How To Choose The Right Blade Steel
There are many different types of steel available for knife blades. The first thing to do before you purchase a knife is determine how it will be used. Is it going to be a collector piece? Are you going to use it for hunting purposes? Will it be used around salt water when cleaning fish? Will it serve as a general purpose pocket knife? The reason that there are so many different types of blade steel is because it is not a “one size fits all” proposition. It is suggested to research the different blade steels that are available to find the one that is best suited for your intended use. And remember… the blade is where the work is done so choose wisely!
The Making & Shaping of Steel
Steel is essentially a combination of iron and carbon. All steels contain certain other elements in small controlled amounts, like Manganese, Sulfur, Silicon, and Phosphorus. If nothing else is present, the steel is referred to as plain carbon steel. Steels used for knife blades are enhanced with additional elements and are called alloy steels. It is these additions that give different types of steel their special properties. Alloy steels that have additions to make them corrosion-resistant are labeled stainless steels, and these are the steels most frequently used in making knife blades.
The making of stainless steel begins by melting steel in a furnace. Alloying elements are added to the melt, and the molten steel is poured into molds called ingots. Once the ingots have solidified, they are processed in a mill to make usable shapes and sizes such as plates and coils. Plates are turned into knife components by laser cutting and coils are shaped into components using a fine blanking press.
Properties of Steel
The selection of steel for specific applications is based on the properties of the steel and other factors like manufacturability—if the steel is difficult to fabricate, then it is not practical for use in a manufacturing environment. These properties are established by the alloys added to steel and by the methods used in its manufacture.
Some of the important properties of blade steel are:
- Hardness: A measure of the steel's ability to resist permanent deformation (measured on a Rockwell Scale)
- Hardenability: The ability of a steel to be hardened (through the heat-treating process)
- Strength: The steel’s ability to resist applied forces
- Ductility: The steel's ability to flex or bend without fracturing
- Toughness: The steel’s ability to absorb energy prior to fracturing
- Initial Sharpness: The sharpness of the blade "out of the box"
- Edge Retention: The ability of the steel blade to hold an edge without frequent resharpening
- Corrosion Resistance: The ability of the steel to resist deterioration as a result of reaction with its environment
- Wear Resistance: The ability to resist wear and abrasion during use
- Manufacturability: The ease with which steel can be machined, blanked, ground, and heat-treated (made into a blade) Since no single material is superior in all property categories, manufacturers select materials that offer the optimum properties for the purpose intended.
The nomenclature used to describe the types of steel and their properties is often derived from the internal structure of metals. As steel is heated and cooled, its internal structure undergoes changes. The structures formed during these changes are given names like Austenite and Martensite. Martensite is a very hard structure that can be formed by rapidly cooling certain types of steel during heat-treating. Steels that are capable of forming Martensite are called martensitic steels, and it is this type of steel that is of most interest to the cutlery industry. S30V, BG-42, 154CM and 420HC are all martensitic stainless steels.
The properties of steel can be altered by the addition of certain elements to the steel during the melting process. The alloying elements that are important to knife-making are listed with a brief description of how they affect the steel's properties.
Carbon - is not an alloying element since it is present in plain carbon steels. Nonetheless, increasing carbon increases hardness.
Chromium - improves hardenability, wear resistance, and corrosion resistance. It is a major element in martensitic stainless steels, which are most commonly used for sports cutlery applications.
Molybdenum - improves hardenability, tensile strength, and corrosion resistance, particularly pitting.
Nickel - improves toughness, hardenability and corrosion resistance. Nickel is a major element in Austenitic stainless steel that is sometimes used for dive knives.
Vanadium - improves hardenability and promotes fine grains. Grain structure in steels is another important factor in wear resistance and strength. Generally, fine grain structures are desirable.
Some Popular Types of Steel
A. Non-stainless Steels (carbon, alloy, and tool steels):
· A2 Tool Steel is a high carbon steel that is very tough and abrasion resistant. It responds very well to cryogenic treatment (see Knife Terminology) for maximum edge retention.
· 10-series -- 1095 (and 1084, 1070, 1060, 1050, etc.) Many of the 10-series steels for cutlery, though 1095 is the most popular for knives. When you go in order from 1095-1050, you generally go from more carbon to less, from more wear resistance to less wear resistance, and tough to tougher to toughest. As such, you'll see 1060 and 1050, used often for swords. For knives, 1095 is sort of the "standard" carbon steel, not too expensive and performs well. This is a simple steel, which contains only two alloying elements: .95% carbon and .4% manganese. 1095 High Carbon Tool Steel, is also known as “Cutlery Spring Steel”. This steel is well known for its use in manufacturing commercial saw blades and recognized for its cutting and edge holding ability. It hones to an unbelievable edge (better than any stainless steel), retains its edge (better than most stainless steels) , and easier to sharpen, (compared to stainless steel). Be aware, this steel will discolor over time and is susceptible to rust. It is recommended to keep the blade oiled, but discoloration and/or rust will not affect blade performance.
· D-2 is sometimes called a "semi-stainless". It has a fairly high chrome content (12%), but not high enough to classify it as stainless. It is more stain resistant than the carbon steels mentioned above, however. It has excellent wear resistance. D-2 is much tougher than the premium stainless steels like ATS-34, but not as tough as many of the other non-stainless steels mentioned here. The combination of great wear resistance, almost-stainlessness, and good toughness make it a great choice for a number of knife applications.
· 5160 is a steel popular with forgers and it is popular now for a variety of knife knowledges, but usually bigger blades that need more toughness. It is essentially a simple spring steel with chromium added for hardenability. It has good wear resistance, but is known especially for its outstanding toughness. This steel performs well over a wide range of hardnesses, showing great toughness when hardened in the low 50s Rc for swords, and hardened up near the 60s for knives needing more edge holding.
B. Stainless Steels:
· AEB-L steel is a steel developed by Uddeholm in Sweden for razor blades years ago and has now become very popular with knife makers. It is often a misunderstood steel in that it needs to be heat treated correctly to bring out its best characteristics and when done correctly, it performs on par with the new so called "super steels" at a much lower cost.
Many times you will hear people state that AEB-L steel is similar to 440B or 440A. The only similarities shared by AEB-L and 440B or 440A is the carbon content. Because of the fact that AEB-L has only 13% chromium by weight compared to 16-17% in 440A and 440B, the steels are quite a bit different. A stainless 52100 steel would compare more to AEB-L than to 440A or 440B.
AEB-L naturally forms what is called a K2 carbide which
is the harder of the two chromium carbides of which K1 is the other. The K1
carbide is formed in steels such as 440C. The K2 carbide is about 79 on the
Rockwell C scale as compared to 72 for the K1 carbide. The key for AEB-L steel
is the heat treat. With proper heat treating, AEB-L produces fine, evenly
distributed K2 carbides. AEB-L steel lands almost exactly on what is called the
"Carbon Saturation Line". This means that all of the carbides formed
are precipitated carbides, not primary carbides like form in 440C plus there is
more carbon and a similar amount of chromium in solution as compared to 440C.
Primary carbides are very large. So when you combine the proper heat treatment
with its balanced composition, AEB-L has excellent toughness, edge retention,
workability, ease of sharpening, and ease of polishing.
· 420 has a lower carbon content (<.5%) than the 440 series which makes this steel extremely soft, and it doesn't hold an edge well. It is used often for diving knives, as it is extremely stain resistant. Also used often for very inexpensive knives. Outside salt water use, it is too soft to be a good choice for a utility knife.
· 420HC is a stainless steel that provides excellent rust resistance, is easy to re-sharpen and has good edge retention. It is a higher carbon version of standard Type 420 martensitic stainless steel. The Carbon content, combined with the high Chromium content, provides good abrasion resistance and edge-holding. This steel is not to be confused with standard 420 stainless steel. 420HC is an excellent general purpose knife steel and is roughly comparable to 440A.
· 440A, 440B and 440C steels are some of the most popular stainless steels used today. The carbon content (and hardenability) of this stainless steel goes up in order from A (.75%) to B (.9%) to C (1.2%). 440C is an excellent, high-end stainless steel, usually hardened to around 56-58 Rc, very tough and with good edge-holding at that hardness. All three resist rust well, with 440A being the most rust resistant, and 440C the least. 440C is fairly ubiquitous, and is generally considered a very good general-use stainless, tougher and more stain resistant than ATS-34 but with less edge-holding and weaker. If your knife is marked with just "440", it is probably the less expensive 440A; if a manufacturer had used the more expensive 440C, he'd want to advertise that. The general feeling is that 440A (and similar steels, see below) is just good enough for everyday use, especially with a good heat treat . 440-B is a very solid performer and 440-C is excellent.
· 425M and 12C27 are very similar to 440A. 425M has .5% carbon. 12C27 has .6% carbon and is a Scandanavian steel that is used often in Finish puukkos and Norwegian knives. 12C27 is said to perform very well when carefully heat treated, due to its high purity. When done right, it may be a slighter better choice than 440A and similar steels.
· AUS-6, AUS-8, AUS-10 (aka 6A 8A 10A) are Japanese stainless steels, roughly comparable in carbon content to 440A (AUS-6, .65% carbon) and 440B (AUS-8, .75% carbon) and 440C (AUS-10, 1.1% carbon). AUS-6 is used by Al Mar, and is a competitor to low-end steels like 420J. Cold Steel's use of AUS-8 has made it pretty popular, as heat treated by Cold Steel it won't hold an edge like ATS-34, but is a bit softer (and therefore weaker) and tougher. 8A is a competitor of middle-tier steels. AUS-10 has roughly the same carbon content as 440C but with slightly less chromium, so it should be a bit less rust resistant but perhaps a bit tougher than 440C. It competes with higher-end steels, like ATS-34 and above. All 3 steels have some vanadium added (which the 440 series lacks), which will improve wear resistance and refines the grain for both good toughness, and the ability to sharpen to a very keen edge. Many people have reported that they are able to get knives using steels that include vanadium, like 8A, sharper than they can get non-vanadium steels like ATS-34.
· ATS-34 and 154-CM stainless steels. ATS-34 was the hottest high-end stainless in the 1990s. 154-CM is the original American version, but for a long time was not manufactured to the high quality standards knifemakers expect, so knifemakers switched over to ATS-34. CPM is again making high-quality 154-CM, and some companies seeking to stick with American-made products are using it. ATS-34 is a Hitachi product that is very, very similar to 154-CM. Normally hardened to around 60 Rc, it holds an edge very well and is tough enough even at that high hardness. Not as rust resistant as the 400 series above. Many custom makers use ATS-34, and Spyderco (in their high-end knives) and Benchmade are among the production companies that use it.
· VG-10 is another vanadium-containing high-end stainless steel. Due to the vanadium content, VG-10 takes a killer edge, just like other vanadium steels like BG-42 and AUS-8. VG-10 is also tougher and more rust-resistant than ATS-34, and seems to hold an edge better.
· BG-42 is somewhat similar to ATS-34, with two major differences: It has twice as much manganese as ATS-34, and has 1.2% vanadium (ATS-34 has no vanadium), so look for significantly better edge-holding than ATS-34. The addition of vanadium and the clean manufacturing process (VIM/VAR) also gives BG-42 better toughness than ATS-34.
· S30V is an excellent blade steel. It is a high vanadium stainless steel with even higher edge retention.
Steel makers follow a precise recipe to ensure that each time they make a particular alloy it has correct properties. The recipes are known as Specifications, and they specify the amount of each alloy. Each alloy recipe or type is named according to a number convention. Martensitic stainless steels, for example, have numbers like Types 410, 420, and 425.
What is Rockwell Hardness?
The hardness of steel or other metals is usually measured on a scale called the "Rockwell Scale", this scale gives a number value to the hardness. This number is preceded by the letters Rc (for example Rc58). High numbers indicate harder material. If a knife is too "soft" meaning it has too low a Rockwell hardness, it will probably not hold an edge and will bend quite easily. If a knife is too "hard" meaning it has too high a Rockwell hardness, it will probably be very brittle and difficult to re-sharpen. When a knife is designed, it is important to determine from the beginning what kind of hardness will be required for its ultimate purpose. This will affect the choice of steel. Once the steel is chosen, a heat treatment sequence must be devised to result in the exact hardness needed in the final knife. (See “Rockwell Hardness Test” in the Knife Terminology section)
Damascus Steel Blades
Damascus steel blade knives are growing in popularity due to their beautiful looks, functionality and nostalgia. Collectors are drawn to the unique and very pleasing patterns that can be created with Damascus steel. The usefulness of Damascus steel blade knives is very attractive as well. The idea of welding layers of steel together can also be thought of as “laminated steel”. This is much like the concept of laminating wood which produces a stronger piece of material. Usually two or more different types of steel are used in the layers so the characteristics of each type will contribute to the necessary qualities that are desired for a high quality knife blade. A good quality Damascus steel blade will have all of the necessary features to provide a lifetime of excellent service… it can be sharpened to a very sharp edge, it will hold an edge very well and it will be tough. If properly taken care of, a good Damascus blade knife can be passed down from generation to generation. Producing Damascus steel blade knives is very labor intensive and it takes about 20 hours to produce one of good quality.
Historians have found that Damascus steel, formally known as Wootz steel, originated in Asia over two thousand years ago, (200 BC). Damascus steel gained popularity throughout the Roman era and was commonly used to make armor and weapons. Damascus steel regained its popularity in the mid 18th century when a Swedish scientist discovered that the original wootz steel contained carbon as the dominant element in the ancient steel. Swedish companies began reproducing Damascus steel on an industrial scale and began using Damascus steel to make gun barrels. Thus, the first crucible steel manufacturing began in 1774.
How a Damascus Steel Blade is Made
There are various techniques that bladesmiths and blacksmiths use to create Damascus knife blades. The following is a “snap shot” of a basic method for a fixed blade knife:
Making the Billet:
The process is started with five pieces of steel. Two high carbon pieces and three medium carbon pieces. (If forging a larger blade such as a sword, seven pieces may be used instead of five.) The thick ness, width and length of the pieces depends on the size of the blade that is being made. One of the me dium carbon pieces is made longer in order to handle the billet in the forge. Impurities are cleaned from the pieces of steel, then sandwiched together with the two high carbon pieces between the medium car bon pieces. The pieces are then tied together with wire or arc welded together on the end. The wire or weld material is removed after the first forge weld.
The Welding Process:
The billet is placed in a forge and brought to a cherry red color. It is then removed and covered with bo rax. The borax becomes fluid on the hot steel and acts as a flux to help clean the steel and keep it from oxidizing. The pieces are then hammered together with a hammer and anvil and/or a mechanical ham mer.
Drawing, Cutting, Folding and Welding:
The piece is then drawn out to twice the original length. Skill and care are very important at this step as the billet must be struck properly to avoid splitting. The billet is struck straight down to draw it out and not to "push" the steel out. The billet is heated several times during this process. The billet is then cut in the middle, leaving a little material to keep it together until the next weld. The billet is then turned on its side and hammered enough to swell the center of the billet. This provides a convex surface for welding. The billet is then folded back on itself. The billet is covered with Borax again and heated to welding temperature and then welded.
The draw, cut, fold and weld process is repeated the necessary number of times to achieve the desired number of layers. Starting with 5 pieces of steel, it requires 5, 6, or 7 times in order to obtain 160, 320 or 640 layers respectively. The more layers that there are, the more skill that is required for a good look ing and high quality blade.
Next the billet is hammered or cut and ground into the shape of the desired blade.
The blade is then heated to a orange-red color (1,400 to 1,500 degrees), then quenched in a quenching oil (sometimes a brine solution is used). The blade is then tempered by slowly heating to around 400 degrees.
The blade is then sanded multiple times using finer and finer grit. It is then polished and sharpened.
The blade is then dipped in an acid bath. The acid reacts differently with the two steels which brings out the distinctive grain pattern.
The blade is now finished.