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Hi!
We know TTT can be quite confusing and convoluted sometimes (much like people), and it may take some noggin energy to understand where they come from (also like people).
When you finish reading our website, you will master the arts of the two fish and lemon juice.
Shown above is a simplified version of a typical TTT diagram for an iron-carbon alloy, and it's our goal here to see how it's plotted.
This may seem overwhelming at first, but don't worry! Hold my trout hand-fin and I'll walk you through this every singular step of the way, and I do this just for you, all 7 billion of you.
[Trout]
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Let's start by understanding the kinetics of solid-state transformation.
We’ve seen temperature dependence of phase transformation, but it is also important to consider the time dependence of a transformation and this is often referred to as the kinetics.
To do this, the Avrami Equation (a.k.a. JMAK equation) is used.
Where, K and n are predefined constant (for those interested in Excel-ing this, use n=4 and k=1E-15).
We can now plot the fraction of transformation as a function of time at a constant temperature and produce the iconic S-shaped graph.
Notably, rate of growth is slow in the beginning immediately after nucleation (the first curve of S) and towards the end of growth (the second curve).
By convention:
The rate of transformation is defined as the reciprocal of time required for the transformation to proceed half way.
and this can be indicated nicely on the kinetic graph (read from 50 percent).
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However, temperature dependence of solid-state transformation tells us that at higher temperature we expect lower rate transformation (note that the time-axis is logarithmic!), longer nucleation period (indicated by the blue dot in the diagram), and vice versa;
As you can see it’s quite inconvenient/impractical when you want to know the progress of transformation over multiple changes in temperature, especially when you're trying to read it from a snake this wiggly.
Moreover, there’s also a little problem with phase diagrams, when eutectic phase transformation occurs it’s not clear the pattern of fraction of transformation over time, while equilibrium cooling is the assumed case yet it’s impractical and non-achievable realistically. Therefore non-equilibrium cooling (realistic cooling) is inaccurately represented
To solve these issues, a graph with both temperature and time axis is devised AKA the TTT plot, and since we make the necessary assumption that temperature is held constant throughout a transformation (isothermal condition), the plot is also referred to as isothermal transformation diagrams
So let’s fulfill the isothermal part and sit that wiggly snake down in our pre-prepared empty plot.
Hey, that's pretty good.
We've now successfully transposed a set of transformation data at a constant temperature onto our empty plot. For visibility, 3 dots shown are the starting, the half-way, and the end point of that transformation, in that order, left to right.
Now we just need to do this a whole bunch lot of time until we have a meaningful range of data for our plot, from which we get 2 curvy lines indicating the starting and ending, and also commonly shown, a dotted line in the middle, the halfway points of the transformation.
But that's not all!
We can now label phase compositions in the region that we're certain about (in this case, A (Austenite), P (Pearlite), B (Bainite), and M (Martensite)).
Simultaneously, we also know that transformation has to happen below a certain temperature (eutectic temperature), and some phases only form under some circumstances and temperature (such as Bainite and Martensite).
Let's go ahead and do all that:
And there we have it!
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