highway paint to make it reflective, except, when used for blasting, they’re useful because they clean the surface of a material without mechanically removing any of the material itself. What’s left is a flat, matte-like finish. It’s a process that’s often used in the tool and die industry for the removal of heat treatment scale and to clean parts destined for further operations like thread rolling or plating.
Steel shot blasting, as its name suggests, uses steel shot to clean a part. As with glass bead blasting, steel shot blasting does not mechanically remove any part of the original material, but removes heat treating scale or other debris that may be left on the surface of a part. Steel shot blasting is similar to shot peening, but the type of steel shot and control of the media is not as rigid as with shot peening, so it does not induce the same amount of stress.
Shot blasting is typically used in applications where materials aren’t highly machined, and instead remain rough, such as in casting or forging, but still need to be cleaned of oxides, heat treatment scale and other debris.