Swords and iron

Question 1

What is the process of carburisation?

Answer 1

Heating in a C-rich atmosphere, e.g. from charcoal; diffusion of C from exterior to interior microstructure

Question 2

What does carburisation do to properties of the object?

Answer 2

Improves local mechanical properties; in combination with quenching, forms a (strong) martensitic exterior and (ductile) pearlite core

Question 3

What is an alternative to carburisation?

Answer 3

Nitridisation

Question 4

What are the three stages of the boiling curve?

Answer 4

Nucleate boiling, transition boiling, film boiling

Question 5

What happens during nucleate boiling?

Answer 5

Water penetrates surface flaws, but surface tension prevents from reaching apex of flaw; as temp increases, bubble becomes larger and is released; cold water collapses onto surface to replace the bubble; hot material against the cooler water allows for heat transfer by conduction/ convection

Question 6

What happens during transition boiling?

Answer 6

Vapour film forms and collapses

Question 7

What happens during film boiling?

Answer 7

Film vapour (~0.5cm) covers surface and insulates liquid from solid, so must use conduction as heat transfer

Question 8

What is heat flux?

Answer 8

Heat transfer rate

Question 9

Where is the critical heat flux?

Answer 9

At the top of the curve

Question 10

What is pattern welding?

Answer 10

A by-product of using welding sheets with different individual C content

Question 11

Why are swords not made of a homogeneous material?

Answer 11

Properties at edge should be different to at centre; thin sheets of carburised iron are homogeneous as no quenching occurs due to thinness, but welding different sheets together gives mechanical advantage

Question 12

What is an example of a heterogeneous pattern-welded sword’s composition?

Answer 12

Core has medium-C content (0.5%); either side of core backed with soft iron strip (0.1%C); core and sides are piled and twisted together; cutting edge welded to core and sides is plain C-steel (0.3%C)

Question 13

What is Damascening?

Answer 13

A method formed in Damascus; same as pattern welding but with finer and more steel strips; takes up to 75 hours and 128 reheats to make; uses layers of higher-C (cementite) steel or cast iron fragments, mixed with low-C bloom steels

Question 14

Outline the process of cementation.

Answer 14

Wrought iron bars into furnace; heat to 1100 Celsius; heat for a week; carbon diffuses into steel; blister steel forms (0.1% C); bars cut, forged, and reheated; more homogeneous composition of shear steel formed

Question 15

What is Huntsmans’ crucible steel?

Answer 15

Liquid Fe with high C

Question 16

What are two overarching methods of making steel?

Answer 16

Carburising wrought iron; de-carburising cast iron

Question 17

How was Huntmans’ crucible steel formed from carburisation of wrought iron?

Answer 17

Use coke (devolatilised coal) to obtain a high furnace temperature (1200-1300 Celsius); blister steel melts in pots with a flux; cast to shape; due to liquid form, C pick-up from coke was high; C content of steel increased

Question 18

How was Huntsmans’ crucible steel formed from decarburisation of cast iron?

Answer 18

Use a Bessemer converter: blow in air; O2 reacts with C in iron –> CO2; C content of steel decreased, but O2 still dissolved in melt which would make brittle; deoxidise to remove embrittling O2

Question 19

What are the four stages of modern steel-making?

Answer 19

Blast furnace; oxygen blowing; deoxidising; alloying

Question 20

What occurs in the blast furnace in modern steel-making?

Answer 20

Iron found as Fe2O3 and Fe3O4 is reduced by reaction with C from coke; occurs at high temperatures; Fe2O3 + CO –> 2Fe + 3CO2; high C content liquid iron formed

Question 21

What occurs in oxygen blowing in modern steel-making?

Answer 21

Lance blows O2 through liquid steel; O2 + 2C –> 2CO; reduced C content, high O content liquid iron formed

Question 22

What occurs in deoxidising in modern steel-making?

Answer 22

Add elements with high reactivity with O2 (Si, Al); deoxidation products float out of steel and removed with slag

Question 23

What occurs in alloying in modern steel-making?

Answer 23

Held under vacuum to degas: N removed; reaction with slag: removes impurities (S and P); addition of ferroalloys (Mn, Cr, Ni): alters composition and properties; stir with Ar gas: makes temperature and composition uniform

Question 24

When was malleable iron invented?

Answer 24

In China c. 300BC; was patented in England in the 1670s

Question 25

What was malleable iron of 1670s England like?

Answer 25

White cast iron with pearlitic matrix and blocky cementite which decomposed to spheroidal graphite

Question 26

What are the three forms of malleable iron?

Answer 26

Blackheart, whiteheart, pearlitic

Question 27

How is blackheart malleable iron formed?

Answer 27

Heated to 900 Celsius in a neutral atmosphere; iron carbide decomposed to graphite rosettas which provided better mechanical properties than flakes

Question 28

How is whiteheart malleable iron formed?

Answer 28

Heated to 900 Celsius in an oxidising atmosphere; results in surface decarburisation

Question 29

How is pearlitic malleable iron formed?

Answer 29

Heated to 900 Celsius then rapidly quenched; results in pearlitic or martensitic matrix