Ferrous Alloys
Metallurgy
Chapter 3
Ferrous Alloys
3.1. Introduction:
Most engineering metallic materials are alloys.
Metals are alloyed to enhance their properties, such as strength,
hardness or corrosion resistance, and to create new properties, such
as shape memory effect.
Ferrous alloys could be classified as shown in Figure 2.5.
Fig.2.5: Ferrous alloys classification.
Ferrous alloys containing Fe as the main element.
The most important ferrous alloy system (Fe-C system)
Alloys of this system can be further divided into steels and cast
irons.
Steels contain less Carbon (generally most common <1.4
wt%C) than cast irons (generally 2.14~4.3wt%C), Figure 2.6.
All steels solidify into a single γ-Fe structure first and then
experience the complex eutectoid reaction.
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Fig.2.6
Therefore, heat treatment processes, which alter the
eutectoid reaction, are vitally important for controlling
microstructure and properties of steels.
Cast irons experience complex eutectic reaction during solidification,
due to the formation of graphite or cementite.
Solidification control is the most important single factor for
properties of cast irons.
3.2. Steel Alloys:
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Steel is classified into two types.
1. Plain Carbon Steel:
Host or base metal is iron and alloying element is carbon.
2. Alloy Steel:
Base metal is iron and other alloying elements are other nonferrous metals in small amount other than carbon.
Plain Carbon Steels are further classified into:
1. Low Carbon steels,
2. Medium Carbon steels,
3. High Carbon steels.
4. Tool Steel.
Alloy steels are further classified into:
1. Low Alloy Steels,
2. medium Alloy Steels,
3. high Alloy Steels
A. Plain Carbon Steel:
(0.15% carbon steel)
❖ Low carbon steels (mild steels):
Composition of 0.05%-0.25% carbon and up to 0.4% manganese.
It is a low-cost material that is easy to shape.
While not as hard as higher-carbon steels, carburizing can
increase its surface hardness.
Has High formability, high ductility: elongation: ~30% relatively.
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Has Low strength: yield strength: 250~400 MPa.
Has excellent weldability.
Cannot be strengthened by heat treatment.
Usually strengthened by cold working.
Typical applications: pipes, panels, sheets, wires, I-beams etc.
❖ Medium-carbon steels (structural steels):
Composition
of
0.25%-0.54%
carbon, with
0.55%-1.65%
manganese.
(0.4% carbon steel)
Medium carbon steel has long-wearing properties.
Good combination of strength and ductility.
Yield strength: 300~600MPa
Tensile strength: 400~800MPa
Elongation: ~25%
Strengthenable by heat treatment.
Weldable; weldability deteriorates with increasing C%.
Applications: Used for load-bearing applications, crankshaft, bolts,
gears, heavy-duty machinery, mining equipment, cranes.
❖ High strength low alloy steels (HSLA):
Medium carbon steels have desired mechanical properties for
structural
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applications
but
suffer
from
welding-induced
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Metallurgy
embrittlement (‫ )التقصف او الهشاشة‬due to the formation of
martensite.
To overcome this problem:
Carbon content in these steels is reduced (<0.3%),
And the loss of strength is compensated by increasing Mn
content (>1%),
And by micro-alloying with Nb, V, Ti, Cr and Cu.
This leads to the development of HSLA steels.
Applications: These steels are widely used for manufacturing
large welded structures, such as Sydney harbor bridge, ocean
liners and cargo ships (‫)عابرات المحيط و سفن الشحن‬, oil drilling rigs ( ‫منصات‬
‫ )حفر آبار النفط‬and platforms, large mining and earth moving equipment,
and pressure vessels and storage tanks.
❖ High carbon steels (Spring steels):
Composition
of
0.55%-0.95%
carbon, with
0.30%-0.90%
manganese.
It is very strong and holds shape memory well, making it
ideal for springs and wire.
Mostly (Predominately ‫ )في الغالب‬eutectoid pearlite at room
temperature.
Often strengthened and hardened by heat treatment.
High strength and moderate toughness.
Have poor weldability and poor machinability.
❖ Tool steels:
Composition of 0.8~1.2%C.
Very high hardness, low toughness, very difficult to machine.
Applications: used for chisels, hammers, knives, saw blades, drills,
dies, punches, cutlery, chine tools and wear resistant applications.
❖ Stainless Steels:
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Stainless steel is a steel alloy with increased corrosion
resistance compared to carbon/alloy steel.
Common alloying ingredients include chromium (usually at
least 11%), nickel, or molybdenum.
Adding (Cr → Cr2O3) formation, an oxide which protects the
underlying alloy.
❖ Three basic classes, specified by microstructure:
Ferritics: Fe-Cr alloys (12~25%Cr), can be cheap.
BCC structure Typical.
alloy Fe-15Cr-0.6C, used in quench and tempered conditions
strengthened by carbide precipitation
Uses rust-free ball bearings, scalpels (‫)المشارط‬, knives.
Austenitics: Fe-Cr-Ni alloys (18Cr-8Ni),
FCC structure (stabilized by adding Ni).
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Corrosion resistance Precipitation hardened, high strength and
hardness.
Typical alloy Fe-18Cr-8Ni-1Mn-0.1C.
Disadvantage: work hardens rapidly so more difficult to shape
and machine.
Advantages of all FCC metals and alloys.
➢ ↑ Toughness.
➢ ↑ Ductility.
➢ ↑ creep resistance
Martensitics: Fe-Cr alloys, low Cr, hard, cutting tools.
BCT structures (body centered tetragonal) and they are classified as a
hard Ferro-magnetic group.
Due the addition of carbon, they can be hardened and
strengthened by heat treatment, in a similar way to carbon steels.
The main alloying elements are chromium (10.5 % to 18 %),
molybdenum (0.2 % to 1 %), no nickel (except for two grades), and
carbon (0.1 % to 1.2 %).
Martensitic steels are suitable for applications where the material
is subjected to both corrosion and wear.
Applications: used for surgical and dental instruments (‫األدوات‬
‫)الجراحية وطب األسنان‬, wire, screws, springs, blades and cutting tools,
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fasteners, gears and ball bearings and races, gauge blocks, moulds and
dies etc.
They are also used in the petrochemical industry for steam and gas
turbines blades and buckets.
Typical other applications are aerospace, automotive, hydroelectric
engines, cutlery (‫)أدوات تناول الطعام‬, defense, power hand tools, pump parts,
valve seats, chisels, bushings, shafts, and sporting equipment industry
etc.
Duplex: (18Cr-5Ni) mixed ferrite & austenite.
Duplex stainless steels are called “duplex” because they have a twophase microstructure consisting of grains of ferritic and
austenitic stainless steel.
The picture shows the yellow austenitic phase as “islands”
surrounded by the blue ferritic phase.
- When duplex stainless steel is melted it solidifies from the liquid
phase to a completely ferritic structure.
- As the material cools to room temperature, about half of the
ferritic grains transform to austenitic grains (“islands”).
- The result is a microstructure of roughly 50% austenite and 50%
ferrite.
Duplex Stainless Steels have roughly twice the yield strength of
their counterpart austenitic grades.
- This allows equipment designers to use thinner gauge material for
vessel construction.
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❖ Classification of steel by alloy content:
-
Plain carbon steels,
Manganese steels,
Silicon-manganese steels,
Chromium steels,
Chromium-nickel (stainless) steels,
Tungsten-chromium-vanadium (tool) steels:
etc.
❖ Steel Designation
based on steel composition:
Proposed by organizations such as American Iron and Steel
Institute (AISI) and the Society of Automotive Engineers
(SAE).
A basic four-digit system is used by the SAE/AISI system
to designate the chemical composition of alloy steels and
carbon steels.
Schematic Representation of AISI/SAE Steel Designation
System
4-numeral designation system:
- 1st digit Type of steel (1 for plain C steels, 2 for nickel steels, etc.).
- 2nd digit Approx. % of predominant (‫ )غالب‬alloying element.
- 3d & 4th digit Mean carbon content divided by 100.
Example:
1. SAE 1018: indicates plain carbon steel containing 0.18% of carbon.
2. SAE 5130: indicates a chromium alloy steel containing 1% of
chromium and 0.30% of carbon.
An additional letter is sometimes added between the second and
third digits of the code groups such as 11L41, 12L14 or 50B40.
The letter L: indicates the addition of lead (between 0.15% and
0.35%) to improve the machinability of the steel.
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The letter B: indicates the addition of boron (between 0.0005%
and 0.003%) to low carbon steels to enhance the
hardness of the steel.
Further, the merchant (commercial) quality steels used as hot-rolled
steels bars in production of non- critical parts of machinery and
structure are designated with prefix M.
The alloy steels with prefix E indicate electric furnace steel and
suffix H indicate that the steel has been produced to the required
hardenability limits.
Given below is a table illustrating the four-digit index classification of
alloy steels by the SAE-AISI system.
Tab. 2.1: Four-Digit Index Classification of Alloy Steels
SAE Designation
1xxx
2xxx
3xxx
4xxx
5xxx
6xxx
7xxx
8xxx
9xxx
❖
Type
Carbon steels
Nickel steels
Nickel-chromium steels
Molybdenum steels
Chromium steels
Chromium-vanadium steels
Tungsten steels
Nickel-chromium-vanadium steels
Silicon-manganese steels
Steel Microstructures (As influenced by carbon content)
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Tab. 2.2: Commercial plain carbon steels with common uses of these alloys.
Type of
steel
Percentage
carbon
Dead mild
0.05-0.15
0.90-1.00
1.00-1.10
1.10-1.20
Chain, stampings, rivets, wire, nails, seam-welded pipes, mattresses,
hot- and cold-rolled strip for many purposes
Structural steels, RSJ, screws, machine parts, tin-plate, case-hardening,
drop-forgings, stampings
Machine and structural work, gears, free-cutting steels, shafting, levers,
forgings
Connecting-rods, shafting, wire, axles, fish-plates, crane hooks, hightensile tubes, forgings
Crankshafts, axles, gears, shafts, die-blocks, rotors, tyres, heat-treated
machine parts
Loco tyres, rails, laminated springs, wire ropes
Drop-hammer dies, set-screws, screw-drivers, saws, mandrels, caulking
tools, hollow drills
Band saws, anvil faces, hammers, wrenches, laminated springs, car
bumpers, small forgings, cable wire, dies, large dies for cold presses
Cold chisels, shear blades, cold setts, punches, rock drills, some hand
tools
Springs, high-tensile wire, axes, knives, dies, picks
Drills, taps, milling cutters, knives, screwing dies
Ball bearings, dies, drills, lathe tools, woodworking tools
1 .20-1 .30
Files, reamers, knives, broaches, lathe and wood-working tools
1.30-1.40
Saws, razors, boring and finishing tools, machine parts where resistance
to wear is essential
0.10-0,20
Mild
0.20-0.30
0.30-0.40
Medium
carbon
0.40-0.50
0.50-0.60
0.60-0.70
High carbon
0.70-0.80
0.80-0.90
Tool steels
Uses
3.3. Cast Iron:
Cast irons are manufactured by melting pig iron in a cupola or other
melting furnaces.
Little or no change in composition takes place during melting.
On the Fe-C system, these are to the right of steels,
With carbon between 2.14 & 5.3 %, but more usual 2.5 to
4%.
Really is tertiary alloy system, with the third element silicon.
▪ The microstructures present depend strongly on the chemical
composition (%Si) and the cooling rate of the cast.
Disadvantage: BRITTLE, due to Presence of high carbon, which
limits their formability.
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Advantages: can be sand cast to intricate shapes using basic
technology. So casting is the only way of making any
product using cast irons
❖Some general properties:
Low melting point and excellent fluidity.
Very good corrosion resistant.
Excellent wear resistant.
Excellent machinability.
❖Classification of Cast Iron:
Type of
Graphite
cast iron
White
No
Gray
Flake
Anneal:
Malleable
flake to
nodule
Nodular
Type of
iron
Grey
Ductile
White
Malleable
Nodular
C
2.5-4.0
3.0-4.0
1.8-3.6
2.2-2.9
Ductility
No
No
Fast cooling rates
Slow cooling rates
Yes
white iron + annealing heat
treatment
Yes
additions made so that
nodules of graphite form
instead of flakes
Si
1.0-3.0
1.8-2.8
0.5-1.9
0.9-1.9
Composition, %
Mn
P
0.2-1.0
0.002-1.0
0.1-1.0
0.01-0.1
0.25-0.8
0.06-0.2
0.15-1.2
0.02-0.2
S
0.02-0.25
0.01-0.03
0.06-0.20
0.02-0.20
❖Factors influencing which type will form:
%C.
%Si.
Temperature (cooling rate).
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Ferrous Alloys
A. White
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Metallurgy
Cast Iron:
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Ferrous Alloys
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Produced by rapid cooling of molten gray CI.
Carbon presents as extremely hard and brittle iron carbide
(Fe3C) compound in pearlite.
Because of this carbide phase, the fractured surface takes on a
white appearance.
Properties:
Excellent wear resistance.
High compressive stress.
Un-machinable.
Not workable.
Have very little commercial uses.
It uses is limited to only where a hard wear-resistant surface is
required and for rollers.
Fe-2.8wt%C-1.8wt%Si (White cast iron)
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B.Grey cast iron:
If we put in 2 to 3 % Silicon, and cool the iron reasonably
slowly (don’t quench it) the Si will cause the carbon to form as
graphite flakes – Gray Cast Iron.
If we put in more Silicon and cool slowly we can get
virtually all the carbon out of the austenite and into the
flakes, so the matrix is ferrite, or we have a ferritic gray
CI.
Because of carbon presents as free “flaky” graphite, the fractured
surface takes on a grey appearance.
Properties of grey cast iron:
Cheap.
Low melting point.
Fluid– easy to cast, especially advantageous into large complex
shapes.
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Excellent machinability.
Excellent bearing properties.
Good compressive strength, making them suitable for
damping applications..
Excellent wear and corrosion resistance.
Can be heat treated (surface hardened etc.).
Can be alloyed etc.
Grey irons are weak and brittle due these graphite flakes, which
act as cracks.
❖ the properties of grey cast iron are strongly dependent on the
shape of the graphite flakes:
Flakes.
Spheroids: relatively high toughness and ductility; formed
by adding Cu or Mg.
Grey Irons – Application:
Grey irons are by far the most produced among all cast irons.
Grey irons are used primarily for their low cost and excellent
castability.
Typical applications include:
➢
Engine cylinders, pistons, gear box casing, transmission casing,
machine tool bases, balance weight of large cranes, large diameter
underground pipework.
➢
They are used always under compressive loading conditions.
➢
They are unsuitable for taking tensile loads or bending loads.
C. Malleable Cast Iron:
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If we heat white Cast Iron above its critical line, normally between
900 to 1000 °C for 20 hours we’ll make the carbide
(cementite) convert to ferrite (almost pure iron) and graphite),
And it will produce a rough clusters of graphite, kind of
between a flak and a nodule (agglomerate “‫)”كتل‬, - malleable CI.
the decomposed graphite grouped into clusters (‫)عناقيد‬, known as
“tempered carbon”
Because of this clustered form (not flakey form), cast iron
becomes more malleable and ductile than grey iron.
Properties of Malleable cast iron:
These cast irons are stronger, tougher and much more ductile
than grey irons, compatible to nodular irons.
They have certain capacity to take shock loading, bending and
tension.
They are suitable for castings of thin thickness.
They are expensive to produce, largely due to the heat
treatment.
Typical applications include:
Gear box casing, transmission casing, and differential casing.
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D. Nodular or Ductile Cast Irons:
Carbon presents as graphite “nodules” in the ferrite-pearlite
matrix.
Produced directly by melting low-sulphur grade pig/grey iron
and adding a small amount of magnesium or cerium before
casting.
The graphite formed as nodules or spheroids instead of
flake in presence of magnesium or cerium.
Properties of nodular cast iron:
Ductile/nodular/spheroidal graphite irons are the most ductile
and malleable cast irons, having mechanical properties approaching to
steels.
These irons are much stronger and tougher than grey irons.
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They are more expensive than grey irons.
Typical applications:
They are produced
applications.
and
used
for
high
specification
Include: gears, crankshafts, pump bodies, pressure valves,
rollers.
Typical properties and applications of cast irons
Types of
Cast Iron
Grey Cast
Iron
White Cast
Iron
Malleable
Cast Iron
Ductile
Cast Iron
Hardness
(BHN)
Tensile
Strength (ksi)
150 – 270
20 – 60
> 320
5
< 135
50 – 65
140 – 270
60 – 90
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Typical Cast Iron Applications
Engine block, cylinder heads, piston, heavy
machine beds, manhole covers
Car wheels, rolls, grinding media balls,
production of malleable irons
Pipe fittings, valves, connecting rods,
transmission gears and differential cases
heavy-duty machinery (gears, dies, rolls),
pressure castings (pumps, valves, shockresisting parts), railway inserts, pipe fittings
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