Metallurgy in that "making a file" thread

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In Tai Goo's thread on making a knife from a file, instead of having us do the usual annealing process, he has us do what seems to me to be simply tempering to dead soft by heating to black heat several times.
In terms of metallurgy, what exactly is happening when you do that?
I haven't really seen this process discussed in the "rosetta stone" sticky or, for that matter, anywhere else, and it seems like a very useful technique...though perhaps only to be used on files?
Thanks for any insight on this!
Andy Gladish
 
That is the sub-critical anneal needed for hyper-eutectoid steel. It will make a sheroidal annealed pearlite.
 
Thanks Stacy, very cool.
So you're making spheroidal pearlite without a quench, or is quenching irrelevant at that temperature?
And is the martensite completely undone without heating to critical?
That makes me wonder- if you can go straight from Martensite to spheroidal pearlite, why normalize or anneal hypereutectoid steel at a higher temp at all?
Andy G.

edit- looks like I THOUGHT I understood something about annealing and normalization...time to read up, thanks for your reply
 
Well, I probably should have left the word spheroidal out,since I can't be sure of what you started with.... but you are making pearlite.
As you raise martensite above 400F, it will start to convert to pearlite. That is sort of what tempering is.
At the upper end of the pearlitic range, near the critical point, the steel has become all pearlite. If you avoid long holds at this temperature, and cool fairly quickly ( air is fine, quenching is better), you will get soft pearlite. Cooling too slow ( in vermiculite of ashes overnight) will give you a file having a pearlite structure that can be hard to file and drill.

Tai has you heating to this range and cooling down reasonably fast ( air), and then repeating a few times. That gives you a good pearlite structure. His method is just a simplified sub-critical annealing.

I personally would do a full spheroidal annealing on a file:

Heat to about 100F above non-magnetic - 1450-1500F
quench in oil
repeat at just a tad above non-magnetic - 1425-1450F
quench in oil
Heat to just non-magnetic - 1350F
Quench in oil
Heat to just below non-magnetic - 1200F
cool to black heat (900F), and quench
Heat to sub-critical again, cool to black, quench.

What this does is give you a fine grain structure with spheroidal carbides in fine pearlite. When you work the blade it will drill and file easily. When you re-harden the blade, it will start from a good place. If working with simple heat treating tools ( torch, one brick forge, etc.) this is the best approach.
 
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Thanks for the explanation Stacy.

I didn’t say that the method I gave was the only way or the best, but just trying to keep it as simple and fool proof as possible for the newbies as I could.
 
Tai,
I think your method will work fine, and is simple enough for the task at hand.
Tai and I both have said that there is a lot of guess work in a file knife, and his HT takes that into consideration.
 
Have any of you makers made a file from a knife ??? Just to be different .LOL
 
So in my imagination it works like this: Making martensite (quenching from the proper transformation temp) gets me fine grain, or preserves the fine grain already in a file.
Over heating allows the carbon to clump up thus the grain to grow.
Converting martensite to pearlite relaxes the stressed structure so that it can be drilled.

I know there's a lot else going on, but am I on track so far?

There's a confusing line in the Wikipedia article on Pearlite ( http://en.wikipedia.org/wiki/Pearlite ) where it says that it is "hard and strong but is not particularly tough."
I'm missing something, because don't we make martensite to increase hardness, not toughness?

The Rosetta Stone gives great info for technique but for some reason I'm not getting a mental picture of exactly what's happening...what I THINK is that we're trying to distribute the carbon as evenly and finely as possible, then temper it to relieve the excess stress in the martensite.

Thanks for the great info, it's like taking a metals class without having to drive fifty miles!
Andy

Mete- I've certainly taken some knives and "filed" them...in the scrap bin...but that's different
 
Hard and strong are relative.
Compared to SOFT pure iron, pearlite is HARD. It is strong compared to Brittle Martensite.

You have the basic idea, but there is no grain growth at 1200F. Grain growth starts at temperatures above Ac1 ( 1400F), and mainly gets large above 1600F. We locked in the fine grain at the quench from just above non-magnetic.
At this point we formed fine grained martensite. We do not want to go back to austenite again if our goal is making soft pearlite.

Pearlite is a lamellar arrangement of ferrite (IRON) and cementite (Iron Carbide). It forms when a eutectic mix of carbon and iron are cooled at a rate that allows separation of the two crystals.The carbides form about 10% of the mix. Any extra carbon gets tied up as more carbides.The ferrite and cementite forms in layers ( lamellae) that look like mother of pearl under a microscope - thus the name. The layers can be coarsely laid down, or finely packed.
When coarse, the carbides can be pushed around easily,and the layers disrupted with less energy. Thus, the metal is soft - allowing drilling and filing. When tightly packed,it takes alot more energy to move the arrangement. Thus, fine pearlite can mimic martensite and be hard and even somewhat brittle.

Simply:
If you cool pearlite quickly from its upper range, the layers are randomly arranged, and it is softer. If the metal is held at the upper pearlite range for long enough, and cooled slowly, everything gets laid out in nice tight layers....which are hard.

Now, what about those rock hard extra Iron Carbides? When we did the quench from austenite, we locked them into place. By leaving them alone, they can't grow any larger without going above the solution temperature again. Thus, we do a SUB-CRITICAL annealing, at which we allow the pearlite to get in its softest state without making larger carbides.
This is done at a temperature just below Ac1. We need to hold the temperature there long enough to allow the carbides to ball up with each other ,then cool it quickly enough to avoid fine pearlite.
When this state is reached, you have spheroidal annealed soft steel.

I am going to point out that again that all pearlite is not soft like butter. It can skate a file if the carbides are well distributed ( not spheroidized). It can burn up a drill if it is too fine. Avoiding both these situations is the whole point of spheroidally annealing hyper-eutectoid steel.

If we remove the extra carbon, and stay at or below the eutectoid, then there is much less problem with slow cooling the steel.

An interesting thing is that if we form austenite, and cool it at a rate fast enough to avoid forming pearlite, but stop the cooling before we would form martensite,....we get bainite. Which has some features of both martensite and pearlite. Despite some marketing publicity, bainite has little or no use in blades, as properly tempered martensite will surpass it.
 
Holy smokes, THANK YOU Stacy!
I hope some other folks are listening in, this is great info.
The big insight for today is that it's not an either/or situation, that steel isn't either soft or martensite, that there are other structures that exist at room temp with varying degrees of hardness/toughness/strength...I know you've been telling us that for years...:confused:
I have to be careful about getting started on the subject of steel when talking with people- regular people, I mean, not blacksmiths and bladesmiths- it's so exciting to find out what's going on beneath the surface of common materials that we'd otherwise take for granted.
Andy G.
 
It was said that this is for hyper-eutectoid steel. Is the process different for eutectoid (1080/84) and hypo-eutectoid steel?

- Paul Meske
 
That is what I meant when I said there is much less worry at or below the eutectoid.

Without the excess carbon, you have 12% max cementite. The lower the carbon, the softer the steel.

Eutectoid and hypo-eutectoid steels can be annealed in vermiculite.
 
Stacy, I'm a little bit confused by the temps given. I thought 1414F was the non magnetic (curie) point? You say "...repeat at just a tad above non-magnetic - 1400F
quench in oil, Heat to just non-magnetic - 1350F Quench in oil" -- are the temps different for a hyper-eutectoid or are you being as general as possible?
 
I was giving the rounded off temp of non-magnetic. 1414F would be the exact temp for non-magnetic. I didn't really think anyone heat treating a file would be using a temperature controlled oven, so I posted an approximate temp ( which is almost always overshot when using a magnet and eyball). I have changed it to read 1425-1450F.
 
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