How to set critical points on a dilatometric curve-determination of CCT of a steel

Thermally activated processes start immediately after passing the equilibrium temperature.

AFAIR "beginning" means "1% transformed".

Michael Dahms

Look at the article by George Eldis titled "A critical review of data sources for isothermal transformation and continuous cooling diagrams" in the symposium book "Hardenability Concepts with Applications to Steel", (edited by D. V. Doane and J. S. Kirkaldy TMS-AIME 1978, pages

126-157

Pittsburgh Pete

Hi Metalo,

A good starting point could be an equilibrium phase diagram that can be computed from Gibbs energies. Have a look at Thermocalc

Best wishes,

Evgenii Rudnyi

Thank your for the replies.

Now, more specifically, having the dilatometric curve and its derived one, I normally put the tangents on the curve to get the critical points. However, from time to time the derived curve is amazingly rough and oscillating, thus it's pretty hard to place a line on the curve. Do you have any trick to overcome this?

Thanks, Metalo

You need a physical model in order to obtain reasonable physical data from a curve. Just doing math isn't worth much.

Michael Dahms

What model do you use to treat experimental results?

Is the paper

is similar to your work?

Evgenii

At the moment I use no model, just "by sight" I lay lines on the curve.

But thanks for your hints, I will have a look at the article you sent me.

Metalo

You shouldn't do that.

Michael Dahms

Hi! The tangent method should work pretty well. I've used this method before and its fairly accurate. If you have a quench dilatometer, quench at different points of the curve to try to capture the phase forming by metallography. Although this may be hard for acicular and bainitic structures and work intensive. The other thing to look at is the gain setting on the LVDT, I don't quite know why your curve is choppy. It should be smooth linear thermal contraction or expansion followed by the phase changed. Otherwise there may be controler issues.

Oh yeah, I know of two research groups to look up.

1) Prof. Van der Zwaag has published some neat papers on dilatometry. 2) Prof. Boyd has done some excellent work on the characterization of bainitic and acicular microstructures using dilatometry.