It is a strange thing to re-create skills and knowledge in the age of the internet. There is at once too much information and then not enough that is applicable. And then there is the annoyance of blogging about something apparently so obscure that your search queries return your own content.
If you want to make a file and have the limitation of a simple forge for heat treating, prepare for a challenge. But what a great challenge for learning this stuff, let me explain. You’ll need to forge your own file cutting chisels, they’re pretty much the unicorn of metal working chisels at this point. So you get the experience of heat treating an edge, and then testing that edge by repeatedly hammering it into a bit of steel. Right away you know if your work is not good enough, the edge tells you if the grain of the steel or temper is bad from how it chips or folds or tears.
I’ve been testing my chisels on a bit of mild 1/4″ bar stock, which is buttery soft compared to the tool steel I’ve been working on, for a number of interesting reasons. Jason dropped a comment in my last post that was manna from heaven, annealing tool steel is not as straight forward as ninety percent of the information out there might lead one to believe. Having previously tested all of my chisels, seeing how many inches of teeth they could cleanly cut with a given bevel angle, I re-tempered.
Normalize, normalize, normalize. That is what I was missing with my heat treatment for these chisels. It was obvious that even though I was hitting the right temps to harden, the grain growth was really bad, leading to an edge that would blow out large pieces. Moving the chisel through several rounds of normalizing (above non magnetic, and then a bit of soak, and then air cool to no longer glowing) made a tremendous difference in edge holding. I probably also did a better job of quenching at not too high of a temperature. What can I say, you’re kind of fucked trying to judge metal temps even in the shade during daylight.
I’ve been drawing the temper to a basic straw color after hardening, seems to be the right range for engraving tools, which I consider these file cutting chisels.
So now I have a few chisels that I feel have basically decent edge holding, how about my problems cutting the file blanks? I knew that my simple annealing process using wood ash as an insulator wouldn’t be the same as a full kiln annealing, but its so often spoken of as adequate I didn’t consider that there was more going on in the structure of the steel. Certainly heating my file steel up to non-magnetic and allowing to cool in an insulated medium softened the steel, that was clear from how easily it bent. The unintended consequence, best explained by Kevin Cashen here:
“….,the ideas of overnight cooling all work fine with 1084, 1080 and other steels with less carbon, but will give you no end of troubles with a hypereutectoid steel like 1095, and is exactly why you found it less than pleasant trying to drill your holes. Heating to above critical (or even non-magnetic) and allowing the steel to slow cool is called a lamellar anneal and it forms coarse pearlite within the metal. This state is made up of extremely hard carbide segregated out into sheets between areas of soft iron, so such a blade will bend and seem soft but will rip drill bits up! No amount of reheating (tempering type) short of a full normalization will have any effect on these sheets. But of even more concern is that leaving hypereutectoid steel that is in solution to slow cool will allow all the carbon in excess of .8% to fill the grain boundaries and cause a very weak an brittle condition that would be rather bad for a knife.
Hypereutectoid steels, and forgive me for not explaining the term which simply means steel that has more carbon than the eutectoid or .8%, really should be spheroidized if you are going to be doing any machining with them. It is best done with well controlled heat treating equipment but it can be approximated in a simpler shop as well. To accomplish this with just a forge you should heat the steel up and normalize it well to homogenize things and get any carbide our of the grain boundaries that may be there. Next reheat it a couple of times at lower temperatures to refine the grain and on the last cycle heat it to just above non-magnet and then quench it in any oil you may have.
Now for the anneal- reheat the hardened blade to red but never allowing it to lose magnetism, do this several times and it will ball all that extra carbon that you trapped in the quench up into little spheres suspended in a soft iron matrix. The steel will be soft and bendable and when a drill or mill hits those little spheres they will just move out of the way and allow you to keep cutting . ”
This jives with how the chisel felt cutting the file blank previously, little hard spots that wreak havoc on the chisel edge.
So after grinding and hand shaping a couple of files I went through the above normalization and spheroidization. I didn’t wind up with the buttery consistency of my mild steel test piece, but it was an improvement.
My work holding is way off from historical president, same with anvil shape, but you use what you have and make it work. If I can actually get a file to harden properly I’ll have to think about refinement here.
Only last night did I read back through a book written by the early Nicholson File Co, “A Treatise on Files and Rasps Descriptive and Illustrated“, which of course speaks only favorably about their new at the time “increment” cut file that reproduces some of the valuable irregularity in hand cut files. It does however mention one important detail about sharpening the chisel for hand cutting, that of rounding off the newly honed edge by dragging a few strokes of the stone across the edge, much as one might sharpen a plane blade for working abrasive material like bamboo that might otherwise lead to chipping.
Another small detail, but important not only to edge retention, but gullet formation for the file tooth. In some sense then the sharp edge of a file tooth is formed by upsetting the steel, not merely cutting into the file blank.
It took a hell of a lot of sharpening to cut these three files. I then mixed up a thin paste of flour and water, coating the file and wiping off the excess with a swipe of my fingers. In a mortar and pestle I ground some table salt to a powder, which was then used to liberally coat the sticky files, as much as I could get on there. After drying they kind of looked like incense sticks, I figured a heavy coating of salt was better than too little. I didn’t include any pictures of this yet because I need to work with the technique more. Supposedly I should be able to see the salt subliming as it reaches non-magnetic temps.
In practice I ran into a simple problem. If I’m moving the file around in the fire to check its heat, try to heat it evenly, the salt coating gets worn off. I resorted to laying the prepared file on the coal bed, covering it over, and trusting in a bit of intuition to remove it a the right moment for quenching.
Such is the way of things that my first file warped quite noticeably in the water quench. I’ve read that if they’re pulled out at just the right moment with enough residual heat its possible to straighten. Of course, a tiny file like this cools very, very rapidly, so I pulled it out an promptly snapped it in half trying to straighten. At least I know my hardening was good!
And it gives the chance to look at the grain structure, which doesn’t look too terrible thanks to the normalization.
The above photo shows a couple of interesting things. The salt coating protected the top half of the file quite nicely, but fell off too soon on the bottom and scale developed. Testing for this file proved immediately destructive of the thin edges. Perhaps I should try an oil quench for a forging so thin and delicate of proportion?
This file (with 135 deg. face angles for sharpening chone-gake on maebiki-oga) seemed to have the best annealed state as you can see by how cleanly the teeth were cut, and didn’t warp during quenching. Both of which I’m attributing to the more even cross-section. I still need to clean it up with an acid wash followed by a neutralizing alkali wash.
Finally, finally I get to the point of my first hardened files! I’ve learned more about blacksmithing and heat treating tool steel in the past week than in the previous seven years, seriously.