[en] The design and construction of heat exchangers, especially in reactor engineering, with metal melts such as sodium or sodium/potassium used as a heat transfer medium brings up the problem of carbon transport between various materials through this medium. If the conventional ferritic and austenitic chrome -nickel steels are used side by side, carburization of the austenitic steels will take place whereby structural elements, such as the fuel element and moderator jackets, are adversely affected. This phenomenon is due to different activities of the carbon contained in the ferrite and austenite, and it is sufficiently known from the use of welded joints of the ferritic-austenitic type. It was therefore necessary to determine whether a suitable ferritic material with an adequately low carbon activity could be developed for sodium circuits. Compared with austenite, such a material has the advantage of a higher rate of economy, a fact to be taken into account along with the low material costs and also with its higher thermal conductivity and lower thermal expansion. Proceeding from the heat-resistant 2 1/4 chrome and 1% molybdenum steel, the effect of carbideforming elements upon the decarburization tendency when brought into contact with chrome -nickel austenite was studied at temperatures up to 700°C. Additions of niobium and titanium prevent decarburization, if the concentration is sufficient to stabilize the entire carbon and nitrogen. Diffusion tests showed that this condition is satisfied if the titanium or niobium content corresponds to the stoichiometrical composition of the respective carbides and nitrides. It was surprising to find that the carbide-forming element, vanadium, even at very high concentrations, did not effectively prevent carbon diffusion. Subsequently, preference was given to the Nb-stabilized material because niobium is much more suitable than titanium to obtain a sufficiently high stress -rupture strength. Because of the association of carbon with niobium and of the ferrite-forming effect of the latter element, it was expected that the properties of the conventional chrome-molybdenum steel would be altered. Examinations were therefore made to determine the influence of rising niobium contents on the transformation and precipitation properties. First, a quasi-binary section of the ironcarbon- chromium-.molybdenum-niobium system was established. This quasi-binary section, which by itself does not allow any conclusions to be drawn regarding the time-dependence of the transformation processes, was supplemented by a continuous TTT -diagram that helped to investigate fully the heat treatment characteristics of various cross -sections. Next, detailed examinations were carried out to determine the influence of niobium concentration on the technological properties. A close relationship was found to exist between the precipitation of Nb-carbide or iron-niobide and, in particular, the impact properties and yield strength. As the niobium acts unfavourably upon impact properties of this type of steel, limitation of the Nb -content and additions of suitable alloying elements, such as manganese or nickel, are necessary to ensure a sufficient degree of resistance to brittle fracture. Having due regard to all these factors, a ferritic material was produced with a sufficiently low carbon activity to meet the requirements of sodium-cooled reactors, as has been borne out by the processing of commercial melts into tubes, sheet metal and forgings. The properties essential for the manufacture and application of this steel are indicated, such as notched-bar impact strength, yield point at elevated temperature, stress-rupture strength and scaling properties in air and steam, as well as weldability when using electrodes made of a material identical with the parent metal. (author)