Rolling

flat rolling

to produce long, flat shapes

produced at high speeds

good surface finish due to sliding

requires high level of capital investment due to expensive rollers and machines

low labor cost

what should we change about a material if we speed up the rollers?

hot work material more

what happens at the neutral point on rollers?

frictional forces oppose each other

“no-slip” the velocities of the roller and material are the same

located near the exit

rolling produces good surface finish…

because of sliding

motion at the entrance of rollers

roll moves faster than the material, friction pulls slab into rollers

what’s the relationship between frictional forces in rolling?

must have F_entrance > F_exit in order to pull material through

what happens if the entrance friction is less than the exit friction?

the neutral point is then located at the entrance of rollers, the material will get kicked back

what if there too little entrance friction?

sliding without motion occurs

friction without movement of the strip, we would have to pull on the piece to get it through the rollers which ends up being a drawing process (tension)

shape rolling

produces straight, structural shapes

produced at high speeds

very high roller/equipment costs

geometry causes non-uniform forces, elongation and thermal zones; leads to warping and cracking

what material property affects the amount of roll-flattening?

Young’s Modulus; stiffness

pack rolling

to get extremely thin sheets

increases speed of production

rolling 2 sheets at once

what are the disadvantages of pack rolling?

all deformation takes place on outside (part touching the roller)

non-symmetric residual stresses can lead to curling

ring rolling

to increase a rings diameter and decrease it’s thickness

thickness is reduced by moving rollers closer together as they continue rotating

benefits of ring rolling

good for short production runs, no material waste, close dimensional tolerances, and favorable grain flow

thread and gear rolling

cold-forming process, threads rolls by passing between rotating dies

what are the benefits of thread and gear rolling?

very high production rate, no material wasted, greater strength than machined threads, better fatigue life

why does thread and gear rolling result in greater strength than machined threads?

cold work increases strength, grain flow pattern increases strength

whereas machined threads cut through the grain-flow lines of the material, weakening it

hot rolling phenomenon

100-200ºF above T_{recrystallization}

fine and uniform grains, increase in yield strength (not more than CW) and less residual stress

why are alloys theoretically weaker when we cast them?

because of porosity; dendrites

disadvantages of hot rolling phenonmenon

non-metallic inclusions (mill scale) will not recrystallize and alloying elements that have high T_{recrystallization} may cause the same effect

non-uniform cooling can create residual stresses → warping

final surface is rougher and difficult to remove high-temp oxides

problems with rolling

high roller forces

roll deflection

rol flattening

material spreading

how do you reduce rolling force?

lower friction by modifying lubricant parameters (type, amount, application)

reduce roll radius (inc. efficiency by lowering energy)

take smaller reductions (less deformation per pass)

increase workpiece temperature; ductility increase, yield strength decrease

reduce speed; reduces strain rate

back tension

tensile force applied at the entrance reduces the hight of a piece, controlled by a breaking system in the uncoiler (payoff reel)

front tension

tensile force applied at exit, causes drawing and controlled by the torque on the coiler (delivery reel)

how do you fix roll deflection?

cambered rolls

• deflects to flat (straight)

• custom to a single amount of force, certain load, width of strip, temperature etc.

• more expensive (precision required to fabricate)

  • economic for large batches

thermal camber

heat generated during rolling can cause rolls to become barrel shaped

hot rolling w/ uneven temperature distribution in the roll (causing diameter to vary)

how do you fix thermal camber?

by adjusting lubrication parameters

roll flattening

roll forces elastically flatten rolls

how do you reduce roll flattening?

choose roller with higher young’s modulus (E), or reduce roll force

what size rollers are more efficient

small

spreading

as width thickness ratio decreases, forces cause strip to spread

how do you decrease spreading?

increasing width/thickness ratio; increase ratios of roll radius to strip thickness

decrease friction and decrease roll force

wavy edges

caused by crowning; non-uniform original material thickness

thick center of rolls produces piece with edges longer than the center, the edges buckle and wave

cracks in the center

caused by crowning; but specifically the longer edges force the center to separate

cracks at edge

caused by thermal camber; inverse crown and material spreading

think: rolling out cookie dough and getting cracks at the edges

alligatoring or curling

caused by non-uniform deformation; residual stress build up

too much material sticking to rollers

why can residual stress be good in some cases?

leaves surface in compression

what does the difference in ductility based on temperature cause?

internally sheared material when worked; i.e. center is too hot

equipment

two-high and three-high rolls

cluster mill

tandem rolling (train)

overall rolling advantages

produces mostly flat, long, shapes

high efficiency, low labor cost

high speed

no additional machining or finishing

good surface finish

economical (almost no waste)

improves material properties

overall rolling disadvantages

can be $$$, especially cambered rollers

high level of capital investment