Ferrous alloys

For S4 students of Kerala University

1. Metallurgy & Metallurgy &
Material ScienceMaterial Science
Dr.S.Jose
Dept of Mechanical Engg.,
TKM College of Engineering, Kollam
[email protected]

2. 2
Ferrous Materials
Non – ferrous alloys
Bearing Materials
Fusible alloys
Composites
Metal matrix composites
Smart Materials.
Module III

3. Ferrous Materials
In engineering applications, lion’s
share is served by ferrous materials.
Factors account for it
availabilityof abundant raw materials
economical extraction
ease of forming
versatile mechanical and physical
properties.

4. Ferrous Materials
Drawbacks of ferrous materials:
poor corrosion resistance
high density & low specific strength
low thermal and electrical conductivities
Classification
steels and cast irons – categorized based
on carbon content.
Steels: %C is upto 2.14%
Cast irons: %C is above 2.14%

5. Ferrous Alloys
Cast irons are called so because they are
usually manufactured through casting
technique owing to their brittle nature
due to presence of iron carbide.
Steelsare serving major part of present
engineering applications.
However, cast irons mostly serve as
structural components.
eg: automobile motor casings, lathe bed,
sliding guides in machinery

6. Steel
 An alloy whose major component is iron,
with carbon being the primary alloying
material.
Carbon acts as a hardening agent,
preventing iron atoms from sliding past one
another.
Carbon atoms occupy interstitial sites of Fe.
Varying the amount of carbon and its
distribution in the alloy controls the
qualities of the resulting steel.

7. Steel
Steel with increased carbon content can
be made harder and stronger than iron,
but is also more brittle.
Currently there are several classes of
steels in which carbon is replaced with
other alloying materials, and carbon, if
present, is undesired.
A more recent definition is that steels
are iron-based alloys that can
be plastically formed

8. Steel - classification
Steels are classified based on their C
content/alloying additions
Plain-carbon steel
Mild (low carbon) steel: < 0.3 wt % C
Medium carbon steel: 0.3 to 0.6 wt% C
High carbon steel: 0.6 to 2.14 wt% C
Alloy steels
HSLA steel
Tool steels
Stainless steel

9. Plain-carbon steel
An alloy of iron and carbon, where other
elements are present in quantities too small to
affect the properties.
Steel with a low carbon content has the same
properties as iron, soft but easily formed. As
carbon content rises the metal becomes harder
and stronger but less ductile.
Carbon steels which can successfully undergo
heat-treatment have a carbon content in the
range of 0.30% to 1.70% by weight.

10. Plain-carbon steel
A limitation of plain carbon steel is the very
rapid rate of cooling needed to produce
hardening.
In large pieces it is not possible to cool the
inside rapidly enough and so only the surfaces
can be hardened.
This can be improved with the addition of other
elements resulting in alloy steels
Trace impurities of various other elements can
have a significant effect on the quality of the
resulting steel

11. Low-carbon steel
Steel with a low carbon content has the same
properties as iron, soft but easily formed.
As carbon content rises the metal becomes
harder and stronger but less ductile.
Carbon present is not enough to strengthen
them by heat treatment, hence are
strengthened by cold work.
They are easily weldable and machinable.
Typical applications: tin cans, automotive body
components, structural shapes, etc.

12. Mild steel
The most common form of steel as it
provides material properties that are
acceptable for many applications.
Mild steel has medium carbon content (up
to 0.3%) and is therefore neither
extremely brittle nor ductile.
It becomes malleable when heated, and so
can be forged. It is also often used where
large amounts of steel need to be formed,
for example as structural steel.

13. Mild Steel
They are less ductile and stronger
than low carbon steels.
Heat treatable (austenitizing,
quenching and tempering).
Hardenability is increased by adding
Ni, Cr, Mo.
Used in various tempered conditions.
Typical applications: gears, railway
tracks, machine parts.

14. High Carbon Steels
They are the strongest and hardest of carbon
steels.
Heat treatable. Used in tempered or hardened
conditions.
Toughness, formability and hardenability are
quite low.
Not recommended for welding.
Alloying additions – Cu, V, W, Ni, Cr, Mo
Typical applications: Knives, razors, hacksaw
blades, etc where high wear resistance is the
prime requirement.

15. Alloy Steels
Limitations of plain carbon steel are
overcome by adding alloying elements
The alloying elements improve various
properties
HSLA steel
Tool steels
Stainless steel

16. HSLA steel
High strength low alloy steel is a type
of steel alloy that provides many benefits over
regular steel alloys.
HSLA alloys are much stronger and tougher
than ordinary plain carbon steels.
They are used in cars, trucks, cranes, support
columns, pressure vessels, bridges and other
structures that are designed to handle a lot
of stress.

17. HSLA steel
A typical HSLA steel may contain 0.15% carbon,
1.65% manganese and low levels (under
0.035%) of phosphorous and sulphur.
It may also contain small amounts of Cu, Ni, V,
Cr, Mo, Si or Zi.
HSLAs are therefore also referred to as
'microalloyed', as they are indeed alloyed in
extremely small amounts in comparison to other
main commercial alloy steels.
HSLA steels are also more resistant to rust than
most carbon steels.

18. Stainless steel
An iron-carbon alloy with a minimum of
12% chromium along with other alloying elements
Highly corrosion resistant owing to presence of
chromium oxide.
The name originates from the fact that stainless
steel does not stain, corrode or rust as easily as
ordinary steel
It is "stains less", but is not actually stain proof.
Stainless steel is 100% recyclable.

19. Stainless Steels
Its resistance to corrosion and staining, low
maintenance, relative inexpense, and familiar
lustre make it suitable for a host of commercial
applications.
There are over 150 grades of stainless steel, of
which fifteen are most common.
Typical applications– cutlery, surgical knives,
storage tanks, domestic items, jewellery.
Three kinds
Ferritic& hardenable Cr steels
Austenitic and precipitation hardenable
Martensitic

20. Stainless Steels - types
Ferritic steels are principally Fe-Cr-C alloys with
12-14% Cr with small additions of Mo, V, Nb, Ni.
Austenitic steels contain 18% Cr and 8% Ni plus
minor alloying elements.
Martensitic steels are heat treatable. Major
alloying elements are: Cr, Mn and Mo.
Ferritic and austenitic steels are hardened and
strengthened by cold work because they are not
heat treatable.
Austenitic steels are non-magnetic as against
ferritic and martensitic steels, which are
magnetic.

21. Tool Steels
A variety of alloy steels that are particularly
well-suited to be made into tools.
Their suitability comes from their
distinctive hardness, resistance to abrasion,
their ability to hold a cutting edge, and/or their
resistance to deformation at elevated
temperatures (red-hardness).
Tool steel is generally used in a heat-
treated state.
Carbon content between 0.7% and 1.4%,

22. Tool Steels - Types
 Water-hardening grades
 Air-hardening grades
 Cold-working grades
 Shock resisting grades
 High speed grades
 Hot-working grades

23. Effects of Alloying Elements
Barrier to Dislocation movement
Polymorphic transformation temperature
Strengthening of ferrite
Formation and stability of carbides
Displacement of the eutectoid point
Retardation of the transformation rates
Lowering of critical cooling rate
Improvement in corrosion resistance

25. Main Alloying Elements in Steel
Manganese
Chromium
Nickel
Molybdenum
Titanium
Phosphorus
Silicon
Copper
Sulphur
Cobalt
Aluminium
Vanadium
Tungsten
Lead
Colubium
Boron

26. Manganese (Mn)
Added to steel to improve hot working
properties and increase strength, toughness
and hardenability.
Improves ductility and wear resistance.
Eliminates formation of harmful iron sulfides,
increasing strength at high temperatures.
Manganese, like nickel, is an austenite forming
element
Usually present in quantities from 0.5% to 2%

27. Chromium (Cr)
Chromium is added to the steel to increase
resistance to oxidation.
This resistance increases as more chromium is
added.
'Stainless Steel' has approximately 12%
chromium
When added to low alloy steels, improves
hardenability and strength.
Resists abrasion and wear (with high carbon).

28. Nickel (Ni)
Added in large amounts, over about 8%, to
high chromium stainless steel to form the most
important class of corrosion and heat resistant
steels, the austenitic stainless steels.
Increases toughness and strength at both high
and low temperatures.
Improves resistance to oxidation and corrosion.
Increases toughness at low temperatures when
added in smaller amounts to alloy steels.
Strengthens unquenched or annealed steels.
Quantity addition is from 1 to 4%

29. Molybdenum (Mo)
When added to chromium-nickel austenitic steels,
improves resistance to pitting corrosion especially
by chlorides and sulphur chemicals.
When added to low alloy steels, it improves high
temperature strengths and hardness.
When added to chromium steels it greatly
diminishes the tendency of steels to decay in
service or in heat treatment.
Increases hardenability and strength.
Enhances corrosion resistance in stainless steel.
Forms abrasion resisting particles.
used in small quantities from 0.10 to 0.40%.

30. Titanium (Ti)
Improves strength and corrosion resistance,
limits austenite grain size.
The main use of titanium as an alloying
element in steel is for carbide stabilisation.
It combines with carbon to form titanium
carbides, which are quite stable and hard to
dissolve in steel.
Reduces martensitic hardness and
hardenability in medium Cr steels.
Prevents formation of austenite in high Cr
steels.

31. Phosphorus (P)
Phosphorus is usually added with sulphur to
improve machinability in low alloy steels
When added in small amounts, aids strength
and corrosion resistance.
Phosphorus additions are known to increase
the tendency to cracking during welding.
Strengthens low-carbon steel.
Increases resistance to corrosion.

32. Silicon (Si)
Improves strength, elasticity, acid resistance
and promotes large grain sizes, which cause
increasing magnetic permeability.
Used as a deoxidising (killing) agent in the
melting of steel.
Silicon contributes to hardening of the ferritic
phase in steels.
Alloying element for electrical and magnetic
sheet.
Increase hardenability of steels.
Strengthens low-alloy steels.
Used in the range of 1.5% to 2.5%

33. Copper (Cu)
Copper is normally present in stainless
steels as a residual element.
It is added to a few alloys to produce
precipitation hardening properties.
Improves corrosion resistance.
Usually 0.15 to 0.25% added

34. Other Alloying Elements
Sulphur (S)
When added in small amounts improves
machinability
used in the range 0.06 to 0.30%.
Cobalt (Co)
Improves strength at high temperatures and
magnetic permeability.
Aluminum (Al)
Dexodises and limits austenite grain growth
Alloying element in nitriding steel.

35. Alloying element Range of
percentage
Important functions
Sulphur < 0.33
Improves machinability, reduces weldability and ductility
Phosphorus <0.12
Improves machinability, reduces impact strength at low temperature.
Silicon 1.5 to 2.5
Removes oxygen from molten metal, improves strength and toughness, increases
hardenability, magnetic permeability
Manganese 0.5 to 2.0
Increases hardenability, reduces adverse effects of sulphur.
Nickel 1.0 to 5.0
Increases toughness, increases impact strength at low temperatures
Chromium 0.5 to 4.0
Improves resistance to oxidation and corrosion. Increases high temperature
strength
Molybdenum 0.1 to 0.4
Improves hardenability, enhances the effect of other alloying elements, eliminates
temper brittleness, improves red hardness and wear resistance.
Tungsten 2.0 to 3.0
Improves hardenability, enhances the effect of other alloying elements, eliminates
temper brittleness, improves red hardness and wear resistance.
Vanadium 0.1 to 0.3
Improves hardenability, increases wear and fatigue resistance, elastic limit.
Titanium < 1.0
Improves strength and corrosion resistance.
Copper 0.15 to 0.25
Improves corrosion resistance, increases strength and hardness
Aluminium 0.01 to 0.06
Removes oxygen from molten metal
Boron 0.001 to 0.05
Increases hardenability
Lead < 0.35
Improves machinability
Cobalt 5 to 10
Improves red hardness, improves wear and corrosion resistance.

36. Wrought Iron
Wrought iron is commercially pure iron
Wrought iron literally means worked
iron.
It is so named because it is worked from
a "bloom" of porous iron mixed with slag
and other impurities.
Carbon content not more than 0.15%

37. Wrought Iron
It is a fibrous material with many
strands of slag mixed into the metal.
These slag inclusions give it a "grain"
resembling wood, with distinct
appearance when etched or bent to the
point of failure.
It is tough, malleable & ductile and is
easily welded.
It is too soft for blades.

38. Wrought Iron
The fibers in wrought iron give it properties not
found in other forms of ferrous metal.
Hammering a piece of wrought iron cold causes
the fibers to become packed tighter, which
makes the iron both brittle and hard.
Wrought iron lacks the carbon content
necessary for hardening through heat
treatment,
wrought iron cannot be bent as sharply as
steel, for the fibers can spread and weaken the
finished work.

40. Cast Irons
They have low melting temperatures, in the
range 1150-1300
O
C with good fluidity for taking
good casting impressions.
It is a low cost material, as it can be produced
relatively easily using low cost raw materials and
technology.
Though brittle and have lower strength compared
to steels, cast irons have higher compressive
strength, ability to absorb vibrations (damping
capacity), better wear and abrasion resistance,
rigidity and machinability.

41. Cast Irons
With suitable composition and heat
treatments, a variety of microstructures
can be developed with varying properties.
Alloy cast irons possess high corrosion
and heat resistance.
However, they are not ductile to be rolled,
drawn or mechanically worked
Hence the only manufacturing process
applicable is casting, and so the name.

42. Classification of Cast Irons
Cast irons are generally classified
based on the metallurgical structure
and appearance. The following factors
control the structure and appearance.
Carbon content
Presence of other elements
Cooling rate during and after
solidification
Heat treatments

44. White & Gray Cast Irons

45. Malleable Cast Iron

46. Ductile cast iron

48. Thank you