Hypereutectoid steel

K

Kevin R. Cashen

Joined
Sep 9, 2003
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I thought I would start a new thread for this since similar questions have been asked in at least two threads and in some e-mails I have received.

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Steel with carbon contents in excess of the eutectoid are called “hyper-eutectoid”, and require a little more thought and car in their heat treatment than eutectoid steel. When you have all this carbon .80% of it will be used up in the making of the desired phases, but then there will be leftover carbon to deal with. This leftover carbon can be useful or can be a great liability depending on where you put it.


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The grain boundary carbides make the steel very brittle causing intergranular fracture .Here's what the fracture surface looks like .
 

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O.K. now that I have scared you to death with all the problems that can arise, I would like to now explain how you can avoid them and how hypereutectoid steels can be our friend and very useful for the right type of knives.

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Kevin,
Could you expound on how forging temps for hypereutectiod steels play into this mix?

J. Scott
 
J. Scott said:
Kevin,
Could you expound on how forging temps for hypereutectiod steels play into this mix?

J. Scott
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of my students will burn some steel on the first day and then get so gun shy ectoid steels, a carbon and pre{{{{{{{{pare it for the heat treat that will come later on.
 
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Thank you, now I understand why the vermiculite advice I gave the other day was bad for 1095.

Keep it coming! (pencils at ready!)

-Page
 
Kevin R. Cashen said:
So when you have a steel well above .8% carbon be certain to normalize well without overly slow cooling, use sub critical anneals by cycling above 1100F but below non-magnetic (not only will the steel love you, your mills and drills will as well), and be careful to keep the temperatures below Accm in hardening. Unfortunately the best way to accomplish that last one is to have a well calibrated heat source. Lower temperatures and longer soaks are much better for hypereutectoid steels.
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You didn't mention about soak time.

When to do it and at which temperature?
 
Kevin, thank you for helping me become less stupid every time I read your writings :thumbup:

Ever thought about writing a knifemaker's version of the tool steel book you recommend?
 
Kevin,
Thanks again for another great post!:thumbup:

You did forget to mention that with the extra carbon content, you have to add more bacon grease to the candle wax and axle grease quenching media.:D

Stacy
 
Thanks for those answers, Kevin.
I look forward to the day when that information will be on your site and we can all just post a link!
 
It's good to understand the iron-carbon equilibrium diagram even though we rarely see equilibrium in the real world. Even though the lines may be moved about due to time and alloying , the principles remain the same.
Those who don't forge have to guess what the original structure of the steel is. Numerous times on this forum someone mentions problems cutting or drilling the steel even though the steel should be 'annealed'. Obviously the steel is not what it should be .The process then is to make it proper by heat treating applying the methods Kevin mentions above.
 
Kevin,

Thanks a lot for answers.

Kevin R. Cashen said:
Remember with hypereutectoids it is better to go a little lower and stay there longer. Let me explain why- your soak temperature will determine how much carbon you will put into solution, your time will determine how evenly it will be distributed. If you have an entire grain (incredibly small by our standards, yet a very, very large space by a carbon atoms standards) to fill evenly, you may get the carbon you want in solution really quick with your temperature but now you have to move it though the grains to equalize the concentrations. You can move it quicker by bumping up the heat but this will automatically also put more carbon into solution than you may want.
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Between grain refining vs carbon distribution, which one we should do first and why?

Kevin R. Cashen said:
Let’s go back to the carbide island in a lake analogy. This is the classic condition you will have in hypereutectoid steel when you buy it from the supplier or mill- spheroidized. With this condition you will have your lakes (grains) of room temperature ferrite littered with hundreds of little carbide islands. This condition will absolutely require a good soak time for the reasons previously mentioned, but the smaller and more numerous the islands, the less time it will take for the lake to be evenly saturated.
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To complete the picture, I know you have been sharing this before but I would like to ask you again to provide the example of your O-1 heat treatment along with this thread for both with spheroidized and non-spheroidized so we are clear exactly what, how, and why.

Thanks, bambright
 
its refreshing to read your take on this.. .. actually its colorful compared to the texts... ;)

i feel that you got a decent take on the old steels... even the hypereutectoids in the past were working towards different goals... (such as crucible steels. )... such as producing very coarse carbides .... which is fun in a different way but not the goal of modern knives...

i know that dealing with modern products give you a much better chance at predictability and dependability... which gives the maker a running start at quality... ( in my opinion, that is a heck of an advance in tech )

question...
since i'm not equipped for controlled subcritical anneal.. .. i'm curious... because once or twice i've drilled some steel barstock that was suppose to have been annealed... and the drillbit chattered alot... ..which is excactly what i expect from the crucible steel barstock ( i'm vary familiar with that )... but this was from the factory... huh...

can the subcritical anneal produce carbides that coarse if the process wasn't watched closely.... what are the limits to the process... or can they run the size of this up to what i'm used to seeing?

another thing... with the popularity of hyperetectoid steels in knifemaking.. why hasn't there been more discussion on the " what to do with the extra carbon "... ... is it just superfluous to the modern knife... ! maybe our expectation of carbon steel is lower due to the popularity of stainless steel.

another question... what about tempering carbides.. i know there small but can they be a problem.. .. pending on temp and where they are?
( sorry bout my ignorance but i don't know much about this at the moment )

G
 
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