Figure 5.1: Rolling process description. Page 1 of 12

Transcription

1 Eperiment ( Rolling Operations Dr. Moammad Al-taat Department o Industrial Engineering. Universit o Jordan. Lab. O Manuacturing Processes. Course No: 964, 964. Objective: Te main objective o tis eperiment is to stud te process o rolling o metals and to eamine te deerent actors inluence te process.. Background: For more inormation about te subject o te eperiments, it is recommended or te student to review section 6. o capter si o te tet.. Teor It is te irst process in converting a cast material (ingot into a inised wrougt product, rolling process can be deined as: te Bulk deormation process o reducing te tickness or canging te cross-section o a long work-piece b compressive orces applied troug a set o rolls (mills similar to rolling doug wit a rolling pin to reduce its tickness as demonstrated in igure 5.. Figure 5.: Rolling process description. Rolling accounts or about 9% o all metals produced b metal orming. Te process irst developed in te late 5 s te basic operation is lat rolling (simpl were te rolled products are lat plate and seet. Plates: aving greater tan 6mm. m tickness. Seets: aving less tan.6 mm. A scematic outline o various lat and sape rolling process are sown in igure 5. te igure implies te sequence o operations needed to convert an ingot or a continues casting into a useul product like strip, plate, bar etc. Page o

2 Figure 5. Scematic Outline O various lat-and sape rolling processes. Mecanics o Rolling Strip Sliding. Scematic illustration or lat rolling is sown in igure [5.]. Because o volume constanc, te velocit o te workpiece (strip must increase as it moves troug te roll gap. At te eit o te roll gap, te velocit o te strip is V. because V r is constant along te roll gap, sliding occurs between te roll and te strip. FIGURE [5.]: Scematic illustration o te lat rolling process. A greater volume o metal is ormed b rolling tan b an oter metalworking process. Neutral point. At a certain point a long te arc o contact strip velocit and roll velocit are te same. It is known as neutral point, or no slip point to te let o te neutral point, roll moves aster tan te workpiece, and to rigt te workpiece moves aster tan te roll. Page o

3 FIGURE [5.4]: Relative velocit distribution between rolls and strip suraces. Note te dierence in te direction o rictional orces. Te arrows represent te rictional orces acting on te strip. Forward slip. In rolling is deined in terms o te eit velocit o te strip V and te surace speed o te roll V r as: Forward slip (V V r /V r State o Stress in Rolling and Roll Pressure Te calculation o orces and stress distribution in lat rolling is more involved tan in upsetting because o te curved surace o contact. In addition, te material at te eit is strain ardened, so te low stress at te eit is iger tan tat at te entr. FIGURE [5.5]: Stresses on an element in rolling: (a entr zone and (b eit zone. Te stresses on an element in te entr and eit zones are sown in igure [5.5]. Using te slab metod o analsis or plane strain. Figure [5.6]: Forces on te element. From te equilibrium o te orizontal orces on te element in igure 5.6, Page o

4 Page 4 o ( ( cos (sin cos (sin cos sin ( or unit widt (w cos ( sin ( ( µ µ µ µ m m prd d d prd d d d d prd prd d d w prd w w prd w d d ± ± FIGURE [5.7]: Stresses on an element in plane-strain compression (Rolling between two rolls. In general [ ] [ ] [ ] ( ( ( υ ε υ ε υ ε E E E z Te maimum value or v (or tat value or wic volume cange is zero is.5 i.e. (In te elastic range < ν <.5 and tere is a volume cange. Since we ave plain strain state, ten we can write ε, tere we obtain [ ] as seen in igure 5.7.5(...5(.5( z z z e i E ε (5. According to te distortion-energ criterion or plane strain, we ave ( ( ( ( ( ( z z - (5. Or p - - p (5. (5.4 Assume te ollowing equation R R H tan (5.6

5 ` mµ H p C e R At entr, ø α; ence, H H o wit ø replaced b α. At eit, ø ; ence; H H. Also, at entr and eit, p `. Hence, in te entr zone, R mµ H C e i ` p o, In te eit zone, R C e µ ( H o H (5.7 ` µ H p e (5.8 FIGURE 5.8 Pressure distribution in te roll gap as a unction o te coeicient o riction. Note tat, as riction increases. Te neutral point sits toward te entr. Witout riction, te rolls slip and te neutral point sits completel to te eit. Te eect o reduction in tickness o te strip on te pressure distribution is sown in Fig 5.9. As reduction increases, te lengt o contact in te roll gap increases, wic in turn increases te peak pressure. Te curves sown are teoretical; actual pressure distributions, as determined eperimentall, ave smooter curves wit teir peaks rounded o. Page 5 o

6 FIGURE 5.9 Pressure distribution in te roll Gap as a unction o reduction in tickness. Note te increase in te area under te curves wit increasing reduction in tickness, tus increasing te roll-separating orce. Front and Back Tension. Te roll orce F can be reduced b various means, suc. A slower riction,. Smaller roll radii,. Smaller reductions, and 4. Higer workpiece temperatures. 5. Reduce te plain compressive ield stress o te material b appling longitudinal tension. FIGURE 5. Pressure distribution as a unction o ront and back tension. Note te siting o te neutral point and te reduction in te area under te curves wit increasing tension. Page 6 o

7 Roll Forces. Te area under te pressure-contact lengt curves (igures 5.8, 5.9, 5. multiplied b te strip widt, w, is te roll orce, F, on te strip. Tis orce can be epressed as: F n α ω. p. R. d w. p. R. d (5.9 n A simpler metod o calculating te orce is multipl te contact area wit an average contact area wit an average contact stress: F L. w. p average (5. Were L is te arc o contact, it can be approimated as in te ollowing epression. L R. (5. R is te roll radius, is te dierence between te original and inal tickness o te strip (drat. p average µ. L. average, Low rictional, Higer rictional conditions, Large L conditions, Small Ratio. Ratio L Were is te average low stress in plane strain o te material in te roll gap, see igure 4.. Roll Torque and Power. Te roll torque T or eac roll can be calculated analticall rom te epression FL T (5. Te power required per roll is Power Tω (5. Were ω πn and N is te revolutions per minute o te roll. Consequentl, te power per roll is πfln Power Kw (5.4 6 Were F is in Newton s, L is in meters, and N is te rpm o te roll. We can also epress te power as Page 7 o

8 πfln Power p (5.5 Were F is in lb and L is in t. Eample 6.4: Power required in rolling A 9 in. wide 66- aluminum strips is rolled rom a tickness o. in. to.8 in. I te roll radius is in. and te roll rpm are, estimate te orsepower required or tis operation. SOLUTION. Te power needed or a set o two rolls is given b Eq. (5.5 as: πfln π ( 97(.( Power p 75p F L. w. p Ib average p average L L R. 75psi R., (. (..8.55in.. t. ln o ε ln..8 n K( ε From tables K n. (.. 85 psi, n. Roll bending and lattening. Roll orces tend to bend te rolls, as sown in Fig. 5.a, wit te result tat te strip is ticker at its center tan at its edges (crown. Te usual metod o avoiding tis problem is to grind te rolls so tat teir diameter at te center is sligtl larger tan at te edges. Tis is known as camber. Page 8 o

9 FIGURE5. (a Bending o straigt clindrical rolls because o te roll separating orce. (b Bending o rolls, ground wit camber, tat produce a seet o uniorm tickness during rolling. Forces also tend to latten te rolls elasticall, muc like te lattening o tires on automobiles. Flattening o te rolls increases roll radius and ence ields a larger contact area or te same reduction in tickness. Tus te roll orce F increases. Spreading o Rolled Metals te widt increases considerabl during rolling. Tis increase in widt is known as spreading Spreading decreases wit increasing widt-to-tickness ratios o te entering material, decreasing riction, and increasing ratios o roll radius-to-strip tickness.. Miscellaneous Rolling Operations. Sape Rolling. Tis process is used to produce straigt structural b passing a bloom troug a number o pairs o speciall designed rollers as seen in igure 5.. FIGURE 5. Stages in sape rolling o an H section part. Tis process also rolls various oter structural sections, suc as cannels and I-beams. Page 9 o

10 Ring Rolling. A small-tick diameter or a ring is epanded into a larger one (tinner diameter. Te ring to be epanded is placed between two rolls, one o wic is driven and te oter is idler as seen in igure 5., te ring tickness is reduced b bringing te rolls closer as te rotate. FIGURE 5. Scematic illustration o a ring-rolling operation. Tickness reduction results in an increase in te part diameter. Tread and Gear Rolling. Treaded Screws and treaded bolts are ormed on round rods or work pieces b passing tem between reciprocating or rotating dies as in te igure 5.4 below: Figure 5.5: Tread and gear rolling operation Figure 5.6: Required diameter s or bot Macined and rolled tread. Page o

11 Rotar Tube Piercing. Rotar tube piercing is used to make long and tick-walled continuous tubing, as sown in igure 5.7. FIGURE 5.7 Cavit ormation b secondar tensile stresses in a solid round bar and its utilization in te rotar tube piercing process. Tis is te principle o te Mannesmann mill or seamless tube making. Te mandrel is eld in place b te long rod, altoug tecniques ave been developed in wic te mandrel remains in place witout te rod. Tube Rolling. Te diameter and te tickness o tubes and pipes can be reduced b tube rolling using saped rolls, eiter wit or witout mandrels.. Deects in Rolling Deects ma be on te suraces o te rolled plates and seets, or te ma be structural deects witin te material. Surace Deects. Surace deects ma result rom inclusions and impurities in te material, scale, rust, dirt, roll marks, and oter causes related to te prior treatment and working o te material. Torc (scaring, is a preconditioning process to remove scale rom te suraces o some ot rolled product. Structural Deects. Some tpical deects are sown in Fig Bending o te rolls causes wav edges. Te cracks sown in Fig. 5.8b and c are usuall caused b low ductilit and barreling. Alligatoring is a comple penomenon resulting rom inomogeneous deormation o te material during rolling or deects in te original cast ingot, suc as piping. Also Residual stresses considered as a major deect. FIGURE 5.8 Scematic illustration o tpical deects in lat rolling: (a Wav edges; (b Zipper cracks in center o strip; (c Edge cracks; (d Alligatoring. Page o

12 4. Materials: Commercial pure lead (98%. 5. Equipments: Rolling Mill, Vernier Caliper and measuring instruments, Set o lat lead seets. 6. Procedures:. Adjust te gap between te two rolls.. Measure te diameter o te rollers.. Set te rolling mill or operation. 4. Measure te tickness o te lead seet beore and ater roling. 5. Measure te widt o te lead seet beore and ater rolling. 6. Measure te lengt o te lead seet beore and ater rolling. 7. Roll te lead seet b setting te lat seet between te two rollers. 8. Repeat te rolling sequence or dierent ratio o seet widt to tickness. 9. Repeat te rolling sequence or dierent reduction wit initial seet tickness constant.. Repeat te rolling sequence ied reduction ratio wit dierent initial seets tickness. 7. Requirements:. Describe te rolling process illustrating wit sketc.. Make a line diagram sowing te metod o operation o te rolling mill.. Find te reduction ratio (r or te rolled seet. r [t l -t / t, were t l initial seet tickness beore rolling, t inal seet tickness ater rolling. 4. Find te ratio o initial seet widt to tickness (W / t, or plain strain condition were tere is no or ver sligtl cange in seet widt beore and ater rolling. 5. Measure main rolling deects. 8. Questions. Discuss rolling metods and rolling deects? Page o