Steels- Introduction

Question 1

Ferrite (α) information

Answer 1

BCC
Solid solution of C in α-Fe
Max solubility of C is 0.02 wt% at 727°C
Solubility of C decreases with temperature (0.008 wt% at 0°C)
Curie temperature 768°C so above this is paramagnetic
Soft and ductile

Question 2

Austenite (γ) information

Answer 2

FCC
Solid solution of C in γ-Fe
Maximum solubility of C is 2.1 wt% at 1147°C (a lot more than α)

Question 3

Cementite (Fe3C) information

Answer 3

Intermetallic compound of fixed composition (6.67 wt% C)
Orthorhombic with 12 Fe atoms and 4 C per unit cell
Hard and brittle

Question 4

Ferrite (δ) information

Answer 4

BCC (but different lattice parameter to α)
Solid solution of C in δ-Fe
Maximum solubility of C is 0.09 wt% at 1495°C

Question 5

Eutectoid reaction

Answer 5

At 727°C
γ(0.8%C) -> α(0.02%C) + Fe3C(6.67%C)

Question 6

How do steel designations work?

Answer 6

Use 4 or 5 digit number
First two digits refer to the major alloy additions (don’t have to remember which alloying elements correspond to number)
Last 2 or 3 digits give C concentration in wt%. Divide the number by 100 to get the weight percentage
E.g XX120 has 1.2wt% C

Question 7

Mechanisms of transformation of austenite on cooling for different cooling rates

Answer 7

Highest T and slowest is nucleation and growth.
Next is massive
Then bainitic
Lowest T and fastest is martensitic

Question 8

Lines A3 and A1 on the iron-cementite phase diagram

Answer 8

A sub 1 is eutectoid temperature line (normally horizontal) above the mixed ferrite and cementite zone.
A sub 3 is the boundary between austenite and α ferrite (normally like exponential decay)

Question 9

Nucleation and growth mechanism

Answer 9

Operates at slow cooling rates below the A3. Diffusion controlled mechanism. Smooth polygonal ferrite grains. Low dislocation density. Incoherent boundaries

Question 10

Massive (acicular ferrite)

Answer 10

Transformation occurs at T under 740°C. Increased dislocation density (5x10^9 cm-2). Occurs by nucleation and short range atomic transport across α/γ interface (hard to distinguish from nucleation and growth). Product differs from N and G in that boundaries are irregular due to ferrite having both coherent and incoherent boundaries with parent austenite. Doesn’t produce surface relief

Question 11

Bainite

Answer 11

Transformation occurs at about 640°C. Produces surface relief. Hybrid transformation

Question 12

Martensite

Answer 12

Generic term that does not define a crystal structure. Occurs by cooperative movement of atoms (no diffusion and constant composition). Produces surface relief and high density of lattice defects (high dislocation density)

Question 13

Displacive transformations

Answer 13

Athermal, diffusionless, shear or martensitic. Atoms move by less than 1 interatomic spacing and by making and breaking interatomic bonds. Atoms move one after another in precise sequence (military transformation). Speed of transformation roughly equal to velocity of lattice vibrations through crystal and not function of T. Extent of transformation depends only on T. No composition change. Always specific crystallographic relationships between product and parent phase.

Question 14

Diffusional transformations

Answer 14

Atoms move over 1-10^6 interatomic spacings. Thermally activated diffusion from site to site. Atoms move randomly from one lattice site to another (civilian transformation). Speed of transformation is function of T. Extent of transformation depends on T and time. Composition of phases can change. Sometime crystallographic relationships between product and parent phase

Question 15

Two groups of alloying elements

Answer 15

Austenite stabilisers (increase austenite phase field)
Ferrite stabilisers (decrease austenite phase field)

Question 16

Austenite stabiliser for open phase field

Answer 16

Ni, Mn, Co used for steels (Rh, Pd, Pt, Ir have same effect but not used).
These don’t tend to combine with other elements in the steel to form compounds so austenite phase field has right lower barrier removed.

Question 17

Austenite stabiliser for expanded phase field

Answer 17

C, N use in steels (Cu maybe, Zn, Au have same effect but not used).
These can combine with other elements in the steel to form compounds and new phases so right lower barrier included in enlarged austenite phase field

Question 18

Ferrite stabilisers for closed phase field

Answer 18

Cr, Ti, V, Mo, Si, Al, Be, P
Cause austenite to contract to a small area (gamma loop back down to y axis in shape of crescent). Alloys with this type of behaviour don’t respond well to heat treatments.

Question 19

Ferrite stabilisers for contracted phase field

Answer 19

B, Ta, Nb, Zr
Still like a loop but right line of crescent is split by other things.
See page 12 lecture 1 for diagrams

Question 20

What do austenite and ferrite formers do to the enthalpy of formation of that phase?

Answer 20

They increase the enthalpy of formation of the phase they are stabilising, better ones increase it by more

Question 21

Effect of increasing amount of ferrite stabiliser on austenite phase field

Answer 21

See page 13 lecture 1 for diagrams for Cr and Ti