Engineering Materials Exam Notes

Steel Microstructures

• Pearlite: A microstructure of steel.

• Bainite: Another microstructure of steel.

• Martensite: A very hard and brittle microstructure of steel; the hardest among the three.

Heat Treatment for 1050 Steel

• To achieve a uniform microstructure and hardness of HRC 23 in a 1050 steel axle, refer to the TTT diagram to determine the appropriate heat treatment.

Flame and Induction Hardening

• Working Principle: These methods involve heating the surface of the steel and then rapidly cooling it.

• Low Carbon Steel: These methods are not very effective on low carbon steels because martensite formation is dependent on the carbon content.

Martensitic Hardening Conditions

• Conditions:

  • Sufficient carbon content.

  • Austenitizing temperature.

  • Quenching rate rapid enough to avoid the formation of pearlite or bainite.

Surface Hardening Processes

• Old Batch Steel: Suggest a process based on its composition.

• New Batch Steel: Suggest a different process based on its composition and justify the selection.

Stainless Steel Hardenability

• Ferritic and Austenitic Stainless Steels: These are non-hardenable by heat treatment due to their stable crystal structures and inability to undergo martensitic transformation (no phase transformation).

Passivity in Stainless Steels

• Passivity: Achieved through the formation of a chromium oxide layer on the surface.
  $Cr + O<em>2 \rightarrow Cr</em>2O_3$

Sensitization Prevention in Austenitic Stainless Steels

• Material Selection Approaches:

  • Use low-carbon grades.

  • Stabilize the steel with elements like niobium or titanium.

Heat Affected Zone Microstructures

• (i) Low Hardenability Steel (Cooling): Microstructure 'a'.

• (ii) High Hardenability Steel (Cooling): Microstructure 'b'.

• (iii) Hypoeutectoid Steel During Welding: Microstructure 'c'.

Steel and Aluminum Cable Comparison

• Steel Cable:

  • Diameter: $12.5mm$

  • Yield Strength: $483 MPa$

  • Density: $7.87 g/cm^3$

  • Area: $A_{steel} = \pi r^2 = \pi (6.25)^2$

• Aluminum Alloy:

  • Yield Strength: $248 MPa$

  • Density: $2.70 g/cm^3$

• Calculations:

  • Maximum load that the steel cable can support:
    $Load = \sigma<em>{yield} \times A</em>{steel}$
    $Load = 483 MPa \times \pi (6.25)^2 mm^2$

  • Diameter of the aluminum-manganese alloy required to support the same load:
    $A<em>{Al} = \frac{Load}{\sigma</em>{yield}} = \frac{483 \times \pi (6.25)^2}{248}$
    $r = \sqrt{\frac{A}{\pi}}$
    $d = 2r$

  • Weight per meter:
    $Weight = Density \times Area$
    Compare the weight per meter of the steel cable versus the aluminum alloy cable.

Age-Hardened Aluminum Component

• Effects of Exhaust Gases: Exposure to $260°C$ for 1 hour would likely cause over-aging.

• Mechanical Properties:

  • Yield strength decreases.

  • UTS decreases.

  • Hardness decreases.

  • Ductility increases.

Aluminum Weld Strength

• Weaker Weld: The heat from welding dissolves precipitates, and the slow cooling results in a softer, weaker material in the weld zone.

Aluminum 6061 Options

• 6061-O: Annealed condition (lower strength).

• 6061-T6: Heat-treated for higher strength (higher yield strength and UTS).