JCS-018立時加工中心主軸箱及進給系統(tǒng)設計【說明書+CAD+SOLIDWORKS】
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南華大學機械工程學院畢業(yè)設計
英文原文: Steels
Steel is one of the most valuable metals known to man; approximately 200 million tons can be produced in the United States annually. In 1900, US capacity was but 21 million tons. Although the process of steelmaking is familiar to most engineers, a review of this process would be appropriate at this time.
Iron ore, limestone, and coal are the principal raw materials used in making iron and steel. Coke is produced by heating bituminous coal in special ovens. Skip cars go up the skip hoist with loads of iron ore, coke, and limestone and dump them into the top of the blast furnace. Hot air from the stove is blown into the furnace near the bottom. This causes the coke to burn at temperatures up to 3000°F. The ore is changed into drops of molten iron that settle to the bottom of the blast furnace. The limestone that has been added joins with impurities to form a slag that floats on top of the pool of liquid iron. Periodically , the molten iron is drained into a ladle for transporting to either the Bessemer converter, electric furnace or open-hearth furnace. The slag is removed separately so as mot to contaminate the iron.
The making of steel from iron involves a further removal of impurities. Regardless of which process is used for making steel-open-hearth, Bessemer-converter, or electric-furnace-steel scrap is added along with desired alloying elements and the impurities are burned out.
Liquid steel removed from the furnace is poured into ingot molds. The ingots are then removed to “soaking pits” where they are brought to a uniform rolling temperature.
At the rolling mill, the white-hot steel passes through rolls that form the plastic steel into the desired shape: blooms, slabs, or billets. These three semifinished shapes then go to the finishing mills where they are rolled into finished forms as structural steel, plates and sheets, rods, and pipes.
Steel is the basic and most valuable material used in apparatus manufactured today. Its application is based on years of engineering experience, which serves as a guide in choosing a particular type of steel. Each variable, such as alloy, heat treatment, and processes of fabrication has its influence on the strength, ductility, machinability, and other mechanical properties, and affects the type of steel selected. The following basic concepts also assist in determining which steel should be used:
1. The modulus of elasticity in tension falls within the range of 28×106to 30×106lb/in2, regardless of composition or form; therefore, sizes as determined by deflection remain the same regardless of the steel chosen.
2. Carbon content determines the maximum hardness of steel regardless of alloy content. Therefore, the strength desired, which is proportional to hardness, can determine the carbon content.
3. The ability of the steel to be uniformly hardened throughout its volume depends on the amount and kind of alloy. This is more complex, but does not necessarily change the calculation of the size of the part.
4.Ductility decreases as hardness increases.
The preliminary choice of steel for a part as well as for other factors, such as notch sensitivity, shrinkage, blowholes, corrosion, and wear, is simplified when based on the above principles. The final selection is made by matching the material with the process of manufacture used in order to obtain the shape, surface, and physical requirements of the part. The selection may be made from among low-carbon steels, low-alloy steels, high-carbon steels, and high-alloy steels.
Steel is one of the few common metals that has an endurance limit. You will recall that fatigue is the failure of a material due to repeated loading. Most metals become tired as they are subjected to stress over and over again. The stress a material can withstand under constant loading is much less than under static loading. As steel is continually loaded, it will reach a lower limit of strength. This property is quite pronounced in wire shapes. Common copper and aluminum wire can easily be broken by flexing the wire in a local spot. Normally after a few dozen flexes, the wire breaks. Steel wire, however, is very tough and flexing the wire simply cold works the material making the process futile for the unknowing person trying to break a steel wire. At some point steel will resist weakening due to repeated loading. This is known as an “endurance limit”. The endurance limit of steel is around 60% of its original strength.
This property of having an endurance limit makes steel invaluable for use in structural applications like bridges, springs, struts, beams, etc. Of course, there are many factors that effect the endurance limit of a material. A primary factor is the surface quality of the material and/or the manufacturing process used to produce the specimen.
Fatigue is attributable to the initial material mot being an ideal homogeneous solid. In each half cycle, irreversible minute strains are produced. Fatigue failure usually develops from:
1.Repeated cyclic stresses that cause incremental slip and cold working locally in the material.
2.Gradual reduction of ductility of the strain hardened areas that develop into cracks.
3.A notching effect from submicroscopic cracks.
The endurance limits of steels create some very desirable physical properties. These properties can be detrimental to the manufacturability of the material. For instance, in the cold rolling of steel the endurance limit creates a limitation on the amount of cold working that can be input to any part. After this limit has been reached the material must be heated above its critical temperature to permit further cold working.
Plain carbon steels represent the major proportion of steel production. Carbon steels have a wide diversity of application, including castings, forgings, tubular products, plates, sheets and wire products, structural shapes, bars, and tools. Plain carbon steels, generally, are classified in accordance with their method of manufacture as basic open hearth, acid open hearth, or acid Bessemer steels, and by carbon content.
The principal factors affecting the properties of the plain carbon steels are the carbon content and the microstructure. The microstructure is determined by the composition of the steel (carbon, manganese, silicon, phosphorus, and sulfur, which are always present, and residual elements including oxygen, hydrogen, and nitrogen) and by the final rolling, forging, or heat-treating operation. However, most of the plain carbon steels are used without a final heat treatment and , consequently, the rolling and forging operations influence the microstructure.
Carbon steels are predonminantly pearlitic in the cast, rolled, or forged conditions. The constituents of the hypoeutectoid steels are therefore ferrite and pearlite, and of the hypereutectoid steels are cementite and pearlite.
Alloy steel is an alloy of iron and carbon containing alloying elements, one or more of which exceeds the following: manganese, 1.65 percent; silicon, 0.60 percent; copper, 0.60 percent; and/or specified amounts of other alloying elements, including aluminum, boron , and chromium up 3.99 percent; cobalt, niobium, molybdenum, nickel, tungsten, vanadium, zirconium, or other elements added in sufficient quantity to give the desired properties of the steel.
Since there are more elements , some expensive, to be kept within the specified ranges in alloy steel than are required in carbon steel , alloy steel requires more involved techniques of quality control and, consequently, is more expensive.
Alloy steel can give better strength, ductility, and toughness properties than can be obtained in carbon steel. Consequently, the engineer should consider alloy steels I designs subject go high stresses and/or impact loading.
Almost all alloy steels are produced with fine-grain structures. A steel is considered to be fine-grained if its grain size is rated 5, 6, 7, or 8. Number1 grain size shows 1 .5 grains/in. of steel area examined at 100diameters magnification. Fine-grain steels have less tendency to crack during heat treatment and have better toughness and shock-resistance properties. Coarse grained steels exhibit better machining properties and may be hardened more deeply than fine-grained steels.
To select the alloy steel that is best suited for a given design, the effects of the principal alloying elements must be taken into account. They are as follows.
1. Nickel provides toughness, corrosion resistance, and deep hardening.
2. Chromium improves corrosion resistance, toughness, and hardenability.
3. Manganese deoxidizes, contributes to strength and hardness, decreases the critical-cooling rate.
4. silicon deoxidizes, promotes resistance to high-temperature oxidation, raises the critical temperature for heat treatment, increases the susceptivity of steel to decarburization and graphitization.
5. Molybdenum promotes hardenability, increases tensile and creep strengths at high temperatures.
6. vanadium deoxidizes, promotes fine-grained structure.
7. Copper provides resistance to corrosion and acts as strengthening agent.
8. Aluminum deoxidizes, promotes fine-grained structure, and aids nitriding.
9. boron increases hardenability.
The term “stainless steel” denotes a large family of steels containing at least 11.5percent chromium. They are not resistant to all corroding media.
Stainless steel competes with nonferrous alloys of copper and nickel on a corrosion-resistance and cost basis and with light metals such as aluminum and magnesium on the basis of cost and strength-weight ratio. Stainless steel has a number of alloy compositions and there are many supplies. Information on its properties and fabrication can be obtained readily. Sound techniques have been evolved for casting, heat treating, forming, machining, welding, assembling, and finishing stainless steel. It will be found that this material usually work-hardens(which makes machining, forming, and piercing more difficult), and it must be welded under controlled conditions and under inert gas. It has desirable high strength, corrosion resistance, and decorative properties.
A bright, clean surface is essential for best corrosion resistance. Traces of scale and foreign matter should be removed by machining, pickling, or polishing. Dipping in nitric acid will ensure the formation of a good oxide film on new pieces. Stainless steels may be electroplated and electropolished, anodically etched covered with porcelain enamel, or given colored coatings through the dying of surface oxides. Highly polished sheets may be purchased directly from stainless-steel producers. A coating of plastic may be used to protect the surface during fabrication.
Stainless steel can be made very hard and its strength can be more than doubled by cooling to 300°F and simultaneously rolling under high pressure, then heating to 750°F for 24hours.
Corrosion resistance is the most important single characteristic of the stainless steels. This quality is due to a thin transparent film of chromium oxide that forms on the surface. It will withstand oxidizing agents such as nitric acid , but will be attacked by reducing agents such as hydrochloric acid or any of the halogen salts. Scaling and corrosion are accelerated in applications in which the oxide layer is constantly being broken. Repeated heating and cooling, with the accompanying expansion and contraction, cracks off the oxide layers. Since the straight-chromium grades of stainless steel have lower thermal expansion than the chromium-nicket grades, they serve best where constant heating and cooling is involved. Most stainless steels show good short-time strength at 1500°F and a few special types are good at 2000°F. Compare this with ordinary carbon steels, which lose their usefulness above 900 to 950°F. The heatconducting properties of stainless steel are poor, so copper cladding is often used in cooking utensils to distribute heat.
中文:
鋼
鋼是人們所熟悉的最有用的金屬材料之一；美國每年大約要生產(chǎn)2億噸鋼，1990年美國的鋼生產(chǎn)能力只有2100噸。雖然大多數(shù)工程師都有熟悉煉鋼的過程，然而這里有必要回顧一下鋼的生產(chǎn)過程。
鐵礦石，石灰石和煤都是煉鋼的主要原材料。焦炭是在特定的爐子里將煙煤加熱燃燒后生產(chǎn)出來的。上料車通過箕斗提升機將鐵礦石，焦炭及石灰石從鼓風爐的頂部倒入爐內(nèi)。
熱風從底部吹入爐內(nèi)，這就使焦炭在高達３０００°Ｆ的溫度下燃燒，鐵礦石則變成熔化的鐵水沉積在鼓風爐的底部。加入的石灰石與雜質(zhì)一起形成爐渣并漂浮在池中的鐵水上。熔化的鐵水定期地排入鐵水包中然后再送到酸性轉(zhuǎn)爐，電爐或者平爐中。為了不使鐵含雜質(zhì)爐渣則被分別排除。
由換煉成鋼則要進一步去除雜質(zhì)，不論是用平爐煉負還是酸性轉(zhuǎn)爐或是電爐煉鋼，乾要將廢鋼與所需的合金元素一起加入以使雜質(zhì)燃燒掉。
鋼水從爐中排出后便注入鋼錠模中，然后將鋼錠送到均熱爐內(nèi)以達到均勻的軋制溫度。
在軋制車間，白熾狀的鋼通過軋輥將塑性狀態(tài)的鋼軋制成所需的形狀：鋼坯，扁鋼坯或坯段。這三種半成品通過精軋后被軋制成最終的形狀，如結(jié)構(gòu)用鋼材，板材，薄板，棒料或管材。
今天 在設備制造中鋼是基本的也是最重要的材料，基應用是基于多年的工程經(jīng)驗，而這些經(jīng)驗也可作為選用鋼材的規(guī)則。鋼的合金元素，熱處理及制造過程等都會影響其強度，塑性，可加工及其它機械性能，也影響鋼的選擇。下面列出有助于選取鋼材的一些概念。
１．無論鋼的成份與結(jié)構(gòu)形狀如何，其拉伸彈性模量均在于２０×１６６lb／in之間，因此，由變形所確定的結(jié)構(gòu)尺寸都是相同的，而與所選擇的鋼無關。
２．鋼的最高硬度由含碳量決定，而與合金含量無關。因而由所要求的強度可確定鋼的含碳量。
３．鋼的均勻淬透能力取決于合金元素的種類與含量，這是個較復雜的問題，但這與零件尺寸的計算無關。
４．隨著硬度的增加其塑性降低。
在上述原則的基礎上，為某一零件以及考慮其他因素初步選擇鋼材時可以簡單一些，但最終選擇要使材料與為了得到零件的形狀，表面和物理要求所采用的制造工藝相匹配?？梢赃x取低碳鋼，低合金鋼，高碳鋼及高合金鋼。
鋼是少數(shù)幾種具有疲勞極限的常用金屬之一，人們所稱的疲勞是由于重復加載而引起的材料失效。大多數(shù)金屬材料由于反復承受應力而變得疲勞。材料在恒定載荷下所承受的應力比靜載時的應力小得多。當對鋼連續(xù)加載時，其強度極限變得較小，這個特性已在鋼材加工中得到了完全的驗證。普通的鋼絲和鋼線在某一局部反復彎曲則很容易折斷。
正常情況下，鋼材經(jīng)過數(shù)十次反復彎曲后就會折斷。然而，鋼絲的韌性很好，在簡單的冷作條件下使鋼絲彎曲并將其折斷是不可能的。如果在某些點反復加載則鋼會阻止弱化，這就是眾所周知的“疲勞極限”。鋼的疲勞極限約為其強度的６０％。
鋼具有疲勞疲勞極限的特性在結(jié)構(gòu)應用中是非常重要的。當然，影響材料疲勞極限的因素很多，主要的因素是材料的表面質(zhì)量和／或試件的制造工藝過程。
疲勞可歸于原始材料理想的均勻體，在每半個應力循環(huán)中不可避免地會產(chǎn)生微應變。疲勞斷裂通常由下列原因所引起：
１． 反復的交變應力會使材料的局部滑移增加或產(chǎn)生冷作現(xiàn)象。
２． 應變硬化區(qū)的塑性將逐步下降而發(fā)展成裂紋。
３． 由亞微裂 紋會形成缺口效應。
鋼的疲勞極限產(chǎn)生了某些非常有用的物理性能，這些性能對材料的可加工性是不利的。例如，在鋼的冷軋過程中，其疲勞極限會限制對零件施加的冷作加工量，當達到疲勞極限后，材料就必須加熱到臨界溫度以上，使之能進一步冷作加工。
鋼產(chǎn)品的大部分是普通碳素鋼。碳素鋼應用廣泛，有鑄件，鍛件，管材，板材，薄鋼板，帶材，線材及鋼絲制品，結(jié)構(gòu)型材，棒料和工具。通常，碳素鋼根據(jù)其制造方法可分成平爐鋼，酸性平爐鋼或酸性轉(zhuǎn)爐鋼，或者由含碳量來區(qū)分。
影響碳素鋼性能的主要因素是含碳量和顯微組織。顯微組織由鋼的成分以及最終的軋制，鍛造或熱處理等方法所決定。但是大多數(shù)碳素鋼不需最終的熱處理，因此，軋制及鍛造加工影響其顯微組織。
在鑄造，軋制或鍛造條件下，碳素鋼的主要組織是珠光體，亞共析鋼是由鐵素體和珠光體組成的，而過共析鋼則是由滲碳體和珠光體組成的。
合金鋼是含有合金元素的鐵碳合金，其中一種或多種元素超過下列含量：錳，１.65%；硅，0.60%;銅，0.60%，和／或一定量的其它合金元素，鋁，硼和鉻的含量達到3.99%；也可加入足夠的鈷、鈮、鉬、鎳、鎢、釩、鋯、或其它元素以使鋼得到所要求的性能。
由于合金鋼含有較多合金元素，與碳素鋼相比，合金鋼更需要把合金元素含量保持在規(guī)定范圍內(nèi)，且合金鋼的質(zhì)量控制技術(shù)更復雜，所以價格更高。
與碳素鋼相比，合金鋼具有較好的強度、塑性和韌性。因此，工程師在設計承受大應力和/或沖擊載荷的零件時應考慮選用合金鋼。
幾乎所有合金鋼的晶粒都較細，若其粒度為5、6、7、8號則為細晶粒鋼，在直徑為100范圍內(nèi)檢查，1號粒度的晶粒大小為1.5粒/in2。在熱處理過程中細晶粒結(jié)構(gòu)鋼出現(xiàn)裂紋的趨勢較小，且韌性和抗沖擊性能都更好。粗晶粒鋼具有較好機加工性，且其淬透性比細晶粒鋼好。
為了對某一設計選定最合適的合金鋼，應考慮所含的主要合金元素的影響，它們有下列特性：
1．加入鎳使鋼的韌性好，耐腐蝕，淬硬性好。
2．鉻可改善鋼的耐腐蝕性，韌性及淬硬性。
3．錳可以脫氧，提高鋼的強度和硬度，降低其臨界冷卻速度。
4．硅可以脫氧，增強鋼的抗高溫氧化能力，提高熱處理時的臨界溫度，增強鋼對脫碳和石墨化的敏感性。
5．鉬具有較好的淬透性，可提高高溫時鋼的抗拉強度和抗蠕變強度。
6．釩可以脫氧，細化鋼的晶粒。
7．銅可改善鋼的搞腐蝕性，起強化劑的作用。
8．鋁可脫氧，細化鋼的晶粒，有助于氮化處理。
9．硼可提高淬透性。
術(shù)語“不銹鋼”代表鋼的一種在家族，其含鉻量至少為11.5%但它并不是對所有的介質(zhì)都有抗腐蝕的能力。
從抗腐蝕性和價格兩方面來衡量，不銹鋼可與有色金屬銅合金和鎳相比；而從價格和強重比來衡量，不銹鋼右與輕金屬鋁、鎂相比。不銹鋼含水量有許多合金元素，也有許多生產(chǎn)廠家，因此，很容易獲得其性能與制造方面的資料。已經(jīng)成功地開發(fā)了不銹鋼的鑄造、熱處理、成形加工、機加工、焊接、裝配及精加工等技術(shù)?，F(xiàn)已發(fā)現(xiàn)這種材料通常會產(chǎn)生加工硬化，且這種材料只有在受控條件及惰性氣體中才能焊接，它具有所希望的高強度、耐腐蝕性和裝飾。
光亮清潔的表面是獲得最好的抗腐蝕性的基本要求。劃痕及雜質(zhì)可通過機加工、酸洗或拋光等方法去除。在硝酸中浸漬可使新顯露的表面形成好的氧化膜。不銹鋼也可以進行電鍍、電拋光、陽極侵蝕，用搪瓷涂覆或者去除表面氧化層后刷上彩色涂料。也可以從不銹鋼廠家直接購買經(jīng)過精細拋光的不銹鋼薄板，在制造過程中還可使用塑料涂層來保護其表面。
若將不銹鋼冷卻至-300°Ｆ，同時在高壓下滾壓，然后再加熱到750°Ｆ并保溫柔4小時，則可獲得很高的硬度，且其強度可增加一倍以上。
耐腐蝕是不銹鋼最重要的一個特性，這是由于在表面形成了一薄層透明的氧化鉻膜所致。它也可經(jīng)受如硝酸等氧化劑的侵蝕，但不能經(jīng)受如鹽酸或鹵鹽等還原劑的侵蝕。如果使用過程中氧化層不斷地被去除的話，則會加速生成氧化皮用腐蝕。對其反復進行加熱和冷卻，則伴隨著產(chǎn)生熱膨脹和收縮，氧化層就會破裂脫落。由于純鉻不銹鋼的熱膨脹比鎳鉻不銹鋼的小，因此，它最適合于不斷加熱和冷卻的場合。大多數(shù)不銹鋼在職500°Ｆ溫度下短時強度較高，而只有幾種特殊不銹鋼在2000°Ｆ溫度下短時強度好。而普通碳素鋼在900到950°Ｆ以上便不能使用。不銹鋼的導熱性差，所以為了導熱常在其上包一層銅以制成炊具。
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