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Microstructural Analysis and Observation of the Carburized Layer

From the metallographic photographs of the carburized layer, it can be observed that the low-magnification structure of the deep carburized layer of the gear should exhibit a favorable morphology, free from network-like structures or defects. At higher magnification, the microstructure consists of dark gray needle-like features and white blocky regions. The dark needle-like structure is identified as tempered high-carbon martensite, while the white regions represent retained austenite. Under high magnification, the majority of the needle-like martensite displays a typical lenticular morphology characteristic of high-carbon martensite, distributed within the retained austenite matrix. This indicates a significant presence of retained austenite in the carburized layer. According to relevant carburizing metallographic standards, the carbide rating is Grade 1 (Grades 1–3 are considered acceptable), and the martensite and austenite rating ranges from Grade 2 to 3 (Grades 1–4 are acceptable), meeting the microstructural requirements for the carburized layer of related products.

The transition zone of the carburized layer exhibits a mixed microstructure of high-carbon martensite and low-carbon martensite. The core structure is relatively refined, consisting of gray low-carbon lamellar tempered martensite, white blocky regions of ferrite, and some austenite. Between the tempered martensite laths, a small amount of retained austenite is present. High-magnification observations reveal distinct precipitates within the tempered martensite structure. Based on metallographic evaluation standards, the dispersed ferrite in the core is rated at Grade 2–3 (Grades 1–4 are acceptable).

In summary, the materials used for manufacturing gears are primarily low-carbon alloy carburizing steels such as Cr-Ni, Cr-Ni-Mo, and Cr-Mn-Ni-Mo series. These alloys exhibit excellent carburizing properties, strength, toughness, and formability, as well as good machinability. For gear drill bits, the external microstructure of the carburized layer consists of high-carbon tempered martensite and retained austenite, while the core comprises low-carbon tempered martensite with a minor amount of retained austenite. The surface hardness of the carburized layer reaches HRC 58–59, and the core hardness ranges from HRC 44–45, fulfilling the technical requirements for the carburized layer of carburizing steel. These properties ensure that the material meets the performance demands of gear applications, balancing hardness and toughness effectively.