Heat treatment of metals: HSC Industrial Technology Metal and Engineering

What this dot point is asking

Heat treatment lets you change the properties of a steel without changing what it is made of, simply by heating and cooling it in controlled ways. NESA expects you to explain the main processes, hardening, tempering, annealing and normalising, and to describe how the rate of heating and cooling alters hardness, toughness and machinability. This explains why a tool steel can be both hardened to cut and softened to machine, and it informs production decisions in your Major Project.

Why heat treatment works

Steel can exist in different internal structures depending on temperature, and how fast you cool it decides which structure is locked in. Cool slowly and the steel becomes soft and ductile; cool quickly and it becomes hard and brittle. Heat treatment exploits this by heating a steel to the right temperature and then controlling the cooling rate to produce the properties you want. Only steels with enough carbon respond strongly to hardening, which links this content back to carbon content.

Hardening

Hardening makes steel hard and wear-resistant. The steel is heated until it reaches the correct structure, then quenched, cooled very rapidly in water or oil. The fast cooling traps a hard, strong but brittle structure. A hardened part resists wear and holds an edge, but on its own it is too brittle for most uses because it can crack or shatter under shock.

Tempering

Tempering follows hardening to restore toughness. The hardened part is reheated to a lower temperature and then cooled, which relieves the brittleness and trades a little hardness for much greater toughness. The tempering temperature, often judged by oxide colours on bright steel, sets the final balance: a cutting tool is tempered less to stay hard, a spring is tempered more to stay springy and tough. Hardening and tempering are almost always done as a pair.

Annealing

Annealing makes steel as soft, ductile and stress-free as possible. The steel is heated and then cooled extremely slowly, usually in the furnace. This produces a soft structure that is easy to machine, bend and form, and it removes internal stresses left by previous work. Annealing is used before heavy machining or forming, and to reverse work hardening.

Normalising

Normalising also relieves stress and refines the grain, but the steel is cooled in still air rather than slowly in the furnace. The result is a steel that is stronger and tougher than annealed steel but softer than hardened steel, with a uniform fine grain. It is often a preparatory step before machining or further treatment.

Case hardening

Case hardening produces a hard, wear-resistant outer skin on a tough, shock-resistant core, ideal for parts like gears and shafts. Low-carbon steel is enriched with carbon at its surface and then hardened, so only the skin becomes hard while the core stays tough. This combines two properties that would otherwise conflict in a single piece.

Selecting the right treatment

Choose the treatment by the property you need. Harden and temper a cutting tool so it is hard yet not brittle. Anneal stock before heavy bending or machining so it works easily. Normalise to refine grain and relieve forming stresses. Case harden a part that needs a wear-resistant surface over a tough core. Always pair hardening with tempering, and state the property each step delivers when you justify your process in the folio.