Eutectoid steel

K

Kevin R. Cashen

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Sep 9, 2003
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O.K. we covered may details of working hypereutectoid steel in the other thread, so now why not talk about eutectoid steel fro a comparison. We so often recommend 1084 or 1080 to makers who are getting started why not explain why that is? In the other thread I avoided including the iron-carbon equilibrium diagram to stay away from making things too technical, but it turns out that it would have been beneficial in understanding many of the topics discussed. So to get a better idea of this topic I will go ahead and put his here:

fefe3c.jpg


I am not sure where I originally got this image but it has been tucked away on my site for some time after some discussion in the past.

The iron-carbon equilibrium diagram charts out how iron and carbon will combine under different carbon concentrations and temperatures. Along the bottom you will find from left to right increasing percentages of carbon. Along the left side you will find increasing temperature from bottom to top. Virtually everything we are concerned with in knife steels will fall well to the 2% mark that divides steel from cast irons, and the vast majority of simple knife steels will fall to the left of the 1% carbon mark. The only thing worth mentioning that is to the right of the 2% mark is the “V” shaped line that occurs at around 4.3% carbon and 2000F this deals with the eutectic and I only point it out in order to remind you to never confuse it with the eutectoid. The eutectic deals with actually moving from liquid metal to solid, the eutectoid occurs entirely within solid metal but deals with solid solutions with that metal.

Back to steels, and back the thread topic. At .83% carbon you will notice a very prominent dotted line ascending all the way up and dividing the entire range of steels almost neatly in half. This is the eutectoid. Anything to the right is hyper-eutectoid; anything to the left is called hypo-eutectoid. When steels to the left are slow cooled they will have some pearlite with leftover ferrite (iron) that was not filled with carbon to make that pearlite. When steels to the right of that line are slow cooled they will be pearlitic with leftover carbon that didn’t get used. Right on the line the steel will go entirely pearlitic with no leftovers at all.

If you pick a point in the hypereutectoid zone, let’s say a steel of 1% carbon, and make an imaginary line straight up like the dotted line at the eutectoid you will be tracing things that happen to that steel with increasing temperature. The first line you will cross will be A1 at 1333F, since we are going up in temp we should actually add a “c” and call it Ac1, I could explain all of this but it is a bit involved and actually involves French terminology that is not really necessary for this thread. A1 is the point where the steel will begin to move carbon around and start going into solution. The point of greatest solubility of carbon into iron at this temp is .83% so that much carbon will readily go into solution at this temperature, but .17% carbon of our 1% will still be left undissolved. Increasing the heat will result in more an more of that carbon going into solution until you encounter the line that rises on an angle from the eutectoid line up to the right and is labeled Acm. This is the line that show when you have dissolved all that extra carbon and put in into solution. On the left hand side of the eutectoid you will see a line that is very similar to Acm but is called A3. A3 is where all the iron (ferrite) has been filled with the carbon that is below the eutectoid. It is also worth noting the line labeled A2 this is the Currie point or where the steel loses magnetism at 1414F. It is VERY important to take not that this A2 line is flat a horizontal, and thus not affected by the carbon differences like A3 or Acm. This is exactly why it cannot be stressed enough that the magnet is NOT an absolute or foolproof method of determining heat treat for every alloy.

Now back to our thread topic. Right in the center of these two lines (A3 and Acm) is the eutectoid point. A steel falling near this line will make total pearlite on slow cooling with no problematic leftovers, and on heating will go totally into solution at the lowest temperature of any steel. Because of this if all you have is a forge or a torch 1080 or 1084 will cooperate with you more readily than any other steel. Simple heating tools will make it much more difficult to pinpoint and hold temperature between A1 and A3 or Acm in order to avoid the problems that those extra components can cause. The safest way to get the best results with these tools is to use 1080 or 1084, but if you are going to work with steels to the left or particularly to the right a very good knowledge of heat treatments that can be done prior to hardening to set things up can be invaluable.
 
Excellent information, on both threads. I'm going to sift over this for a while
 
I'm trying to keep up but I need to read, reread, rereread then repeat. Thank you for sharing the knowledge Kevin
 
Will the advantage of a forum thread over just getting your hands on a metallurgy book is that you can ask questions. If something doesn't make sense the first time the ability to discuss it and have it explained in a different way is something a book can't do. So by all means feel free to take advantage of that.
 
Please discuss why one still wants to take eutectoid steel above A1 in order to heat treat properly. Since the diagram is for "neat" iron and carbon, what effect if any does the presence of the manganese have on the solubility and mobility of the carbon?
Finally, since A1 and Acm are essentially at the same point for the eutectoid, does this indicate that it is much more sensitive to grain growth?
Thank you.
 
A1 - you must transform the steel to austenite to HT -that means above A1.
I have to assume manganese has little effect on carbon solubility or diffusion.[I'd have to research to be sure]. Remember that carbon is much smaller than most of the alloying elements in steel ,it can move easily between other atoms and that's where it stays in the crystals.The big effect of Mn is in hardenability.
The eutectoid might be a bit more sensitive to grain growth as the carbides in the hypereutectoid would tend to slow down grain boundary movement. Nothing that a pinch of vanadium wouldn't cure, but that would help all of them !!
 
mete said:
A1 - you must transform the steel to austenite to HT -that means above A1.
Click to expand...

The reason I asked my question was to clarify this statement of Kevin's:
"A1 is the point where the steel will begin to move carbon around and start going into solution. The point of greatest solubility of carbon into iron at this temp is .83% so that much carbon will readily go into solution at this temperature"

I understand from HT data that it needs to be heated above Ac1, but since Acm and Ac1 are the same point on the graph at the eutectoid, how do we deduce where the proper temp is for converting to austenite?
 
Theoretically Acm and A1 are the same point for .83% carbon and iron. The problems arise when reality enters the picture.
The transfer of heat is not a 100% situation, nor is it instantaneous. In an analytical oven with good insulation, and good regulation, Eutectiod steel could convert to Austenite at 1 degree above A1/Acm.In reality, it would take a very long time for the solution to fully occur at the exact point. Most charts will add 50F to assure a full and reasonably fast conversion.

Most new smiths have basic and primative equipment, no control ,and are guessing the temperature. Thus, giving the temperatures in a higher range and allowing a short soak are designed to prevent failures by erring on the side of caution.

I am glad that Kevin posted this. I almost always recommend 1080/1084 as a good starter steel. The reasons are usually lost on the newbies, and augured by some of the others. The very reasons that make 1080/1084 a good starter steel make 1095 a much less desirable starter steel.

Stacy
 
Thanks, Stacy, that's a nice pragmatic explanation but I asked rhetorically so that perhaps we could get a little deeper into it. We know that the carbon starts moving around at Ac1. But we also know that there is an actual solid state phase change that it has to undergo to become austenite. This is an equilibrium diagram, steady-state, so it is understood to be unlike real-world conditions.
My question is more along the lines of what is the next step in the transition after initiating carbon movement? What is transpiring at the point it becomes non-magnetic but still too low a temp for full austenitization? Is it a gradual transformation to austenite over the next 50*, or rapid and global once a certain energy threshold is achieved? What is the difference of whether it starts out as martensite versus pearlite?
Since we hopefully have an opportunity to have a thread go a little deeper than has been typical recently, I hope to stimulate Kevin and mete to give us a dose of the heavier stuff. :)
 
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Stacy is correct .This where equilibrium is taken over by the dynamics of HT.
 
fitzo said:
Please discuss why one still wants to take eutectoid steel above A1 in order to heat treat properly. Since the diagram is for "neat" iron and carbon, what effect if any does the presence of the manganese have on the solubility and mobility of the carbon?
Finally, since A1 and Acm are essentially at the same point for the eutectoid, does this indicate that it is much more sensitive to grain growth?
Thank you.
Click to expand...

Good question Fitzo, so good that it has to be answered carefully since it would be very easy to spin off into some very technical discussions involving the physics involving energy equilibrium in grain boundary movement. But to keep it simple, there are at least three things working for you on controlling grain growth in a steel that goes into solution at Ac1. First you still have diffusion time at temp working for you which brings us back to our friend the soak time. Next there is the fringe benefit of the aluminum killing process that was used to deoxidize the steel resulting in aluminum nitrides stabilizing the grain boundaries, most modern poured steels are would have this and are actually used to be referred to as “fine grained” steel as opposed to the silicon killed steels that would grow grains much sooner, yet another HUGE difference between the steel the ancients worked with and ours. These particles will hold the grain boundaries in place until enough temperature is thrown at them to defeat the effect and then the boundaries will move very quickly like rubber band that suddenly got freed, this is why grain growth surprises us so much when it happens and gets out of hand so quickly. Also manganese while not forming stable carbides in the boundaries will still have an effect as a substitutional atom in the matrix and slowing the movement of the grain boundaries as well (i.e. solute drag).

Another thing to remember about grain growth is that it is all about transfers of energy and when that energy levels off the grains boundaries tend to become stable until more energy is introduced, this is why some steels will tend toward a particular ASTM grain size. When I consider this it brings to mind yet another possible advantage of a controlled soak, Uneven grain size can be a problem and time at a set temperature could give the grains the opportunity to go from a mixed grain size to a more uniform one, but you need to be sure that the larger grains are still a decent size since that is what things will more toward.


As for desired temp above Ac1, well when you consider the previous paragraphs and how much stability in grain size you really can have, then you can see why a eutectoid steel is the most heat treat friendly of them all. If all you have is a magnet and a forge, a bit above 1414F (Ac2 or the "Currie point") will still work fine for you, if you have the ability to hold at 1500F even better, and you can assure total solution without any of the headaches that can arise with the hypereutectoid steel. Simply put all that carbon into solution and quench away!:thumbup:

Now is a great time for another plug for Aldo and his 1084;)
 
Thank you for the explanation, Kevin. It helps explain why we can hold even a simple steel at a given temp for an "extended" period of time (with good control) and not experience runaway grain growth. Phillip established this for us with his experiments with a tool steel. Your explanation helps to understand how it extends to plain steel, also, with trace levels of stuff from the smelting process. Still, I understand the mechanism is complex, as smaller grains will have to continue to grow faster than the large ones if the size tends to homogenize.

I appreciate you trying to answer technical questions in detail but in terms we can all understand. It is enjoyable trying to understand and picture what is going on in that steel at the microscopic level as it is having huge amounts of energy pumped into it.
 
So if I understant all of this grain growth, then it is more a product of temperature than a product of time. HIgher the temp more growth, right. We can hold a metal, say 1084, at 1500 all day long and not really expect any growth in the grain.
 
Wade, yes ! That's why we speak of temperature so much more than time .For those who HT with a torch we warn about not putting the torch near the edge and especially near the tip. The tip can very easily be overheated and the grains can grow rapidly even in short times....If you're a math type - temperature is usually a linear function and time a logrithmic function.Look at a TTT curve ,it's demonstrated there.
 
This grain-coarsening thing... I've got one reference on it (Metallurgy by B.J. Moniz). There is a short paragraph and a graph on p. 190.

The grain-coarsening temperature of fully killed steel is 925C to 980C (1700F to 1800F) and is dependent on a variety of factors. For example, it is dependent on the amount othe steel has been cold worked. One advantage of fully killed steel is the resistance to grain-coarsening during high-temperature heat treating operations (for example, carburizing... as high as 925C to 955C - 1700F to 1750F)

Under the graph is this...

The grain size of coarse-grain steels gradually increases with increasing temperature, whereas fine-grain (fully killed) steels maintain fine-grain size until the grain-coarsening temperature is exceeded.

I've guessed the tool steels used for forging and maybe the other common forging steels like 1095, 1084, 1075, 5160, etc. are all fully-killed. Is that so?

Mike
 
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