Post by Buck Conner on Feb 10, 2021 7:03:21 GMT -7
An old friend sent me this article he wrote a few years ago, he a "nut job" for doing thing correctly.
Old Files and New Knives
Jim Kelly, January, 1987
"Used files are the handies 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 carelessly, the result can be brittle tools with crumbly edges.
Doing it right means forging and hardening from controlled, moderate to low temperature.
First, some light background in steel metallurgy. Most of you know that steel is hardened by heating to red heat, then quenching in oil or water. Do this to low carbon steel and nothing much happens. It will remain rather soft. This is because steel must have a certain amount of carbon in it before it will harden. Very roughly speaking, the higher the carbon, the higher the as quenched hardness and wear resistance, but the lower in toughness.
Some examples. Commonly available cold drawn steel is generally AISI 1018. The '10' means plain carbon steel, and the '18' means a nominal 0.18%by weight carbon content. Heat this steel red hot, quench in cold water and it will harden a little, but not enough to be useful.
Normally the only heat treatment done to 1018 would be to case carburize it. Common machine shafting, axles, and some modern shotgun barrels are AISI 1035 (nominally 0.35% carbon). This steel responds to heat treatment. If small pieces are water quenched, and then tempered peacock blue, they will attain a spring hardness (but, no, this does not mean that 1035 is useful spring steel). AISI 1060 (0.55 to 0.65% carbon) is used for sledge hammers and cold chisels, its 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 (that is, resistance to being dented) only slightly> But wear resistance does continue to improve with sill higher carbon, above 0.6%. The excess carbon can form many fine, hard iron carbide particles which help retain sharp edges and resist abrasion. Common wood saws and older power lawnmower blades are AISI 1070 to 1080 (0.7 to 0.8% carbon). Axes are 1086 and str drills are 1095.
1095, with carbon ranging from 0.90 to 1.03%, is the highest carbon plain carbon steel you can find in steel warehouses. Since the beginning of this decade Nicholson/Black Diamond files have been 1095. 1095 steel makes good general purpose knives, daggers, firesteels and wood working tools.
Back to files. Obviously, file teeth must be extremely hard and wear resistant to stay sharp while cutting metal. For centuries, files have been made of either soft iron, which is then case hardened, or 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 Narural Magik', 1589, G.B. Della Porta describes 'ways whereby Iron may be made extra 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 on the place, or air. The powder being prepared, make your Iron like to a file; then cut it chequer wise, and crosswayes, 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 aolocer: then put on the cover, and lute will the chinks with clay and straw, that the smoak of the powder may not breath out; and lay a heap of burning coals all over it, that if 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 pincers, and plunge the files into very cold water, and so they will become extra hard. This is the usual temper for files; for we fear not if the files should be wrested by cold waters.'
This four hundred year old process is metallurgically entirely correct. The chimner soot, of course, is a nearly pure source of carbon the carburize or case harden the surface. 'Ox hoofs', along with carbon, provide nitrogen which further improves sliding wear resistance. Salt and glass melt the whole mess together to provide intimate contact with the iron. They also probably catalyse or enable the high temperature chemical reactions which get the carbon into the iron. And, even today the 'usual temper for files' is just as they come from the quench, with no further reheat(or draw).
Until recent years when the EPA became overly concerned, modern practice was to coat high carbon steel files with a roughly similar mixture before hardening from a protective lead bath [heated quite red, to provide the heat--lead is a commonly used heat treat heating bath, as is molten salt--Mike]. 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, as it is necessary to case harden the 1035 steel from which they are made.
While the currently used 1095 processes into a very good file, I suggest you get ahold of 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 van be found anywhere else. Other than your memory there a couple of ways of telling which are the older files. One is that files marked only 'Black Diamond' were made in 1975 or earlier, and are difinitely 10130 (1.3% carbon) steel.
After that year the manufacturer stamped 'Black Diamond' on one side and 'Nicholson' on the other. The other idstinction is the tang. It used tobe that to same metal the tangs were hot'rolled from the file bland with little waste. [these tangs generally taper BOTH in length AND thickness--Mike] By 1978 it was more economical to use a longer blank and shear off metal to form the tang. [these tangs taper only in the length--Mike]. A sheared tang will appear obviously different than a hot rolled tang.
The ultra high carbon of these 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 threat than, say, an AISI 1070 spring steel. If you overheat and coarsen the grain of your file steel it will not be capable of taking the finest edge.
That old Nicholson file in your scrap bod was heated for about five minutes in a molten lead bath at 1440 Degrees F., quenched in brine to get maximum hardness, and then NOT tempered at all. Let's do the simplest job first, and make a straight wood chisel
or wood turning tool out of that old file. Most commercially available wood chisels are not really meant either to take or hold a fine edge. The best on the maker these days are, sad to say, Japanese, such as Oiichi or Iyoroi. Well, you can match these chisels. Just grind your file to the shape you want, usually with a 25 to 30 degree bevel. Use plenty of water so not to soften it, or, worse yet, crack it. The best bet is to first scrub off all old oil and grease. This is necessary to preserve domestic tranquility as you are about to temper the thing at 350 degrees in Wife's oven, for a good hour. This will relieve a lot of internal stress like taking one small glass of wine) and prevent 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 degrees F, which is very light straw.
After tempering for an hour the straw may deepen somewhat. Do not trust the temperature settings on kitchen ovens, try a scrap piece first and go by the temper color, or by a Tempilstik. You now have a wood chisel which will hold as good or better edge than anything on the market today.
This choice of seel and temper will not make a good bowie knife, although it would be great for a skinning knife. That 350 degree F temper leaves the metal rather too brittle to handle the rough use of our average camp knife. A skinning knife must hold a good 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 and firesteels I'd first suggest using a post-1980 file of 1095 steel. This will still hold a good edge but be a little tougher than the older 10130 steel. Temper a bit before grinding to shape. For most knives I suggest tempering at 500 degrees F. That is a mottled brown to purple temper color. Again, I'd use the household oven for about an hour. And don't trust the temperature setting on that dial!
We have about come to the point where we've gotta get serious about temperature measurement. Pyrometers, thermocouples and electronically controlled electric heat treat furnaces ust aren't in your budget, are they? Do forget about judging temperature, at red heat, by eye. Even professional heat treaters can be off by 200 degrees F when checked against the cold, unfriendly pyrometers. Well, there is hope for the low budget shop. That is, an inexpensive and very accurate metod of temperature measurement, called a Tempilstik. This thing is a calibrated crayon. Let's say you sprung $6 or so for a 500 degree f temperature indicating crayon. First, mark your workpiece with this crayon. Heat the etal. The crayon mark will change color, that means nothing. But, as soon as the metal reaches 500 degrees F, that dry opaque mark will change to a distinct melted mark. Really, you should also mark it with a 525 degree F Tempilstif so you will know if it got too hot.
These things are easy to use and they are accurate. The 500 degree F Tempilstik is accurate to +/- 5 degrees F. Above 700 degrees F or so, the procedure is to stroke the hot metal with the Tempilstik now and again during heating. When the metal reaches temperature, the crayon will leave a liquid smear [the 'feel' of the stroke changes too. This lets you work kinda blind, inside your fire--Mike]. The cost of Tempilstiks is currently $5.90 in quantities from 1-9. They are carried by welding supply distributors, or may be had direct from: Tempil Division, Big Three Industries, Inc., 2901 Hamilton Blvd., south Plainfield, N.J. 07080 (201-757-8300. Minimum order is three Tempilstiks, and include some postage. .
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. Forging high carbon steel is a very different matter than working mild steel. Yes, you blacksmith types know it is easier to burn high carbon steel. But even if you don't burn it, forging a 1.3% Carbon file from a white heat and slow cooling it can coarsen the grain and make it rather brittle at any temper. One problem is all that carbon. When steel contains more than 0.8% carbon, it is easy to get a brittle iron carbide network. Huh?? Well, the excess carbon can form a brittle carbide layer around each grain. Back up further! all metals, including steel, consist of millions of tiny crystals, all stuck together. Imagine a pile of graped that have been pressed together without breaking the skins. each grape roughly the same shape as a metal crystal or 'grain'. If you overheat and slow cool a file, the 'skins' of each of the little 'grape' will be hard and glass brittle. That translates into a cutting edge that easily becomes radded or crumbles.
I suggest forging that file, especially if it is the old 10130 steel, as if it was a true Damascus (Wootz) steel.
That is, heat to a much lower temperature than usual. This means fore muscle but it will also keep the metal fine grained, which is most important. damascus, which could be as high as 2% carbon, is forged starting at 1550-1600 degrees F and finished at a blood red, about 1200 degrees F.
There is really no way you can guage a 1660 degree F starting temperature by eye. If you are serious about fine work and don't like throwing hours of your time and material into the scrap bin, invest in a couple of Tempilstiks, rated at say, 1550 and 1600 degrees F. 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 is to heat the blade 1300-1400 degrees F for half an hour or so, and bury it in ashes or lime to cool slowly.
[Omitting this pre-hardening anneal is about the single greatest cause of amateur smith's failures in hardening steel--mike] Most steels are annealed from a higher temperature. I am suggesting this lower anneal to keep the fine grain structure and fine carbide distribution you got by forging that old 1.3% carbon file down in the cherry to blood red range. [sub-critical anneal--breaks up any fine carbide network--Mike]
Now that you have forged and annealed the blade, grind it all over to near final shape. you MUST grind off the scaled, decruburized surface or it will crack in heat treatment. When you heat in the forge, some of their on oxidizes to a blue-gray scale. that is obvious. But, some of the carbon literally burns out of the steel's surface. This is just the opposite of case hardening. If you leave that thin, soft, low carbon skin on the blade, paradoxically enougth it WILL crack when quenched in water or brine.
To harden that file steel, heat to 14401450 degrees F for about 4-5 minutes and brine quench. Again, invest in a 1425 and a 1450 Tempilstik. Do NOT overheat it!
The quench 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. However, today there is a better way, particularly for apartment dwellers and other Urban Folk. That is salt brine. Mix just 13 ounces of common salt in a gallon of fresh water. That makes a 9% brine solution. This brine will quench twice as good as water. [extract heat from the steel at twice the rate--Mike]. It reduces the chances of cracking and warping and makes a harder part. Brine is, of course, less fragrant than the aforementioned animal product. Keep your brine cool or at no more that room temperature.
Don't quench file steel in oil. That won't harden it well at all. How you dunk the knife in the brine quench is important. If you belly-flop it in, it is guaranteed to warp, and it may crack. Plunge the hot knife straight into the brine, and it will come out reasonably straight. It will be more straight if you quench in brine than if you quench in water. this is because brine quenches the steel more uniformly all over. Move the knife in a figure 8 motion while it sizzles in the brine. This gives a more uniform quench. Remove it while it is still warm and temper as soon as possible.
Tempering quickly is important, lest the metal decide to crack while resting quietly on your workbench. [that tell tale 'click' in a quiet shop is VERY disheartening--Mike]. The word 'temper' these days means to reheat a quenched part to make it softer and tougher. for knives of file steel I temper at least at 450 degrees F, preferable 500 degrees F.
If you forged a firesteel out of an AISI 1095 steel file, I would temper 500 to 600 degrees F. You might temper first at 500 degrees F and try striking spark. If it seems too hard, retemper at 600 degrees F (the blue beyond purple).
If the knife blade is crooked, temper it first before you try to straighten it!! [Cool to room temperature after all tempering or annealing operations-mike]. Then heat it to 300 degrees F again (straw to brown) and straighten while hot. DO NOT heat the steel a beautiful blue and try to straighten at this temperature. Steel has a peculiar brittleness while it is in the blue range (550-600 degrees F). [This range is known as the blue-brittle range for a reason, and is avoided like the plague in industry--there is also a black heat, 885 degrees F known as 'black shearing heat' where a two inch bar of steel can be cut cleanly with a 2# hammer and a cold chisel--Mike]. This toughness, by the way, is important in a farriers rasp.
When shoeing a horse I understand it is occasionally necessary to get the beast's attention. This is accompolished by rapping them on the hoof.
High carbon farrier's rasps tend to break during this endeavor. Hence, they are made of tough 1035, lightly cased. [anyone comfirm this? --Mike] you can forge the rasp steel from high temperatures without embrittling itl when you're finished, take the forging and normalize ti by heating to 1575-1650 degrees F and air cool. This refines the grain which may have coarsened in the forge. It is very important that the steel be fine grained. Fine grain steel in remarkably tougher than coarse grain. All steel forgings should be annealed or normalized to the toughness.
To harden that rasp steel, heat 1525-1600 degrees F and quench in cold brine. For a tough throwing knife, temper about 700 degrees F. This discussion holds for American made Nicholson or Black Diamond rasps. If you are wont to purchase asian products I might suggest you contact an Asian metallurgist regarding how best to treat them.
I would like to thank Jim for this article, and hope your patience on this thread was not exhausted by the length. Jim Kelly works for Rolled Alloys, in Michigan, and is an excellent metallurgist, blackpowder shooter, and amateur smith. I don't think he will use a skinning knife of me, and this is an excellent article.