Fig. 2 shows a cold chisel being used to cut a piece of metal on an anvil. It is made in a shop. All the bits are smooth. It can be fixed if it breaks. It c...

(a) The type of metal used for the cutting edge of a cold chisel is high carbon steel (tool steel). [1] High carbon steel can be hardened by heat treatment, giving it the hardness needed for a cutting tool that must maintain a sharp edge when struck against other metals.

(b) The process of case hardening:

1. The chisel is packed in a carbon-rich material such as charcoal, carbon powder, or a proprietary case-hardening compound, inside a sealed metal container. [1]
2. The container is heated to a high temperature (around 900-950 degrees Celsius) and held at this temperature for several hours to allow carbon atoms to absorb (diffuse) into the outer surface layer of the steel. [1] The longer the soaking time, the deeper the carbon penetration.
3. The component is then quenched in water or oil to harden the carbon-enriched outer layer. [1]

Case hardening is suitable for the cold chisel because the chisel needs a hard, wear-resistant outer surface at the cutting edge to maintain sharpness and resist abrasion, while the core must remain tough and ductile to absorb the repeated impact of hammer blows without fracturing. Case hardening achieves exactly this combination: a hard case surrounding a tough core.

(c) One disadvantage of case hardening compared with through hardening:

Only the outer surface layer is hardened, so with heavy or prolonged use the hard layer can wear through, exposing the softer core beneath. [1] The depth of the hardened case is relatively thin and difficult to control precisely, so the protection may not last as long as a component that is hardened uniformly throughout its full cross-section (through hardening).

(d) The heat treatment process carried out after hardening the chisel to prevent it from being too brittle is tempering. [1] Tempering involves reheating the hardened steel to a controlled lower temperature and quenching it again, which relieves internal stresses and reduces brittleness while retaining most of the hardness.