Knife Steel Guide
Getting ready to invest in a new knife can feel like a crash course in metallurgy. You know the importance of the blade, and the alloy from which it's made, in determining how your knife will perform. You plunge into a plethora of information about chemical elements, properties, and alloy designations. By the time you come up for air, you can't decide what you need, let alone what to buy.
Remember that the most important determiner of knife selection lies in matching the blade to your intended use. Use this guide to gain an understanding of some of the basics behind knife materials and their performance, and to compare some of today's best-known, most-popular alloys.
Properties of knife steel types
Rockwell scales designate the hardness of steel and other materials, based on a differential-depth test in which heavy force drives an implement called an indenter into a sample placed on an anvil within a piece of laboratory equipment. The Rockwell scale typically applied to knife steels is the C scale, referenced by the abbreviation HRC, which includes the first and last initials of Hugh Rockwell, co-inventor of the hardness tester, which was patented in 1914. The HRC scale uses 150 kgf (kilogram-force) of load on a spheroconical indenter made from industrial diamond.
Less standardized than measurements of hardness, measurements of toughness indicate a steel's resistance to damage under heavy use and its ability to bend rather than break. Damage includes the chips, cracks, and breakage that always prove difficult, if not impossible, to repair. As toughness increases, hardness decreases, and vice versa.
3. Wear resistance
Two forms of wear contribute to shortening the working life of steel tools. Abrasive wear defines what happens to a softer surface that encounters a rough surface. Adhesive wear consists of the transfer of material dislodged from one surface onto another. Most steels that show good hardness also demonstrate good wear resistance, but the exact chemical composition of the alloy plays a large role in determining behavior under wear.
4. Corrosion resistance
Oxidation occurs when steel interacts with elements in its environment, including salt and moisture. Corrosion resistance designates a steel's ability to withstand exposure without developing rust. Alloy steels incorporate chromium or, less commonly, copper to increase corrosion resistance. As corrosion resistance increases, edge retention drops, and vice versa.
5. Edge retention
The unstandardized measurement of a blade's ability to remain sharp through use has become a popular criterion for the evaluation of knives. Like any subjective, abstract quality, edge retention is difficult to pinpoint, and claims of this property should be subjected to appropriate skepticism.
Alloy ingredients in knife steel types
• Iron (Fe): The basic building block of all steels, purified in a furnace.
• Boron (B): Increases hardness in low-carbon steels.
• Carbon (C): All steel contains at least some carbon, which transforms iron into steel, increases hardness and wear resistance, but reduces toughness and introduces brittleness. You'll see steels described as low, medium, or high carbon. Low equals 0.05 to 0.3 percent, medium ranges between 0.3 and 0.6 percent, and high correlates with 0.6 to 1.5 percent.
• Chromium (Cr): All stainless steel contains chromium, which increases corrosion resistance. In knives, look for a minimum of 12 or 13 percent. Chromium also increases steel's tensile strength and hardness, but too much chromium decreases toughness.
• Cobalt (Co): Promotes hardness by making it possible to cool steel rapidly at high temperatures during the manufacturing process. In steel alloys with complex chemistries, cobalt enhances the effects of other ingredients.
• Copper (Cu): In some instances, alloys incorporate copper to increase corrosion resistance.
• Lead (Pb): Increases steel's machinability.
• Manganese (Mn): Increases hardness, tensile strength, and wear resistance. Too much manganese can lead to brittle steel.
• Molybdenum (Mo): Enhances high-temperature strength and edge retention.
• Niobium (Nb): Increases toughness and edge retention.
• Nickel (Ni): Increases toughness and low-temperature strength.
• Nitrogen (N): Sometimes used instead of carbon in steel production. Added to increase corrosion resistance, hardness, and edge retention.
• Phosphorus (P): Increases hardness.
• Silicon (Si): Removes oxygen from metal during production to prevent pitting, and increases the hardness of the material produced.
• Sulfur (S): Makes steel easier to machine, but reduces toughness.
• Tungsten (W): Enhances wear resistance. Typically incorporated with either chromium or molybdenum.
• Vanadium (V): Increases toughness through production of fine-grained steel during heat treatment, and promotes wear resistance and edge retention.
The overall usefulness and behavior of a knife depend as much on the heat treatment it receives during production as they do on the specific type of alloy from which it's made. Heat treatment fundamentally alters the grain structure of metal, and both the duration and the temperature of the treatment can affect the value and practical performance of a knife. The proper sequence of repeated heating and cooling cycles can yield superior usefulness from a mid-level alloy or turn a superior alloy into an unremarkable blade.
Classification systems for knife steel types
At least eight standards organizations develop and maintain coding systems that classify various forms of steel according to the ingredients used in creating them and the physical properties they demonstrate.
In the United States, the Society of Automotive Engineers (SAE) and the American Iron and Steel Institute (AISI) developed a numerical coding system starting in the 1930s and 1940s. It identifies carbon and alloy steels' chemical composition using sequences of four digits. The first two reference the combination of elements present, with the initial number representing an overall category, such as carbon steels or nickel steels, and the second digit indicating the presence or absence of secondary elements. The concluding two digits indicate how much carbon the steel contains. In some cases, a letter appears between the first and second groupings of digits: B to indicate the inclusion of boron to enhance hardness or L to represent the presence of lead to enhance machinability. For some chromium steels, the SAE/AISI designation expands to five digits, with three digits instead of two used to indicate carbon content.
SAE/AISI classifies stainless steels with a sequence of three digits followed by one or more optional capital letters. The system's tool steel classifications combine a capital letter with a digit.
Knife steel types
1. Carbon steel fares well in rough-handling situations that call for a tough, durable blade. Its lack of chromium makes it likely to rust unless it's oiled or coated.
• 1045, 1095: Inexpensive and durable, steels designated with SAE/AISI classifications that begin with 10, including 1045 and 1095, constitute the most-common forms of steel found in knives, especially 1095. 1045 contains 0.45% carbon; 1095 contains 0.95% carbon. Steels in this range contain less than 1% manganese, although 1045 includes more than 1095. 1095 provides excellent edge retention, sharpens easily, but offers little corrosion resistance, which some knife manufacturers counteract with surface coatings. 1095 also tends toward brittleness in thin blades.
2. Alloy steel takes carbon steel a step further, with the addition of other elements in specific combinations. In addition to more manganese than carbon steel contains, alloy steel incorporates vanadium or molybdenum.
• 5160: A carbon steel with the addition of a small amount of chromium to produce a high toughness, but not enough to call this alloy "stainless." Contains less carbon than the most popular carbon steels (between 0.56% and 0.64%, as opposed to the 0.95% in 1095).
3. Tool steel incorporates tungsten, molybdenum, and other elements in hard alloys popular in cutting tools.
• D2: A high-carbon (1.5%) tool steel with high wear resistance and good corrosion resistance, the latter thanks to a high chromium content that just misses the threshold to qualify as stainless steel. Compared to other tool steels, this alloy lacks toughness because of its high carbon content. It's difficult to sharpen but demonstrates good edge retention.
• 52100: At 0.98% to 1.1% carbon, this is a high-carbon tool steel alloy frequently used in hunting knives. Lower corrosion resistance coupled with great edge retention.
• A2: High toughness, which promotes its use in custom-made combat knives, but with low wear resistance for a tool steel. Low chromium content reduces corrosion resistance and promotes the use of coatings to protect this tool steel.
Crucible Particle Metallurgy, or CPM: The trade name applied to a process, patented by Crucible Industries, that results in alloy stability in a homogeneous metal. Molten metal pours through a nozzle and is transformed into a spray by gas under high pressure, then cooled into a powder that's pressed into steel.
• CPM 10V: A highly wear-resistant tool steel with good, but not great, toughness.
• CPM 3V: Another Crucible product with high wear resistance in a tough tool steel alloy.
• CPM M4: A high-carbon (1.42%) tool steel alloy that includes molybdenum, vanadium, and tungsten for great toughness and wear resistance. Lower chromium levels equal relatively low corrosion resistance. Difficult to sharpen.
• L6: A tough tool steel with good edge retention and relatively poor corrosion resistance that requires care and maintenance. A favorite for use in cutlery.
• M2: A high-carbon (0.85%) tool steel with high heat resistance, excellent edge retention, and a tendency to brittleness, especially over large blade surfaces.
• O1: A popular high-carbon (0.85% to 1%) tool steel with good hardness and edge retention but low corrosion resistance, giving it a tendency to oxidize quickly.
• O6: A high-carbon tool steel with greater toughness than O1, high hardness, and excellent edge retention.
• W2: A high-carbon tool steel with additional carbon for greater hardness. Good edge retention.
4. Stainless steel includes a minimum of 12% to 13% chromium for high corrosion resistance and greater strength. Steels with less chromium may demonstrate good corrosion resistance without qualifying as stainless steel. Note that the term "stainless" actually is a misnomer, as any steel allow will show some corrosion over long exposure to the elements.
• 420: An inexpensive, soft low-carbon (0.38%) stainless steel with low edge retention and a high propensity to chip. Its high corrosion resistance makes it an ideal alloy for knives that come into contact with salt water.
• 420HC: Similar to 420 but with higher levels of carbon in a lower-mid-range alloy with good edge retention and excellent corrosion resistance.
• 425M: Essentially a medium-carbon (0.5%) version of 420 contained in Buck knives.
• 440A: An inexpensive high-carbon (0.65% to 0.75%) stainless steel with high corrosion resistance. Of the three types of 440 steel, 440A resists corrosion the best.
• 440B: A higher-carbon (0.75% to 0.95%) version of 440A with less corrosion resistance but greater wear resistance and edge retention.
• 440C: A high-carbon (0.95% to 1.2%) stainless steel with less corrosion resistance than 440A and 440B. Good hardness and wear resistance make this a desirable alloy for knives. This high-end version can be mistaken for its less-expensive variations, especially because some knife makers mark the numerical designation without the letter grade.
• BG 42: Excellent corrosion resistance in a newer stainless steel that has gained popularity among custom knife makers.
• Bohler M390 (Bohler-Uddeholm): A high-carbon (1.9%) stainless steel with high hardness, corrosion resistance, edge retention, and wear resistance, often used in surgical blades and for implements that require a mirror finish. It owes its hardness to the addition of vanadium. Manufactured by Bohler-Uddeholm, a merger of Austrian and Swedish companies.
• Bohler N680 (Bohler-Uddeholm): A medium-carbon (0.54%) stainless steel with high hardness and corrosion resistance thanks to 0.20% nitrogen and more than 17% chromium. Good edge retention, but less than 154 CM. Well suited for use in divers' blades used in salt water.
• Bohler N690 (Bohler-Uddeholm): A high-carbon (1.07%) stainless steel made in a small factory in Austria and comparable to 440C at the higher end of 440C's carbon-content range. Contains cobalt, molybdenum, and vanadium.
• Elmax (Bohler-Uddeholm): A high-carbon (1.7%) powdered stainless steel with high corrosion resistance, wear resistance, and edge retention.
• GIN 1 (Gingami 1): A high-carbon (0.8% to 0.9%) stainless steel with excellent corrosion resistance. A favorite for diving knives used in salt water.
• 154 CM: A high-carbon (1.05%) stainless steel made by Crucible Industries. Depending on how it's heat treated, this stainless steel offers high hardness and edge retention with good toughness that earns it wide use in production. Often compared to ATS 34.
ATS series (Aichi, Japan)
• ATS 34: A high-carbon (1.05%) stainless steel classified as a super steel. Popular for use in custom knives. Very similar to 154 CM, which is made in the U.S. Excellent edge retention with slightly less corrosion resistance than 440C
• ATS 55: A high-carbon (1%) stainless steel without the vanadium found in ATS-34. Good edge retention with less corrosion resistance than ATS 34.
AUS series (Japan)
• AUS 6: A high-carbon (0.65%) low-quality soft stainless steel comparable to 420.
• AUS 8: A high-carbon (0.75%) mid-grade stainless steel with vanadium for improved edge retention, toughness, and wear resistance.
• AUS 8A: A high-carbon (0.7% to 0.75%) upgraded version of AUS 8.
• AUS 10: A high-carbon (1.1%) stainless steel with greater toughness but less corrosion resistance than 440C thanks to the addition of vanadium and the subtraction of chromium from the alloy.
• CTS-BD1: Similar to AUS 8 and 8Cr13MoV, but with better edge retention and corrosion resistance. A vacuum-melted stainless steel made in the U.S.
• 8Cr14MoV (China): A high-carbon (0.75%) stainless steel manufactured in China and comparable to AUS 8. Reasonable quality in a lower-grade alloy.
• 9Cr13CoMoV (China): A high-carbon (0.85%) stainless steel comparable to 440, but with additional cobalt for hardness.
• AEB-L: A high-carbon (0.67%) stainless steel similar to 440B and originally developed for use in razor blades.
• 13C26 (Sandvik): A variation on AEB-L with similarities to 440A, although with more carbon for greater hardness and wear resistance at the expense of corrosion resistance.
• 14C28N (Sandvik): An upgraded version of 13C26 with more corrosion resistance thanks to added nitrogen.
• H1 (Myodo Metals, Japan): A low-carbon (0.15%) precipitation-hardened premium stainless steel with extraordinary corrosion resistance that makes it virtually rustproof. Popular in diving knives used in salt water. Insufficient edge retention for use in an everyday knife.
• VG-10 (Takefu, Japan): A premium stainless steel with high hardness, edge retention and corrosion resistance. Added vanadium bolsters toughness, although this alloy can be somewhat brittle and prone to chip. Similar to 154 CM and ASTS-34, but additional chromium and vanadium give it more toughness.
Crucible Industries SxxV series: Strong, corrosion and wear resistant, with excellent edge retention. Difficult to sharpen and to polish to a mirror finish.
• CPM S30V: A high-carbon (1.45%) stainless steel that contains 3% vanadium in a highly tough alloy with excellent wear resistance and good hardness.
• CPM S35V: Includes niobium added to CPM-S30V to produce a variation created for knifemaker Chris Reeve.
• CPM S60V: A high-carbon (2.15%) stainless steel with 6% vanadium. High edge retention and wear resistance. Not widely implemented.
• CPM S90V: A high-carbon (2.3%) ultra-high vanadium (9%) alloy with nearly three times the vanadium of S30V or Elmax. Unbeatable wear resistance and edge retention in an alloy that's almost impossible to sharpen. Typically used only in custom knives.
• VG 10 (Japan): A high-carbon (0.95%-1.05%) stainless steel with vanadium for increased toughness. Excellent hardness, edge retention, and corrosion resistance, but occasionally brittle. Similar to 154 CM and ATS 34, but with additional chromium and vanadium.
• X15 (France): A low-carbon (0.4%) stainless steel developed to provide high corrosion resistance in airplanes. High hardness, but not particularly tough.
• Z60CDV14 (Sweden): A low-carbon (0.4%) stainless steel similar to 440A, with allegedly better edge retention.
• ZDP-189 (Hitachi, Japan): A high-carbon (3%) super steel with very high (20%) chromium content, resulting in extreme hardness and superior edge retention. Not very good at corrosion resistance, and very difficult to sharpen.
4. Miscellaneous and rare materials
• Damascus steel: Hand made in the Middle East for millennia, today this exotic metal is pattern welded from multiple layers of various steels folded together and acid etched, producing swirled patterns on its surface. Expensive, beautiful, and largely custom made.
• Ceramics: Non-metallic and therefore rustproof, with extreme hardness that rarely requires sharpening. Brittle, and surpassingly difficult to sharpen.
• Titanium: Lightweight and tough, but with poor edge retention. Found in some custom knives and divers' blades.
• Stellite 6K: A highly tough, wear resistant cobalt alloy made with chromium, molybdenum, and tungsten. Prone to a pitting form of corrosion.