Metallurgy and manufacture Flashcards
What are the main phases of steel in the iron-carbon phase diagram?
ferrite BCC
austenite FCC
cementite
ferrite has what kind of structure?
Body centred cubic
austenite has what kind of structure?
face centred cubic
ferrite has what kind of structure?
Body centred cubic
What do I mean when I say iron is allotropic?
can have different structures under different conditions
Steel goes from [] -> [] structure when melted at []C
BCC -> FCC
912C
the solubility of carbon in ferrite is how low?
0.02 wt %
solubility of carbon in austenite is much more than ferrite at . . .
2.1 wt %
As you cool down in the ferrite austenite phase reigon, an increasing amount of [] is produced.
ferrite can’t contain carbon, so it must move from [] to []
ferrite
ferrite to austenite
The three types of microstructure are . . .
They are defined by . . .
hypo eutectoid <0.8wt% C
eutectoid 0.8wt% C
hyper eutectoid >0.8wt% C
defined by their level of C relative to the eutectoid microstructure
What phases are present in a hypoeutectoid steel microstructure?
firstly, austenite
ferrite precipitates a steel cools, carbon content of austenite increases
at 723C austenite has a eutectoid structure
below 723C austenite cools into pearlite ( a ferrite cementite compound)
remember how eutectoid reactions simultaneously produce two solid phases? ta da
What phases are present in a eutectoid steel microstructure?
firstly, austenite
at 723C austenite has a eutectoid structure, and instantly becomes pearlite
pearlite is a ferrite cementite compound
What phases are present in a hypereutectoid steel microstructure?
firstly, austenite
then a phase of austenite + cementite, as cementite precipitates at grain boundaries.
Carbon moves from austenite to cementite
past 723, remaining austenite has a eutectoid structure and become pearlite
pearlite is a ferrite cementite compound
What type of cooling do phase diagrams represent?
slow cooling that produces stable phases
what may form in rapid cooling?
metastable phases
if cooling is too rapid for carbon to diffuse out of [] to [], we get []
out of ferrite
to cementite
we get martensite
what is martensite? what does it contain?
a steel phase formed through rapid cooling
it contains excess carbon
What is the structure of martensite?
BCT body centred tetragonal (4.3% larger in volume)
The microstructure of martensite can be described as
hard, brittle, ceramic-like
many interfaces
high, strong varying local stresses
To make the overly brittle + hard martensite useful, we can heat treat it at which temperature in order to allow what to diffuse out?
200-600C
Carbon diffuses out
The tempering of martensite produces what?
ferrite and cementite in a uniform distribution
all with high strength and toughness
pearilte has a fine lamellar (many layers) structure of alternating layers of [] and []
ferrite and cementite
many binary alloys of Fe & C with 0.8wt% C also contain:
manganese 0.45 -0.9 %
phosphorous 0.025 - 0.6 %
sulphur 0.03 - 0.05%
Hypereutectoid steel may have improved properties through
heat treatment
