Steel

Question 1

What are the four phases of steel that form under slow cooling

Answer 1

Cementite, pearlite, ferrite, austenite

Question 2

What is the phase of steel that form under fast cooling

Answer 2

Martensite (and its tempered version)

Question 3

Why is iron considered allotropic

Answer 3

It can have different structures at different temperatures

Question 4

At what temperature does pure iron transform from face centred cubic to body centred cubic

Answer 4

912 degrees

Question 5

What is hypoeutectoid, eutectoid and hypereutectoid steel. What phases are formed in what order, starting at austenite

Answer 5

Hypoeutectoid steel is one that has lower carbon percentage than eutectoid steel. Austenite to austenite + ferrite to ferrite + cementite (pearlite)
hypereutectoid steel is one that has higher carbon percentage than eutectoid steel. Austenite to austenite + cementite to ferrite + cementite (pearlite)
Eutectoid steel is with a carbon percentage that allows it to remain as austenite at the lowest temperature (0.8% carbon). Austenite to ferrite + cementite (pearlite)

Question 6

Where does the ferrite and cementite precipitate when austenite cools down and at what temperature has all the austenite been transformed

Answer 6

At the grain boundaries, 723 degrees

Question 7

What is the difference between ferrite and cementite

Answer 7

Cementite has a higher carbon content and thus is harder and more brittle

Question 8

Why do alternating strips of cementite and ferrite form from cooling austenite

Answer 8

As the phases grow out of the grain boundary the carbon from the ferrite goes into the phase next to it, creating cementite. The cementite then takes carbon from the phase adjacent to it, forming ferrite.

Question 9

What would be the difference in structure between eutectoid, hypoeutectoid and hypereutectoid steels

Answer 9

Eutectoid has alternating strips of cementite and ferrite phases and will have an equal amount of both.
Hypoeutectoid will have a layer of ferrite from first at the grain boundaries then have alternating strips of ferrite and cementite.
Hypereutectoid will have a thinner layer of cementite from first at the grain boundaries then have alternating strips of ferrite and cementite.

Question 10

What is the structure of martensite if austenite is face centred cubic

Answer 10

body centred tertragonal

Question 11

What is the property of martensite

Answer 11

Extremely hard but more brittle

Question 12

How is tempered martensite formed and what is the result

Answer 12

The steel is heated to 200-600 degrees to allow some of the carbon to escape out of the martensite phase, forming some cementite. This increases strength and toughness (opposite of brittle)

Question 13

What element makes stainless steel resistant to corrosion? Explain how this is accomplished and give the composition percentage of this element in stainless steel

Answer 13

11 wt%. Chromium. This forms a layer of chromium oxide Cr203

Question 14

With what fuel source is iron oxide melted with

Answer 14

Coke, a purer form of coal

Question 15

What is the first stage of creating steel

Answer 15

Iron ore (iron oxide) is mixed with limestone (to remove impurities) then heated by carbon in blast furnace to form molten iron of around 4% carbon and slag (waste)

Question 16

What is the second stage of creating steel

Answer 16

The molten iron (which currently has too high carbon content) has the carbon removed by heating it in an oxygen vessel, forming an iron with less than 1.5% carbon

Question 17

What is the alternate way of creating steel, not using iron

Answer 17

Using recycled steel, this steel is placed in an electric arc furnace where the steel is melted by passing high current through it.

Question 18

does steel have a high embodied energy

Answer 18

compared to other materials, not really.