In this article we are going to learn about Hot working processes, Methods involved and what are the advantages and disadvantages of hot working.
What is Hot Working ?
Hot working process is the plastic deformation of metal which is carried out above their recrystallisation temperature.
Hot working is the process of shaping a metal under pressure at a relatively high temperature. This material can be hot worked at temperatures ranging from 2150 F to 1800 F. Hot working is the process of shaping a metal under pressure at a relatively high temperature.
Hot–working Process is a type of metal forming process distinguished by the temperature at which the deformation is carried out. Recrystallization temperature is the lowest temperature at which plastically deformed metals form new grains or crystals within a given time period.
What is recrystallisation temperature ?
The temperature at which new stress-free grains are formed in a metal is known as recrystallisation temperature.
Methods of Hot Working
In other words, it is the process of metal formation occurs when metal is deformed plastically above the recrystallization temperature but below the burning point.
The principal methods of hot working are as follows :
1. Hot Rolling
Hot rolling is the process of shaping steel, aluminum, and other metals by reducing the transverse section by applying pressure to one or more rolling stands. Hot rolling takes advantage of the metal’s ductility at high temperatures to achieve large section reductions.
Rolling is the most rapid method of forming metal into desired shapes by plastic deformation in between rolls. The crystals are elongated in the direction of rolling, and they start to reform after leaving the zone of stress, but in cold rolling they retain substantially the shape given to them by the action of the rolls.
In the hot rolling process, metal in a hot plastic state is passed between two rolls revolving at the same speed but in opposite directions. As the metal passes through the rolls, it is reduced in thickness and increased in length.
2. Piercing or Seamless Tubing
Piercing is employed to produce seamless tubing which is the natural form from which is made any thin-walled round objects. Seamless tubing is a popular and economical raw stock for machining because it saves drilling and boring of parts. The process of making hot-pierced tubing consists of passing a hot-rolled billet between two conical-shaped rolls and over a mandrel which assists in the piercing and controls the size of the hole as the billet is forced over it .
Reduction in a diameter. The material to be drawn is reduced in diameter for a short distance at one end by swaging, to permit it into the die orifice and a gripped in the jaws. The process requires a very large forces in order to pull the metal through the die. To reduce the frictional force between the die and the metal the die is kept well lubricated.
Parts that have circular cross-sections can be made by spinning them from sheet metal. The principle of metal spinning is illustrated in Fig. 7.8. A heated circular blank of sheet metal is lightly held against a chuck by the pressure of a freely rotating pad on the lathe tail stock. This chuck may be made of plaster, wood, or metal and is revolved on the spindle of a lathe. A rounded stick or roller is pressed against the revolving piece and moved in a series of sweeps. This displaces the metal in several steps to conform to the shape of the chuck.
Once the operation is started, considerable frictional heat is generated which aids in maintaining the metal at a plastic state. Spinning is a highly specialized art. To avoid excessive thinning of the metal, the pressure of the forming tool (rounded stick) should be directed sometime backward toward the tail stock as well as forward toward the headstock.
During “spinning on air” (not against the chuck), with large parts made from relatively thin metal, a hardwood bar is commonly used as back-up support opposite the spinning tool to avoid wrinkling at the outer edge.
Extruding is a process in which a heated billet or slug of metal is forced by high pressure through an orifice that is shaped to provide the desired form to the finished part. An everyday analogy is the squeezing of toothpaste from a collapsible tube. Because of the large forces required in extrusion, most metals are extruded
Blank hot under conditions where the deformation resistance of the metal metal is low.
Forging is a manufacturing process that involves shaping metal with localized compressive forces. The blows are delivered with a hammer or a die. Forging is frequently classified according to the temperature at which it is performed: cold forging, warm forging, or hot forging
Difference Between Hot Working and Cold Working
To know the difference between hot working and cold working : Click Here
Hot Working Process Explanation
Metal hot working is a form of plastic deformation at high temperatures (above the re-crystallization point of metal). During hot working of metals, re-crystallization occurs concurrently with deformation.
Recrystallization is a phenomenon that occurs at sufficiently high temperatures and accelerates the growth of new grains in the metal structure, resulting in a metal that is entirely composed of new grains.
The hot working temperature of a metal is determined by its re-crystallization temperature; for example, carbon steel requires a temperature greater than 1000°C (1832° F) for hot working.
Because of the formation of new strain-free grain during hot metal working, distorted grain structure and strain hardening caused by deformations are rapidly eliminated (a result of recrystallization). The maximum temperature for hot metal working is set to be 50 °C below its melting point.
When metal is worked at or near its melting point, it will crumble into pieces during deformation (known as hot shortness).
Metal hot working operations are carried out in a series of parallel steps. The metal is heated to recrystallization temperature in the first step. Following that, the intermediate (hot working) and finishing temperatures are set to produce a product with the greatest number of fine grain sizes.
To maintain a fine recrystallized grain size, the finishing temperature is kept just above the metal’s recrystallization temperature, and the deformation produced in the final step is relatively large.
Advantages of Hot Working
- Hot-working is suitable for mass production work.
- Metal’s shape and size can be easily altered.
- Because the material is above the recrystallization temperature, any work on metal can be done without the stress of hardening.
- Because the material is at a high temperature, it has more ductility, which means there is no limit to hot-working metal.
- Metals with a refined grain structure have a result of hot working. As a result, mechanical properties improved.
- The porosity has been significantly reduced.
- Hot-work will not affect strength, hardness, corrosion, or other properties if there are no errors.
- Because higher temperatures reduce shear stress, much less force is required for necessary deformation.
- Larger deformation is possible because the metal is in a plastic state.
- It has the ability to improve mechanical, physical, and chemical properties.
- This is quick, cheap, and dependable.
Disadvantages of Hot Working
- Due to oxidation, it leads to a poor surface finish.
- There is a loss in carbon sometimes leads to a weaker strength because of oxidation.
- Maintaining and handling the hot working setup is difficult.
- Requires expensive units like a gas furnace or induction heater.
- Dimensional accuracy is difficult to achieve. It is due to the thermal expansion of metal.
- It is not suitable for all types of metals.
Applications of Hot Working
1. The hot working process is used to manufacture different types of products like tubes, pipes, metal sheets, etc.
2. Different types of products we encounter daily are manufactured using this method like the many types of equipment of automobile, aerospace, architecture, home decor, etc.
Q. Hot working process is the plastic deformation of metal which is carried out
- At temperature below the recrystallisation temperature
- At temperature above the recrystallisation temperature
- At temperature equals to boiling point of water
- None of the above
ANSWER: At temperature above the recrystallisation temperature
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