4. Factors Effecting Work Hardening Characteristics Assoc.Prof.Dr. Ahmet Zafer Şenalp Mechanical Engineering Department Gebze Technical University ME 612 Metal Forming and Theory of Plasticity

4.1. 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 Hardening Characteristics

4.1. 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. Dr. Ahmet Zafer Şenalp ME 612 3Mechanical Engineering Department, GTU 4. Factors Effecting Work Hardening Characteristics

4.2. 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. Dr. Ahmet Zafer Şenalp ME 612 4Mechanical Engineering Department, GTU 4. Factors Effecting Work Hardening Characteristics

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 Cold, Warm and Hot Forming Dr. Ahmet Zafer Şenalp ME 612 5Mechanical Engineering Department, GTU 4. Factors Effecting Work Hardening Characteristics Figure 4.1. The effect of cold and hot forming. (Elasticity modulus does not change)

4.2. Cold, Warm and Hot Forming 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. Dr. Ahmet Zafer Şenalp ME 612 6Mechanical Engineering Department, GTU 4. Factors Effecting Work Hardening Characteristics

4.2. Cold, Warm and Hot Forming 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. Dr. Ahmet Zafer Şenalp ME 612 7Mechanical Engineering Department, GTU 4. Factors Effecting Work Hardening Characteristics

4.3. 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. Dr. Ahmet Zafer Şenalp ME 612 8Mechanical Engineering Department, GTU 4. Factors Effecting Work Hardening Characteristics

4.3. Transition Temperature and Creep 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. Dr. Ahmet Zafer Şenalp ME 612 9Mechanical Engineering Department, GTU 4. Factors Effecting Work Hardening Characteristics

4.3. Transition Temperature and Creep Dr. Ahmet Zafer Şenalp ME Mechanical Engineering Department, GTU 4. Factors Effecting Work Hardening Characteristics 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 to 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. Dr. Ahmet Zafer Şenalp ME Mechanical Engineering Department, GTU 4. Factors Effecting Work Hardening Characteristics Figure 4.3. The effect of temperature to the engineering stress engineering strain graph

4.4. The Effect of Temperature to Material Properties Dr. Ahmet Zafer Şenalp ME Mechanical Engineering Department, GTU 4. Factors Effecting Work Hardening Characteristics Figure 4.4. Stress-strain curves obtained at different temperatures. ( 1/s)

4.4. The Effect of Temperature to Material Properties Dr. Ahmet Zafer Şenalp ME Mechanical Engineering Department, GTU 4. Factors Effecting Work Hardening Characteristics Figure 4.5. The effect of temperature to elasticity modulus

4.4. The Effect of Temperature to Material Properties Work hardening power is also effected from temperature. The increase of temperature causes work hardening power to decrease. Dr. Ahmet Zafer Şenalp ME Mechanical Engineering Department, GTU 4. Factors Effecting Work Hardening Characteristics 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. 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. Dr. Ahmet Zafer Şenalp ME Mechanical Engineering Department, GTU 4. Factors Effecting Work Hardening Characteristics (4.1) (4.2)

4.5. Effect of Strain Rate to Material Properties Dr. Ahmet Zafer Şenalp ME Mechanical Engineering Department, GTU 4. Factors Effecting Work Hardening Characteristics Figure 4.7. The effect of strain rate to yield point

4.5. Effect of Strain Rate to Material Properties Dr. Ahmet Zafer Şenalp ME Mechanical Engineering Department, GTU 4. Factors Effecting Work Hardening Characteristics Figure 4.8. Stress-strain curves obtained at diffeerent strain rates. ( C)

4.5. Effect of Strain Rate to Material Properties Work hardening power; n decreases with increasing strain rate. Dr. Ahmet Zafer Şenalp ME Mechanical Engineering Department, GTU 4. Factors Effecting Work Hardening Characteristics Figure 4.9. The effect of strain rate to work hardening power Cold rolled steel (A. Sexana - D. A. Chatfıeld, SAE Paper , 1976).

4.5. Effect of Strain Rate to Material Properties 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 coefficient m is strain rate sensitivity power. Increase of temperature causes m value to increase In cold forming m < 0.05, In hot forming m = , For superplastic materials m = Dr. Ahmet Zafer Şenalp ME Mechanical Engineering Department, GTU 4. Factors Effecting Work Hardening Characteristics (4.3)

4.5. Effect of Strain Rate to Material Properties Superplastic materials have the ability to elongate uniformly with a large amount without rupture. For example a lead-tin alloy uniform elongation value is % (Taplin, D.M.R., Dunlap, G.L., Langdon, T.G.: "Flow and Failure of Superplastic Materials," Ann. Rev. Mater. Sci., 9, 1979, pp ). The examples of superplastic materials are hot glass and polymers, very fine grain zinc -aluminum and titanium alloys. Dr. Ahmet Zafer Şenalp ME Mechanical Engineering Department, GTU 4. Factors Effecting Work Hardening Characteristics

4.5. Effect of Strain Rate to Material Properties Dr. Ahmet Zafer Şenalp ME Mechanical Engineering Department, GTU 4. Factors Effecting Work Hardening Characteristics Figure The effect of m value to percent rupture elongation in tensile test. Temperature range °C (D. Lee - W. A. Backofen, Trans. AIME, vol. 239, 1967, pp ).

4.5. Effect of Strain Rate to Material Properties 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. Dr. Ahmet Zafer Şenalp ME Mechanical Engineering Department, GTU 4. Factors Effecting Work Hardening Characteristics

4.5. Effect of Strain Rate to Material Properties Dr. Ahmet Zafer Şenalp ME Mechanical Engineering Department, GTU 4. Factors Effecting Work Hardening Characteristics Figure The change of m value with yield point in hot and cold rolled low carbon steels. (Room temperature)

4.5. Effect of Strain Rate to Material Properties 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. Dr. Ahmet Zafer Şenalp ME Mechanical Engineering Department, GTU 4. Factors Effecting Work Hardening Characteristics

4.6. The Effect of Hyrodstatic Pressure to Material Properties Bridgman’s tensile experiments up to, 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. Dr. Ahmet Zafer Şenalp ME Mechanical Engineering Department, GTU 4. Factors Effecting Work Hardening Characteristics

4.6. The Effect of Hyrodstatic Pressure to Material Properties Dr. Ahmet Zafer Şenalp ME Mechanical Engineering Department, GTU 4. Factors Effecting Work Hardening Characteristics Figure The effect of hydostatic pressure to true stress-true strain curves