Microstructure changes responsible for the degradation of the 10CrMo9-10 and 13CrMo4-5 steels during long-term operation

Abstract

The paper presents results of microstructure and mechanical testing examinations performed using optical and transmission electron microscopy, tensile tests and Charpy tests on 10CrMo9-10 and 13CrMo4-5 steels, before and after they were long-term operated at elevated temperatures in a steam heater. In the 10CrMo9-10 steel, the optical microscopy detected a degradation of original bainite that was accompanied by the formation of ferrite, precipitates and micropores. The transmission electron microscopy revealed that the precipitates are M 23 C 6 and M 7 C 3 type carbides, which are located mainly at the boundaries of former austenite grains, and M 3 C type carbides, which appear inside the grains. The 13CrMo4-5 steel contained a relatively high amount of ferrite in the ferritic-bainitic/perlitic microstructure already in the originally state. The degradation of the microstructure was less serious than for the 10CrMo9-10 steel. The thermal treatment of the 13CrMo4-5 steel led mainly to the precipitation of carbides. The M 23 C 6 and M 7 C 3 type carbides form in perlitic-bainitic areas, while M 3 C and M 6 C type carbides precipitate in ferrite. The higher density of the grain boundary precipitates in the long-term operated 10CrMo9-10 steel facilitated the formation of creep-induced micropores and contributed to the hardness reduction.

Fatigue damage development in 10CrMo9-10 steel for power plant pipes in as-received state and after 280,000 h of exploitation

“…3a-b). On the other hand, the exploited material was characterized by degraded bainite, a small amount of ferrite and precipitated carbides concentrated at the grain boundaries [8] (Fig. 3c-d).…”

“…It was reported that the long-term operation of power engineering steels leads to significant changes in their microstructure which mainly include the diffusion of alloying elements out of the bainite, formation and coagulation of carbide precipitates at the grain boundaries, and partial transformation of original bainite to ferrite [8,15]. It was…”

“…Furthermore, such comparative studies clearly indicate whether the exploited material could be further used without a decrease of safety and reliability of the power engineering constructions. Gwozdzik et al [8] applied optical and transmission electron microscopy as well as standard tensile tests and Charpy tests on 10CrMo9-10 and 13CrMo4-5 steels (low-carbon chrome-molybdenum grade) before and after long-term operation at elevated temperatures in a steam heater. Golasinki et al [9] provided a detailed description of the microstructural changes in the high-chromium martensitic steels that occurred under creep conditions to further extend their time of safe operation in the particular structural elements of power systems.…”

High-Temperature Oxidation of Boiler Steels at 650 °C

“…Small amounts of molybdenum (up to 0.35 wt.%) and chromium (up to 0.3 wt.%) provide heat resistance [9]. 13Cr (13CrMo4-5) is a higher-alloyed steel that contains up to 1 wt.% Cr and up to 0.6 wt.% Mo compared to 16Mo3 and has better oxidation and creep resistance [10].…”