Cold working by Mechanical Working of Materials.pdf

Cold working operations on materials

1. ROLLING

9. Flat Rolling
•Initial thickness h
o
•Final thickness h
f
•Roll gap L
•Surface speed of rolls V
r
•Entry velocity of strip V
o
•Final velocity of the strip V
f
•Neutralpoint,no-slippoint–pointalongcontactlengthwhere
velocityofthestripequalsvelocityoftheroll

10. •Draft: h
o –h
f
•Maximum draft possible: h
o –h
f= μ
2
R
•Coefficient of friction μ
•Roll radius R
•The strip thickness is reduced at each rolling pass and the
strip width increases slightly (around 2%)

11. •Roll Force: F= LwY
avg
•Roll-strip contact length L
•Contact length
•Average strip width w –despite the fact that spreading,
or an increase in width, may actually occur if edger
mills are not used
•Average true stress of the strip in the roll gapY
avg
•Assumes no friction and thus predicts lower roll force
than the actual value 
fhhRL 
0

12. Power per roll (SI units)= πFLN/ 60,000 kW
Where F is in Newtons, L is in meters, and N is rpm of roll

22. ROLL PRESSURE DISTRIBUTION

25. •Average flow stress:
•In rolling:1)1(
0
1
0














n
k
n
k
dk
Y
kY
f
f
n
f
n
ave
n
f
f







 f
f
h
h
0
ln

26. PROBLEM
Anannealedcopperstrip228mmwideand25mmthickis
rolledtoathicknessof20mminonepass.Therollradius
is300mm,andtherollsrotateat100rpm.Calculatethe
rollforceandthepowerrequired.
Forannealedcopper,ithasatruestressofabout80Mpa
intheunstrainedconditionandatatruestrainof0.223,
truestressis280Mpa.

27. FOUR HIGH ROLLING MILL

37. PROBLEMS AND DEFECTS IN ROLLED
PRODUCTS
Porosity,Cavity,BlowHolesoccurredinthecastingotwillbe
closedupduringtherollingprocesses.
Longitudinalstringersofnon-metallicinclusionsorpearlite
bandingarerelatedtomeltingandsolidificationpractices.In
severalcases,thesedefectscanleadtolaminationswhich
drasticallyreducehestrengthinthethicknessdirection.

42. Residual Stresses Developed in Rolling
(a)Residualstressesdevelopedinrollingwithsmall-diameterrollsoratsmall
reductionsinthicknessperpass.(b)Residualstressesdevelopedinrollingwithlarge-
diameterrollsorathighreductionsperpass.Notethereversaloftheresidualstress
patterns.

43. Production of Steel Balls
(a)Productionofsteelballsbytheskew-rollingprocess.(b)Productionofsteel
ballsbyupsettingacylindricalblank.Notetheformationofflash.Theballsmade
bytheseprocessessubsequentlyaregroundandpolishedforuseinballbearings.

44. Ring-Rolling
(a)Schematicillustrationofaring-rollingoperation.Thicknessreductionresultsin
anincreaseinthepartdiameter.(b-d)Examplesofcross-sectionsthatcanbe
formedbyring-rolling.

45. Thread-Rolling Processes
Thread-rollingprocesses:(a)and(c)reciprocatingflatdies;(b)two-rollerdies.(d)
Threadedfasteners,suchasbolts,aremadeeconomicallybytheseprocessesathigh
ratesofproduction.Source:CourtesyofCentralRolledThreadDieCo.

46. Machined and Rolled Threads
(a)Featuresofamachinedorrolledthread.Grainflowin(b)machinedand(c)
rolledthreads.Unlikemachining,whichcutsthroughthegrainsofthemetal,the
rollingofthreadsimpartsimprovedstrengthbecauseofcoldworkingand
favorablegrainflow.

47. Cavity Formation in Bar
Cavityformationinasolid,roundbaranditsutilizationintherotarytube-piercing
processformakingseamlesspipeandtubing.
Mannesmann Process