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Engineering LibreTexts

17.8: Questions

  • Page ID
    31625
  • Quick questions

    You should be able to answer these questions without too much difficulty after studying this TLP. If not, then you should go through it again!

    What would happen to a brittle metal such as white cast iron, if it were formed by closed die forging?

    Answer

    It would shatter when the load was applied at high loading rate, and the final object would be composed of shards pushed together � it would not be strong enough.

    Discuss how the metal pieces of this article were made.

    Answer

    The walls of the filing cabinet were made by rolling. The components of the lock were forged. The rails inside (that hold the drawers) were extruded.

    Deeper questions

    The following questions require some thought and reaching the answer may require you to think beyond the contents of this TLP.

    The melting temperature of a low-carbon steel is 1534°C. Above what temperature can we use hot working to form it, and why?

    Answer

    Hot working requires the temperature to be >0.6Tm. In the case of this steel, Tm is 1807 K so 0.6 Tm is 1084.2 K � hot working can be performed above this temperature. Above this temperature, recovery, recrystallisation and grain growth occur as the deformation process is happening. It is also interesting to note that the range of temperature in which hot working can occur is mostly in the austenite field. Once a sample of this steel has been hot worked for long enough for the transformation to take place, it is austenitic and will again undergo the ferritic transformation on cooling to room temperature. The properties of the steel will change between the end of hot working and the piece�s use.

    What processes could be used to make this shape? What else could affect the choice of process?

    Answer

    Deep drawing, closed die forging and open die forging could be used. The cylindrical part could also be extruded and the bottom welded on afterwards.

    The choice of process could be influenced by:

    • The size of the shape; a small cup could be made by deep drawing, a 10m diameter tank must be open die forged.
    • The metal used to make the shape; some metals are better suited to processes The application the shape will be used for; the work hardening exerted during cold working processes may make the shape more suitable to its final purpose (e.g. if it were a pressurised tank).

    At 30 ms–1, what weight of 1 mm diameter copper wire could be drawn in an hour? The density of copper is 8920 kg m–3.

    Answer

    One metre of the wire has volume of π × r2 × 1 m = 7.85 × 10-7 m3. To find the mass per metre, we multiply by the density to get 7.01 × 10-3 kg per metre of wire. If 30 metres are drawn in a second, then 30 × 60 × 60 = 108000 metres are drawn in an hour. Thus 757 kg of wire can be drawn per hour.

    Do you think steel reinforcing bars for concrete are drawn or extruded?

    Answer

    They are extruded. If they were drawn they would end up coiled on a large drum; they would then need to be unrolled and cut into lengths at the site. This would be difficult because they are by necessity very stiff. If they are extruded they can be cut as they emerge and this makes them easier to make into buildings.

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