Metals and Alloys
Ferrous and Non-Ferrous Metals
All metals used in engineering fall into one of two families. Ferrous metals contain iron. This includes all the steels and cast iron. Because they contain iron, most ferrous metals are magnetic and will corrode in moist air if left unprotected — a thin layer of rust forms on the surface and, unlike the oxide layer on aluminium, it does not protect the metal underneath. Non-ferrous metals do not contain iron. Examples include aluminium, copper, lead and zinc. They are generally corrosion-resistant and non-magnetic, making them valuable in electrical, marine and food-contact applications.
Ferrous metals contain iron and are mostly magnetic and corrosion-prone: cast iron, low carbon steel, high carbon steel, stainless steel. Non-ferrous metals contain no iron and are mostly corrosion-resistant and non-magnetic: aluminium, copper, lead, zinc.
Steels and the Role of Carbon
Steel is iron with a controlled amount of carbon added. Increasing the carbon content raises hardness and strength but reduces ductility and toughness. Low carbon steel (also called mild steel) contains around 0.1% to 0.3% carbon. It is tough, ductile and easy to form and weld, so it is the most widely used structural metal — think RSJ beams, car body panels and the bracket used to mount a gate post. Low carbon steel does not harden well through heat treatment, which means it cannot be used for cutting tools.
High carbon steel contains approximately 0.6% to 1.4% carbon. The extra carbon makes it much harder and it responds well to heat treatment (hardening and quenching), so it is used for springs, chisels, files, drill bits and cutting tools. The trade-off is reduced ductility and weldability — high carbon steel can crack if welded without careful preheat. Stainless steel is an alloy of iron, carbon and at least 10.5% chromium. The chromium forms a passive oxide layer that improves corrosion resistance, but stainless steel can corrode in some environments. It is used for surgical instruments, food-processing equipment and architectural cladding.
Cast iron contains 2% to 4% carbon — much more than any steel. This makes it very hard, but also brittle: it cannot be bent, drawn or welded without cracking. What it can do is pour beautifully into moulds, picking up fine surface detail, and it damps vibration extremely well. These properties make it ideal for machine beds (the heavy base of a lathe that must absorb machining vibrations), engine blocks and manhole covers. Cast iron has good compressive strength but poor tensile strength, so it must never be used in tension.
Non-Ferrous Metals in Engineering
Aluminium is the most widely used non-ferrous metal in engineering. It has a density of around 2.7 g/cm3 — roughly one third that of steel — giving an excellent strength-to-weight ratio. It is highly ductile, corrosion-resistant (a natural oxide layer forms on the surface) and a good electrical conductor. These properties make it the default choice for aircraft fuselage panels, vehicle wheels, drink cans and overhead power cables. Copper is an outstanding electrical and thermal conductor, highly ductile and corrosion-resistant. It is drawn into electrical wiring, shaped into heat-exchanger tubing and used in plumbing. Lead is very dense, soft, malleable and highly resistant to corrosion and radiation — used for X-ray shielding and roofing flashings. Zinc is used mainly for the galvanising of steel: a zinc coating corrodes preferentially, protecting the steel beneath.
Aluminium has roughly one-third the density of steel but retains good strength, corrosion resistance and ductility. This strength-to-weight ratio makes it essential wherever reducing mass is critical — aircraft, high-performance vehicles and bicycle frames.
Alloys and How They Change Properties
An alloy is formed by melting two or more elements together, usually to improve on the properties of the base metal alone. The additional atoms disrupt the regular crystal lattice of the metal, making it harder for dislocations (the defects that allow slip) to move — this is why alloys are almost always harder and stronger than the pure metals they are made from. Brass (copper + zinc) is harder than either copper or zinc alone, and easier to machine into precision components than pure copper. Bronze (copper + tin) is harder than copper and more resistant to wear, making it ideal for bearings, propellers and bushings. Brass is also corrosion-resistant and has a gold appearance, so it is used for plumbing fittings, musical instruments and decorative hardware.
Properties are not fixed at casting — they can be changed by working and heat treatment. Cold working (pressing, drawing or rolling at room temperature) distorts the crystal grains, introducing dislocations that tangle and block further movement. The result is a harder, stronger but less ductile metal. This is why cold-drawn steel rods are stronger than the equivalent hot-rolled material. If ductility needs to be restored, annealing (heating and slow cooling) recrystallises the grains and relieves the internal stresses.
For high carbon steel, hardening by quenching involves heating the steel above its critical temperature (typically above 723°C) and then plunging it into water or oil. The rapid cooling traps a hard, brittle crystal structure called martensite. Tempering follows: the steel is reheated to a lower temperature (150°C to 600°C) to allow some internal stress relief and recovery of toughness, at the cost of a small reduction in hardness. Reducing grain size also increases strength: finer grains mean more grain boundaries to impede dislocation movement.