The Behaviour of Weldings in Large 9

摘要

In large castings for steam turbine components manufacturing, as well as fabrication, weldings are standard cycles in manufacturing process. It is, therefore, of great importance to know about the behaviour of welds during the logn term service of the castings. A soft zone in the weld connection ,negatively influencing the creep behaviour of welded joints, was detected and localised in the intercritical zone of the heat affected zone, limiting the long term creep strength of welds. Energy-filtering transmission electron microscopy (EFTEM) has een used for imaging of precipitates and grain boundary phases in the soft zone of the HAZ and in the weld deposit of advanced ferritic martensitic Cr steels. Resulting from the different creep duration times at a temperature of 600 deg C, the joint contains many different precipitates and grain boundary phases such as chromium carbides (M_23C_6), vanadium nitrides, niobium carbonitrides (MX), Mo-W Laves phase and a new type of Cr-V-Nb-N rich precipitate the so called modified Z-Phase. The appearance of the Cr and V rich Z-Phase was followed by decrease of the population of M_23C_6 and MX precipitates. According to these results a decrease of the creep strength in the soft zone of the HAZ beginning after about 10000 h, was found which seems to be in connection with the observed microstructural changes. In the design of welded components made from this type of materials, this effect must be taken into account.~1-4