Effect of Shot Blasting on Workpiece Surface Roughness

Influencing Factors

Shot Blasting Parameters

1. Shot diameter

   The larger the shot diameter, the greater the impact force exerted on the workpiece surface, generating larger, deeper indentations and higher surface roughness.

   For instance, 1.0 mm shot can achieve a surface roughness of approximately Ra6.3 μm; 0.5 mm shot yields only around Ra3.2 μm.
2. Shot shape

   Spherical shot forms regular indentations upon impact, delivering more consistent surface roughness. Irregular angular shot produces complex scratches and pits, raising surface roughness further.
3. Shot hardness

   Harder shot resists deformation during impact and efficiently removes surface contaminants such as mill scale and rust, yet it also increases roughness.

   As an example, shot with HRC60 hardness will create surface roughness 1–2 μm higher than shot of HRC40 hardness when processing identical workpieces.

Shot Blasting Process Parameters

1. Shot velocity

   Higher shot speed translates to greater kinetic energy and stronger impact force, resulting in elevated surface roughness. Generally, every 10 m/s rise in shot speed increases roughness by 0.5–1 μm.
2. Blasting duration

   Insufficient blasting time fails to fully clear surface contaminants, leading to negligible roughness improvement. Excessively long blasting time causes overly high roughness and even fatigue damage. For carbon steel workpieces, 5–15 minutes of blasting is typically optimal.
3. Blasting angle

   Perpendicular shot impact maximizes contaminant removal yet generates relatively high roughness. Adjusting the blasting angle changes impact direction and force, altering roughness accordingly. The blasting angle is normally set between 45° and 90° for optimal surface treatment results.

Workpiece Material

1. Material hardness

   Softer workpiece materials deform easily under shot impact, leading to a more dramatic rise in roughness. Harder materials resist deformation with minimal roughness variation. For example, aluminum alloy surfaces see far greater roughness increases after blasting compared to cast iron.
2. Original surface condition

   Workpieces covered with mill scale, rust, oil and other contaminants require full removal via blasting, which drastically boosts roughness. Clean raw workpieces experience only minor roughness changes after blasting.

Specific Effects

1. Increased roughness

   Shot blasting raises microscopic surface roughness and its numerical value. A workpiece originally at Ra1.6 μm roughness can reach Ra3.2–6.3 μm post-blasting.
2. Altered surface texture

   Uniformly distributed pits and scratches form across the workpiece surface, creating a more pronounced texture. This texture change modifies visual appearance, yet benefits components requiring high surface friction or superior coating adhesion.
3. Improved roughness uniformity

   Optimized blasting process parameters deliver consistent surface roughness. Inside the blasting chamber, shot impacts the workpiece evenly, erasing localized surface irregularities and achieving homogeneous roughness.

Action Mechanisms

1. Plastic deformation

   High-speed shot impact induces plastic deformation on the workpiece surface, forming pits and protrusions and increasing roughness. The degree of plastic deformation depends on shot kinetic energy and the workpiece’s mechanical properties.
2. Contaminant removal

   Shot blasting strips mill scale, rust, oil and other impurities for a cleaner substrate. Friction and collision between shot and workpiece during cleaning simultaneously abrade the surface and raise roughness.
3. Surface strengthening

   Shot blasting introduces compressive residual stress on the workpiece surface, boosting surface hardness and fatigue resistance. Plastic surface deformation refines grain size to further enhance mechanical performance. This surface strengthening process is inherently accompanied by changes in surface roughness.