Abstract
The precipitates obtained by EPE technology from the 9Cr-1.7W-0.4Mo-Co ferritic heat-resistant steel subject to isothermal aging were investigated using SEM, TEM and XRD. The particle size distribution and the coarsening kinetics of M23C6 with duration of isothermal aging were also analyzed with or without consideration of Laves phase. The results show that the isolated dislocations were detected in delta ferrite interior, and the precipitates on delta ferrite and martensite boundaries are obviously larger than other locations. Fe2W-Laves phase can only be found as duration of aging time to 2000 h, and is preferential to form adjacent to M23C6 particles. The small M23C6 particles firstly coarsen, but the large M23C6 are relatively stable during short aging. The total coarsening rate of M23C6 precipitates is 9.75 × 10−28m3s−1, and the coarsening of M23C6 depends on the formation of Laves phase.
Highlights
Laves phase, composed by [(Fe, Cr)[2] (W, Mo)], only appears during aging/creep[16, 17]
The migrations of alloying elements from the interior to the grain boundaries of delta ferrite phase lead to the formation of precipitates with a large number, and new dislocations located in the interior of delta ferrite phase might be emerged
The precipitates in the 9Cr-1.7W-0.4Mo-Co ferritic heat-resistant steel subjected to isothermal aging at 973 K were investigated using scanning electron microscope (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD)
Summary
Laves phase, composed by [(Fe, Cr)[2] (W, Mo)], only appears during aging/creep[16, 17]. In the case of without considering the evolution of MX carbonitrides, the coarsening of M23C6 particles and the formation of Laves phase, both as the key factors that will affect the microstructural stability of 9~12 wt.% Cr ferritic creep steels during aging/creep, especially 9 wt.% Cr creep strengthening steel with addition of Co, should be given more enough data to find ways for improving the aging/creep strength of the steels. The content of delta ferrite in 9~12 wt.% Cr ferritic creep steels can be influenced by the alloy elements as W, Mo, Co, etc., especially as C29. With the situation of high content, delta ferrite plays important roles on solid solution strengthening of alloying elements and precipitation behavior of secondary phases, such as M23C6 and Laves phase. The particle size distribution and the coarsening kinetics of M23C6 precipitates were analyzed
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