About the temperature of polymorphic phase transformation of austenite into ferrite of 12% chromium steel

Determining the temperature of the polymorphic phase transformation of austenite into δ-ferrite (Ac4) is of great practical importance for ensuring deformability during hot plastic deformation of high-chromium martensitic and martensitic-ferrite steels, as it allows to exclude the formation of cracks in the steel billet during deformation. By limiting the heating temperature for hot plastic deformation, it is possible to prevent the segregation of δ-ferrite, the formation of which leads to the appearance of cracks along the boundaries of its section with austenite. In this article, the temperature of phase transformation was determined for the steel grade 07‑Cr12‑Ni-Mo-V-Nb martensitic ferritic class. Three industrial smelting with different content of alloying elements within the steel grade composition were investigated. Ac4 temperature was determined by differential scanning calorimetry (DSC), X-ray phase analysis and dilatometric method. Dilatometric and DSC studies were conducted under the same heating rate and showed similar results in the temperature range 1155 –1181°C for all smelting. The temperature of transformation of austenite into δ-ferrite under continuous heating is identified more clearly as a result of measuring the heat flow of the DSC method than by measuring the elongation of the sample by the dilatometric method. X-ray phase analysis showed lower temperature values <1150°C, however, they can be considered the most accurate, since this method is a direct method for determining the phase composition of materials due to the comparability of the wavelength of the X-ray radiation and the size of the crystal lattice, thereby obtaining diffraction reflections not from atoms, but from different planes of the crystal lattices. The increased values of DSC and the dilatometric method are associated with continuous heating, during which the fixation of the phase transformation temperature occurs in the presence of a certain content of the new phase. Thus, as a result of research it is established that the temperature of heating for hot plastic deformation should not exceed the temperature of 1150°C.