Describe the following heat treatment processes: (i) tempering (ii) carburizing – NABTEB QUESTION

(i) Tempering is often called drawing or drawing the temper. It is heat treatment process which relieves internal strain in hardened steel and this increases it toughness. Steel can be tampered by heating it to a temperature below the lower critical temperature and then quenching. The tempering temperature depends upon the use to which the article is to be put. Tools are usually tempered at low temperatures. These temperature are judged by the temper colour which appear on the freshly polished surface of steel when it is heated.

(ii) Carburizing This is the process of inducing carbon into steel. This is done by packing the steel components in boxes containing a carbon rich material e.g. carbon. The lids of the boxes are sealed with clay to exclude air. The boxes are heated at 900°C to 950°C for three to eight hours depending upon the depth of case required. After carburizing the components are allowed to cool slowly in the boxes.