Stress relieving is done by subjecting the parts to a temperature of about 75 ºC (165 ºF) below the transformation temperature,line A1 on the diagram, which is about 727 ºC (1340 ºF) of steel-thus stress relieving is done at about 650 ºC (1202 ºF) for about one hour or till the whole part reaches the temperature. This removes more than 90% of the internal stresses. Alloy steels are stress relieved at higher temperatures. After removing from the furnace, the parts are air cooled in still air. This is by far the most frequently used form of heat treatment which will confront the authorized inspector. As a result of welding processes used to join metals together, the base materials near the weldment, the deposited weld metal and, in particular, the heat affected zones transform through various metallurgical phases. Depending upon the chemistry of the metals in these areas, hardening occurs in various degrees, dependent mainly upon carbon content. Again, this is particularly true in the heat affected zone (HAZ) adjacent to the weld metal deposit where the highest stresses due to melting and solidification result. Stress relieving, as the name implies, is designed to relieve a proportion of these imposed stresses by reducing the hardness and increasing ductility, thus reducing danger of cracking in the vessel weldments. techmat
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