Why shot blast steel plate before cutting or welding?

Mill scale is an adhered oxide that interferes with thermal processes. When heated during welding it releases gases that get trapped in the bead and create porosity; its impurities can form non-metallic inclusions, and its higher melting point can prevent proper fusion between the base metal and the filler material. On clean plate, by contrast, the bead comes out sounder and more uniform. It is no minor detail: structural welding codes such as AWS D1.1 require removing mill scale on critical joints. The impact is greater with automatic or robotic welding, calibrated to work on clean steel, where mill scale causes repetitive defects and rework.

Shot blasting as preparation applies to thick hot-rolled plate, where mill scale is thicker and uneven. In plasma, oxy-fuel and especially laser cutting, that irregular scale makes the beam lose focus, peels off during the process and causes unstable cuts or piercing defects. A blasted, even surface gives a more consistent cut. On thin plate, by contrast, the usual preparation is pickling; shot blasting is reserved for thick gauges, where there is no risk of deforming the part.

It has a different electrochemical potential from the base steel, so when it is trapped under paint it causes galvanic corrosion and the coating peels prematurely. It is also brittle and poorly adhered: any paint applied over it comes off together with the scale. That is why blasting before fabrication, while the plate is still flat —the simplest geometry to treat—, leaves the surface uniform and ready.

Most small shops do not apply shop primer over that plate, but the benefit carries through anyway: if a final blasting of the welded structure is needed later —which the small shop usually does by hand, in a blast room— that manual work is much faster, because the surface is already pre-treated and only the welding contaminants have to be removed, not the base oxide of the whole part.

Blasting plate as raw material can be done with different equipment depending on the scale of the shop: from a portable floor blasting machine for low volumes, to dedicated automatic lines for profiles, structures and plate in continuous production. The method changes; the fundamentals are the same.