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Strength

  Effect of precipitates     4 of 5
 
Most precipitates in steels are relatively large (compared to GP zones in Al-Cu alloys for example) and strong so dislocations have to travel between them rather than cut through them. This is called Orowan bowing. The effectiveness of the precipitates in the steel depend on their composition (as this controls their thermodynamic stability), size, volume fraction and distribution.

Have a go!Slowly increase the applied force on the dislocation by holding the button down and see how a dislocation bows around precipitates of different sizes and spacing.  Plot a graph of precipitate bowing stress v average separation.

Control of the precipitate type, size and distribution can be achieved in plate products using controlled reheating, rolling and cooling schedules and careful alloying additions. The typical alloying additions that are used to generate precipitate strengthening are Ti, V, Nb and Al. These additions can be made individually but more commonly are made in combination depending on the requirements of strength, toughness etc. from the customer. The reason that these additions are used is that their precipitates (carbides and nitrides) have high thermodynamic stability at elevated temperatures which means they also give rise to grain refinement during processing in addition to strengthening at room temperature.  (Find out more about the grain refinement...).

An example of how alloying additions made in combination affect the strength (and toughness) is given below for Al and V additions to a C-Mn ferrite + pearlite steel. The formation of AlN particles causes grain refinement on heat treatment and some strengthening and the VC particles provided strengthening. Note that the toughness decreases as the strength increases except for the strengthening effect of grain refinement which is also beneficial to toughness.

  • Nb is used as a microalloying addition to control grain size and provide some precipitation strengthening.
  • V is used to provide precipitation strengthening. The strengthening that is obtained by V precipitates will depend upon their size but a guideline is that for every 0.1 wt% V addition an increase of 50 - 60 MPa is seen in strength.
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