Lecture 22a Wire Drawing 3w2f48

WIRE DRAWING

• Method of producing wires from rods by pulling through stationary die is called Wire drawing.

• Die angle : 12° to 48°

• (12 for hard materials and 48 for soft materials) • L= length of approach (length of deformation zone)

Wire drawing in brief • The required deformation is obtained by drawing the wire through the conical bore section of the die, reducing the diameter through plastic deformation. • During deformation, a thin film of lubricant between wire surface and die surface is essential to minimise friction, to reduce die wear, and to keep the die cool. • For good wire deformation, it is necessary to select a drawing die tool with the appropriate profile, designed for either ferrous or non-ferrous materials.

• Entry zone:

• Friction force(in backward inclined direction) acts on interface when wire is pulled through the die • Since friction force is inclined, it has horizontal and vertical components • Horizontal- Increases force to be applied in front • Vertical- tries to cause failure of die • So, lubrication is introduced, and the material is guided to the deformation zone (approach) of the die. • Lubricants may be liquid(mineral oils, vegetable oils) or solid powders(graphite powder, glass powder)

• Approach zone or deformation zone:

• • • • • •

Required deformation for converting rod into wire is attained in this zone. Bearing zone(sizing zone) Converts elastic deformation present in material to plastic deformation. Exit zone(safety zone): The zone in which the deformed wire leaves the die tool Back relief, is provided to allow the wire to exit smoothly from the bearing of the die.

Drawing load calculation-Probem • Find the drawing load required for drawing of 60mm diameter steel rod to 40mm diameter. Coefficient of friction (µ) is given to be 0.2 and die angle is 20°. The yield stress of steel is 300MPa.

Wire area reduction • In products where subsequent draws are needed to reach the desired finish diameter an average area reduction per die of about 20-30% is usual. • Steel wire work hardens during plastic deformation and the ductility (the degree of elasticity) is reduced while the tensile strength increases. • The degree of total area reduction possible without intermediate annealing depends on the composition of the steel, i.e. the work hardening characteristic of the steel quality (=grade). • In general it is possible through subsequent or sequential es through smaller dies to reduce the cross section area of a wire between 85-95%. Further area reduction will require an intermediate anneal to restore ductility.

• • • • • • • •

•

Die Materials Overview Tungsten Carbide: Lowest cost, shock resistance, ease of production, large sizes available. Lower life expectancy. Natural Diamonds: Wear resistance, gives excellent wire surface, high thermal conductivity, longer life expectancy Susceptible to fractures from shock or wear, limited availability in required high quality and quantity, constantly escalating price. Polycrystalline Diamond: Excels in life expectancy, wear resistance of diamond, shock resistance of carbide, high availability, cost effectiveness Higher drawing force, may be damaged by temperatures above 700°C, wire surface condition less than from natural diamond.