Pearlite Flashcards
Ferrite contains __wt% carbon
0.02
Cementite contains __wt% carbon
6.67
Ferrite is ____, while cementite is _____
soft, ductile; hard, brittle
Pearlite contains ____wt% ferrite and ___wt% cementite
88; 12
Because ferrite and cementite have similar densities, ferrite lamellae are ______ than cementite lamellae
seven times thicker
Steel with finer inter-lamellar spacing has ____ hardness
higher
How does hypo-eutectoid steel transform to pearlite?
pro-eutectoid ferrite forms at GB, austenite transforms to pearlite
How does hyper-eutectoid steel transform to pearlite?
pro-eutectoid cementite forms at GB, austenite transforms to pearlite
Does the carbon content of pearlite and ferrite change change with carbon content of steel?
no, the relative amounts do
As CC increases in steel, relative amount of pearlite ___ and amount of ferrite _____
increases; decreases
Hardness and strength of steel _____ with ____ CC of steel because the relative amount of pearlite _____ with CC of steel
increases; increases; increases
Higher the austenite GB area/vol and finer the grain sizer, the higher the ____ ____ of pearlite
nucleation rate
Why does the nucleation rate for pearlite have a maximum at an intermediate temperature?
> free energy change ∆Gv (driving force) of the eutectoid transformation is zero at eutectoid point and gets more negative at lower temperatures
nucleation requires atomic jumps of C, and the diffusion coefficient of C decreases exponentially as temp decreases
nucleation rate is zero at eutectoid temp and rapidly decreases at very low temp due to diffusion coefficient
Growth rate of pearlite is ______ of austenite grain size
independent
Growth rate of pearlite is _____ _____ to inter-lamellar spacing. Interlamellar spacing _____ as transformation temperature decreases, which ______ the growth rate
inversely proportional; decreases; increases
Growth rate is ____ at eutectoid temperature as Cα-Ccm (diffusion flux) is zero. The diffusion flux Cα-Ccm ______ as temperature decreases, which increases growth rate.
zero; increases
Diffusion rate of C _____ exponentially with temp, which ______ growth rate at low temp
decreases; decreases
Why aren’t the curves for pearlite initiation and completion shown below 540C?
The austenite to bainite transformation takes over at lower temperatures
What variables affect the IT curve for steel?
> austenite grain size: finer austenite grains, higher nucleation rate of pearlite, and IT curves shift to LEFT
alloys: most alloys slow kinetics and shift IT curves to RIGHT
Co alloys increases nuc and growth and shift curves to LEFT
What variables affect the IT curve for hypo and hyper steel?
> austenite grain size: pro-eutectoid ferrite, cementite, and pearlite heterogeneously nucleate on austenite GB, IT curves shift to LEFT
CC: carbon is an austenite stabilizer, increase in CC shifts IT curves to the RIGHT
alloys: Mn, Cr, V and W slow transformation and shift IT curves to the RIGHT. Co increases rates of nuc and growth and shift IT curves to the LEFT
Why aren’t isothermal TTT diagrams applicable to steel? Why do we use continuously cooling diagrams?
Because steel is continuously cooled from austenitic temperature, so the transformation occurs over a range of temperatures rather than isothermally