Influence of the focus wobbling technique on the integrity and the properties of electron beam welded MarBN steel

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.