OBRABOTKAMETALLOV TECHNOLOGY Vol. 25 No. 1 2023 The process of titanium carbonitrides TiCxNy formation begins at temperatures around 1,425 oC. The higher the initial nitrogen content in the steel, the more titanium is consumed to form the nitride component of this phase, the less titanium binds carbon to carbide. Thus, conditions for the release of this carbon in the form of Cr23C6 carbides are created. With a significant nitrogen content (N = 0.1 %), chromium carbides are formed at 960 oC and reach up to 1.15 wt.%, with a decrease in nitrogen to 0.05 %, the formation temperature drops by 100 oC and the final mass is 0.41 wt.%. Metallographic studies confirm that with an increase in the nitrogen content, there is an increase in the mass of chromium carbides Cr23C6, provoking intergranular corrosion. Without interacting with chromium, nitrogen indirectly increases the number of carbide phases by binding titanium. The carbon content naturally and significantly affects the number of Cr23C6 compounds, so, with an increase in the carbon content, their mass and the temperature of the formation beginning grow up. The calculation results show that the critical value is C = 0.05 %, at which there is no release of chromium carbides, and a significant part of the carbon binds to titanium. Thus, in order to minimize the formation of defects associated with intergranular corrosion, it is recommended to reduce nitrogen and carbon to 0.05% at the stage of liquid steel ladle treatment. It is also recommended to keep the titanium content in this type of steel not less than 0.3 % within the permissible composition according to GOST 5632-2014. In case of defect detection in the metal structure at the stage of input control, it is necessary to carry out the austenization operation of steel at temperatures specified by thermodynamic calculation for an exact composition, and amounting to about 1,050–1,100 °C. Study of the causes of the martensitic α-phase formation Metallographic analysis of the martensitic α-phase was carried out on samples of 12-Cr18-Ni10-Ti steel rods obtained after the drawing stage. Samples with varying degrees of this defect manifestation were studied, technological parameters of cold forming were recorded in parallel. Calculations show that in stainless steel of this type, martensite is formed at negative (Celsius degrees) temperatures, therefore its origin is most likely by deformation mechanism. Figure 2 shows an example of the martensite, formed by this mechanism in a metallurgical semi-finished product. Measurements of the metal hardness show that a significant hardening follows the presence of martensite in the structure – the samples have a hardness of about 370 HB. Such values correspond to increased reduction of steel during the manufacture of rods. The analysis of technological parameters showed that with reduction of more than 50 % in the process of manufacturing a semi-product, there is an excessive amount of martensitic α-phase. Thus, the appearance of this defect in rods made of 12-Cr18-Ni10-Ti steel is associated with the stage of cold forming and is possible when the critical reduction of the billet is exceeded. During its formation, an additional step of metal austenization is required. Investigation of the ferrite δ-phase in 12-Cr18-Ni10-Ti steel Samples of different thickness were studied to investigate the causes of the δ-phase formation in 12-Cr18-Ni10-Ti steel. Figure 3 shows an example of the rod microstructure with a diameter of 3 mm in the direction of drawing. Thermodynamic calculations show the presence of a ferrite phase in a wide temperature range from 1,250 to 1,450 °C, depending on the specific composition of steel, in particular the content of carbon, chroFig. 2. Occurrence of martensitic α-phase in steel 12-Cr18-Ni10-Ti, rod Ø4 mm
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