Forging, Casting, Heat Treatment and Finishing: notes

Forging

• Forge: a hearth that heats metal to a bright cherry red (about 800 to 900 degrees C for mild steel) before it is hammered.
• Anvil: the heavy steel block for hammering. Tongs: hold the hot work.
• Operations: drawing down (making longer and thinner), upsetting (making shorter and thicker), bending and punching.

Heat treatment

• Annealing: heat then cool slowly to soften metal and relieve stress.
• Hardening: heat high carbon steel to cherry red then quench (cool fast) in water or oil. It becomes hard but brittle.
• Tempering: reheat hardened steel gently to reduce brittleness. The colour of the oxide film shows the temperature: pale straw for scribers and brown for chisels.
• Normalising: heat and cool in still air to refine the grain.

Casting

• Sand casting: a pattern is pressed into moulding sand to leave a cavity, molten metal is poured through the sprue, and the casting is removed once cold.
• Core: a sand shape that makes a hollow in the casting.
• Draft: a slight taper on the pattern so it lifts out of the sand.

Finishing

• Rust: forms when iron meets water and oxygen. Paint, oil, galvanising (zinc coat) and plating keep these out.
• Polishing: removes marks with abrasive cloth and polish.