Wire Drawing Defects and Residual Stresses

Typical defects in a drawn rod or wire are similar to those observed in extrusion especially center cracking another major type of defect in wire drawing is seams, which are longitudinal scratches or folds in the material.

Seams may open up during subsequent forming operations (such as upsetting, heading, thread rolling, or bending of the rod or wire), and they can cause serious quality-control problems.

Various other surface defects (such as scratches and die marks) also can result from improper selection of the process parameters, poor lubrication, or poor die condition.

Because they undergo nonuniform deformation during drawing, cold-drawn products usually have residual stresses. For light reductions, such as only a few percent, the longitudinal-surface residual stresses are compressive (while the bulk is in tension) and fatigue life is thus improved.

Conversely, heavier reductions induce tensile surface stresses (while the bulk is in compression). Residual stresses can be significant in causing stress-corrosion cracking of the part over time.

Moreover, they cause the component to warp if a layer of material subsequently is removed such as by slitting, machining, or grinding.

Rods and tubes that are not sufficiently straight (or are supplied as the coil) can be straightened by passing them through an arrangement of rolls placed at the different axe.

Article Source: Wire Drawing Defects and Residual Stresses

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