Mechanical descaling by shot blasting

Hot-rolled products —wire rod, bars, rebar— develop a layer of iron oxides on their surface known as mill scale. This layer is brittle and fragile, with mechanical characteristics different from the base steel, which is ductile and deformable. If the mill scale is not removed before drawing, it damages the drawing dies, deteriorates the surface quality of the finished wire and compromises the adhesion of subsequent coatings. Descaling is therefore a mandatory step prior to wire drawing.

Shot blasting is an impact surface treatment technique: it consists of bombarding the steel surface with thousands of steel shot particles launched at high speed, between 65 and 110 meters per second. The kinetic energy released at each impact breaks the mill scale and detaches it from the base steel. Unlike methods that rely on bending the material, shot blasting attacks the entire surface uniformly without deforming or hardening the wire, achieving a degree of cleanliness that bending methods do not reach.

CYM Materiales manufactures two shot blasting lines for descaling according to the processing format. The COIL line processes complete wire coils in batch: the coil is mounted on a rotating mandrel and the turbines project the shot over its entire surface, processing coils of up to 3,000 kg in cycles of 7 to 20 minutes depending on the type of steel and wire diameter. The BAR line processes wires, bars and billets in continuous pass, at speeds of up to 250 m/min, a single strand or several simultaneous strands, and can be integrated directly into the drawing line. The batch format supplies several drawing machines from a central point; the continuous format integrates in line with a single drawing machine.

Shot blasting completely eliminates the problems of acid descaling. It requires no effluent treatment plants or chemical waste management. It generates no unwanted chemical reactions on the surface, avoiding the risk of hydrogen embrittlement —a phenomenon in which the hydrogen released by the acid diffuses into the steel and makes it brittle—. It produces no acid vapors harmful to personnel health or corrosive to facilities. And unlike acid, it generates a controlled surface roughness that improves the retention of the drawing lubricant and grip in the drawing benches, with a brighter final finish.

Reverse bending mechanical descaling —which breaks the mill scale by bending the wire rod over a system of pulleys— is an effective and low-cost method, but its degree of cleanliness does not exceed commercial grade (SA2). Shot blasting reaches higher degrees of cleanliness and treats the entire surface homogeneously, including geometries that bending does not fully cover. Both methods are complementary: in many installations, reverse bending performs the initial roughing and shot blasting completes the cleaning to the degree required by the final product. The choice depends on the degree of cleanliness each application demands.

The decision between batch (COIL) and continuous (BAR) shot blasting depends on the production logic: if descaling supplies several drawing machines from a central point, the coil batch format is more flexible; if it integrates in line with a single drawing machine, the continuous format eliminates intermediate handling. The required degree of cleanliness, the type of steel, the wire diameter and the production volume define the turbine configuration and process speed. The CYM Materiales engineering team sizes the line according to each customer's process, with the possibility of prior tests at the plant.