Engineering Materials: Classification, Properties and Alloys
Engineering materials is the domain that asks you to classify what a workshop item is made of, name the property that justifies the choice, and read carbon or alloy percentages correctly. Expect one-mark classification questions plus a short answer on properties or on a named alloy.
Classification
- Ferrous metals have iron as the base: pig iron (about 3.5–4.5 % carbon), cast iron (2–4.5 % C), wrought iron (under 0.15 % C) and steels (up to about 1.5 % C).
- Non-ferrous: copper, aluminium, zinc, lead, tin. Brass is copper–zinc; bronze is copper–tin; solder is tin–lead; duralumin is aluminium with copper, magnesium and manganese.
- Non-metallic: timber, rubber, plastics, ceramics. Composites combine two materials, for example glass-reinforced plastic.
Properties you must name precisely
Strength resists load; hardness resists indentation or scratching; toughness absorbs shock without fracture; ductility allows drawing into wire; malleability allows hammering into thin sheets; elasticity returns to the original shape; brittleness fractures with little deformation; weldability and machinability describe behaviour in the shop. Cast iron is hard and brittle but excellent in compression; mild steel is ductile and weldable.
Alloys worth memorising
Stainless steel: about 18 % chromium and 8 % nickel, corrosion resistant. High-speed steel 18-4-1: 18 % tungsten, 4 % chromium, 1 % vanadium; it retains hardness when red hot, so it is used for drill and lathe tools.
Worked example
A shaft steel is stated as 0.45 % carbon. Since 0.45 % lies between 0.3 % and 0.6 %, it is medium-carbon steel. That grade responds to hardening and tempering, which is why it suits shafts and gears rather than car-body panels (about 0.1 % mild steel, ductile for pressing) or files (0.8–1.2 % high carbon, hard and wear resistant). Give the percentage band and the reason, not just the name.
Traps
Do not swap malleability (sheets, hammering) with ductility (wire, drawing). Remember that higher carbon means higher hardness but lower ductility and weldability. Do not call stainless steel non-ferrous; it has an iron base. Never state that cast iron can be forged - it cannot.