1. Materials Engineering
Lecture 9:
Metals and alloys.
Iron alloys

2. Metals and Alloys
There are no really pure metals! Neither there should be.
An alloy can be:
Mechanical mixture: no chemical bond between components, variable composition, heterogeneous
Compound: chemical bond between components, constant composition, homogeneous
Solid solution (substitutional or interstitial): some chemical bond between components, variable composition, homogeneous
Pearlite (mechanical mixture)

3. Iron-Carbon Phase diagram
a-ferrite: BCC, stable at T<912ºC
g-ferrite or austenite: FCC, stable at 912ºC<T<1394ºC
d-ferrite: BCC, stable at T>1394ºC
Fe3C cementite
6.67 wt% of C = 100% of Fe3C (cementite).
Invariant points:
4.30% C, 1147ºC: eutectic:
L ↔ g + Fe3C
0.76% C, 727ºC: eutectoid:
g(0.76%C) ↔ a (0.022%C) + Fe3C

4. Eutectoid structure: Pearlite
Pearlite is an a-Fe3C solid solution having grain (colony) microstructure
Ductile, easily formable, cheap;
Not hard, susceptible to corrosion
Cables, wires, rails

5. Hypoeutectoid structures
Ferrite + pearlite
Cheap plain steels
Mild, ductile, easily formable
Very easily corrode

6. Hypereutectoid structures
Pearlite + Cementite
Hard
Brittle
The cementite forms a brittle network around the pearlite. This presents difficulty in machining the hypereutectoid steels.
Old hypereutectoid structure (1st century CE, Spain)

7. Austenite
Hard and Ductile; non-magnetic
Stabilization: Mn, Ni

8. Austenite transformations
Austenite is unstable at T<912ºC. As time goes austenite structure can transform:
Cementite + Pearlite
Cementite + Bainite (very fine 2 phase needle structure; stronger and harder that pearlite)

9. Spheroidite
Pearlite or bainite steel is heated to ~700ºC (below eutectoid point) and left at that temperature for hours
Soft and “gummy”
This structure is desirable when minimum hardness, maximum ductility and maximum machinability are required.

10. Martensite Austenite hot steel is rapidly quenched  martensite
Non-diffusional transformation => occurs instantaneously
Non-equilibrium structure
Martensite: BCT (body-centered tetragonal)
Hardest and strongest of all steels; the most brittle;
can crack on quenching
Martensite structure in a 1 century BCE knife
Martensite needles in austenite matrix (x1220)

11. Tempered martensite Martensite is heated to 250-650ºC
Martensite (BCT, 1 phase)  tempered martensite (a + Fe3C)
Extremely small cementite particles
Almost as hard and strong as martensite, but ductile and tough

12. Phase transformations for iron alloys (review)
Austenite
Pearlite
(a + Fe3C)
+ proeutectoid
Bainite
Martensite
(BCT)
Tempered martensite
Slow cooling
Rapid cooling
Reheat

13. Ferrous alloys Steels (<2%C) Cast Irons (2 – 6.7%C)
Low-Carbon (iron, <0.25%C)
Medium-Carbon ( %C)
High-Carbon ( %C)
Cast Irons (2 – 6.7%C)

14. Low-Carbon steels Armco Iron: < 0.02%C Mild Steels: 0.1 – 0.2%C
All have pearlite-ferrite structure; no martensite
Strengthening by cold work
Soft, weak, ductile, tough;
Good for machining, welding; very cheap
Used in automobiles, structural shapes, sheets

15. Plain low-carbon steels High-Strength Low-Alloy Steels
applications
Ductility (%EL in 50 mm)
Yield Str., MPa
Tens. Str., MPa
%Other
%Mn %C
UNS
AISI/SAE, ASTM
Plain low-carbon steels
Automobile panels. Nails, wire 28
180
325
0.45
0.10
G10100
1010
Pipes, sheets, structural 25
205
380
0.20
G10200
1020
Structural (bridges, buildings) 23
220
400
0.20Cu
1.00
0.29
K02600
A36
High-Strength Low-Alloy Steels
Bolted structures 21
290
435
<0.30Si, >0.20Cu
1.35
0.28
K12810
A440
Structures for low ambient temperatures
520
0.30Si, 0.08V, 0.02N, 0.03Nb
0.22
K12002
A633 Grade E
Truck frames, railway carriages 15
552
655
0.60Si, 0.1V, 0.20Al, 0.015N
1.60
0.18
K11804
A656 Grade 1
AISI: Americal Iron and Steel Institute, SAE: Society of Automotive Engineers;
ASTM: American Society for Testing and Materials
UNS: Uniform Numbering System

16. Medium-Carbon and High-Carbon Steels
Hardening by austenitizing, quenching then tempering.
Structure (for tempered): tempered martensite
Strong, poor ductility and toughness
High-carbon steels are especially wear-resistant, holding a sharp-cutting edge

17. Medium and High-Carbon Steels (2)
applications
Ductility (%EL in 50 mm)
Yield Str., MPa
Tens. Str., MPa
%Other
%Mn %C
UNS
AISI/SAE, ASTM
Plain steels
Bolts, crankshaft
33-19
<1.00
0.40
G10400
1040
Chisels, hammers
24-13
0.80
G10800
1080
Knives, blades
26-10
0.95
G10950
1095
Alloy Steels
Springs, hand tools
24-4
Ni, Cr, Mo
0.63
G40630
4063

18. Stainless Steels >11% Cr, (Ni, Al)
Martensitic, ferritic and austenitic
Aplication
Duct., EL% in 50 mm
Y. Str., MPa
Tens. Str, MPa
Condition
%Comp
UNS
AISI
Ferritic
Exhaust components, tanks for agricultural sprays 20
205
380
annealed
0.08C, 11.0Cr, 1.0Mn, 0.50Ni, 0.75Ti
S40900
409
Valves, combustion chambers
275
515
0.20C, 25Cr, 1.5Mn
S44600
446
Austenitic
Chemical and food processing, cryogenic vessels 40
0.08C, 19Cr, 9Ni, 2.0Mn
S30400
304
Martensitic
Rifle barrels, jet engine parts 12
620
485
825
Annealed, Q+T
0.15C, 12.5Cr, 1.0Mn
S41000
410

19. Cast Irons >2.14%C (practically, 3.0-4.5%C)
Fe3C  3Fe(a) + C (graphite)
Gray iron
Ductile (nodular) iron
White iron
Malleable iron

20. Gray Iron C: 2.5 – 4.0% Si: 1.0 – 3.0% Graphite flake structure
Brittle and weak
Especially brittle when chilled
Damping vibrations
High wear resistance
Very cheap

21. Nodular (ductile) iron
Addition of Mg or Ce to gray iron
Strong and ductile (almost as a steel)
Valves, pump bodies, gears etc.

22. White and Malleable Iron
<1.0% Si, rapid cooling  no graphite, only cementite: white carbon
Extremely hard, but very brittle; unmachinable
Rollers in roller mills
Heating white iron at ºC for a prolonged time  malleable iron
Structure: graphite clusters in pearlite (ferrite) matrix
High strength and some ductility
Rods, gears, flanges etc.

23. Musts of this lecture Iron phases (ferrites, austenite, cementite);
Iron microstructures (pearlite, bainite, spheroidite, martensite, tempered martensite);
Steels and cast irons;
Low-carbon steels (properties, application);
Medium and High-carbon steels (properties, application);
Stainless steels
Cast irons