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Iron Carbon Phase diagram and TTT diagrams Questions and Answers Graded A+
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Iron Carbon Phase diagram and TTT diagrams
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Iron Carbon Phase diagram and TTTdiagrams
how to distinguish between steels and cast iron - answer- steels contain les than 1.5 wt
%C
- cast irons contain 2-6 wt% C
eutectic - answer alloys that we can see in the phase diagram that there is the lowest
melting point and that they get solidified straight from liquid phase to 2 solid phases
eutectoid reaction - answer- the point in an iron carbon phase diagram which is 0.8 wt%
C that we have the formation of pearlite at 723°C
- from austenite, we get ferrite and cementite alternating plates- this microstructure is
pearlite
formation of pearlite - answer- initial cementite nucleus forms at the gamma boundary
- then after the cementite plate has grown, α is now nucleated
- then after α plate has grown, we have a new cementite that starts nucleating again
- then new cementite is now nucleated with different orientation
- finally we have a new colony of cementite and α ferrite at advanced stage of growth
- from pure iron to 0.8 wt% C we get the lamellar structure of pearlite (black areas are
impurities and grey areas are α ferrite
temperature transformation diagrams - answer- plots time for transformation against
temperature
- what determines when martensite will form is the cooling rate (rapid)
- the temperature at which the transformation occurs depends on the carbon content
- in order to form martensite we must prevent the eutectoid reaction whilst cooling
austenite to room temperature
bainite - answer- at cooling rates too fast for pearlite to form but too slow for martensite
to form we get a microstructure called bainite
- the bainite microstructure contains fine aggregates of ferrite plates (or laths) and
cementite particles
- the phases present in bainite are the same as those formed in pearlite
- it is a very refined microstructure compared to pearlite that is both strong and tough
bainite and tempered martensite - answerboth have similar structures and properties
but:
- tempered martensite has potentially higher toughness as the lath size is finer
- bainite needs isothermal cooling, it is sometimes difficult to achieve bainite structure
- bainites don't generally need to temper so there is a potential energy saving
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