MECH2636: Rolling Processes Quiz

Course Information

• Course title: MECH2636: Design and Manufacture 2
• Course topic: Solid Deformation Processes
• Course Institution: University of Leeds

Rolling

• Steel is the most common material in engineering structures and components.
• Steel production is primarily through ingot casting or continuous casting.
• Rolling is a steel processing step where the material is forced through rolls.
• Rolling generates plates, bars, rods, pipes, and various other shapes.
• Rolling is typically sequential on multiple rolls to reduce force and deformation at each stage.
• Rolling processes use hot or warm metal, with finishing often done cold.
• Rolling speed at exit must be higher than the speed at entry.
• Contact between roll and workpiece has two areas where the roll is faster than the workpiece and vice-versa respectively.
• The area of contact length is ~√(roll radius × (initial thickness - final thickness)).
• Force generated on the rolls increases with the flow stress of the material, contact length, width of work, average thickness, and the friction coefficient.
• Formula for force: F ≈ σy(LW)(1 + μL/2h_avg)
• High forging force can distort the workpiece and the roles.
• Torque is equal to ~ F x (length of contact /2),
• Power required = Torque × Rotational speed.

Rolling Mills

• Rolling mills have different arrangements (two high, three high, four high, Sendzimir).
• Two high mills are the simplest.
• Three high mills offer higher productivity per roll.
• Four high mills use a small inner roll, reducing forces and distortion.
• Sendzimir mills use many rolls for a very rigid process, usually for cold rolling.
• Lubrication is used to reduce friction between rolls and workpiece.
• Shape rolling produces various structural shapes across multiple passes.

Extrusion & Drawing

• Extrusion and drawing are related processes used to produce long lengths of material with specific cross-sections.
• Extrusion pushes a billet through a shaped or tapered die.
• Drawing pulls the material through the die.
• Cross-sectional area reduces during either process and the shape of the billet emerges as per the shape of the die.
• Forward extrusion applies pressure.
• Pressure applied to the billet is expressed by: F = σy A0 ln (A0/Af), where σ is the plastic flow stress of material, A0 is the initial cross-sectional area and Af is the final cross-sectional area.
• Factors that increase the ideal force in reality involve friction and redundant work.
• Friction is minimized by using lubricants or tapered dies.
• Redundant work from unnecessary shearing increases as the die angle increases.

Backward Extrusion

• Backward extrusion is a variation of forward extrusion where the direction is reversed.
• Hydrostatic extrusion uses pressurized fluid on the billet to force it through a tapered die, reducing friction.

Die Drawing

• Die drawing is similar to forward extrusion, except the work is pulled rather than pushed.
• Drawing is often used as a finishing operation and is mostly employed for small cross-sections, such as tubes, wires, and rods.
• Ideal drawing force = σ y A0 ln (A0/Af) (where σ is material flow stress, A0 is initial and Af is final area)

Forging

• Forging is a bulk deformation process, typically by compressing metal between two surfaces.
• Forging can be at room temperature or elevated temperature.
• Open die forging, the most common process, involves placing a workpiece between two die surfaces and compressing it.
• The resultant shape depends on the die surfaces' friction.
• High friction leads to 'barrelling' of the workpiece.
• Hot workpiece between cold dies also causes barrelling
• Upsetting force formula: F ≈σy (πr²) (1+(2µr)/3h)).
• Sequential forging may occur when a press is unable to perform all deformation in one go.
• Heading and Coining are other forging operations.
• Flaws in forging can include surface cracks from over-straining, laps from buckling thin sections, and cold shuts when the die design improperly guides the metal.
• Design rules for forgings involves choosing fillet and corner radii, good draft angles and understanding of material anisotropy.
• Parting line should pass through greatest cross section area in the die when possible. Minimum thicknesses for ribs & webs depends on the material. Punchout holes are to reduce weight & add clearance.

Cold Working

• Cold working operates below ~30% of the material's absolute melting temperature.
• Cold working increases strength and hardness but decreases ductility.
• Cold working gives better dimensional accuracy and surface finish better than hot working.

Hot Working

• Hot working operates above ~60% of the absolute melting temperature.
• Hot working improves mechanical properties, refining the grain structure, and decreasing the forces required by increasing the flowability of the material.