How the properties of materials are enhanced before manufacture, including heat treatment of metals (hardening, tempering, annealing, normalising and case hardening), work hardening and alloying, the seasoning and preservative treatment and lamination of timber, and the addition of admixtures and reinforcement to polymers and composites.

What this dot point is asking

AQA wants you to explain how a material's properties are deliberately improved before or during manufacture, and to match each enhancement to a reason. The key idea is that you can change how a material performs (harder, tougher, more stable, more flexible) without swapping it for a different material.

Heat treatment of metals

The processes you must know form a connected family:

• Hardening heats high-carbon steel above its critical temperature, then quenches it rapidly (water or oil). The result is very hard but brittle, so it is rarely the final state.
• Tempering reheats hardened steel to a lower temperature and cools it. This trades a little hardness for a large gain in toughness, which is why a chisel or knife is hardened and then tempered.
• Annealing heats the metal and cools it very slowly (often in the furnace). This makes it as soft and ductile as possible, relieving internal stresses so it can be cut, bent or machined.
• Normalising heats and air-cools steel to refine the grain and relieve the stresses left by working, giving a uniform, stable structure.
• Case hardening adds carbon to the surface of low-carbon (mild) steel and then hardens it, producing a hard, wear-resistant skin (the case) over a tough, shock-absorbing core. It is used on gears and spanners.

Work hardening and alloying

Work hardening strengthens a metal by deforming it cold (hammering, rolling, bending). The repeated working distorts the structure and makes the metal harder and stronger but less ductile, so it eventually becomes brittle and can crack. Annealing reverses work hardening when more forming is needed. Alloying mixes a base metal with other elements to gain properties the pure metal lacks: adding chromium to steel gives stainless steel (corrosion resistance), adding carbon increases hardness, and adding copper to aluminium increases strength. The examiner expects you to link an alloying element to the property it delivers and a typical product use.

Enhancing timber

Timber is enhanced in three main ways:

• Seasoning reduces the moisture content of green timber to a stable level. This increases strength and resistance to decay and stops the wood warping, splitting or shrinking after manufacture. Air seasoning is cheap but slow; kiln seasoning is fast and controllable but uses energy.
• Preservative treatment protects timber from fungal attack (rot) and insect attack, by pressure-impregnating or coating it with preservatives, which matters most for exterior timber such as fencing and decking.
• Lamination glues thin layers (laminae) of timber together, often with the grain aligned. This increases strength and stability and lets curved forms be made (laminated beams, bent chair frames) that solid timber could not achieve without snapping.

Enhancing polymers and composites

Polymers are enhanced by adding admixtures (additives) during processing:

• Plasticisers make a rigid polymer flexible (rigid PVC pipe versus flexible PVC cable sheathing).
• Pigments add colour without a separate finishing stage.
• Stabilisers resist degradation from UV light and heat, extending outdoor life.
• Fillers add bulk cheaply and can improve stiffness or impact resistance.
• Flame retardants slow ignition, required in furniture and electrical housings.

Composites are enhanced by reinforcement: embedding strong fibres (glass, carbon, aramid) in a polymer matrix gives a far higher strength-to-weight ratio and toughness than either material alone, which is why carbon fibre reinforced polymer is used in performance products.