The fractography features of fracture of the 2,25CR-1MO steel after creep test
Journal Title: Вісник Тернопільського національного технічного університету - Year 2015, Vol 79, Issue 3
Abstract
The unstable properties of steels for the large construction during long term operation are one of the reasons of loss of oil equipment workability. High operating temperature, strain and hydrogenating technological environment are among the main factors of their degradation. The reactor damage causes huge destructions due to high processing parameters (temperature – 450 °C and pressure – 15 MPa) and large its dimensions (diameter up to 4 m, wall thickness up to 0.25 m and height ~20 m). Because that the reactor damage is very dangerous for service staff and environment. Stability of the mechanical properties of the steel of the oil hydrocracking reactor body during long-term exploitation is very important to guarantee it save workability. That’s why it is very significant to know metal properties after different service life for prediction of the material workability. The 2,25 Cr–1Mo steel in the initial state and after high-temperature degradation in hydrogenated environment under working conditions of hydrocracking reactor service for 6•104 hours (as the specimen-witnesses) were creep tested. The steady creep rates for both variants of heat-resistant steel were estimated in hydrogen and in air. It was revealed that steady creep rate of virgin metal is lower compared with metal exploited into oil hydrocracking process. The degradation of the metal in operating conditions increases creep strain rate in air and hydrogen compared with unexploited metal. More over the steady creep rate of the 2,25Cr-Mo steel in virgin state and serviced one creep tested in hydrogen is higher than in air. It firstly means that initial properties of the steel are exhausted during service, and secondly that hydrogen simultaneously effects on the degradation and creep processes too. The hydrogen intensities of the diffusion process and consequently accelerate of the steel degradation and makes easy of the creep process. The fractography features after creep test of the 2,25Cr-1Mo steel in the initial and after the operation during 6•104 hours in the process of oil hydrocracking process were analyzed. It was found the creep fracture occurs according to the classic ductile mechanism of nucleation, growth and association of pores. Special fractographical feature of degraded steel is large and at the same time deep holes caused by decohesion nonmetallic inclusions from the matrix. The next feature is the big and at the same time plate disc-like dimples with the small micro pores around carbides on their bottom. Both types of holes were initiated under the influence of hydrogen accumulated by bulk metal during operation. They become more visible at the creep test in hydrogen because their initiation is additionally stimulated by hydrogen, absorption by metal from the tested environment.
Authors and Affiliations
Halyna Krechkovska, Leontiy Babiy, Oleksandra Student
Keywords
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