I like to watch Forged in Fire, but sometimes the metallurgist in me just wants to crawl right into the TV & explain to those guys why their blade cracked or warped during heat treat, and why it snapped during some of those interesting tests.
So, here is a l-o-n-g thing I wrote on steel for home-shop guys.
Old Files and New Knives
Used files are the handiest source of high carbon steel available to the home craftsman. Treat them right and they’ll make wood chisels finer than any on the market, as well as excellent knives. But, if you forge them like mild steel and heat treat carelessly, the result can be brittle tools with crumbly edges. Doing it right means forging and hardening files from controlled, moderate to low temperatures.
Mild steel though, is indeed properly forged from much higher temperature, well over 2000F..
First, let us go over some light background in the Art & Science of steel metallurgy. Most of you know that steel is hardened by heating until it glows red, then quenching in oil or water. Do this to low carbon steel, such as a 20d nail, and nothing much happens. The nail will remain soft. This is because in order for steel to harden, it must have a certain amount of carbon in it. The nail has no more than about 1/10% carbon, which is not enough. Very roughly speaking, the higher the carbon, the higher the heat- treated hardness and wear resistance, but the lower the toughness.
Some examples. Commonly available cold drawn steel is usually AISI 1018. The “10” is a code that means it is a plain carbon steel. That is, there are no deliberate alloy additions such as nickel, chromium or molybdenum to this steel. The “18” means the nominal carbon content is 0.18%. Heat this steel red hot, quench in cold water and it will get just a little bit harder, but not enough to be useful. Normally the only heat treatment done to 1018 would be to carburize it, which means to add some carbon just to the surface. Common machine shafting and axles may be made of AISI 1035, which is nominally 0.35% carbon. This steel does respond to heat treatment. If small pieces are water quenched, and then tempered to a peacock blue color, they will attain a spring hardness (but no, this does not mean that 1035 is particularly useful for mainsprings, or indeed springs in general). AISI 1060 (0.55 to 0.65% carbon) is used for sledge hammers and cold chisels, which must be hard yet take a beating. As the carbon content of steel increases, the as-quenched hardness also increases, up to a level of about 0.6% carbon. Above this, higher carbon increases hardness only slightly. But wear resistance does continue to improve with still higher carbon, above that 0.6%. The additional carbon can form many fine, hard iron carbide particles which help retain a sharp edge and resist abrasion. Common wood saws and older power lawnmower blades have been AISI 1070 to 1080 (0.7 to 0.8% carbon). Steels 1070 to 1075, by the way, are good, forgiving choices for gun springs. Axes from the old Kelly Axe Factory (no relative) in Charleston, West Virginia, were 1086, and star drills are 1095.
1095, with carbon in the range of 0.90 to 1.03%, is the highest carbon steel readily available from steel warehouses. The tool steel W1 has roughly the same or slightly higher carbon. I wish you good luck in getting the warehouse to tell you exactly which carbon level, or “temper”, of W1 they supply.
Since about 1980 U.S. made Nicholson files have been 1095. This 1095 steel makes good general purpose knives, daggers, fire steels, tomahawk bits and wood working tools. It can make a good spring if treated right, though you may find 1070 preferable.
Let us discuss files, specifically. Obviously, file teeth must be extremely hard and wear resistant to stay sharp while cutting metal. For centuries, files have been made either of soft iron, which is then case hardened, or of high carbon steel. Even high carbon steel files may be coated with some mixture to lightly case harden them. This is so the razor fine cutting edge doesn’t burn off when heated cherry red to harden it.
In
The Thirteenth Book of Natural Magick, 1589, G.B. Della Porta describes “ways whereby iron may be made extream hard”. To harden files he says:
Take Ox hoofs, and put them into an Oven to dry, that they may be powdered fine; mingle well one part of this with as much common Salt, beaten Glass, and Chimney-soot, and beat them together, and lay them up for your use in a wooden Vessel hanging in the smoak; for the salt will melt with any moisture of the place or Air. The powder being prepared, make your iron like to a file; then cut it chequerwise, and crossways, with a sharp edged tool: having made the iron tender and soft, as I said, then make an iron chest fit to lay up your files in, and put them into it, strewing on the powder by course, that they may be covered all over: then put on the cover, and lute well the chinks with clay and straw, that the smoak of the powder may not breath out; and then lay a heap of burning coals all over it, that it may be red-hot about an hour: when you think the powder to be burnt and consumed, take the chest out from the coals with iron pinchers, and plunge the files into very cold water, and so they will become extream hard. This is the usual temper for files; for we fear not if the files should be wrested by cold waters.
This four-centuries old process is metallurgically entirely correct. The chimney soot, of course, is a nearly pure source of carbon to carburize or case harden the surface. “Ox hoofs”, provide nitrogen which, along with the carbon, further improves sliding wear resistance. Salt and glass melt the whole mess together to provide intimate contact with the surface, and “activate” the surface to make it easier for the iron to absorb carbon. This is really 21st Century stuff! Even today the “usual temper for files” is just as they come from the quench, with no further reheat/temper/draw.
Until recent years when our EPA became overly concerned, modern practice was to coat high carbon steel files with a roughly similar mixture before hardening from a protective molten lead (now bismuth) bath. The modern version of “Ox Hoofs” is potassium ferrocyanide, K4Fe(CN)6. This is mixed with flour and bone black, and all are boiled together in salt water.
Both files and rasps were coated with this “cyanide loaf”. Rasps still are, or at least were in 1987, as it is necessary to case harden the 1035 steel from which they are made.
I have used a mix of potassium ferrocyanide and actual wood or bone charcoal as a home-made Kasenit. Spelling may be important to your health, the chemical should be ferr
ocyanide. Wash your hands after using the stuff, it might not be all that harmless. Well, I wrote this in 2009. I have since learned that Our Gov’t permits “yellow prussiate of potash” to be used as anti- caking agent in table salt. One must hunt to find a brand of sea salt without its dose of cyanide. On a more positive note Our Gov’t’s total incompetence appears to be independent of political party, so election results are unlikely to change it. Yeah, wash your hands but read the ingredients also.
While the currently used 1095 processes into a very good file, you might want to get ahold of some pre-1980 files to make the best edge holding tools. These older files will be of 1.2 to 1.4% carbon steel, which is higher carbon than can be found anywhere else. There are a couple of ways of telling which are the older files. One is that files marked only “Black Diamond” were made 1975 are earlier and are definitely something like 10130 steel. After that year the manufacturer stamped Black Diamond on one side and Nicholson on the other. One more distinction is the tang. It used to be that to save metal the tangs were hot-forged from the file blank, with very little waste. By 1975 it was more economical to use a longer blank and shear off metal to form the tang. A sheared tang will appear quite different than a hot rolled/forged tang. I believe the change in steel to 1095 may have come about because Republic Steel in Cleveland, possibly the only source for 10130 steel bars, stopped making it.
The ultra high carbon of those older files makes the steel capable of taking and holding a razor edge, and suits it for the very finest of wood cutting tools. This high carbon also makes the steel more tricky to forge and heat treat than, say, an AISI 1070 spring steel. If you overheat and coarsen the grain of your file it will not be capable of taking, and holding, the finest edge.
Nicholson file blanks were heated for about five minutes in a molten lead (now bismuth) bath at 1440°F, quenched straight down into brine to get maximum hardness, and then not tempered at all.
Let’s talk about the simplest job first, making a straight wood chisel or wood turning tool out of that old file. Many commercially available wood chisels are not really meant to either take or hold a fine edge. You can make a better one. Just grind your file, gently, to the shape you want, usually with a 25° to 30° bevel. Use plenty of water so not to soften the metal or, worse yet, crack it.
The best bet is to first scrub off all old oil and grease. This is necessary for Domestic Tranquility, as you are about to temper the thing 350°F in Wife’s oven, for a good hour. This drops the hardness a little, but it will relieve internal stress and help keep the fine edge from chipping.
The safest thing to do is temper before you grind it to shape, so it is less likely to crack in grinding. I suggest 300-350°F, at which temperature the metal will develop a light straw color. After tempering for an hour, the straw color may deepen somewhat.
Do not trust temperature settings on the kitchen oven, get a separate thermometer. You might also try a scrap piece first and go by the temper color developed. Better yet, buy a few Tempilstik° temperature indicating crayons. To be sure, I’d be inclined to repeat that tempering treatment after the chisel had been ground to its final shape.
For a chisel, draw back the temper on and near the tang to make sure it doesn’t break in normal use.
You now have a fine wood chisel, as good or better than anything on the market today.
This choice of temper, and a very high carbon old Black Diamond file, will not make a good bowie knife, although it would be a fine skinning knife. That 350°F temper still leaves the metal rather too brittle to handle the rough use of your average camp knife. A skinning knife, on the other hand, must hold its edge through a lot of cutting. Hopefully, you all won’t use your skinning knives to chop wood, bone and tin cans as well.
For bowies, daggers, tomahawk bits, etc., I’d suggest using the more available post-1980 file of 1095 steel. Temper a bit before grinding to shape. For most knives I would suggest a 500°F temper. That will leave bright metal a mottled brown to purple temper color. Again, I’d use the kitchen oven for about an hour, likewise don’t trust the temperature setting to be at all close.
We have about come to the point where we’ve got to get serious about temperature measurement. Pyrometers, thermocouples and electronically controlled electric heat treat furnaces are great, but not necessarily in everyone’s budget. Judging temperature by how bright the metal glows is not very accurate. Nevertheless, if that is how you will be heat treating, you will find plain carbon steels more forgiving of errors than are alloy steels. I might offer for your consideration that making springs of 1070, and cutting tools of 1095, w1, or best, W2, offers the best chance of success.
Frizzens may be faced with any of these. Tools steels and alloy steels, such as D2, O1, 4140, etc do best heat treated with reasonable temperature control.
A practical, relatively inexpensive means of temperature measurement is to use a temperature indicating crayon, specifically the Tempilstik° brand. Each crayon is marked with a temperature.
Mark the metal with this crayon, and when it has reached the specified temperature, that mark very clearly melts. You may find details of what is available & how to use them at
www.tempil.com . Some Tempilstik°s are available through welding supply shops, a full range may be found on the internet. Get several, so you can tell if your bowie knife is too hot, too cold or just right. For temperatures under 600°F I myself am inclined to judge how hot the metal is by whether it is straw, brown or blue in color, but a Tempilstik° will be more accurate.
Springs need to be tempered 800°F or higher, and there is just no way to tell that temperature by color or any other traditional way, except a Tempilstik°.
Use your lead pot for casting bullets. The finest craftsman of our generation more than once sent me a beautifully finished, but broken, mainspring which was simply too hard.
Back to files. It is more fun to forge a blade to shape than to grind it, so let’s get into the nitty gritty.
By the way, you forge the blade because:
1. You want to
2. It is a good way to get the shape you want.
Your forging procedure does not improve the metal one bit, and is more likely to degrade it. Beating on it with your little 5# hammer does not “compact the metal” any more than already done by the steel company’s enormous rolling mill.
Forging high carbon steel is a different matter than working mild steel. You blacksmith types know it is easier to burn high carbon steel, for one thing. But even if not burnt, forging file metal from white heat and slow cooling it can coarsen the grain and make it rather brittle at any temper. The reason is all that carbon. When steel contains more than 0.8% carbon, it is easy to get a brittle carbide network . . . Huh? The carbon above 0.8% can form a brittle iron carbide layer around each grain.
Grain?
All metals, including steel, consist of millions of tiny crystals all stuck together. Imagine a pile of grapes that have been pressed together without breaking the skin. Each grape is roughly the same shape as a metal crystal, or “grain”. If you overheat and slow cool a file, the “skins” of each little “grape” will be hard and glass-brittle. That translates into a cutting edge that easily chips and becomes dull. Wear of a sharp cutting edge occurs by fine chipping.
I would suggest forging that file, especially if it is the old 10130 steel, as though it were a true Damascus (Wootz) steel. Heat it to a much lower temperature than for forging mild steel. This means more muscle but it will also keep the metal fine grained, which is most important.
Ancient Wootz, which could be as high as 2% carbon, was forged starting at 1550-1600°F and finished at a blood red, about 1200°F. It would be nice to have a few Tempilstik°s around so you don’t have to guess this temperature.
Once the blade has been forged you should always anneal it to reduce chances of warping and cracking during the hardening operation. The safest way to do this with high carbon steel is to heat maybe 1500°F for a while, then bury the thing in ashes, sand or vermiculite to cool slowly.
Most steels are annealed from a higher temperature. I am suggesting a lower temperature anneal to keep the fine grain structure and fine carbide distribution you got by forging that old file down in the cherry, down to blood red, temperature range.
Now that the forging is annealed, grind it all over to near final shape. Use cooling water. You must grind off the scaled, decarburized surface or it is very likely to crack when quenched.
Decarburized?
What does that mean?
When you heat steel in the forge some of the iron oxidizes to a blue-gray scale. That is obvious. Also, some of the carbon literally burns out of the steel surface—the metal is “de-carburized”. This is just the opposite of case-hardening. If you leave that thin, soft low carbon skin on the blade, paradoxically enough it will crack when quenched in water or brine.
To harden the file steel, heat to around 1440-1450°F for 4 to 6 minutes, and then quench straight down into salt water. Overheating is bad, use a Tempilstik° or the electric furnace you bought from Brownell’s.
The quench liquid is important. If you want to be really traditional, you could use the urine of a three-year old goat fed only ferns for three days.
Or, you could use a modern mix, more suited to Urban Folk. And that is salt brine. Mix just 13 ounces of common salt in a gallon of fresh water. That makes a 9% salt brine. This brine will quench twice as good as water. It reduces the chances of cracking and warping, and the part will be harder. It is, of course, a less interesting quench than the aforementioned animal product.
I have heard different numbers for the salt concentration. One man years ago told me that when quenching steel in a remote mid-eastern location he used just enough salt to float a potato.
File steel does not harden well when quenched in oil. That isn’t necessarily all bad, but the end result will not be quite as good as it might have been.
How you dunk the knife in the brine quench matters. If you belly-flop it, it will surely warp, if not crack. Plunge the hot knife straight into the quench and it will come out reasonably straight. This my old metallurgy professor noted, from his experience with bayonets at Springfield Armory.
Likewise the blade will be straighter if you quench in brine than if you quench in water. This is because brine reduces the steam blanket which otherwise forms over red-hot steel immersed in water. The steam blanket forms, collapses, then forms again somewhere else. Steam does not cool as fast as does water, so the quench is spotty. You may be aware that when the old smith quenched something in his slack tub, he moved the piece around in a figure-8 pattern. This was to help break up that steam blanket.
Remove the blade from the quench while still a little warm and temper as soon as possible. The word “draw” also means to temper. For a knife of brine-quenched file steel I would suggest temper at least 450°F (brown), preferably 500°F (purple). That may leave AISI 1095 at about Rockwell C57 hardness, close to the Ideal RC 58 of the Knife Guys.
Fire steels might work better if a little softer, tempered 500°F (purple) to 600°F (blue).
Rasps are a different matter. Nicholson rasps are, or were in the 1980’s, made of AISI 1035 steel with a lightly case-hardened surface. This steel does not have enough carbon to make a knife that will hold a good edge. But it is tough, and a good choice for throwing knives that take something of a beating. I might suggest quenching in brine from 1575°F, then tempering about 400°F (dark straw). This will leave the metal hard enough to do some cutting, with reasonable toughness.
©2009, minor upgrade 2019 James Kelly (retired metallurgist)
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