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The early wear of carburized gears often occurs in the tens of micrometers on the tooth surface
2026-09-29
Shortly after the installation and operation of the gear, abnormal wear occurred in the transition zone between the tooth surface pitch circle and the tooth root. After cutting, grinding, and etching along the tooth surface, place it under a metallographic microscope. The outermost layer of the tooth surface is covered with an undeserved dark tissue, with a thickness of tens to hundreds of micrometers. This is what the industry calls the 'non martensitic microstructure layer', which is often accompanied by oxidation within grain boundaries and is one of the common root causes of early wear in carburized gears.
1. First, observe the phenomenon: wear in the transition zone of the tooth surface, pointing towards the surface of the infiltration layer
The wear is concentrated on the tooth surface pitch circle and tooth root transition surface, as well as the bottom of the tooth root circle - these positions have both contact stress and relative sliding, making them the most susceptible to wear. Failure analysis usually involves cutting along the tooth width, observing the entire infiltration layer, and making a hardness gradient at the most severely worn area.
2. Looking at the organization again: What is the dark layer of tissue on the surface
At high magnification (20 μ m scale), this dark tissue is not a single phase, but a mixed structure of ferrite, bainite, and martensite, collectively referred to as the "non martensitic tissue". The normal carburizing and quenching surface should be high carbon tempered martensite with a small amount of residual austenite; This layer of mixed tissue is equivalent to a layer of "soft shell" underneath the hard shell.
Left field of view: The thickness of the non horse layer on the surface is about 44.25 μ m, and the total influence layer of the transition zone below is about 141.87 μ m;
Right field of view: The thickness of the non horse layer on the surface is about 56.26 μ m, and the total affected layer is about 111.57 μ m.
The thickness of the non horse layer is 44-56 μ m, and together with the total influence layer of the transition zone below, it is about 110-140 μ m, covering the main stress layer on the tooth surface.
3. Data Speaking: Microhardness Gradient, Quantifying "Soft Shell"
Micro Vickers hardness was applied at a distance of 0.03 mm (30 μ m) from the surface in the transition zone between the tooth surface pitch circle and the tooth root. The results were obtained from three points:
• 570 HV ≈ 54 HRC
• 505 HV ≈ 50 HRC
• 584 HV ≈ 54 HRC
The depth is falling within the non horse layer range. The three-point hardness of 505-584 HV (approximately 50-54 HRC) is significantly lower than the surface hardness requirement of conventional carburized gears (generally ≥ 58 HRC, subject to the drawing), and the hardness difference comes from this layer of non horse tissue. The indentation is a regular square shape, indicating that the sample is flat and the data is reliable.
4. Mechanism: How did this "soft shell" grow in the furnace
This layer of organization was already buried during the carburizing stage. The core is internal oxidation: trace amounts of oxygen infiltrate along the austenite grain boundaries in the carburizing atmosphere, forming oxides with alloying elements such as Cr, Mn, Si, etc; The nearby matrix alloy elements are consumed, and the hardenability decreases. After quenching, it cannot transform into martensite and instead forms a mixed structure of bainite, martensite, and ferrite. The low surface hardness is often not due to insufficient quenching, but rather due to the oxidative components in the furnace gas weakening the surface layer first.
5. How to view standards: Daily metallographic examination should focus on these three points
Daily metallographic examination focuses on three points:
At first glance, the surface is not a horse layer. According to GB/T 25744 rating, the depth of non horse layer and internal oxidation level must fall within the scope of the drawing/technical agreement, and cannot be solely based on the total depth of the infiltration layer.
Take a look at the hardness gradient. From the surface to the inside, the hardness should transition smoothly; An obvious trough within 0.05 mm is considered abnormal.
Check the sampling location three times. Poor gas exchange and slow cooling between the tooth root and transition zone furnace, and non horse tissues are often heavier. Sampling only at the tooth tip or center of the pitch circle is prone to missed detection.
The entire process of sampling, embedding, grinding, erosion, microscopic observation, and hardness testing in this case was completed by Naibo metallographic equipment and hardness tester. The automatic grinding and polishing machine ensures the quality and repeatability of grinding and polishing, and the metallographic microscope clearly distinguishes the tissue differences of tens of microns on the surface layer. The micro Vickers hardness tester produces regular indentation.
LABTT can not only provide a series of metallographic equipment and hardness testers covering the entire process, but also provide corresponding technical support. It is the stability of the equipment and the standardization of the sample preparation process that directly determine what this metallographic structure diagram can explain.
