Microstructural evolution in austenitic heat-resistant cast steel 35Cr25Ni12NNbRE during long-term service

Insight to the Microstructure Characterization of a Hp Austenitic Heat Resistant Steel After Long-Term Service Exposure

“…Besides, a small quantity of M6C carbide formed bordering to M23C6. Some literatures [3,[12][13][14] OR between the two phases. These two types of carbides present similar morphology with fine particle, and they are not easy to be distinguished using OM or SEM.…”

“…(almost 87% relative area fraction) and a continuous network of primary precipitation (around 13%), located at the boundaries of grains and dendrites. The typical microstructure of HP alloys is an austenite matrix with intergranular eutectic-like primary chromium carbides with the stoichiometric composition of M7C3 and/or, M23C6 and MC type of niobium carbides [3,8]. The SEM micrograph of the long-term (around 10 years) serviced sample (Alloy X) is presented in Fig.…”

“…In the as-cast condition, three types of carbides can be detected as primary carbide: Cr7C3 and Cr3C as chromium carbides and NbC as niobium carbide [9]. J. Liu et al [3] reported that M7C3 carbides were clearly identified only in the as-cast sample but not in the aged and/or in the serviced sample, they suggested that the M7C3 carbides formed during the solidification and transformed into other phases after aging, this phenomenon also can be seen in Fig. 5(b).…”

“…DOI Although this alloy steel castings are designed for use at elevated temperatures, metallurgical degradation, aging and phase transformations can occur after extended service exposure above 800 °C, potentially resulting in embrittlement. The typical microstructure of as-cast alloy is an austenite matrix with intergranular eutectic-like primary chromium-rich carbides (M7C3 and/or M23C6 types) and niobium carbides (MC type) [3][4]. Long-term service at high temperature of heat-resistant steels can bring about decomposition of austenite and forming of carbides and intermetallic compounds such as sigma, chi, Laves, and epsilon phases [5].…”

“…Although a number of studies, e.g. [1][2][3][4][5][6][7][8][9], have been conducted subsequently to evaluate of phase changes during service and the influence of these changes on the failure mechanisms on 25Cr-35Ni-type heat-resistant cast steel, there is still a lacking of detailed investigations on the microstructural evolution after long-term service by using EBSD, such as phase orientation, and phase map. In this paper, the microstructural evolution of two HP steels in as cast condition and after long time service exposure have been characterized by means of EBSD techniques.…”