me 612 metal forming and theory of plasticity 4...

4. Factors Effecting Work Hardening Characteristics

Yrd.Doç.Dr. Ahmet Zafer Şenalpe‐mail: [email protected]

Makine Mühendisliği Bölümü

Gebze Yüksek Teknoloji Enstitüsü

ME 612Metal Forming and Theory of Plasticity

mailto:[email protected]

4.1. Recrystallization

After cold plastic deformation grain structure of material changes, internal stressesand anisotropy occurs, mechanical and physical properties change.

With annealing the properties of the material before forming can be regained.Crystallization temperature is called temperature at which this process is completed inone hour.

If melting temperature of the metal is Te ( Kelvin) recrystallization temperature isapproximately 0.4xTe ( Kelvin).

Some materials can even crystallize at room temperature. For example lead, tin, zincand cadmium recrystallize at room temperature.

Dr. Ahmet Zafer Şenalp ME 612

2GYTE‐Makine Mühendisliği Bölümü

4. Factors Effecting WorkHardening Characteristics

4.1. Recrystallization

With annealing a cold formed material under recrystallization temperature internalstresses can be revealed. At this time hardness does not change and microstructuredoes not change. But physical quantities change back to original state before theforming process. This is called recovery.




4.2. Cold, Warm and Hot Forming

If a plastic deformation occurs under recrystallization temperature it is called coldforming else called hot forming.

Forming at below recrystallization temperature but above room temperature is calledwarm forming.

In cold forming crystal structure and grain continuously disrupted, hardness andstrength values increase (work hardening), ductility and electrical conductivitydecrease.

The forces needed in cold forming are higher than in hot forming. In contrast to this incold forming better dimensional tolerances and better surface quality is obtainedcompared to hot forming.




In cold forming the decrease of ductility can cause material to damage beforereaching to the desired form (Figure 4.1). In this case after a certain deformationannealing is applied. After than cold working can be continued. During the productionseveral annealing processes may be necessary.





Figure 4.1. The effect of cold and hot forming . (Elasticity modulus does not change)


In warm forming recrystallization is not observed but less force is required comparedto cold forming and material damage risk decreases.

Hot formed materials have higher dimensional tolerances compared to cold formedmaterials. In addition to that heating expenditures increase production cost. In hotforming materials are coated with oxide. The thickness of this coating can bedecreased by controlling the heating furnace. During forming process the oxides canresult poor surface quality.





In metals strain rate sensitivity (m) increases with temperature. Hence the increase oftemperature decreases forming force and increases m value. Increasing m valueincraeses forming force.

Metals like lead, tin and zinc which recrystallize at room temperature do not workharden at this temperature. So rigid perfectly plastic material model is used for thesecases. But it is important that these metals are very sensitive to strain rate at roomtemperature.

If plastic deformation is applied in certain period of temperature and time it can yieldto high strength properties for steels. For this time‐ temperature conversion diagramsare used. This process is call thermomechanical process.




4.3. Transition Temperature and Creep

In body‐centered cubic and some hexagonal closed‐packed metals toughness dependson temperature firmly and in a narrow band of temperature ductile fracture changesto brittle fracture.

This situation is not seen on nickel, copper, aluminum, austenitic steel which are face‐centered cubic. Transition from ductile to brittle is called below transitiontemperature and transition from brittle to ductile is called above transitiontemperature. If below or above are not mentioned average value for transitiontemperature is used.

Transition temperature depends on factors such as composition, micro structure,grain size, the status of the surface and part geometry. Strain rate also effectstransition temperature. High strain rates, sharp changes in the geometry and surfacetick marks increase transition temperature.





Creep deformation: Deformation that occurs in high temperature under constantstress state. Important in nuclear stations, turbines... .

Creep: The deformation criteria that even occurs under constant stress statedepending on temperature. Depends on material’s recrystallization temperature. Atthis temperature internal stresses reveal. For tin creep occurs even at roomtemperature.








Figure 4.2. Transition temperature of some metall and alloys, (p,r) rupture contraction in simple tensioon; (p,e) Rupture elongation in simple tension;(c) Charpy sharp notch

breaking energy.

4.4.The Effect of Temperatureto Material Properties

Generally increase of temperature increases ductility and toughness, decreaseselasticity modulus, yield point and tensile strength. The effect of temperature tomaterial properties can be seen in the figure below.




Figure 4.3. The effect of temperature to the engineering stress engineering strain graph

4.4. The Effect of Temperatureto Material Properties




Figure 4.4. Stress‐strain curves obtained at different temperatures. ( 1/s)310ε −≈&





Figure 4.5. The effect of temperature to elasticity modulus


Work hardening power is also effected from temperature. The increase oftemperature causes work hardening power to decrease.




Figure 4.6. The effect of temperature to work hardening power. Materail: pure aluminium.

4.5. Effect of Strain Rate to Material Properties

Strain rate shows different characteristics in every metal forming operation. Forexample in press works strain rate is comparatively low whereas in operations withhigh energy higher strain rates are observed.True strain rate;

(unit: time‐1)and engineering strain rate;

was previously defined.

For compression process for constant press speed increasing strain rate is obtained. Inorder to keep strain rate constant the speed of the press should be decreased. Fortension the reverse is valid. This extraction is obtained by investigating true strain rateequation. Yield point and tensile strength increases with increasing strain rate.




/ 1dH H dH Vdt H dt H

ε = = − = −&

0

0 0

/ 1dH H dH Vedt H dt H

= = − = −&

(4.1)

(4.2)





Figure 4.7. The effect of strain rate to yield point





Figure 4.8. Stress‐strain curves obtained at diffeerent strain rates. (10000C)


Work hardening power; n decreases with increasing strain rate.




Figure 4.9. The effect of strain rate to work hardening powerCold rolled steel

(A. Sexana ‐ D. A. Chatfıeld, SAE Paper 760209, 1976).


The effect of strain rate to strenght under constant temperature and strain is given by

relation. Here;

C is a material constant similar to K strebght coefficientm is strain rate sensitivity power.Increase of temperature causes m value to increaseIn cold forming m < 0.05,In hot forming m = 0.05...0.4,For superplastic materials m = 0.3...0.85




mCσ ε= && (4.3)


Superplastic materials have the ability to elongate uniformly with a large amountwithout rupture. For example a lead‐tin alloy uniform elongation value is % 4850.(Taplin, D.M.R., Dunlap, G.L., Langdon, T.G.: "Flow and Failure of SuperplasticMaterials," Ann. Rev. Mater. Sci., 9, 1979, pp. 151‐189).

The examples of superplastic materials are hot glass and polymers, very fine grain zinc‐aluminum and titanium alloys.








Figure 4.10. The effect of m value to percent rupture elongation in tensile test. Temperature range 20‐1000 C

(D. Lee ‐W. A. Backofen, Trans. AIME, vol. 239, 1967, pp. 1034‐1040).


In tensile test m value has an important effect on contraction. Experimentalobservations show that for high m value material elongates in a high amount beforethe rupture. This means high m value delays contraction. At the starting of contractionin this region strength is higher compared to other regions due to work hardening.

As in contraction region elongation is faster, strain rate is higher compared to other regions of the workpiece. This is a factor that increases the strength of contraction region. In contraction region the increase of material strength will obstructcontraction occurrence. As a result high m value will delay contraction occurrence and increase total elongation amount before the rupture.








Figure 4.11. The change of m value with yield point in hot and cold rolled low carbon steels. (Room temperature)


In metals m value decreases with increasing strength. The effect of strain rate toductility is not easily investigated. However generally it can be said that withincreasing strain rate ductility decreases.




4.6. The Effect of Hyrodstatic Pressureto Material Properties

Bridgman’s tensile experiments up to, 25000 atmosphere hydrostatic pressure showsthat the effect of hydrostatic pressure to yield point can be neglected unless very highpressures are applied.

The most important effect of hydrostatic pressure is the increase of ductility andhence obtaining large deformations before rupture. Hydrostatic pressure do not havean effect on uniform elongation that occurs until the beginning of contraction and onmaximum load.

The metals under hydrostatic pressure are experimentally observed that mechanicalproperties do not change.




4.6. The Effect of Hyrodstatic Pressureto Material Properties




Figure 4.12. The effect of hydostatic pressure to true stress‐true strain curves

me 612 metal forming and theory of plasticity 4...

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