3. and 4. - incorrect, Mn and
Mo do have a lower diffusivity in steel compared
to C and N however this has little effect on the
yield strength which depends on dislocations breaking
away from pinning sites rather than atoms diffusing
to pin the moving dislocations. There is an effect
in some steels (for example strip steels) where
the C and / or N atoms do diffuse and pin previously
mobile dislocations thereby raising the yield
strength - this can occur during a controlled
low temperature heat treatment (˜150&#deg;C)
such as for the paint bake hardened automotive
strip steels. Try again.
1. only - incorrect in that this
is not the only reason why C and N affect the
yield strength much more than Mn and Mo. Try again.
2. only - incorrect, C and N
atoms are much smaller than Mn and Mo atoms. Try
again.
1. and 3. - correct, interstitial
elements tend to produce a greater strengthening
effect because they cause more strain in the lattice
than substitutional elements in steels. The fact
that C and N have a smaller atomic mass number
does mean that for the same weight % addition
there are more C or N atoms introduced into the
steel and hence more lattice strain.
4. only - incorrect, Mn and Mo
do have a lower diffusivity in steel compared
to C and N however this has little effect on the
yield strength which depends dislocations breaking
away from pinning sites rather than atoms diffusing
to pin the moving dislocations. There is an effect
in some steels (for example strip steels) where
the C and / or N atoms do diffuse and pin previously
mobile dislocations thereby raising the yield
strength - this can occur during a controlled
low temperature heat treatment (˜150&#deg;C)
such as for the paint bake hardened automotive
strip steels. Try again.
Try again.
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of the answer on your notepad.
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NOTEPAD
Well done!
1. C and N are interstitial elements rather than substitutional. 2. C and N are much bigger atoms hence introduce more
strain energy. 3. C and N have a much smaller atomic mass number therefore
for the same weight % addition you are adding a higher atomic
%. 4. Mn and Mo have much lower diffusivities in steel
therefore cannot pin the moving dislocations.
Select
the correct statements from the following list.
3. and 4. - incorrect, Mn and
Mo do have a lower diffusivity in steel compared
to C and N however this has little effect on the
yield strength which depends on dislocations breaking
away from pinning sites rather than atoms diffusing
to pin the moving dislocations. There is an effect
in some steels (for example strip steels) where
the C and / or N atoms do diffuse and pin previously
mobile dislocations thereby raising the yield
strength - this can occur during a controlled
low temperature heat treatment (˜150&#deg;C)
such as for the paint bake hardened automotive
strip steels. Try again.
1. and 3. - correct, interstitial
elements tend to produce a greater strengthening
effect because they cause more strain in the lattice
than substitutional elements in steels. The fact
that C and N have a smaller atomic mass number
does mean that for the same weight % addition
there are more C or N atoms introduced into the
steel and hence more lattice strain.
