TTT diagram Flashcards
Where is the TTT diagram valid?
Only for iron-carbon alloys of eutectoid composition.
How is the transformation rate related to temperature in a TTT diagram?
The transformation rate increases with increasing temperature.
At temperature right below eutectoid point - very long times. At 540 degrees only 10s to reach 50% completion (see diagram).
How does the thickness of the layers in the pearlite influence the strength of the material and how does that relate to temperature?
Thicker cementite layers (coarse perlite) - high temperatures - high diffusion rates - more cementite - stronger material.
Thinner cementite layers (fine perlite) - low temperatures - low diffusion rates - less cementite - more ductile material.
What is the relation between the phases in the TTT diagram (pearlite, bainite, martensite)?
The transformations are competitive with each other, once some portion of an alloy has transformed into either of the phases, transformation to the other micro-constituents is not possible without REHEATING THE MATERIAL TO THE AUSTENITE REGION.
What is bainite?
A micro-constituent.
Product of austenite transformation at rapid cooling - some degree of diffusion is possible.
Forms as plates or needle depending on temperature.
What is martensite?
A micro-constituent.
Formed at very fast cooling rates (quenching) to prevent carbon diffusion.
Because diffusion doesn’t occur the transformation is instant and time-independent. If any diffusion happens - the formation of ferrite and cementite.
Carbon atoms remain as interstitial impurities - they can transform rapidly if the temperature is raised enough to cause diffusion.
The crystal structure changes from FCC austenite to BCT martensite.
The grain size is dependent on the former austenite grain size because martensite forms in the same grains.
It forms like needles or plates.
Nucleation sites are at the grain boundaries.
It is a hard and brittle material.
Tempering is required to use martensite (too brittle).
How are cooling rates related to hardness?
Cooling slowly - not much hardness
Cooling rapidly - harder material - closer to martensite.
