Steel is a mixture, or alloy, of iron and carbon, combined during the smelting process. Specifically, steel is iron that has a carbon content of 1.7 percent or less. The characteristics of steel can be modified by the addition in varying amounts of other metals, principally chromium, manganese, molybdenum, vanadium, nickel, or tungsten, and by adjusting the carbon content.
Specific traits of the steel can be further enhanced through physical manipulation of the metal by heat treatment, quenching, hardening and tempering. The science of metallurgy is devoted to developing properties in steel and other metals to optimize their performance in specific applications.
Heat treating is the process that gives steel its hardness, as well as toughness, strength, durability, wear resistance, and ductility. As a generalization, the process involves thoroughly preheating a component, such as knife steel, to 1400-1500 degrees Fahrenheit for approximately 30 minutes. It is then raised to the range of 1850-1950 degrees Fahrenheit for 30 minutes to one hour.
To achieve a high degree of hardening, the steel is then subjected to rapid cooling. Stainless steels are typically air cooled at room temperature. Tool steels are generally "cooled" in warm oil. The beneficial changes in steel that occur as a result of the heat treating process do not actually take place during the heating cycle, but rather stem from the rapid cooling down or "quenching" cycle. The abrupt fall from high temperature changes carbon particles in the metal into hard carbide crystals.
As soon as the steel is quenched, it is tempered. This involves again heating the steel, this time to the tempering temperature, which is a function of the desired hardness one would like to achieve. Tempering temperatures range from 400 degrees Fahrenheit for most tool and stainless steels to 950 degrees Fahrenheit for some premium stainless steels. Hold time at the tempering temperature ranges from 30-60 minutes. After holding the steel at the requisite temperature for a corresponding specific period of time, the steel is allowed to cool. The tempering process is normally repeated a second time.
Shallow cryogenic tempering, performed as part of the initial quenching cycle in heat treating, involves bringing cooling temperatures down to -110 degrees Fahrenheit. Deep cryogenic quenching, involving gradual cooling to below -240 degrees Fahrenheit, is sometimes subsequently used to relieve stress in steel blades, thereby increasing durability, strength, and performance.
Typical high performance stainless steels are hardened to RC 58-60 on the Rockwell scale. Steels that are more stainless are somewhat softer, in the range of 55-58. To provide even more toughness, high carbon tool steels are typically hardened even less, to RC 52-58 or so. These knives are well suited to prying, digging or chopping, and will hold an edge better than stainless steels even though they are softer.
Several features are desirable in the steel used to make the blade of a knife. The intended use of the knife must be known, however, because steels well suited for one purpose may not perform well in another. Some steels represent a "compromise," and may function adequately, but not optimally, in a variety of applications.
Properties desirable in a knife blade include the following:
edge holding ability;
toughness, strength and flexibility;
resistance to corrosion.
High carbon steel is steel with 0.5 percent or more carbon content. It requires at least 0.5-0.6 percent carbon for steel to be sufficiently hard to keep an edge. Edge holding ability is produced by use of such high carbon or "hard" steel, with low chromium content. Cutlery grade steel is typically of this composition.
Hard steel will produce a sharp, long-lasting edge. A blade that is extremely hard will stay sharp for a long time. When it does lose its edge, however, considerable effort will be required to restore it. The addition of carbon makes steel harder. However, toughness is sacrificed because the steel becomes more brittle, less malleable, and less able to withstand shock and stress. Extremely hard blades can sometimes snap because they lack the toughness provided by a medium carbon content.
Toughness and strength is a characteristic associated with the medium carbon content steels. This chemical composition produces a "soft" or flexible steel capable of better withstanding bending and impacts. Soft steel is tougher and easier to sharpen. Soft steel, however, will not be hard enough to provide superior edge holding ability.
Resistance to corrosion is a characteristic associated with "stainless" steel, even though this and all steels will stain or discolor if subjected long enough to adverse conditions or hard use. The stainless property of steel is produced by reducing carbon content and adding chromium.
In order to be considered stainless, a steel must exhibit a chromium content of at least 13 percent. Steel becomes increasingly stainless as chromium content increases and carbon content decreases. The tradeoff with stainless steels involves edge holding performance. As steel becomes more stainless, the ability for a blade to hold an edge decreases and it becomes increasingly difficult to sharpen. As steel becomes less stainless, edge holding ability increases, but resistance to corrosion is degraded. While high carbon tool steel makes excellent, rugged knives, the trend is to use high chromium stainless steel in the production of modern knife blades.
Metallurgists have produced various types of stainless steel in the attempt to achieve compromises between edge holding ability/sharpness versus toughness, while still maintaining resistance to corrosion.
The 420 stainless steel series is very resistant to corrosion, quite ductile, and tough under extreme circumstances. It is found in less expensive production knives. This stainless steel series, however, doesn't hold an edge as well as other tactical or premium stainless knife steels. Other than for salt water use as a diving knife, 420 is too soft a steel to be suitable for utility knife blades. 425 stainless is an improved form of 420.
The stainless series 440A, 440B, and 440C are the stainless steels which set the standard for better quality, yet relatively inexpensive, stainless steel production knives. Carbon content, and thus the ability to harden this series, increases in order from 440A to 440C. All three steels in the 440 series exhibit excellent corrosion resistance, with 440A being best, 440B better, and 440C good. Consider 440A to be suitable for every day knife use, especially when subject to good heat treatment. If a knife is marked with just "440", it is likely that the blade is made from 440A, rather than the more expensive 440C. Stainless 440B steels produce proven, dependable knife blades.
Although 440A and 440B are in the same series as 440C, these alloys are not nearly as good as 440C for knife making. Higher carbon steels like 440C afford better edge retention than generic stainless steels but more resistance to corrosion than high carbon tool steels. 440C stainless was among the first of the stainless steels to be generally accepted for quality knife making and it still remains very popular. Technologies regarding tempering have further enhanced the toughness of 440C. Cryogenic treatment is a sub-zero quenching procedure that greatly contributes to the toughness and edge holding ability of air hardened 440C stainless. 440C produces an excellent, serviceable and durable knife. Consider it to be the "next to the best" choice in production knife steel.
AUS 6A is a popular Japanese stainless steel of medium to high carbon content roughly equivalent in performance between U.S. made 420 and 440A: very resistant to corrosion, reasonably tough, and easy to sharpen.
Higher performance stainless blade steels include ATS 34, a cleaner Japanese copy of 154-CM. Both of these steels have a definite advantage in hardness and toughness over 440C. ATS 34 has a well deserved reputation for superb edge holding, good resistance to corrosion, and toughness. It is the preferred steel for high-end production and custom knives. ATS 34 is vacuum melted, a desirable quality consideration, while 154-CM is not. For the time being, consider ATS 34 to be the "best" knife steel, especially when cost is not a significant consideration.
425M and 12C27 are both stainless steels with properties similar to 440A. Buck knives often feature 425M. 12C27 is a Scandinavian stainless.
AUS 8A stainless is a premium Japanese blade steel. It is of medium to high carbon content. This steel represents an excellent compromise between toughness/strength, edge holding ability, and resistance to corrosion. Slightly softer and a bit tougher than ATS 34, AUS 8A won't hold an edge as well as the latter steel will. In performance, AUS 8A falls between 440A and 440B.
AUS 10A exhibits approximately the same carbon content as 440C, but with slightly less chromium. Hence, it should be slightly tougher than 440C, while somewhat less corrosion resistant.
ATS 55 is a high performance stainless specifically made for cutlery applications. It is well suited for longer, heavier blades utilized for chopping or hacking. This steel provides the excellent edge holding ability of ATS 34 with the benefit of added blade toughness.
BG-42 is fast becoming the "premium" stainless steel. Due to its higher manganese content and trace of vanadium, it should hold an edge even better than ATS 34. It may replace the latter steel as the "ultimate" high end knife steel.
CMP 440 produces a blade which holds an edge even better that ATS 34. This steel is correspondingly difficult to sharpen in the first place, however, and will require extra effort to restore the edge when it becomes dull.
Simple and basic, high carbon tool steels make excellent, tough knives. Tool steel O-1 is probably the most popular knife steel of the last century. It produces a rugged, tough blade of excellent quality. Edge holding ability is also exceptional. It tends to rust easily, if not properly cared for.
The high carbon series 1095, 1084, 1070, 1060, and 1050 represent tool steels that are often used in cutlery applications, although 1095 is very popular for making knife blades. These series of tool steels exhibit the following characteristics, ranging from 1095 to 1050 in descending order: more carbon to less carbon; best edge holding to better edge holding to good edge holding; and tough to tougher to toughest. Because of their toughness, 1060 and 1050 are often found in swords. 1095 is a common knife tool steel that is not too costly yet performs in a superior manner. It holds an edge well and is functionally tough. Similar to other tool steels, it is subject to rust. Most basic USMC type fighting/field knives produced by Ka-bar, Camillus, and Ontario are made from the extremely durable 1095.
A-2 is an air hardened tool steel frequently used in the construction of combat type knives. Toughness is superb, complemented by good edge holding ability.
D-2 is a high grade tool steel gaining a following among higher end production and custom knife makers. Although not a true stainless steel, the "semi-stainless" D-2 is more resistant to rust and corrosion than other tool steels. It possesses superb wearing properties and excellent edge retention, but is not quite as tough as other commonly used tool steels.
Refer to the following table for a summary of the chemical composition of the various knife steels.*Carbon**Manganese**Chromium**Molybdenum**Nickel**Vanadium**Tungsten*
Several locking mechanisms are available on modern folding knives. The most common is the classic lock back. This mechanism utilizes a spring steel back strap which operates vertically and snaps into a cutout in the tang of the blade. Release of the lock is accomplished by pressing on the spring steel on the back of the knife handle, a two-handed operation. The lock back mechanism is reliable and strong.
The liner lock, a mechanism which utilizes a section of the liner to spring horizontally from the side, snaps against the back of the blade tang. Although not quite as secure as the lock back, the liner lock allows for one handed opening and closing of the knife blade, a feature desirable in tactical knives. The liner lock is readily adapted to "assisted opening" mechanisms which automatically deploy the blade once it has been partially opened. These knives are legal to own in most jurisdictions because they are not true automatics.
The spine of the blade is the top portion, or back. It may or may not be sharpened, depending upon the blade type. The belly of the blade is the curving section under the point. The more pronounced the belly, the better the knife's ability to slice, cut and slash. The tradeoff in knife performance results from a reduced ability to pierce, since the point is less pronounced. More slicing ability equates to less stabbing ability, and vice versa.
The classic dagger, with little belly, is ideal for piercing, but is poor for slashing. Some designs, such as the Western Knife Co. Bowie and the venerable military specification USMC fighting/field knives by Camillus, Ka-bar, and Ontario, provide a clip point to improve the knife's stabbing ability, while retaining belly sufficient for slicing and slashing.
Innumerable blade shapes exist. Common types include the clip blade, drop point blade, hawk bill blade, spear point blade, and tanto blade.
Clip Point: Spine of blade tapers downward in a straight line or concave arc toward the tip. Controllable, sharp tip. Good belly. Useful for slicing, stabbing and skinning.
Drop Point: Spine of blade tapers downward in a convex arc toward the tip. Controllable, strong tip. Ample belly. Useful for most cutting applications.
Hawk Bill Point: Spine of blade hooks downward in a convex arc, exaggerated. Useful for slashing and ripping, such as that required for rescue or utility purposes.
Spear Point: Spine of blade tapers downward in a convex arc to meet the belly at the midpoint of the blade. Spine may or may not be sharpened. Controllable, strong tip. Little belly. Useful for stabbing.
Tanto Point: Spine of blade is straight, or curves downward in a slight convex arc. Tip is aligned with the spine. A secondary tip is formed where the straight lower edge intercepts the straight front edge at a sharp angle. This Japanese inspired design produces an extremely strong, reinforced tip. Less control of tip. Slicing difficult. Slashing aided by secondary point. Useful for powerful thrusting against hard materials.
A plain edged blade is easier to sharpen than a blade with serrations. A knife with serrations will still cut well, however, even if it has been dulled somewhat. The serrations in the blade are useful for slicing, sawing types of cuts.
In a flat ground blade, the front profile of the blade tapers from a thicker spine to the thinner edge in a straight line or slight convex arc. A saber grind is similar to the flat grind, except that the grind starts in the mid section of the blade, producing a thicker, stronger edge section. The flat grind produces a knife with good slicing and chopping ability. It represents an excellent compromise between strength and performance, and is the grind of most kitchen utility knives. The saber grind sacrifices some cutting ability for strength and toughness. The saber grind is well suited for chopping and other chores, and is found on military type fighting/field knives such as the USMC.
A hollow ground blade has concave sides, which produces a blade with an extremely thin edge, ideal for slicing. Because of its thinness, this type of blade is correspondingly weaker than its huskier counterparts. It is ideal for slicing applications where cuts don't have to be deep, such as field dressing game. Hence, many hunting type knives utilize hollow ground blades. Since the shape of the blade rapidly becomes thicker as it nears the spine, the hollow ground profile is not good for chopping food, due to its inability to penetrate deeply.
Snurd wrote:You get what you pay for.
MarshallDodge wrote:Snurd wrote:You get what you pay for.
Tenuki wrote:Go get you a benchmade and you will never regret it.
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