New results of the tempering behavior of T24 between 200°C and 700°C-Their interpretation regarding service and additional repair

摘要

Between 2005 and 2007 a lot of new coal-fired power plant projects were initiated in Germany. That was not only the start of normal investment. It was also the start for the new class of the 600/620°C supercritical steam generators in Central Europe. With the increase of the end temperature in the water-steam-cycle, there is also an increase in the end temperature of the membrane wall. Therefore it was decided to use the qualified material T24. The T24 is a high temperature steel equally described as 7CrMoVTiB10-10 (1.7378). For most mechanical, physical and chemical loads it has the same behaviour as T23. But different damage was found when using T24; this was particularly the case in the heat affected zone (HAZ) of the weld during first commissioning. Different causes were investigated. One of them was the influence of the welding process on the material structure. The change in the HAZ could increase the stress corrosion cracking (SCC) susceptibility by tension stress level, structure and lower ductility [1, 2]. One part of the technical solution to prevent additional damage was the stress relief heat treatment (SRHT) of the complete membrane wall in one technological step also called “boiler heating”. The question was raised as to which processes take place inside the material to reduce the risk. Different investigations concluded that the reduction of the local residual tension stress and the homogenising of the stress state around the weld provide the main cause of the reduction of SCC susceptibility. From the point of view of material science, it was assumed that additional mechanisms take place. In order to answer that question, a program was started to evaluate the generation of the HAZ structure when welding. After these investigations, calorimetric and dilatometric measurements were performed to identify the assumed material change.