Heat treatment of welded joints in the repair of steam boiler drums

摘要

Until recently, the heat treatment of welded joints in steam boiler drums in Russia was carried out in accordance with RD 34.26.608 standard. As a result of the publication of new rules SO 153-84.26.608-2003, the requirements on heat treatment have changed only slightly. These rules apply to drums made of 22K, 20K, 20B, 20, 60M, 15M, 16GNM and 16GNHMA steels. The drums of boiler systems have the form of horizontal pressure vessels up to 25 m long with the external diameter of up to 2.0 m and the wall thickness up to 120 mm, working at temperatures of up to 340 deg C and pressures of up to 16 MPa. In the process of long-term service for 250 000 hours at 300-400 start-ups, damage in the form of cracks, corrosion crevices, and surface cracking appears m different parts of the drums (the orifices, the area as a welding fixing devices for intra-drum systems, nozzles, lids, the shells of the body, etc). After repair of these defects by welding, it is necessary to carry out high-temperature tempering with heating to 620-620 deg C (in the case of 16GNM and 16GNMA steals) and 600-630 deg C (in the case of steel 22K, 20K, 20B, 20, 16M, 15M) at a rate of 40-50 deg C/h with holding for 5 h and cooling to 150 deg C at a rate of 20-30 deg C/h, followed by tempering under a layer of heat-insulating material (Table 1). Temperature is controlled using thermocouples installed on the external and internal size of the drum (Fig. 1). The temperature gradient in the thickness of the wall of the drum should not be greater than 30 deg C. The duration of the cycle of heat treatment is up to 36 h.