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35.8: Questions

  • Page ID
    34955
  • Three low alloy steels, which differ only in their carbon content (0.1, 0.3 and 0.7 wt% carbon) are characterised using the Jominy end quench test. Select the plot of hardness variation along the test specimen that best describes their behaviour.

    Plots of hardness against distance

    a Plot (a)
    b Plot (b)
    c Plot (c)
    d Plot (d)
    Answer

    C. Increasing the carbon content increases the hardenability (depth of hardening) and also the hardness of the untempered martensite. However, at high carbon levels (0.7%), the martensite can auto-temper, resulting in a softer microstructure.

    Two specimens of a low alloy steel with 0.3wt% carbon are characterised using the Jominy end quench test. One was austenitised at 950°C, and the other was austenitised at 1100°C. Select the plot of hardness variation along the test specimen that best describes their behaviour.

    Plots of hardness against distance

    a Plot (a)
    b Plot (b)
    c Plot (c)
    d Plot (d)
    Answer

    D. Increasing the grain size with the high temperature anneal has two effects. It increases the hardenability (hardness in depth) by reducing the nucleation rate of ferrite at grain boundaries, which encourages martensite formation, but decreases the hardness of the non-martensitic microstructure (ferrite and pearlite) due the Hall-Petch effect.

    Three medium carbon steels (0.3wt%) that differ only in their Chromium content (0.25, 0.5 and 1 wt%) are characterised using the Jominy end quench test. Select the plot of hardness variation along the test specimen that best describes their behaviour.

    Plots of hardness against distance

    a Plot (a)
    b Plot (b)
    c Plot (c)
    d Plot (d)
    Answer

    B. Increasing the Cr content increases hardenability (hardness in depth) by reducing the diffusion controlled transformation rate of ferrite and pearlite and thus encouraging martensite formation. There is also a small increase in strength due to solid solution strengthening by the Cr.

    You have three steels. Select the most appropriate steel to achieve the necessary levels of mechanical properties, residual stress and distortion in a 1mm diameter wood-working drill.

    a 1% C, 0.4% Si, 1% Mn, 5% Cr, 1% Mo
    b 0.4% C, 0.4% Mn, 0.3% Si
    c 0.5% C, 4% Cr, 6% Mo
    Answer

    B. A low hardenability steel (cheap) suitable for water quenching small component with transformation to martensite across the full section.

    Again, you have three steels. Select the most appropriate steel to achieve the necessary levels of mechanical properties, residual stress and distortion in an injection moulding die for a mobile phone plastic case.

    a 1% C, 0.4% Si, 1% Mn, 5% Cr, 1% Mo
    b 0.4% C, 0.4% Mn, 0.3% Si
    c 0.5% C, 4% Cr, 6% Mo
    Answer

    A. An oil-hardening steel (moderately high hardenability), suitable for transforming to martensite through thickness with a slow quench, which reduces the level of distortion in the mould. This is essential for high precision injection mouldings.

    Again, you have three steels. Select the most appropriate steel to achieve the necessary levels of mechanical properties, residual stress and distortion in a tool for high speed milling of steel components.

    a 1% C, 0.4% Si, 1% Mn, 5% Cr, 1% Mo
    b 0.4% C, 0.4% Mn, 0.3% Si
    c 0.5% C, 4% Cr, 6% Mo
    Answer

    C. A high hardenability steel, which maintains its hardness at high operating temperatures when tempered (stable, secondary hardening Cr-Mo carbides).

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