BARR KNIVES

ESSAYS ON HEAT TREATING AND FORGING

• Heat Treating
• Machinery's Handbook
• The Forge

Steel has the property of malleability when hot (forgeability) and also the potential for different room-temperature-strength characteristics by the controlled application and removal of heat (heat-treating). Its versatility is what makes steel so important and its efficient manufacture was a driving force in the industrial revolution.

It has been said and that the heat treatment is the soul of the blade and without proper heat treatment you at best have no more than a pretty piece of metal. Nothing wrong with pretty pieces of metal, but sometimes people use knives to cut with, and it is in the cutting that the heat treatment shows. Knifemakers use this philosophy to justify both doing their own heat treating and sending their work out to a professional heat treating shop. I am very firmly in the former camp, because well, I'm not sure, but when it comes down to paying someone else to do something versus buying myself a new tool I am pretty predictable. That is I have a lot of tools. I am also an information junkie and must know how to use my tools so as to avoid embarrassment when dealing with someone who does.

I group forging in with heat treating because what I know about forging is mostly academic, and heat treating once you are armed with a good temperature control becomes a mostly academic procedure. I have broken perhaps two dozen sweats doing actual forging and have heat treated more than 100 knives, and still have the very first. Since I didn't learn forging first, I still see forging as a small part of the process of making knives. I have beaten blades by hand from 5160 spring steel, 52100 bearing steel, and 1084 spring steel. The 5160 came from a new coil spring for an RV and the 52100 from a large ring shaped bearing race that was cut in half, one half given to each of two students in a weekend forging seminar. The 1084 is one of the layinaround steels in my shop.

One of the biggest things for me was the physical difficulty of rough shaping the steel. My technique was poor and I am sure I wasted a lot of energy, but that 52100 didn't get very soft even at 1500 degrees. The hammer seemed to make only the barest impression in the steel. My classmate, a Neurosurgeon (Yes a Neurosurgeon) from Hilton Head, SC, and I took turns holding each other's work while the other used the ten pound sledge hammer on it. I have it on Video and it's not pretty. My Dr. buddy did fine, and was not about to smash any of his fingers, which led to a conservative hammer usage and good overall technique. I was trying too hard to swing the hammer instead of just lifting and letting it drop. We wailed on our respective chunks of bearing for about six hours on the second day and I ended up with a chunk of steel that is only vaguely knife-like and would require the removal of at least half of its mass in order to become anything resembling a knife. I still have it and use it as a preheater for my quenching oil. I heat it up in the furnace and drop it in the oil and it heats the oil up to a better temperature for quenching.

I still have the other blades that I forged that weekend, but have never made them into knives. I just haven't been inspired to work around their inherent crudeness.

Steel is a wonderful thing; heat it up and it becomes pliable and bendy, not quite smooshy, though. It requires a very satisfying amount of effort to shape steel. Some metals are harder to forge than others and the characteristic is called hot-hardness. I bought some stainless sheet to make some brackets for motorcycle parts and found that it formed better corners when cold than hot, and Titanium 6Al4V, when used for similar purposes is best formed hot. Titanium is very flexible but it just breaks at the end of its flexibility instead of taking a set. But lots of metals are great for hot forming into knives or anything else. I've heard it said by a blacksmith that with a forge you can make anything in a shop, and I understand on one hand and am skeptical on the other. Many of the things in my shop consist of fairly large castings with a number of aligned flat surfaces for reference to other parts. Achieving flat surfaces requires the aid of a machinist, which is often considered a separate vocation from blacksmithing. In modern times it is, but historically blacksmiths have been inventors or by necessity the implementers of invention.

The historical and romantic implications aside, forging is fun. Hearing the ring of the anvil and seeing the steel dent and change shape under the hammer. The action of the hammer on the steel is a complex one since the steel doesn't actually compress under the hammer. It spreads away from the hammer blow in a complex pattern as determined by the shape and force of the hammer penetration as well as characteristics of the steel at that temperature.

A flat bar of steel will bend upward into a U-shape if you just hit in the same spot without turning it over, and forging on sheet metal must be done meticulously because every blow has a side effect that needs to be counteracted by the next.

I begin forging by heating up the knife blank and hitting it with the hammer along what will eventually be its edge. This begins forming the blade bevels, but the effect that I am really looking for is the steel bending slightly away from the hammer blow. Even blows all along the edge gave it the nice gentle curve that I wanted for it. A few strikes (OK maybe 50, but they were gentle ones) in the blade-edge area of my project knife blank, and it developed a nice, gentle curve. I dropped it once while it was hot, and not only did it bend because of its pliability but it also branded its shadow into one of the pink carpet scraps on my shop floor.

HEAT TREATING

This knife is painted with a coating to inhibit scaling, or rapid oxidization. Ten seconds at 1400 degrees will do about the same as six months in the weeds. In the upper left of the photo is the infamous 52100 bearing knife that already has six hours of work in it and still looks like that.

This knife underwent other thermal treatments as well. After forging the curve, I softened it by heating it to a high forging temperature and cooling it slowly over a period of about 8 hours. Getting it up to temperature is called normalizing, and it is the temperature at which all the internal stressed generated by forging or machining relieve themselves. The slow cooling process is called annealing and has the opposite effect of quenching and makes the metal as soft as it can be.

The knife came out of the oven dead soft and a little bent, but the softness made getting rid of the bend easy. Then I ground the blade close enough for heat-treating and heat-treated the blade. Unfortunately while I was quenching it, my grip on the blade with the tongs slipped and the temper line ended up not quite where I wanted it to be. Soooo, normalize, anneal, grind, paint, and harden again. This time the temper line looked good. There was a crack on the edge of the blade, but even after grinding away the crack the temper line was still positioned OK, so it was good for further treatment. The last step in practical knife heat treatment is called tempering and it consists of heating the steel to 450 degrees and holding at that temperature for an hour. This softens the hardened edge of the knife just a bit to an optimum balance between edge holding and ease of sharpening.

MACHINERY'S HANDBOOK

One of the things that go along with the educational process is a willingness to purchase $75 books. The Machinery's Handbook is such a costly little book and contains lots of useful information for knifemakers and metalworking hobbyists. My Grandfather has a copy of the eighteenth edition of this book dated 1968 and he probably got it right before he came to Washington to open his own machine shop. It is 2293 pages of pure manufacturing knowledge and I just love it. I use it for breakfast reading sometimes. But it is the nuts and bolts knowledge of how to make gears, threads, springs, any mechanical contrivance whatsoever. I bought my own copy of the twenty-fifth edition, last year, and the differences between the two are striking.

The Ready Reference Index on the inside front cover of Grandpa's is a quick guide to specific topics in the book that might be of frequent use. Subjects include gears and gear ratios, cylindrical fits, gearing, helical an herringbone, ball and roller bearing types and sizes, machine elements such as springs, belts and chains, materials including all the commonly worked metals of the time, and also drills, broaches, reamers, taps, machine tapers and grinding wheels. Bear in mind that this index pointed not to the suggested uses of or theoretical applications for these items, but to how to make them. For someone like me who is working with some machinery that is older than I am, performing many of those same processes, the Ready Reference Index is just perfect.

My edition has a table of conversion factors: metric to English, for area and volume. I've not had to make many metric parts in my life, so this inside front cover is next to useless to me. Besides, I know that one inch equals 25.4 millimeters, and that's all I need to make those types of conversions at the rate I need to in my shop (which is never). The only time I ever need to convert cubic inches to cubic centimeters is when I'm trying to compare the displacement of an oinking Harley Davidson motorcycle in comparison to my svelte Moto Guzzi motorcycle.

Some of the of things I have needed to look for in the Machinery's Handbook are cylindrical fits, heat treating, strength characteristics and other properties of various metals, threading information and whatever else looks interesting. Sometimes I just use it for recreational reading. I found all of the subjects I needed by looking in the regular index in the back of the book, but that Ready Reference Index from Grandpas edition would be nice. I also found a section in Grandpa's book on coloring metals, commonly called patinization, which has methods for giving different surface colors to brass and steel, a decorative process deemed unnecessary in the more theoretically oriented later edition. Also included in the earlier edition is a section on forging complete with a design for a set of tongs, the beginning blacksmith's first project. I haven't yet made my own tongs, as I inherited a large clumpy set of tongs from my father-in-law. The good thing about them being oversized was that I could heat them up and beat them into the shape most useful for me at the time.

The section on mechanics is also more interesting in the 1968 edition with its extensive discussion of flywheel design and usage. None of these subjects is covered as extensively in the later edition, and are much more pertinent to the hobbyist/metal artist like myself. In fact the later edition has no information at all on forging, as if the hot forming of metal was no longer useful.

Steel is the generic term for a large family of iron-carbon alloys, and steel with enough carbon in it can be hardened. Steel suitable for knifemaking is usually in the .5 to just over 1 percent carbon range. The carbon in the steel is what gives it hardenability. In fact, the maximum hardness attainable by a steel type is determined by its carbon content and is moderated by other elements in the mix.

A big question that I can only partially answer is "What happens to steel when it is hardened?" One of the purposes of heat-treating is to manipulate the distribution of carbon and other alloys in the metal. The carbon in steel exists in tiny pockets of compounds throughout the metal, but when the steel is heated to its transformation range the carbon and other alloying elements dissolve themselves evenly throughout the mix. This doesn't happen instantly when the temperature is reached, but must be allowed some time. With knives, the cross-sectional thickness of the workpiece is thin enough that soak times are at a maximum of 60 minutes for the most heavily alloyed steels. Once all the alloys are in solution, and the carbon is fully dissolved throughout, the state of the steel is called austenite. At this point the steel is cooled as quickly as possible from the austenitic temperature to keep the carbon in even solution throughout. If cooled at the correct rate for the steel, the carbon forms the finest possible crystalline structure in the steel. The crystalline structure is common to steel, but it is the relative size of the grains that is related to its hardness and strength. Cooled at the proper rate, the grains are as small as possible. When cooled rapidly from the austenitic phase, the structure formed is called martensite. Martensite has the optimum potential hardness for a steel, and will represent the hardest the steel will get. The transformation from austenite to martensite is not instantaneous or complete once the steel cools to room temperature.

Some of the retained austenite will transform into martensite over a period of months after the original quench. This aging process usually results in some change in size over the time period as well. Using a subzero-cooling period can speed up the process. The subzero phase is usually accomplished by immersing the part in liquid nitrogen overnight. In the past few years the subzero treatment has become very popular with knifemakers, particularly the ones that do their own heat-treating and blade testing. This makes sense, since they can test the blade for optimum performance right after the liquid nitrogen phase, and see if what they are doing works in terms of edge holding and sharpening ease. I will be getting some liquid nitrogen, as it is a necessary step in heat treating some stainless steels.

Once the steel is quenched and at its maximum possible hardness, it needs to be drawn back, or slightly softened to be at the right hardness for a particular blade. This process is called tempering and is accomplished by holding the blade at some temperature between 350-600 degrees for at least an hour. The tempering temperature is determined by the steel and the desired hardness or temper of the blade.

Some of my earliest blades never got tempered, leaving them in as-quenched condition. I still have one of them and it is just as hard as glass. It positively rings when I scrape my finger across its edge. I occasionally use it for leather cutting because it is unbelievably sharp and thin. I could never let this knife go, because it is so brittle it would likely break if dropped. Plus, it's hideous.

When making knives, I have to avoid heating the blade at any time outside the heat treating process. If I grind too aggressively after heat treating, I can destroy the temper of the knife by localized overheating. What really happens is that the steel gets softer when heated beyond its tempering temperature, because tempering is a softening process and the higher the temperature applied, the softer the steel becomes as a result.

THE FORGE

Here are my current tools for heat treating and forging. The propane-fired forge at left is of my own manufacture and lives on its own little cart. The little yellow box is a temperature gauge that I use with the forge and to cross check against the temperature control on the Paragon knifemakers furnace at right. It's hard to see in the picture but that furnace is 36 inches deep. Long enough to do a full-length sword, although I haven't made one that long yet. For that I'll need a 36-inch long quench tank.

For now, I am content with using the large turkey-sized quench tank shown here. The temperature shown in the furnace is 1370 degrees, down a bit since the door is open from the 1450 degrees used for quenching this knife steel.

Here is an example of quenching. I had to do this twice for that knife, because my grip slipped slightly the first time around. The knife is only partly immersed in the oil. That's what is called a differential quench. Notice also the Giant Silver Sucking Snake (GSSS), which is sucking away all the burning oil smoke that if left untended would undoubtedly lead to grievous marital discord.

The GSSS is my own application of a large exhaust system for my shop. It's really like a huge vacuum cleaner, able to suck up a tee-shirt in the blink of an eye (don't ask). It just has the one hose, which I put wherever Big Suction is needed (like removing the smoke from oil quenching or profile grinding hardwoods or other plumes of toxic smoke/dust).