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100-527: Knives can serve various purposes. Hunters use a hunting knife , soldiers use the combat knife , scouts, campers, and hikers carry a pocketknife ; there are kitchen knives for preparing foods (the chef's knife , the paring knife, bread knife , cleaver ), table knife ( butter knives and steak knives ), weapons ( daggers or switchblades ), knives for throwing or juggling, and knives for religious ceremony or display (the kirpan ). A modern knife consists of: The blade edge can be plain or serrated , or

200-458: A tantō , a common Japanese knife. An athame , a ceremonial knife, is used in Wicca and derived forms of neopagan witchcraft. In Greece , a black-handled knife placed under the pillow is used to keep away nightmares. As early as 1646 reference is made to a superstition of laying a knife across another piece of cutlery being a sign of witchcraft . A common belief is that if a knife is given as

300-494: A certain degree of ductility too. Tempering was originally a process used and developed by blacksmiths (forgers of iron). The process was most likely developed by the Hittites of Anatolia (modern-day Turkey), in the twelfth or eleventh century BC. Without knowledge of metallurgy, tempering was originally devised through a trial-and-error method. Because few methods of precisely measuring temperature existed until modern times,

400-451: A combination of both. Single-edged knives may have a reverse edge or false edge occupying a section of the spine. These edges are usually serrated and are used to further enhance function. The handle, used to grip and manipulate the blade safely, may include a tang , a portion of the blade that extends into the handle. Knives are made with partial tangs (extending part way into the handle, known as "stick tangs") or full tangs (extending

500-402: A cradle, to protect the baby; knives were included in some Anglo-Saxon burial rites, so the dead would not be defenseless in the next world. The knife plays an important role in some initiation rites, and many cultures perform rituals with a variety of knives, including the ceremonial sacrifices of animals. Samurai warriors, as part of bushido , could perform ritual suicide, or seppuku , with

600-548: A gift, the relationship of the giver and recipient will be severed. Something such as a small coin, dove or a valuable item is exchanged for the gift, rendering "payment." Some types of knives are restricted by law, and carrying of knives may be regulated, because they are often used in crime, although restrictions vary greatly by jurisdiction and type of knife. For example, some laws prohibit carrying knives in public while other laws prohibit possession of certain knives, such as switchblades . Hunting knife A hunting knife

700-453: A layered structure, combining the attributes of both. For example, a harder, more brittle steel may be pressed between an outer layer of softer, tougher, stainless steel to reduce vulnerability to corrosion. In this case, however, the part most affected by corrosion, the edge, is still vulnerable. Damascus steel is a form of pattern welding with similarities to laminate construction. Layers of different steel types are welded together, but then

800-451: A little less strong, but need to deform plastically before breaking. Except in rare cases where maximum hardness or wear resistance is needed, such as the untempered steel used for files , quenched steel is almost always tempered to some degree. However, steel is sometimes annealed through a process called normalizing , leaving the steel only partially softened. Tempering is sometimes used on normalized steels to further soften it, increasing

900-418: A long time, will begin to turn brown, purple, or blue, even though the temperature did not exceed that needed to produce a light-straw color. Oxidizing or carburizing heat sources may also affect the final result. The iron oxide layer, unlike rust , also protects the steel from corrosion through passivation . Differential tempering is a method of providing different amounts of temper to different parts of

1000-489: A longer time. Tempering times vary, depending on the carbon content, size, and desired application of the steel, but typically range from a few minutes to a few hours. Tempering quenched steel at very low temperatures, between 66 and 148 °C (151 and 298 °F), will usually not have much effect other than a slight relief of some of the internal stresses and a decrease in brittleness. Tempering at higher temperatures, from 148 to 205 °C (298 to 401 °F), will produce

1100-400: A low carbon content. Likewise, tempering high-carbon steel to a certain temperature will produce steel that is considerably harder than low-carbon steel that is tempered at the same temperature. The amount of time held at the tempering temperature also has an effect. Tempering at a slightly elevated temperature for a shorter time may produce the same effect as tempering at a lower temperature for

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1200-557: A number of different materials, each of which has advantages and disadvantages. Handles are produced in a wide variety of shapes and styles. Handles are often textured to enhance grip. More exotic materials usually only seen on art or ceremonial knives include: Stone, bone, mammoth tooth, mammoth ivory, oosik (walrus penis bone), walrus tusk, antler (often called stag in a knife context), sheep horn, buffalo horn, teeth, and mop (mother of pearl or "pearl"). Many materials have been employed in knife handles. Handles may be adapted to accommodate

1300-473: A period that may last from 50 to over 100 hours. Precipitation-hardening alloys first came into use during the early 1900s. Most heat-treatable alloys fall into the category of precipitation-hardening alloys, including alloys of aluminum , magnesium , titanium , and nickel . Several high- alloy steels are also precipitation-hardening alloys. These alloys become softer than normal when quenched and then harden over time. For this reason, precipitation hardening

1400-416: A portion of the blade, usually the spine, or the center of double-edged blades. For single-edged blades, the heat, often in the form of a flame or a red-hot bar, is applied to the spine of the blade only. The blade is then carefully watched as the tempering colors form and slowly creep toward the edge. The heat is then removed before the light-straw color reaches the edge. The colors will continue to move toward

1500-422: A quench and self-temper (QST) process. After the bar exits the final rolling pass, where the final shape of the bar is applied, the bar is then sprayed with water which quenches the outer surface of the bar. The bar speed and the amount of water are carefully controlled in order to leave the core of the bar unquenched. The hot core then tempers the already quenched outer part, leaving a bar with high strength but with

1600-643: A rounded point as to not damage the skin as it is being removed. Hunting knives include the puukko , the Yakutian knife , and the Sharpfinger . Most American designs are based on a smaller version of the Bowie knife . Knifemaker Bob Loveless popularized the drop point hunting knife and William Scagel popularized the Camp knife. Tempering (metallurgy) Tempering is a process of heat treating, which

1700-427: A sharp edge for years with no maintenance at all, but are fragile and will break if dropped on a hard surface or twisted in use. They can only be sharpened on silicon carbide sandpaper and appropriate grinding wheels. Plastic blades are not sharp and are usually serrated to enable them to cut. They are often disposable. Steel blades are commonly shaped by forging or stock removal. Forged blades are made by heating

1800-445: A single piece of steel, then shaping the metal while hot using a hammer or press. Stock removal blades are shaped by grinding and removing metal. With both methods, after shaping, the steel must be heat treated . This involves heating the steel above its critical point, then quenching the blade to harden it. After hardening, the blade is tempered to remove stresses and make the blade tougher. Mass manufactured kitchen cutlery uses both

1900-530: A slight reduction in hardness, but will primarily relieve much of the internal stresses. In some steels with low alloy content, tempering in the range of 260 and 340 °C (500 and 644 °F) causes a decrease in ductility and an increase in brittleness, and is referred to as the "tempered martensite embrittlement" (TME) range. Except in the case of blacksmithing, this range is usually avoided. Steel requiring more strength than toughness, such as tools, are usually not tempered above 205 °C (401 °F). Instead,

2000-427: A small amount of carbon. It is not able to take quite as sharp an edge as carbon steel, but is highly resistant to corrosion. High carbon stainless steel is stainless steel with a higher amount of carbon, intended to incorporate the better attributes of carbon steel and stainless steel. High carbon stainless steel blades do not discolor or stain, and maintain a sharp edge. Laminated blades use multiple metals to create

2100-508: A specific temperature range, the steel experiences an increase in hardness and a reduction in ductility, as opposed to the normal decrease in hardness that occurs on either side of this range. The first type is called tempered martensite embrittlement (TME) or one-step embrittlement. The second is referred to as temper embrittlement (TE) or two-step embrittlement. One-step embrittlement usually occurs in carbon steel at temperatures between 230 °C (446 °F) and 290 °C (554 °F), and

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2200-600: A stud, hole, disk, or flipper located on the blade, all of which have the benefit of allowing the user to open the knife with one hand. The "wave" feature is another prominent design, which uses a part of the blade that protrudes outward to catch on one's pocket as it is drawn, thus opening the blade; this was patented by Ernest Emerson and is not only used on many of the Emerson knives, but also on knives produced by several other manufacturers, notably Spyderco and Cold Steel . Automatic or switchblade knives open using

2300-411: A utility tool the knife can take many forms, including: The knife plays a significant role in some cultures through ritual and superstition , as the knife was an essential tool for survival since early man. Knife symbols can be found in various cultures to symbolize all stages of life; for example, a knife placed under the bed while giving birth is said to ease the pain, or, stuck into the headboard of

2400-445: A variation in hardness is usually produced by varying only the tempering time. When increased toughness is desired at the expense of strength, higher tempering temperatures, from 370 to 540 °C (698 to 1,004 °F), are used. Tempering at even higher temperatures, between 540 and 600 °C (1,004 and 1,112 °F), will produce excellent toughness, but at a serious reduction in strength and hardness. At 600 °C (1,112 °F),

2500-414: A very malleable state through annealing , or it can be hardened to a state as hard and brittle as glass by quenching . However, in its hardened state, steel is usually far too brittle, lacking the fracture toughness to be useful for most applications. Tempering is a method used to decrease the hardness, thereby increasing the ductility of the quenched steel, to impart some springiness and malleability to

2600-472: A wavy, scalloped or saw-like blade. Serrated blades are more well suited for tasks that require aggressive 'sawing' motions, whereas plain edge blades are better suited for tasks that require push-through cuts (e.g., shaving, chopping, slicing). Many knives have holes in the blade for various uses. Holes are commonly drilled in blades to reduce friction while cutting, increase single-handed usability of pocket knives, and, for butchers' knives, allow hanging out of

2700-429: Is a knife used during hunting for preparing the game to be used as food: skinning the animal and cutting up the meat . It is different from the hunting dagger which was traditionally used to kill wild game. Some hunting knives are adapted for other uses in the wild , such as a camp knife, which hunters may use as machetes or hatchets when those specific tools are not available. In this case, their function

2800-468: Is a pick axe which was found in Galilee , dating from around 1200 to 1100 BC. The process was used throughout the ancient world, from Asia to Europe and Africa. Many different methods and cooling baths for quenching have been attempted during ancient times, from quenching in urine, blood, or metals like mercury or lead, but the process of tempering has remained relatively unchanged over the ages. Tempering

2900-532: Is a much tougher microstructure. Lower bainite is a needle-like structure, produced at temperatures below 350 °C, and is stronger but much more brittle. In either case, austempering produces greater strength and toughness for a given hardness, which is determined mostly by composition rather than cooling speed, and reduced internal stresses which could lead to breakage. This produces steel with superior impact resistance. Modern punches and chisels are often austempered. Because austempering does not produce martensite,

3000-419: Is a technique used to form pure bainite, a transitional microstructure found between pearlite and martensite. In normalizing, both upper and lower bainite are usually found mixed with pearlite. To avoid the formation of pearlite or martensite, the steel is quenched in a bath of molten metals or salts. This quickly cools the steel past the point where pearlite can form and into the bainite-forming range. The steel

3100-434: Is affected in a localized area by the heat from the welding process. This localized area, called the heat-affected zone (HAZ), consists of steel that varies considerably in hardness, from normalized steel to steel nearly as hard as quenched steel near the edge of this heat-affected zone. Thermal contraction from the uneven heating, solidification, and cooling creates internal stresses in the metal, both within and surrounding

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3200-428: Is also performed on normalized steels and cast irons, to increase ductility, machinability, and impact strength. Steel is usually tempered evenly, called "through tempering," producing a nearly uniform hardness, but it is sometimes heated unevenly, referred to as "differential tempering," producing a variation in hardness. Tempering is an ancient heat-treating technique. The oldest known example of tempered martensite

3300-435: Is an OTF (out-the-front) switchblade, which only requires the push of a button or spring to cause the blade to slide out of the handle and lock into place. To retract the blade back into the handle, a release lever or button, usually the same control as to open, is pressed. A very common form of sliding knife is the sliding utility knife (commonly known as a stanley knife or boxcutter). The handles of knives can be made from

3400-407: Is called normalized steel. Normalized steel consists of pearlite , martensite , and sometimes bainite grains, mixed together within the microstructure. This produces steel that is much stronger than full-annealed steel, and much tougher than tempered quenched steel. However, added toughness is sometimes needed at a reduction in strength. Tempering provides a way to carefully decrease the hardness of

3500-431: Is done in an inert gas environment, so that the decomposing carbon does not burn off. Instead, the decomposing carbon turns into a type of graphite called "temper graphite" or "flaky graphite," increasing the malleability of the metal. Tempering is usually performed at temperatures as high as 950 °C (1,740 °F) for up to 20 hours. The tempering is followed by slow cooling through the lower critical temperature, over

3600-429: Is followed by a slow cooling rate of around 10 °C (18 °F) per hour. The entire process may last 160 hours or more. This causes the cementite to decompose from the ledeburite, and then the carbon burns out through the surface of the metal, increasing the malleability of the cast iron. Ductile (non-porous) cast iron (often called "black iron") is produced by black tempering. Unlike white tempering, black tempering

3700-474: Is necessary for things like wrenches and screwdrivers . On the other hand, drill bits and rotary files need to retain their hardness at high temperatures. Adding cobalt or molybdenum can cause the steel to retain its hardness, even at red-hot temperatures, forming high-speed steels. Often, small amounts of many different elements are added to the steel to give the desired properties, rather than just adding one or two. Most alloying elements (solutes) have

3800-442: Is often referred to as "aging." Although most precipitation-hardening alloys will harden at room temperature, some will only harden at elevated temperatures and, in others, the process can be sped up by aging at elevated temperatures. Aging at temperatures higher than room-temperature is called "artificial aging". Although the method is similar to tempering, the term "tempering" is usually not used to describe artificial aging, because

3900-417: Is permanent, and can only be relieved by heating above the upper critical temperature and then quenching again. However, these microstructures usually require an hour or more to form, so are usually not a problem in the blacksmith method of tempering. Two-step embrittlement typically occurs by aging the metal within a critical temperature range, or by slowly cooling it through that range, For carbon steel, this

4000-440: Is reversible. The embrittlement can be eliminated by heating the steel above 600 °C (1,112 °F) and then quickly cooling. Many elements are often alloyed with steel. The main purpose for alloying most elements with steel is to increase its hardenability and to decrease softening under temperature. Tool steels, for example, may have elements like chromium or vanadium added to increase both toughness and strength, which

4100-424: Is similar to a survival knife . Hunting knives are traditionally designed for cutting rather than stabbing, and usually have a single sharpened edge. The blade is slightly curved on most models, and some hunting knives may have a blade that has both a curved portion for skinning , and a straight portion for cutting slices of meat . Some blades incorporate a gut hook. Most hunting knives designed as "skinners" have

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4200-466: Is then held at the bainite-forming temperature, beyond the point where the temperature reaches an equilibrium, until the bainite fully forms. The steel is then removed from the bath and allowed to air-cool, without the formation of either pearlite or martensite. Depending on the holding temperature, austempering can produce either upper or lower bainite. Upper bainite is a laminate structure formed at temperatures typically above 350 °C (662 °F) and

4300-453: Is then held at this temperature until the temperature of the steel reaches an equilibrium. The steel is then removed from the bath before any bainite can form, and then is allowed to air-cool, turning it into martensite. The interruption in cooling allows much of the internal stresses to relax before the martensite forms, decreasing the brittleness of the steel. However, the martempered steel will usually need to undergo further tempering to adjust

4400-407: Is to cause the cementite within the ledeburite to decompose, increasing the ductility. Malleable (porous) cast iron is manufactured by white tempering. White tempering is used to burn off excess carbon, by heating it for extended amounts of time in an oxidizing environment. The cast iron will usually be held at temperatures as high as 1,000 °C (1,830 °F) for as long as 60 hours. The heating

4500-466: Is transferred from the hook on the blade's tang to the hook on the rocker bar and thence to the small rocker pin. Excessive stress can shear one or both of these hooks rendering the knife effectively useless. Knife company Cold Steel uses a variant of the lock back called the Tri-Ad Lock which introduces a pin in front of the rocker bar to relieve stress on the rocker pin, has an elongated hole around

4600-408: Is typically between 370 °C (698 °F) and 560 °C (1,040 °F), although impurities like phosphorus and sulfur increase the effect dramatically. This generally occurs because the impurities are able to migrate to the grain boundaries, creating weak spots in the structure. The embrittlement can often be avoided by quickly cooling the metal after tempering. Two-step embrittlement, however,

4700-408: Is used frequently on steels such as 1045 carbon steel, or most other steels containing 0.35 to 0.55% carbon. These steels are usually tempered after normalizing, to increase the toughness and relieve internal stresses. This can make the metal more suitable for its intended use and easier to machine . Steel that has been arc welded , gas welded , or welded in any other manner besides forge welded ,

4800-414: Is used to increase the toughness of iron -based alloys . Tempering is usually performed after hardening , to reduce some of the excess hardness , and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of

4900-412: Is used to mechanically strengthen the knife. Knife blades can be manufactured from a variety of materials, each of which has advantages and disadvantages. Carbon steel , an alloy of iron and carbon , can be very sharp. It holds its edge well, and remains easy to sharpen, but is vulnerable to rust and stains. Stainless steel is an alloy of iron, chromium , possibly nickel , and molybdenum , with only

5000-453: Is usually accompanied by an increase in ductility , thereby decreasing the brittleness of the metal. Tempering is usually performed after quenching , which is rapid cooling of the metal to put it in its hardest state. Tempering is accomplished by controlled heating of the quenched workpiece to a temperature below its "lower critical temperature ". This is also called the lower transformation temperature or lower arrest (A 1 ) temperature:

5100-479: Is usually used as cast, with its properties being determined by its composition. White cast iron is composed mostly of a microstructure called ledeburite mixed with pearlite. Ledeburite is very hard, making cast iron very brittle. If the white cast iron has a hypoeutectic composition , it is usually tempered to produce malleable or ductile cast iron. Two methods of tempering are used, called "white tempering" and "black tempering." The purpose of both tempering methods

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5200-407: The liner lock , an L-shaped split in the liner allows part of the liner to move sideways from its resting position against the handle to the centre of the knife where it rests against the flat end of the tang. To disengage, this leaf spring is pushed so it again rests flush against the handle allowing the knife to rotate. A frame lock is functionally identical but instead of using a thin liner inside

5300-399: The alloy and on the desired properties in the finished product. For instance, very hard tools are often tempered at low temperatures, while springs are tempered at much higher temperatures. Tempering is a heat treatment technique applied to ferrous alloys , such as steel or cast iron , to achieve greater toughness by decreasing the hardness of the alloy. The reduction in hardness

5400-402: The alloy will usually soften somewhat proportionately to carbon steel. However, during tempering, elements like chromium, vanadium, and molybdenum precipitate with the carbon. If the steel contains fairly low concentrations of these elements, the softening of the steel can be retarded until much higher temperatures are reached, when compared to those needed for tempering carbon steel. This allows

5500-463: The benefit of not only increasing hardness, but also lowering both the martensite start temperature and the temperature at which austenite transforms into ferrite and cementite. During quenching, this allows a slower cooling rate, which allows items with thicker cross-sections to be hardened to greater depths than is possible in plain carbon steel, producing more uniformity in strength. Tempering methods for alloy steels may vary considerably, depending on

5600-404: The blade is not released by means of a button or catch on the handle; rather, the blade itself is the actuator. Most assisted openers use flippers as their opening mechanism. Assisted opening knives can be as fast or faster than automatic knives to deploy. In the lock back , as in many folding knives, a stop pin acting on the top (or behind) the blade prevents it from rotating clockwise. A hook on

5700-568: The carbon content in the martensite decreases. If tempered at higher temperatures, between 650 °C (1,202 °F) and 700 °C (1,292 °F), or for longer amounts of time, the martensite may become fully ferritic and the cementite may become coarser or more spherical. In spheroidized steel, the cementite network breaks apart and recedes into rods or spherical-shaped globules, and the steel becomes softer than annealed steel; nearly as soft as pure iron, making it very easy to form or machine . Embrittlement occurs during tempering when, through

5800-406: The carbon content, it also contains a certain amount of "retained austenite." Retained austenite are crystals that are unable to transform into martensite, even after quenching below the martensite finish (M f ) temperature. An increase in alloying agents or carbon content causes an increase in retained austenite. Austenite has much higher stacking-fault energy than martensite or pearlite, lowering

5900-401: The carbon content. However, they are usually divided into grey and white cast iron, depending on the form that the carbides take. In grey cast iron, the carbon is mainly in the form of graphite , but in white cast iron, the carbon is usually in the form of cementite . Grey cast iron consists mainly of the microstructure called pearlite , mixed with graphite and sometimes ferrite. Grey cast iron

6000-466: The crystals, providing less-stressful areas for the carbon atoms to relocate. Upon heating, the carbon atoms first migrate to these defects and then begin forming unstable carbides. This reduces the amount of total martensite by changing some of it to ferrite. Further heating reduces the martensite even more, transforming the unstable carbides into stable cementite. The first stage of tempering occurs between room temperature and 200 °C (392 °F). In

6100-562: The desired balance of physical properties. Low tempering temperatures may only relieve the internal stresses, decreasing brittleness while maintaining a majority of the hardness. Higher tempering temperatures tend to produce a greater reduction in the hardness, sacrificing some yield strength and tensile strength for an increase in elasticity and plasticity . However, in some low alloy steels , containing other elements like chromium and molybdenum , tempering at low temperatures may produce an increase in hardness, while at higher temperatures

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6200-417: The edge for a short time after the heat is removed, so the smith typically removes the heat a little early, so that the pale yellow just reaches the edge, and travels no farther. A similar method is used for double-edged blades, but the heat source is applied to the center of the blade, allowing the colors to creep out toward each edge. Interrupted quenching methods are often referred to as tempering, although

6300-537: The field, but may seem rather vague when viewed from the outside. Terms such as "hardness," "impact resistance," "toughness," and "strength" can carry many different connotations, making it sometimes difficult to discern the specific meaning. Some of the terms encountered, and their specific definitions are: Very few metals react to heat treatment in the same manner, or to the same extent, that carbon steel does, and carbon-steel heat-treating behavior can vary radically depending on alloying elements. Steel can be softened to

6400-436: The first stage, carbon precipitates into ε-carbon (Fe 2,4 C). In the second stage, occurring between 150 °C (302 °F) and 300 °C (572 °F), the retained austenite transforms into a form of lower-bainite containing ε-carbon rather than cementite (archaically referred to as "troostite"). The third stage occurs at 200 °C (392 °F) and higher. In the third stage, ε-carbon precipitates into cementite, and

6500-403: The forging and stock removal processes. Forging tends to be reserved for manufacturers' more expensive product lines, and can often be distinguished from stock removal product lines by the presence of an integral bolster, though integral bolsters can be crafted through either shaping method. Knives are sharpened in various ways. Flat ground blades have a profile that tapers from the thick spine to

6600-488: The full length of the handle, often visible on top and bottom). There is also the enterçado construction method present in antique knives from Brazil, such as the Sorocaban Knife , which consists in riveting a repurposed blade to the ricasso of a bladeless handle. The handle may include a bolster, a piece of heavy material (usually metal) situated at the front or rear of the handle. The bolster, as its name suggests,

6700-419: The grey-blue color the iron oxide loses its transparency, and the temperature can no longer be judged in this way, although other alloys like stainless steel may produce a much broader range including golds, teals, and magentas. The layer will also increase in thickness as time passes, which is another reason overheating and immediate cooling is used. Steel in a tempering oven, held at 205 °C (401 °F) for

6800-424: The handle material uses a thicker piece of metal as the handle and the same split in it allows a section of the frame to press against the tang. A sliding knife is a knife that can be opened by sliding the knife blade out the front of the handle. One method of opening is where the blade exits out the front of the handle point-first and then is locked into place (an example of this is the gravity knife ). Another form

6900-468: The handle of the knife on both sides allowing the user to slide the bolt backward freeing the knife to close. The Axis Lock used by knife maker Benchmade is functionally identical to the bolt lock except that it uses a cylinder rather than a rectangle to trap the blade. The Arc Lock by knife maker SOG is similar to the Axis Lock except the cylinder follows a curved path rather than a straight path. In

7000-533: The hardness and toughness, except in rare cases where maximum hardness is needed but the accompanying brittleness is not. Modern files are often martempered. Tempering involves a three-step process in which unstable martensite decomposes into ferrite and unstable carbides, and finally into stable cementite, forming various stages of a microstructure called tempered martensite. The martensite typically consists of laths (strips) or plates, sometimes appearing acicular (needle-like) or lenticular (lens-shaped). Depending on

7100-434: The hardness will decrease. Many steels with high concentrations of these alloying elements behave like precipitation hardening alloys , which produces the opposite effects under the conditions found in quenching and tempering, and are referred to as maraging steels . In carbon steels , tempering alters the size and distribution of carbides in the martensite, forming a microstructure called "tempered martensite". Tempering

7200-412: The hot steel in water, oil, or forced-air. The quenched steel, being placed in or very near its hardest possible state, is then tempered to incrementally decrease the hardness to a point more suitable for the desired application. The hardness of the quenched steel depends on both cooling speed and on the composition of the alloy. Steel with a high carbon content will reach a much harder state than steel with

7300-504: The layer. This causes a phenomenon called thin-film interference , which produces colors on the surface. As the thickness of this layer increases with temperature, it causes the colors to change from a very light yellow, to brown, to purple, and then to blue. These colors appear at very precise temperatures and provide the blacksmith with a very accurate gauge for measuring the temperature. The various colors, their corresponding temperatures, and some of their uses are: For carbon steel, beyond

7400-417: The malleability and machinability for easier metalworking . Tempering may also be used on welded steel, to relieve some of the stresses and excess hardness created in the heat affected zone around the weld. Tempering is most often performed on steel that has been heated above its upper critical (A 3 ) temperature and then quickly cooled, in a process called quenching , using methods such as immersing

7500-461: The metal, such as shear strength , yield strength , hardness , ductility , and tensile strength , to achieve any number of a combination of properties, making the steel useful for a wide variety of applications. Tools such as hammers and wrenches require good resistance to abrasion, impact resistance, and resistance to deformation. Springs do not require as much wear resistance, but must deform elastically without breaking. Automotive parts tend to be

7600-411: The metal. This allows the metal to bend before breaking. Depending on how much temper is imparted to the steel, it may bend elastically (the steel returns to its original shape once the load is removed), or it may bend plastically (the steel does not return to its original shape, resulting in permanent deformation), before fracturing . Tempering is used to precisely balance the mechanical properties of

7700-492: The needs of people with disabilities. For example, knife handles may be made thicker or with more cushioning for people with arthritis in their hands. A non-slip handle accommodates people with palmar hyperhidrosis . As a weapon, the knife is universally adopted as an essential tool. It is the essential element of a knife fight . For example: A primary aspect of the knife as a tool includes dining, used either in food preparation or as cutlery . Examples of this include: As

7800-494: The processes are very different from traditional tempering. These methods consist of quenching to a specific temperature that is above the martensite start (M s ) temperature, and then holding at that temperature for extended amounts of time. Depending on the temperature and the amount of time, this allows either pure bainite to form, or holds off forming the martensite until much of the internal stresses relax. These methods are known as austempering and martempering. Austempering

7900-428: The rocker pin to allow the mechanism to wear over time without losing strength and angles the hooks so that the faces no longer meet vertically. The bolt in the bolt lock is a rectangle of metal that is constrained to slide only back and forward. When the knife is open a spring biases the bolt to the forward position where it rests above the tang of the blade preventing the blade from closing. Small knobs extend through

8000-402: The same effect as heating at the proper temperature for the right amount of time, and avoided embrittlement by tempering within a short time period. However, although tempering-color guides exist, this method of tempering usually requires a good amount of practice to perfect, because the final outcome depends on many factors, including the composition of the steel, the speed at which it was heated,

8100-488: The sharp edge in a straight or convex line. Seen in cross section, the blade would form a long, thin triangle, or where the taper does not extend to the back of the blade, a long thin rectangle with one peaked side. Hollow ground blades have concave , beveled edges. The resulting blade has a thinner edge, so it may have better cutting ability for shallow cuts, but it is lighter and less durable than flat ground blades and will tend to bind in deep cuts. Serrated blade knives have

8200-425: The steel does not require further tempering. Martempering is similar to austempering, in that the steel is quenched in a bath of molten metal or salts to quickly cool it past the pearlite-forming range. However, in martempering, the goal is to create martensite rather than bainite. The steel is quenched to a much lower temperature than is used for austempering; to just above the martensite start temperature. The metal

8300-438: The steel may experience another stage of embrittlement, called "temper embrittlement" (TE), which occurs if the steel is held within the temperature range of temper embrittlement for too long. When heating above this temperature, the steel will usually not be held for any amount of time, and quickly cooled to avoid temper embrittlement. Steel that has been heated above its upper critical temperature and then cooled in standing air

8400-787: The steel to maintain its hardness in high-temperature or high-friction applications. However, this also requires very high temperatures during tempering, to achieve a reduction in hardness. If the steel contains large amounts of these elements, tempering may produce an increase in hardness until a specific temperature is reached, at which point the hardness will begin to decrease. For instance, molybdenum steels will typically reach their highest hardness around 315 °C (599 °F) whereas vanadium steels will harden fully when tempered to around 371 °C (700 °F). When very large amounts of solutes are added, alloy steels may behave like precipitation-hardening alloys, which do not soften at all during tempering. Cast iron comes in many types, depending on

8500-403: The steel, thereby increasing the toughness to a more desirable point. Cast steel is often normalized rather than annealed, to decrease the amount of distortion that can occur. Tempering can further decrease the hardness, increasing the ductility to a point more like annealed steel. Tempering is often used on carbon steels, producing much the same results. The process, called "normalize and temper",

8600-663: The steel. The method is often used in bladesmithing , for making knives and swords , to provide a very hard edge while softening the spine or center of the blade. This increased the toughness while maintaining a very hard, sharp, impact-resistant edge, helping to prevent breakage. This technique was more often found in Europe, as opposed to the differential hardening techniques more common in Asia, such as in Japanese swordsmithing . Differential tempering consists of applying heat to only

8700-409: The stock is manipulated to create patterns in the steel. Titanium is a metal that has a better strength-to-weight ratio, is more wear resistant, and more flexible than steel. Although less hard and unable to take as sharp an edge, carbides in the titanium alloy allow them to be heat-treated to a sufficient hardness. Ceramic blades are hard, brittle, lightweight, and do not corrode: they may maintain

8800-475: The stored energy from a spring that is released when the user presses a button or lever or other actuator built into the handle of the knife. Automatic knives are severely restricted by law in the UK and most American states. Increasingly common are assisted opening knives which use springs to propel the blade once the user has moved it past a certain angle. These differ from automatic or switchblade knives in that

8900-405: The tang of the blade engages with a hook on the rocker bar which prevents the blade from rotating counter-clockwise. The rocker bar is held in position by a torsion bar. To release the knife the rocker bar is pushed downwards as indicated and pivots around the rocker pin, lifting the hook and freeing the blade. When negative pressure (pushing down on the spine) is applied to the blade all the stress

9000-400: The temperature at which the crystalline phases of the alloy, called ferrite and cementite , begin combining to form a single-phase solid solution referred to as austenite . Heating above this temperature is avoided, so as not to destroy the very-hard, quenched microstructure, called martensite . Precise control of time and temperature during the tempering process is crucial to achieve

9100-468: The temperature was usually judged by watching the tempering colors of the metal. Tempering often consisted of heating above a charcoal or coal forge , or by fire, so holding the work at exactly the right temperature for the correct amount of time was usually not possible. Tempering was usually performed by slowly, evenly overheating the metal, as judged by the color, and then immediately cooling, either in open air or by immersing it in water. This produced much

9200-413: The type and amount of elements added. In general, elements like manganese , nickel , silicon , and aluminum will remain dissolved in the ferrite during tempering while the carbon precipitates. When quenched, these solutes will usually produce an increase in hardness over plain carbon steel of the same carbon content. When hardened alloy-steels, containing moderate amounts of these elements, are tempered,

9300-428: The type of heat source ( oxidizing or carburizing ), the cooling rate, oil films or impurities on the surface, and many other circumstances which vary from smith to smith or even from job to job. The thickness of the steel also plays a role. With thicker items, it becomes easier to heat only the surface to the right temperature, before the heat can penetrate through. However, very thick items may not be able to harden all

9400-449: The user's hand, folding knives typically have a locking mechanism. Different locking mechanisms are favored by various individuals for reasons such as perceived strength (lock safety), legality, and ease of use. Popular locking mechanisms include: Another prominent feature of many folding knives is the opening mechanism. Traditional pocket knives and Swiss Army knives commonly employ the nail nick, while modern folding knives more often use

9500-399: The way through during quenching. If steel has been freshly ground, sanded, or polished, it will form an oxide layer on its surface when heated. As the temperature of the steel is increased, the thickness of the iron oxide will also increase. Although iron oxide is not normally transparent, such thin layers do allow light to pass through, reflecting off both the upper and lower surfaces of

9600-411: The way when not in use. A fixed blade knife, sometimes called a sheath knife , does not fold or slide, and is typically stronger due to the tang, the extension of the blade into the handle, and lack of moving parts. A folding knife connects the blade to the handle through a pivot , allowing the blade to fold into the handle. To prevent injury to the knife user through the blade accidentally closing on

9700-485: The wear resistance and increasing the chances of galling , although some or most of the retained austenite can be transformed into martensite by cold and cryogenic treatments prior to tempering. The martensite forms during a diffusionless transformation , in which the transformation occurs due to shear stresses created in the crystal lattices rather than by chemical changes that occur during precipitation. The shear stresses create many defects, or " dislocations ," between

9800-618: The weld. Tempering is sometimes used in place of stress relieving (even heating and cooling of the entire object to just below the A 1 temperature) to both reduce the internal stresses and to decrease the brittleness around the weld. Localized tempering is often used on welds when the construction is too large, intricate, or otherwise too inconvenient to heat the entire object evenly. Tempering temperatures for this purpose are generally around 205 °C (401 °F) and 343 °C (649 °F). Modern reinforcing bar of 500 MPa strength can be made from expensive microalloyed steel or by

9900-401: Was historically referred to as "500 degree [Fahrenheit] embrittlement." This embrittlement occurs due to the precipitation of Widmanstatten needles or plates , made of cementite, in the interlath boundaries of the martensite. Impurities such as phosphorus , or alloying agents like manganese , may increase the embrittlement, or alter the temperature at which it occurs. This type of embrittlement

10000-504: Was often confused with quenching and, often, the term was used to describe both techniques. In 1889, Sir William Chandler Roberts-Austen wrote, "There is still so much confusion between the words "temper," "tempering," and "hardening," in the writings of even eminent authorities, that it is well to keep these old definitions carefully in mind. I shall employ the word tempering in the same sense as softening." In metallurgy , one may encounter many terms that have very specific meanings within

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