Iron and steel making

Question 1

What does cementite stabilise to?

Answer 1

Alpha and graphite

Question 2

What crystal structure is austenite?

Answer 2

Face centre cubic

Question 3

What crystal structure is martensite?

Answer 3

Body centre tetragonal

Question 4

How is martensite formed?

Answer 4

Quenching prevents nucleation/ growth by diffusional transformations; produces phase in which atomic moements < interatomic spacing; forms super-saturated form of C in alpha ferrite; martensite grows in laths/ plates

Question 5

What are the mechanical properties of martensite?

Answer 5

High TS but brittle; tempering to ~600 Celsius allows some martensite to be relieved, and carbides are precipitated

Question 6

What are isothermal transformation diagrams?

Answer 6

Show to what extent transformations occur if a steel is held at one temperature; shows what happens with unlimited time, when all transformations are allowed to process to completion

Question 7

What is a TTT diagram?

Answer 7

Time, temperature, transformation

Question 8

What does the nose show on an isothermal transformation diagram?

Answer 8

Shows period of time and temperature of pearlite forming

Question 9

How is a TTT diagram determined?

Answer 9

By study of quenched-in microstructures, which reveal extent of the transformation

Question 10

What is CCT?

Answer 10

Continuous cooling transformation

Question 11

How is a CCT diagram determined?

Answer 11

From study of quenched-in microstructures

Question 12

What is annealing?

Answer 12

Leaving object in furnace after heating is finished and furnace is switched off; very slow cooling

Question 13

What is normalising?

Answer 13

Cooled in air after removing from furnace; cooled by convection

Question 14

Why is oil quenching slower than water quenching?

Answer 14

Oil is very viscous, so not very effective at convection

Question 15

What is air quenching?

Answer 15

Forced air blast

Question 16

What does the critical cooling rate do?

Answer 16

Produces 100% martensite

Question 17

What happens at the martensite start?

Answer 17

Any pearlite remaining is transformed to martensite

Question 18

Is tit possible to have austenite and martensite at RTP?

Answer 18

Yes, small amounts of austenite might be present

Question 19

What is spheroidisation?

Answer 19

Steel held below A3 transformation temperature; Fe3C particles in pearlite will gradually form spheres, giving increased strength and ductility

Question 20

What are the implications of a greater cooling rate?

Answer 20

Greater risk of failure due to residual stress or excessive distortion (thermal strain); residual stresses arise from the thermal stresses induced by differential contraction within a part

Question 21

What alloying elements push nose to right, and which pushes to the left?

Answer 21

Mn and most TMs push nose to right, allowing some austenite to persist down to RTP; Co is abnormality and pushes to the left

Question 22

Where was ferrous metallurgy originated?

Answer 22

40-50,000 years ago: sea levels dropped; earthquake sealed off connection between Mediterranean and Black Seas. 20,000 years ago: glacial period ends; glaciers melt; sea levels rise; Black Sea remains same level. 5,600 years ago: another earthquake dislodges what was blocking passage; forms waterfall 120m tall; Mediterranean empties into Black Sea; took 50 years for seas to equilibrate; displaces people along River Danube; form Vinca culture at the Iron Gates; were protoliterate and invented ferrous metallurgy; carried knowledge up Danube into Western Europe

Question 23

What was likely the original method of extraction of ores?

Answer 23

Digging out of outcrops; open-cast mining; e.g. where a valley was carved out at end of last Ice Age, exposing intersected mineral bands

Question 24

What other ores contained useful amounts of iron ore?

Answer 24

Cu or Pb ores

Question 25

Why was iron smelting invented relatively late?

Answer 25

Due to the much lower temperature where reduction with CO takes place (800 Celsius)

Question 26

What is another word for slag?

Answer 26

Fayalite