Damascus steels from powder metal
Damascene steel is an advanced form of blacksmithing recognizable by the decorative pattern on the metal surface. The look comes from stratified layers of steel with varying compositions that have been etched to reveal distinctive swirl patterns of light-colored regions on nearly black backgrounds. The damascene art went through three golden ages before it became a lost art that has only recently been rediscovered. And if powder metal specialists at Damasteel AB, Söderfors, Sweden, have any sway, this legendary art is on the cusp of its fourth golden era.
The golden ages for damascene (Damascus) steel have been the result of metallurgical development, explains Damasteel's Per Billgren. "Throughout the history of iron and steel production, smiths sought to increase the purity of the finished product. Nonmetallic slag inclusions create weak spots from which cracks initiate. The larger and more numerous the inclusions, the lower the fracture strength of the metal."
Early Damascene Steels
The art of pattern (damascene) forge welding likely developed naturally from primitive iron manufacturing, explains Billgren. "Ancient Hittite smiths around 1500 B.C. were some of the first to reduce iron from bog iron and red earth (iron ochre). Using primitive blast pits they produced iron with high slag content." When heated to forging temperatures some of the slag would become molten. The smiths hammered (forged) and kneaded (folded or tilted) the porous iron fragments to remove liquid slag impurities from the surface. The remaining slag deformed or shattered when forged out lengthwise as the metal was compacted with blows from the hammer. Tilting crushed the hard solid slag decreasing inclusion size. "This aggressive handling of the metal also was a way to physically even out hardness irregularities due to compositional variations in carbon and phosphorous content," he continues.
Modern rupture mechanics gives a mathematical equation for hard steels:
where = fracture strength, d = size (thickness) of the rupture initiating slag inclusion, and K = constant depending on the type of steel and heat treatment.
"Cutting the size of the slag inclusion by 75%," says Billgren, "doubles fracture strength." Forging also stretches the slag lengthwise decreasing its surface area and boosting its tensile strength in the longitudinal direction. "The smiths used this to their advantage, and were careful to turn the metal in the right direction when forging."
As the smiths worked the metal to improve properties they also began manipulating the striations that developed because of different iron compositions. "Higher phosphorous-containing materials resulted in layered regions that kept their brilliant luster over time, while their phosphor-free neighbors turned rust red," continues Billgren. Smiths learned to vary the rust attach and color by etching the metal with acids to create more striking swirl patterns on the metal surface. "They also experimented with deliberately tailoring component (bog iron and iron ochre) percentages then twisting, stamping, and in other ways working the metal to produce artistic damascene results. Early sword blades were usually built from several bars creating a tough core, decorative sides, and harder edges," says Billgren.
Around 800 A.D. crystallization (Wootz or Oriental) Damascus developed in the Middle East and India. Smiths employing this early form of liquid metallurgy combined iron and carbon powders in closed crucibles that were heated and slowly cooled inside blast-fired furnaces."Key to this technique is that the iron ore selected contained 1 to 1.5% carbon and had substantially lower melt temperatures than pure iron. As the molten metal slowly cooled in the crucible, the 'Wootz' billets were interlaced with a course network (striation) of carbide crystals that were visible to the naked eye," says Billgren.
The smiths worked the metal via secondary-pattern-forging operations of torsion-twisting, pattern coining, and pattern-cutting to skillfully create a host of decorative swirl patterns. "The Wootz blades exhibited exceptional strength, toughness, and edge-holding properties. And their distinctive look was in effect an ancient quality certificate that set them apart from other metals of the day," says Billgren. The dramatic look coupled with the hardenability and elastic springiness of Wootz-sword blades made them highly prized by emperors and revered men and feared by those who stood against them in battle.
The art of damascening spread throughout Europe from Romanian workshops in the Rhine area with some of the most highly esteemed swords coming from Scandinavian smiths. Damascene steels were also common during the Roman Iron Age and Merovingian and Viking eras (500 to 1000 A.D.).
Larger furnaces, water-powered blast bellows, and refractory ceramics made the development of blast furnaces possible around 1100 A.D., continues Billgren. "Liquid pig iron at 2,400°F could be poured from the furnaces. Impurities floated to the surface and were more easily removed. The improved cleanliness substantially improved material properties." To transform the pig iron to forgeable steel,carbon content was burnt away in the hearth furnaces. Lower carbon content increased the melting point and refining was done in a half liquid state. This period is sometimes referred to as the half liquid metallurgy era and was the golden age for pattern-welded damascene steels.
Pattern-welded Damascus steels were made by forge welding two steel grades into a sandwich. The number of layers multiplied as the smiths repeatedly tilted and forge-welded the metals (at temperatures near their melting point) with blows from a hammer. The laminations underwent similar pattern-forging operations of torsion twisting, pattern coining, and pattern cutting. The resulting dramatic damascene patterns were proof that the metal was heavily hotworked and would possess incredible strength and toughness to meet the demands of the most challenging applications including swords, broad axes, and most notably hunting firearms.
"Firearms were developed in the 1500s but over time, weapon smiths had problems making gun barrels capable of withstanding increasing gunpowder loads: The original longitudinal forgeweld down the barrel length was a weak link," says Billgren. During the 1700s damascene barrels developed in Turkey were forge welded as a spiral. The spiral weld increased the strength in the transverse direction. The Damascus was made of soft iron and hardenable carburized steel. "The result was a primitive toughhardening steel with a middle carbon content in the range of modern gun barrel steels. The spiral welding turned the forging grain in a favorable direction and the resulting damascene pattern again served as a quality certificate setting the damascene barrels apart and making them the barrel of choice throughout the 1800s," says Billgren.
The art of damascene, however, became a lost art as the science of metallurgy advanced during the mid to late 1800s. Ingot steels made carburization in the old forging hearth furnace obsolete. And handling of molten steel made alloy additions possible. New steel alloys for better machinability, corrosion and abrasion resistance, and cold working along with high-speed steels developed in the early 1900s made traditional metalworking tasks of tilting and forging unnecessary.
Metallurgy advances continued in the quest to minimize the occurrence and size of slag inclusions. Improvements included deoxidation, vacuum degassing of molten steel, gas-protected teeming, white-slag treatment in an electric furnace, ladle furnace treatment, calcium injection, and electro slag remelting. Nevertheless, almost every crack that forms within modern steels begins as a defect that is most likely a slag inclusion.
Rapdily Solidified Powder Metals
Efforts to prevent slag inclusions and other defects continue to be a priority in the steel industry. "In the 1970s, a new high-speed steel was produced by Swedish steel maker Erasteel Kloster AB using rapidly solidified powder (RSP) and hot-isostatic pressing (HIPing), says Billgren. "The technique employs an electric refining furnace placed on top of a nitrogenfilled chamber. The liquid steel from the furnace runs through a small nozzle into the nitrogen-filled chamber. There is rapid solidification when powerful gas beams atomize the alloy into fine 0.01-mm-diameter powder. The inclusions that manage to form during the spilt-second solidification are one-hundredth the size of those formed in a traditional ingot."
The powder is then vacuum sealed in steel capsules and heated to a temperature just below the alloy melting point, continues Billgren. The capsules go into the HIP where temperatures reach 2,012 to 2,192°F under 1,000 atmospheres of pressure. Here the powder grains weld together via a solid-state transformation to near theoretical densities.
The resulting billets can then be forged or rolled into any dimension. "Due to the extremely small carbide inclusion size, fracture strength of the HIPed RSP steels rises radically to more than 870 kpsi compared to traditionally made high-speed steels with fracture strength on the order of 435 kpsi," he says. "The substantially smaller carbides in the RSP materials inhibit fracture initiation until the stress level is nearly doubled."
To create damascene patterned steels using RSP requires that two powder grades be carefully placed into the steel capsules in a laminated or mosaic pattern cross section. Most modern steels can be used to create RSP Damascus. The choice of components is based on the following principles:
- Chemical composition of both components must correspond to standard industry-grade steels. Machining qualities and heat treatment for each alloy must also be similar.
- The easily diffusible elements, carbon and nitrogen, must not be accumulated in one of the steels. Both steels must have the same carbon-nitrogen potentials in all hotworking temperatures. Otherwise the properties will vary out of control.
- The transformation temperatures must be similar. If the components harden at different temperatures there is a risk of deformations and dimensional errors.
- The deformation stresses at forging must be similar. Differences too large result in increased hardness and cracks.
- Alloy chemistry must differ so each alloy responds differently to the etching bath. One alloy must have galvanic protection from the other metal.
The best combinations of RSP alloys for many applications (hunting and cutlery knives, jewelry, decorative iron-work, gun-receiver hardware, golf putters, axes, and door handles) include 936 martensitic stainless-steel and 958 austenitic stainless-steel Series.
The 936 Series is made from RWL 34 (1.05% C, 14% Cr, 4% Mo and 0.2% V) and PMC 27 (0.6% C and 13.5 Cr) alloys. This RSP damascene steel can be hardened to a maximum of 3HRC over conventional steels without losing toughness (measured as fracture energy in both compressive and bed loaded edge). The material is, however, sensitive to overheating when forged at 1,920 to 2,120°F. That's because the material starts to melt at 2,230°F.
Electrical or gas-fired furnaces are recommended to better control forging temperatures. Compared to normal lowalloy carbon steels, the martensitic RSP stainless steels have higher, almost double deformation stresses. Hand forging must, therefore, take place on relatively small dimensions. Long heating times can also damage the material because of decarburization and scale formation. And slow cooling after hot working prevents crack formation at the martensitic formation temperature (400°F). Because of the cracking risk, no cutting or machining should be done after hot working until the material is annealed for 5 hr at 1,380 to 1,440°F.
Austenitic 958 damascene steel is an alternative to silver. It has good corrosion resistance and is made from two nonhardenable steel grades (316L and 304L) welded together in more than 100 layers. Typical applications include table cutlery, jewelry, and decorative ironwork.
For damascene-steel gun barrels there are two series. The first, 926, are low-alloy hardenable (300 and 400 HRB) carbon steels made from AISI 4140 (0.4% C, 1% CR, and 0.2% Mo) and AISI 4340 (0.4% C, 1% Cr, 0.2% Mo, and 0.2% Ni) alloys. They are designed for bluing and browning operations. The second, 968, are hardenable (300 and 400 HRB) stainless steels made from AISI 416 (0.22% C, 13% CR, and 0.2% S) and AISI 431 (0.23% C, 16% Cr, and 3% Ni) alloys that work well for etched-pattern designs.
Damascene patterns can be made in a variety of ways. Random patterns depend on the power of the forging hammer blows and the feed steps. The irregularities look like randomly distributed waves in the linear pattern. More predictable patterns come from three other methods:
Torsion twisting creates a distinctive swirl pattern after the metal is etched. The twist blank surface shows a threadlike striated pattern. After grinding or cutting deeper into the surface two spectacular details emerge. The "crossroads" pattern is a band of crosses that follow the line where the grinding depth is at its maximum. The "eyeline" pattern is a band of "eyes" that follow the line where the grinding depth is minimal. The twist patterns come in three grades, Sparse Twist, Twist, and Dense Twist. The density is expressed as the helix angle or the twist. In most cases the bar is twisted to 80° helix angle. The round bars are forged and tolled into flat bars. The bars are then stretched, the resulting twist angle drops to between 50 and 70°.
Coining is a second method for developing a variety of patterns into a flat laminate. The coining can be either by hand or by tools in a forging press. Surfaces will need to be ground and etched. Typical Damasteel-coined pattern designs include Odin's Eye, Rose, Hakkapella, and Muhammed's ladder.
Pattern cutting is yet another method for developing distinctive patterns. Here after cutting the pattern into the flat laminate it is forged or rolled flat again. A grinding step smooths away rough edges before the bar is etched. This method is often employed to put company logos directly into the pattern.