|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.
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.