Lesson 3 - Impurity Elements And Their Effects Upon Steel Properties

Question 1

What are the main impurity elements in carbon steel?

Answer 1

Oxygen
Aluminium
Silicon
Phosphorus
Sulphur
Copper

Question 2

Rough outline of involvement of oxygen in steelmaking (iron ore to pig iron)

Answer 2

• Since iron ore exists in iron oxides, carbon is needed to extract iron in the steelmaking process
• Limestone (CaCO3) helps remove impurities by forming slag
• Oxygen (highly reactive) is injected to oxidise impurities to remove them
• Intermediate product (pig iron) is produced as a raw material for steel production
• By product (Slag): A mixture of metallic oxides formed during the steelmaking process including calcium oxide (CaO), silica (SiO2), iron oxide (FeO) with additional components.

Question 3

Rough outline of involvement of oxygen in steelmaking (Pig Iron to Steel)

Answer 3

• Pig iron is re-melted and oxygen is blown through to reduce carbon content to the desired level by producing CO and CO2
• As carbon is removed from the molten steel, it becomes increasingly possible for oxygen to dissolve in the molten steel
• Dissolved oxygen reacts with carbon when the steel is solidified, carbon monoxide is produced. This dissolved gas causes blowholes, which can significantly reduce the strength of steel
• Also a risk of producing oxide inclusions at higher oxygen concentrations

Question 4

What is a steel with all oxygen removed called and how is it characterised?

Answer 4

Killed Steel
More uniform structure and properties than other types of

Question 5

What are inclusions?

Answer 5

What is produced in steel ingots when the temperature drops during steelmaking when the molten impurities/products solidify.

Question 6

Why might aluminium be added to steel? And how does it affect the material properties?

Answer 6

• Added for deoxidising and grain refining in steel
• Can improve hardness, fatigue strength and wear resistance
• May cause problems in surface heat treatments

Question 7

Is aluminium difficult to remove from steel

Answer 7

Yes, difficult to completely remove, but on balance its effects are not such a big problem

Question 8

What is silicon used for in steel making

Answer 8

One of the principal deoxidizers

Question 9

How does silicon content effect steel?

Answer 9

• Influences both strength, hardness and toughness of steel (depending on multiple factors)
• Increases tendency for carbon to be deposited as graphite
• Makes steels more fluid when molten

Question 10

Implications of phosphorus in steel? And how to contain phosphorus content in steels?

Answer 10

• Increases brittleness
• Forms ‘ghost bands’ which largely reduce the mechanical strength
• Only way to control phosphorus content is by the manipulation of steelmaking conditions

Question 11

Effectivement of sulphur on mechanical properties of steel?

Answer 11

• FeS forms brittle film around metal grains which reduces mechanical strength and fracture toughness.
• Makes steels hard to roll and from

Question 12

What is the problem related to limiting phosphorus and sulphur content in steelmaking? And what is the other answer to removing sulphur?

Answer 12

• It is not possible to minimise both phosphorus and sulphur contents simultaneously during steelmaking
• In practice phosphorus is minimised and another method is used to remove sulphur.
• This is to add manganese to form MnS which is fairly ductile and does not harm mechanical properties

Question 13

What does the solubility of copper(Cu) depend on?

Answer 13

Temperature

Question 14

What is precipitation hardening?

Answer 14

Heat treatment technique used to increase the yield strength of malleable materials by controlling the solid solubility to produce fine particles of an impurity phase which impedes the dislocation movements.

Question 15

What does the presence of copper do to a steel?

Answer 15

• Can improve the corrosion resistance of mild steel
• Small amounts allows the limited precipitation hardening of steel.
• Copper does not interact and mix with carbon so all copper in mild steel is dissolved or precipitated in ferrite, resulting in hardening effect with decrease of ductility.

Question 16

What are the copper weathering steels?

Answer 16

• Group of steel alloys which were developed to eliminate the need for painting, and form a stable rust-like appearance after several years’ exposure to weather
• Tradename: COR-TEN for corrosion resistance and tensile strength

Question 17

Disadvantages of copper weathering steels

Answer 17

• Bad effect on workability
• Cannot be welded using standard oxyacetylene techniques

Question 18

How can copper in steel be minimised?

Answer 18

• Choice of steel making conditions
• Copper dilution by addition of pure, new steel to the melt
• Use of vacuum distillation
• Better scrap sorting