Transformation products of ferrous alloys

Question 1

What are equilibrium phases of the Fe-Fe3C diagram?

Answer 1

Austenite, ferrite, graphite

Question 2

What is a metastable phase of the Fe-Fe3C diagram?

Answer 2

Cementite

Question 3

What is a dual phase of the Fe-Fe3C diagram?

Answer 3

Pearlite

Question 4

What is a metastable phase (definition)?

Answer 4

Phase is quasi-equilibrium and will stabilise to a stable, equilibrium phase

Question 5

What does cementite stabilise to?

Answer 5

Fe and C in form of graphite

Question 6

How did the Inuit work iron?

Answer 6

Basalt was crushed, cooled down, and formed crystals; small grains of telluric Fe was found inside; telluric Fe beat out into disk; edges were sharpened; Fe mounted onto and into bone

Question 7

What is the composition of Type 1 telluric Fe alloy?

Answer 7

Up to 4% C, 4% Ni; C is locked up as cementite crystals or pearlite, so is a brittle alloy which cannot be cold-worked; is a Ni-bearing cast iron

Question 8

What is the composition of Type 2 telluric Fe alloy?

Answer 8

Up to 4% Ni, <0.7% C; has lots of ferrite, with small bit of pearlite, so can be cold-worked

Question 9

How did meteoritic Fe become a large source of Fe on earth?

Answer 9

Likely due to two proto planets colliding; silicate external surface stripped away; leave Fe-Ni core in space (16 Psyche)

Question 10

What are the two phases within meteoritic Fe?

Answer 10

Kamacite and taenite

Question 11

What is the composition of kamacite?

Answer 11

5-10% Ni, 90-95% Fe; can be readily cold-worked

Question 12

What is the composition of taenite?

Answer 12

20-65% Ni, rest Fe; can be cold-worked

Question 13

How do the phases exist within meteoritic Fe?

Answer 13

Phases form Widmanstatten structure; interleaved lamellae of kamacite and taenite; occurs due to very slow cooling; can be revealed by etching with HNO3

Question 14

How did the ancient Egyptians use meteoritic Fe?

Answer 14

Meteoritic Fe dagger found in Tutankhamen’s tomb (~14th C BC); is thought that the first pyramidion was comprised of meteoritic Fe, before being stolen and replaced with stone

Question 15

What are the three types of transformation?

Answer 15

Diffusion dependent with no phase change; diffusion dependent with phase change; diffusion-less

Question 16

What is an example of a diffusion dependent transformation with no phase change?

Answer 16

Alteration of grain structure by recrystallisation and grain growth; may become weaker

Question 17

What is an example of a diffusion dependent transformation with phase change?

Answer 17

Eutectoid transformation

Question 18

What is an example of a diffusionless transformation?

Answer 18

Formation of a metastable phase; e.g. when undergoes quenching

Question 19

When proeutectoid ferrite undergoes low undercooling, what types of structures form?

Answer 19

Alpha ferrite nucleates at austenite grain boundaries; blocky structures form; grain boundary allotriomorphs

Question 20

When proeutectoid ferrite undergoes high undercooling, what types of structures form?

Answer 20

Alpha ferrite nucleates on austenite grain boundaries; ferrite plates grow into grains; Widmanstatten structure forms

Question 21

What is a Widmanstatten structure?

Answer 21

Plates growing perpendicularly to one another within the grain

Question 22

When proeutectoid cementite undergoes low undercooling, what types of structures form?

Answer 22

Low cooling rates = grain boundaries are suitable nucleation site; blocky structures form; coarse cementite forms

Question 23

When proeutectoid cementite undergoes high undercooling, what types of structures form?

Answer 23

Plates of cementite nucleate on grain boundaries and grow into the grains; Widmanstatten structure forms

Question 24

What happens when pearlite is a transformation product?

Answer 24

@ eutectoid point: Fe3C nucleates on austenite grain boundary; formation of Fe3C sucks C from austenite; depletion of C enables ferrite to nucleate adjacent to cementite; plates grow cooperatively into grain interior as a pearlite colony

Question 25

What is the difference between structure when pearlite is formed with rapid cooling compared to slow cooling?

Answer 25

Rapid cooling = very fine plates of alpha and cementite; slow cooling = coarse plates