【正文】 esistance, 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 heattreating procedures, but the addition of small amounts of other elements makes some of them hardenable by a solutionprecipitation reaction.TOOL AND DIE STEELS The greatest tonnage of tools ( other than cutting tools ) and dies are made from plain carbon or low alloy steels. This is true only because of the low cost of these materials as their use has a number of disadvantages. They have low hardenability, low ductility associated with high hardness, and do not hold their hardness well at elevated temperature. Manganese Steels. Manganese tool and die steels are oil hardening and have a reduced tendency to deform or crack during heat treatment. They contain from 85～100 points of carbon, ％～％ of manganese to improve hardenability, and small amounts of chromium, vanadium, and molybdenum to improve hardenability, and small amounts of chromium, vanadium, and molybdenum to improve hardness and toughness qualities. Chromium Steels. High chromium tool and die steels are usually quenched in oil for hardening, but some have sufficient hardenability to develop hardness with an air quench. One group of the high chromium steels, called high speed steel, has substantial addition of tungsten, vanadium, and sometimes cobalt to improve the hardness in the heat range.致謝 將近兩個(gè)月的畢業(yè)設(shè)計(jì)即將結(jié)束了，在畢業(yè)設(shè)計(jì)的過(guò)程中，我遇到了很多的困難和疑問(wèn)，在這里特別感謝我的指導(dǎo)老師XXX老師，感謝她給予的幫助和耐心的講解，有時(shí)已經(jīng)到了下班的時(shí)間，因?yàn)檫€沒(méi)有講解完畢仍然繼續(xù)為我講解，直到我明白為止。還有XXX老師，在這次設(shè)計(jì)中也給以很大的幫助，對(duì)我的制圖指出很多的關(guān)鍵的錯(cuò)誤并給予糾正。在此，我衷心的感謝兩位老師，老師，你們辛苦了！ 通過(guò)這次設(shè)計(jì)，我學(xué)到了很多以前從沒(méi)有學(xué)過(guò)的東西，使自己受益菲淺。在今后的日子里我仍然會(huì)嚴(yán)格要求自己，把自己的力量回報(bào)于社會(huì)，為母校增光添彩！