Steels- Formation of Austenite Flashcards
First step in the heat treatment of steel
Formation of austenite
Temperature Vs time graph for austenitising heat treatment
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
Mechanism of austenite formation starting with spheroidal Fe3C in ferrite
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
How does percentage of carbon vary from centre of Fe3C particle outwards?
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
What is growth rate of austenite influenced by?
Concentration gradient (decreases as time increases).
Rate of diffusion of C in austenite.
Both increase with temperature
Two most important variables influencing the rate of γ formation
Temperature and initial structure
What is the A1 temperature (number)
727°C
Effect of temperature on rate of austenite formation
%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
Diffusivity of carbon in austenite equation
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
Temperature Vs diffusivity of C in γ graph
Concave curve up from A1 on y axis to A3 temperature further right
What is pearlite spacing?
Also interlamellar spacing. In pearlite have alternating layers of ferrite and cementite. The distance between centres of two closest ferrite layers is the spacing.
The effect of pearlite spacing on growth rate of ausentite
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.
The effect of carbide size and dispersion on growth rate of austenite
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
Progression of pearlite into homogeneous γ on T Vs t graph
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
Rule of thumb for austenitising thicker specimens
For every 25mm thicker hold at temperature for 1h longer
Temperature Vs nucleation rate (I) graph
Concave curve starting from y axis at A1 temperature
Mathematical relationships for the effect of temperature on nucleation rate
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
Problems of austenitisation of alloy steels
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
Why are steels heated into the γ range?
To facilitate hot working.
As initial step in heat treatment.
The temperature range in each case is very different
What happens if austenitising temperature Tγ is too low?
Incomplete dissolution.
Times involved are too long so more expensive.
What happens in Tγ is too high?
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
