Process analysis and physicochemical test heat treatment process analysis The heat treatment process of the drive gear is carburizing (protection measures to prevent carburization at the thread), quenching in a multi-purpose furnace (hydrogen protection atmosphere), and then tempering at a low temperature (19010) for 6 hours. . (a) gas carburizing (b) protective gas quenching (c) air tempering active gear carburizing, quenching, tempering process curve diagram After the main gear breaks, the quality record of the heat treatment subcontractor is inspected and found to be active The three furnaces with broken gears have a tempering temperature of 150160 and the holding time is 4h.
Material analysis was taken at the thread of the broken part of the driving gear to analyze the chemical composition of the material. We also sampled the fractured and qualified drive gears at the same location and analyzed the hydrogen content separately. The results showed that the hydrogen content was 00001 (ie 1 ppm). Metallographic analysis of metallographic and fracture analysis The metallographic specimens were taken from the thread of the broken part of the drive gear. The surface of the 500x fracture at the fracture site was observed by light microscopy. The surface structure was martensite and retained austenite. The top of the thread is carbide grade 6, and the core structure is low carbon martensite.
In addition, the depth test was performed using a microhardness tester: the hardened layer at the top of the thread was 170 mm deep (measured to HV550), and the surface hardness was HV739 (ie, HRC61), and the carburized layer was evenly distributed. The fracture distribution of carbides at 100x232 fractures was analyzed by scanning electron microscopy. The results show that the fractures of the external fractures are characterized by dimple fracture, and the internal fractures are mainly in the coexistence zone of intergranular and transgranular dimples. External Fracture High-Evaluation Internal Fracture Morphology 24 Comparative Test and Analysis In order to further understand the effect of hydrogen embrittlement on the driving gear, we selected three sets of samples and tested the hydrogen content and mechanical properties under three different heat treatment conditions.
Determination of hydrogen content We have made three sets of different heat-treated driving gears into 350mm samples for hydrogen content determination. Sample number heat treatment state and requirements 0 carburizing multi-purpose furnace quenching (hydrogen shielding gas) 190 tempering (2h) 1 carburizing multi-purpose furnace quenching (hydrogen shielding gas) 190 tempering (2h) dehydrogenation (6h) 2 carburizing salt Bath furnace quenching 190 tempering (2h) sample number hydrogen content mechanical properties determination Three sets of different heat treatment state of the drive gear were made into standard mechanical samples for determination.
RESULTS AND DISCUSSION (1) From the scanning electron microscopy analysis of the fracture of the driving gear after carburizing and quenching, and the hydrogen content and comparison test of different heat treatment conditions, it is proved that whether in the hydrogen protection atmosphere, quenching or increasing dehydrogenation and tempering, or in non-hydrogen The quenching in the salt bath furnace with protective atmosphere has no significant difference in hydrogen content, and there is no significant difference in mechanical properties. The fracture characteristics of the drive gear indicate that although quenched in a large amount of hydrogen atmosphere, it is not a hydrogen brittle fracture.
(2) From the heat treatment process, metallographic structure and depth measurement of the driving gear, it can explain the abnormal fluctuation of the heat treatment state, especially the insufficient tempering time and the effective protection of the thread during the heat treatment. The main cause of the drive gear breaking.
(3) After the above test and analysis, find out the cause of the broken gear of the driving gear, and strengthen the quality monitoring of the heat treatment. Adopting a series of effective corrective and preventive measures, more than one year of production practice has proved that the failure of the drive gear after heat treatment has been eliminated, the intrinsic quality of the product has been further improved and the reliability of the whole product has been guaranteed. .
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