Abstract
Problem Statement (Relevance): Within the framework of a comprehensive project to create a hi-tech production process, which is carried out by the Nosov Magnitogorsk State Technical Uni-versity in cooperation with the Magnitogorsk Iron and Steel Works and funded by the Russian Min-istry of Education and Science, work is being carried out to develop a prototype process to produce ultra cold-resistant nanostructured steel sheets that could substitute the imported materials, includ-ing cryogenic materials designed for application in critically low temperatures, highly aggressive environments and in the Arctic. The use of special simulation equipment, the property of the Ther-modeform-NMSTU research centre and the Institute of Nanosteel, secures successful implementa-tion of the project. With the help of the above facilities one can experiment with different produc-tion processes in order to find the way to yield new cold-resistant steels and steel sheets with import substitution in mind. Objectives: The objective of this research is to understand the effect of a mul-ti-stage heat treatment process on the microstructure of steel sheets made of cryogenic structural steel with high cold resistance. Methods Applied: Using the capabilities of the Thermodeform-NMSTU laboratory complex, ingots were produced with given chemical compositions, which were hot-rolled and heat-treated under various modes. The following metallography equipment was used: the Meiji optical microscope with the Thixomet PRO image analyzer and the JSM 6490 LV scan-ning electron microscope. Differential scanning calorimetry (DSC) was performed on the Iupiter 449 F3 synchronous thermal analyzer. A Buchler Mikromet microhardness meter was used for hardness tests as per GOST 9450-60, which calls for the use of a pyramid indenter with a 136° angle between opposite faces. Findings: The critical temperatures (points) of the 0Н9А (9% Ni) steel were determined, which appeared to be lower in comparison with traditional carbon steels. They were found to be as follows: Ac1 ≈ 624°С and Аc3 ≈ 720°С. It was found that, following double hardening, austenite was enriched with alloying elements, which caused an additional decrease in the point Ac1 by 20°C. The effect of single and double hardening and the following high-temperature tempering (at 500, 550, 600°C) was studied on the alloy and its microstructure. After simple hardening followed by tempering within the studied temperature range, a structure is formed consisting of tempered martensite, residual austenite, α-phase and carbide particles, which precipitate predominantly at the grain boundaries leading to embrittlement. This is confirmed by fractorgraphic studies. After double hardening followed by tempering within the specified tempera-ture range, a disperse lamellar duplex structure is formed consisting of an α-phase, a “new” marten-site, a tempered martensite, and a residual stable austenite with the volume fraction of about 4%, which ensures fracture toughness at cryogenic temperatures. Practical Relevance: The established regularities are of interest not only because they give a general understanding of the structural pro-cesses in ferritic nickel steels but also in terms of the relationship between the duplex structures ob-tained and the fracture mechanisms affecting the target alloys at cryogenic temperatures. Such al-loys are used to make tanks for liquified gas. Using the new scientific data obtained one can devel-op new and improve the existing heat treatment modes applicable to the specified alloys.
Keywords
Sheet steel, cryogenic structural steel, simple hardening, double hardening, multi-stage heat treatment, tempering, cold resistance, hardness.
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