Hot Working

Introduction area

In this article, we will read in detail about hot working. Mechanical working and Casting are the best methods by which one can shape the metal into the required form.

The mechanical working is a method of shaping metals to desired forms where casting cannot be used. It includes the shaping of metals by either cold working or hot working.

Metals are subjected to mechanical working to reduce the original block or ingot to the finished dimensions of the part, thus saving the material cost and time. At the same time, it improves the mechanical properties of the metal.

Hot Working Definition:

The process of working above the recrystallization temperature (lower critical temperature i.e the temperature at which the metal can be easily worked upon to give it the desired shape) but lower than the burning point is termed as Hot Working.

During crystallization in hot working the yield strength ( the magnitude of the stress at which an object stops showing elastic properties and transforms to a plastic state) and hardness is low. At the same time ductility is high.

Residual stresses are not produced in the hot working process which is there in cold working.

Hot Working Methods:

The various methods of the hot working are as follows:

i) Rolling.

ii) Forging.

iii) Extrusion.

iv) Piercing

v) Drawing( also known as Cupping).

vi) Spinning.

Let us now discuss them in detail:

A) Rolling:

Rolling is one of the best methods used for Hot Working. It is more economical than forging when a long uniform cross-section of metal is required.

Working:

i) In hot rolling, the metal is the first cast into Ingot molds of suitable form. which is a rectangle, square, or round).

ii) The ingot is then removed from the mold after its solidification, then is placed in a gas-fired furnace called the soaking pits, and then is taken to the rolling mills.

iii) The rolling process includes compressing and lengthening the metal as it is fed between the two rolls rotating in opposite directions.

iv) Primary rolling is done in either a two-high reversing mill or in three high continuous rolling mills.

v) The final product from the rolling includes plates, strips, and a wide variety of bars and sections.

Advantages of Hot Rolling:

T0he various advantages of hot are:

i) The finished product is made up of uniform size.

ii) The end product produced is stronger, tougher, and defect free.

iii) Speed of the process is good adding economic advantage.

Limitations:

Every process comes with certain advantages and disadvantages, so does Hot rolling. Some of them are:

i) Dimensional tolerance control is less precise.

ii) The surface finish depends upon the metal’s chemical composition.

Defects In Rolling:

a) Surface Defect: It originates due to impurities in the material, rusting, and scale formation.

b) Overheating of metal could result in the formation of excessive grain growth.

c) Incorrect geometric shape and dimensions of the mill.

d) Vertical Misalignment of the rolls produces blank with unequal sides and deformed sections.

e) Horizontal Misalignment of grooves of a pass causes undercut and twisting of the strip.

f) Edge cracks may be developed because of low ductility originating due to non-homogeneous deformation of metal during rolling,

B.) Forging:

Forging is defined as the controlled plastic deformation of metals at increased temperatures into the required size or shape using compressive forces by making use of some kind of die or hammer or by an upsetting machine.

The raw material for the forging is usually a bar, billet or blank.

It includes altering the shape and the size of the steel specimen by hammering at a temperature of 1000 degrees Celcius. The steel flows under pressure.

It is generally employed for those components which require high strength and resistance to shock or vibration and uniform properties.

Forging improves the mechanical properties and the structure of the metal. The forging produces high strength and ductility and provides great resistance to impact and fatigue load.

The various operations involved during the formation of shape by forging includes the following given below operations. A combination of such operations can be used.

some of them are:

i) Upsetting: This process involves increasing the cross-sectional area usually by pressing or hammering.

In this process, the rod or shaft is held is gripped in the dies, and is shaped to give the desired dimensions.

ii) Punching: It is a process of producing holes using hot punch over a cylindrical die.

iii) Setting down: The local thinning down operation done by using a hammer.

iv) Bending

v) Welding: Metals are welded by pressing together two surfaces.

vi) Fullering: It is similar to setting down and consists of reducing the cross-section of the workpiece or lengthening a preparation of the stock.

vii) Coining and ironing:

Coining is basically a pressing operation and the flow of the metal takes place only at the top layer and not for the complete volume.

Pressure of about 1300 to 3200 MPa makes the metal to flow to each nook and corner of the die cavity.

It is used for the production of badges, currency medals etc.

Methods of forging:

The various methods of forging are:

Defects in the forged Parts:

The various defects that might occur during forging are:

a) Cracks and scabs might form on the ingot.

b) Dirt, slag and blow holes resulting from melting.

c) Improper output might produce due to incorrect alignment of the dies.

d) Shallow surface depression might occur if the scale is not removed from the die.

e) Improper heating and the cooling of the forging might result in burnt metal, decarburized steel, and flakes.

C) Extrusion:

Extrusion is defined as the process of pushing the heated billet or slag of metal through the orifice provided into a die, thus forming a part that is similar to the shape of the orifice die.

In extrusion the pressure is applied either hydraulically or mechanically.

Aluminum and its alloys are greatly used for extrusion directly at elevated temperatures.

Some example of the products made from extrusion are:

Rods, tubes, molding trims, structural shapes, cable sheathing, hose, casing, aircraft parts, and hardware items.

Three different ways in which the metal can be extruded are:

a) Direct and the forward extrusion:

Direct Extrusion is a popular method of extrusion as it is mechanically simpler.

In this case, the hot metal is pushed by the press, operating a ram in the cylinder into the die through a small restricted opening called orifice.

Solid wire is also made by the extrusion cylinder.

B) Indirect Extrusion( Also called the backward extrusion):

Indirect extrusion is similar to direct extrusion with the difference that the extruded metal is forced through the hollow ram.

The equipment used is more complicated in the case of the indirect extrusion and the force required to compress the metal is less.

Advantage: Fast process, low tooling cost and is cheaper than die casting. The final structure produced is very dense.

C) Impact Extrusion:

Impact extrusion a type of cold extrusion process in which the punch impacts the workpieces rather than the pressure.

Impacting can be carried out as forward extrusion, backward extrusion, or a combination of both.

It is used for making short tubes of short alloys, such as the toothpaste containers.

D)Stepped Extrusion:

Stepped extrusion is mostly used when the stepped diameter components are required to be formed.

In this process two dies are used major and the minor die.

When the smaller diameter cross-section has been extruded, the process is stopped for a while, the smaller die is removed and the extrusion is completed by forcing the metal through the major die to obtain stepped diameter.

Overall Advantages of the extrusion process are:

i) Surface quality: Extrusion produces good surface quality.

ii) Setup- Cost:set-up cost is low.

iii) Good quality seamless tubing can be produced.

iv) Small lots can be produced economically.

Iv) Rotary Piercing

Rotary Piercing method is best used for making of seamless tubes .

The solid billet is first center punched and then heated to a temperature of 1100 degrees Celsius.

During this method the billet at 1100 degree Celsius fixed between the two rolls is made to spin at high speed and over a mandrel which helps in piercing and controlling the size of the bore as the billet is forced over it.

The tube is then passed through a reeling machine which helps in straightening and sizing to the final dimensions.

V) Drawing:

Drawing is a type of cold working process during which a wire, tube, or rod is pulled through a tapered hole of a die thereby, reducing its diameter.

The drawing operation can produce a reduction of 50% percent of the original area.

No mechanical and mechanical properties are impaired.

Vi) Spinning:

Spinning is used for getting circular and desired contours.

The operation is performed on a lathe Machine which imparts rotational motion to the job.

The circular blank of sheet metal after heating is fixed in the chuck and is rotated on the spindle of the lathe.

The very hard and smooth rotating tool is pressed against the blank so that the workpiece acquires the required shape

As compared to drawing, the tool and the equipment cost is low in drawing. It is suitable for low volume production.

Hot Working in detail:

In this topic we are going to read hot working in more detail.

The metal forming process is an important part of the manufacturing industry.

Metal forming is a process in which the required shape and size are obtained by the plastic deformation in the material.

The stresses produced during the forming process are more than the yield strength but at the same time lesser than the strength of the material fracture.

The forming process can be divided into two categories:

a) Cold Working  b) Hot Working.

Cold working is a process in which the working temperature is below the recrystallization temperature.

Here we will only focus on Hot working.

Hot-working is a metal forming process in which the desired shape of the metal is obtained when the material undergoes plastic deformation above the recrystallization temperature.

The recrystallization temperature of steel is 800 degrees Celsius and for lead, tin or zinc is at or near room temperature.

Plastic deformation takes place through a slip mechanism. The stress required to continue deformation increases with deformation is known as strain hardening.

In addition to increasing strength, plastic deformation at room temperature also causes anisotropy which is different properties in different directions.

Both the above effects are eliminated by recrystallization of metal at elevated temperatures.

The growth of the grains takes place above the recrystallization temperature, the grains are broken up and the parts are deformed into small and numerous crystals.

At this stage, metals possess little elasticity and low load is required to shape the metal as the strength and the hardness decreases at elevated temperature.

Hot Working Advantages:

The various advantages of Hot Working are:

i) Metalworking is done above the recrystallization temperature so the metal posses little elasticity and a low load is required to shape the metal.

ii) Residual stresses are not produced in Hot working.

iii) Directional property resulting from a fiber structure is obtained.

iv) The porosity of the steel ingot can be eliminated to a great extent.

v) The mechanical properties such as toughness, ductility, elongation, and reduction in the area are improved due to the refinement of grains.

vi) Power consumption is less while finishing the part from ingot.

vii) It can be used on most metals because it is a rapid and economical process.

Hot Working Disadvantages:

The various disadvantages of the hot working are:

i) Poor surface Finish:

Rapid oxidation or scaling of the surface due to the high temperature of the metal results in poor surface finish.

ii) Close Tolerances: Close tolerances cannot be maintained.

iii) Tooling and the handling costs are high.

iv) Tool life is less as they are used at a high temperature.

Hot Working Application:

The various applications of the hot working process are:

i) It helps in the production of plates, strips, and a wide variety of bars and sections.

ii) Seamless Pipe and tube production.

iii) Rods, tubes, molding trims, structural shapes, cable sheathing, hose, casing, aircraft parts, and hardware items.

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