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    Grain Size

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    Hoang
    Decreasing the grain size of steel increases both its strength and toughness by lowering the ductile-brittle transition temperature. This is because smaller grain sizes provide more barriers to crack propagation. Compositional changes can also affect toughness, either by altering the microstructure, such as increasing pearlite content, or through other mechanisms like alloying elements. Controlling alloying additions to form fine precipitates during processing helps refine the grain size in the final steel product.

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    Grain Size

    Uploaded by

    Hoang
    23 views3 pages
    Decreasing the grain size of steel increases both its strength and toughness by lowering the ductile-brittle transition temperature. This is because smaller grain sizes provide more barriers to crack propagation. Compositional changes can also affect toughness, either by altering the microstructure, such as increasing pearlite content, or through other mechanisms like alloying elements. Controlling alloying additions to form fine precipitates during processing helps refine the grain size in the final steel product.

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    Grain size

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    GRAIN SIZE

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    Decreasing the grain size of steel increases both its strength and toughness by lowering the ductile-brittle transition temperature. This is because smaller grain sizes provide more barriers to crack propagation. Compositional changes can also affect toughness, either by altering the microstructure, such as increasing pearlite content, or through other mechanisms like alloying elements. Controlling alloying additions to form fine precipitates during processing helps refine the grain size in the final steel product.

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    © All Rights Reserved
    Download as DOCX, PDF, TXT or read online from Scribd
    Download as docx, pdf, or txt
    23 views3 pages

    Grain Size

    Uploaded by

    Hoang
    Decreasing the grain size of steel increases both its strength and toughness by lowering the ductile-brittle transition temperature. This is because smaller grain sizes provide more barriers to crack propagation. Compositional changes can also affect toughness, either by altering the microstructure, such as increasing pearlite content, or through other mechanisms like alloying elements. Controlling alloying additions to form fine precipitates during processing helps refine the grain size in the final steel product.

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    GRAIN SIZE – STEEL

    Decreasing the grain size increases the toughness of a steel, i.e. decreases the ductile-brittle transition
    temperature (DBTT), as can be seen in the diagram below for predominantly ferritic steels.

    In fact decreasing the grain size is the only mechanism by which both the strength and the toughness of a steel is
    increased. Other mechanisms of increasing strength lead to a decrease in toughness.

    To control the ferrite grain size two different approaches can be used (separately or in combination)

    The effect of microstructure on toughness is linked to the effect of composition since generally the composition
    alters the microstructure, however, there are some compositional changes that have only minor effects on the
    microstructure and yet affect toughness, and these are dealt with separately under compositional effects.

    For example increasing the amount of pearlite in a ferritic steel will change the strength and toughness of the
    material

    Select a carbon level from the drop-down list to see how changing the amount of pearlite affects the ductile-brittle
    transition (DBTT) curve of a ferrite-pearlite steel.

    This effect, along with other compositional changes, can be related to the DBTT by the equation below for ferrite
    and pearlite steels:

    DBTT = -19 + 44 (%Si) + 700 (%Nf1/2) + 2.2 (%pearlite) - 11.5 (d-1/2)

    - Where d is the grain size in mm and Nf is the free nitrogen content in the steel.
    Alloying elements, which do not have a significant effect on microstructure, can have an effect on toughness:

     Ni additions decrease the Ductile-Brittle Transition Temperature (DBTT). Small additions can be made
    to an alloy to increase toughness to meet customer specifications. Unfortunately Ni is expensive so
    there is a cost penalty.
     Nb, V, Ti additions tend to increase the DBTT - they increase the strength of the alloy by precipitate
    strengthening but decrease the toughness.
     Si, Mn additions tend to increase the DBTT - they strengthen the alloy by solid solution strengthening
    but decrease the toughness.

    CONTROLED ALLOYING ADDTION

    Alloying additions of fine precipitate forming elements can be used to refine the grain size of a steel. A fine
    precipitate distribution in a steel will restrict the growth of austenite grains at high temperature and will retard
    recrystallization of deformed austenite grains. If the combination of precipitates and rolling schedule is used then
    a 'pancake' structure of deformed austenite grains is created during rolling. These deformed grains provide
    many nucleation sites for subsequent ferrite formation resulting in a fine grained ferritic structure in the final steel
    product. The choice of alloying elements is important as the precipitates must be stable at the high temperatures
    of processing in order to pin the grain boundaries. The typical alloying additions used in High Strength Low Alloy
    steels (HSLA) are Nb, Al, Ti and / or V. They can be added in isolation or in combination with one another.

    Using the diagram above which alloying element gives you the most thermodynamically stable precipitate at
    1150°C?

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