Steels- Formation of Austenite

Question 1

First step in the heat treatment of steel

Answer 1

Formation of austenite

Question 2

Temperature Vs time graph for austenitising heat treatment

Answer 2

Steep rise in temperature from below the A1 to just above the A3. Held there for a bit. Then a slow exponential decay curve. Higher curve for furnace cooling (annealing), medium curve for air cooling (normalising), lower curve for water quenching

Question 3

Mechanism of austenite formation starting with spheroidal Fe3C in ferrite

Answer 3

Small regions of austenite nucleate at at the α/Fe3C interface. These grow outwards until they start overlapping. Fraction of austenite increases over time. All done above A3

Question 4

How does percentage of carbon vary from centre of Fe3C particle outwards?

Answer 4

Is constant at 6.67% until edge of Fe3C particle. Then vertical drop down a little to C3. Then exponential decay down to C2 across the austenite region. Then larger vertical drop down to C1 and constant with distance into ferrite region. At small time exponential bit steeper and second vertical drop closer to centre and goes slightly lower than after longer time

Question 5

What is growth rate of austenite influenced by?

Answer 5

Concentration gradient (decreases as time increases).
Rate of diffusion of C in austenite.
Both increase with temperature

Question 6

Two most important variables influencing the rate of γ formation

Answer 6

Temperature and initial structure

Question 7

What is the A1 temperature (number)

Answer 7

727°C

Question 8

Effect of temperature on rate of austenite formation

Answer 8

%γ Vs logt. Starts at 0 at some time along X axis then follows cumulative frequency shape surve up to 100%. For greater temperature above the A1 the whole curve starts earlier but same shape if same initial structure

Question 9

Diffusivity of carbon in austenite equation

Answer 9

Dc^γ=Do exp(-Q/RT)
Q is the activation energy of diffusion.
D0 is the frequency factor
R is universal gas constant 8.31J/mol.K
T is temperature in K

Question 10

Temperature Vs diffusivity of C in γ graph

Answer 10

Concave curve up from A1 on y axis to A3 temperature further right

Question 11

What is pearlite spacing?

Answer 11

Also interlamellar spacing. In pearlite have alternating layers of ferrite and cementite. The distance between centres of two closest ferrite layers is the spacing.

Question 12

The effect of pearlite spacing on growth rate of ausentite

Answer 12

Same shape graph of %γ Vs logt. For finer spacing the graph is shifted left so faster growth rate. As spacing decreases the Fe3C/α interfacial area per unit volume increases which increases nucleation rate.

Question 13

The effect of carbide size and dispersion on growth rate of austenite

Answer 13

Same graph shape. For finer size (e.g when quenched and tempered) graph shifts left compared to larger size (e.g spheroidised). Finer carbides give higher surface/volume ratio increasing nucleation rate

Question 14

Progression of pearlite into homogeneous γ on T Vs t graph

Answer 14

There are 4 exponential decay like curves from left to right. Always above A1. Starts pearlite between y axis and first line. Then pearlite+γ+residual carbides. Then γ+residual carbides. Then γ+carbon heterogeneities. Then homogeneous γ

Question 15

Rule of thumb for austenitising thicker specimens

Answer 15

For every 25mm thicker hold at temperature for 1h longer

Question 16

Temperature Vs nucleation rate (I) graph

Answer 16

Concave curve starting from y axis at A1 temperature

Question 17

Mathematical relationships for the effect of temperature on nucleation rate

Answer 17

ΔT is T-A1. So as T increases so does ΔT. Free energy change ΔG* is proportional to 1/ΔT^2 so decreases as T increases. This means the number of critical nuclei formed increases, n*. So nucleation rate, I, increases

Question 18

Problems of austenitisation of alloy steels

Answer 18

Alloy elements segregate and are difficult to homogenise due to low diffusivity.
Steels containing strong carbide/nitride forming elements must be austenitised at higher temperatures to dissolve them.
Segregation during solidification gives uneven transformation characteristics and microstructures

Question 19

Why are steels heated into the γ range?

Answer 19

To facilitate hot working.
As initial step in heat treatment.
The temperature range in each case is very different

Question 20

What happens if austenitising temperature Tγ is too low?

Answer 20

Incomplete dissolution.
Times involved are too long so more expensive.

Question 21

What happens in Tγ is too high?

Answer 21

γ grain size increases.
Cause of large quenching strains.
Localised melting (e.g at T over 1147°C).
Excessive oxidation/decarburisation.
Excessive fuel costs/pollution control