Module 6.1 Aircraft materials - ferrous
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Module 6 (Materials and hardware) is a large paper: 52 questions (A), 80 (B1 and B3), 60 (B2/B2L) under EASA.
Knowledge levels
| Item | A | B1 / B3 | B2 / B2L |
|---|---|---|---|
| 6.1(a) Characteristics, properties and identification of alloy steels; heat treatment and applications | 1 | 2 | 1 |
| 6.1(b) Testing for hardness, tensile strength, fatigue strength, impact resistance | — | 1 | 1 |
| 6.1(c) Repair and inspection of ferrous materials, structures and airframes | — | 2 | 1 |
Iron and steel
- Ferrous metals are based on iron. Steel is iron with up to about 2 % carbon; cast iron has more (2-4 %).
- Carbon content controls hardness and strength:
| Steel | Carbon | Properties / uses |
|---|---|---|
| Low carbon (mild) | up to about 0.3 % | ductile, weldable, not hardenable by quenching; safety wire, non-structural parts |
| Medium carbon | 0.3-0.6 % | stronger, hardenable; shafts, forgings |
| High carbon | 0.6-1.0 %+ | very hard, less ductile; springs, cutting tools |
Alloying elements
| Element | Effect |
|---|---|
| Nickel | toughness, strength, corrosion resistance |
| Chromium | hardness, wear resistance; above about 12 % gives stainless steel |
| Molybdenum | strength at high temperature, reduces temper brittleness |
| Vanadium | fine grain, toughness, fatigue strength (springs, tools) |
| Manganese | strength, deoxidiser |
| Silicon | strength, magnetic properties (transformer steels) |
| Tungsten / cobalt | hot hardness (tool steels, high-temperature alloys) |
SAE/AISI identification
A four-digit number: first digit = main alloy type, second = approximate % of that alloy, last two = carbon in hundredths of a percent.
| Series | Type |
|---|---|
| 10xx | plain carbon steel |
| 23xx, 25xx | nickel steels |
| 31xx | nickel-chromium |
| 41xx | chromium-molybdenum ("chrome-moly") |
| 43xx | nickel-chromium-molybdenum |
| 52xx | chromium (bearing steel) |
| 61xx | chromium-vanadium |
| 92xx | silicon-manganese (springs) |
Key fact: SAE 4130 = chromium-molybdenum steel with 0.30 % carbon - the classic welded tubular fuselage and engine-mount steel. 4340 = nickel-chrome-moly, used for high-strength landing gear and bolts.
Stainless and heat-resistant steels
- Austenitic (300 series, e.g. 304, 321, 347): non-magnetic, not hardenable by heat treatment, good corrosion and heat resistance - exhaust systems, firewalls, safety wire.
- Martensitic (400 series, e.g. 410, 440): magnetic, hardenable - bearings, cutting edges.
- Precipitation-hardening stainless (e.g. 17-4PH): high strength with corrosion resistance.
- Nickel-based superalloys (Inconel, Nimonic) for turbine sections are non-ferrous but often taught alongside.
Heat treatment of steel
| Process | How | Result |
|---|---|---|
| Hardening | heat above the critical temperature, then quench (water, oil, air) | very hard, brittle martensite |
| Tempering | reheat hardened steel to a lower temperature, cool | reduces brittleness and stress, slight loss of hardness, gains toughness |
| Annealing | heat above critical, cool very slowly in the furnace | softest, most ductile state, relieves stress |
| Normalising | heat above critical, cool in still air | refines grain, relieves stresses (e.g. after welding) |
| Case hardening | carburising, nitriding, cyaniding, induction or flame hardening | hard wear-resistant surface with a tough core (gears, cam shafts, cylinder barrels - nitriding) |
Key fact: Annealing (furnace cool) gives maximum softness; normalising (air cool) gives refined grain; hardening needs a quench; tempering always follows hardening.
Material properties
- Hardness: resistance to indentation/wear. Brittleness: breaks without deforming. Malleability: can be hammered/rolled. Ductility: can be drawn into wire. Toughness: resists shock (impact). Elasticity: returns to shape. Strength: tensile, compressive, shear. Fatigue strength: resistance to repeated (cyclic) loads. Density, fusibility, conductivity.
Testing ferrous materials
| Test | Measures | Method |
|---|---|---|
| Brinell | hardness | hardened steel or carbide ball pressed with a set load; diameter of the indentation measured (BHN) |
| Rockwell | hardness | depth of penetration by a diamond cone (C scale, hard steel) or steel ball (B scale, softer metals); read directly |
| Vickers | hardness | diamond pyramid; diagonals of the indentation measured; good for thin and very hard materials |
| Tensile test | yield strength, UTS, elongation, modulus | specimen pulled to failure in a testing machine |
| Fatigue test | endurance under cyclic loading; S-N curve | rotating-beam or axial loading machines |
| Impact test (Izod, Charpy) | toughness | notched specimen broken by a pendulum; energy absorbed |
Steel has a fatigue (endurance) limit: below a certain stress it can survive an indefinite number of cycles. Aluminium alloys have no true endurance limit.
Repair and inspection
- Inspect for cracks (magnetic particle inspection is ideal for ferromagnetic steels), corrosion (rust, pitting), wear, dents and distortion.
- Remove corrosion and restore protection; check remaining thickness against SRM limits.
- High-strength steels are susceptible to hydrogen embrittlement after plating or acid cleaning - bake (de-embrittlement) as specified.
- Welding of steel tube structures follows approved procedures; heat-treated parts must not be welded unless the data allow it (heat ruins the treatment).
- Never straighten heat-treated or high-strength steel parts cold unless the manual permits.
Exam tips
- SAE: first two digits = alloy type, last two = carbon in 0.01 %.
- Austenitic stainless is non-magnetic - magnetic particle inspection will not work on it.
- Brinell = ball and diameter; Rockwell = depth; Vickers = diamond pyramid.