The Influence of Low-Temperature Restorative Heat Treatment on the Structure and Mechanical Properties of Boiler Drum Metal after Long-Term Operation

Abstract

The drum metal properties of the BKZ-160-100FB and TP-85 boilers installed at the Irkutsk TETs-11 combined heat and power plant were investigated after these boilers had been in operation for 259 000 and 224 000 h, respectively, and after they had been subjected to low-temperature restorative heat treatment, which was carried out with steam produced by an external boiler after replacing all unions installed in the drum’s water volumes. “Plugs” made of grade 22K carbon steel and of grade 16GNM low-alloy steel cut from the drum shell rings were used as research subjects. The samples were subjected to metallographic analysis, tensile tests, impact bending test, and hardness measurement test. The results of tensile tests carried out at room temperature and at the metal operation temperature are presented. The results of drum metal U-notched specimens impact strength tests obtained at room temperature are given. Impact strength tests on V-notched specimens were carried out for samples after their metal had been subjected to low-temperature restorative heat treatment in the temperature range from –10 to +80°C. Based on the results of the V-notched specimens tests, the ductile constituent in the fractures was determined, and the drum metal ductile-to-brittle transition temperature was determined. The microstructure and mechanical properties that the drum metal had after having been in operation and having passed subsequent low-temperature restorative heat treatment were in compliance with the requirements of the relevant regulatory documents. After the boiler drum metal had been subjected to low-temperature restorative heat treatment, the studied steel samples showed a higher proof strength both at room temperature and at the operation temperature. Based on the data obtained from the investigations of boiler drum metal after it had been in operation and then subjected to low-temperature restorative heat treatment, technical diagnostics, and numerical strength analysis, it was recommended to extend the service life of the drums at their design (certificate) parameters.