Steel for Cutting Tools

Steel is malleable iron. Although iron is the fourth most common element in the Earth's crust, its extraction is a relatively new chapter in the history of technology.

Due to its affinity for oxygen, iron only naturally exists in the form of minerals which must be reduced via controlled burning to obtain elemental iron.

The Hittites first used a simple smelting furnace to melt iron about 3,700 years ago. The resulting forged spears, blades and tools, which were harder than bronze tools, established the military supremacy of the Hittites in the Near East. Even in the Roman Empire, as well as among the Celts and Vikings, the parallels between cultural superiority and the development of the craft of blacksmithing are obvious.

In order to create well malleable and, in particular, hardenable steel, it is crucial that a specific carbon content (0.2 to 2 %) be obtained. The first smelting furnace operators solved this problem intuitively by controlling the iron ore, fuel and air supply variables during the melting process. This material, which is also called "refined steel", was usually highly porous and contaminated with slag; it therefore had to be condensed and homogenised by repeated forging.

It was not until the late Middle Ages that bloomeries were increasingly replaced by blast furnaces, which made steel production on an industrial scale possible. The refining processes that were developed in the late 18th century used blowing air or remelting the iron in a crucible (puddling steel, crucible steel), making it possible to adjust the carbon content more accurately, thereby producing carbon steel of a consistent quality.

In Japan, the extreme requirements of forging Samurai swords led to the development of the so-called "tamahagane" process in which high-purity steel was directly smelted to a unique bloom as early as the 7th century.

The increasing demands of the weapon and tool industries acted as a decisive boost for metallurgy at the end of the 19th century. The effect of alloy elements on the corrosion resistance and strength of steel was now understood. In 1913 stainless steel, which revolutionised the cutlery industry at the very least, was almost simultaneously developed in Sheffield, England and by Krupp. Durability and heat resistance were extremely important characteristics for industrial machining processes. This led to the development of alloyed tool steels as well as, more recently, high-alloy high-speed steel (HSS) and steels produced by powder metallurgy (PM), which are also in part found in the cutlery industry. With these steels, the steel matrix is only a carrier material for the hardness and durability of the responsible carbides which, chemically speaking, are ceramics. Modern non-metallic blade materials such as zirconia ceramics as well as ceramic coatings should be mentioned in this context.

Steel is a so-called alloy, i.e. it is an iron alloy that is malleable by forging or milling. It contains 0.2 to 2.0 % carbon and is therefore hardenable. Steel which contains only carbon is referred to as carbon steel. Alloy elements, such as chromium, molybdenum, cobalt, vanadium, etc., may also be added to steel to modify its properties. It is then referred to as alloy tool steel.

Like all metals, steel is a crystalline material. Its matrix (structure) is composed of areas of homogeneous crystallites which are also referred to as grains. Its properties are mainly determined by its average grain size. Essentially, the smaller and more uniform the grain size in the structure, the better the mechanical and chemical properties of the steel (resistance to breaking, impacts, corrosion, as well as possible sharpness and durability with knife blades).

The grain size in turn depends on the chemical composition of the steel, the heat treatment and the processing (transformation). It is therefore possible to refine the grain through:

• Normalising
• Hardening
• Forging
• Cold forging (cold hardening)
• Adding certain alloying elements
  
  

With cutting steel, the grain size and the size of the embedded carbides (see alloying elements) is the decisive factor for the possible sharpness of the blade (blade width, phase angle). Roman Landes measured a minimum blade width of 0.1 for razor blades.

Hardening steel uses a special heat treatment to generate internal stresses in the structure which significantly increases its durability. The lattice distortion, which in turn generates these residual stresses, is on the one hand caused by the carbon atoms which occupy certain lattice sites, as well as by other confounding factors such as voids, dislocations, etc. So-called hard phases, which occur during the hardening process, play an essential role in improving the abrasion strength. With more than 0.8 % C, iron carbide, also known as cementite (Fe3C), is separated out. If other alloying elements are present, chromium carbide, nitride, boride, etc. will be formed.

The lattice structure of the steel and its solubility for carbon and other elements depends on the temperature. To harden the steel, it is heated over the so-called transformation temperature (approx. 780 °C for carbon steel or approx. 1,050 °C for stainless steel) and then quenched in a medium (water, oil or air). This creates a new, very fine crystal structure called martensite which is extremely hard but also brittle in this state. To give it a defined hardness and make it tougher so it can be used as cutting steel, the material is then tempered. During this heat treatment (180–300 °C), the residual stresses are reduced, thereby decreasing the risk of breakage. A specific heat treatment can be used to precipitate the special carbides from steel with a higher alloy content; this results in a so-called secondary hardening (temper hardening).

However, these hard phases must not take up too large a proportion of the basic matrix, nor be too big or unequally distributed, as this would cause the blades to break.

Chromium (Cr):

The formation of chromium carbides increases sharpness and durability. The hardenability (formation of martensite) is improved. Up to 11 % chromium is "consumed" to produce chromium carbides; in addition, it forms a passive film of chromium oxide which prevents corrosion. Stainless steel has at least 13 % chromium.

Manganese (Mn):

This element reduces the negative effect of sulphur, thereby improving the hardenability (depth of hardening), tensile strength, malleability and weldability of the steel.

Molybdenum (Mo):

This element refines the grain and reduces the brittleness of alloyed steel. It forms stable carbides, increasing the durability and ductility of the steel and also improves the corrosion resistance of stainless steel.

Vanadium (V):

This element forms stable carbides with a significantly higher tempering temperature, thereby increasing their temperature stability. It refines the grain and improves the weldability of malleable steel with a higher alloy content.

Nickel (Ni):

This element improves the ductility of the steel at both high and very low temperatures. 7 % Ni combined with at least 13 % Cr forms purely austenitic highly corrosion-resistant (acid-resistant), non-magnetic steel. However, since nickel counteracts the formation of martensite and the refinement of the grain, it is hardly ever used in cutting steel.

Cobalt (Co):

This element is used to refine the grain and increase the heat resistance; it is especially used in HSS and is less relevant for knife steels. It does not form carbides.

Wolfram (W):

This element forms very hard carbides with increased heat resistance; it is especially used in HSS.

Phosphorous (P):

Even the smallest proportion of this element makes steel brittle due to segregation at the grain boundaries.

Sulphur (S):

Like phosphorous, this element leads to significant segregation; the formation of iron sulphide causes dreaded red brittleness in hardenable steel.

Both sulphur and phosphorous have a high affinity for iron and are difficult to remove from the bath. On the other hand, even the lowest proportions are very harmful. Purity of well below 0.03 % (S+P) is an essential characteristic of high-quality cutting steel, although this often does not receive sufficient attention!

Hardness:

Generally speaking, the Rockwell hardness of knife steel is read on the "C" Rockwell scale. This test method consists of using a specific hardness tester to measure the depth of penetration of a diamond cone at a defined load (10 kg).

Tensile strength:

This value specifies the tension at which the steel is permanently deformed.

Elongation at break:

This value specifies how much the steel will deform during a tensile test before breaking.

Notched impact resistance:

This value provides information on the ductility of the steel in the event of a sudden bending load. In this context, special attention must be paid to the possible notch effect of microscopic fissures and structural defects (e.g. forging flaws). Hardening generally increases the hardness and tensile strength while decreasing the elongation at break and the notched impact resistance.

• Carbon steel

  Carbon steel is the classic steel for cutting tools. It contains 0.2 to 1.7 % carbon and no or only small amounts of alloying elements – it is therefore not stainless. However, during the hardening process, it forms a very fine-grained martensite structure which makes a high level of sharpness possible. If its carbon content is greater than 0.8 %, it also contains hard phase cementite that also increases the sharpness. This cementite has a lower hardness level than other carbides (compared to chromium carbide, for example); they are usually also distributed more finely and uniformly, which makes the blades less likely to break. Since highly hardened carbon steel (more than 60 Rockwell) is relatively brittle, it is mostly processed as a laminate. (See list of carbon steels)

• High-Alloy Steel

  For cutting steel, "high-alloy" usually means a chromium content of at least 13 %, which makes the blades "stainless". It would be more accurate to speak of "rust resistance", as using the dishwasher, salt water or acid can all cause intergranular corrosion. Susceptibility to corrosion and the risk of brittle breaking increase with increasing hardness. These steels are therefore rarely hardened over 60 Rockwell.
  
  Regarding their suitability as cutting steel, there are two problems with high-alloy steel:

  1. The alloying elements, particularly chromium, lead to an intensive production of carbides. This can cause the blades to break, especially if the carbides are unevenly distributed and too large. This problem is, of course, all the more evident the thinner the blade is ground.

  2. Chromium makes it difficult to use water stones to sharpen blades, as it tends to smear. Sharpening is further complicated when the basic matrix is too soft, which causes strong burrs to form.

  Efforts are made to compensate for these disadvantages by fine-tuning and adding additional alloying elements, especially Mn, Mo and Co. Another important factor for these types of steel is also a very precise heat treatment which ensures the carbide precipitation is uniform and fine during the secondary hardening process (age hardening). (See list of high-alloy steels)

• PM (Powder Metallurgy) Stainless Steel

  The development of materials with extreme strength properties has always been the focus of powder metallurgy. In this process, molten steel is sprayed so that it transforms into a powder. This powder is then compacted during a hot isostatic pressing process near its melting point into a wrought material which is then processed in a conventional way (forging, rolling, etc.). This has the advantage of allowing much more freedom in the composition of the alloy, since this special annealing technique reduces the usual risk of segregation and decomposition.
  
  Although development used to focus more on achieving wear resistance than a fine grain, the Japanese steel types SG2 and Cowry X were the first to also produce high-quality PM cutting steel. These steel types provide an unprecedented combination of extremely high hardness with ductility, tensile strength and corrosion resistance.
  
  In addition, a finer carbide is present, as with conventional high-alloy steels. The disadvantage is that it is relatively difficult to sharpen due to the high carbide content and somewhat coarse-grained structure. (See list of PM steels)

• Multi-Layer Steel

  1. Damascus or Wootz Steel

     The term "Damascus steel" has recently been heard more and more frequently. This notion should therefore be more precisely defined.

     
     

     Where does this name come from?

     Steel with an organic pattern has always been named after the capital of Syria – Damascus. Since ancient times, this was one of the major trading centres for precious metals, iron and steel. The material itself was not necessarily made in Syria; rather, it was often imported from Asia Minor.

     
     

     How was it made? How is it made now?

     What sets Damascus steel apart is its organic pattern. Recent research has shown that these patterns were not originally created intentionally by forging various types of steel or iron. On the contrary, they resulted from the decomposition or segregation process when the crude steel was annealed in the crucible. This led to the formation of areas with varying carbon content and different crystal structures (austenite, martensite, pearlite). The material thus obtained is called wootz. After continued processing, i.e. forging and folding, the blade would then obtain its characteristic pattern, often reminiscent of the structure of wood grains, which would be made even more apparent by etching. The blade would have harder and softer structures.

     
     

     Only later was the steel known as forged Damascus steel developed in Central Europe whereby steels with different compositions were welded in a forge and then folded or twisted to deliberately create a specific pattern. This method is still common today, whereas wootz is now only produced experimentally.

  2. Piled steel

     The term "piled steel", often referred to as "laminated steel", is usually used to describe a blade made out of two or three layers whose cutting edge consists of hardenable steel whereas the rest of the blade is usually made of non-hardenable material. The goal is to use flexible, tougher layers to support the hard but less resistant cutting steel, thereby reducing the risk of the blade breaking. This also improves the blade's sharpening capability, since the there is less hard material to be processed in the profile. This method of constructing blades is frequently found in outdoor Scandinavian knives, as well as many Japanese kitchen knives.
     
     Visually, it is often easily recognisable due the varying reflectivity of the harder (lighter) cutting layer and the softer (duller) surrounding layers; etching enhances this effect.
     
     This type of steel is either produced traditionally by hand, whereby the layers are fire welded in a forge, or industrially by laminating the steel prior to hot rolling it in a rolling mill.

  3. Japanese Sword Steel

     Despite frequent claims, Damascus and suminagashi steel have little in common with the structure of traditional Japanese swords. Japanese sword blades were also made of folded steel, but in a much more elaborate manner. The steel used is known as tamahagane, a material produced from iron sand in the charcoal embers.
     
     As with Damascus steel, this material is folded during the forging process, but many more times. Many historic katana blades are made up of more than a million layers. The individual layers are no longer readily apparent to the naked eye. The purpose of this folding process is therefore not its aesthetic effect, but only to produce a homogeneous structure with as few impurities as possible. This method is therefore closer to the original wootz steel.
     
     Characteristic of katana blades is the tempering line, also called “hamon”. Unlike multi-layer steel, this line is not created when two different types of steel converge; it is a result of how the clay is applied to the blade profile during the hardening process. The profile of the blade can be made of soft and hard steels, which are obtained from different parts of the tamahagane blooms. The structure is quite variable (e.g. hard shell and soft core, hard cutting layer and soft back) and is usually not visually recognisable.

  4. Japanese Multi-Layer Steel

     Many Japanese knife blades are made of multi-layer steel. Since its pattern is often reminiscent of ink paintings, this "Japanese Damascus steel" is often referred to as suminagashi (ink drawing) steel. As with two- or three-ply steel, this steel is either produced traditionally by folding and fire welding during the forging process, or industrially in specialised mills. With the latter technique, block bonds of two or more different types of steel are processed into multi-layer metal sheets. It is also possible to combine stainless with non-stainless steel; more recently, it is even possible to add in nickel or other metals in plants using a shielding gas.

Forging is a transformation process that not only determines the outer shape, but also alters the internal structure of the steel. The structure becomes finer, the average grain size smaller, and specific techniques can even be used to give the blade a texture.

1. Improving resistance to breaking:

   During the forging process, the lattice is oriented according to the stress, and the blade becomes more resistant. It is therefore beneficial to finish by forging a knife blade upright on the anvil, thereby improving the structure in the cutting area.

2. Improving ductility and sharpening capability:

   Impurities (e.g. unwanted alloys such as aluminium, phosphorous and sulphur) often settle at the grain boundaries while the steel melt is solidifying. They increase the risk of breakage along the blade as well as corrosion. Proper forging refines the grain, which reduces or better distributes these separated elements, thereby improving the ductility and sharpening capability of the blade.

3. Dimensioning stress:

   Another advantage of forging is that it makes it possible to precisely dimension the stress of the material. With knife blades, this means decreasing the thickness of the blade from the tang to the tip, which not only makes it lighter, but also improves the balance.

4. Unique crafting:

   Finally, forging always creates individual and unique pieces where the signature of the blacksmith is reflected in the structure and execution of each part. A one-off is obtained that has a completely different character than a uniform mass product.

The crystal structure can only be seen under a microscope (etched micrograph, magnification approximately x 300–500). It is possible, however, to identify most forged blades from their shape, e.g. the thickness of the blade increases or decreases from tang to tip.

Yes. During the machine forging process, the shape is usually produced in only one or a few transformation processes in the drop forge after punching. Due to the resulting high degree of transformation, the material must be heated to a relatively high temperature to prevent fissures from forming. This, in turn, is detrimental to the structure (decarburisation along the edges).

Hand forging (whereby the smith holds the material under the forging hammer with his own hands) involves a lower degree of transformation and temperature, whereby the power and frequency are metered as needed. To obtain optimum results, an experienced smith will always utilise his experience in his work, especially for the heat treatment phase, e.g. using desulphurised coke, applying paste to prevent decarburisation, quenching in accordance with the geometry of the material and possibly cold hammering to further refine the grain (cold working).

The blanks for most Japanese knives in the lower and middle price segments are now made of several layers of wrought material that has been fire welded and rolled in a steel mill. Due to modern process controls and gas shielding, this does not necessarily lead to an inferior product compared to manual welding. However, during manual welding, an experienced blacksmith will always individually adjust the dimensions of the carbon steel to the size and thickness of the blade and to the intended use, thereby optimising its function.

The vast offering and large number of steel names makes it often difficult for consumers to choose the right type of steel. This is partly due to the fact that the perfect cutting steel does not really exist, as the desired characteristics often cannot be combined in one type of steel. For example, easy-care stainless steel is usually not easy to sharpen and dulls more quickly than non-stainless steel.

The following charts compare the main cutting steels, giving consumers an overview of their properties and how they may meet their needs.

1. Roman Landes, Messerklingen und Stahl (Knife Blades and Steel - item no. 713166)
2. Havald Bergland: Die Kunst des Schmiedens (The Art of Forging - item no. 713715)
3. Stahlschlüssel (Key to Steel), paperback, 21st edition
4. Volker Läpple, Wärmebehandlung des Stahls (Heat Treatment of Steel)