After gear grinding for running gear reducer in a factory, cracks were found on several parts in the circumferential direction of the inner hole keyway hole wall of the gear and the end face of the gear.
The gear material is 18CrNiMo7-6.
The production process: rough machining → carburizing and quenching+tempering → finishing (keyway opening, etc.).
In order to determine the cause of gear crack, a series of tests and analyses were carried out.
1. Test process and results
1.1 Macro inspection
Long cracks were found on the outer circumference of the gear, the circumferential direction of the inner hole keyway hole wall and the end face, and both sides of some end face cracks were cocked up, as shown in Fig. 1.
After opening along the crack, the fracture morphology was observed.
Most of them showed silver metallic luster of fine porcelain, and no old fracture was found.
The crack source was at the root corner of the keyway.
Fig. 1 Gear Crack Location and Morphology
From the crack source, obvious radial patterns and tear edges can be seen around, which is the high stress brittle fracture morphology, as shown in Fig. 2 and Fig. 3.
The keyway is rough and the trace of wire cutting is clear.
Fig. 2 Macro morphology of gear fracture
Fig. 3 Location of Gear Crack Source
1.2 Chemical composition inspection
The chemical composition of the gear was tested by ICP inductively coupled plasma atomic emission spectrometer, and the results met the requirements of EN 10084-2008 Technical Delivery Conditions for Carburizing Steel.
See Table 1 for the test results.
Table 1 Chemical Composition of Gear (Mass Fraction) (%)
1.3 Hardness and metallographic inspection
The depth of the carburized layer of the gear is about 1.58mm, the average hardness of the tooth surface is 725HV1, and the hardness of the center is 43.0HRC, all in accordance with the technical requirements of the drawing.
According to method B in GB/T 10561-2005 Determination of the Content of Non metallic Inclusions in Steel – Micrographic Examination by Standard Rating Chart, all kinds of non-metallic inclusions are better than grade 0.5;
According to GB/T6394-2017 Determination of Average Grain Size of Metals, the grain size is 6.5.
The gear carburized layer is composed of a small amount of fine granular carbide+coarse acicular martensite+more retained austenite.
It is evaluated as Grade 1 carbide, Grade 5 martensite and Grade 6 retained austenite according to GB/T 25744-2010 Metallographic Examination of Carburizing, Quenching and Tempering in Steel, which does not meet the requirements of GB/T 3480.5-2008 Calculation of Bearing Capacity of Spur and Helical Gears – Part 5: Strength and Quality of Materials for carburized steel surface structure and retained austenite content, as shown in Fig. 4.
Fig. 4 Carburized layer structure (500 ×）
The sample was cut from the crack source, polished and then corroded by 4% nitric acid alcohol corrosion solution.
According to the microscope, there is a white bright layer on the surface of the keyway, the surface of the keyway is rough, the root is irregular, and there are microcracks.
There is no carburization or decarburization on both sides of the microcrack, as shown in Fig. 5.
Fig. 5 Wire cutting white bright layer (500 ×）
2. Analysis and discussion
The gear of the reducer was tested, and the results showed that its material composition, inclusion, grain size, hardness and penetration depth all met the requirements.
The main causes of gear cracking are as follows.
1) Because the wire cutting speed is too fast, the gear keyway is rough, the machining trace is clear, and the shape of the transition fillet is irregular, which aggravates the stress concentration at the keyway, so a crack source is formed here, and the crack gradually expands to the gear crack under the effect of grinding stress.
In addition, too fast wire feeding speed causes a white layer on the surface of the keyway, and there are many microcracks in the white layer, which leads to cracks in the subsequent process.
During the manufacturing and use of parts with sharp concave corners, convex edges or notches, great stress concentration will occur at the transition of sharp concave corners, notches or convex edges and cracks may be formed.
In addition, during the processing of parts, the rough tool marks on the machined surface caused by operation, tool edge shape, machine tool accuracy, etc. will cause stress concentration and deteriorate the performance.
After inspection, it was found that the surface of the keyway of the gear was rough, the machining tool marks were clearly visible on the surface, the shape of the fillet at the root of the keyway was irregular, and the stress concentration was large.
There is a layer of white bright layer produced by wire cutting on the surface of keyway, which is a quenching layer containing a large amount of residual austenite.
The residual austenite is an unstable structure, which can continue to be transformed into martensite and generate large stress.
When the stress is concentrated in the transition fillet, the stress concentration of the transition fillet will be increased, resulting in microcracks.
2) The martensite needle in the carburized layer is thicker, and the residual austenite content is too much, which reduces the strength and increases the brittleness of the gear.
In addition, a large amount of retained austenite will continue to transform into quenched martensite at room temperature, which will produce large residual internal stress and accelerate the crack growth.
3. Conclusions and Suggestions
As the speed of gear wire cutting is too fast, the surface of the keyway is rough, the shape of the transition fillet is irregular, and the white bright layer of wire cutting is generated on the machined surface, which intensifies the stress concentration at the keyway and forms a crack source at the keyway.
Under the action of grinding stress and residual stress of the workpiece, the cracks gradually expand to cracks.
It is recommended to mill the keyway before carburizing.
If the keyway is opened by wire cutting after carburizing, the wire speed of wire cutting should be controlled to ensure the transition fillet shape at the root of the keyway and improve the processing quality of the keyway;
If the white bright layer produced by wire cutting cannot be avoided, it shall be polished and removed manually after wire cutting.