4. factors effecting work hardening characteristics e-mail: assoc.prof.dr. ahmet zafer Şenalp...

4. 4. Factors Effecting Factors Effecting WorkWork Hardening Characteristics Hardening Characteristics

Assoc.Prof.Dr. Ahmet Zafer Şenalpe-mail: e-mail: [email protected]@gmail.com

Mechanical Engineering DepartmentGebze Technical University

ME 612ME 612 Metal Forming and Theory of Plasticity Metal Forming and Theory of Plasticity

mailto:[email protected]

4.1. Recrystallization4.1. Recrystallization

After cold plastic deformation grain structure of material changes, internal stresses and 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 in one hour.

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

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

Dr. Ahmet Zafer Şenalp ME 612

2Mechanical Engineering Department, GTU

4. Factors Effecting Work 4. Factors Effecting Work Hardening Characteristics Hardening Characteristics

4.1. Recrystallization4.1. Recrystallization

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




4.2. Cold, Warm and Hot Forming4.2. Cold, Warm and Hot Forming

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

Forming at below recrystallization temperature but above room temperature is called warm forming.

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

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




In cold forming the decrease of ductility can cause material to damage before reaching to the desired form (Figure 4.1). In this case after a certain deformation annealing is applied. After than cold working can be continued. During the production several 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 compared to cold forming and material damage risk decreases.

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





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

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

If plastic deformation is applied in certain period of temperature and time it can yield to high strength properties for steels. For this time- temperature conversion diagrams are used. This process is call thermomechanical process.




4.3. Transition Temperature and Creep4.3. Transition Temperature and Creep

In body-centered cubic and some hexagonal closed-packed metals toughness depends on temperature firmly and in a narrow band of temperature ductile fracture changes to 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 transition temperature and transition from brittle to ductile is called above transition temperature. If below or above are not mentioned average value for transition temperature 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 effects transition temperature. High strain rates, sharp changes in the geometry and surface tick marks increase transition temperature.





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

Creep: The deformation criteria that even occurs under constant stress state depending on temperature. Depends on material’s recrystallization temperature. At this temperature internal stresses reveal. For tin creep occurs even at room temperature.








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 Temperature 4.4.The Effect of Temperature to Material Propertiesto Material Properties

Generally increase of temperature increases ductility and toughness, decreases elasticity modulus, yield point and tensile strength. The effect of temperature to material 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 Temperature 4.4. The Effect of Temperature to Material Propertiesto Material Properties




Figure 4.4. Stress-strain curves obtained at different temperatures. ( 1/s) 310





Figure 4.5. The effect of temperature to elasticity modulus


Work hardening power is also effected from temperature. The increase of temperature 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 Properties4.5. Effect of Strain Rate to Material Properties

Strain rate shows different characteristics in every metal forming operation. For example in press works strain rate is comparatively low whereas in operations with high 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. In order to keep strain rate constant the speed of the press should be decreased. For tension the reverse is valid. This extraction is obtained by investigating true strain rate equation. Yield point and tensile strength increases with increasing strain rate.




/ 1dH H dH V

dt H dt H

0

0 0

/ 1dH H dH Ve

dt 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




mC (4.3)


Superplastic materials have the ability to elongate uniformly with a large amount without 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 Superplastic Materials," 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. Experimental observations show that for high m value material elongates in a high amount before the rupture. This means high m value delays contraction. At the starting of contraction in 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 obstruct contraction 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 to ductility is not easily investigated. However generally it can be said that with increasing strain rate ductility decreases.




4.6. The Effect of Hyrodstatic Pressure 4.6. The Effect of Hyrodstatic Pressure to Material Propertiesto Material Properties

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

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

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




4.6. The Effect of Hyrodstatic Pressure 4.6. The Effect of Hyrodstatic Pressure to Material Propertiesto Material Properties




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