Ch-2 Rolling.pdf
- 1. 14-08-2015 1 Rolling Hot Rolling Cold Rolling Rolling Definition: The process of plastically deforming metal by passing it between rolls. Most widely used, high production and close tolerance. Friction between the rolls and the metal surface produces high compressive stress. Hot-working (unless mentioned cold rolling.) GATE-2013 In a rolling process, the state of stress of the material undergoing deformation is (a) pure compression (b) pure shear (c) compression and shear (d) tension and shear
- 2. 14-08-2015 2 Hot Rolling Done above the recrystallization temp. Results fine grained structure. Surface quality and final dimensions are less accurate. Breakdown of ingots into blooms and billets is done by hot-rolling. This is followed by further hot-rolling into plate, sheet, rod, bar, pipe, rail. Hot Rolling Video Cold Rolling Done below the recrystallization temp.. Products are sheet, strip, foil etc. with good surface finish and increased mechanical strength with close product dimensions. Performed on four-high or cluster-type rolling mills. (Due to high force and power) Cold Rolling and Grain StructureVideo
- 3. 14-08-2015 3 Ring Rolling Ring rolls are used for tube rolling, ring rolling. As the rolls squeeze and rotate, the wall thickness is reduced and the diameter of the ring increases. Shaped rolls can be used to produce a wide variety of cross-section profiles. Ring rolls are made of spheroidized graphite bainitic and pearlitic matrix or alloy cast steel base. Ring Rolling Video ISRO-2009 Ring rolling is used (a) To decrease the thickness and increase diameter (b) To increase the thickness of a ring (c) For producing a seamless tube (d) For producing large cylinder Sheet rolling In sheet rolling we are only attempting to reduce the cross section thickness of a material. Roll Forming
- 4. 14-08-2015 4 Roll Bending A continuous form of three-point bending is roll bending, where plates, sheets, and rolled shapes can be bent to a desired curvature on forming rolls. Upper roll being adjustable to control the degree of curvature. Roll Bending Video IES – 2006 Which one of the following is a continuous bending process in which opposing rolls are used to produce long sections of formed shapes from coil or strip stock? (a) Stretch forming (b) Roll forming (c) Roll bending (d) Spinning Shape rolling Shape Rolling Video Pack rolling Pack rolling involves hot rolling multiple sheets of material at once, such as aluminium foil. A thin surface oxide film prevents their welding.
- 5. 14-08-2015 5 Thread rolling Used to produce threads in substantial quantities. This is a cold-forming process in which the threads are formed by rolling a thread blank between hardened dies that cause the metal to flow radially into the desired shape. No metal is removed, greater strength, smoother, harder, and more wear-resistant surface than cut threads. Thread Rolling Video Thread rolling contd…. Major diameter is always greater than the diameter of the blank Blank diameter is little larger than the pitch diameter of the thread. Restricted to ductile materials. Manufacture of gears by rolling The straight and helical teeth of disc or rod type external steel gears of small to medium diameter and module are generated by cold rolling. High accuracy and surface integrity. Employed for high productivity and high quality. (costly machine) Larger size gears are formed by hot rolling and then finished by machining. Fig. Production of teeth of spur gears by rolling Roll piercing
- 6. 14-08-2015 6 It is a variation of rolling called roll piercing. The billet or round stock is rolled between two rolls, both of them rotating in the same direction with their axes at an angle of 4.5 to 6.5 degree. These rolls have a central cylindrical portion with the sides tapering slightly. There are two small side rolls, which help in guiding the metal. Because of the angle at which the roll meets the metal, it gets in addition to a rotary motion, an additional axial advance, which brings the metal into the rolls. This cross-rolling action makes the metal friable at the centre which is then easily pierced and given a cylindrical shape by the central-piercing mandrel. Force and deflection reduces Use in high strength material and cold working process Backing rolls Backing rolls En sab tam jham sae Buckling nhi hoti Use for Supper alloy and cold working process Advantage of small roller in 4-High or cluster A small roll diameter is advantageous because less roll is in contact with the material which results in a lower FORCE and ENERGY requirement. P=F/A A B 1. Same material 2. Same final Height
- 7. 14-08-2015 7 Let pressure required for final reduction 10N/m2 For Fig A For Fig. B F=100N A= 10m2 P=F/A=100/10 = 10N/m2 F=? Since roller size is small, let it will be 5m2 P=F/A 10=F/5 =>F=50N Force reduce from 100N to 50N conclusion: Area involment is very-2 high in bigger roller compare to smaller roller. Power supply always given to working roller, back up roller rolls due to Friction Problem with small roller Reduction of stiffness using Back UP roller How to overcome Planetary mill Consist of a pair of heavy backing rolls surrounded by a large number of planetary rolls. Each planetary roll gives an almost constant reduction to the slab as it sweeps out a circular path between the backing rolls and the slab. As each pair of planetary rolls ceases to have contact with the work piece, another pair of rolls makes contact and repeat that reduction. The overall reduction is the summation of a series of small reductions by each pair of rolls. Therefore, the planetary mill can reduce a slab directly to strip in one pass through the mill. The operation requires feed rolls to introduce the slab into the mill, and a pair of planishing rolls on the exit to improve the surface finish. Flatness The roll gap must be perfectly parallel to produce sheets/plates with equal thickness at both ends a b
- 8. 14-08-2015 8 Camber Camber can be used to correct the roll deflection (at only one value of the roll force). Lubrication for Rolling Hot rolling of ferrous metals is done without a lubricant. Hot rolling of non-ferrous metals a wide variety of compounded oils, emulsions and fatty acids are used. Cold rolling lubricants are water-soluble oils, low- viscosity lubricants, such as mineral oils, emulsions, paraffin and fatty acids. Defects in Rolling Defects What is Cause Surface Defects Scale, rust, scratches, pits, cracks Inclusions and impurities in the materials Wavy edges Strip is thinner along its edges than at its centre. Due to roll bending edges elongates more and buckle. Alligatoring Edge breaks Non-uniform deformation
- 9. 14-08-2015 9 GATE – 2009 (PI) Anisotropy in rolled components is caused by (a) changes in dimensions (b) scale formation (c) closure of defects (d) grain orientation Formula in Rolling Geometry of Rolling Process The largest angle that will just grip a lump between the jaws, rolls, or mantle and ring of a crusher. Also know n as angle of bite; nip. Angle of nip = 2 α Draft Total reduction or “draft” taken in rolling. Usually, the reduction in blooming mills is about 100 mm and in slabbing mills, about 50 to 60 mm. Maximum Draft Possible 0 f h=h -h =2(R-Rcos )=D(1-cos ) 2 max h = R Numberof pass needed
- 10. 14-08-2015 10 Therefore Free engagement/for unaided entery will occur when µ > tan α Continuity Equation o Generally rolling increases the work width from an initial value of bo to a final one of bf and this is called spreading. o The inlet and outlet volume rates of material flow must be the same, that is, hobovo = hfbfvf where vo and vf are the entering and exiting velocities of the work GATE‐2014 A mild steel plate has to be rolled in one pass such that the final plate thickness is 2/3rd of the initial thickness, with the entrance speed of 10 m/min and roll diameter of 500 mm. If the plate widens by 2% during rolling, the exit velocity (in m/min) is …… ……… 14.706 Torque and Power Where λ is 0.5 for hot-rolling and 0.45 for cold-rolling. The total rolling load is distributed over the arc of contact in the typical friction-hill pressure distribution. However the total rolling load can be assumed to be concentrated at a point along the act of contact at a distance a from the line of centres of the rolls. The ratio of the moment arm a to the projected length of the act of contact Lp can be given as The torque MT is equal to the total rolling load P multiplied by the effective moment arm a. Since there are two work rolls, the torque
- 11. 14-08-2015 11 Calculation Projected length (Lp) = R sinα = √RΔh, mm Projected Area(Ap)= Lp × b, mm2 Roll Separating Force (F )= σ0 × Lp × b, N Armlength (a in mm)=0.5 × Lp for hot rolling =0 45 × Lp for cold rolling Torque per roller (T )= F ×a/1000, Nm Total power for two roller=2Tω, in W In rolling a strip between two rolls, the position of the neutral point in the arc of contact does not depend on Material of the rolls GATE‐1990 (PI) While rolling a strip the peripheral velocity of the roll is ….A…..than the entry velocity of the strip and is ……B …..the exit velocity of the strip. (a) less than/greater less (b) Greater than/less than GATE-2008 In a single pass rolling operation, a 20 mm thick plate with plate width of 100 mm, is reduced to 18 mm. The roller radius is 250 mm and rotational speed is 10 rpm. The average flow stress for the plate material is 300 MPa. The power required for the rolling operation in kW is closest to (a) 15.2 (b) 18.2 (c) 30.4 (d) 45.6 GATE-2007 The thickness of a metallic sheet is reduced from an initial value of 16 mm to a final value of 10 mm in one single pass rolling with a pair of cylindrical rollers each of diameter of 400 mm. The bite angle in degree will be (a) 5.936 (b) 7.936 (c) 8.936 (d) 9.936
- 12. 14-08-2015 12 GATE-2004 In a rolling process, sheet of 25 mm thickness is rolled to 20 mm thickness. Roll is of diameter 600 mm and it rotates at 100 rpm. The roll strip contact length will be (a) 5 mm (b) 39 mm (c) 78 mm (d) 120 mm GATE – 2012 Same Q in GATE – 2012 (PI) In a single pass rolling process using 410 mm diameter steel rollers, a strip of width 140 mm and thickness 8 mm undergoes 10% reduction of thickness. The angle of bite in radians is (a) 0.006 (b) 0.031 (c) 0.062 (d) 0.600 GATE-2006 A 4 mm thick sheet is rolled with 300 mm diameter rolls to reduce thickness without any change in its width. The friction coefficient at the work-roll interface is 0.1. The minimum possible thickness of the sheet that can be produced in a single pass is (a) 1.0 mm (b) 1.5 mm (c) 2.5 mm (d) 3.7 mm GATE – 2011 (PI) The thickness of a plate is reduced from 30 mm to 10 mm by successive cold rolling passes using identical rolls of diameter 600 mm. Assume that there is no change in width. If the coefficient of friction between the rolls and the work piece is 0.1, the minimum number of passes required is (a) 3 (b) 4 (c) 6 (d) 7 IES – 2001 A strip is to be rolled from a thickness of 30 mm to 15 mm using a two-high mill having rolls of diameter 300 mm. The coefficient of friction for unaided bite should nearly be (a) 0.35 (b) 0.5 (c) 0.25 (d) 0.07 IES – 1993 In order to get uniform thickness of the plate by rolling process, one provides (a) Camber on the rolls (b) Offset on the rolls (c) Hardening of the rolls (d) Antifriction bearings Ans. (a)
- 13. 14-08-2015 13 IES – 1993 The blank diameter used in thread rolling will be (a) Equal to minor diameter of the thread (b) Equal to pitch diameter of the thread (c) A little large than the minor diameter of the thread (d) A little larger than the pitch diameter of the thread Ans. (d) IES – 1992 Thread rolling is restricted to (a) Ferrous materials (b) Ductile materials (c) Hard materials (d) None of the above Ans. (b) IAS – 2001 Consider the following characteristics of rolling process: 1. Shows work hardening effect 2. Surface finish is not good 3. Heavy reduction in areas can be obtained Which of these characteristics are associated with hot rolling? (a) 1 and 2 (b) 1 and 3 (c) 2 and 3 (d) 1, 2 and 3 Ans. (c) IAS – 2000 Rolling very thin strips of mild steel requires (a) Large diameter rolls (b) Small diameter rolls (c) High speed rolling (d) Rolling without a lubricant Ans. (b) IAS – 2003 In one setting of rolls in a 3-high rolling mill, one gets (a) One reduction in thickness (b) Two reductions in thickness (c) Three reductions in thickness (d) Two or three reductions in thickness depending upon the setting IAS – 2007 Consider the following statements: Roll forces in rolling can be reduced by 1. Reducing friction 2. Using large diameter rolls to increase the contact area. 3. Taking smaller reductions per pass to reduce the contact area. Which of the statements given above are correct? (a) 1 and 2 only (b) 2 and 3 only (c) 1 and 3 only (d) 1, 2 and 3 Ans. (c)
- 14. 14-08-2015 14 GATE 2011 The maximum possible draft in cold rolling of sheet increases with the (a) increase in coefficient of friction (b) decrease in coefficient of friction (c) decrease in roll radius (d) increase in roll velocity Ans. (a)