CAMSDM Lecture 6:

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CAMSDM Lecture 6 Properties of Steels II Dr
Tim Sercombe School of Mechanical Engineering
The University of Western Australia
Room 2.12 Phone [phone] email
tim.sercombe_at_uwa.edu.au
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Recap

• Steel is an alloy of?
• Classes of steel
• Plain carbon (low, med or high C)
• Low alloy (typical high strength grade)
• High alloy (tool and stainless steels)
• Strength/hardness increases with C
• Toughness and ductility decrease.

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Heat Treatment

• These are equilibrium (slow cooled)
  microstructures.
• We can alter structure (and therefore properties)
  by heat treatment
• Re-heat treatment can also erase prior
  treatments.

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Quenching

• Heat to austenitic region and quench rapidly
• Forms martensite
• Extremely hard, but brittle
• Not particularly useful
• Temper
• Re-heat to 200-500oC for up to 10h
• Small decrease in hardness/strength
• Regain some ductility and toughness

Quench to room temperature
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Example Properties
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Steel Products

• Nearly all carbon and alloy steels can be
  produced either as cast products or wrought
  products.
• Castings Casting is used to produce near-finish
  components.
• Wrought products are produced more as
  semi-processed raw materials for further
  fabrication into components.
• usually produced by any or combination of
  rolling, extrusion, drawing and forging.
• Common forms available are bars, tubes, sheets,
  coils, wires, I-beams, angles.
• Most structural and mild steels, stainless steels
  are found in these forms.
• Tool steels usually produced by casting and
  forging and are available as bars, rods and
  forged shapes.

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Classes of Metals
Metal Alloys
Grey iron Nodular iron White iron Malleable iron
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Fe-C Phase Diagram
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Cast Irons

• High C content 2-4
• Lower melting temperature than steel (300oC
  lower) makes them more suitable for casting
• Developed in China 500 BC.
• Not widely used in western world until 1700 and
  the development of the steam engine

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Types of Cast Iron

• From a microstructural view point, cast irons can
  regarded as steels embedded with a carbon-rich
  phase.
• This C-rich phase be either graphite or
  cementite (Fe3C).
• Graphite offers no strength nor toughness and
  acts like voids in the iron.
• Cementite is very hard and brittle.
• Formation of graphite of Fe3C depends on alloying
  content
• Eg. Addition of 2 Si causes the Fe3C to become
  unstable and graphite forms instead.

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Types of Cast Iron
Normally, a cast iron product is grey iron, but
it is not the only type available
Grey iron graphite flakes embedded in ferrite
matrix, cutting the continuity of the matrix and
rendering the metal high brittleness.
White iron cementite plates embedded in pearlite
matrix, renders the metal high hardness and
brittleness.
Malleable iron graphite clusters converted from
decomposing cementite in white iron,
significantly improving toughness and ductility
of the metal
Nodular iron graphite nodules in ferrite matrix,
significantly improving toughness and ductility
of the metal
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Grey Cast Irons

• Fe-1.8Si-0.5Mn-3.4C
• Properties
• Cheap
• excellent castability
• high damping capacity
• good metal-metal wear resistance when lubricated
• strength much higher in compression than in
  tension
• brittle in tension.
• UTS lt200MPa, ductility lt1
• Hardness 180 HB

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Grey Irons - Application

• Grey irons are by far the most produced among all
  cast irons.
• Used primarily for their low cost and excellent
  castability.
• Typical applications include
• engine cylinders,
• pistons,
• gear box casing,
• transmission casing,
• machine tool bases,
• balance weight of large cranes,
• large diameter underground pipework.
• They are used always under compressive loading
  conditions. They are unsuitable for taking
  tensile loads or bending loads.

Mild steel
Stress
Grey iron
Strain
Tensile stress-strain behaviour of grey cast iron
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Nodular (Ductile) Irons

• Fe-1Ni-0.4Mn-0.06Mg-3.4C
• Produced by inoculating (addition of RE or Mg to
  molten metal just before casting) grey iron. This
  converts graphite flakes into granules or nodules

• These irons are much stronger and tougher than
  grey irons, but more expensive
• They are produced and used for high specification
  applications.

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Nodular Irons - Applications

• Typical applications include
• gears,
• Crankshafts
• pump bodies
• pressure valves
• rollers.
• Properties
• Yield Strength 365MPa
• UTS 500MPa,
• ductility 18
• Hardness 170 HB

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White Cast Irons

• Fe-0.7Si-0.6Mn-3.4C
• White irons contain relatively less C and Si.
• Cementite (Fe3C) is formed instead of graphite.
  This makes the alloy very hard and brittle,
  practically useless as structural material.

• The high hardness, however, renders them high
  resistance to abrasive wear.

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White Cast Irons - Applications

• White irons are produced mainly for two purposes
• Intermediate product for producing malleable
  irons
• As abrasive wear resistant components, such as
• ball mill lining tiles,
• slurry pipe elbows,
• slurry pump bodies
• Properties
• Yield Strength lt200MPa
• Ductility 0
• Hardness 450 HB

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Malleable Cast Irons

• Fe-1Si-0.55Mn-2.5C
• Produced by heat treating white iron at elevated
  temperatures for long times.
• Such treatment decomposes cementite into ferrite
  and graphite.

• Consequently, these cast irons are stronger,
  tougher and much more ductile than grey irons
  (comparable to nodular irons).
• They have certain capacity to take shock loading,
  bending and tension.

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Malleable Cast Irons - Applications

• They are suitable for castings of thin thickness
• But they are expensive to produce, largely due to
  the heat treatment.
• Typical applications include
• gear box, and diff casings,
• clamps
• pipe fittings
• Properties
• Yield Strength 230MPa
• UTS 360MPa
• Ductility 12
• Hardness 130 HB

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Cast Iron Summary

• Cast Irons are a class of ferrous alloys with
  very high (2-4) carbon contents.
• Classed depending on microstructure
• Grey Graphite flakes in ferrite matrix
• Brittle in tension, good in compression
• Cheap
• Nodular graphite nodules instead of flakes
• Much higher strenght and ductility
• White Fe3C forms instead of graphite
• Very hard, but brittle
• Usually only used in wear applications
• Malleable Heat treating white CI decomposes Fe3C
  into graphite
• Increased strength and toughness.