On this page
Metals are split into two groups by one ingredient: iron. Ferrous metals are iron-based: all steels and cast irons. Non-ferrous metals contain little or no iron: aluminium, copper, titanium, magnesium, nickel, zinc and their alloys. The split is useful because it predicts a lot at once: ferrous metals are usually stronger, cheaper and magnetic, and most of them rust; non-ferrous metals are usually lighter or more corrosion-resistant, and cost more.
Ferrous metals
Carbon steels
Iron with up to about 2 % carbon (in practice under about 1 %). Carbon is the main thing that sets the steel’s strength and hardness.
| Type | Carbon | Properties | Typical uses |
|---|---|---|---|
| Low carbon (mild) steel | below 0.25 % | Ductile, easy to weld and bend, cannot be hardened much | Sheet metal, frames, brackets, structural sections |
| Medium carbon steel | 0.25–0.6 % | Stronger, can be hardened and tempered | Shafts, axles, gears, crankshafts |
| High carbon steel | 0.6–1.0 % and more | Hard and wear-resistant, less ductile | Springs, blades, cutting tools, wire |
Alloy steels
Carbon steels with chromium, nickel, molybdenum or vanadium added. The alloying elements let thicker sections harden through and improve toughness and fatigue strength. 42CrMo4 (4140) and 34CrNiMo6 (4340) are common choices for highly loaded shafts, gears and bolts.
Stainless steels
Steels with at least 10.5 % chromium, which forms a thin, self-repairing oxide film that stops rust.
- Austenitic (304, 316): the most common; excellent corrosion resistance, good to weld, non-magnetic when annealed. 316 adds molybdenum for salt water and chemicals.
- Ferritic (430): cheaper, magnetic, moderate corrosion resistance; used for trim and appliances.
- Martensitic (410, 420, 440C): can be hardened; used for knives, valve parts and bearings.
- Duplex (2205): a mix of austenite and ferrite; about twice the yield strength of 304 with better resistance to stress-corrosion cracking.
Tool steels
High-carbon alloy steels made to be hardened, for cutting tools, dies and moulds. Examples are D2 (cold-work dies), H13 (hot-work and die-casting dies) and M2 (high-speed steel drills and taps).
Cast irons
Iron with 2–4 % carbon, which makes it melt at a lower temperature and flow well into moulds.
- Grey cast iron: the carbon forms graphite flakes, which give excellent vibration damping and machinability but low tensile strength and ductility. Used for machine beds, engine blocks and brake discs.
- Ductile (spheroidal graphite) iron: the graphite forms round nodules, so it is much tougher and can bend before breaking. Used for crankshafts, gear housings and suspension parts.
Non-ferrous metals
| Metal | Density (kg/m³) | Key strengths | Typical uses |
|---|---|---|---|
| Aluminium alloys | 2700 | Light, corrosion-resistant, conducts heat, easy to machine and extrude | Frames, housings, enclosures, heat sinks |
| Copper | 8960 | Best common conductor of electricity and heat | Wiring, bus bars, heat exchangers |
| Brass (copper–zinc) | about 8500 | Machines very easily, corrosion-resistant, attractive | Fittings, valves, fasteners, decorative parts |
| Bronze (copper–tin) | about 8800 | Wears well against steel, resists sea water | Bushes, bearings, worm wheels, marine parts |
| Titanium alloys | 4430 | Very high strength per kg, corrosion-proof, body-friendly | Aerospace, medical implants, racing parts |
| Magnesium alloys | about 1800 | Lightest structural metal | Light castings, laptop and camera bodies |
| Nickel alloys | about 8200–8900 | Keep their strength very hot, resist corrosion | Turbine parts, exhaust systems, chemical plant parts |
| Zinc alloys | about 6600 | Low melting point, very accurate die castings | Small die-cast housings, handles; coatings on steel |
Aluminium is the non-ferrous metal you will use most in machine design; it has its own comparison with steel.
Ferrous vs non-ferrous: the comparison
| Ferrous | Non-ferrous | |
|---|---|---|
| Main element | Iron | Aluminium, copper, titanium, nickel and others |
| Magnetic | Yes (except austenitic stainless) | No (except nickel and cobalt) |
| Corrosion | Rusts, unless stainless or coated | Generally good; most form protective films |
| Density | High (about 7200–8000 kg/m³) | From 1800 (magnesium) to 8900 (copper, nickel) |
| Strength and stiffness | High | Lower on average, but titanium and some alloys are very strong |
| Conductivity | Low | High for aluminium and copper |
| Cost | Low | Higher, sometimes much higher |
| Recycling | Easy to sort with magnets | Valuable; sorted by composition |
Choosing between them
- Function and load. Heavily loaded structures, shafts and gears are usually steel because it is strong, stiff and cheap. Use non-ferrous metals where another property matters more: low weight, conductivity, low friction or appearance.
- Environment. Outdoors, near water or with food, choose stainless steel, aluminium, brass or bronze, or coat the carbon steel. At high temperature, steels and nickel alloys outperform aluminium and copper alloys.
- Cost. Steel is the cheapest strong metal. Weigh the extra cost of non-ferrous metals against savings in coating, machining time and weight.
- Manufacturability. Brass and aluminium machine fastest; mild steel welds most easily; aluminium and zinc cast and extrude best.
- Specification. Always give the grade, not just the family: “brass” covers alloys from free-machining CW614N to cold-forming CW508L.
Example: a small worm gear pair often uses a hardened steel worm running against a phosphor bronze wheel. The steel worm carries the load and resists wear; the bronze wheel slides smoothly against it without galling.
Key points
- Ferrous means iron-based: carbon, alloy, stainless and tool steels, and cast irons.
- Carbon content sets a steel’s strength and hardness; chromium above 10.5 % makes it stainless.
- Non-ferrous metals trade cost for lightness, conductivity or corrosion resistance.
- Pick by function, load, environment, cost and manufacturing, then specify the exact grade.