Local heat treatment is usually carried out with electrical heaters placed close to the weld seam, but gas
may be used. In special cases, such as standard pipe work, joints are heated by specially prepared
exothermic pads, which are placed round the weld and ignited. The heat generated during combustion is
calculated to heat the joint to the required stress relief temperature without any possibility for control once
the operation has been initiated. Generally, local heat treatment of structural items is covered by the
requirements of BS 5500 1976, ‘Specification of Unfired Fusion Welded Pressure Vessels’, which details
the procedure necessary to ensure satisfactory stress relief. Figure 6 shows the closing seam of a trunnion
ring for a large basic oxygen steel-making furnace being prepared for site stress relief. The heating elements
which were attached to the weld seams to control the preheat cycle were also used for the subsequent stress
relief. Local weather protection was erected over the weld area.