19. Heat Treatment of Metals

Question 1

Give three reasons why we heat treat materials

Answer 1

1. to relieve stresses caused by dislocations and grain boundaries
2. to produce specific properties by manipulating the micro structure
3. to use non-equilibrium heating and cooling to create micorstructures which do no appear on the phase diagram

Question 2

Define Austenitize

Answer 2

heat to a temperature region where 100% austenite is formed

Question 3

Define quench

Answer 3

to rapidly cool in water or oil

Question 4

Define tempering

Answer 4

after quenching, reheating to decrease hardness and regain some ductility

Question 5

Cooling to temperatures farther from the euctectoid temperature result in rapid cooling and therefore _____ transformation times

Answer 5

faster

Question 6

What does TTT stand for?

Answer 6

Time-Temperature Transformation

Question 7

Why does a faster cooling rate generate finer pearlite?

Answer 7

faster cooling rate = shorter time for transformation to occur = less diffusion can happen = thinner layers

Question 8

The microstructure of bainite is _____-like

Answer 8

needle

Question 9

Bainite is a mix of ____ and ____

Answer 9

ferrite and cementite

Question 10

What is the name of the phase of the needle like structures in bainite?

Answer 10

Cementite

Question 11

When does bainite form instead of pearlite?

Answer 11

when the quenching temperature is sufficiently low and therefore the cooling rate is sufficiently fast

Question 12

How is martensite formed?

Answer 12

cooled very rapidly such that no diffusion can occur

Question 13

Martensite is ____ and ___

Answer 13

hard and brittle

Question 14

Martensite is stuck part way between FCC and BCC. This structure is called ____ ____ ____ (___)

Answer 14

Body Centered Tetragonal (BCT)

Question 15

Martensite is thermodynamically _____

Answer 15

unstable

Question 16

Martensite is the ____, and the _____ but consequently also the most ____ micro structure of steel

Answer 16

hardest, and the strongest

also the most brittle

Question 17

What is tempering?

Answer 17

reheating a material to a moderate temperature after quenching to regain ductility and toughness

Question 18

Do tempered and untempered martensite have the same microstructure?

Answer 18

no

Question 19

Is the formation of tempered martensite diffusionless like the untempered martensite?

Answer 19

no, it is a diffusion governed process

Question 20

Tempering ____ yield and ultimate tensile strength

Answer 20

decreases

Question 21

tempering _____ ductility and toughness

Answer 21

increases

Question 22

What unit cell does martensite have?

Answer 22

BCT

Question 23

what unit cell does tempered martensite have?

Answer 23

BCC

Question 24

What phases are present in tempered martensite

Answer 24

very small cementite precipitates in a alpha matrix

Question 25

What is the microstructure formed by tempering pearlite?

Answer 25

spheroidite

Question 26

How does the microstructure of pearlite change to become spheroidite?

Answer 26

Cementite layers become spheres in alpha matrix

Question 27

are the cementite spheres larger in spheroidite or in tempered martensite?

Answer 27

They are larger in spherodite

Question 28

Which is stronger: pearlite or spheroidite? Why?

Answer 28

Pearlite is stronger because it have more grain boundary area