Chapter 11 - Mirco-structure and Metallography | Iron-Carbon Alloys

Question 1

Alloys

Answer 1

Mixture of metals or mixture of metal and another element.

Question 2

Steel

Answer 2

alloy of steel and carbon.

Question 3

Microstructure & Properties of Plain Carbon Steels

Answer 3

Fe-C Diagram

- Great variability in the microstructures we form in steels ( due to various phases)

Question 4

Fe-C Phase Diagram

Answer 4

allows us to process alloys by conrol temperature & Concentration of Fe & C

Question 5

Pure Iron 1

Answer 5

Solidification form at 1534 degrees: BCC Phase δ

Question 6

Pure Iron 2

Answer 6

2 Polymorphic changes (first: 1400ºC, FCC austentite γ phase; then at 910ºC, BCC ferrite a phase

Question 7

Pure Iron 3

Answer 7

Max solubility of C in δ, γ and α is 0.1, 2.0, & 0.02 wt.%

Question 8

Pure Iron 4

Answer 8

Alloys with C content <2.06 wt.% = Steels
C content > 2.06 wt.% = Cast Irons
Majority of Commercial steels have < 1wt.%

Question 9

Ferrite

Answer 9

α solid solution of small amount of Carbon dissolved in α (BCC) iron.
Stable form of iron below 912°C
Softest structure on Phase diagram

Question 10

Ferrite 2

Answer 10

Tensile Strength = 2.75x107 Pa
Elongation = 40% in 2 in
Hardness = > Rockwell C 0 or
> Rockwell B 90

Question 11

Austentite 1

Answer 11

solid solution of Carbon dissolved in gamma (FCC) iron
High Formability
Normally not stable at room temperature.

Question 12

Austentite 2

Answer 12

• Tensile strength = 1.03x109 Pa
• Elongation = 10% in 2 in
• Hardness > Rockwell C 40

Question 13

Cementite 1

Answer 13

AKA Iron Carbide
Very hard, brittle intermetallic compound (6.67% C)
Low tensile strength (approx. 3.44x107 Pa)
High Compressive strength

Question 14

Ledeburite

Answer 14

mixture of astentite and cementite

formed at 1130°C & contains 4.3% C

Question 15

Pearlite

Answer 15

Mixture of ferrite & cementite

0.80% C and is formed at 723°C

Question 16

Pearlite 2

Answer 16

Tensile strength = 8.27 x108 Pa
Elongation = 20% in 2 in
• Hardness = Rockwell C 20 or
Rockwell B 95-100 or
BHN 250-300

Question 17

Formation of pearlite

Answer 17

1) You have austentite
2) Cementite Nucleus Grows on A Boundary
3) Ferrite grows either side of C Nucleus
4) Expands up and out the way until all austentite is covered.

Question 18

Martensite

Answer 18

Super saturated solution of carbon in ferrite

Question 19

Martensite Formation

Answer 19

Formed when steel is cooled so quickly that the change from austentite to pearlite is suppressed.

Question 20

Martensite 2

Answer 20

The interstitial carbon atoms distort the BCC ferrite
into a BC-tetragonal structure (BCT).; responsible
for the hardness of quenched steel
• Needlelike structure

Question 21

Phase: Ferrite

Answer 21

Crystal Structure: BCC

Characteristics: Soft, Ductile, Magnetic

Question 22

Phase: Austentite

Answer 22

Crystal Structure: FCC

Characteristics: Soft, Moderate Strength, Non-Magnetic

Question 23

Phase: Cementite

Answer 23

Crystal Structure: Compound of FE & C - Fe3C

Characteristics: Hard & Brittle

Question 24

Carbon Content: 0.01 - 0.1

Answer 24

Soft, ductile, can be work hardened but not heat hardened, weldable,
Used where high formability is needed

Question 25

Carbon Content: 0.1 - 0.25

Answer 25

Soft, ductile, hardened by normalizing, work hardened, weldable, Ductile-Brittle transition just below room temp
General engineering uses for a mild steel

Question 26

Carbon Content: 0.25- 0.6

Answer 26

Very Strong, heat treating produces a wide range of properties, Difficulte to weld, brittle below room temp
used for bars and forgings: rods, springs, hammers & axles shafts

Question 27

Carbon Content: 0.6 - 0.9

Answer 27

Strong (without being heat treated, Ductility will be lower with less carbon
Used for Max strength items (tools, wear resistant components)

Question 28

Carbon Content: 0.9 - 2.0

Answer 28

Wear resistant (V hard = less tough & ductile), No welding, tend to be brittle
Used for cutting tools; wood chisels, files and saw blades.