Wrought Products: Rolling, Forging, Extrusion and Drawing

Learning outcomes

The learner should be able to explain hot and cold working, compare major wrought processes, trace fibre and inclusion direction, identify process discontinuities and align NDT coverage with worked geometry.

10.1 Deformation and reduction

Wrought processing reshapes solid material through plastic deformation. Hot working occurs above a temperature where recovery and recrystallisation can offset strain hardening. Large reductions can close voids, break down cast dendritic structure and refine grains. Cold working occurs below that range and increases strength, surface finish and dimensional control while adding work hardening and residual stress.

Reduction ratio helps describe how extensively original cast section has been worked. More reduction can improve structure, but closure of a cavity does not guarantee clean bonding when oxides or inclusions remain on its surfaces.

10.2 Rolling and plate products

Rolling compresses stock between rotating rolls. Flat rolling produces plate, sheet and strip; shaped rolls produce rails, beams and other sections. Rolling elongates grains and inclusions. Central casting porosity or segregation can become planar laminations or bands parallel to the product surface.

Edge cracks, alligatoring, laps, seams, scale pits, rolled-in material, thickness variation and residual stress can arise from temperature, reduction, roll condition or starting-stock problems. Flame cutting, levelling and forming add further local history.

Straight-beam UT is commonly used for lamination detection because the likely planes are parallel to plate surfaces. Angle-beam coverage may be needed for differently oriented cracking or weld regions.

10.3 Forging

Open-die forging uses simple tools to reduce and shape large stock. Closed-die forging fills a shaped cavity and can produce near-net components. Proper working develops grain flow around the component contour and can improve fatigue performance. Inadequate stock control can create laps, folds, seams, bursts, flakes or unworked central zones.

Hydrogen flakes are internal crack-like conditions associated with hydrogen, stress and susceptible heavy sections. Forging bursts can be internal or surface connected. Die mismatch and underfill are dimensional. Grain flow near complex geometry can complicate UT and interpretation.

10.4 Extrusion, drawing and tube manufacture

Extrusion forces material through a die and can produce long constant sections. Drawing pulls wire, bar or tube through a die, reducing section and improving finish. Discontinuities include central burst, surface cracking, seams, die lines and residual stress.

Seamless tube can be pierced and rolled over a mandrel. Welded tube begins as strip and contains a longitudinal or helical seam. Tube history determines likely orientation: longitudinal seams and cracks, transverse fatigue, laminations in strip, incomplete seam fusion and dimensional eccentricity require different techniques.

10.5 Practical checkpoint

For plate, a forged shaft and a seamless tube, draw the original cast axis and final material-flow direction. Mark likely discontinuity planes. Select probe direction or magnetisation direction to cross each plane as close to normal as practical. Document inaccessible or poorly oriented regions.

Common errors and misconceptions

Several common errors recur in this topic: assuming a smooth wrought surface means a sound interior; scanning only along the working direction; calling every plate indication a lamination without mapping extent; treating seamless tube as free of seams or internal laps; and ignoring cold-work residual stress before service assessment.

Chapter summary

Working improves cast structure and controls shape, but also stretches discontinuities and creates process-specific laps, bursts and seams. Material flow is a map for NDT orientation. Product form and working direction must be known before coverage can be claimed.