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发表于 2012-9-19 09:16
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本帖最后由 陈醉 于 2012-9-19 09:24 编辑
这是附件
Knife Steel
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. 420HC is a higher carbon version of standard 420. With .44% C carbon and 12-14% chromium, 420HC is considered the standard for mass-produced, less expensive factory-made knives.
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 hardenability of 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. |
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