UNIVERSITI MALAYSIA PAHANG
BMM3643 (Sem II 2012/13)
a) Define the various regimes of cold working,warm working,and hot working in terms of the melting point of the material being formed. b) Indicate some of the advantages of cold working relative to warm and hot working. c) What are some of the negative aspects of hot working? (8 marks)
a) The temperatures required for hot working generally exceed 0.6 times the melting point of the material on an absolute temperature scale. Cold working generally requires temperatures below 0.3 times the melting point, and warm working is the transition region, between 0.3 and 0.6 times the melting point. b) Advantages of cold working are;
i) better accuracy,
ii) better surface finish,
iii) increased strength due to work hardening,
iv) possible directional properties due to grain flow, and v) no heating of work required.
c) Disadvantages associated with hot working involve the reactions which may be promoted by elevated temperature, such as rapid oxidation. Tolerances are poorer and the metallurgical structure will be nonuniform if the amount of deformation or thermal history varies throughout the product.
a) What is the difference between open-die and impression-die forging? b) Explain the reasons why the flash assists in die filling, especially in hot forging. c) Why are heated dies generally employed in hot-press forging operations? (8 marks)
a) Open-die forging does not confine the flow of metal in all directions, so the final shape is dependent upon the manipulation and skill of the equipment operator. Impression-die forging operations confine metal flow in all directions to provide good repeatable control of size and shape. b) The flash is excess metal which is squeezed out from the die cavity into the outer space between the two dies. The flash cools faster than the material in the cavity due to the high a/h ratio and the more intimate contact with the relatively cool dies. Consequently, the flash has higher strength than the hotter workpiece in the die cavity and, with higher frictional resistance in the flash gap, provides greater resistance to material flow outward through the flash gap. Thus, the flash encourages filling of complex die cavities. c) Heated dies are usually employed in press forging because the long time of die contact with the hot workpiece would otherwise permit considerable surface cooling and could produce cracking of the surface.
d) List some of the products produced on a rolling mill. e) In rolling of steel, what are the differences between a bloom, a slab, and a billet? f) Rolling may be described as a continuous forging operation. Is this description appropriate? Explain. (8 marks)
a) Rolled products include flat sheet and plate stock, round bar and rod stock, rails, structural shapes such as Ibeams and channels. b) A bloom is a rolled steel workpiece with a square cross section of about 150 mm by 150 mm. The starting work unit for a bloom is an ingot heated in a soaking pit. A slab is rolled from an ingot or a bloom and has a rectangular cross section of about 250 mm by 40 mm. A billet is rolled from a bloom and has a square cross section of about 40 mm by 40 mm. c) This is a good analogy. Consider the situation of forging a block to a thinner cross section through increments (as in incremental forming). As the number of stages increases, the operation eventually approaches that of the strip profile in rolling.
g) Distinguish between direct and indirect extrusion.
h) What is centerburst defect? How would you go about preventing centerburst defects in extrusion?
i) What are some of the attractive features of the extrusion process? (8 marks)
a) In direct...