Rolling of Metals Introduction This chapter describes Flat

Rolling of Metals

Introduction Ø This chapter describes üFlat rolling ü Shape rolling üRolling Mills üProduction of seamless tubing & pipe üDefects in rolled products

Introduction • The Process of Plastically deforming metal / alloy by passing it between rolls is known as ROLLING. • Rolling done either at Hot or cold. • The metal is drawn into the opening between the rolls by frictional forces. • Work piece is subjected to high compressive forces due to squeezing action of rolls, resulting in reduced area of cross-section and increased length.

Cold Rolling Hot Rolling

TERMINOLOGY INGOT BLOOM [ 150 x 150 mm] or [ 250 x 300 mm] BILLET [ 50 x 50] or [ 125 x 125 ] Slab B = 300 -1500 mm PLATE [ t > 6. 35 mm ] SHEET [ t < 6. 35] STRIP [ t = 6. 35 & b = 600 mm ] FOIL [ t < 1. 5 mm & b= 300 mm ]

TERMINOLOGY Contd. , • Angle of contact or Angle of bite – The Angle subtended at the centre of the roll by arc AB ( Metal in contact with the roll) • Rolling Pass – The feeding of material between the rolls once • Rolling mill – Consists of rolls , bearings& Housings and Drive for applying power to rolls

Flat Rolling Process

Flat Rolling Process • • Metal strip enters the roll gap The strip is reduced in size by the metal rolls The velocity of the strip is increased the metal strip is reduced in size Factors affecting Rolling Process – Frictional Forces – Roll Force and Power Requirement

GEOMETRY OF ROLLING

• The angle of bite α can be expressed as; tan α = L/R • The length of contact, (2) -h )} L= sqrt{R(h o f h 0 is the strip thickness at start and R is the roll radius • h 0 -hf = µ 2 R

Schematic Illustration of Various Roll arrangements

Schematic Illustration of various roll arrangements : (a) two-high; (b) three-high; (c) four-high; (d) cluster mill

Shape-Rolling Operations • Various shapes can be produced by shape rolling – Bars – Channels – I-beams – Railroad rails • Roll-pass design requires considerable experience in order to avoid external and internal defects

Stages in Shape Rolling of an H-section part. Various other structural sections such as channels and I-beams, are rolled by this kind of process.

Ring Rolling • A thick ring is expanded into a large diameter ring – The ring is placed between the two rolls – One of which is driven – The thickness is reduced by bringing the rolls together • The ring shaped blank my be produced by: – Cutting from plate – Piercing – Cutting from a thick walled pipe Various shapes can be produced by shaped rolls • Typical applications of ring rolling: – Large rings for rockets – Gearwheel rims – Ball-bearing and roller-bearing races • Can be carried out at room temperature • Has short production time • Close dimensional tolerances

RING ROLLING (a) Schematic illustration of Ring-rolling operation. Thickness reduction results in an increase in the part diameter. (b) Examples of cross-sections that can be formed by ringrolling

Thread Rolling • Cold-forming process • Straight or tapered threads are formed on round rods by passing the pipe though dies • Typical products include – Screws – Bolts

Production of Seamless Pipe & Tubing • • Rotary tube piercing (Mannesmann process) – Hot-working process – Produces long thick-walled seamless pipe – Carried out by using an arrangement of rotating rolls Tensile stresses develop at the center of the bar when it is subjected to compressive forces

Defects in Rolled Plates & Sheets • • Undesirable – Degrade surface appearance – Adversely affect the strength Sheet metal defects include: – Scale, Rust, Scratches, Gouges, Pits, & Cracks – May be caused by impurities and inclusions Wavy edges – result of roll bending Alligatoring – complex phenomenon