dr.r.narayanasamy - effect of microstructure on formability of steels - modified

Effect of Microstructure on

Formability of steels.

By

Dr. R. Narayanasamy, B.E.,M.Tech.,M.Engg.,Ph.D.,(D.Sc.),

Professor,

Department of Production Engineering,

National Institute of Technology,

Tiruchirappalli- 620 015 ,

Tamil Nadu, India.

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Microstructure

• Microstructural variable of plain carbon steels:

Amount of pearlite content in the steel (hard phase which enhances brittleness in metal)

Amount of ferrite content in the steel (soft phase which enhances ductility in metal)

• In low carbon steels – increase in perlite increases the flow stress and decreases yield extension.

• Yield extension controls the flow stress (at low strain) due to high work hardening rate at such strains.

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• The ferrite grain has no effect on uniform strain ( *). This is because the fine grains increases flow stress as well as work hardening rate.

Effect of grain size on *

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Effect of Pearlite on *

• Pearlite has more

effect on uniform

extension.

• Pearlite effect is

large on flow

stress than work

hardening rate.

This is the reason

for decrease in

uniform strain

( *) by pearlite.

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Effect of Spheroidization

• Spheroidization of pearlite: (when compared to conventional annealing)

– Decreases flow stress

– No effect in work hardening

– Increase uniform elongation (with carbon content)

– Total ductility ( T)at fracture increases

– Total ductility decreases with volume fraction of carbide. (In general)

– Spheroidization depends on carbon content.

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Spheroidization

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Effect of Spheroidization on *

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Effect of Spheroidization on T

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• It is possible to identify microstructural parameters which will give low yield stress values and work hardening rates.

• For low forging pressure:

a low pearlite content (low carbon content)

a coarse ferrite grain size.

at slow cooling rate through ferrite range to overage precipitated iron carbides

low solute contents are desirable.

• Controlled high finishing temperature with slow cooling rate in low carbon steel is beneficial

Microstructure cont… Low carbon steels

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Effect of Pearlite on Forging Pressure

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Effect of ferrite grain size on forging

pressure

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• If interlamellar spacing is finer, flow stress and work hardening increases.

• Interlamellar spacing has no effect on uniform strain ( *).

• Interlamellar spacing has effect on total fracture strain. Fine carbide lamellae can deform whereas coarse carbide lamellae crack and form cavitation.

• Total ductility ( T) is greater for fine interlamellar spacing with delayed cavitation (due to ductile fracture).

Microstructure cont… Eutectoid steels

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Microstructure cont…

Eutectoid steels

• Coarse grain steels – poor mechanical

properties – tendency towards tensile & shear

flow failure- during forging & drawing.

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• Austenitic stainless steel types:

a. Stable austenitic stainless steel (no phase transformation occurs)

b. Unstable austenitic stainless steel (Unstable austenite when deformed plastically strained martensite is formed)

• Austenitic grain size – improves strength.

• ferrite (second phase) – notable strengthening effect.

• ferrite due to strain concentration in softer austenitic phase, work hardens to strain more than nominal (0.2%) and gives higher flow stress.

• In case of stable austenitic steel – martensite does not form (at 0.2% strain). So proof stress is not affected.

Microstructure cont… Austenitic stainless steels

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• Austenitic Stainless steel containing up to 35%

martensite (solution treated condition) has no

effect on 0.2% proof stress. But 20%

martensite may lower it.

• This effect is because of formation of

martensite (unstable austenitic stainless steel)

when strain is applied.

Microstructure cont… Austenitic stainless steels

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• 35 % martensite phase itself becomes load

bearing phase in Stainless Steels.

• The tensile strength is based on twin spacing

which is present in austenitic structure.

(Tensile strength does not relay in grain size.)

• Twin spacing reflects staking fault energy

rather than major strengthening from twin

boundaries.

Microstructure cont… Austenitic stainless steels

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• Low stacking fault energy + small twinning space = high work hardening rate + high tensile strength.

• In total : forging of austenitic stainless steel will be easier with increase in austenitic twin space.

• Hot forging: flow strength (steel) decreases & ductility increases. This is expected in forging of steels.

Microstructure cont… Austenitic stainless steels

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• In austenitic stainless steel (SS): Blue

brittleness do not occur like other steels. Flow

stress decreases at low temperature.

• Preheating of Austenitic SS (200 - 300°C) will

be good.

• Full recrystallization do not occur as low as

400°C but with deformation at 500°C

recrystallization takes place.

Microstructure cont… Austenitic stainless steels

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Effect of Second phase particles on

formability

• Presence of second phase particles – reduces total elongation.

• Increase in sulfur/MnS (Manganese sulphide) – reduces total ductility.

• The shape and distribution of second phase particles have major effect.

• Voids nucleate – cracking of second phase particle/ decohesion of metal particle interface.

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Effect of oxide, carbide,sulfide on total

ductility

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• Void growth follows – due to strain concentration

– caused by presence of crack/voids.

• Interparticle spacing, void coalescence are

employed to denote the rapid growth of ductile

crack.

• Increasing (Length – width ratio of MnS) of

second phase particles – improves high ductility

(tensile). This condition exists when the long axis

of inclusion is parallel to tensile axis.

Effect of Second phase particles on formability

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• Failure of Austenitic SS – in forming operation –

due to ductile fracture (void nucleation &

growth).

• The non metallic inclusions and second phase

particles is also important.

• Non metallic inclusions – controlled by – sulfide

content (using sulfur addition to stable 16% - 25%

Ni (Nickel) steel).

• The maximum uniform strain ( *) also decreases.

Effect of Second phase particles on

formability

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Effect of inclusions on total ductility

16% - 25% Ni steel

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• The austenitic steels give lower ductility than ferritic steels.

• The reasons are: (a) angular shape of oxides in austenitic steels – will give increased strain concentration and more rapid void growth.

(b) Segregation of sulfides, causing high volume fractions of particles so void coalescence occurs at smaller strains than randomly distributed particles.

Effect of Second phase particles on formability

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Shape of inclusion on formability

• Manganese sulfides (MnS) in hot formed product form elongated stringers.

• Stringers (have low transverse ductility) – bad for forming operation (cold heading & upsetting operation) – it will end up with longitudinal crack along the tensile edge of the bulge.

• Addition of small quantity of cerium, calcium or zirconium prevent elongated stringers and enhance globular sulfides (less stress concentration in transvers direction)which improves transvers ductility.

• Globular sulfides cause less void growth.

• Calcium – modify plasticity of oxide inclusions (beneficial).

• Complete sulfide shape modification – occurs at cereium to sulfur ratio of 1.5

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