Forging materials and facilities
Forging materials and facilities
Forging materials need to be the ones that can transform easily under sharp processing loads.
The properties that are suitable for forging is called malleability.
Malleability refers to the ability to transformation without cracking.
Many testing methods were developed to quantify malleability, but there is no universal method. General testing uses upsetting test which uses a compressed cylindrical ring to observe any cracks formed on the sides. The metal has good malleability when it can be transformed into many shapes before it cracks. Upsetting test can be done with different temperature and transformation speed. With higher notch of the material sensitivity, the surface defects develop into cracks, and affects the material. Traditional surface defects include length, direction, filler, and folds form previous processing step.
Hot-twist test twists a round material in the same way until it break. Test is performed with different temperatures and many specimen to count the number of twist until it breaks. The optimum forging temperature is found this way.
Hot-twist test is a godd method for steel materials. Higher malleability among steel, copper, aluminum can be easily forged, but machinery materials need to be considered for the correct intensity, price and conditions needed for the product, therefore the materials with proper mechanical properties are used although the malleability is not so perfect.
Usually the metals transform easily in higher temperature, making the forging easier, but there are two limitations in this. One is the maximum forging temperature, and the other is forging termination temperature. When the materials is over heated, the material becomes weak and vulnerable to crack.
When forging the overheated material, the temperature remains even after forging, the particles grow to decrease the mechanical quality. On the other hand, if the finishing temperature is lower than the recrystallization temperature, the forging would have great internal stress, and will crack easily.
Therefore, the finishing temperature of forging needs to be a little bit higher than recrystallization temperature. If the temperature for steel forging goes below 800°C, forging needs to continue with reheating so that there is no internal stress remaining.
Also, steel has blue brittleness around 300°C, forging around this temperature needs to be avoided.
Various forging machines with different quantity, speed and administrative properties are used in forging(Table 1). Forging machines are divided into press and hammer.
|Hydraulic press||0.06 - 0.30|
|Machine press||0.06 - 1.5|
|Screw press||0.6 - 1.2|
|Gravity fall hammer||3.6 - 4.8|
|Power fall hammer||3.0 - 9.0|
|Counter fall hammer||4.5 - 9.0|
< Table 1 - Processing speed of forging machine >
Hydraulic press is operated with regular speed, and has maximum load as a limit. In other words, overloading can stop the machine.
The administration and speed can be controlled with the same load, and great energy can be delivered to the material.
Compared to other forging machines, hydraulic press takes longer time, so the material can cool down easily when non-preheated swages are not used.
Hydraulic press is slower than mechanical press, costs a lot to start up, but has good maintenance.
Machine press is basically a crank tool or one-sided type, and reaches the maximal speed at the middle of administration, and becomes zero at the end of administration.
crank spindle can be used instead of one-sided one to make ram move up and down. Machine press will have limited movement range. The press will have energy delivered through a large motor-rotating fly wheel.
Fly wheel and one-sided spindle are connected by a clutch. Rotation changes to linear movement through connecting rode.
Knuckle joint press can load very heavily by specially designed link device. The load acquirable from the machine press depends on the administration location, and the load increases greatly at the bottom dead point.
Therefore, in order to avoid damaging the swage or compartment of the device, they need to be properly aligned. Mechanical press has high production rate, and atomization is useful, and requires lower skill level of the workers.
Screw press gets limited energy from the fly wheel.
Forging load is delivered to the screw in a perpendicular form.
When the energy saved in the flywheel is all used, the ram stops. When the swage is not completely closed after one cycle, the forging work is repeated. Screw press is used in various free forging and swage forging, and it is suitable for small number of elaborate products such as turbine blades.
Hammers get energy by transforming the potential energy of ram to kinetic energy, and gets limited energy.
Unlike hydraulic press, hammers operate in high speed, and has short working hours, so the cold speed is minimized for forging.
Because it has small cold speed, the complex shapes, especially the thin and deep parts can be used for forging.
Until the forging finishes, the same swage gets multiple impacts. Hammers have various types and low-costed forging machine.
Gravity drop hammer
Gravity drop hammer (free forging) gets energy from the free falling ram. This type of hammer get twice the height of lifting during falling. Ram weighs from 180kg to 4,500kg and the total energy is 120KJ.
Count blow hammer
Counter blow hammer has 2 rams that approach the forging in horizontal or perpendicular planes, In free forging, the material needs to be rotated to make the wanted form. Counter blow hammer operates in high speed but doesn't vibrate the supporter. Maximum amount is 1200KJ
In high-energy-rate machine, high-pressure gas speeds up in very high speed and the product is forged with single blow. There are various types in this machine, but maintenance, swage damage and safety-related problems are limiting the use.
Choosing the forging machine
In choosing the forging machine, load cost, energy, size, shape of forging, complexity, intensity of material, transformation speed of material and sensitivity need to be considered.
Other considerations need to include productivity rate, accuracy, maintenance, skills needed, noise level and price. Generally, aluminum magnesium, beryllium, bronze, brass are used for press. Hammers are used for steel titanium, copper and hot metal alloy.