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The 'invisible killer' of carburized gears: how to cause early failure due to excessive non horse and internal oxidation?
2026-06-11
In the process of gear manufacturing, carburizing heat treatment is a key step in improving surface hardness and wear resistance. However, if there are surface non martensitic structures (non martensite) and excessive internal oxidation after carburizing, the service life of the gear will be greatly reduced, and even early rapid wear may occur, seriously affecting the reliability and safety of the transmission system.
These three metallographic microscope images clearly show the typical defect characteristics of the gear surface after infiltration, and are also direct evidence of failure:
Figure 1/Figure 2 (corroded state)
The dark area near the surface below the picture is a typical manifestation of excessive non horse tissue and internal oxidation.
The dark mesh/strip area is an internal oxide layer formed by the reaction of carbon, nitrogen, and other elements with oxygen on the surface of the carburized layer, with a thickness close to 30 μ m, far exceeding conventional technical requirements;
Accompanying internal oxidation are needle shaped/network like non martensitic structures (such as martensite, bainite, etc.), which have much lower hardness than qualified martensite and become the "weak spots" on the surface.
Figure 3 (uncorroded state)
The point like and linear dark defects below the surface are oxide inclusions formed by internal oxidation, which directly cut the continuity of the matrix and become stress concentration sources during service, accelerating crack initiation.
These defects are like honeycomb coal on the surface of gears. The seemingly hard surface is actually covered with "soft spots" and "crack sources", which cannot withstand the alternating loads and friction wear during service, ultimately leading to premature failure of the gears.
Non horse and internal oxidation are not invisible "hidden defects" that do not exist, but rather "invisible killers" that can cause early failure. To run gears for a long time, you have to start from every degree and breath in the stove. Don't let the defects in the process become a disaster for the equipment.
