Metallurgy and manufacture

Question 1

What are the main phases of steel in the iron-carbon phase diagram?

Answer 1

ferrite BCC
austenite FCC
cementite

Question 2

ferrite has what kind of structure?

Answer 2

Body centred cubic

Question 3

austenite has what kind of structure?

Answer 3

face centred cubic

Question 4

ferrite has what kind of structure?

Answer 4

Body centred cubic

Question 5

What do I mean when I say iron is allotropic?

Answer 5

can have different structures under different conditions

Question 6

Steel goes from [] -> [] structure when melted at []C

Answer 6

BCC -> FCC
912C

Question 7

the solubility of carbon in ferrite is how low?

Answer 7

0.02 wt %

Question 8

solubility of carbon in austenite is much more than ferrite at . . .

Answer 8

2.1 wt %

Question 9

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 []

Answer 9

ferrite

ferrite to austenite

Question 10

The three types of microstructure are . . .

They are defined by . . .

Answer 10

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

Question 11

What phases are present in a hypoeutectoid steel microstructure?

Answer 11

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

Question 12

What phases are present in a eutectoid steel microstructure?

Answer 12

firstly, austenite

at 723C austenite has a eutectoid structure, and instantly becomes pearlite

pearlite is a ferrite cementite compound

Question 13

What phases are present in a hypereutectoid steel microstructure?

Answer 13

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

Question 14

What type of cooling do phase diagrams represent?

Answer 14

slow cooling that produces stable phases

Question 15

what may form in rapid cooling?

Answer 15

metastable phases

Question 16

if cooling is too rapid for carbon to diffuse out of [] to [], we get []

Answer 16

out of ferrite

to cementite

we get martensite

Question 17

what is martensite? what does it contain?

Answer 17

a steel phase formed through rapid cooling

it contains excess carbon

Question 18

What is the structure of martensite?

Answer 18

BCT body centred tetragonal (4.3% larger in volume)

Question 19

The microstructure of martensite can be described as

Answer 19

hard, brittle, ceramic-like

many interfaces

high, strong varying local stresses

Question 20

To make the overly brittle + hard martensite useful, we can heat treat it at which temperature in order to allow what to diffuse out?

Answer 20

200-600C

Carbon diffuses out

Question 21

The tempering of martensite produces what?

Answer 21

ferrite and cementite in a uniform distribution

all with high strength and toughness

Question 22

pearilte has a fine lamellar (many layers) structure of alternating layers of [] and []

Answer 22

ferrite and cementite

Question 23

many binary alloys of Fe & C with 0.8wt% C also contain:

Answer 23

manganese 0.45 -0.9 %
phosphorous 0.025 - 0.6 %
sulphur 0.03 - 0.05%

Question 24

Hypereutectoid steel may have improved properties through

Answer 24

heat treatment

Question 25

Stainless steel has how much chromium?

Answer 25

11wt%

Question 26

Why do we put chromium in stainless steel?

Answer 26

forms chromium oxide layer which is corrosion resistant

Question 27

stainless steels are austenitic, what does this mean?

Answer 27

non magnetic

stabilise austenite phase to room temp

Question 28

what steps are involved in the production of molten steel?

Answer 28

limestone and iron ore sintered together

coal + carbon becomes coke

coke and iron+limestone are put into the blast furnace

the pig iron from the blast furnace is put into a basic oxygen converter to remove excess carbon from it.

meanwhile recycled steel is put through an electric arc furnace which can also be used in the next phase

After this, secondary steel making results in molten steel

Question 29

molten steel cools slowly in the shapes of

Answer 29

blooms
billets
slabs

Question 30

What specifically happens in the blast furnace?

Answer 30

(limestone+iron ore) + coke enter blast furnace at 200C

O2 in air reacts with coke, makes CO

Iron oxide becomes Pig iron (4-4.6wt% C) at 2000C

molten slag is extracted and then so is pig iron

Question 31

What specifically happens in a basic oxygen converter?

Answer 31

pig iron enters, along with O2 and N2 gas.

High speed and pressure O2 reacts with C to make CO2.

The excess carbon is now removed from pig iron, and we can now put the product through secondary steel making

Question 32

what specifically happens in the electric arc furnace?

Answer 32

pig iron or scrap steel is fed in

lid of arc furnace has electrodes

current goes through lid and material, forming an arc

O2 blown in whilst steel, lime fluorspar help form slag

furnace tilts to remove slag

molten steel tapped into a lade for secondary steel making

Question 33

How much steel can an electric arc furnace make in 90 minutes?

Answer 33

150Tonnes