6.1 — Aircraft Materials — Ferrous

Introduction to Ferrous Materials

Ferrous materials are metals and alloys whose primary constituent is iron (Fe). The word "ferrous" comes from the Latin ferrum, meaning iron. Ferrous materials are characterised by their high strength, hardness, and — with the notable exception of stainless steel — their susceptibility to corrosion (rust). In aircraft construction, ferrous materials are used selectively where their superior strength and wear resistance are essential, despite their relatively high density compared to aluminium alloys.

The most important ferrous material in aviation is steel — an alloy of iron and carbon (typically 0.05% to 2.0% carbon by weight). By adding other alloying elements and applying specific heat treatments, the properties of steel can be tailored to a wide range of requirements.

Properties of Ferrous Materials

Property	Description
Tensile strength	Resistance to being pulled apart — steel has very high tensile strength, typically 400–2000 MPa depending on alloy and treatment
Hardness	Resistance to surface indentation or scratching — can be greatly increased by heat treatment and alloying
Ductility	Ability to be drawn into wire or deformed without fracturing — low-carbon steels are very ductile, high-carbon steels less so
Malleability	Ability to be hammered or rolled into shape without cracking
Toughness	Ability to absorb energy and deform plastically before fracturing — a combination of strength and ductility
Elasticity	Ability to return to original shape after load is removed — steel has a well-defined elastic limit
Fatigue strength	Resistance to failure under repeated cyclic loading — critical in aircraft structures
Corrosion resistance	Plain carbon steels rust readily; stainless steels and surface treatments improve corrosion resistance
Density	Approximately 7,850 kg/m³ — about 2.8 times heavier than aluminium

Types of Steel

Plain Carbon Steel

Plain carbon steel contains iron and carbon with only small amounts of other elements (manganese, silicon, sulphur, phosphorus). It is classified by carbon content:

Type	Carbon %	Properties	Aircraft Applications
Low carbon (mild steel)	0.05–0.30%	Soft, ductile, easily welded; low strength	General fittings, non-structural brackets, wire, welded tube structures (some light aircraft fuselages)
Medium carbon	0.30–0.50%	Harder, stronger; responds to heat treatment	Bolts, studs, axles, forgings
High carbon	0.50–1.50%	Very hard after heat treatment; brittle; holds sharp edge	Springs, cutting tools, wire rope; not common in primary structures

Alloy Steels

Alloy steels have deliberate additions of other elements to improve specific properties. Common alloying elements in aircraft steels:

Element	Effect
Chromium (Cr)	Increases hardness, wear resistance, and corrosion resistance; key element in stainless steel (≥10.5% Cr)
Nickel (Ni)	Increases toughness and impact resistance; improves low-temperature properties; corrosion resistance
Molybdenum (Mo)	Increases strength at elevated temperatures; reduces temper brittleness; improves hardenability
Vanadium (V)	Refines grain structure; increases strength, toughness, and wear resistance
Tungsten (W)	Increases hardness at high temperatures; used in high-speed tool steels
Manganese (Mn)	Increases strength and hardness; improves hardenability; counteracts sulphur brittleness
Silicon (Si)	Improves strength and elasticity; used in spring steels

Important Aircraft Steel Alloys

Designation	Composition	Properties	Typical Aircraft Use
SAE 4130 (chromoly)	Cr-Mo steel (0.30% C, 1% Cr, 0.2% Mo)	Excellent strength-to-weight ratio; good weldability; responds well to heat treatment	Engine mounts, fuselage tubing (light aircraft), landing gear, structural fittings
SAE 4340	Ni-Cr-Mo steel (0.40% C, 0.8% Cr, 1.8% Ni, 0.25% Mo)	Very high strength (up to 1800 MPa heat-treated); excellent fatigue strength and toughness	Landing gear components, crankshafts, connecting rods, high-stress structural fittings
Stainless steel (18-8)	18% Cr, 8% Ni (austenitic type 304/321)	Excellent corrosion resistance; non-magnetic; good high-temperature properties	Exhaust systems, firewalls, high-temperature zones, fasteners
Maraging steel	18% Ni, plus Co, Mo, Ti	Ultra-high strength (up to 2400 MPa); tough; good fatigue resistance	Landing gear, high-performance structural parts

Identification of Ferrous Materials

Aircraft steels are identified by several methods:

• Designation systems: SAE/AISI (USA), BS (British), DIN (German), or specification numbers (e.g. AMS, MIL-S)
• Colour coding: Bars and sheets may be colour-coded on the end or edge per specification standards
• Spark test: Grinding the steel on a wheel produces characteristic spark patterns — carbon content affects spark length, branching, and colour. High carbon produces more branching and bursting sparks
• Magnetic test: Most ferrous materials are magnetic (attracted to a magnet). Exception: austenitic stainless steels (18-8 type) are non-magnetic
• Markings: Part numbers stamped or etched on components; material certificates and traceability documentation

Heat Treatment of Steel

Heat treatment is the controlled heating and cooling of metals to alter their mechanical properties without changing the shape. It is one of the most important processes in aircraft steel manufacture and maintenance.

Hardening (Quenching)

The steel is heated to above its upper critical temperature (typically 750–900°C depending on alloy) until the crystal structure transforms to austenite. It is then rapidly cooled (quenched) in oil, water, or air. The rapid cooling traps the carbon atoms in the crystal lattice, forming martensite — an extremely hard but brittle structure.

Result: Maximum hardness and strength, but very low ductility and toughness. The steel is too brittle for most applications in this state — it must be tempered.

Tempering

After hardening, the steel is reheated to a moderate temperature (150–650°C) and held for a period, then cooled. Tempering reduces brittleness while retaining most of the hardness gained from quenching. The higher the tempering temperature, the softer and tougher (but less hard) the steel becomes.

Result: The desired balance between hardness and toughness. Different applications require different tempering temperatures — springs are tempered at around 300–400°C; cutting tools at 150–250°C.

Annealing

The steel is heated to above its critical temperature and then cooled very slowly (typically in the furnace itself). This produces the softest possible condition with maximum ductility.

Purpose: To soften the steel for machining, forming, or cold working; to relieve internal stresses; to refine the grain structure.

Normalising

Similar to annealing, but the steel is cooled in still air (faster than furnace cooling but slower than quenching). This produces a finer, more uniform grain structure than annealing.

Purpose: To refine grain structure after hot working (forging, welding); to produce a uniform structure; to improve machinability.

Case Hardening

A process that hardens only the outer surface of the steel while leaving the core soft and tough. This is ideal for components that need a wear-resistant surface but must withstand shock and fatigue loads (like gears and bearing races). Methods include:

• Carburising: The steel is heated in a carbon-rich environment (solid, liquid, or gas). Carbon diffuses into the surface, increasing the surface carbon content. The part is then quenched to harden the carbon-rich surface layer.
• Nitriding: The steel is heated in an ammonia atmosphere at around 500°C. Nitrogen diffuses into the surface, forming extremely hard nitride compounds. No quenching is needed — the surface is hard as-treated. Excellent for fatigue resistance.
• Induction hardening: The surface is rapidly heated by electromagnetic induction and then quenched. Only the heated surface layer hardens.

Testing of Ferrous Materials

Hardness Testing

• Brinell test: A hardened steel or tungsten carbide ball is pressed into the surface under a known load. The diameter of the indentation is measured and converted to a Brinell Hardness Number (BHN). Suitable for large, rough surfaces.
• Rockwell test: A diamond cone (Rockwell C scale) or steel ball (Rockwell B scale) is pressed into the surface. Hardness is read directly from the machine dial based on depth of penetration. Fast and widely used.
• Vickers test: A diamond pyramid indenter is pressed into the surface. The diagonal of the resulting square indentation is measured. Very accurate; suitable for thin materials and surface-hardened layers.

Tensile Testing

A standard test specimen is pulled in a tensile testing machine until it breaks. The test measures:

• Ultimate tensile strength (UTS): Maximum stress the material can withstand
• Yield strength: Stress at which permanent deformation begins
• Elongation: Percentage increase in length at fracture (measure of ductility)
• Reduction of area: Percentage decrease in cross-section at the fracture point

Fatigue Testing

A specimen is subjected to repeated cyclic loading (bending, torsion, or axial) at a specified stress level until it fails. The number of cycles to failure is recorded. Testing at various stress levels produces an S-N curve (stress vs number of cycles). Steel typically has a definite fatigue limit — a stress level below which it can endure an infinite number of cycles without failing.

Impact Testing

• Charpy test: A notched specimen is placed as a beam and struck by a heavy pendulum. The energy absorbed in breaking the specimen (measured in joules) indicates toughness. Low values indicate brittle material.
• Izod test: Similar to Charpy but the specimen is held as a vertical cantilever. Less commonly used than Charpy in aviation.