Sorbitic Structure? 1

[Salvatio]

in a recent research on heat treatment of plain carbon steel, a sample of AISI 1045 was used. After austenizing for 30mins at 1050degC, it was water quenched for 5 seconds, followed by isothermal treatment in a furnace for 3 hours at 450 degC.

under tensile test, it gives a value of around 1180MPa, ductile failure, which i believe it's relatively high for this grade of steel.

checking under the microscope, widmanstatten side plates were found in quite an amount, about 40%. However, in the case of the matrix, it looks something like pearlite, but with a globular appearance. Could this matrix be sorbitic-pearlite? and would this be the reason of yielding such high tensile strength, with a ductility comparable to that of a fully annealed sample too.

Thank you.

 

[metengr]

Salvatio;
I believe that what you have in the matrix is an aggregate of ferrite and cementite, as a result of tempering martensite at 450 deg C. Based on your described conditions for heat treatment, the formation of Widmanstatten ferrite side plates makes perfect sense upon water quenching. The sorbite term is misleading, and has been dropped by most metallographers. From what I have found regarding sorbite - some refer to this as tempered martensite.

In reviewing the CCT diagram for this steel, you would have formed a combination of proeutectoid ferrite and martensite from the austenite decomposition because you quenched in water, and dropped below the MS temperature. Since you only quenched for 5 seconds, and immediately reheated the steel to 450 deg C, you have tempered whatever martensite had formed, and any residual austenite (I doubt that you would have converted all of the austenite to martensite in 5 seconds unless you had very small specimens) had decomposed to either upper bainite and/or pearlite. At 450 deg C you are right at the inflection point of the pearlite/bainite transformation.

I included the attached article that provides some additional information on this subject;

http://www.key-to-steel.com/Articles/Art12.htm

 

[unclesyd]

I agree that the trend is to is to call sorbite; tempered martensite, but it will always be sorbite to met
Check out this website using the terms “sorbite”, “sorbitic”, and “Jominy”
Check out the “patented wire C 60".

http://www.metallograf.de/start-eng.htm

Here is patent for producing a spring wire. A little different approach.

http://spore.patentmatrix.com/Projects/WNCCTC/Comp093/..\BOS\5302216.html

 

[arunmrao]

I did not think that in US this term would be familiar. Sorbite, Troostite, etc were terms most likely found in Russian metallurgy books which we referred to as students. Now with US invasion into Indian minds I think it has faded out here too!

 

[Salvatio]

sorry about the confusion caused. I still cant get any clear confirmation that whether the structure is sorbide or not. However, from what I can observe, there are two things that can be seen.
The first one is a grain of dull appearance, as compared to the others.
The second one is that, within each grain, it looks something like pearlite, but not a lamellated layer one. It's somehow shows to be much globular.
I've search through some books, in which the just named the second phase as "globular pearlite". Little infomation was found on sorbitic-pearlite, in which by defination, should give the same appearance. According to some books that I reffered, "globular pearlite" should form under slow cooling.
As for the first phase, it shows no exact signs of tempered martensite. Initially I thought it was just some large grains of ferrite, but with a few trials of etching time, the difference starts to show. Ferrite may remain to show bright, while the latter starts to get dull.
From this, I predict that there would be carbide finely disperse throughout the grain. This would suits the defination of sorbide.

The reason of why I use the term sorbide, is because I have reallly no idea what those phase could be, or even to call them..haha..

Could it have been that the cementite has coalesence, to become more globular, instead of layered, during isotherm decomposition?

 

[unclesyd]

arunmrao,
Sorbite and Troostite, to a lesser degree, are still used in the US. This is especially true for the wire and strip producers. Another area where you will hear the terms is in reference to mill rolls.