Optimization of the Heat Treatment of the Material T24
The latest generation of coal-fired power plants in Europe predominantly employs the material 7CrMoTiB10-10 (T24) as tube material in membrane walls. Due to the occurrence of damages on T24 tube butt welds in membrane walls of newly constructed power plants, the VGB Working Group T24 discussed whether a heat treatment of tube butt welds in manufacturing, erection or repair may be advisable.
Several projects carried out heat treatments in different manner after the erection, but as well in manufacturing. Heat treatments have been conducted as stress relief treatment at low temperatures of about 500°C, at approx. 600°C but as well as annealing treatment in the temperature range according code. The latter has been partly carried out after a previous heat treatment at 600°C.
Ordinary member companies building or operating the latest generation of coal-fired power plants with the material T24 need a reliable decision base and a specification for the heat treatment of T24 tube butt welds used in manufacturing, erection or repair.
Heat treatment of tube butt welds can be carried out according different methods. The investigation aims at finding an optimized method.
Therefore tensile loaded and constrained tube samples with tube butt welds made of T24 with two chemical compositions will be heat treated at different temperatures following different procedures to identify essential influences on microstructure. For comparison a tube sample made of high temperature alloy shall also be tested.
The investigations are supervised by the VGB Working Group T24 upon assignment of the VGB-Technical Committee “Materials and Quality Assurance”.