Pearlite

Question 1

Ferrite contains __wt% carbon

Answer 1

0.02

Question 2

Cementite contains __wt% carbon

Answer 2

6.67

Question 3

Ferrite is ____, while cementite is _____

Answer 3

soft, ductile; hard, brittle

Question 4

Pearlite contains ____wt% ferrite and ___wt% cementite

Answer 4

88; 12

Question 5

Because ferrite and cementite have similar densities, ferrite lamellae are ______ than cementite lamellae

Answer 5

seven times thicker

Question 6

Steel with finer inter-lamellar spacing has ____ hardness

Answer 6

higher

Question 7

How does hypo-eutectoid steel transform to pearlite?

Answer 7

pro-eutectoid ferrite forms at GB, austenite transforms to pearlite

Question 8

How does hyper-eutectoid steel transform to pearlite?

Answer 8

pro-eutectoid cementite forms at GB, austenite transforms to pearlite

Question 9

Does the carbon content of pearlite and ferrite change change with carbon content of steel?

Answer 9

no, the relative amounts do

Question 10

As CC increases in steel, relative amount of pearlite ___ and amount of ferrite _____

Answer 10

increases; decreases

Question 11

Hardness and strength of steel _____ with ____ CC of steel because the relative amount of pearlite _____ with CC of steel

Answer 11

increases; increases; increases

Question 12

Higher the austenite GB area/vol and finer the grain sizer, the higher the ____ ____ of pearlite

Answer 12

nucleation rate

Question 13

Why does the nucleation rate for pearlite have a maximum at an intermediate temperature?

Answer 13

> 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

Question 14

Growth rate of pearlite is ______ of austenite grain size

Answer 14

independent

Question 15

Growth rate of pearlite is _____ _____ to inter-lamellar spacing. Interlamellar spacing _____ as transformation temperature decreases, which ______ the growth rate

Answer 15

inversely proportional; decreases; increases

Question 16

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.

Answer 16

zero; increases

Question 17

Diffusion rate of C _____ exponentially with temp, which ______ growth rate at low temp

Answer 17

decreases; decreases

Question 18

Why aren’t the curves for pearlite initiation and completion shown below 540C?

Answer 18

The austenite to bainite transformation takes over at lower temperatures

Question 19

What variables affect the IT curve for steel?

Answer 19

> 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

Question 20

What variables affect the IT curve for hypo and hyper steel?

Answer 20

> 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

Question 21

Why aren’t isothermal TTT diagrams applicable to steel? Why do we use continuously cooling diagrams?

Answer 21

Because steel is continuously cooled from austenitic temperature, so the transformation occurs over a range of temperatures rather than isothermally