Week 13 - TTT diagrams, martensite and
bainite as well as hardenability
If a eutectoid steel is quenched (avoiding the nose of the TTT curve) to a temperature
below the martensitic finish temperate the microstructure will consist of? – answer
Martensite
Pearlite transformation - answer This transformation in steels involves the movement of
both iron and carbon atoms
A thermal reaction - answer The extent of the martensite reaction is found to be virtually
independent of time and only dependent on temperature, i.e. the fraction of martensite
on transformation from austenite is given by: 1-(Va, fraction of martensite)=beta
constant*(Ms, Martensite start temperature - temperature at or below Ms)
Fully hardened (i.e, quenched) carbon steel - answer Relatively brittle, and the
presence of residual stresses makes it use in this condition inadvisable unless extreme
hardness is required. For these reasons, it is usual to reheat, the hardened component,
so that stresses are relieved, and, at the same time, brittleness and extreme hardness
are reduced, i.e, tempering.
Metastable (non-equilibrium phases) - answerMartensite, lower bainite and upper are
non-equilibrium phases.
Increasing austenite grain size with high temperature annealing - answerIncreases
harden ability (hardness in depth) by reducing nuclear ion rate of ferrite at grain
boundaries. This encourages martensite formation but decreases hardness of the non-
martensitic microstructure (ferrite and pearlite). This is because there is less
inculcations sites for Pearlie and ferrite leading to a larger size which in turns reduces
strength owing to the Hall-patch effect.
Reconstructive transformation - answerDiffusion of all atoms during nucleation and
growth. Atoms movements from the parent to the product lattice sites occur by random
diffusional jumps.
Displacing transformation - answerInvolves cooperative movements of large numbers of
atoms in a diffusion-less process i.e. there is no change in chemical composition at any
point. Large shear deformation component.
Martensite (FeC) - answerHigh strength but brittle
Bainite - answerGood strength, not as hard as martensite, ductile and tough
bainite as well as hardenability
If a eutectoid steel is quenched (avoiding the nose of the TTT curve) to a temperature
below the martensitic finish temperate the microstructure will consist of? – answer
Martensite
Pearlite transformation - answer This transformation in steels involves the movement of
both iron and carbon atoms
A thermal reaction - answer The extent of the martensite reaction is found to be virtually
independent of time and only dependent on temperature, i.e. the fraction of martensite
on transformation from austenite is given by: 1-(Va, fraction of martensite)=beta
constant*(Ms, Martensite start temperature - temperature at or below Ms)
Fully hardened (i.e, quenched) carbon steel - answer Relatively brittle, and the
presence of residual stresses makes it use in this condition inadvisable unless extreme
hardness is required. For these reasons, it is usual to reheat, the hardened component,
so that stresses are relieved, and, at the same time, brittleness and extreme hardness
are reduced, i.e, tempering.
Metastable (non-equilibrium phases) - answerMartensite, lower bainite and upper are
non-equilibrium phases.
Increasing austenite grain size with high temperature annealing - answerIncreases
harden ability (hardness in depth) by reducing nuclear ion rate of ferrite at grain
boundaries. This encourages martensite formation but decreases hardness of the non-
martensitic microstructure (ferrite and pearlite). This is because there is less
inculcations sites for Pearlie and ferrite leading to a larger size which in turns reduces
strength owing to the Hall-patch effect.
Reconstructive transformation - answerDiffusion of all atoms during nucleation and
growth. Atoms movements from the parent to the product lattice sites occur by random
diffusional jumps.
Displacing transformation - answerInvolves cooperative movements of large numbers of
atoms in a diffusion-less process i.e. there is no change in chemical composition at any
point. Large shear deformation component.
Martensite (FeC) - answerHigh strength but brittle
Bainite - answerGood strength, not as hard as martensite, ductile and tough
