Effect of aging temperatures on the microstructural stability and mechanical properties of Super304H steel
Aging tests at 650 °C and 700 °C for up to 5000 h were performed on Super304H steel tubes. The microstructural evolution during aging was observed by optical microscopy (OM) and scanning electron microscopy (SEM). Thin foils were prepared to further identify precipitates using transmission electron...
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| Main Authors: | , |
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| Format: | Article |
| Language: | English |
| Published: |
Elsevier
2025-01-01
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| Series: | Journal of Materials Research and Technology |
| Subjects: | |
| Online Access: | http://www.sciencedirect.com/science/article/pii/S2238785424030667 |
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| Summary: | Aging tests at 650 °C and 700 °C for up to 5000 h were performed on Super304H steel tubes. The microstructural evolution during aging was observed by optical microscopy (OM) and scanning electron microscopy (SEM). Thin foils were prepared to further identify precipitates using transmission electron microscopy (TEM) equipped with selected area electron diffraction (SAED) and energy dispersive spectroscopy (EDS). Precipitation kinetic curves and equilibrium phase fractions of the second phases in the Super304H steel were calculated using JMat-Pro and Thermal-Calc. Hardness and impact tests were performed on the aged specimens. By comparing the microstructure at 700 °C with that at 650 °C, the stability of the second phases were concluded and the strengthening and toughening mechanisms were revealed. The hardness of the Super304H steel increased during the short-term aging process at both temperatures, whereas the impact toughness decreased rapidly. With the extension of aging time, the hardness and impact toughness remained relatively stable. With increasing aging temperature from 650 °C to 700 °C, the increase of hardness decreased, while the impact toughness decreased more obviously. During aging at 700 °C/5000 h, the Super304H steel retained fine grains. In addition to the Nb(C,N) and Cu-rich phases, some σ phase was observed, and a few M23C6 shells precipitated around the primary Nb(C,N) inside the grains due to the similar configuration with the {111} plane of the interface between Nb(C,N) and the austenite matrix. The stress field around the primary Nb(C,N) particles facilitated the precipitation and growth of the M23C6 shells. During aging at 700 °C, the coarsening rate of Cu-rich phase was significantly faster than that at 650 °C. When the diameter of the Cu-rich phase exceeded the critical value of 36.36 nm, the strengthening mechanism of the Cu-rich phase changed from a cutting mechanism to an Orowan mechanism. The dimensions of the M23C6 carbides on the grain boundaries at 700 °C were larger than those at 650 °C, which facilitated crack propagation and decreased the crack propagation energy during impact. The hard and brittle σ phase that precipitated on the grain boundaries also deteriorated the impact toughness. |
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| ISSN: | 2238-7854 |