Metallographic analysis of active helical gear

Test equipment: MH-6 micro Vickers hardness tester, TH300 Rockwell hardness tester, DMI5000M metallographic microscope

Reference Standards: Metallographic examination of QC/T262-99 automotive carburized gear, general technical conditions for heat treatment of JB/T5944-91 construction machinery, technical requirements provided by bridge supplier, etc.


(1) Non-martensite structure on tooth surface is 0.04mm, non-martensite structure on tooth root surface is 0.06mm, and bainite and a little troostite have appeared on the surface of carburizing layer.

Non-martensite structure is too deep on the surface of the carburized layer, bainite and a little trough appear in the surface structure of the carburized layer, and the hardness of the core is too low (detection is 26H RC, the technical requirement provided by the supplier is 29-46 H RC). The surface of gear parts is prone to flaking and tooth collapse during the working process.It is indicated that there are oxidation factors in Carburizing and quenching process and the cooling capacity of quenching is insufficient. The cooling performance of carburizing gas and quenching medium should be checked to meet the process requirements.

(2) The core structure is mainly sorbite with unqualified hardness (26HRC tested and 29-46 HRC provided by the supplier), which indicates insufficient quenching cooling capacity;

(3) The carbides on the top of the main helical teeth are Grade 7 and distributed in a continuous network, which seriously affects the performance of the top of the teeth and reduces the fatigue strength, resulting in wear and tear on the top of the teeth.

(4) The effective hardening layer of the main helix is not deep enough, the hardness gradient of the carburized layer is steep, and the hardness decreases too fast, reaching a maximum of 86HV/0.1mm, which easily leads to the spalling of the carburized layer.

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