Heat treatment and forging
This domain covers annealing, normalising, hardening, tempering, case hardening and the forging operations used to shape hot metal. The exam tests temperatures, cooling rates, tempering colours and safe forging practice.
Core rules
- Annealing softens steel: heat above the critical temperature, soak, then cool very slowly, for example in a furnace or in ash.
- Normalising refines grain structure: heat above the critical temperature and cool in still air.
- Hardening: heat high-carbon steel to its hardening temperature, then quench in water, oil or brine according to the steel. High-carbon steel is prone to cracking in a severe water quench.
- Tempering reduces brittleness after hardening. Polish the surface, heat gently and judge the temper colour: pale straw about 220 degrees C, straw about 230, brown about 240, purple about 270, blue about 300.
- Case hardening adds carbon to a low-carbon steel surface, then quenches to give a hard case with a tough core.
- Forging operations include drawing out, upsetting, bending, punching and twisting. Mild steel forges roughly in the 1100-1300 degrees C range; stop before it becomes dull red.
Worked example
A cold chisel is made from high-carbon steel. Heat the cutting end to about 780-800 degrees C, which is cherry red, and quench it in water. Polish the cutting end, then heat the shank gently and watch the temper colour run to the edge. Stop at straw to brown, about 230-250 degrees C, and quench. Reason: full hardening makes the edge hard but brittle; tempering softens it slightly and gives toughness, so the edge resists shock without chipping.
Traps that cost marks
- Quenching a high-carbon tool in water and causing cracks.
- Tempering beyond blue, which leaves the tool too soft.
- Forging below the correct temperature, causing cracks.
- Confusing annealing with normalising.
- Heating a tool unevenly, so the temper colour is misleading.