Study of the influence of long-term high-temperature operation on the structure and properties of austenitic chromium-nickel-molybdenum steel

K.A. Okhapkin, A.S. Kudryavtsev show affiliations and emails
Received 21 September 2021; Accepted 24 November 2021;
This paper is written in Russian
Citation: K.A. Okhapkin, A.S. Kudryavtsev. Study of the influence of long-term high-temperature operation on the structure and properties of austenitic chromium-nickel-molybdenum steel. Lett. Mater., 2022, 12(1) 21-26
BibTex   https://doi.org/10.22226/2410-3535-2022-1-21-26

Abstract

The paper investigates the effect of long-term operation at a temperature of 515 °C on the structure and properties of steel grade 08Cr16Ni11Mo3. As a result of the study, it was found that during operation at a temperature of 515 °C for 195,000 hours in steel grade 08Cr16Ni11Mo3, the structure changes with the formation of secondary phases, initiated by the release of elements with limited solubility from the supersaturated solid solution. A change in the phase composition during heat aging leads to embrittlement of steel - a decrease in plasticity.The effect of long-term operation at a temperature of 515°C on the structure and properties of steel 08Cr16Ni11Mo3 (grade 316) is studied. Data on the structure and phase composition of the steel were obtained using optical and scanning electron microscopy. The phase composition of steel in the equilibrium state was determined by thermodynamic modeling in the FactSage software package. As a result of the study, it was found that there are the structure changes with the formation of secondary phases initiated by the precipitation of elements with limited solubility from the supersaturated solid solution during operation at a temperature of 515°C for 195 000 hours in steel 08Cr16Ni11Mo3 (grade 316). The presence of the following secondary precipitates in the structure of the solid solution of austenite was revealed: carbide of the М23С6 type (M — Cr, Mo), ferrite (α), intermetallic phases. Based on a comparison of the thermodynamic modeling data and the experimental determination of the phase composition, it was found that the structure of steel is close to equilibrium. The results of static tensile tests have shown that a change in the phase composition during heat aging leads to embrittlement of steel, i. e. a decrease in ductility. In general, the mechanical properties of the material in the state after operation are close to the original ones. The tensile strength at room temperature is 5 % higher than the initial one, and in the range of elevated temperatures of 350 – 600°С it is 5 –10 % lower than the initial level. The yield strength at temperatures of 20, 500 and 600°С after operation increases by 5 –10 % of the initial level, and at temperatures of 350 and 550°С it practically does not change. In the temperature range 350 – 600°C, the ductility values of steel 08Cr16Ni11Mo3 (grade 316) decrease by 5 –17 % from the initial level. The main contribution to the change in the mechanical characteristics of steel is made by secondary precipitates of carbide and intermetallic phases.

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Funding

1. Ministry of Science and higher Education - RF----2296.61321X0014