19. Heat Treatment of Metals Flashcards
Give three reasons why we heat treat materials
- to relieve stresses caused by dislocations and grain boundaries
- to produce specific properties by manipulating the micro structure
- to use non-equilibrium heating and cooling to create micorstructures which do no appear on the phase diagram
Define Austenitize
heat to a temperature region where 100% austenite is formed
Define quench
to rapidly cool in water or oil
Define tempering
after quenching, reheating to decrease hardness and regain some ductility
Cooling to temperatures farther from the euctectoid temperature result in rapid cooling and therefore _____ transformation times
faster
What does TTT stand for?
Time-Temperature Transformation
Why does a faster cooling rate generate finer pearlite?
faster cooling rate = shorter time for transformation to occur = less diffusion can happen = thinner layers
The microstructure of bainite is _____-like
needle
Bainite is a mix of ____ and ____
ferrite and cementite
What is the name of the phase of the needle like structures in bainite?
Cementite
When does bainite form instead of pearlite?
when the quenching temperature is sufficiently low and therefore the cooling rate is sufficiently fast
How is martensite formed?
cooled very rapidly such that no diffusion can occur
Martensite is ____ and ___
hard and brittle
Martensite is stuck part way between FCC and BCC. This structure is called ____ ____ ____ (___)
Body Centered Tetragonal (BCT)
Martensite is thermodynamically _____
unstable
Martensite is the ____, and the _____ but consequently also the most ____ micro structure of steel
hardest, and the strongest
also the most brittle
What is tempering?
reheating a material to a moderate temperature after quenching to regain ductility and toughness
Do tempered and untempered martensite have the same microstructure?
no
Is the formation of tempered martensite diffusionless like the untempered martensite?
no, it is a diffusion governed process
Tempering ____ yield and ultimate tensile strength
decreases
tempering _____ ductility and toughness
increases
What unit cell does martensite have?
BCT
what unit cell does tempered martensite have?
BCC
What phases are present in tempered martensite
very small cementite precipitates in a alpha matrix
What is the microstructure formed by tempering pearlite?
spheroidite
How does the microstructure of pearlite change to become spheroidite?
Cementite layers become spheres in alpha matrix
are the cementite spheres larger in spheroidite or in tempered martensite?
They are larger in spherodite
Which is stronger: pearlite or spheroidite? Why?
Pearlite is stronger because it have more grain boundary area
