Selecting Metals & Heat Treatment

Factors Influencing Metal Selection

Choosing the right metal for a product is rarely straightforward. Designers must balance a range of sometimes competing factors to arrive at the most appropriate choice. No single property determines the decision — a metal that is mechanically perfect may be too expensive, too heavy or impossible to source in the required stock form.

The principal factors are:

Function — the mechanical demands placed on the product. A bridge cable must have very high tensile strength; a cooking pot must conduct heat efficiently and resist corrosion from food acids. The function defines which properties are non-negotiable.
Cost — raw material cost, ease of machining and finishing all affect the final product cost. Mild steel is cheap and easy to process; titanium offers outstanding strength-to-weight ratio but costs approximately 50 times more per kilogram.
Aesthetics — appearance matters for consumer products. Copper has a warm, distinctive colour and develops a patina; polished stainless steel projects a modern, hygienic image. Anodised aluminium can be given vivid colours for consumer electronics and sporting equipment.
Availability — even the ideal metal is useless if it cannot be sourced in the required quantity, lead time or stock form. Exotic alloys may only be available from specialist suppliers with long lead times, disrupting production schedules.
Environment — the operating environment determines corrosion resistance requirements. A boat hull operates in salt water; a storage rack in a dry warehouse faces very different corrosion risks. Materials used in food contact must comply with food-safety regulations.

Key Fact

A complete metal-selection answer must address at least two factors: the key functional property (e.g. tensile strength, corrosion resistance) and one additional factor (cost, availability, aesthetics or environment). Answers that only cite one property will not reach the higher mark bands.

Stock Forms

Metals are supplied commercially in a range of stock forms. Selecting the appropriate stock form reduces the amount of material that must be removed and can simplify manufacturing. Common metal stock forms include:

Sheet and plate — flat, rolled metal. Sheet is typically under 3 mm thick; plate is 3 mm and above. Used for car body panels (sheet steel), structural decking (plate steel) and roofing (copper sheet).
Bar and rod — solid sections with circular, square, hexagonal or rectangular cross-sections. Hexagonal bar is used for nuts and bolt blanks; round bar is turned on a lathe to produce shafts and spindles.
Tube — hollow cylindrical or square section. Used in bicycle frames (lightweight tubing), handrails and scaffolding. The hollow section gives a high strength-to-weight ratio.
Angle section — L-shaped cross-section, used in shelving frameworks, brackets and structural frames. Provides rigidity without excessive weight.

Selecting the most appropriate stock form from the outset saves significant machining time and reduces material waste. If a component is essentially a hollow cylinder (such as a handrail or a bicycle fork), starting from tube rather than solid rod eliminates the need to bore out the centre — a time-consuming and wasteful operation. Similarly, using angle section for shelf brackets means the required L-shaped profile already exists; cutting it from flat plate would waste roughly half the material. Designers should always ask which stock form most closely matches the final component geometry before choosing a process.

Forces, Stresses and Work Hardening

When metals are in service they experience various forces. Understanding these helps designers choose the right metal and design to minimise failure:

Tension — a stretching force pulling the material apart (e.g. a lifting cable). Requires high tensile strength.
Compression — a squashing force (e.g. a column supporting a load). Cast iron performs well in compression.
Shear — forces acting in opposite directions across a cross-section (e.g. the force on a rivet holding two plates together).
Torsion — a twisting force (e.g. a drive shaft transmitting rotary motion from an engine to the wheels).

Work hardening occurs when a metal is repeatedly deformed at room temperature. Each deformation increases the dislocation density in the metal’s crystal structure, making it harder but less ductile. A paperclip repeatedly bent back and forth becomes harder and eventually snaps — this is work hardening in action. It can be useful (cold-drawing wire to increase its strength) or problematic (a sheet of metal becoming too brittle to bend further during forming).

The mechanical properties of steel are not fixed; they can be deliberately altered by carefully controlled heating and cooling. This is because the crystal structure of iron changes at specific temperatures. By controlling the temperature and the rate of cooling, engineers can produce steels ranging from very soft and ductile (for forming) to extremely hard (for cutting tools). Heat treatment therefore gives designers a second layer of material selection: not just which steel to use, but which state to use it in.

Hardening and tempering are most commonly applied to medium and high carbon steels and to high-speed steels. They do not work on mild steel, which holds too little carbon to respond. Mild steel can still be annealed, normalised or case hardened. Hardening also cannot make non-ferrous metals such as aluminium hard in the same way, though aluminium alloys can be heat-treated to improve strength by a different mechanism (precipitation hardening).

Heat Treatment Processes

The main heat treatment processes for steel are annealing, hardening, tempering, normalising and case hardening. Each involves different temperatures and cooling rates, producing different outcomes:

Annealing — the steel is heated to a specific temperature (typically 700–900 °C depending on carbon content), held there to allow the crystal structure to reorganise, then cooled very slowly (usually by leaving it in the furnace to cool). The result is a soft, ductile metal that is easy to machine or form. Annealing is used to relieve internal stresses after welding or to restore ductility to work-hardened metal.
Hardening — the steel is heated above its critical temperature (typically 760–900 °C) until it glows orange/yellow, then quenched (plunged rapidly into water, oil or brine). The rapid cooling prevents the carbon atoms from migrating, locking a hard but brittle crystal structure (martensite) in place. The steel is now very hard but too brittle for most uses and must be tempered.
Tempering — always follows hardening. The hardened steel is reheated to a lower temperature (150–400 °C, depending on the required toughness), held briefly, then allowed to cool in air. This reduces some of the brittleness introduced by hardening while retaining most of the hardness. The temperature is judged by oxide colours that appear on polished steel: pale yellow (∼220 °C) for scrapers, blue (∼300 °C) for springs. Tempering produces a balance of hardness and toughness.
Normalising — similar to annealing but the steel is cooled in still air rather than in the furnace. This produces a slightly harder and stronger result than full annealing. Normalising is used to refine the grain structure after forging or hot rolling, producing a more uniform microstructure across the material.
Case hardening — used specifically for mild steel, which has too little carbon to be hardened by quenching alone. The surface is enriched with carbon (by heating in a carbon-rich medium such as charcoal powder) and then quenched. The result is a hard outer case with a tough, ductile core. This combination is ideal for gear teeth and camshafts, which need a wear-resistant surface but a core that can absorb impact without shattering.