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CIVE40007 – Materials > Metallurgy and manufacture of steels > Flashcards

Metallurgy and manufacture of steels Flashcards

1
Q

What does a eutectoid reaction in the Fe-C system entail?

A

A eutectoid reaction in the Fe-C system describes the phase transformation of one solid into two different solids, specifically austenite into ferrite and cementite at 0.8 wt% C and 723°C

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2
Q

At what temperature does the bcc ↔ fcc transformation of iron occur?

A

912°C

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3
Q

What is the maximum carbon content for steel?

A

The maximum carbon content for steel is less than 2%

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4
Q

What are the three different types of steels based on the iron-carbon phase diagram?

A

The three different types of steels are hypoeutectoid, eutectoid, and hypereutectoid steels

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5
Q

What is pearlite, and how does it form?

A

Pearlite is a fine lamellar structure formed directly from austenite in the (α + γ) region of the phase diagram, consisting of alternating layers of ferrite (which is soft and ductile) and cementite

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6
Q

What is the texture of cementite?

A

hard and brittle

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7
Q

What is the microstructure of low carbon steel?

A

ferrite formed directly from austenite, with pearlite present as well

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8
Q

How does the cooling rate affect the microstructure of steel?

A

Affects the fineness of the pearlite in steel; faster cooling rates generally lead to finer pearlite structures

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9
Q

What happens to the carbon during the transformation from austenite to ferrite and cementite?

A

During the transformation, carbon is expelled from the ferrite and is taken up by the cementite, increasing its concentration there

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10
Q

What is the difference in carbon content between ferrite and cementite?

A

Ferrite has little carbon and is a body-centered cubic structure, while cementite is a carbide with a higher carbon content and is hard and brittle

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11
Q

What are the two different types of iron?

A

Allotropic
Polymorphic

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12
Q

What is pearlite like?

A

Pearlite is analogous to a metal-ceramic nano-composite material

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13
Q

What are the textures of ferrite?

A

Ferrite is relatively soft and ductile

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14
Q

What are grain boundaries occupied by in eutectoid condition?

A

Pearlite

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15
Q

What is the microstructure of hypereutectoid steel?

A
  • Fe3C forms at austenite grain boundaries
  • Continuous brittle phase
  • Cementite surround pearlite
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16
Q

What are steels primarily composed of in general engineering use?

A

Steels in general engineering use are essentially binary alloys of iron and carbon, often with less than 0.8 wt.% C

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17
Q

What other elements are commonly found in carbon steels?

A

Carbon steels also typically contain manganese (~0.45-0.9%), phosphorus (0.025-0.060%), and sulphur (0.030-0.050%)

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18
Q

How can the properties of steel be enhanced and controlled?

A

The properties of steel can be enhanced and controlled through heat treatments

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19
Q

What happens to the phases shown in the phase diagram when steel is cooled rapidly?

A

If cooling is rapid, the phase diagram becomes invalid as stable phases shown during slow cooling may not form, and instead, metastable phases like martensite may develop

20
Q

What is martensite, and how is it characterized?

A

Martensite is a very hard and quite brittle phase that retains the amount of carbon present during its formation. It is a ceramic-like material with many interfaces and high, strongly varying local stresses

21
Q

For what kind of applications can as-quenched martensite be used?

A

As-quenched martensite can be used for products like crankshafts, spanners, and high-tension bolts due to its hardness

22
Q

What does the tempering process involve?

A

Tempering involves a heat treatment at 200-600˚C, which allows carbon atoms to diffuse out of martensite

23
Q

How does the hardness of as-quenched martensite compare to air-cooled (pearlitic) steel?

A

As-quenched martensite is much harder than air-cooled (pearlitic) steel, as indicated by the steeper curve on the hardness versus carbon content graph

24
Q

What is the typical hardness range of martensitic steel with a carbon content of 0.6 wt.%?

A

600-800 Vickers (VPN)

25
Q

What effect does carbon content have on the yield stress of steel?

A

The yield stress of steel increases with carbon content, as indicated by the yield stress axis on the graph

26
Q

What influences the microstructure of tempered martensite?

A

The microstructure of tempered martensite depends on the tempering temperature, which leads to the formation of very fine precipitates of α (alpha) ferrite and Fe3C (cementite)

27
Q

What is the defining characteristic of stainless steel?

A

Stainless steel is defined by having greater than 11 wt% chromium (Cr), and may also contain elements like nickel (Ni) and manganese (Mn)

28
Q

How does chromium protect stainless steel from corrosion and oxidation?

A

Chromium contributes to the formation of an adherent Cr2O3 film that provides protection against corrosion and oxidation

29
Q

Why are most stainless steels non-magnetic?

A

Most stainless steels are austenitic, which is a non-magnetic phase of steel

30
Q

What happens in a normal oxidising environment to stainless steel?

A

In a normal oxidising environment, stainless steel automatically forms a tightly bonded chromium-rich oxide film which enhances its corrosion resistance

31
Q

Compare the corrosion resistance of austenitic stainless steel to other types.

A

The corrosion resistance of austenitic stainless steel is not as good compared to other types of stainless steel.

32
Q

What is low alloy steel, and what is its carbon content?

A

Low alloy steel contains less than 10 wt% alloying elements and typically has low carbon content

33
Q

What materials are used in the production of molten steel?

A

The production of molten steel involves limestone and iron ore, which go into a sinter plant, and coal that is transformed into coke for the blast furnace

34
Q

Why doesn’t a blast furnace produce steel?

A

A blast furnace doesn’t produce steel because the iron extracted has too much carbon

35
Q

How is the carbon level reduced in molten steel?

A

In a basic oxygen vessel, oxygen is blasted into molten steel, which reacts to form CO2 and reduces the carbon level in the molten steel

36
Q

What might be added during secondary steel making, and what is another method of steel production?

A

During secondary steel making, elements like chromium might be added, and another method of steel production is using electric arc furnaces which primarily use recycled steel

37
Q

What is the purpose of a Basic Oxygen Converter (BOC) in steel production?

A

Used to transfer molten iron into the vessel and blast oxygen into it to further reduce the carbon content to between 0 and 1.5%

38
Q

Describe the physical characteristics of a Basic Oxygen Converter.

A

A Basic Oxygen Converter is a cylindrical vessel approximately 8 meters in diameter and 11 meters high, lined with about 1 meter of magnesite bricks

39
Q

How is energy reclaimed in the Basic Oxygen Converter process?

A

Energy is reclaimed in the BOC process by burning the exhaust gas to heat a boiler

40
Q

What is the starting material for an Electric Arc Furnace (EAF), and what is its capacity?

A

The starting material for an Electric Arc Furnace is typically scrap steel or iron, and an EAF can typically make 150 tonnes of steel in around 90 minutes

41
Q

How is an electric arc created in an Electric Arc Furnace?

A

An electric arc is created in an Electric Arc Furnace by passing an electric current through electrodes when the lid containing the electrodes is swung into position

42
Q

What are the processes involved in steel casting and hot rolling?

A

Steel casting and hot rolling are forming processes that contribute to microstructure formation in steel production

43
Q

How much steel was produced in 2018?

A

1,808 million tonnes of steel were produced

44
Q

How much carbon dioxide is emitted from making steel each year?

A

2,730 million tonnes

45
Q

What makes steel a sustainable material?

A
  • completely recyclable
  • possesses good durability
  • has low embodied energy
  • lightweight steel construction can save energy and resources

CIVE40007 – Materials (16 decks)

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