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 + O2 \rightarrow Cr2O_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{yield} \times A{steel}
      Load = 483 MPa \times \pi (6.25)^2 mm^2

    • Diameter of the aluminum-manganese alloy required to support the same load:
      A{Al} = \frac{Load}{\sigma{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).