Properties of metals
Choosing, inspecting and repairing aircraft materials starts with a common vocabulary. These terms appear constantly in Module 6 and in structural repair manuals.
- Hardness — resistance to wear, cutting and indentation. Hardness generally rises with strength, so hardness tests are used to estimate strength.
- Strength — resistance to deformation and failure. Tensile (pulling), compressive, shear (sliding layers), bearing (crushing at a fastener hole) and torsional strength are quoted separately. Yield strength is where permanent deformation begins; ultimate tensile strength (UTS) is the maximum stress before failure.
- Ductility — ability to be drawn out (into wire or tube) without breaking. Malleability — ability to be hammered or rolled into sheet without cracking. Both are needed for forming skins and fittings.
- Elasticity — ability to return to the original shape when the load is removed, up to the elastic limit. Structures are designed to stay within it.
- Toughness — ability to absorb energy and resist tearing or shock. Brittleness is the opposite: a brittle metal breaks with little or no bending (cast iron, glass-hard steel).
- Fusibility — ability to melt and fuse (welding). Conductivity — of heat and electricity (important for bonding and lightning paths). Thermal expansion — growth with temperature (aluminium expands about twice as much as steel).
- Density — mass per unit volume: steel about 7.85 g/cm³, titanium 4.5, aluminium 2.7, magnesium 1.74. The useful figure is strength-to-weight ratio.
- Fatigue strength — resistance to repeated loading. Most aircraft structural failures are fatigue failures that start at stress concentrations such as holes, notches and corrosion pits.
Iron and steel
Ferrous metals are those based on iron. Pure iron is soft; adding carbon makes steel, and carbon content controls how hard and strong it can be made:
| Steel | Carbon | Character and uses |
|---|---|---|
| Low carbon (mild) | ≈ 0.10–0.30% | Soft, ductile, easily welded and formed — safety wire, some fittings, non-structural parts |
| Medium carbon | ≈ 0.30–0.50% | Can be hardened — shafts, bolts, gears |
| High carbon | ≈ 0.50–1.05% | Very hard — springs, cutting tools, files |
Above about 2% carbon the material is cast iron — hard, brittle and used only for parts such as some piston rings and brake drums.
Alloy steels and their numbers
Alloying elements add specific properties:
- Chromium — hardness, wear and corrosion resistance (above about 11–12% it makes stainless steel).
- Nickel — strength and toughness, without much loss of ductility.
- Molybdenum — strength, hardenability and resistance to temper brittleness; combined with chromium in the chrome-moly steels.
- Vanadium — fine grain, toughness and fatigue resistance (springs, tools).
- Manganese — deoxidiser and strengthener; silicon — strength and elasticity (spring steels); tungsten and cobalt — hardness at high temperature.
The most common aircraft steels: 4130 (chromium-molybdenum) for welded tube structures, engine mounts and fittings — easily welded and heat-treatable; 4340 (nickel-chromium-molybdenum) for highly stressed parts such as landing gear components, bolts and gears; 8630 and similar nickel-chromium-molybdenum steels; and plain 1020/1025 for low-stress parts.
Corrosion-resistant (stainless) steels
- Austenitic (300 series, for example 302, 304, 321, 347 — the "18-8" types with about 18% chromium and 8% nickel): non-magnetic, excellent corrosion resistance, cannot be hardened by heat treatment — only by cold working. Exhaust collectors, firewalls, safety wire, fluid lines, clamps.
- Martensitic (400 series, for example 410, 420, 440C): magnetic and heat-treatable — bearings, valves, cutting parts.
- Precipitation-hardening (17-4 PH, 17-7 PH, 15-5 PH): high strength with good corrosion resistance — structural fittings, springs, actuator parts.
Stainless steel is "passive" because of a thin chromium-oxide film. It can still corrode when that film is starved of oxygen (crevices, under seals) or attacked by chlorides.
Heat treatment of steel
Heat treatment changes the internal (grain) structure of a metal by controlled heating, soaking (holding at temperature so the whole part reaches it) and cooling. The rate of cooling is what makes the difference.
- Hardening: heat to just above the critical temperature, soak, then quench. Brine quenches fastest, then water, then oil (slower, less distortion and cracking). The result is hard but brittle.
- Tempering (drawing): reheat a hardened part to a temperature below the critical range and cool. It relieves internal stress and reduces brittleness; hardness and strength fall slightly while toughness rises. Hardened steel is almost always tempered.
- Annealing: heat above the critical range, soak, then cool very slowly (in the furnace). Produces the softest, most ductile condition — for forming and machining.
- Normalising: heat above the critical range and cool in still air. Relieves the stresses of welding, forging or machining and refines the grain. Welded 4130 assemblies are often normalised.
Case hardening
Some parts need a hard, wear-resistant surface on a tough, ductile core — gears, cylinder bores, cam followers, bearing surfaces.
- Carburising — the part is heated in a carbon-rich solid, liquid or gas so carbon diffuses into the surface, then it is hardened by quenching.
- Nitriding — heated (around 510 °C) for many hours in ammonia gas; nitrogen forms very hard nitrides at the surface. No quench, so distortion is minimal — used for engine cylinder barrels and crankshafts.
- Cyaniding (carbon and nitrogen), and induction or flame hardening, where only the surface is rapidly heated and quenched.
Forming processes
- Forging — hammering or pressing hot metal into shape. The grain flows with the shape, giving strong, tough parts (crankshafts, landing gear, high-stress fittings).
- Casting — pouring molten metal into a mould. Complex shapes, but generally lower strength and more prone to porosity.
- Extruding — forcing metal through a shaped die (stringers, angles, channels) — mostly aluminium.
- Cold working — rolling, drawing or bending below the recrystallisation temperature. It work hardens the metal, raising strength and hardness but reducing ductility; too much causes cracking. Annealing restores ductility.
Never substitute a different steel, heat-treatment condition or hardness without approved data. A stronger but harder steel may be more brittle or more prone to stress-corrosion cracking; a softer one may yield. The structural repair manual or approved data specifies the exact material and condition.