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Jaz

Austenization, Hardening, 100% Martensite. Various questions.

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Firstly, I haven't been on here in a long time. But summer is coming around and I want to get back to finishing my forge.

Anyways. I'm a mechanical engineering student, and I've had quite a bit of course material relevant to metallurgy, but I still have a few practical questions.

First of all, when Looking at a time temperature transformation phase diagram of eutectoid steel (0.76 wt % C), before cooling we always assume that the material is 100% austenite. If I have a piece of eutectoid steel, so with 100% pearlite microstructure, at room temperature, assuming negligable time to reach above the eutectoid temperature (~727C) how long does the specimen take to reach 100% austenite? I know it will depend on the size of the specimen, for this case assume its a typical size bowie. Looking at a phase diagram it's impossible to tell, since all the points on the phase diagram are based on the assumption of equilibrium being reached.

Also, when hardening, is the general goal to have 100% martensite when quenched, I know this would result in the highest hardness, but I don't know what the typical procedure is in most cases. In my view it would seem simpler when tempering to know that you are starting out with 100% martensite than with say, 50% pearlite, 50% martensite. Since then you get into issues with the pearlite becoming coarser as diffusion accelerates, etc.

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Your question is a little out there. Here's why, most of the charts I've seen are based on 1085 simple carbon steel. The various alloys can drastically affect transformation, temper and everything else. Just changing the carbon content will change these. What steels are you using, if it is to complex of an alloy for me I'll be some one here works with it.

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Ahhh why are you not discussing this with your professors? That's what you are paying them for! (and even if you are not taking a class with the correct professor; few will turn down the chance to lure you over into their specialty if you ask them a question nicely!)

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Like a lot of higher education classes this one needs some lab work. You need to make some blades and do some heat treating to understand how this applies in the shop. Pick one of the streels you have and interest in and make several blades, you can do less that a perfect finish on them. Try different times at heat and then break them so you can see the grain structure. Strict record keeping is a must for this as years later info may become foggy if not documented. If you do not get repeatable results you have to solve that problem before continuing. The same procedure should produce the same result. ABS has a testing procedure that will give you guidelines for this. If you have failures with the test, all the better. That will give you more lab work to see wot went wrong. And of course if you move to another steel you will need to repeat the above as it may be different.

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If you are trying to understand heat treat, and are going to break the pieces anyways, order thin stock that is near dimensions to your intended finished blade, grind a simple bevel on it, saw it into samples and heat treat the samples.

Yes, there is value in making blades that will be destroyed in testing, but if you are not at the point of making blades, this can expedite understanding heat treat. Even better if you can get credit for it!

Phil

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Jaz,

I'll take a stab at this one as best I can. If you have a eutectoid composition the transition to austenite is instantaneous once the material has crossed the A1. In practice nobody is going to assume instantaneous transformation, but this is the real implication of the eutectoid composition.

Yes, as a rule most people are striving for a fully martensitic transformation. In practice you rarely get it. The size at which you get it will take you into concepts like the DI value, etc. In practice in the blacksmith world, you will quench for hardening and have to work out tempering experimentally for your application. Your as hardened structure is very unlikely to ever be fully martensitic, but that does not mean there is any failure in your process.


Tim

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These are good questions. I am sure one of your professors can give you a theoretical answer to the time-to-100%-austenite question. (It's not just a function of the size of the piece. It's also a function of alloying elements. Highly alloyed steels convert to austenite slowly, which is why many of the high speed/air hardening tool steels require very long soaks at high temperatures for full hardening.) However, the practical answer to your first question is that smart metallurgists have already figured this stuff out and written it down for the rest of us. Walk over to your university library and find a good reference book on heat treating -- one that specifies soak times for different alloys. The point (or at least one of the points) of soak time is to ensure 100% austenite formation. If they don't have such a book, I'm positive they can get it for you.

Generally speaking, yes, the goal is to form maximum martensite (I don't think it's ever actually 100%, but you can get very close), and then temper back from there to get the desired characteristics. This is much easier and more repeatable than trying to achieve a particular mix of structures straight out of the quench, by not fully austenitizing or by quenching in a coolant that isn't fast enough for the steel. An exception would be differential hardening of blades, in which you try to achieve for a martensite edge, a pearlite spine, and a transition zone of mixed structures.

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Actually I proposed the test be done on a specific shape, I chose blade shape even though he did not mention wot he was going to craft. It could have also been for a punch a chisel or any number or items. Lab work should have a specific goal in mind. On of the issues with heat treating metals is that if yoiu keep samples of metal you heat treated when you first started and later on repeat the same test on the same kind of material you may see differences in grain size between first and last, Many things can cause this. But one that I see most often and had to learn to not only get my act together but then to teach others, is the physical moves to get the piece from the heat to the quench.without losing the correct heat. If youi use thin pieces of stock as suggested above and are not going to use that size for your project that heat loss may be quite different and skew the results. If the piece is not consistant in thickness it can also be an issue. A blade, or punch or chisel may lose heat along the thin part if the moves are not rapid to the quenchant. A destructive test may show a different grain size on the thin area. And yes I have done this, broke a lot of steel and looked at the grain. There are a lot of ways to skin a cat I have ones that work for me. Simply put this can be a fortified learning about heat treat but not only in the class room but in the shop also.

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I agree that experimentation is important. I didn't mean to suggest otherwise. But to borrow some terminology from my days as an artilleryman, a good reference book can save a lot of adjusting rounds. In other words, it can help you get in the right ballpark.

Industry has come up with some fascinating ways to overcome the problem of getting from the furnace to the quenchant without losing heat. E.g., sometimes the quench tank is directly under the furnace. When the part is fully austenitized, the bottom of the furnace opens and dumps the charge straight into the quench bath. I'm not suggesting blacksmiths necessariy need something that fancy, but it does get you thinking. :)

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Hey thanks for the replies, I've been busy this last week and so I didn't get to see the comments until now.

I asked my teacher the question about austenization and he reaffirmed that it depends on many variables, I forgot the exact number for an "average" steel that he gave me, it may have been 20m, or possibly 2h haha, so I can't remember which. Either way, he is a good guy, but his knowledge on this subject seemed sketchy, I'm pretty sure it doesn't take that long.

In terms of doing experiments in the lab:
The class isn't a metallurgy class by any means, its a materials class, and covers different topics relating to materials science, a few of the units we covered had information pertaining to blacksmithing, which I ate up :) We covered phase diagrams and calculations involving phase diagrams, knowing what phases are present and in what concentrations, and where, as well as the compositions of each phase (lever rule stuff). Strengthening mechanisms were covered as well, like why alloying elements add strength, grain boundary size, dislocation density etc. We did some stuff on phase transformations, which included the time temperature transformation curves etc.
So in our labs we never get to do any real practical heat treating, since that is such a small part of the course. We did a lab where we performed tensile tests with different steels, and had a lab where we tested some different treated steels as well (it was pretty cool). Otherwise we did some fractography stuff in the last lab, which was really interesting. Our labs are always prepackaged, I could probably get some time with some of the machines if I really put some effort into it, like a hardness testing maching (don't we all wish we had one? :) ) as well as the microscopes and tensile test machines. Generally access is limited to people working on their fourth year design projects, as otherwise every guy who liked working with tools would be using the machines, and they are needed by the staff.

If you guys have any theoretical questions or calculations you're interested in, feel free to ask. I understand the material we covered, but I know if I don't use it I'll lose it, so that would be an excellent way to keep me fresh, and also a way to learn the details better.

Thanks again for the replies.

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Jaz I understgand about the program you are involved. They surely do have a schedule to keep. Wot I really meant by labs is to get a feel for wot metal does youi should watch some steel heat up and then try quenching,,,shop test for hardness, break open and see grain..then do it again at different temps, different quenching mediums. etc,,You get the point. Simply put is that a book knowledge will certainly be a strong backgrouind,,and again I do not know where you are going to use this knoledge in the future. But we all tend to forget things in a short time after learning them. The lab work woiuld help you assemble a package of knowledge that might stick aroind a while.

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Jaz,

Get yourself a copy of "Heat Treatment of Steels " by M. A. Grossman. This book has a whole chapter on the development of microstructures based on carbon content, hold times and cooling rates in plain carbon steels. The book was first published in 1935 and the 5th edition came out in 1965. That one is coauthored by Edgar Bain (of Bainite fame). Of all the metallurgy texts I have seen and used, this is the absolute best one for a basic intruduciton to steel metallurgy. It is now out of print but used copies are available online for not much money. I have 3 copies which I loan to my interns and collegues at work. I have found this to be an invaluable resource, especially when dealing with folks who are are not classically trained in metallurgy. I think this will answer the bulk of your questions without the need to repeat experitments which were done almost a century ago (though I'm not suggesting experimentation is not worth doing).

Patrick (professional metallurgist and avid blacksmith)

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Jaz,

I'm a recent ME grad (2009) and I would definitely recommend utilizing any resources the school has to try this all yourself. Including hardness testing and finding methods of polishing for examination under the microscope. This is all done outside of class hours but that's what it takes to learn and is how I learned so much when I was in school. Good luck.

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Keep in mind the the phase diagram you probably used is a BINARY phase diagram. Iron and carbon are the only elements evaluated. Ternary metallic systems get a lot more complex and a real alloy is jus too complex to graph. The scientists created the phase diagram. As an engineer, you have to learned how to apply it.

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