Wire Drawing

Download Wire Drawing

Post on 22-Nov-2014

804 views

Category:

Documents

6 download

Embed Size (px)

TRANSCRIPT

<p>Suranaree University of Technology Jan-Mar 2007Drawing of rods, wires Drawing of rods, wires and tubesand tubes Introduction/objectives Rod and wiredrawing Analysis of wiredrawing Tube drawing processes Analysis of tube drawing Residual stress in rod, wire and tubesChapter 5Subjects of interestTapany UdompholSuranaree University of Technology Jan-Mar 2007ObjectivesObjectives This chapter provides fundamental background on processes of drawing of rods, wires and tubes. Mathematical approaches for the calculation of drawing load will be introduced. Finally drawing defects occurring during the process will be highlighted and its solutions will be included.Tapany UdompholSuranaree University of Technology Jan-Mar 2007Introduction : wire drawing Wire drawing involves reducing the diameter of a rod or wire by passing through a series of drawing dies or plates. The subsequent drawing die must have smaller bore diameter than the previous drawing die.www.e6.com UndrawnwireDrawn wireDrawing diehttp://en.wikipedia.orgTapany UdompholSuranaree University of Technology Jan-Mar 2007Introduction : Tube drawing Tube drawing involves reducing the cross section and wall thickness through a draw die.Brass tubes for heat exchanger cheap, strong, good corrosion resistant The cross section can be circular, square hexagonal or in any shapes.Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Drawing operations involve pulling metal through a die by means of a tensile force applied to the exit side of the die. The plastic flow is caused by compression force, arising from the reaction of the metal with the die. Starting materials: hot rolled stock (ferrous) and extruded (non-ferrous). The metal usually has a circular symmetry (but not always, depending on requirements). Bar wire and tube drawing are usually carried out at room temperature, except for large deformation, which leads to considerable rise in temperature during drawing. Introduction Material should have high ductility and good tensile strength.Tapany UdompholSuranaree University of Technology Jan-Mar 2007Rod and wiredrawingRod and wiredrawing Same principals for drawing bars, rods, and wire but equipment is different in sizes depending on products.Rods relatively larger diameter products. Metal wiresMetal rods Reducing the diameter through plastic deformation while the volume remains the same.Wires small diameter products &lt; 5 mm diameter.Tapany UdompholSuranaree University of Technology Jan-Mar 2007 Rods which can not be coiled, are produced on drawbenches.Insert though the dieRod is swagedClamped to the jaws of the drawheadThe drawhead is moved by a hydraulic mechanismMachine capacity : 1 MN drawbench 30 m of runout 150-1500 mm.s-1draw speedRod drawingDraw headMetal stockTapany UdompholWire drawing dieConical drawing die Shape of the bell causes hydrostatic pressure to increase and promotes the flow of lubricant into the die. The approach angle where the actual reduction in diameter occurs, giving the half die angle . . . . The bearing region produces a frictional drag on the wire and also remove surface damage due to die wear, without changing dimensions. Suranaree University of Technology Jan-Mar 2007 The back relief allows the metal to expand slightly as the wire leaves the die and also minimises abrasion if the drawing stops or the die is out of alignment. The die nib made from cemented carbide or diamond is encased for protection in a thick steel casing.Drawing dieTapany UdompholExample of wiredrawing diesA drawing of wire drawing dieSuranaree University of Technology Jan-Mar 2007UndrawnwireDrawn wireDrawing diehttp://en.wikipedia.orgTapany UdompholExample of wiredrawing diesWire drawing die made from cemented tungsten carbide with polycrystalline diamond core.Suranaree University of Technology Jan-Mar 2007 Tapany UdompholSuranaree University of Technology Jan-Mar 2007Drawing die materials Most drawing dies are cemented carbide or industrial diamond (for fine wires). Cemented carbides are the most widely used for drawing dies due to their superior strength, toughness, and wear resistance. Polycrystalline Diamond (PCD)used for wire drawing dies for fine wires. Longer die life, high resistance to wear, cracking or bearing. img.tradekey.comhttp://www.scob.de Cemented carbide is composed of carbides of Ti, W, Ni, Mo, Ta, Hf.Tapany UdompholSuranaree University of Technology Jan-Mar 2007Wire drawing equipment The wire is first passed through the overhead loop and pulley, brought down and then inserted through the die of the second drum and drawn through this die for further reduction. Thus, the wire is drawn through all the wire drawing drums of the set in a continuous manner to get the required finished diameter of the wire. Speedof each draw block has to be synchronised to avoid slippage between the wire and the block. The drawing speed ~ up to 10 m.s-1for ferrous drawing~ up to 30 m.s-1for nonferrous drawing.Bull block drawing machines Multiple bull block machines - commonTapany UdompholSide view of bull blockcoildiewireTop viewBull blockHot rolled rodPickling, descalingLubricatingRemove scale -causing surface defects. Cu and Sn are used as lubricants for high strength materials. Or conversion coating such as sulphates or oxalates. Oils and greases for wire drawing Mulsifiable oils for wet wire drawing Soap for dry drawing. drawing Area reduction per drawing pass is rarely greater than 30-35%. Bull block drawing allows the generation of long lengthsSuranaree University of Technology Jan-Mar 2007Wire drawing process100 1 % 2</p> <p>|||</p> <p>\| =InletOutletDDRATapany UdompholSuranaree University of Technology Jan-Mar 2007Example: Drawing of stainless wire Larger diameter stainless wire is first surface examined, tensile and hardness tested, diameter size measured. Surface preparation by pickling in acid (ferrictic and martensitic steels) and basic solutions (austenitic steels). The prepared skin is then coated with lubricant. Cold drawing is carried out through diamond dies or tungsten carbide dies till the desired diameter is obtained. Stainless steels: 304, 304L, 316, 316L Applications: redrawing,mesh weaving, soft pipe,steel rope, filter elements,making of spring.Stainless steel ropeStainless steel mesheswww.metalwire-mesh.comwww.metalwiremesh.com Cleaning off oil/lubricant is then carried out and the wire is heat-treated (annealing at about 1100oC or plus skin pass).Tapany UdompholSuranaree University of Technology Jan-Mar 2007Stepped-cone multiple-pass wiredrawing More economical design. Use a single electrical motor to drive a series of stepped cones. The diameter of each cone is designed to produce a peripheral speed equivalent to a certain size reduction.Tapany UdompholSuranaree University of Technology Jan-Mar 2007Heat treatments Nonferrous wire / low carbon steel wire Tempering (ranging from dead soft to full hard). This also depends on the metal andthe reduction involved. Steels (C content &gt; 0.25%) normally 0.3-0.5% requirePatenting heat treatment before being drawn. Patented wire have improved reduction of area up to 90% due to the formation of very fine pearlite.Heating above the upper critical temp T~970oCCooling in a lead bath at T~315oCGood combination of strength and ductility. Provide austenitic structurewith rather large grain size. Rapid cooling plus small cross section of wire change microstructure to very fine pearlitepreferably with no separation of primary ferrite.Tapany UdompholSuranaree University of Technology Jan-Mar 2007Defects in rod and wiredrawingDefects in rod and wiredrawingDefects in the starting rod (seams, slivers and pipe).Defects from the deformation process, i.e., centre burst or chevron cracking (cupping).Centre burst or chevron cracks This defect will occur for low die angles at low reductions. For a given reduction and die angle, the critical reduction to prevent fracture increases with the friction.Tapany UdompholSuranaree University of Technology Jan-Mar 2007Analysis of wiredrawingAnalysis of wiredrawingFrom the uniform-deformation energy method, a draw stress is given byr AAoaboxa= =11ln ln_ Eq.1(This however ignore friction, transverse stress and redundant deformation.)Tapany UdompholSuranaree University of Technology Jan-Mar 2007Consider the problem of strip drawing of a wide sheet, (Dieter p. 509) A wide strip is being drawn through a frictionless die with a total included angle of 2 . Plane strain condition is applied (no strain in the width direction.)wha xadx 2 2 2 2 pxx = 0hbh x+d x xdh/2dh/2ppp cos dxdx/cos The equilibrium of forces in the x direction is made up of two components1) Due to the change in longitudinal stress with x increasing positively to the left.( )( ) hw w dh h d x x x + +2) Due to the die pressure at the two interfaces.||</p> <p>\|cossin 2 dxw pTaking the equilibrium of force in the x direction and neglect d xdh0 tan 2 = + + dx p hd dh x x Eq.2Tapany UdompholSuranaree University of Technology Jan-Mar 2007ha x x+ d xppu uu upu uu uphb Taking equilibrium of forces in the x direction Eq.2 then becomes0 ) (tan 2 0 2 tan 2= + + + = + + +u u pdx hd dh pdx dx p hd dhx xx xSince h = 2x tan , and dh = 2 dx tan . then 2dx = dh/tan 0 ) cot 1 ( = + + + dh p hd dh x x u dxWe now haveWe shall now consider the problem of strip drawing where a Coulomb friction coefficient u uu u exists between the strip and the die.The equilibrium now includes 2u uu updx.Eq.3Tapany UdompholSuranaree University of Technology Jan-Mar 2007Since the yield condition for plane strain is x+ p = oand B = u uu u cot , the differential equation for strip drawing ishdhB B do xx=+ ) 1 (' If B and oare both constant, Eq.4 can be integrated directly to give the draw stress xa.Eq.4| |BoBbao xa rBBhhBB) 1 ( 1111 ' ' +=</p> <p>|||</p> <p>\|+= For wiredrawing conducted with conical dies,</p> <p>|||</p> <p>\|+= Bbao xaDDBB 211 Eq.5Tapany UdompholSuranaree University of Technology Jan-Mar 2007Analysis for wiredrawing with friction by Johnson and RoweThe surface area of contact between the wire and the die is given by sin a b A AS =p is the mean normal pressure on this area.Pdis the draw force.PdSAbAau uu upS u uu upS cos pS sin pSBalancing the horizontal componentsof the frictional force and the normal pressure.( )( ) ) 1 )( ( 1 cot ) (sin cossin) sin cos (sin cos_ __ __ _B A A p A A p PA Ap S p PS p S p Pa b a b da bdd+ = + =+= + =+ = u u u uEq.6Eq.7Tapany UdompholSuranaree University of Technology Jan-Mar 2007In the absence of friction, B = 0 andabaoa b dAAA A A p P ln ) ( _ _ = =) 1 ( ln_BAAAPaboadxa + = = The draw stress with friction is given byEq.8Eq.9Tapany UdompholSuranaree University of Technology Jan-Mar 2007Example: Determine the draw stress to produce a 20% reduction in a 10-mm stainless steel wire. The flow stress is given by o= 1300 0.30(MPa). The die angle is 12oand u u u u = 0.09.MPanK rBno63730 . 1 ) 223 . 0 ( 13001223 . 02 . 0 1 1ln11ln8571 . 0 6 cot ) 09 . 0 ( cot30 . 01_1= =+===== = = umm r D D mm Db ab8 ) 8 . 0 ( 10 ) 1 (10= = ==MPa BAAaboxa 527 ) 8571 . 1 (8 . 00 . 1ln 637 ) 1 ( ln 22_= = + = | | MPaDDBB Bbaxa 438 8 . 0 18571 . 0 8571 . 1637 11 8571 . 0 22_= ||</p> <p>\|=</p> <p>|||</p> <p>\|||</p> <p>\| += ~ 20% differenceFrom Eq.5From Eq.9Tapany UdompholSuranaree University of Technology Jan-Mar 2007If the wire is moving through the die at 3 m.s-1, determine the power required to produce the deformation.timece disforce Power tan =Drawing forcekN A P a xa d 02 . 22 ) 8 (4438 2= = = PowerkW Power 06 . 66 3 02 . 22 = =Tapany UdompholSuranaree University of Technology Jan-Mar 2007If redundant work is included in Eq.9, the expression becomes) 1 ( ln BAAabo xa + = Where is a factor for the influence of redundant work, which can be defined as( ) *, = = r fWhere = the redundant work factor. * = the enhanced strain corresponding to the yield stress of the metal, which has been homogeneously deformed to a strain .Eq.10Eq.11Tapany UdompholSuranaree University of Technology Jan-Mar 2007Procedure for determining redundant deformation of drawn wireFlow curve for drawn wire Stress *=Basic flow curveAnnealedStrain The flow curve of a drawn wire is superimposed on the flow curve for the annealed metal. The origin of the curve for the drawn metal is displaced along the strain axis = drawing reduction, = ln (Ab/Aa) = ln [1/(1-r)]. Due to redundant work, the yield stress of the drawn metal is above the basic flow curve . To determine , the flow curve for the drawn metal is moved to the right to * where the curves coincide.Tapany UdompholSuranaree University of Technology Jan-Mar 2007For drawing of round wire | |22 1) 1 ( 1 rr + = Where = the approach semi-angle, in radiansr = the drawing reductionCommercially, is in the range 6 to 10oand r of about 20%.and the redundant work is related to by4 . 48 . 02 1 + + = C C = h/L = mean thickness / the length of the deformation zoneFor strip, sis based on a plane-strain reduction rs= 1-(h1/h0)For wire or rod, wis based on an axisymmeteric reduction rw= 1-(d1/d0)whereBased on deformation-zone geometryEq.12Eq.13Tapany UdompholSuranaree University of Technology Jan-Mar 2007The effect of die angle on the total energy required to cause deformationUTUP *UfUr UComponents of total energy of deformationIdeal work of plastic deformation UPindependent of die angle . Work to overcome friction Uf Redundant work UrThe summation of Up, Ufand Urgives the total energy UT.This has a minimum at some optimum die angle *.The reduction and the friction Tapany UdompholSuranaree University of Technology Jan-Mar 2007A1Ao dEntrance Exit = lnAo/A1Tapany UdompholSuranaree University of Technology Jan-Mar 2007Development of limit on drawabilityFlow curve xa d = = = = max d d1For steady-state wiredrawing xacan be expressed most simply by= dxa 1Where the efficiency of the deformation process, = Up/UTIdeal process (no friction/redundant work)Actual drawing processAt a given strain = ln (Ab/Aa) draw stress xaand the flow stress .As the material is being deformed through the die, strain hardening occurs and if the material is severely strain-hardened necking fracture.The drawing limit is reached when d= Eq.14strainstressTapany UdompholSuranaree University of Technology Jan-Mar 2007If the material follows a power-law hardening relationship = K n, then( ) ( ) 1 11+=+= +n nK nd Substituting the criterion for the maximum drawing strain in a single pass, that is, d= ,) 1 (max + = n Since = ln (Ab/Aa), ( ) 1max+=|||</p> <p>\| nabeAA And by the definition of the reduction r = 1- (Aa/Ab)) 1 (max 1 + = ne r For repeated reductions through a series of dies, n...</p>

Recommended

View more >