Martensitic 9-12% chromium steel is a favored material group for critical components in thermal power plants. However, during welding of these steels, a fine-grained zone is formed within the heat-affected zone, which is subject to type IV cracking during creep exposure. Due to the MarBN alloying concept, which is characterized by the controlled addition of boron and nitrogen, the formation of the fine-grained heat-affected zone can be suppressed and the minimum creep rate can be significantly decreased. First, EB welding studies were carried out, and the influence of the process parameters on the weld seam quality, geometry, microstructure, and hardness was investigated. However, independent of the used parameters, microfissures were detected within the fusion zone of the welds. To reduce the hot crack formation within the fusion zone, focus wobbling was used. Results showed an improvement with respect to accumulated number and length of the microfissures, but the formation could not completely be prevented. Despite this problem, the mechanical properties of joint welds were determined and creep investigations were conducted. Results showed that the existence of microfissures within the fusion zone has no major influence on the mechanical and the creep properties.

Martensitic 9-12% chromium steels present the most preferred material group for high-temperature components in thermal power plants. Previous investigations revealed that due to the use of a creep-resistant martensitic steel strengthened with boron and nitrogen (MarBN), the minimum creep rate can significantly be decreased. Furthermore, the formation of the fine-grained heataffected zone (FGHAZ) due to welding can be suppressed. This FGHAZ is subject to the most dominant failure mode (type IV cracking) of welded joints during creep exposure. By using electron beam welding, the total width of the heat-affected zone (HAZ) can be reduced compared to conventional arc welding processes. Preceding investigation on electron beam welding of MarBN steel showed recurring difficulties with hot cracking within the fusion zone. Various approaches were tried to produce defect-free welds without the use of any filler metal, but no satisfactory results were achieved. In this investigation, the chemical composition of the fusion zone was modified by the addition of conventional 9% chromium creep-resistant steel as a filler material. By using the filler material, the fusion zone was locally (de-)alloyed and defect-free joints of MarBN steel were produced.

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