本科专业外文文献翻译
Heat treatment of metal
金属热处理
Heat treatment of metal
In industry today there are more than a thousand different metals being used to manufacture products. The modern automobile has more than one hundred different metals used in its construction. An attempt will be made in this passage to give an understanding of the basic classification of metals.
Metals were formerly thought to be those elements that had a metallic luster and were good conductor of heat and electricity. Actually, metals are generally defined as those elements whose hydroxides from bases (such as sodium or potassium).the nonmetals’ hydroxides from acids (such as sulphur). Metals may exist as pure elements. When two or more metallic elements are combined,they form a mixture called an alloy
The term alloy is used to identify any metallic system. In metallurgy it is a substance, with metallic properties, that is composed of two or more elements, in timately mixed. Of these elements one must be a metal. Plain carbon steel, in the sense, is basically an alloy of iron and carbon. Other elements are present in the form of impurities. However, for commercial purposes, plain carbon steel is not classified as an alloy steel.
Alloy maybe further classified as ferrous and nonferrous. Ferrous alloys contain iron. Nonferrous alloys do not contain iron.All commercial varieties of iron and steel are alloys. The ordinary steels are thought of as iron-carbon alloys. However, practically all contain silicon and manganese as well. In addition, there are thousands of recognized alloy steels. Examples are special tool steels, steels for castings, forgings, and rolled shapes. The base metal for all these is iron.
临沂师范学院本科专业职业生涯设计外文文献翻译
Steels are often called by the principal alloying element present. Examples are silicon steel, manganese steel, nickel steel, and tungsten steel. Even nonferrous alloys may contain iron in a small amount, as impurities. Some of the nonferrous alloys are bronze, brass, and monel.
Although pure metals solidify at a constant temperature, alloys do not. The first nuclei have a tendency to form at a higher temperature than that at which complete solidification occurs. Each element in an alloy has its own peculiarities relative to temperature. Thus, the change in temperature as solidification progresses causes the solid being formed to change in chemical composition.
Many alloying elements dissolve in the base metal in different proportions in liquefied and solidified steels. The proportion of the alloying element that remains in solid solutions has a tendency to vary with the temperature and grain structure of the alloy that is formed.
Nonferrous metals are seldom formed in the pure state. They must be separated from the gangue before the ore can be reduced. Thus, a process known as ore-dressing is performed. Metals and metal compounds are heavier than the gangue. They settle to the bottom if such a mixture has been agitated in water. This process is similar to the method used by the early miners who panned for gold. However, refinements have been developed to speed up the accumulation of metal compound of metal compounds by using this principal.
The reverberatory furnace is the type most often used in the smelting of nonferrous metals. This furnace is constructed of refractory brick with a steel structure on the outside. The charge is placed in the furnace and heated indirectly by the flame. Slag inducers or fluxes are added to the charge to reduce oxidation.
Properties of metals
Metals have properties that distinguish them from other materials. The most important of these properties is strength, or the ability to support weight without bending or breaking. This property combined with toughness, or ability to bend without breaking, is important. Metals also have advantages regarding resistance to corrosion. They are responsive to heat treatment.
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临沂师范学院本科专业职业生涯设计外文文献翻译
Metals can be cast into many shapes and sizes. They can be welded, hardened,and softened. Metals also possess another important property-recycling and reuse. When a particular product is discarded, it can be cut into convenient sections. These sections can be put into a furnace, remelted, and used in another product.
The properties of metals may be classified in three categories: chemical properties, mechanical properties, and physical properties. Here we will emphasize the primary mechanical properties of metals. In understanding the related areas of metalworking and methods used today, the mechanical properties of metals are of the utmost importance.
The hardness of metals varies greatly. Some, like lead, can be indented easily. Others like tungsten carbide, approach diamond hardness. They are of great value as dies for cutting tools of various types. Heat treatment causes changes in the hardness. Annealed tool steel can readily be machined. Often, this is difficult after it has been hardened and tempered. Annealed brass is comparatively soft. When cold-worked the hardness is greatly increased.
A tough metal possesses very high strength. It also has the capability to deform permanently and resist rupture. Toughness enables the metal to survive shock or impact without fracture.
The strength of a metal is its ability to resist deformation or rupture. In certain items, a combination of strength and plasticity is desirable. Machine tools are an example.
Annealing
The word anneal has been used before to describe heat-treating processes for softening and regaining ductility in connection with cold working of material. It has a similar meaning when used in connection with the heat treating of allotropic materials. The purpose of full annealing is to decrease hardness, increase ductility, and sometimes improve machinability of highcarbon steels that might otherwise be difficult to cut. The treatment is also used to relieve stresses, refine grain size, and promote uniformity of structure throughout the material.
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临沂师范学院本科专业职业生涯设计外文文献翻译
Machinability is not always improved by annealing. The word machinability is used to describe several interrelated factors, including the ability of a material to be cut with a good surface finish. Plain low carbon steel, when fully annealed, are soft and relatively weak, offering litter resistance to cutting, but usually having sufficient ductility and toughness that a cut chip tends to pull and tear the surface from which it is removed, leaving a comparatively poor quality surface, which results in a poor machinablity of many of the higher plain carbon and most of the alloy steels can usually be greatly improved by annealing, as they are often too hard and strong to be easily cut at any but their softest condition.
The procedure for annealing hypoeutectoid steel is to heat slowly to approximately 60℃ above the Ac3 line, to soak for a long enough period that the temperature equalizes throughout the material and homogeneous austenite is formed, and then to allow the steel to cool very slowly by cooling it in the furnace or burying it in the maximum ferrite and the coarsest pearlite to place the steel in its softest, most ductile, and least strained condition.
Normalizing
The purpose of normalizing is somewhat similar to that of annealing with the Exceptions that the steel is not to its softest condition and the pearlite is left rather fine instead of coarse. Refinement of grain size, relief of internal
stresses, and improvement of structural uniformity together with recovery of some
ductility provide high toughness qualities in normalized steel. The process is frequently used for improvement of machinability and for stress relief to reduce
distortion that might occur with partial machining or aging.
The procedure for normalizing is to austenitize by slowly heating to approximate 80℃ above the AC3 or Accm3 temperature for hypoeutectoid or hypereutectoid.
Steels, respectively; providing soaking time for the formation of austenite; and cooling slowly in still air. Note that the steels with more carbon than the eutectoid
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临沂师范学院本科专业职业生涯设计外文文献翻译
composition are heated above the Accm instead of the Ac13 used for annealing. The purpose of normalizing is to attempt to dissolve all the cementite during austenitization to eliminate, as for as possible, the settling of hard, brittle
iron carbide in the grain boundaries. The desired decomposition products are smallgrained, fine pearlite with a minimum of free ferrite and free cementite.
Spheroidizing
Minimum hardness and maximum ductility of steel can be produced by a process called spheroidizing, which causes the iron carbide to form in small spheres or nodules in a ferrite matrix. In order to start with small grains that spheroidize more readily, the process is usually performed on normalized steel. Several variations of processing are used, but all require the holding of the steel near the A1 temperature (usually slightly below)for a number of hours to allow the iron carbide to form on its more stable and lower energy state of small, rounded globules.
The main need for the process is to improve the machinability quality of high carbon steel and to pretreat hardened steel to help produce greater structural uniformity after quenching because of the lengthy treatment time and therefore rather high cost, spheroidizing is not performed nearly as annealing or normalizing.
Hardening of steel
Most of the heat treatment hardening processes for the steel is the based on the production of high percentages of martensite .The first step, therefore, is that
Used for most of the other heat-treating processes-treatment to produce austenite.
Hypoeutectoid steels are heated to approximately 60℃ above the Ac3 temperature and allowed to soak to obtain temperature uniformity and austenite homogeneity. Hypereutectoid steels are soaked at about 60℃ above the Ac1 temperature, which leaves
Some iron carbide present in the material.
The second step involves cooling rapidly in an attempt to avoid pearlite transformation by missing the nose of the I-T curve. The cooling rate is determined by the temperature and the ability of the quenching media to carry heat away from the
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