Special carburizing and quenching process of planetary shaft

Planetary transmission mechanism is adopted for transmission products, and most planetary shafts are required to be carburized and quenched with high-quality carburized carbon steel. However, the planetary shaft of one transmission not only has special material requirements, but also has special heat treatment standards. Because of confidentiality, the original designer is unwilling to provide heat treatment standards and requires to purchase them from their factory.

In order to reduce the cost and realize localization, we have carried out multiple rounds of test improvement on the test results of standard OEM parts, and finally achieved the heat treatment results consistent with OEM parts in all aspects. We have passed the strict bench test and entered the stage of stable mass production.

1. Special materials and special heat treatment results of planetary shaft

(1) The product metal material is specially made by the original company and requires high purity and hardenability. The grain size is required to be ≥ grade 5. The inclusion requirements are shown in Table 1. The di value (calculated according to chemical composition) should be 77.5 – 100.3mm. The end quenching test requirements are shown in Table 2. The residual element requirements are: the maximum calcium residue is 20ppm (1ppm = 10-6, the same below), and the maximum oxygen content is 12ppm.
Table 1 inclusion requirements

Type	A		B		C		D
	Department of fine	Department of coarse	Department of fine	Department of coarse	Department of fine	Department of coarse	Department of fine	Department of coarse
Maximum value/level	2.5	1.5	1.5	0.5	0	0	0.5	0.5

Table 2 End quenching test requirements

End quenching test measurement distance /mm		1.5	7	15
Hardness value is HRC	The biggest	48	–	36
	The minimum	43	39	–

(2) The product drawing only has the requirements of hardness and hardened layer depth (surface hardness ≥ 60HRC, hardened layer depth 2.2 ~ 2.8mm). The special heat treatment standard marked in the drawing cannot be obtained. We conducted sampling inspection on OEM of two batches respectively, and the test results are shown in Table 3.
Table 3 test results

Project	Surface hardness HRC	Hardened layer depth / mm	Retained austenite	Surface bainite	Subsurface bainite	Intergranular oxidation	Core ferrite	Decarbonization	Carbide
Batch A	65.5	2.51	A1	B0	BB0	O0	F1	D0	C3
Batch B	65	2.45	A1	B0	BB0	O0	F1	D0	C4

By comparing the drawing requirements, test results and common carburizing and quenching specifications, it can be found that there are uniformly dispersed carbides on the surface and sub surface of the part (see Figure 1), and the rest are basically the same, except for the high requirements for surface hardness and hardened layer depth.

Fig. 1 uniformly distributed dispersed carbide on the surface

2. Material selection

According to the special material requirements of the drawing, the general carburized steel is difficult to meet the requirements at the same time, so the material a commonly used by our company which is close to the requirements is selected.
The grain size of material a is grade 7, the residual calcium is 8ppm, the oxygen content is 9.2ppm, and the measured value of Di is 68.8mm. The inclusions are shown in Table 4, and the end quenching test is shown in Table 5.
Table 4 inclusions

Type	A		B		C		D
	Department of fine	Department of coarse	Department of fine	Department of coarse	Department of fine	Department of coarse	Department of fine	Department of coarse
Maximum value/level	1.5	0	0.5	0	0	0	0.5	0

Table 5 end quenching test

End quenching test measurement distance /mm		1.5	7	15
Hardness value is HRC	The biggest	48	–	36
	The minimum	43	39	–
Material measured value		43.2 – 44	36 – 38.5	unmeasured

From the above data, except that the di value and the end quenching hardness value at 7mm are lower than the material requirements of the drawing, all others meet the requirements.

3. Experimental research and comparative improvement

Compared with the common carburized and quenched parts, this part needs to focus on two aspects: one is deep carburization, and the other is to obtain uniform and uniform dispersed carbide distribution on the surface and sub surface. The size and shape of carbides have a significant impact on the fatigue resistance of steel. According to literature, coarse and network carbides will reduce the critical shear stress required for cleavage cracks at carbide grain boundaries. Large angular and grain boundary like carbides seriously reduce the fatigue life of deep carburized workpieces. On the contrary, the spherical and evenly distributed carbides can improve the fatigue limit. The finer and more evenly distributed carbides are, the better. The control of carbide morphology is mainly the result of the comprehensive action of carburizing temperature, time, carbon potential and alloy elements. Generally, when the carburizing temperature is high, the interstitial atoms and replacement atoms can conduct long-range diffusion, and the diffusion results in the formation of stable carbides. During carburizing, the carbides directly precipitated from high-temperature austenite generally nucleate along the grain boundary, and then grow and aggregate to become carbides with special morphology.
As we all know, deep carburizing is a time-consuming and energy consuming process in heat treatment production. With the increase of carburizing layer depth, the time and cost are doubled. Due to the limitations of equipment, carburizing temperature and control methods, it is basically impossible to greatly shorten the process time. Due to the influence of many factors, carbides are very difficult to control for carburizing and quenching process, and it is easy to produce coarse network carbides and other harmful structures. Therefore, the generation of carbides should be strictly controlled in common carburizing and quenching.
According to the above theory and practical experience, we believe that the commonly used direct quenching after carburizing can not obtain the desired carbide morphology, so we need to carburize the workpiece first, and then conduct low-temperature diffusion, so that the carbide can precipitate along the grain boundary during diffusion and cooling, and then reheat the part to the quenching temperature and conduct thermal insulation diffusion for a long time, so as to make the carbide precipitated in the previous steps more uniform, Then carry out quenching, hardening and tempering.
All the equipment used in the following tests are aixielin box type multipurpose furnace, model vkes5 / 2-90 / 85 / 150cn. The furnace temperature uniformity can reach ± 5 ℃, and the carbon potential uniformity can reach ± 0.05%. The cleaning and tempering processes are also completed on the same production line.
The results show that the depth of the hardened layer exceeds the requirements of the drawing, and the dispersed carbide is not obtained on the deep surface. In order to obtain the appropriate hardened layer depth, two tests were carried out respectively. Except that the strong penetration and diffusion time of carburizing process were shortened, the others remained unchanged. It can be seen that with the shortening of carburizing time, the hardened layer depth gradually decreased, but the metallographic structure remained unchanged, and no uniform carbide was obtained.
The average carburizing depth is about 830 ℃ after quenching at the surface layer, but it only needs to be reheated at the standard carburizing depth of about 830 ℃ μ m. And there are a lot of network carbides, which has a bad effect on the performance of the product. With the increase of holding time, the diffusion depth of carbide is deeper, and the average depth can reach 160 when holding for 150 min μ M, and the average depth can reach 260 when holding for 300 min μ M, and with the increase of time, the network carbides gradually decrease and slowly transform into more round carbides. At the same time, with the increase of time, there are fine spherical carbides uniformly precipitated from the surface to the infiltration layer of 1mm depth.

4. Bench test

Small batch production and assembly are carried out according to OEM and the above successful test process, and then the gearbox is installed for bench test to verify the performance and service life. Figure 5A shows the inspection photos of 300h after one round of bench test, and figure 5B shows the inspection photos of 600H after two rounds of bench test. After verification, the performance and service life all meet the test standards.

     （a）                                     （b）
Fig. 5 photos after bench test

5. Conclusion

• (1) Deep carburizing and obtaining uniform dispersed carbide on the surface is feasible, but it is more time-consuming and laborious than the commonly used carburizing process.
• (2) The parameters in the carburizing process mainly affect the control of the depth of the hardened layer, while the acquisition of surface dispersed carbide is mainly controlled by the secondary heating and quenching process.
• (3) In order to obtain uniform carbides and avoid harmful structures such as network carbides, a long holding time is required.

Source: China Pipe Fitting Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at [email protected]