TUGAS PROSES MANUFAKTUR DAN PEMILIHAN MATERIAL
MARDIANSYAH PRATAMA (1406533094) TRIANA YUSMAN (1406533081) DEPOK, 21 MARET 2016 TUGAS 5
l3.l. What is the difference between a plate and a sheet?
Plates, which are generally regarded as having a thickness greater than 6mm (114 in.), are used for structural applications, such as machine structures, ship hulls, boilers, bridges, and nuclear vessels. Plates can be as much as 0.3 m (12 in.) thick for the supports for large boilers, 150 mm (6 in.) thick for reactor vessels, and 100-125mm (4-5 in.) thick for battleships and tanks. Sheets are generally less than 6 mm thick; they are provided to manufacturing facilities as flat pieces or as strip in coils for further processing into various products. They are used for automobile and aircraft bodies, appliances, food and beverages containers, and kitchen and office equipment. l3.3 What factors contribute to spreading in flat rolling?
Traditionally, the initial material form for rolling is an ingot. As stated in Section 5.4, however, this practice is now being rapidly replaced by that of continuous casti ng and rolling, at much higher efficiency and a lower cost. l3.7. List the defects commonly observed in flat rolling.
Pada flat rolling kita dapat membagi defect kedalam 2 kategori yaitu :
Surface defect : termasuk didalamnya rusting and scaling, surface scratches, surface caracks, dan juga lubang yang tersisa pada permukaan.
Structural defect : termasuk didalamnya wavy edges, zi pper cracks, edge cracks, centre split, alligatoring, folds, dan laminations.
l3.l0. Why is the surface finish of a rolled product better in cold rolling than in hot rolling?
Karena semua bentuk, batang, lembaran dan strip diproduksi oleh rolling panas biasanya selesai dengan rolling dingin. dingin lembar bergulir dan strip membuat bagian penting dari keseluruhan produksi baja. Hal ini terjadi akibat/karena berkurang/kurangnya skala pada proses.
l3.l3. Flat rolling reduces the thickness of plates and sheets. It is possible, instead, to reduce their thickness simply by stretching the material? Would this be a feasible process? Explain.
Although stretching reduces the thickness of materials, there are several limitations associated with it as compared to rolling. Stretching process is a batch process and it cannot be continuous as it is in rolling. The reduction in thickness is limited by necking of the sheet, depending on its strain-hardening exponent, n (see Section 2.2.4 on p. 61). Furthermore, as the sheet is stretched, the surface finish becomes dull due to the orange-peel efect. Stretching the sheet requires some means of clamping the material at its ends which, in turn, will leave marks on the sheet. l3.l6. Describe the factors that influence the magnitude of the roll force, F, in Fig. 13.2c.
As can be deduced by observing the equations on p. 319, the roll force, F , is influenced by the roll radius, strip width, draft (hence the roll-strip contact area), coeficient of friction, and the strength of the material at the rolling temperature. If the material is strain-rate sensitive (i.e., high m value), the rolling speed would also influence the roll force; this is particularly important in hot rolling. l3.20. Describe the methods by which roll flattening can be reduced. Which property or properties of the roll material can be increased to reduce roll flattening?
Flattening is elastic deformation of the roll and results in a larger contact length in the roll gap; therefore, the elastic modulus of the roll should be increased, for example, by making it from materials with high modulus of elasticity, such as carbides (see Tables 2.1 on p. 56, 2.2 on p. 58, and 22.1 on p. 602. Roll attening also can be reduced by (a) decreasing the reduction per pass and (b) reducing friction at the roll-sheet interface. |3.25. Describe the importance of controlling roll speeds, roll gaps, temperature, and other process variables in a tandem-rolling operation, as shown in Fig. 13.11. Explain how you would go about determining the distance between the stands
Referring to the tandem rolling operation shown in Fig. 13.11 on p. 326, we note that mass continuity has to be maintained during rolling. Thus, if the roll speed is not synchronized with the strip thickness in a particular stand, excessive tensions or slack may develop between the stands; some rolls may slip. Also, if the temperature is not controlled properly, strip thickness will change, thus affecting reduction per pass and, consequently, the roll forces
involved. This, in turn, will also affect the actual roll gap and roll deflections. Complex control systems have been developed for monitoring and controlling such operations at high rolling speeds. l4.l. What is the difference between cold, warm, and hot forging?
Cold Forging is used in the production of parts that are symmetrical and weigh 25 lbs or less. The process is performed when the metal, usually carbon and standard alloy steels, at room temperature goes through impression die forging or true closed die forging. The dies used in cold forging are usually circular and require lubricant. The end products ar e draftless, closetolerance parts. Cold forging typically improves mechanical properties of the finished pieces, however the improvement isn’t necessarily useful in many applications. Warm Forging is a cost-effective method of forging that has become very popular as a manufacturing method. By using warm forging methods manufacturers see benefits such as lower tooling and press loads, increased steel ductility, and as-forged properties that eliminate heat treatment. Warm forging also eliminates the need to anneal prior to forging. The process is usually performed when steel is anywhere from just above room temperature to a temperature below recrystallization. More specifically the temperature is defined as 800 to 1,800 degrees Fahrenheit. Hot forging methods involve recrystallization and deformation of steel simultaneously. The temperature used for hot forging is specific to the recrystallization temperature of the metal being used. In some instances the metal and the dies are heated to the same temperature in a process known as isothermal forging. l4.7. Explain what is meant by “load limited,” “energy limited,” and “stroke limited” as these terms pertain to forging machines.
Load limited : a press stops if the load required exceeds its capacit y. Large amounts of energy can be transmitted to a workpiece by a constant load throughout a stroke-the speed of which can be controlled. Stroke limited : Mechanical presses are basically of either the crank or the eccentric type.The speed varies from a maximum at the center of the stroke to zero at the bottom of the stroke; thus, mechanical presses are stroke limited.
Energy limited : Screw presses derive their energy from a flywheel; hence, they are energy limited. The forging load is transmitted through a large vertical screw, and the ram comes to a stop when the flywheel energy is dissipated. If the dies do not close at the end of the cycle, the operation is repeated until the forging is completed. |4.l3. Describe the factors involved in precision forging
Precision forging melibatkan toleransi tinggi dan geometri rinci; ini hanya dapat dicapai dengan intricate dies (sehingga machining dan finishing dies biayanya akan tinggi) dan kekutan forging tinggi (yang memiliki efek buruk pada life die). Presisi biasanya dilakukan pada keadaan dingin, sehingga tidak ada panas ketegangan yang disebabkan warping, dan ini juga berarti kekuatan forging akan tinggi. Juga, pelumasan efektif menjadi perhatian, karena lapisan pelumas tebal dapat mengakibatkan bagian tidak mencapai bentuk die, dan juga dapat mengakibatkan orange peel. l4.l6. What are the advantages and limitations of (a) a cogging operation and (b) isothermal forging?
(a) Benda kerja memiliki sifat mampu bentuk yang lebih baik karena suhu yang tinggi, dan (b) Suhu dipertahankan karena tooling panas tidak melakukan panas dari benda kerja. Keterbatasan proses ini adalah kehidupan yang bisa dibilang rendah dies yang mahal ( yang membutuhkan kekuatan suhu tinggi dan ketahanan aus) karena suhu tinggi yang terlibat dan kesulitan dalam benar pelumas operasi isotermal forging. l4.20. Comment on your observations regarding the grainflow pattern in Fig. 14.12.
Jenis informasi yang diperoleh dari Gambar. 14.12 di p. 344 akan menjadi penting dalam situasi di mana daerah-daerah tertentu dari bagian ditempa harus dikenakan, misalnya, beban tinggi, memakai berlebihan, dan berdampak. Dalam kasus tersebut, setiap upa ya harus dilakukan agar bagian tersebut ditempa sedemikian rupa sehingga daerah-daerah memperoleh sifat-sifat akhir yang diinginkan.
l4.25. Calculate the forging force for a solid, cylindrical workpiece made of 1020 steel that is 90 mm high and 125 mm in diameter and is to be reduced in height by 30%. Let the coefficient of friction be 0.15.
Penempaan kekuatan untuk memulai memberikan hasil dalam materi: Y = 294 MPa yang diperoleh dari Tabel 5.2 di p. 139 (dengan asumsi bahwa benda tersebut anil), r = 62,5 mm, h = 90 mm yaitu
1+ 20,1562.5 = (1+ 2 ) = 294 62, 5 3ℎ 3 90 = 3,86
