Bainitic Steel, AISI 1045 2

[Salvatio]

I have samples of AISI 1045 and 1050 in a 10*10*100 dimension. My question is that will it be possible obtain a fully bainitic stuctured under isothermal cooling. The Bs temperature is around 750K. Will the result be better if the specimen is to be AISI 4340? What will be the best possible means to avoid any formation of the ferrite phase and pearlite?

Any help would be greatful
Thank you.

 

[metengr]

Sorry to ask, but what are the units of the square stock (metric - cm or mm)?

 

[metengr]

As a follow-up, I reviewed the CCT/TTT diagram for an AISI Type 1040 carbon steel and it would appear that the ferrite/austenite upper nose protrudes before the bainite nose, meaning you would form ferrite before the bainite transformation is completed. You just don't have enough alloying elements like Mo, Ni or B in carbon steel to retard the ferrite/pearlite transformation. If you quenched this material, you will form martensite.

The CCT/TTT diagram for 4340 alloy steel will easily allow you to form bainite.

 

[Salvatio]

metengr:
thanks for your reply.
the dimensions are in mm.
In order to reduce the percentage of allotriomorphic ferrites, will it possible if i were to increase the strength of austenite, ie. dislocation density, hence the yeild strength,as I found that the dislocation debris may retard the plastic accomodation in ferrite formation.

now I have another question, in which that you guys could give me a helping hand. Since that I could not form 100% bainitic structure, will I be able to form any at all, if I only have 2 furnace for the heat treatment process. One is for austenizing, the second is for the isothermal process?

from my view, this may seem as an impossible task, as the cooling from one furnace to another, ie. 850 degC to 450 degC, may take too long. From the TTT diagrams I have obtained, the temperature range for the formation of bainite is around 360 to 500 degC. And the time taken to form bainite at 450 degC, the nose of the C curve, is around would be less than 1 minute upon 100% completion.

Pls give your ideas and comments.
Thank you.

 

[metengr]

Salviato;
To answer your first question, increasing the strength of austenite can only be performed by alloy additions. Increasing dislocation density will not prove effective because dislocation density will diminish with increasing temperature - you want to minimize any nucleation sites for ferrite to form upon cooling, which is the reason why coarse grained steels have increased hardenability during heat treatment.

To answer your second question - you might be able to form some residual bainite if you were to have an interrupted quench - similar to an austempering heat treatment. You will still clip the ferrite and austenite nose upon quenching to about 450 deg C. However, holding at this temperature for about 10 minutes will allow for the bainite start transformation based on your bar dimensions. The objective is to avoid complete transformation of pearlite from austenite. My estimate is that you will have ferrite-pearlite and some bainite with an interrupted quench.

 

[Salvatio]

metengr;
thank you again.

I have a this problem now. Due to maintenance problem, the hot salt bath machine is out of order. What is left is only 2 furnace. I have tried to do the whole process on these 2 furnaces, but the resulted microstructure is a complicated 3 phase, ferrite+pearlite+martensite, in which the pearlite phase dominates. I have initally heated the specimens to 900degC, and transfered to the second furnace at 450degC. The transfer process took about 3-5 seconds. During this interval, I estimated that about 50% of the austenite has transformed.

With the due date coming soon, is there a better solution, or a better way to make use of these two furnace to their best? Such as, changing the quenching process.

Would it be better if that during the transfer of the specimen from one to another, the specimens were to be firstly quenched, to apporoximately 450degC, followed by isothermal process in the second furnace? By so, it might reduce the time needed for the specimens to cool from 900 to 450degC in the second furnace, thus reducing the percentage other phases to form.

 

[metengr]

Salvatio;
Yes to your second paragraph question above. In my second response to you regarding the austempering heat treatment, that is exactly what I would do - perform an interrupted quench from the austenitizing temperature of 900 deg C to 450 deg C, and perform an isothermal heat treatment at 450 deg C to allow for complete phase transformation. Keep the specimen about 10 minutes at 450 deg C and cool in air. If you were using the second furnace to perform the austempering operation, you probably would not cool fast enough thus forming pearlite upon the decay in temperature in the lower temperature furnace.

 

[Salvatio]

metengr,
thank you for your reply again.

After through a series of trial and error heat treatments, I found by using an oil quench, after austenizing at 900degC for about 45 minutes, shows better results of bainitic structure.
Although other phases such as pearlite and martensite have been reduced, there are pretty much of acicular ferrite and Widmanstattan ferrite within the microstructure. There is an relative amount of ferritic needle-like plates growing from and within the grain, surrounded by martensite. The large ferrite grain is also precipitated with fine needle-like carbides.

In another attempt, in which i tried to reduce the formation of acicular ferrite, I austenized the specimen at 750degC, which lies in the region between Ac1 and Ac3. Again, the specimen was oil-quenched. This is to reduce the austenitic grain size, which may reduce the point sites for acicular ferrite. However, this attempt has failed. Under optical microscopy, it is as if the whole microstructure is filled with large ferritic grains. Which may be the fact that I left it too long for austenizing, where the allotriomorphic ferrites have nucleated to such size. There are some patches, or fields of martensitic structure, surrounded by these large ferritic grains.
The amount of the latter is comparatively low.

Please advice on the area which I have done wrong, or could be improved.
Thank you for reading.

 

[metengr]

It appears that your first attempt as described above was your best attempt to form bainite given that you don't have an interruptible quench method using a salt bath. During an oil quench from 900 deg C with 100% austenite you hit the nose of the ferrite transformation curve and by the time you cooled thru this nose, you had formed some bainite transformation product.

Your second approach being that you are in a two phase region prior to quenching would not promote any bainite, as I would expect, and as you reported. You need 100% austenite to achieve any possible bainite formation with this steel composition. So, I would not consider your second heat treatment option. The rather large ferrite grains could be explained based on the original structure of the steel (prior to the intercritical heat treatment between Ac3-Ac1). Keep in mind that phase transformation products are driven by reaction kinetics, which are highly dependent on time/temperature, original grain size of the steel and degree of cold work. Remember, the finer the austenite or austenite/ferrite grain size, your rate of nucleation is increased, which in your situation is not what you want to reduce ferrite levels in this steel. You want to delay the rate of ferrite and pearlite transformation products upon cooling to achieve bainite transformation.

 

[Salvatio]

I've carried out several experiment to produce a bainitic structured steel. Specimens were heated to 1050degC. I have tried using lower temperature, at 800-900degC, as austenizing temperature. The lower temp attempt was a failure, as it only produce network of ferrite structures; cellular precipitation, with pearlite within.

with the 1050degC test, there were 3 types, namely as non-interupted quench, interupted quench, and 2 continuous quench.
In the non-interupted quench (NIQ) attempt, specimens were transferred from 1050degC to 450degC after 30mins of austenizing.
As for the interupted quench (IQ), specimens were austenized, then quench at boiling water with different duration; 1-5 seconds.
2 continuous quench (2CQ) is done by quenching the austenized specimens firstly in brine solution, and while the specimen is slightly red hot, it is then quenched in oil to room temperature.

in NIQ, there are significant amount of ferritic strucutre. Also, it comes with secondary Widmanstatten ferrite plate, growing into pearlitic matrix.

in IQ, there are some visible secondary Widmanstatten ferrite plates, but much less as compared to NIQ. Allotriomorphic ferrites that formed too, seemed to be less thick than that of in NIQ.

in 2CQ, there seems to be quite an amount of bainitic sheaves that formed, along with a majority of martensitic structure. There are several distinct regions, the martensitic; large ferritic plate-like with precipitation in it. There are also regions where it is free of martensitic phase, where there are mostly bainitic sheaves growing out from grain boundary.

there are some difficulties that I'm facing now. As secondary Widmanstatten ferrite plates may appear to be large wedge, but at lower temperature, it may be of fine wedge, with fine spacing to neighbouring wedges. This resembles the structure of bainite. In the microscopic examination of 2CQ, the sheaves were seperated by what is believed to be cementite, which appeared dark in colour.

Q: How can I differentiate or to make confirmation that the structure i obtained in 2CQ consist of partially bainitic structure? What differs a Widmanstatten ferrite plate from that of a bainitic sheaf?

Thank you for reading.

 

[metengr]

To answer your question, the following web site provides a valuable link to the topic of bainite;

http://www.msm.cam.ac.uk/phase-trans/index.html

There is a free book offered on one of the links by one of the leading experts in bainitic phase transformation, his name is Professor Harry Bhadeshia of the University of Cambridge. His book is titled "Bainite in Steels". I had down loaded the files some time ago and the information related to bainite morphology and ferrite are discussed in detail. This is an excellent reference book on bainite.

 

[unclesyd]

Is it essential that you use a 40 point steel?
If not you could use a lower carbon steel such as 1018 or 1020 and get a 100% Bainite structure.


Go to this website and use the “Keyword Search” for “bainite”, some very good metallographs of varying % bainite. It is interesting that they use Jominy samples.

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

Here are two more site that might be of interest as far the metallography is concerned.

http://www.metallography.com/cgibin/search.pl

http://www.kaker.com/Products/Products.html

 

[Salvatio]

metengr:
Ah..thanks for that. However, I've already downloaded the online book. In fact, most of my research are based on Professor Harry Bhadeshia's work, where I got the idea of producing a partially bainitic structure with a relatively high carbon content steel. ^^ Anyway..Thanks..

As some updates..under microscopic examination done today, to my supprise,I found that there were some amount of acicular ferrite too, which is out of what I predicted. Though that acicular ferrite may also be taken as intragranular bainite, which usually nucleates on impurities, especially on Ti-oxide..

However, though that there may be some acicular ferrite being formed, I still have doubts on the mechanical properties, due to the martensitic martrix. The extend to which both bainite and acicular ferrite may reinforce the matrix is yet to be tested. Hopefully it would have superiority properties over that of martensite and pearlite.

unclesyd:
initially i was thinking of changing my specimens too. but however, instead of producing a fully bainitic structure, it would be of greater interest to study the effects of dual phase or even triple phase steel, with a partial bainitic strucuture.
again, Thank you.