The material used for the boring machine shaft gear produced by a company is 18Cr2Ni4W. The heat treatment process involves carburizing and quenching of the gear teeth, with an effective hardened layer depth ranging from 0.9 to 1.2 mm. The surface hardness should be between 58-62HRC, while the core hardness needs to be within 30-36HRC.
A batch of these gears underwent tooth grinding during the installation test, as illustrated in Figure 1, prompting a detailed failure analysis.
**Chemical Composition**
We rechecked the chemical composition of the materials to ensure they met the required standards. The results, shown in Table 1, indicate that the chemical composition was within the acceptable limits.
**Table 1: Chemical composition of defective parts (mass fraction) (%)**
**Hardness Testing**
(1) **Effective Hardened Layer Depth**
The effective hardened layer depth of the failed part was measured at approximately 0.95 mm, which is at the lower end of the technical specifications.
**Table 2: Effective hardened layer depth and hardness gradient**
(2) **Surface and Core Hardness**
The surface hardness of the failed parts ranged from 53-55HRC, which is below the required 58-62HRC. The core hardness was measured between 36-40HRC, which is slightly higher than the specified range.
Due to the insufficient surface hardness, the gear meshing with this shaft gear had a higher hardness, leading to excessive stress on the shaft gear during operation. This caused the shaft gear to exceed its yield limit, significantly reducing its load-bearing capacity. As a result, plastic deformation occurred under resistance, leading to tooth grinding and a shortened service life.
**Metallographic Examination**
The metallographic examination aimed to identify the nature and distribution of defects and determine their root causes.
(1) **Non-Metallic Inclusions**
According to GB/T10561-2005, the non-metallic inclusions in the failed parts were classified as D3, indicating a significant level of inclusions. These inclusions disrupted the continuity and integrity of the metal matrix.
(2) **Metallographic Structure**
The carburized subsurface structure showed coarse tempered martensite along with a large amount of retained austenite, as seen in Figure 2. The martensite and retained austenite were rated at grade 6, while the carbide grade was 1. The core structure consisted of lath martensite and ferrite, as shown in Figure 3.
**Conclusion**
(1) The metallurgical quality of the gear material was poor, with serious non-metallic inclusions that compromised the structural integrity of the metal.
(2) The surface hardness of the gear teeth did not meet the required specifications.
(3) The presence of coarse martensite and a high amount of retained austenite in the surface layer resulted in inadequate surface hardness, contributing to the failure of the gear.
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