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Due to accelerated creep damage, martensitic 9%Cr welded joints fail prematurely in the fine-grained heat affected zone (FGHAZ). These failures in the FGHAZ are categorized as “Type IV cracking”. In recent years, boron containing 9% Cr steels have been developed that suppress the FGHAZ formation in order to avoid Type IV cracking.
The advanced alloying concept is based on the balanced addition of boron and nitrogen. Creep tests of 9Cr3W3CoVNbBN steel crosswelds have been carried out up to 17000 hours at 650°C. The crossweld creep strength was analyzed and the damage evolution in crosswelds was investigated using synchrotron micro-tomography of creep exposed welded joints. By synchrotron micro-tomography, the 3-dimensional void distribution in the heat affected zone of the creep exposed welded joints was visualized. Additionally, the microstructure of the crossweld specimens was investigated using optical and scanning electron microscopy and electron backscatter diffraction. Creep void formation along the large original prior austenite grain boundaries, at a certain distance to the weld fusion line, in the heat affected zone was observed. Electron backscatter diffraction images revealed the incomplete suppression of grain refinement along the prior austenite grain boundaries. These newly formed small grains have been identified as the main reason for the increased creep void formation in this area.
The combination of the results of long term creep testing with 2D and 3D failure investigations, allows a completely new view on the basic damage evolution at elevated temperatures. Based on these results, further improvements of 9Cr3W3CoVNbBN steels and welded joints are discussed.