Mandrel work shop

Mandrel bar regeneration plants are designed to:

• Dimensionally and geometrically restore worn or damaged mandrel bars, guaranteeing tolerances of shape, diameter, eccentricity and straightness.
• Restore surface roughness and finish, which are essential for the internal sliding of the bar and for the quality of the finished tube.

These plants are integrated downstream of a mandrel bar production department, or as a maintenance island at seamless tube manufacturers.

In retained-mandrel rolling technology, the mandrel bar is the central element that determines the internal diameter, the wall thickness and the surface quality of the seamless tube. During the production process, the mandrel bar works under extremely severe conditions:

• High temperatures (over 1,000 °C in the deformation zone)
• High contact pressures
• Continuous friction between the mandrel bar and the hot steel
• Repeated thermal and mechanical shocks

These conditions inevitably lead to progressive wear and damage.

The main causes of mandrel bar degradation are:

• Abrasive wear → reduction in diameter and loss of the nominal geometry.
• Surface scoring and cracking → deterioration of the internal quality of the tube.
• Ovality and eccentricity → non-uniform wall thickness.
• Thermal deformation and residual stresses → dimensional instability over time.
• Micro-fractures and flaking → risk of mandrel bar failure and plant downtime.

Process

A geometric inspection system helps the operator to understand when the mandrel bar needs to be regenerated. Once the need for regeneration emerges, the operator has to analyse its causes and the defects to be corrected. Several machines can serve this purpose depending on the operator’s requirements, specifically:

• Straightness inspection machine.
• Device for the rapid repair of the mandrel bar, where it can be repaired without the need to dismantle it.
• Mandrel screwing/unscrewing machine, where the mandrel bar has to be removed from the extension and the shank.
• Induction furnace to bring the mandrel bar up to temperature for straightening.
• Device for cooling the mandrel bar during rotation.
• Straightening press.
• Chrome plating machine.
• Horizontal lathe for regeneration or for reducing the diameter to the lower range.