Materials- Metals: Steel Processing

Question 1

What are the stages of processing a steel after extraction of the raw materials?

Answer 1

Ironmaking, steelmaking, secondary steelmaking, casting, primary forming

Question 2

How does ironmaking work?

Answer 2

Carried out using a blast furnace. Iron ore, coke (C) and limestone added at the top. Hot air blown in at the base. Reactions are essentially O2 reacting with C to make CO, iron ore reacting CO to make Fe and CO2. The liquid metal and slag (waste) is removed (tapped) every few hours. Can produce up to 12000 tons per day.

Question 3

How does general steelmaking work?

Answer 3

Metal from blast furnace contains about 5% C so is essentially a cast iron with poor strength and toughness (pig iron). Aims to reduce C% and other impurities. Done using oxygen injection. 2 methods are basic oxygen steelmaking, electric arc furnace.

Question 4

How does basic oxygen steelmaking (BOS) work?

Answer 4

Uses hot metal straight from blast furnace and steel scrap. Water-cooled oxygen lance blows high-purity O2 at v.high pressure on metal. Combines with C and other impurities, removing them from the molten charge. Reactions produce heat so temp controlled by quantity of added scrap. C leaves converter as CO. During ‘blow’, lime added as a flux to help carry other oxidised impurities as a floating slag layer. During tapping, alloy additions to adjust final comp. After tapping, converter inverted so residual slag removed to slag cooling pond.

Question 5

How does electric arc furnace (EAF) work?

Answer 5

Only uses cold scrap steel. Is circular bath with movable roof so 3 graphite electrodes raised or lowered. To start, electrodes withdrawn. Steel scrap charged into furnace from large steel basket. Then roof swung back in position and electrodes lowered into furnace. Large electric current (10^5A) passed through charge, arc created and heat generated melts scrap. Lime and fluorspar (calcium fluorite) added as fluxes and O2 blown into melt. Impurities combine forming liquid slag.

Question 6

What can secondary steelmaking involve?

Answer 6

Further small composition adjustments to create alloys. Can be alloy addition, inclusion removal, inclusion chemistry modification, desulfuration, homogenisation.

Question 7

What factors are considered when choosing fabrication methods?

Answer 7

Properties of the metal, size and shape of finished piece, cost.

Question 8

What are forming operations?

Answer 8

Those in which the shape of a metal piece is changed by plastic deformation which is induced by an external force or stress, the magnitude of which must exceed the yield strength of the material.

Question 9

What is hot working and what are its advantages?

Answer 9

Metal is deformed into a desired shape by plastically deforming above recrystallisation temperature. No strengthening occurs during this deformation allowing almost unlimited plastic deformation. Suitable for forming large parts and metals less brittle at elevated temperatures. Can eliminate imperfections like gas pores and composition differences.

Question 10

Disadvantages of hot working and how they arise

Answer 10

Final properties of hot-worked materials not isotropic. Forming rolls are cooler then metal so surface of metal cools faster and has smaller grain size than centre. Surface oxidation occurs unlike in cold working.

Question 11

Positives and negatives of cold working

Answer 11

Can shape and strengthen metal at same time. Obtain excellent surface finishes and dimensional tolerances. Inexpensive for large numbers of small parts. However
Little cold work can be done on brittle metals. Ductility, electrical conductivity and corrosion resistance reduced. Cold-worked components can’t be used at high temperatures.

Question 12

What is forging?

Answer 12

Mechanically working or deforming a single piece of normally hot metal, either by application of successive blows or by continuous squeezing.

Question 13

Types of forging?

Answer 13

Closed die: force is brought to bear on two or more die halves having the finished shape such that the metal is deformed in the cavity between them.
Open die: two dies having simple geometric shapes are employed instead (normally on large work pieces)

Question 14

What is casting?

Answer 14

Totally molten metal is poured into a mild cavity having the desired shape and upon solidification the metal assumes the shape of the mould but experiences some shrinkage.

Question 15

When is casting used?

Answer 15

The finished shape is so large or complicated that any other method is impractical. A particular alloy is so low in ductility that forming by either hot or cold working would be difficult. In comparison to other fabrication processes, casting is the most economical.

Question 16

Positives of sand casting

Answer 16

Inexpensive mould, complex geometry, all alloys, unlimited size, economical in low quantities

Question 17

Negatives of sand casting

Answer 17

Cost per part is higher, labour intensive, slower production rate, rough surface finish, loose tolerances, requires relatively thick walls.

Question 18

How does investment casting (lost-wax) work?

Answer 18

Pattern max from wax with low melting temperature. A fluid slurry poured around the pattern which sets to form a solid mould (can be ceramic). Mild then heated so pattern melts and is burned out leaving a mild cavity with desired shape which molten metal is poured into.

Question 19

Positives of investment casting

Answer 19

Good dimensional accuracy, relatively inexpensive mold, rapid production rates possible, complex shapes, can cast very high temperature materials (like Ti)

Question 20

Disadvantages of investment casting

Answer 20

Long production cycle. Mold not reusable.

Question 21

How does lost-foam casting work?

Answer 21

Aka expendable pattern casting. A variation of investment casting. The expendable pattern is a foam made from compressing polystyrene beads into desired shape then bonding by heating. Sand then packed around pattern to form mold. Molten metal poured in which replaces the pattern which vaporises. Compacted sand remains in place and metal solidifies to assume shape of mold.

Question 22

Positives and negatives of lost-foam casting

Answer 22

Complex geometries and tight tolerances possible. Simpler, quicker, less expensive than sand casting. Fewer environmental wastes. However
Mold not reusable.

Question 23

How does pressure die casting work?

Answer 23

Liquid metal injected into reusable steel mold, or die, very quickly with high pressures. This differentiates it from other metal casting processes.

Question 24

Positives of pressure die casting

Answer 24

High production rates possible with high level of automation. Very thin walls possible. Good surface finish. Economical in large quantities. Better control of mold quantities.

Question 25

Negatives of pressure die casting

Answer 25

High tooling cost. Long lead times (months). Limited size (<15kg). Limited in alloys (low temperatures only): Zn, Al, Mg, Cu.

Question 26

Ingot casting

Answer 26

Molten steels or alloys are cast into large molds. The resultant ingots are then processed for conversion into useful shapes via thermomechanical processing (e.g hot rolling). Cost effective for smaller batches.

Question 27

Continuous casting

Answer 27

Go from molten metal to a semi-finished shape such as a billet, bloom, slab, plate, etc. Cost effective for many steels, stainless steels and Al alloys but not all alloys can be cast by continuous casting. The molten metal is poured and transported while solidifying to a cutter. All a continuous process.