e 2008 Pearson Education , Inc., Upper Saddl River, NJ. All rights reserved . This material is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited

reproduction. storage in a retrieval system, or transmission in any form or by any means, electronic , mechanical, photocopy ing, recording, or likewise . For information regarding permlsslon(s), wrile to:

Rights and Permissions Department, Pearson Education, Inc.• Upper Saddle River. NJ 0745 8.

,

r

(f) Co oling of the workpiece in the die in hot furging, and its consequen ces regarding die filling and forging for ies ,

(g) Economic considerations.

6.5 Devise an exper imental method whereby yon can measure the lore required for forging oulv t.he flash ill impression-die forgin g . (See Fig. Ei.l5H,,)

An experimental method t o det ermine t he forc es -requirod to forge o nly th e flash (for an axisy n nue t.ric part.) would involve making LlH' die in two concentric p ieces, each with its own loarl cell 1.0 measure I he force. TIle d ie in , he center would (JIIly cover t.he proj ect ed area of th e part its elf, and tIll' outer die (r ing shaped ) would COW l' th e proj ected area of I.IIt' a nnular Has h. Du ring fo rging. tb p loar (·plls a ro moui ­tored indi vidually mel, Ihus. Ilw loads for Ihe par t and the flash, respect ivelv. can be mea­sured independently. St.urlcnts ar e encouraged t.u devise 01her possible and practical methods.

6.6 A manufacturer is successfullv hot forgin g It Cl'r­t. ain pa rt, usiug uiaterial s upplied II,)' Co mpany .4. A Il('W su pply of materi al is ob tnined from Company 1:1, with f.lw same nominal cornposi ­t.ion of I he major alloying eleme ntS a,-; I h il l )1' th e material from Company A. Ilowever. it i. found Iha l tilt' new forgings a n" cracki ng r-ven . th ough t he same procedure is followed l.IS he­fore. What is the prob able reason ?

The probable reason is the presen ce of impu­rities , inc lusions. a nd minor elements (such a,-;

51111'111' in the material su pplied by Company B . Note that t he qu es tion suues that both rna teri­als have the "same nominal composition of th e major alloying elements" . No mention is mad e regarding min or eleme nts or im p urity levels .

,, ­v . 1 Explain why th en' might hI' 11 change in th e density of a forged product 11.'0 compared to I hal of the cas t blank .

If l,h,' origin al material has porosity, such as from a poo r casting with porosity rlue to gases or sh rinkagt· cavilies, its density wiII increase a fter furging bt~al1s e t he pores will close IIl1der

the applied compre;siw stre:;ses. On the 0 1her hand , th e oriRinal hla nk lJIar be free of finy po ros ity bllt due to adverse mater ial ftc1w ;u ,d

state of stress during plastic deformat ion . cavi­t.it ~ may develop (similar t o voids th at devel op in t.he necked recion of a te ns ile- tes t specimen, see Fig. ~.24 on p . 100). Thus, t he density will dt'('r east >a fter furging d ue to void formation .

6. ince I!,la: . is a good lubricant for hot extrusion , would yon use glass for impression-die for ging i1.S well? Expl ain .

Glas,'l. ill var ious forms. is used fur hot forgin g ope ru t ions. Howev er , in impression-die forging, even th in films (beca use glass is incompressib le) will prevent the part from producing th e d ie ge­ometry, and thus develop poor qu ality, an d may p revent s uccess ful forging of intricate shapes. If th e' glass lubricant solidifies ill deep recess es of th e dies , th ey will bp difficult and cos t ly to re ­move.

lEI Describe and expls in t he lactors tha t, iuflu ' I1C(-,

sp read in ro ggin g operations on square bille ts .

A re view of th e events laking place at t Ilt' die­workpi ece interface in cogging indi cates th at. th o Iaet.ors th at influence s prea d ing ar e .

(a) Friction : the lower th e frict ion, th e more I he sp reading because of redueerl l at eral

resistance to materi al flow

(h) Width-to-thirkness ratio uf th ' workpiec ': the higher thi . ratio, th lower th e sp read­ing .

(c) Co ntac t length (ill !'IIP lon gitudinal directionj-t o-workpiece ratio ): the higher this ratio, th e higher th e spreading, Re '1l.1I fhat r he mater ial llows in f h l' direc tio n o f least resh ltpl ce.

n.in Wh~' are I'mI grains ge nera lly uml esirahh- ill forged products'! G ive exa mples of such prorl­u cts .

A: dis cussed in Sec t ion 6.2,5 st.art.inz on p. 28.1. end grains /l.fC' gene rally undesirable be­ca use corros ion occurs preferen! ially along grain boundaries. Titus end grains present. many g rain houndaries a t the surface for cO!1'os ion tu take pInel' . Tn add it ion. they mlL.~· result in obj ect ionable surfac t" appea rance. as well fl.'! m.. ducing the fntiglle life of lh e cumponent beclluse (j f surfll£:e rou ghne: s Lhat results frolll corrosion ,

28

© 2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This mater ial is protected by Copyright and written permission should be obta ined from the publisher prior to any prohtb ited

reproduction, storage In a retrieval system, or transmission in any form or by any means , electronic, mechanical, photocopying, recording, or likewise . For information regard ing permission(s) , write to:

Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River , NJ 07458.

)f.... G. J1 E xp lain why oue cannot produce a finished forgin g in one press s troke , st.ar t.ing vit h a blank.

Forgings nre typically prod uced t hr ou rh a series o f opera tions, such as edging, blo cking, etc .. 11:;

dep icted in F ig. l·i.25 011 p . 285. T his is done for a number of reas ons:

(a) T he force a nd ene rgy requirements on the press are great Iy red uced by pe rforrni Il g

the op era tions seq 1lP.1lI ia llv:

(11) T he part may have to be sub jected to in­termedi ate an nealing. thus a llowing less du ctile ma te rials to be forzed to compli­

r : ca ted sh ap es. A l s~ , t"i55 (c) Rev iewing th e Archard wear law given by

/p .Ze, p Eq, (-4 .6) on p. 1-1 5. it can be seen I hat ( e>'VI Id low die wear rntes can be achieved if th e

(J w';;) P slid ing distan ce and/or th e force i:-; low i ll i st roke. Reviex ing rig. 6.25 o n p . 285,e it r-an be seen t hat each opera! ion will involve II la rge s liding distuno - be tween t.he workpiece and di es, t.hus ca us ing mo l' . wear .

l.12 List the advantages and di.'i/ldvlUlt agl~ of using c lubrica nt in forging operat ions.

T he advantages jnclude:

(it) It redu ction in the force and ene rgy n.."­qnired :

(b ) II.'5s localiz ation of st rain . r .'51111 ing in im­proved Iorgeabilit v:

(c) rhe lubricant ucts as a t hermal harrier , so I h a l. the part CIU I re ma in hot t ' I' lon ger IUll l

thus ]1i!W' more du ctili ty:

(d) the lubricant can pro tect t ill' workpiece fro m the cn vironrn in t., ~p 'dally ill hot forgin g. and also ad as a par t ing agent .

T IH! disadvantages in clude:

(a ) The lubricant adds cost to the operat ion:

(h ) a thi ck Iilm can result ill orange-p eel effect. Oil lI\t' workpiece :

(c) lubrican ts may be entrapped in die cavi­l ies, I hus part dimensions my no t be ceptahle;

(r1) t he lubricant must subseq ueut.ly he re­1110\ '1.'<1 from !he part su rface. a u ad d itionul and difficult oper a t ion;

(e) disposal of 1.111' luhri ;11 111. c a ll present env i­ronmernal sho r tco mings,

6.13 Explai n he reasons why the flash i:l..'i: ists in d ie filling , especially ill hot forging .

T he Hash is excess metal vhich is squeezed ou t fro m the die cavity int o t he outer space between th e two dies . T III~ flash cools fast er than t he ma ­te rial in t he cavity due to the high "/" rat io and I he 1110\'1:' intima te c-ontact with t he relat ively coo l dies . OI ~ equently, th e flash has higher s trength tha n till' hoi tel' workpiece in t he die cavi ty RUlI, wi t h hiaher fric t ional resistan ce in thp llash gap . provides grea ter resistance to rna­IPf IO.I flow outward th rou gh t he llash gap, T hus. t he flash encourages filling of complex die cav ­ities ,

6. 1.! B ' inspect ing some forged product s (s udl as a p ipe wrench or coins), yo u can see I1IH! the leur-ring on I hem i ~ ra ised ra t her than sunk. Offer au expla nat ion as 10 why lIae.v ar« wade that way.

By t he student . It. is much eas ier and e onorn i­ca l to machine cavities in il, d ie (thus prod ucing let tering on /1 forgin g that a re raised from its su rface) than producing pr ot.rus ions (t.nus pro­du cing let tering t.hat are like impressions on the forged su rface) . I Jote th at 1.0 prod uce a protru­sion 0 11 1he .-I iI'. mate rial surroun di ng t.he lette rs he rem oved , a di lficu lt operatio n for mos t. let ­t ,ring. Recall a lso si milar conside rat ion in cas t p roduct s,

Rolling

G.l !j It was s taled Iha l. rhree fal'tr I' I ha t influ­«nee spreading in rolling art.' (II) th e wid th- to­th ickness ratio of th e strip , (h ) friction . an d (c) the ra t io of th e rad ius of I he roll to [he thick­ness of the ~l rip. E xpl a in how each of th ese factors affects spreading.

Th ese par amet er s basically fill contribute to the frict ional resistance in the width direction of th e strip by chan gin g till! as pect ratio of t he con­tac t, area between the roll aMI the s tri p (see also Answe r 6.!J a bove]. '

:m

© 2008 Pearso n Educa tion, Inc., Uppe r Saddle River, NJ. All rights reserved. This material is protected by Copyright and written permission should be obtained from the pUblisher prior to any prohibited

reproduction, stora ge In a retrieval system, or transmission in any form or by any means, electronic, mechanica l, photocopying, reco rding, or likewise. For information rega rding permission (s), write to :

Rights and Permis sion s Department, Pearso n Education, Inc., Upper Saddle River, NJ 07458 .

6.16 Explain how }'0I1 would go about applying front and back tensions 1.0 she!'t rnet.als during rolling .

Fron t tensions life applied and controlled by the jake- lip reel of a rolli ng mill: the higher nIP torq ue to this red or the higher the rotational speed. 1.IIP greater the front tension . Back len­. ion is applied by LIte pay-off reel by increasing the b raki ng torque on the pay-off reel or reduc­in g its rotational spee d,

6.17 It was noted that rolls tend to flaueu under roll forces. Which propertyties) of the rol l material ('a n 1)1' increased I o red 11('e flattening? Why?

F la t tening hi elast ie deformat iOB of the origi­nally ci rcu lar roll cross section. and remits in a la rge r contact lengt h in t.he roll gap. There­fore, the elast.ir modulus of the roll should be increased .

G.18 Describe the methods by which roll flattening can be reduced.

Roll flntten ing can be reduced by:

(a) decreasing I.hl: reduction per pass,

(h ) reducing fri 'lion, and/or

(c ) iuereas mg t he roll stiffness (lor example, by making it from materials with high mod ulus of elasticity, such as carbides] .

6 .19 Explain th e tec hnical anrl economic reasons for taking larger rather than smaller red uct ions per P <lS5 ill flat rol ling.

Eco nomically, it is always beneficial to reduce th e number of operations involved in ruanufac­t uring of products . Reducing tht' number of passes iu rolling achieves this r ~1I11 by lowe r­ing the n umber of required operat ions, This a llows less product iOB t ime 10 acllj('vt':' 1111' fina l t.hic 'nes,'i of the rolled product. Of co urse, any adverse effects of high reduct ions per pass mus t also be considered ,

6.20 List and explain the rnet.horls that can be used 10 red uce LJU' roll force.

In review ' 19 the mechanics of a nat. rolling op­eration, describe d in Section 6.3. 1 s tart ing 011

p. 2!l(), ii , will he apparent I hat I he roll for' • F . can be reducer! by:

3D

(a) \Isin g smal ler-diameter rolls ,

(b) taking lower red uction per pass,

(e) red ucing friction ,

(d) increasing strip temperature. and

(e) applying front nnd/or back tensions, (J f and ab o

G.21 Explain the advantages and limitations of us ing small-diameter rolls ill flat rolling .

The advantages of using smaller diameter rolls in ll, trolling 1\1'1' I.h!' following

(a) compressive residual stresses are devel­oped OIl the workpiece surfa -t'.

(h) lower roll force; are requi red,

(1-) lower power requirements.

(d) It":>:> spreading, and

(e) the smaller diameter rolls are less costly ami easier to rep lace and maintain .

The disndvantages include:

(II) larger roll detlectious, possibly requirrng backup rolls, and

(II) lower possible drafts: see F..41. (6...16) 011

p . 298 .

6.22 A ring-rolling operat.ion is heing used S IlC ­

r: essfully for I he production of bearing races . However, when the bearing race d iam >I.e r is changed. the operation results in verv poor su r­fat:e finish. List th(' possible causes, and r1p.­scribe the type of inves tiga t ion yon WO\l)J r.:OIt­

duct to irlent ily t.he parameters involved and correct the problem.

Surface finish is closely relate) to lubricant film th ickness, I hus init ial investigations should be perfonucd to make SIlIP that I hf' Iilm I hickness is main!aine l I he same for bo th bearing races. SOllie of the ini t ial inves t.iaa t. ions would involve making sure , for example. that the lubricant supply is no t reduced wit h a larger rnce siz '. Also. the higher t he roll ing speed, the greater the film I hickness, so it should be checked that the rolling speed is the same for both cases. For­ward slip should Ill' measured and the rolling sPt't~Js adjnst.ed accordingly.