Recently, an agricultural machinery company received a large number of quality information feedback and complaints: when the user of the DF 300 wheel tractor was engaged in rotary tillage, the power output transmission gear in the transmission assembly (the transmission assembly provided by our company) had a quality problem of tooth collapse.
1. Preliminary investigation and Analysis on the cause of tooth breakage
After receiving this information feedback, the company quickly organized personnel from technical, quality assurance, manufacturing and other departments to conduct failure analysis on the quality problem of DF300 power output transmission gear tooth collapse.
DF300 power output transmission gear: the material was originally 20CrMnTi.
Later, according to the requirements, it was changed to 19CN5 with better mechanical properties.
The outer diameter is Ф131mm ± 0.05mm, the number of teeth is 51, the modulus is 2.5, the pressure angle is 25 º, the surface is C-N co infiltrated and quenched, the infiltrated layer is 0.8 ~ 1.2mm, the effective hardened layer is 0.5 ~ 0.75mm, the surface hardness is 58 ~ 64HRC, and the core hardness is 32 ~ 46HRC.
The metallographic structure shall be in accordance with QCn29018-91 metallographic examination of automotive carbonitriding gear, with carbonitriding compound 1-4, martensite 1-5, retained austenite 1-5 and ferrite 1-4 at the core.
The preliminary inspection found that there was an obvious root step at the root of the failed gear.
The reason was that the single arc hob was not used during gear hobbing, and the step was formed by extrusion during shaving.
The cutting stress at the step was too concentrated, resulting in the formation of micro-cracks in the step area where the stress was concentrated after quenching, which eventually led to the quality accident of tooth collapse during customer use.
2. Metallographic analysis
Representative samples were selected from the failed gear to inspect the gear raw material structure, carburized layer structure and gear core structure.
The surface carburized structure is coarse high-carbon martensite and retained austenite.
The martensite needle is between 4 and 5 grades according to QCn29018-91 metallographic inspection of automotive carbonitriding gear, and the surface hardness is 59HRC;
The core structure is low carbon martensite, and the core hardness is 45HRC.
The analysis shows that although the heat treatment quality of the failed gear is qualified, the martensite needle is relatively large, the amount of retained austenite is large, and the grade is poor;
The core hardness of 45HRC is also poor.
Therefore, the heat treatment process needs to be further improved.
3. Rectification measures
(1) Metalworking
Current situation of tooth root: there is no transition arc at the tooth root of the gear.
At present, double arc hobs are used.
Rectification measures:
① Adopt single arc hob.
② Stock hobs and on-site hobs shall be sent to the tool factory for grinding of single arc (single arc hobs shall be used for all ordered tools in the future).
③ When shaving, control the outer diameter of the razor to prevent the outer diameter of the razor from pushing up to the tooth root.
④ Strictly inspect the tools when they are put into storage, focusing on the arc of the hob tip and the size of the contact angle.
⑤ Improve the grinding quality of hob and reduce the radial error of hob.
Current situation of tooth accuracy:
The processing benchmark is not consistent with the acceptance benchmark.
The processing is positioned with small diameter, and the acceptance is positioned with large diameter;
The knurled key hole is now drawn horizontally with a compound broach, and the end jump is inconsistent and large in size and diameter inspection.
Rectification measures: the current processing technology is adjusted as follows: rough and fine turning → gear hobbing → chamfering and hair removal → gear shaving → spline hole drawing → heat treatment.
Relevant requirements:
① CNC fine turning of round hole gear blank shall ensure that the hole is perpendicular to the end face ≤ 0.03mm. The technical department will issue the rough and fine drawing.
② Positioning hobbing and shaving with circular holes.
③ After gear hobbing, the burr and flanging at the tooth end shall be removed with sand.
④ Strictly prevent the gear end face, that is, the drawing hole reference surface, from being damaged.
⑤ The newly purchased vertical broaching machine and broach ensure that the big and small diameter end jumps are consistent and meet the drawing requirements.
(2) Improvement of heat treatment process
The original process is C-N co infiltration treatment and quenching with Kerun 228 fast bright quenching oil (see Fig. 1).
Fig. 1
Note:
(1) The heating time is uncertain
(2) This process procedure is applicable to the multi-purpose furnace C-N co infiltration heat treatment of 20CrMnTi (or 20CrMo) material.
(3) Kerun 228 fast bright quenching oil is used.
The process after rectification is shown in Fig. 2.
Fig. 2
Note:
(1) The heating time is uncertain.
(2) This process procedure is applicable to the multi-purpose furnace carburizing heat treatment process of 19CN5 material.
(3) Kerun 468 graded quenching oil is used.
4. Rectification effect
(1) Metalworking
Tooth root: there is a transition arc at the root of the gear, and there is no obvious shaving cutting step.
Tooth accuracy: positioning hobbing and shaving with circular holes, and the big and small diameter end jumps meet the requirements of the drawing.
(2) Improvement of heat treatment process
Metallographic examination results show that:
The carburized structure on the surface is fine high-carbon martensite and retained austenite.
The martensite needle is in accordance with QCn29018-91 metallographic inspection of automobile carbonitriding gear, and the surface hardness is 59HRC;
The structure is martensite and retained austenite, which are about grade 3.
The core hardness is 42HRC, and the core structure is low-carbon martensite.
5. Conclusion
The root cause of the quality problem of the broken teeth of the power output transmission gear in the DF 300 wheel tractor gearbox assembly is that the single arc hob is not used during gear hobbing, and an obvious step is formed during gear shaving.
The cutting stress at this step is excessively concentrated, resulting in the formation of microcracks in the step area where the stress is concentrated after quenching, and finally the quality accident of broken teeth during customer use.
19CN5 is a Cr Ni series steel introduced from abroad, which is an important case hardening steel for transmission parts.
It has high strength and toughness, good machinability and hot working performance of forging and rolling, relatively small thermal deformation, but poor weldability and corrosion resistance.
It is used in the transmission box of tractors and automobiles and its accessories to produce gears and gear shafts with high toughness, high strength and medium modulus.
When this material is treated by C-N co infiltration, it often shows coarse structure;
If the nitrogen content is slightly high, black structure will often appear, which will reduce its contact fatigue strength and easily cause hydrogen embrittlement.
Since the thermal deformation is small, the carburizing process is more suitable for the heat treatment of 19CN5 material than the C-N co-carburizing process.
