WHAT IS HEAT TREATMENT
One of the purposes of heat - treating is to relieve stresses caused when two pieces of metal are joint together by welding. As the metal in welded area raised to fusion temperature, the surrounding metal tends to expand but is restrained by the unheated areas adjacent to the welded joint. As a result, the heated portion is place under a compression stress, the magnitude of which may approach and sometimes exceed the yield point of metal.
Upon the cooling, the heated areas contract and set up reverse stresses, with the result that when cooled to room temperature the metal in the weld area is under constant Tensile Stress. To avoid internal stresses, stress-relieving equipment is equally applicable for normalizing, annealing and pre-heating, and electrical heating apparatus is proving successful in carrying out stress relieving requirements, for temperature up to 1100°C.
Control of temperature is an important feature and temperature in range of 600°C to 950°C required to heat-treat the materials can be arrived at a specified rate of temperature increase. The heat can then be adjusted to maintain the area being stress relieved at the temperature and period specified. Reducing the temperature at any given rate controls the cooling period.