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Section 6.1

Ferrous Materials

Carbon and alloy steels, stainless steels, heat treatment and identification.

Notes

Ferrous materials — summary notes

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Main ideas
  • Ferrous materials are iron-based. Plain carbon steel is graded by carbon content: low-carbon/mild (<0.3 % C — ductile, weldable), medium-carbon, and high-carbon (harder, stronger, less ductile). More carbon trades ductility for hardness.
  • Alloy steels add elements to tailor properties: chromium (hardness, corrosion resistance), nickel (toughness), molybdenum (high-temperature strength), vanadium and tungsten (grain refinement/hardness). Nickel-chrome-moly steels such as AISI 4130/4340 are common aircraft structural steels.
  • Stainless steels contain ≥ ~12 % chromium, which forms a self-healing passive oxide film. Austenitic (18/8 Cr-Ni) is corrosion-resistant and essentially non-magnetic (firewalls, exhausts); martensitic types are hardenable (blades, bearings); ferritic types sit between.
  • Heat treatment changes the properties without changing composition: hardening (heat then quench), tempering (reheat to relieve brittleness and stress after hardening), annealing (soften and stress-relieve, slow cool), normalising (refine grain, air cool) and case hardening (hard wear surface over a tough core — gears).
  • Identification uses part numbers, colour codes, the spark test and a magnet — most ferrous metals are magnetic, and ferrous alloys are generally denser than aluminium alloys.
  • ⚠ Exam trap: austenitic (18/8) stainless steel is essentially NON-magnetic and cannot be hardened by heat treatment — only by cold working. Tempering always FOLLOWS hardening, trading a little hardness back for toughness.
Solved examples

  1. A technician checks two bright steel brackets with a magnet: one is strongly attracted, the other barely at all. What can be inferred?

    The strongly magnetic bracket is a plain-carbon or low-alloy/martensitic steel. The non-magnetic one is most likely an austenitic (18/8) stainless steel, which is corrosion-resistant and can only be work-hardened, not heat-treated. The magnet test is a quick first-pass material identification, confirmed against the part number.

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