Forging:
Shaping heated metal by applying sudden blow or steady pressure is called forging. It is done by making use of plastic characteristics of the material. Usually the workpiece is compressed between the two dies using impact or gradual pressure to form the part. Forging may be done either hot or cold. Forging is always stated as hot working unless stated other wise. Typical components made by forging are crank shafts and connecting rods for engines, gears, wheels, bolt heads and hand tools etc.
Advantages of Forging:
• Ability to withstand heavy or unpredictable loads.
• More strength per unit cross-sectional area under static loads, i.e. minimum weight per unit strength.
• Better shock resistance.
• Superior machining qualities.
• Forged product can welded easily.
• Impurities like inclusions gets broken up and distributed throughout the metal.
Limitations of Forging:
• Complex shapes and cored shapes are possible in casting process cannot be forged.
• Forgings costs more than castings.
• Scaling occurs on the surface of forgings due to oxidation at high temperatures.
Mechanical working of metals:
Metalworking process are classified as,
• Hot working process.
• Cold working process.
Hot Working:
It involves deforming a hot metal which is heated above recrystallization temperature [800°C for steel] but below the melting point [1350°C for steel]. It is the process carried out between solid and liquid state i.e., semi solid state. In hot working, the temperature at which the work is completed is most important. Since any extra heat after working will aid in grain growth thus giving poor mechanical properties. Requires less energy to deform than cold working. Hot metal easily deforms due to plasticity. It involves in refinement of coarse grain in to fine grain structure.
Advantages of Hot Working:
• Ductility and impact resistance are improved.
• Large deformations [change in shape and size] in the metal are easily obtained because no strain hardening.
• Porosity in metal is minimized.
• Process is rapid and requires less power.
• Brittle materials can also be hot worked.
• Very favourable grain size can be achieved.
Limitations Hot Working:
• Tooling and handling of hot metals are difficult and costly.
• Close tolerance on dimensions not possible.
• No dimensional stability.
Effects of Hot Working:
• Raising the metal temperature lowers the stresses required to produce deformations of metal.
• Hot working produces the same net results on a metal as cold working and annealing. It does not strain harden the metal.
• In hot working processes, compositional irregularities are ironed out and nonmetallic impurities are broken up into small, relatively harmless fragments, which are uniformly dispersed throughout the metal instead of being concentrated in large stress-raising metal working masses.
• Hot working such as rolling process refines grain structure. The coarse columnar dendrites of cast metal are refined to smaller equaled grains with corresponding improvement in mechanical properties of the component.
• Surface finish of hot worked metal is not nearly as good as with cold working, because of oxidation and scaling.
• Too high a temperature may cause phase change and overheat the steel whereas too low temperature may result in excessive work hardening.
• Defects in the metal such as blowholes, internal porosity and cracks get removed or welded up during hot working.
• During hot working, self-annealing occurs and recrystallisation takes place immediately following plastic deformation. This self-annealing action prevents hardening and loss of ductility.
• The various mechanical properties that can be improved are;
1. Ductility,
2. Malleability,
3. Hardness,
4.T oughness,
5. Strength.
Cold Working:
Deforming a metal to plastic stage at room temperature or below recrystallisation temperature is called cold working. The effect of cold working of steel is to distort or elongate the metal in the direction of applied stress. During the process, grain structure changes and results in fragmentation of grains. Greater loads are required to deform a metal. It does not get permanently deformed until the stress exceeds the elastic limit. Since there is no recrystallisation of grains during this process, no recovery from grain fragmentation takes place which leads to distortion. Residual stress are set up in the cold worked metal piece. Mostly employed as a finishing operation. Metals such as Mild steel, Copper, Bronze, Brass and Stainless steel are easily cold worked in the form of Plates and Sheets.
Advantages of Cold Working:
• Ultimate strength and yield strength increases.
• Hardness increases but ductility decreases.
• Improved surface finish and close tolerances can be obtained.
• Ideal method for increasing hardness of metals which cannot be hardened by heat treatment.
• Surface defects are removed.
• Suitable for mass production and automation, because of low working temperatures.
Limitations Cold Working:
• Only ductile metals can be cold worked.
• Over working of metals results in brittleness and requires annealing.
• Small diameter components are more suitable for cold working. Generally plates less than 5 mm are cold worked .
• Internal stress remains in the metal unless they are removed by annealing.
Effects of Cold Working:
• Grain structure is distorted.
• Increases corrosive resistance.
• Residual stresses are built up inside the metal.
• Heat treatment processes need to be conducted to remove residual stresses
• Strength and hardness increases.
• Ductility decreases.
• Surface finish of the product is improved.
• Close dimensional tolerances can be obtained.
• Recrystallisation temperature for steel is raised.
• Improve wear resistance.
• Directional properties can be imparted.
• Imparted directional properties may be determined
• Excessive cold working leads to crack formation.
• Resistance to corrosion decreases.
• Spring back is an lever present phenomenon.
• Impact strength decreases.
Drop Forging:
In drop forging, impression dies [closed] are used. The upper die is fitted on the friction roller on the ram and the lower die is fitted on the anvil and both dies have impressions. Two rollers are fixed on the board when both rolls rotate opposite to each other, it drives the board upward and lifting the ram. When the rolls are released the ram will falls down and, producing a working stroke. A single blow of press makes small and simple parts and large complicated shapes are made by number of steps. It is used for making: Spanner, Automobile parts, Machine parts etc.
Upset Forging:
A simple type of open die forging is called upsetting. In an upsetting process the work is placed between two flat die and its height is decreased by compressive forces exerted between the two die. Since the volume of a metal will remain constant throughout its deformation, a reduction in height will be accompanied by an increase in width. In real conditions during industrial manufacturing, friction plays a part in the process. Friction forces at the die-work interface oppose the spreading of the material near the surfaces, while the material in the center can expand more easily. The result is to create a barrel shape to the part. This effect is called barreling in metal forging terms.
Open Die Forging:
The work piece compressed between two flat dies. The metal flows without constraint in a lateral dimension relative to the die surfaces. [flat - die forging]
Closed Die Forging:
The ]ob is forced to fill the Cavities or impressions in the Dies, thus forming the required shape. Some times excess metal comes out from the cavity as flashes which is removed by trimming operation. Complex shapes cannot be forged directly from round or rectangular bar into die cavity. Therefore, some pre-forming operations required. Number of forging steps is dependent on the shape and size of the forged part.
Press Forging:
It is a slow squeezing action to shape the hot metal. Slow squeezing action penetrates completely through the metal. Press forging produce more uniform deformation and flow. This process is used for large pieces or thicker products.
Closed Die Forging Operations:
The various types of closed die forging operations are,
• Fullering impression,
• Edging impression,
• Bending impression,
• Blocking impression,
• Finishing impression.
Fullering Impression:
Since drop forging involves only a reduction in cross section with no upsetting, the very first step is to reduce the stock to the desired size. The impression machined in the die to achieve this is called fullering impression. Sometimes it may be possible to obtain this without a special die impression, in open dies.
Edging Impression:
Also called ‘preform’, this stage is required to gather the exact amount of material required at each cross section of the finished component. This is the most important stage in drop forging. Properly designed preform ensures a defect free flow of metal, complete die fill and minimum flash loss.
Bending Impression:
This is required for those parts, which have a bent shape. As shown in figure, the bent shapes can also be obtained without the bending impression, but then, the grain-flow direction will not follow the bent shape and thus the point of bend may become weak. To improve the grain flow, therefore, a bending impression is incorporated after edging impression.
Blocking Impression:
Blocking or semi-finishing impression, resembles final shape with liberal radii at corners. No gutter is provided in blocking. The area at each section is roughly 15 to 20% greater. The height of the blocked forging is large and the breadth is smaller by an amount of the order of 0.8 to 1.5 mm and length remains the same.
Finishing Impression:
The dimensions of the finishing impression are same as that of the final forging desired with the necessary allowances and tolerances. A gutter should be provided in the finishing impression.
Forgeable Materials:
Forgeability of a material is the ability of a material to undergo deformation under compression without rupture. Any metal or alloy which can be brought to plastic stage through heating can be forged. The extent to which a material can be forged is governed by its composition as well as the temperature of forging. Selection of a forging material depends on certain desirable mechanical properties inherent in the material like strength, malleability, resistance to fatigue, durability, shock or bending, machinability, etc. Any material having good malleability can be forged, since compressive forces are involved.
Types of Forging Presses:
Mechanical Press:
Mechanical presses belong to a class of machine tools that encompass a wide range of different machine types. The mechanical press transforms the rotational force of a motor into a translational force vector that performs the pressing action. Therefore, the energy in a mechanical press comes from the motor. These types of presses are generally faster than hydraulic press. In a mechanical press, the application of force varies in both speed and magnitude throughout the distance of the stroke. Mechanical press uses a crank link attached to a drive shaft. This crank link rotates with the drive shaft. The crank link is connected to a connecting rod by means of rotational joints This Crank link convert the rotary motion into linear motion. The connecting rod rocks back and forth during the motion of the crank. A ram is fixed to the bottom end of the connecting rod. This ram can slides up and down by the crank rotation. Through this path the crank press delivers its compression force.
Hydraulic Press:
The ram is actuated by the hydraulic system, that drives large amount of energy at a constant rate throughout the stroke. Press stops if the load required exceeds its capacity, hence they are load limited or load restricted. It depends on the principle of Pascal's law, the pressure exerted by a incompressible fluid in a closed system is constant throughout. The hydraulic press typically consists of a frame with two or more columns, pistons, cylinders, ram and hydraulic pump driven by a electric motor. Oil or water oil emulsion are used as fluids, which are pressurised by high pressure pumps. The pressurised fluid from the hydraulic accumulator, forces the piston gradually downwards and upwards when it is displaced. The pressure can be applied directly from a high pressure pump or through the hydraulic accumulator. The pressurized fluid from the pump is regulated by a relief valve. The pressure can be changed at any point of the stroke by adjusting pressure control valve, there by controlling the rate of deformation. The accumulator drive is more economical because it can be used by several presses. The intensity of the pressure increases as the plastic metal resists deformation. Due to high pressure availability, it can be used for very large capacities.
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