Questions on file knives

[willowbilly3]

Hi, new here and a rank ametuer. I have made scant few knives but do have a little backyard savvy on welding ect. Recently I have decided to make some knives in the spirit of the old trade knives and buffalo skinners. My first two were from files. One is finished but I didn't temper it before I handled it. I used ironwood for the handles and now I am wondering if I might be able to draw that edge out a little without ruining the handle. Any suggestions? I also have another file blade hammered out and have not put a handle on so it should be easier. Oh, and I also have a couple Ulus I made many years ago from skilsaw blades and they wont take an edge either, didn't treat them either. I do not have a forge, I use my oxy/acetelene with a small rosebud clamped in the vise for a forge and a 4x6x12 block of D2 steel for an anvil. thanks

[David Einhorn]

I expect that people will suggest removing the handle prior to heat treating. Your blade should first be normalized, made sure that it is straight and free of stresses, then heat treated.

In heat treating blades are first hardened, then tempered according to the metal used and desired Rockwell hardness.

To say that one wishes to "temper" a blade with the handle on using a rose tip on your torch is pretty much saying that you wish to selectively soften spots on the blade. :D

Edited September 30, 2009 by UnicornForge

[bigfootnampa]

Well it may actually be possible to selectively harden the edge of a knife without removing the handle by taking advantage of the quick heating that is possible on the thinned edge. Having said that though I would suggest that you'd spend your time more productively by starting over. I would even suggest starting by forging a couple of small test blades and practicing your heat treatments before even attempting a blade that you intend to finish. Then when your next keeper blade is fully shaped AND heat treated TEST it's edge BEFORE taking the time to handle it.

[ThomasPowers]

File knives are sometimes made without re-hardening using only drawing the temper from the extreme hardness and brittleness of the file down a bit to make a tougher knife blade that is easier to sharpen.

So when you say drawing the edge out do you mean hammering it out or drawing the temper on it?

[willowbilly3]

I didn't mean to hammer it anymore. Maybe I used the wrong term. What I was thinking, or hoping to hear was maybe something like a quick heat back a bit from the edge until I see straw color at the edge and then quickly quench. Or a case harden of sorts.The ironwood is pretty resilient and will give off odor quite a bit before it actually chars.
After reading a bit on saw blades I realize the Ulus are probably just junk because the saw blades were the carbide tipped kind. My daughter loves them for cutting pizza anyway.

[MattBower]

Are you saying that this blade was ground cool from a file (i.e., never got hotter than a few hundred degrees during shaping)? Or was it forged from a file?

Edited October 1, 2009 by MattBower

[ThomasPowers]

Case hardening a file would be a trifle like gilding a lily! Already very high carbon; adding more won't get you much if anything!

[willowbilly3]

Are you saying that this blade was ground cool from a file (i.e., never got hotter than a few hundred degrees during shaping)? Or was it forged from a file?

Forged from a file. The metal seemed very soft, easy to file by hand but won't hold an edge at all.
I did attempt to reheat the blade to about a straw color and water quench. Uniform heat is a little hard to obtain and I got it blue in spots and redid the process 3-4 times. I have not done anything more to see if I realized any success at all. thanks.the handle didn't burn.

[bigfootnampa]

Thing is you'd have to get it red hot to harden it. You are attempting to temper it and that only works when you have hardened it first. Tempering actually softens the steel and is done to correct excessive brittleness that is created by the hardening process. Unless you harden at red hot heat tempering will be ineffective and unneeded.

[willowbilly3]

Thank you bigfoot, that is very useful information. I think I will just throw that knife in the box and keep it as a reminder of my beginnings. I do have a couple other file knives in the works and will attempt to temper them as soon as I have a better control of the heat. Non-magnetic maybe but that seems rather subjective. Has anyone here ever heard of or used molten salt? That is how Stanley tempered their cutting tools. It is supposed to become molten around 1500f

[bigfootnampa]

1500 degrees is way too hot to temper knives. Read Steve's sticky above on basic heat treating. I have heard of using hot brine as a quench (for hardening... not tempering) which may be what you are referring to... this type of heat treatment is WAAaaay beyond the basics which you have yet to master. Don't confuse yourself with such industrial systems. Hardening and tempering are separate but related steps in heat treatment. For most steels both steps are needed to get a quality cutting edge that will take and hold a good edge.

Back up and start over. Make a couple of small practice blades and practice heat treating them and then test them (BEFORE putting handles on them). If you get spotty results try again. Once you can consistently get nice edges you will be ready to start with learning the handle making end of things.

[MattBower]

Willowbilly, don't take this the wrong way but you need to get your terminology straight; you're confusing folks who're trying to help you. It's not entirely your fault; some of the old-timers had a habit of doing the same thing, and I can be confusing for a new guy to sort out. When the blueprints come back up, go read Quenchcrack's intro to the metallurgy of heat treating. It'll really help you get your head on straight.

What you need to do with your forged knives is harden them. You do this by heating them above a certain "critical" temperature until the steel undergoes a phase change to austenite (yes, in the neighborhood of 1500 degrees F for most simple carbon steels -- we'll get into that more in a minute); this process is called austenitizing. The method of heating the steel can be anything from an electric heat treating oven to a molten salt bath to a propane/coal/charcoal forge to an oxy-acetylene torch; they're not all created equal, but they all can work. (More on that momentarily.) Once the steel is austenitized you remove the it from the heat source and immediately -- while it's still at the critical temp -- quench it in an appropriate medium, known as the quenchant. Depending on the steel that quenchant could be brine, water, oil, air, nitrogen gas, or what have you. For backyard purposes it's going to be one of the first three -- brine, water or oil.

Now, ideally you will soak your blade at the austenitizing temperature for a bit. Eutectoid steels like 1084(ish) require almost no soak, which makes them great for backyard heat treating methods. Hypereutectoild steels like the stuff your file is probably made of (and also including 1095), as well as most higher-alloy tool steels, really want to soak for 10 or more minutes in order to get all the carbon and alloying elements into the solid solution of austenite. Unfortunately, it's very hard to hold steel at a steady temperature in a non-temperature controlled forge. And if you overheat steel during austenitizing, the grain will get too big and make it brittle. So overheating is something to be carefully avoided.

A properly hardened blade will be extremely hard (it should skate a file easily), but very brittle; if you drop it on a hard surface it's likely to break, and the edge is apt to chip during use. To reduce that brittleness you need to temper the blade, which means heating it at relatively low temperature -- typically 300 to maybe 600 F, depending on the steel and what you're using it for. This softens the blade slightly, but makes it far tougher, so it's not so prone to chippage and breakage. This can be done in an oven, with a propane torch, with a special pair of heated tongs, on a heated block of steel, or any number of other ways. Colors such as straw, bronze and peacock blue are oxidation colors that occur on steel heated to a few hundred degrees. They can be used as visual references that give you an approximate idea how hot you've gotten your steel during tempering outside a temperature controlled oven. So when you talk about heating your steel to straw, then quenching, that's supposed to be a tempering step. If your steel hasn't been hardened before you do that, you've accomplished exactly nothing.

High temperature salt baths are one means of heating the steel during austenitizing. They have tremendous advantages in that they're very closely temperature controlled (no chance of overheating, they prevent decarburization of the steel, and they heat the steel quickly and evenly. All of these are extremely desirable qualities. But molten salts are extremely dangerous, and the furnaces are quite expensive. (Of course you can make one yourself, but it takes some skills.) The salts themselves are relatively inexpensive; there are many different combinations of salts with different melting points that can be used in heat treating, but for our purposes you wouldn't want a salt that just melts at 1500 F, would you?

You won't find too many folks on boards like this one who're using salt baths -- some, but not too many. As a newbie who hasn't yet learned the difference between hardening and tempering, high temp salt baths probably shouldn't even been on your radar yet unless you're independently wealthy and have nothing better to do with your money. (And if that is the case: wanna buy me a salt bath?)

It turns out that iron -- the principal component of steel -- becomes non-magnetic (reaches its Curie point) at right around 1415 degrees F. There's nothing "subjective" about this; it's a reliable scientific fact. That's not far off from the usual recommended austenitizing temperature for 1500 F. So it's very common among backyard heat treaters with primitive equipment -- most of us -- to use a magnet to tell them when their steel is almost ready to quench. The conventional wisdom goes along the lines of "take it a shade above non-magnetic, hold for a minute or so, then quench." Now, yes, there is some subjectivity in that advice! And I'm not going to tell you that this is an ideal method of heat treating, because it isn't. If you're serious about getting the best possible performance out of your blades, eventually you'll want to move on to a more closely temperature controlled heating method that allows you to soak at a desired temperature for 10, 20 or more minutes, which many of the higher alloy steels need for best performance. When that time comes you can think about an electric furnace or molten salts. But why don't you make some knives first, and decide if you like this game before you go making those kinds of investments?

Edited October 9, 2009 by MattBower
adding more details

[MattBower]

I Forge Iron - BP0078 The Metallurgy of Heat Treating

[jake pogrebinsky]

Thanks,Matt,it's very generous of you to go into such detail.An effort like this is much appreciated even by those of us that know a little(or hope that they do),about the subject.Very well said,it's quite a struggle to reconcile the science with the backyard/common sense end of things,and your take on it is very helpful.
Cheers,Jake

[MattBower]

Thanks, Jake, but I'm no guru. I know enough to help a new guy, I think, but I'm constantly reminded of how much I don't know! I'm just now getting to the point where I know what ferrite and cementite are without having to look them up every time. ;)

Edited October 13, 2009 by MattBower