Ferrous alloys, specifically various types of steels, are the most important engineering construction materials due to iron being abundant and economical to extract, and steels having many versatile mechanical and physical properties. However, they have poor corrosion resistance and lower thermal/electrical conductivity than other materials. Steels are classified based on their carbon content as low-carbon (<0.25%), medium-carbon (0.25-0.6%), high-carbon (0.6-1.4%) or cast irons (>2.14%). Alloying elements like chromium, nickel, molybdenum are added to steels to improve properties such as corrosion resistance, strength and hardness. Common alloy steels include

1. Ferrous alloys

2. Ferrous Materials
• They’re the most important role of
engineering construction materials.
 Factors account for it:
 Iron containing compounds exist in
abundant quantities in Earth
 economical extraction
 Many-sided mechanical and
physical properties

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

4. Steel
 Iron-carbon alloys
 Carbon acts as a hardening agent,
preventing iron atoms from sliding past one
another.
 Steel with increased carbon content can be
made harder and stronger than iron, but is
also more sensitive.
 According to carbon concentration, steels
are classified as low-, medium-, high-
carbon types.

5. Low-carbon steel
 Steel with a low carbon content (less than
0.25wt%) has the same properties as iron,
soft and weak alloys but have outstanding
ductility and toughness.
 Unresponsive to heat treatments intended
to form martensite, strengthening is
accomplished by cold work.
 They are easily weldable and machinable.
 Typical applications: automotive body
components, sheets that used in pipelines,
buildings, tin cars.

6. 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.

7. HSLA steel
 High strength low alloy steel is a type
of low carbon steel 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.
 Stregthened by heat treatments.
 They’re ductile, formable, machinable.

8. 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 10 wt% amounts of Cu, Ni,
V, Mo.
 HSLA steels are also more resistant to rust than
most carbon steels.

9. Medium steel
 The most common form of steel as it
provides material properties that are
acceptable for many applications.
 Medium steel has carbon concentration
(0.25wt% to 0.6wt%).
 Heat treated by austenitizing, quenching,
then tempering to improve their
mechanical properties.

10. Medium Steel
 Heat treated are stronger than
low carbon steels, but sacrifice of
ductility and toughness.
 Ni, Cr, Mo improve the capacity of
heat-treated, strength ducility
combination.
 Typical applications: gears, railway
tracks, machine parts.

11. High Carbon Steels
 They are the strongest, hardest and the least
ductile of carbon steels. C contents 0.6wt%
to 1.4wt%
 Heat treatable (used in tempered or hardened
conditions).
 Wear resistant and capable of holding a sharp
cutting edge.
 Toughness, formability are quite low.
 Alloying additions – V, Cr, Mo, Trungsten
 Typical applications: Knives, razors, hacksaw
blades, etc where high wear resistance is the
prime requirement.

12. 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 sharp edge.
 Tool steel is generally used in a heat-
treated state.
 Carbon content between 0.6% and 1.4%,

13. 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

14. Stainless Steels
 Highly resistant to corrosion (rusting) in a
variety of environments, especially the
atmosphere.
 Typical applications – cutlery, surgical knives,
storage tanks, domestic items, jewellery.
 Three classes:



Ferritic & hardenable Cr steels
Austenitic and precipitation hardenable
Martensitic

15. Stainless Steels - types
 Ferritic steels are principally Fe-Cr-C alloys with
12-14% Cr with small additions of Mo, V, 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.

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

17. 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%

18. 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).

19. 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%

20. 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%.

21. 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.

22. 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.

23. 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%

24. 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

25. 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.

26. 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, increas
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, eliminat
temper brittleness, improves red hardness and wear resistance.
Tungsten 2.0 to 3.0 Improves hardenability, enhances the effect of other alloying elements, eliminat
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