Svařitelnost a vlastnosti svarových spojů oceli T24 (7CrMoVTiB10-10, 1.7378)

Abstract

The work is focused on the weldability of blunt peripheral weld joints of steel T24 of pipes. This issue has in recent years received considerable attention due to the detected defects on the membrane walls of the thermal power plants in Germany. The issue is also relevant for the Czech Republic due to the construction and already postponed launch of the new power unit of brown coal power Ledvice in the North Bohemia. Introduction of the theoretical part is devoted to the development, utilization and comparison of steel T24 in terms of material properties with similar grade of steel for the energy industry. There is also analysed the influence of individual alloying elements their behaviour on steel T24 and for carbide and nitride former elements are identified and characterized possible types of minority phases. In one sub-chapter there there is especially devoted attention to the secondary hardening of welded joints in the modified low-alloy heat-resistant steel. In the theoretical part there are not neglected knowledge and requirements regarding to the weldability of steel T24 and tempering of welded joints in steel T24. The experimental part of the work is divided into four chapters. In the first chapter there is carried out experimental work aimed at determining the plastic and mechanical properties of in each simulated, heat affected areas of weld joints in steel T24. The measured characteristics are complemented with the analysis of the microstructure using light and electron microscopy. In the next chapter is part of the obtained results validated under real condition of the blent welded joints of steel T24 on pipes. Further on there is an additional assessment of other selected material properties on this joint. The third part is devoted to the secondary hardening steel T24 welded joints during long-term isothermal exposure to a temperature of 575 ° C. Analysis is focused on determining the process of the hardening of weld metal, zone of overheating and unaffected base material. The last part of the experimental work is focused on determining the degree of embrittlement of simulated untempered zone of overheating due to secondary hardening during operational exposure.