Hot Working - Introduction

HOT WORKING OF METALS

Metal forming is also known as mechanical working of metals. Metal forming operations are employed  either to produce a new shape or to improve the properties of the metal. Metal forming is  an intentional and permanent deformation of metals plastically beyond the elastic range of the material. The main objectives of metal working processes are to provide the desired shape and size, under the action of externally applied forces in metals. Such processes  achieve required mechanical properties in the metal and reduce any internal voids or cavities present and thus make the metal dense.

The plastic deformation of a metal takes place when applied forces reaches the yield point.

Plasticity, ductility and malleability are the properties of a material, which retains the deformation produced under applied forces permanently and hence these metal properties are important for metal working processes.

Plasticity is the ability of material to undergo some degree of permanent deformation without rupture or failure. Plastic deformation will take place only after the elastic range has been exceeded. Such property of material is important in forming, shaping, extruding and many other hot and cold working processes.  This property generally increases with increase in temperature.

Ductility is the property of a material enabling it to be drawn into wire with the application of tensile force. A ductile material must be both strong and plastic. The ductility is usually measured by the terms percentage elongation and percent reduction in area often used as empirical measures of ductility. The ductile material commonly used in engineering practice in order of diminishing ductility are mild steel, copper, aluminium, nickel, zinc, tin and lead.

Malleability is the ability of the material to be flattened into thin sheets without cracking by hot or cold working.  The malleable materials commonly used in engineering practice in order of diminishing malleability are lead, soft steel, wrought iron, copper and aluminium. Aluminium,

copper, tin, lead, steel, etc. are recognized as highly malleable metals.

Mechanical working processes which are done above recrystallisation temperature of the metal are know as hot working processes.  If the hot working is completed just above the recrystallisation temperature then the resultant grain size would be fine. For any hot working process the metal should be heated to such a temperature below its solidus temperature, that after completion of the hot working its temperature will remain a little higher than and as close as possible to its rccrystalisation temperature

RECRYSTALLISATION

During the process of plastic deformation in metal forming, the plastic flow of the metal takes place and the shapes of the grains are changed. If the plastic deformation is carried out at higher temperatures, new grains start growing at the location of internal stresses caused in the metal. If the temperature is sufficiently high, the growth of new grains is accelerated and continuous till the metal comprises fully of only the new grains. This process of formation of new grains is known as recrystallisation and is said to be complete when the metal structure consists of entirely new grains. That temperature at which recrystalisation is completed is known as the recrystallisation temperature of the metal. It is this point, which draws the line of difference between cold working and hot working processes. Mechanical working of a metal below its recrystalisation temperature is called as cold working and that accomplished above this temperature but below the melting or burning point is known as hot working.

 HOT WORKING PROCESSES


1. Hot rolling

2. Hot forging

3 . Hot extrusion

4. Hot drawing

5. Hot spinning

6. Hot piercing or seamless tubing

7. Tube Forming and

8. Hot forming of welded pipes

Hot Rolling

Rolling is the most rapid method of forming metal into desired shapes by plastic deformation through compressive stresses using two or more than two rolls. It is one of the most widely used of all the metal working processes. The main objective of rolling is to convert larger sections such as ingots into smaller sections which can be used either directly in as rolled state or as stock for working through other processes.

The coarse structure of cast ingot is convened into a fine grained structure in rolling.  Significant improvement is accomplished in rolled parts in their various mechanical properties such as toughness, ductility, strength and shock resistance. The crystals in parts are elongated in the direction of rolling, and they start to reform after leaving the zone of stress.

The majority of steel products are being converted from the ingot form by the process of rolling. Hot rolling process is being widely used in the production of large number of useful products such as rails, sheets, structural sections, plates etc. There are different types of rolling mills.

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Two-High Rolling Mill

A two-high rolling mill  has two horizontal rolls revolving at the same speed but
in opposite direction. The rolls are supported on bearings housed in sturdy upright side
frames called stands. The space between the rolls can be adjusted by raising or lowering the
upper roll. Their direction of rotation is fixed and cannot be reversed. The reduction in the
thickness of work is achieved by feeding from one direction only. However, there is another
type of two-high rolling mill, which incorporates a drive mechanism that can reverse the
direction of rotation of the rolls. A Two- high reverse arrangement is also there.
In a two-high reversing rolling mill, there is continuous rolling of the workpiece through
back-and-forth passes between the rolls.

Three-High Rolling Mills

It consists of three parallel rolls, arranged one above the other.  The directions of rotation of the upper and lower rolls are the same but the intermediate roll rotates in a direction opposite to both of these. This type of rolling mill is used for rolling of two continuous passes in a rolling sequence without reversing the drives. This results in a higher rate of production than the two-high rolling mill.

Four-High Rolling Mill

It is essentially a two-high rolling mill, but with small sized rolls. Practically, it consists of four horizontal rolls, the two middle rolls are smaller in size than the top and bottom rolls. The smaller size rolls are known as working rolls which concentrate the total rolling pressure over the work piece. The larger diameter rolls are called back-up rolls and their main function is to prevent the deflection of the smaller rolls, which otherwise would result in thickening of rolled plates or sheets at the centre. The common products of these mills are hot or cold rolled plates and sheets.

Cluster Mill

It is a special type of four-high rolling mill in which each of the two smaller working rolls are backed up by two or more of the larger back-up rolls.  For rolling hard thin materials, it may be necessary to employ work rolls of very small diameter but of considerable length. In such cases adequate support of the working rolls can be obtained by using a cluster-mill. This type of mill is generally used for cold rolling work.

Continuous Rolling Mill

It consists of a number of non reversing two-high rolling mills arranged one after the other, so that the material can be passed through all of them in sequence. It is suitable for mass production work only, because for smaller quantities quick changes of set-up will be required and they will consume lot of time and labor.


Applications of Rolling

Rolling mills produce girders, channels, angle irons and tee-irons. Plate mill rolls slabs into plates. The materials commonly hot rolled are aluminium, copper magnesium, their alloys and many grades of steel.

Industrial Engineering and Productivity Management of Hot Rolling

Analysing quality and productivity improvement in steel rolling industry in central India
International Conference on Advances in Engineering & Technology – 2014 (ICAET-2014)
PP 06-11
http://iosrjournals.org/iosr-jmce/papers/ICAET-2014/me/volume-7/2.pdf?id=7622

The rolled product quality depends on the quality of the charge, the construction of a rolling machine, setting of the rolls, a kind and state of armament, temperature and a way of heating as well as the level of training a worker and his experience. Other significant quality parameters which need to be addressed are; Raw Material Inspection and Approval Process, Finished Product quality Approval Process, Geometrical Parameter Test, Physical Parameter Test, Chemical Test.

Productivity and Quality Improvement through Setting Parameters in
Hot Rolling Mill
International Research Journal of Engineering and Technology (IRJET)
Volume: 05 Issue: 04 | Apr-2018
https://www.irjet.net/archives/V5/i4/IRJET-V5I4239.pdf

HIGH-PERFORMANCE HOT ROLLING MILLS
Electrics and Automation
Good information on electrical drives, control of quality parameters
https://www.sms-group.com/press-media/downloads/download-detail/15493/

 Hot Piercing or Seamless tubing

Hot piercing is also known as seamless tubing or roll piercing process. . It is used for making thin-
walled round objects. Seamless tube forming is popular and economical process in comparison to machining because it saves material wasted in boring of parts.

Hot piercing includes rotary piercing to obtain formed tube by piercing a pointed mandrel through a billet in a specially designed rolling mill. The rotary piercing can be performed either on a two-high rolling mill or on a three-high rolling mill. In the former, the two rolls are set at an angle to each other. The billet under the rolls is deformed and a cavity formation is initiated at the centre due to tensile stressing. The carefully profiled shape of the mandrel assists and controls the formation of cavity. In a three-high rolling mill, the three shaped rolls are located at 1200 and their axes are inclined at a feed angle to permit forward and rotary motion of the billet. The squeezing and bulging of the billet open up a seam in its center pass makes a rather thick-walled tube which is again passed over plug and through grooved rolls in a two-high roll mill where the thickness is decreased and the length is increased. While it is still up to a temperature, it is passed on to a reeling machine which has two rolls similar to the piercing rolls, but with flat surfaces. If more accuracy and better finish are desired, the run through sizing dies or rolls. After cooling, the tubes are used in a pickling bath of dilute sulphuric acid to remove the scale.

Seamless steel pipe manufacturing process

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https://www.youtube.com/watch?v=ewA1v-s0Dp4

HOT EXTRUSION

It is the process of enclosing the heated billet or slug of metal in a closed cavity and then
pushing it to flow from only one die opening so that the metal will take the shape of the
opening. The pressure is applied either hydraulically or mechanically. Extrusion process is
identical to the squeezing of tooth paste out of the tooth paste tube. Tubes, rods, hose, casing,
brass cartridge, moulding-trims, structural shapes, aircraft parts, gear profiles, cable sheathing
etc. are some typical products of extrusion. Using extrusion process, it is possible to make
components, which have a constant cross-section over any length as can be had by the rolling
process. The intricacy in parts that can be obtained by extrusion is more than that of rolling,
because the die required being very simple and easier to make. Also extrusion is a single pass
process unlike rolling. The amount of reduction that is possible in extrusion is large. Generally
brittle materials can also be easily extruded. It is possible to produce sharp corners and re-
entrant angles. It is also possible to get shapes with internal cavities in extrusion by the use
of spider dies, which are explained later.

The extrusion setup consists of a cylinder container into which the heated billet or slug of
metal is loaded. On one end of the container, the die plate with the necessary opening is fixed. From
the other end, a plunger or ram compresses the metal billet against the container walls and the
die plate, thus forcing it to flow through the die opening, acquiring the shape of the opening. The
extruded metal is then carried by the metal handling system as it comes out of the die.

The extrusion ratio is defined as the ratio of cross- sectional area of the billet to that
of the extruded section. The typical values of the extrusion ratio are 20 to 50. Horizontal
hydraulic presses of capacities between 250 to 5500 tonnes are generally used for conventional
extrusion. The pressure requirement for extrusion is varying from material to material. The
extrusion pressure for a given material depends on the extrusion temperature, the reduction
in area and the extrusion speed.

Methods of Hot Extrusion

Hot extrusion process is classified as

1. Direct or forward hot extrusion

2. Indirect or backward hot extrusion

3. Tube extrusion




Different methods of extrusion  Each method is described as
under.

Direct or Forward Hot Extrusion

In this method, the heated metal billet is placed in to the die chamber and the pressure is applied through ram. The metal is extruded through die opening in the forward direction, i.e. the same as that of the ram. In forward extrusion, the problem of friction is prevalent because of the relative motion between the heated metal billet and the cylinder walls. To reduce such friction, lubricants are to be commonly used. At lower temperatures, a mixture of oil and graphite is generally used. The problem of lubrication gets compounded at the higher operating temperatures. Molten glass is generally used for extruding steels.

Aluminum Extrusion

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https://www.youtube.com/watch?v=iiGlq7408ME

Indirect or Backward Hot Extrusion

In indirect extrusion, the billet remains stationary while the die moves into the billet by the hollow ram (or punch), through which the backward extrusion takes place. Since, there is no friction force between the billet and the container wall, therefore, less force is required by this method. However

this process is not widely used because of the difficulty occurred in providing support for the extruded part.

Tube Extrusion

This process is an extension of direct extrusion process where additional mandrel is needed to restrict flow of metal for production of seamless tubes. Aluminium based toothpaste and medicated tubes are produced using this process.

HOT DRAWING

Drawing is pulling of metal through a die or a set of dies for achieving a reduction in a diameter. The material to be drawn is reduced in diameter. Fig.  is another method used in hot drawing or shaping of materials where the heated blank is placed over the die opening the punch forces the blank through the die opening to form a cup or shell. The multiple dies are also used to accomplish the stages in drawing process. Kitchen utensils and components of food processing industries are manufactured by this process.

EJP Chain Draw Bench Line DB 120

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https://www.youtube.com/watch?v=INgJSHOgioU

HOT SPINNING

Hot spinning is a process in which pressure and plastic flow is used to shape material. Spinning is generally carried over a spinning lathe. The metal is forced to flow over a rotating shape by pressure of a blunt tool.  The amount of pressure of the blunt tool against the disc controls the generated
heat, which helps in forming processes.

Hot spinning machine for CNG cylinder

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https://www.youtube.com/watch?v=Z39GZYZ5l9U


EFFECT OF HOT WORKING ON MECHANICAL PROPERTIES OF METALS



1. Raising the metal temperature lowers the stresses required to produce deformations
and increases the possible amount of deformation before excessive work hardening
takes place.

2. In hot working processes, compositional irregularities are ironed out and non-metallic 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.

3. Hot working such as rolling process refines grain structure. The coarse columnar dendrites of cast metal are refined to smaller equiaxed grains with corresponding improvement in mechanical properties of the component.

4. Oxidation and scaling take place and hence surface finish of hot worked metal is not nearly as good as with cold working.

5. The temperatures at which  hot work is started and stopped  affects the properties to be introduced in the hot worked metal.

6. Too high a temperature may cause phase change and overheat the steel whereas too low temperature may result in excessive work hardening.

7. Defects in the metal such as blowholes, internal porosity and cracks get removed
or welded up during hot working.

8. During hot working, self-annealing occurs and recrystallization takes place immediately following plastic deformation. This self-annealing action prevents hardening and loss of ductility.

  HOT WORKING - MERITS

1. As the material is above the recrystallisation temperature, any amount of working
can be imparted since there is no strain hardening taking place.

2. At a high temperature, the material would have higher amount of ductility and
therefore there is no limit on the amount of hot working that can be done on a
material. Even brittle materials can be hot worked.

3. In hot working process, the grain structure of the metal is refined and thus mechanical
properties improved.

4. Porosity of the metal is considerably minimized.

5. If process is properly carried out, hot work does not affect tensile strength, hardness,
corrosion resistance, etc.

6. Since the shear stress gets reduced at higher temperatures, this process requires
much less force to achieve the necessary deformation.

7. It is possible to continuously reform the grains in metal working and if the temperature and rate of working are properly controlled, a very favorable grain size could be achieved giving rise to better mechanical properties.

8. Larger deformation can be accomplished more rapidly as the metal is in plastic state.

9. No residual stresses are introduced in the metal due to hot working.

10. Concentrated impurities, if any in the metal are disintegrated and distributed throughout the metal.

11. Mechanical properties, especially elongation, reduction of area and izod values are
improved, but fibre and directional properties are produced.

12. Hot work promotes uniformity of material by facilitating diffusion of alloy constituents and breaks up brittle films of hard constituents or impurity namely cementite in steel.

DEMERITS OF HOT WORKING

1. Due to high temperature in hot working, rapid oxidation or scale formation and surface de-carburization take place on the metal surface leading to poor surface finish and loss of metal.

2. On account of the loss of carbon from the surface of the steel piece being worked the surface layer loses its strength. This is a major disadvantage when the part is put to service.

3 . The weakening of the surface layer may give rise to a fatigue crack which may ultimately result in fatigue failure of the component.

4. Some metals cannot be hot worked because of their brittleness at high temperatures.

5. Because of the thermal expansion of metals, the dimensional accuracy in hot working is difficult to achieve.

6. The process involves excessive expenditure on account of high cost of tooling. This however is compensated by the high production rate and better quality of components.

7. Handling and maintaining of hot working setups is difficult and troublesome.