The document discusses the effect of microstructure on the formability of steels. It states that the amount of pearlite and ferrite phases affect formability, with pearlite decreasing ductility and ferrite increasing it. Finer pearlite and ferrite grain sizes increase strength but decrease formability. Spheroidizing pearlite increases ductility. The presence of inclusions like oxides and sulfides reduces ductility depending on their shape, size, and distribution. Globular inclusions perform better than elongated ones. The document also discusses how these microstructural factors affect the formability of different steel types like austenitic stainless steels.

1. 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.
9/29/2015 1

2. 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.
9/29/2015 2

3. • 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 *
9/29/2015 3

4. 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.
9/29/2015 4

5. 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.
9/29/2015 5

6. Spheroidization
9/29/2015 6

7. Effect of Spheroidization on *
9/29/2015 7

8. Effect of Spheroidization on T
9/29/2015 8

9. • 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
9/29/2015 9

10. Effect of Pearlite on Forging Pressure
9/29/2015 10

11. Effect of ferrite grain size on forging
pressure
9/29/2015 11

12. • 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
9/29/2015 12

13. Microstructure cont…
Eutectoid steels
• Coarse grain steels – poor mechanical
properties – tendency towards tensile & shear
flow failure- during forging & drawing.
9/29/2015 13

14. • 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
9/29/2015 14

15. • 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
9/29/2015 15

16. • 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
9/29/2015 16

17. • 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
9/29/2015 17

18. • 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
9/29/2015 18

19. 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.
9/29/2015 19

20. Effect of oxide, carbide,sulfide on total
ductility
9/29/2015 20

21. • 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
9/29/2015 21

22. • 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
9/29/2015 22

23. Effect of inclusions on total ductility
16% - 25% Ni steel
9/29/2015 23

24. • 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
9/29/2015 24

25. 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
9/29/2015 25