Fig. 5 shows a screwdriver. All the bits are smooth. It works in rain and sun. The blade is thin and sharp. The parts fit well and look neat. The oak does n...

(a) The type of steel used for the blade of the screwdriver is high carbon steel. [1] High carbon steel contains between 0.6% and 1.4% carbon, which allows it to be hardened and tempered to achieve the necessary combination of hardness and toughness for a tool blade.

(b) The blade must be heat treated for two reasons:

• The blade needs to be hardened so that the tip does not wear down or deform when it is repeatedly turned against the slot of a screw under force. [1] Without hardening, the soft steel would quickly round off and become useless.
• After hardening, the blade also needs to be tempered to reduce its brittleness so that it does not snap or shatter under the twisting forces and impact loads that a screwdriver experiences during use. [1] Pure hardened steel is extremely hard but also very brittle.

(c) The heat treatment process of hardening and tempering the blade:

• Hardening: The blade is heated to a cherry red colour (approximately 750-780 degrees Celsius for high carbon steel) and then quenched rapidly by plunging it into water or oil. [1] The rapid cooling locks the carbon atoms in a stressed crystal structure called martensite, which is extremely hard but also very brittle.
• Tempering: The hardened blade is then reheated to a lower temperature (around 230-300 degrees Celsius), which can be judged by the oxide colour that forms on a polished surface (light straw to dark straw for screwdriver blades). The blade is then quenched again. [1] Tempering reduces the internal stresses in the martensite, making the steel slightly less hard but significantly less brittle, giving it the toughness needed to withstand use without snapping.