Hot Working & Cold Working

8/12/2019 Hot Working & Cold Working

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FUNDAMENTALS OF METAL FORMING

4 basic production processes for producing desired shape 1.Casting2.Machining3.Joining (welding, mechanical fasteners, epoxy, etc.)4.Deformation processes

1. Casting process exploit the fluidity of a metal in liquid state as it takesshape and solidifies in a mold.

2. Machining processes provide desired shape with good accuracy andprecision but tend to waste material in the generation of removed portions.

3. Joining processes permit complex shapes to be constructed from simplercomponents and have a wide domain of applications .

4.Deformation processes exploit a remarkable property of metals,which is their ability to flow plastically in the solid state withoutdeterioration of their properties.


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With the application of suitable pressures, the material ismoved to obtain the desired shape with almost no wastage.

The required pressures are generally high and the tools andequipment needed are quite expensive.

Large production quantities are often necessary to justify theprocess.

Plastic deformation


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To deform the metal permanently, the stress must exceed the elasticlimit.

C:Maximum yield stress


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The process in which the desired shape and size are obtainedthrough plastic deformation of a material under the action ofexternally applied forces.

Stresses induced in the process are greater than yieldstrength , but less than fracture strength of material.

Economical process, no loss of material, high production rates.

ROLLING, EXTRUSION, FORGING, DRAWING, SPINNING, PRESSWORKING.


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As a metal is deformed (or formed) into useful shape, itexperiences stresses such as tension, compression, shear, or

various combinations.

Fiber flow lines in forging

Due to plastic deformation the grains of the metal getelongated in the direction of metal flow (Fiber Flow Lines).


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State of the s t resses meta l un dergo d ur ing d eformat ion .


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Overview of Metal Forming

Rolling

Extrusion

Forging

Drawing


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Overview of Metal Forming

Bending Deepdrawing

Shearing


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How better mechanical properties?

Reduces any internal voids or cavities present and makes metaldense . Impurities get elongated with grains and in the process, getbroken and dispersed throughout the metal. This decreases theharmful effect of impurities and improve the mechanical strength.

At room temperature , the metal is in a more rigid state than whenat higher temperature. Thus, to deform the metal greater pressuresare needed when it is in cold state than when in hot state.

When metal is formed in cold state , there is no recrystallization ofgrains.


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When metal is deformed in cold state , severe stressesknown as residual stresses are set up in the material.

These stresses are often undesirable , and to removethem the metal is heated to some temperature below therecrystalline range temperature .

In this temperature range, the stresses are rendered

ineffective without appreciable change in physical propertiesor grain structure.

Metals having large grains are more ductile than thosehaving smaller grains.


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Recovery:

At a temp range below recrystallization temp.

No change in mechanical properties such as hardness and strength

Stresses in the highly deformed regions are relieved

Recrystallisation:

At a certain temp range, new equiaxed and strain free grains areformed, replacing the older grains is called recrystallization.

(0.3Tm to 0.5Tm)

Recrystallisation temp is generally defined as the temp. at whichcomplete recrystallization occurs within approximately one hour.


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Lowers strength and raises ductility

Recrystallization depends on the degree of prior cold work(work hardening).

The more cold work , the lower the temperature required forrecrystallization to occur.

As the amount of cold work increases , the no. of dislocations andthe amount of energy stored in dislocations (stored energy) alsoincreases. This energy supplies the work required for recrystallization.

Under the action of heat and force, when the atoms of metalreach a certain higher energy level, the new crystals start forming .This is called recrystallization .

When this happens, the old grain structure no longer exist ,

instead new crystals which are strain-free are formed.

Recrystalization contd..


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Grain Growth :If we continue to raise the temperature of the metal, the grain

begin to grow and their size may eventually exceed the original grainsize. This phenomenon is know as grain growth and it affectsmechanical properties.

Grain size has a significant effect on the strength of metals. Thesmaller the size, the stronger the metal .


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Properties of C10 steel Hot rolled Cold rolled

Ultimate tensile strength, MPa 427 558

Yield Strength, MPa 220 345

Brinnels Hardness Number 94 174

Cold, Warm & Hot working


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The effects of cold working can be reversed by annealing the

metal:

Heating it in a temperature range for a period of time and thereby allowing successive processes of recovery, recrystallization,and grain growth to take place .

Deformation at room temperature ( cold working ) results inhigher strength but reduced ductility of the metal. It generallycauses anisotropy , a state in which the properties are different indifferent directions.

Metals can be plastically deformed(worked) at room, warm orhigh temperatures. Their behavior and workability depend largelyon whether deformation takes place below or above therecrystallization temperature.

Cold, Warm & Hot working


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COLD AND HOT WORKING OF METALS

Cold Working

Plastic deformation of metals below the recrystallizationtemperature is known as cold working.

It is generally performed at room temperature . In somecases, slightly elevated temperatures may be used.

Cold working offers a number of distinct advantages , andfor this reason various cold-working processes have become

extremely important.

Strengthening occurs because of dislocation movementswithin the crystal structure of the material.


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Cold worked material is harder and stronger than materialdeformed at other temperatures. These harder materials are

advantageous for applications such as machine parts andmechanical supports

While cold-working, a metal will tend to increase its strength,other properties such as ductility or corrosion resistance may benegatively affected.

To remove internal stresses of cold work, it is sometimesdesirable to heat treat the metal after cold working (Annealing).

Cold Working

% Cold Work = {(A o-Ad)/Ao } x 100

Ao- original cross-sectional area Ad- area after deformation%CW is another measure of degree of plastic deformation, like strain


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During cold-working , it may take a considerable amount ofenergy to affect the change in size and shape . Some of theenergy expended will appear in the form of heat .

A considerable amount of the energy will also be stored in thematerial. This stored energy is associated with the defectscreated during the deformation.

This is also called as strain hardening/work hardening/coldworking. Usually carried out in several steps with annealing usedto soften the cold worked metal and restore ductility.

Strain hardening/cold working


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In comparison with hot working, the advantages of cold working are

1. No heating is required

2. Better surface finish is obtained

3.Better dimensional control is achieved; therefore no secondarymachining is generally needed.

4. Products possess better reproducibility and interchangeability.

5. Better strength, fatigue, and wear properties of material.

6. Directional properties can be imparted.

7. Contamination problems are almost negligible.


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Some disadvantages associated with cold-working processes are:

1. Higher forces are required for deformation.

2. Heavier and more powerful equipment is required.

3. Less ductility is available.

4. Metal surfaces must be clean and scale-free.

5. Strain hardening occurs (may require intermediate annealing).

6. Undesirable residual stresses may be produced.

7.Better suited to large-scale production of parts because of thecost of the required equipment and tooling.

8. Some materials are not capable of cold working

d


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AdvantagesIncreases strength and hardness due to strain hardening.

No oxide formation, good surface finish.

Better dimensional accuracy.

It is easy to handle cold parts.

DisadvantagesMaterial has high yield strength at low temperature .Hence, amount of deformation given to it is limited .

Metals get strain hardened. the maximum amount of deformationcan be given is limited.

Excessive cold work will lead to fracture before final size has beenreached.

Metals which are brittle cannot be cold worked . complexity ofshapes is limited.

Summary of Cold working


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Worm working is metal forming at temperatures above theroom temperature but below recrystallization one.

Advantages:

Lower forces and powerMore complex part shapesNo annealing is required

Disadvantages:

Some investment in furnaces is needed.

Warm working or Warm forming


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Hot Working

First step of converting a cast ingot into a wrought product

Plastic deformation of metal carried out at temperature above the recrystallization temperature, is called hot working(with orwithout actual heating).

For Lead, Tin, Cadmium and Zinc, R.T is below room temp.

For Steels, R.T is of order 1000 0C(At 900 0C is also cold working)

In hot working, the temperature at which the working iscompleted is critical since any extra heat left in the materialafter working will promote grain growth, leading to poor

mechanical properties of material.


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The lower limit of the hot working temp. is determined byits recrystallization temp.

The upper limit for hot working is determined by excessiveoxidation, grain growth, undesirable phase transformation

Hot working


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Disadvantages associated in the hot-working of metals are:

1.Heat energy is needed

2.Poor surface finish of material due to scaling of surface

3.Poor accuracy and dimensional control of parts

4.Poor reproducibility and interchangeability of parts

5.Handling and maintaining of hot metal is difficult andtroublesome

6.Lower life of tooling and equipment.

7.Due to high temperature, surface oxidation and

decarburization can not be prevented


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S.No Hot working Cold working

1Above recrystallisationtemp., any amount ofworking can be imparted asthere is no strain hardening(work hardening)

Cold working increasesstrength and hardness of thematerial due to strainhardening , which would bebeneficial in some situations.

2At a high temp, materialwould have higher amount

of ductility and no limit onthe amount of hot workingthat can be done a material.Even brittle materials can behot worked .

Some materials which are

brittle cant be cold worked


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S.No Hot working Cold Working

3 Since shear stresses gets reducedat higher temp , hot workingrequires less force to achieve thenecessary deformation

Since the material has higher yieldstrength at lower temps , theamount of deformation that canbe given is limited by the capabilityof the processes or hammers used.

4 If temp. and rate of working areproperly controlled, a veryfavourable grain size could beachieved giving better mechanicalproperties.

Since the material gets strainhardened, the max. amount ofdeformation that can be given islimited . Any further amount ofdeformation can be given afterannealing .


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S.No Hot working Cold working

5 At higher temp. surface finishobtained is poor

Since the working is done in coldstate, no oxide would form onthe surface and consequently,good surface finish is obtained

6 Because of the thermalexpansion of metals, thedimensional accuracy in hotworking is difficult to achievesince it is difficult to control thetemp. of work pieces

Better dimensional accuracy isachieved .

7 Handling and maintaining of hotmetal is difficult andtroublesome

It is far easier to handle coldparts and it is also economicalfor smaller sizes


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Microstructure as a Function of Manufacturing Method

Schematic illustration of a part made by three different processesshowing grain flow.

(a) Casting by the processes.

(b) Machining form a blank, and

(c) Forging.

Each process has its own advantages and limitations regardingexternal and internal characteristics, material properties, dimensionalaccuracy, surface finish, and the economics of production.

Hot Working & Cold Working

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