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ENGINEERING, PPT 2025 HSC 13 MC

Which row of the table correctly identifies the materials represented by the microstructures shown in the top row of the table?
 

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\(D\)

Show Worked Solution
  • Pearlite + \(\alpha\) (ferrite) — large ferrite grains with pearlite patches = Mild steel; low carbon content means excess ferrite.
  • Pearlite + graphite flakes — distinctive dark flakes in a pearlite matrix = Grey cast iron; graphite precipitates as flakes during slow cooling.
  • Pearlite only — entirely pearlitic structure = High carbon steel; carbon content (~0.8%) produces near-100% pearlite (eutectoid).
  • Fe\(_3\)C + Pearlite — cementite network at grain boundaries = White cast iron; high carbon solidifies as iron carbide rather than graphite.

\(\Rightarrow D\)


♦ Mean mark 47%.

ENGINEERING, PPT 2016 HSC 26c

Structural steel `text{I}`-beams are used to make a crane were hot-rolled then normalised.

Explain why normalising was chosen as the heat treatment process. Support your answer with a labelled sketch of the resulting microstructure.   (4 marks)


 

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Advantages of normalising:

  • An increase in strength. Grain size strengthening is achieved due to the fine uniform grain structure produced by normalising.
  • When compared to other strengthening methods it is relatively economical.
  • Normalising relieves stress.
     

Show Worked Solution

Advantages of normalising:

  • An increase in strength. Grain size strengthening is achieved due to the fine uniform grain structure produced by normalising.
  • When compared to other strengthening methods it is relatively economical.
  • Normalising relieves stress.
     


♦ Mean mark 43%.

ENGINEERING, PPT 2017 HSC 25b

A small truck chassis rail has been made from rectangular hollow section (RHS) steel. The RHS has been cold formed from an alloy steel with a yield strength of 500 MPa. A manufacturer's sign on the chassis rail is shown below.
 

  1. Explain why the chassis rail should normally not be drilled or welded.   (3 marks)
  1. A large reinforcing plate that supports a new attachment is to be welded onto the chassis rail according to the manufacturer's specifications.   
  2. Use the diagram below to draw and label the macrostructure of the weld area, including the surrounding chassis rail and reinforcing plate.   (2 marks)
     

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i.   Welding

  • The chassis rail steel becomes molten when welded at temperatures exceeding the A1 temperature.
  • Columnar grains may form on some parts of the weld upon cooling.
  • Martensite may form on cooling, creating a brittle, hard microstructure with less strength than that of the steel chassis (500 MPa).

Drilling

  • Stress raisers are produced by drilling the flanges.
  • Fatigue failure is initiated by surface roughness that can occur around the drill hole.
Show Worked Solution

i.   Welding

  • The chassis rail steel becomes molten when welded at temperatures exceeding the A1 temperature.
  • Columnar grains may form on some parts of the weld upon cooling.
  • Martensite may form on cooling, creating a brittle, hard microstructure with less strength than that of the steel chassis (500 MPa).

Drilling

  • Stress raisers are produced by drilling the flanges.
  • Fatigue failure is initiated by surface roughness that can occur around the drill hole. 

♦♦ Mean mark (i) 39%.

ii.


♦♦ Mean mark (ii) 27%.

ENGINEERING, PPT 2019 HSC 24b

Normalised high-tensile steel has been chosen for the manufacture of a wing support beam.

  1. Draw and label the microstructures of a normalised high-tensile steel and an annealed high-tensile steel.   (2 marks)
     

  1. Explain how the microstructure produced by normalising high-tensile steel improves the steel's suitability for this application.   (2 marks)
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i.    Microstructures

 

ii.   The strength of the steel is markedly increased.

  • Normalising produces finer and more uniform grains.
Show Worked Solution

i.    Microstructures


♦♦♦ Mean mark (i) 26%.

ii.   The strength of the steel is markedly increased.

  • Normalising produces finer and more uniform grains.

♦ Mean mark (ii) 41%.

ENGINEERING, PPT 2022 HSC 23c

Steel `text{I}`-beams have been used when large, open spans need to be created inside buildings.

Explain how microstructural changes take place in steel when an `text{I}`-beam is formed using the process of hot rolling. You may use a drawing to support your answer.   (4 marks)

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  • During hot rolling the steel is heated to above its recrystallisation temperature.
  • Therefore, due to the pressure of the rollers, the grains change from their original state to become elongated.
  • However, as the steel exits the rollers it is still above recrystallisation temperature, resulting in the recrystallisation of the elongated grains to create finer, equiaxed grains.
Show Worked Solution

  • During hot rolling the steel is heated to above its recrystallisation temperature.
  • Therefore, due to the pressure of the rollers, the grains change from their original state to become elongated.
  • However, as the steel exits the rollers it is still above recrystallisation temperature, resulting in the recrystallisation of the elongated grains to create finer, equiaxed grains.

♦ Mean mark 48%.

ENGINEERING, PPT 2020 HSC 18 MC

The grain structure of a material has changed over time from that shown in microstructure `A` to that shown in microstructure `B`.
 

What heat treatment process has the material undergone to cause this change?

  1. Tempering
  2. Normalising
  3. Quench hardening
  4. Precipitation hardening
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`D`

Show Worked Solution
  • Of the answers presented, precipitation hardening is the only form of heat treatment that causes `alpha` and `beta` to dissolve into a single phase alloy as shown in Microstructure B.

`=>D`


♦♦ Mean mark 33%.