Legendary Damascus steel is good for not only dramatic-looking knives, but also for razor-sharp cutting edges that self sharpen during use.
Beauty And Brawn
by Ray Rantanen
Iron Anvil Forge Co.
Damascus steel is beautiful and tough. If forged from the right materials it is an excellent cutting tool. There are two types Damascus. The first, from which the name originated, is now referred to as wootz or oriental Damascus. The other is called pattern welded. The art of making wootz Damascus was lost over 200 years ago and has only recently been replicated. Pattern-welded blades were also a lost art until U.S. smiths began experimenting in the early 70s with more modern grades of steel.
Pattern-welded Damascus has a distinctive swirl pattern of light-colored regions on a nearly black background. The look comes from alternating sheets of high and low-carbon steels which are repeat-edly drawn, folded, and forge welded together. Forge welding is a blacksmithing operation used to join steel or iron. Two or more clean metal pieces brought nearly to their melting point fuse together when hit with blows from a hammer.
Many types of steel are used in pattern-welded Damascus blades. But most blades today combine hard steels such as 1095 and O-1 tool steel with a soft or mild steel. Layers of hard and soft metals give the blades good edge-holding properties from the hard component and shock resistance from the softer metal.
One problem in forging Damascus, however, is the extremely high temperatures needed to hammer-weld the layers together. At temperatures on the order of 1,700 to 2,000°F, carbon travels easily through the metal's atomic lattice, moving rapidly from the high-carbon steel into the softer metal. The effect is known as carbon migration. If the carbon is allowed to migrate between the different layers, the steels eventually get a homogenized carbon content. This is not desirable because it eliminates the individual properties of each steel when hardened and tempered.
Some knife makers use nickel sheets or a high-nickel steel as one of the layers to thwart carbon migration. Laminations such as these also make a beautifully patterned metal but may not produce the best cutting tools because nickel is not a good knife steel. A nonconventional metal used by the author to minimize carbon migration in his Damascus blades is a high-silicon, high-carbon shock steel. The S-2 steel recycled from spent jackhammer bits contains about 1.5% silicon and nearly 1.5% carbon. Silicon occupies the same lattice sites as the carbon in the metals crystal structure and inhibits carbon migration. With less carbon migration, hardness, wear resistance and other properties of each steel lamination are maintained. Blades from O-1 tool, S-2, and mild steel are tough, sharpen easily, hold an edge, and if bent can be straightened cold. Aesthetically, after etching with muratic acid, the different steels contrast well to each other which gives each blade its own unique dama-scene or layered pattern. This particular combination of metals also makes thin blades possible. Laminated like plywood, the S-2 Damascus blades are strong and see virtually no crack propagation through the various layers. A blade bent to 90° and hammered flat on an anvil, cold, showed no signs of stress or breakage when tested. When bent to 180°, the outer layers cracked, but the blade still didn't break.
Another effect of high temperature is large grain growth within the steel microstructure. At elevated temperatures vacancies in the atomic lattice migrate, as do atoms sitting interstitially. This atomistic movement develops large crystal imperfections called dislocations and is also responsible for increasing the size of the grains.
Dislocations disrupt the crystalline order of the metal. As the metal is worked under the hammer the dislocations are broken down to finer grains. Dislocations and large grains are not desirable on a knife's cutting edge because they are coarse and do not sharpen readily or hold a good edge. To reduce grain size, a nearly finished blade is hammer packed as the blade cools from a visible red to a black heat. During hammer packing, the smith continues to hit the blade, but each successive blow has less power.
A SHARPER EDGE
Another important attribute for Damascus is to have numerous layers cross the blade's cutting edge. This comes via surface manipulation before final shaping of the Damascus billet. The edge is hammer finished which brings a large number of layers near the cutting edge and compacts the steel into a thin cross section. For example, the Damascus made with S-2 steel laminations has 224 layers. This is realized by taking seven different steel layers and folding the lamination five times during the forging process. Hammer finishing the edge to less than one-sixteenth of an inch results in the removal of only a small amount of material during final finishing and makes each of the 224 layers less than 0.0003-in. thick.
A ladder pattern is often used to maximize the frequency with which layers cross the cutting edge. Grooves ground into each side of the lamination zigzag the steel back and forth across the cutting edge during final blade shaping. Another method which increases the number of layers crossing the cutting edge employs twisting the steel, but the cutting edge generally is not as good as the ladder pattern but is the second best blade type.
The goal with any knife blade is to get it as hard as possible so it will remain sharp. For single-steel knives, the hardest state for the blade is too hard.
The blades are extremely brittle and must be tempered to remove some of the hardness. With Damascus the cutting edge is left in a fairly hard state. The brittle components, especially O-1, are protected by thin layers of mild steel and the S-2 steel layers. After quenching in oil from a high heat, where a magnet does not attract the material, the three different steels exhibit three distinct hardness values.
The mild steel tests out at a Rockwell C hardness of 45. This is about the same hardness as metal stampings. The mild steel picks up some carbon from the O-1 and S-2 layers next to it. This increases the toughness of the blade. The S-2 comes out near a hardness of 59 when oil quenched. The O-1 steel with nearly 1% carbon tests out at a hardness of 62 to 64 after quenching.
These three hardness values at the cutting edge give the Damascus blade a unique cutting capability. The mild steel wears away faster than either the O-1 or S-2. The result is a miniature saw at the cutting edge which self sharpens for a time as the mild steel wears away.
Eventually, the small teeth become misaligned. But a gentle stroke against a fine ceramic rod squares them up to restore a razor sharp edge. No attempt should be made, however, to remove any metal during this light honing process. Only after extended use should the blade be honed down and reset.
A LITTLE HISTORY
Damascus was originally made in Damascus, Syria, as early as the 12th century. In the 14th century the Tartan conqueror, Timur Leng, raided Damascus and carried off all the great sword makers. The steel used by the Syrians to create Damascus came from India and is called wootz a relatively high purity iron with about 1.5% carbon.
Wootz steel was formed by heating the components in a closed crucible and allowing the resulting steel ingot, or billet, to solidify slowly. Blades from wootz steel had a streaked appearance. In the 17th century, English smiths tried to capture the appearance of wootz by layering steel and repeatedly folding it. This is referred to as pattern-welded or layered steel and is the type found in many early Japanese swords.
In 1973, American smith Bill Moran introduced the first modern pattern-welded, layered Damascus steel. Since then a multitude of smiths have developed their own Damascus steel. Wootz steel has been made by several American smiths over the last 10 years. It is a valuable material because of its forming capability for gears and other components and several patents have been applied for.
LAYERED BILLET The first steps in making Damascus include forging the S-2 steel jack hammer bit, to size 1-in. wide 3 3 /8-in. thick, grinding scale off all three steels, and cutting them to the appropriate dimensions. Four-inch long pieces of each metal are stacked with the O-1 in the middle, flanked on either side by a mild steel/(S-2)/mild steel combination. This gives a total of seven layers.
Next, a handle is arc welded onto one end of the stack while the other end is welded shut with a small bead. The stacked billet's outside surfaces must be cleaned with a belt grinder before being put into the gas forge. The billet is then brought to a red heat in the forge at which time an anhydrous borax flux is applied.
The billet is brought to weld heat, then hand hammered to ensure the layers stick together. It is reheated in the forge and hammered with a trip hammer to approximately three times the original 4-in. length and a little wider to about 1.5 to 2 in.
FORGE WELDING The smith quickly makes a small cut in the billet on the anvil with a handle-mounted hot cutter. Then he clamps the end near the handle in a vise and bends the billet at the cut. Again the scale must be cleaned from all surfaces. The flux is applied and the fold is hammered shut. Additional flux is added to the outside surfaces and the billet is returned to the forge. At this stage, the metal should still be a high orange color which means that this sequence of steps must take place in quick succession.
The smith reheats and repeats above steps five times. This will result in a 224 layer Damascus. After final fold, he cuts the billet in half lengthwise with a chop saw.
The Damascus billet is rough shaped to nearly half the width and length of the intended knife and about 3 /8 to 1 /2-in. thick.
PATTERN DESIGN The smith grinds grooves in the sides for a ladder pattern and shallow holes on the flat surfaces, then reheats the blade in the forge until the steel glows orange. He'll flux the metal and heat to forge-weld temperature and rough the blade to shape with hammer blows, then cool the metal and use a pattern to trace the knife shape, at which point, he grinds the blank to the outlined shape.
The shaped blade returns to the forge for completion of tapering and hammer packing. The handle section is reshaped and ground flat on a belt grinder. The smith then grinds the blade to almost finished taper and smoothness.
After cooling, the cutting edge is brought to temperature for oil quenching and quenched immediately. After an emery paper clean up, the blade edge opposite the cutting edge is tempered, as is the tang area where the blade handle will eventually attach. Next is finish grinding with sanding belts of several grits. Polishing follows, first lengthwise with a 3M 1-in.-wide deburring wheel, then repeating for the crosswise directions. A final polish uses 3M scotch brite belts.
ETCH AND POLISH After cleaning, the blade goes in preheated muratic acid, and stays there until it takes on the desired etched appearance, usually in 1 /2 to 2 hr.
The blade gets brushed with brass black after removal and rinsing. Another rinse, dry, and sanding cycle brings out the pattern.
At this stage, the blade is ready for attachment of its guard and handle.
Ray Rantanen holds a Ph.D. in physics and is a consultant for the aerospace industry. He has been forging Damascus and other blades since 1974 from recycled steels which include the S-2 steel jackhammer bits, 5160 steel leaf and coil springs, high-carbon railroad spikes, and horse hoof rasps. Rayrant@aol.com