【正文】 在此，我衷心的感謝兩位老師，老師，你們辛苦了！ 通過這次設(shè)計，我學(xué)到了很多以前從沒有學(xué)過的東西，使自己受益菲淺。 for two steels with equivalent hardness and strength, the one with finer grain will have better ductility, which is reflected in the chart as improved toughness. This improved toughness, however, may be detrimental to machinability. Corrosion Resistance. Most pure metals have relatively good corrosion resistance, which is generally lowered by impurities or small amounts of inteneional alloys. In steel, copper and phosphorus are beneficial in reducing corrosion. Nickel bees effective in percentages of about 5％, and chromium is extremely effective in percentages greater than 10％, which leads to a separate class of alloy steels called stainless steels. Many tool steels, which not designed for the purpose, are in effect stainless steels because of the high percentage of chromium present.LOW ALLOY STRUCTURAL STEELS Certain low alloy steels sold under various trade names have been developed to provide a low cost structural material with higher yield strength than plain carbon steel. The addition of small amounts of some alloying elements can raise the yield strength of hotrolled sections without heat treatment to 30％～40％ greater than that of plain carbon steels. Designing to higher working stresses may reduce the required section size by 25％～30％ at an increased cost of 1550, depending upon the amount and the kind of alloy. The low alloy structural steels are sold almost entirely in the from of hotrolled structural shapes. These materials have good corrosion resistance, particularly to atmospheric exposure. Many building codes are based on the more conservative use of plain carbon steels, and the use of alloy structural steel often has no economic advantage in these cases.LOW ALLOY AISI STEELS Improved Properties at Higher Cost. The low alloy American Iron and Steel Institute ( AISI ) steels are alloyed primarily for improved hardenability. They are more costly than plain carbon steels, and their use can generally be justified only when needed in the heattreathardened and tempered condition. Compared to plain carbon steels, they can have 3040 higher yield strength and 1020 higher tensile strength. At equivalent tensile strengths and hardnesses, they can have 3040 higher reduction of area and approximately twice the impact strength. Usually Heat Treated. The low alloy AISI steels are those containing less than approximately 8 total alloying elements, although most mercially important steels contain less than 5. The carbon content may vary from very low to very high, but for most steels it is in the medium range that effective heat treatment may be employed for property improvement at minimum costs. The steels are used widely in automobile, machine tool, and aircraft construction, especially for the manufacture of moving parts that are subject to high stress and wear.STAINLESS STEELS Tonnagewise, the most important of the higher alloy steels are a group of high chromium steels with extremely high corrosion and chemical resistance. Most of these steels have much better mechanical properties at high temperatures. This group was first called stainless steel. With the emphasis on high temperature use, they are frequently referred to as heat and corrosionresistant steels. Martensitic Stainless Steel. With lower amounts of chromium or with silicon or aluminum added to some higher chromium steels, the material responds to heat treatment much as may be hardened by heat treatment similar to that used on plain carbon or low alloy steels. Steels of this class are called martensitic, and the most used ones have 4％ to 6％ chromium. Ferritic stainless steel. With larger amounts of chromium, as great as 30％ or more, the austenite region of the ironcarbon equilibrium diagram is suppressed, and the steel loses its ability to be hardened by normal steel heattreating procedures. Steels of this type are called ferritic and are particularly useful when high corrosion resistance is necessary in coldworked products. Austenitic Stainless Steel. With high chromium and the addition of 8％ or more of nickel or binations of nickel and manganese, the ferrite region of the diagram is suppressed. These steels, the most typical of which contains 18％ chromium and 8％ nickel, are referred to as austenitic stainless steels. They are not hardenable by normal steel heattr