Passage:
Passage-IV The simplest method of welding two pieces of metal together is known as pressure welding. The ends of metal are heated to a white heat — for iron, the welding temperature should be about 1300°C — in a flame. At this temperature the metal becomes plastic. The ends are then pressed or hammered together, and the joint is smoothed off. Care must be taken to ensure that the surfaces are thoroughly clean first, for dirt will weaken the weld. Moreover, the heating o f iron or steel to a high temperature cause oxidation, and a film o f oxide is formed on the heated surfaces. For this reason, a flux is applied to the heated metal. At welding heat, the flux melts, and the oxide particles are dissolved in it together with any other impurities which may be present. The metal surfaces are pressed together, and the flux is squeezed out from the centre o f the weld. A number of different types of weld may be used, but for fairly thick bars o f metals, a vee-shaped weld should normally be employed. It is rather stronger than the ordinary butt weld.
The flux is used to
- Amake the metal plastic
- Bcool the heated metal
- Ccover up any dirt
- Ddissolve oxide and other impurities
Explanation
The passage explains: 'At welding heat, the flux melts, and the oxide particles are dissolved in it together with any other impurities which may be present.' The flux is then squeezed out as the surfaces are pressed together. Its function is therefore to dissolve oxide and impurities, not to make the metal plastic, cool it, or cover up dirt. Hence option (D) is correct.