Dear Prabhash Gokarn

for determination primary austenite grain size you can see standard: ASTM E 112 or ISO 643. Amount of carbon in your steel is very important in choosing method.

Hello Mr. Gokarn,

Yes, it is temperature and deformation depended. Basically, the grain size increases with increase in temperature and decreases with the amount of applied deformation.

You may follow the proper instructions in ASTM E112 according to carbon and/or phosphorus content in steel.

Kind regards,

Emre

Prabhash:

You can determine the prior austenite grain size (presumably you are referring to steels) the same way as you measure any other grain size, using standard line intercept methods or equivalent, in accordance with the ASTM standard mentioned in the preceding replies. The main issue though is not how you measure this grain size but how you actually see the prior austenite grains in the first place. They are not that easy to see. Typical etchants for steels, such as nital and picral, will generally not reveal the prior austenite grain boundaries.

The best way to etch the prior autensite grain boundaries in steels though is to use an etchant of a saturated solution of picric acid containing a few ml of "teepol" (sodium alkyl sulfate) at 75C, although it can work at room temperature as well. This acts to "groove" the grain boundaries, particularly if there has been some segregation there during the austenitizing treatment. Be careful though as the picric acid will stain you hands yellow if you get it on them (you'll then look like you have nicotine stains from smoking too much!), and picric acid can become explosive if it is left too long, so it's wise to dispose of it once you're finished.

With respect to hat changes this grain size, the prior austenite grain size will certainly be affected by temperature but only when the steel is austenitic, e.g., at a temperature above 850C or so. Most steels are austenitized around, or just under, 900C, and if quenched from this temperature, the prior austenite grain size typically will be in the tens of microns. However, if you austenitize the same steel at 1200C, the resulting prior austensite grains will likely be an order of magnitude larger, which can have some rather interesting effects of the resulting mechanical properties.

If you're interested, some of these effects, due to changes in the prior austenite grain size, on the mechanical properties, in particular the toughness, are described in the attached paper.

I hope that this helps.

ROR

Determination of prior austenite grain boundary of steel is not easy, and the difficulty tends to vary with the chemical composition or grade of the steel. In their work, C. Garcı´a de Andre´s, et. al*, found that thermal etching (TE) is the best technique to reveal the prior-austenite grain boundaries in microalloyed steels. The attached article by them may be of interest to you.

We are seeing "grain refinement" of austenite in iron castings due to some inoculation factor. Foundries increasing the percent steel in their charge are noticing this effect by thermal analysis. The effect is similar to grain refinement in Aluminum using TiB. I assume that we are picking up possible titanium from the steel that combine with silicon in the iron to form crystals that nucleate the dendrites. The foundries were using pig iron before, but with China's economic woes, steel scrap prices have dropped $40 a ton and so foundries are substituting more steel for the pig iron. Dendrite size is controlled by many factors including cooling rate. But if you have a consistent process and the dendrite size changes, look at nucleation promoters.

For low carbon micro-alloyed steel with Nb and Ti addition, the former austenite grain boundaries can be revealed using an aqueous solution of picric acid

with sodium dodecylbenzene sulphonate and different additions of hydrochloric acid depending on the chemistry. You might need to repeat etching and polishing several times.

As has been mentioned, saturated solution of picric acid containing a few drops of "teepol" and a few drops of HCl is the best Etchant. I find it works better when the etchant is filter through filter paper. It only works on quenched microstructures, so not for ferrite.

Thermal etching can be done in a dilatometer (small samples and good control of temperature and atmosphere).

The cast austenite grain size will be reduced due to recrystallization when rolling is high enough above the austenitizing temperature. Microalloyed steels prevents recrystallization of austenite, leading too deformed prior austenite grains. Deformed austenite will transform into multiple ferrite grains, so ferrite grain size can be much smaller than prior austenite grain size.

Typically, the smaller the prior austenite size, the smaller the final microstructure.

Recently computation can help to reveal not only grain size but also the orientation of prior austenite... take a look at this article

Article Analysis of Misorientation Relationships Between Austenite P...