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Austenite reversion heat treatment is a proven strategy for enhancing the mechanical properties of 300M steel； however， the phase transformation mechanisms governing austenite evolution during this process remain inadequately understood. This study employs in-situ electron backscatter diffraction （EBSD） to systematically investigate the microstructural evolution and crystallographic characteristics of 300M steel during high-temperature austenite reversion. Results demonstrate that during isothermal holding above the austenitizing temperature， martensite transforms into two distinct austenite morphologies： needle-shaped austenite （γ（A））， which retains the orientation of the original martensite， and spherical austenite （γ（G）） with a random orientation. Notably， γ（A） preferentially nucleates and grows within the grains， maintaining a Kurdjumov-Sachs （K-S） orientation relationship with the surrounding martensitic blocks. Consequently， most of the γ（A） phase shares the same orientation and size as the initial austenite phase， hindering the refinement of the final microstructure. In contrast， γ（G） has a small grain size and nucleates at the grain boundaries of the original austenite as well as within the newly formed γ（A） crystals. γ（G） lacks a K-S orientation relationship with the untransformed martensite and adopts a random orientation， playing a crucial role in refining the final microstructure. Furthermore， heating temperature and duration exhibit significant control over the completeness of martensite-to-austenite transformation， with γ（G） propagating into the matrix via diffusion-driven growth kinetics. These findings elucidate the dual-phase transformation pathways and their distinct roles in microstructural evolution， offering a mechanistic foundation for optimizing heat treatment protocols to achieve tailored microstructures in 300M steel.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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