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How to solve the deformation of internal spline during induction quenching

Due to the structural characteristics of the internal spline, it is impossible to carry out finish machining after heat treatment to ensure the size. Therefore, when preparing the machining process of the internal spline before heat treatment, the influence of heat treatment deformation shall be considered, and a certain amount of change shall be reserved, and the amount of reservation shall be verified by a large number of process tests. Different induction quenching frequency, hardening layer depth and induction heating process will have different deformation; Different part structure and internal spline wall thickness will also have different deformation after induction quenching; In addition, different materials and metallographic structures have different deformation after heat treatment.
Through experimental verification, the influence data of various factors on induction quenching deformation are found out, which can greatly shorten the development cycle of similar internal spline parts and improve the induction quenching quality, which has positive guiding and reference significance for internal spline induction quenching.

Part description

Figure 1 shows two planet carriers with different structures. The material is QT700-2. The top circle diameter of the internal spline is 54mm, the effective length is 53mm, the spline modulus is 1.5875, and the wall thickness is 12mm and 23mm respectively. Induction quenching requires that the hardening layer depth at the root of the internal spline is ≥ 0.4mm and the hardness is ≥ 45HRC. The full length of the internal spline is allowed to be quenched.
20210730110117 76874 - How to solve the deformation of internal spline during induction quenching
Figure.1 internal spline planet carrier structure
In the process of part development, induction heating power supply with different frequencies is tried, and the induction quenching layer depth and deformation data are also different. With different quenching machine tools, the clamping runout of parts is different, and the taper and roundness of internal spline after heating are also different.

Effect of hardened layer depth on internal spline deformation

On the premise of meeting the minimum hardening layer depth and hardness requirements, the lower the frequency of heating power supply, the greater the hardening layer depth; The deeper the influence zone of induction heating, the greater the shrinkage of internal spline during cooling.
On the contrary, the higher the frequency of heating power supply, the smaller the depth of hardening layer; The smaller the depth of the affected zone of induction heating, the smaller the shrinkage of the internal spline during cooling.
With different frequency induction heating power supply, the hardening layer depth is different, and the deformation data are also different. Therefore, for the selection of reserved amount before internal spline heat treatment, the test data must be collected according to the power frequency of induction heat treatment equipment.
In actual production, due to various conditions, the frequency of the equipment used for induction heating will deviate far from the required optimal frequency. Practice has proved that for induction heating of gear and spline parts, the selection of frequency is related to modulus. When the specific power Pb of induction heating is less than 1.5kw/cm2, the optimal frequency is f = 2.5 × 105/m2 (M is gear module).
The internal spline modulus of such parts is 1.5875, so the best frequency selection: F = 2.5 × 105/1.58752=99200Hz=99.2kHz. During the development of induction quenching process for internal spline parts, two power supplies with different frequencies have been used for induction heating and quenching.
(1) Use 30 ~ 40KHz ultra audio frequency induction heating quenching equipment
Because the frequency is low, the thermal energy is transmitted from the root of the internal spline to the top of the internal spline in the actual heating process. When the root hardening layer depth reaches more than 1.5mm (see Fig. 2 and Fig. 3), the top can fully reach the austenitizing temperature. After comparison before and after heat treatment, the average shrinkage of internal spline deformation is 0.19mm.
20210730110257 63833 - How to solve the deformation of internal spline during induction quenching
Figure.2 The depth of root hardening layer is 1.05mm
20210730110314 73884 - How to solve the deformation of internal spline during induction quenching
Figure.3 The depth of root hardening layer is 1.5mm
(2) Use 200 ~ 400kHz high frequency induction heating quenching equipment
Because the frequency is too high, thermal energy is transmitted from the top of the internal spline to the root of the internal spline. At the same hardened layer depth, the deformation data are also different with different frequency selection.
When the frequency is 200kHz, the hardening layer depth at the root of the internal spline is 0.6 ~ 0.8mm, which meets the technical requirements of the drawing. Compared with before and after heat treatment, the average shrinkage of internal spline deformation is 0.14mm.
When the frequency is 235khz, the hardened layer depth at the root of the internal spline is 0.6 ~ 0.8mm, which meets the technical requirements of the drawing. Compared with before and after heat treatment, the average shrinkage of internal spline deformation is 0.10mm (see Table 1).

Table.1 Comparison of internal spline deformation data under different states

Internal spline wall thickness/mm Frequency/kHz Root hardening layer depth/mm Average shrinkage/mm Internal spline taper/mm Roundness of internal spline / mm
12 30-40 1.5-1.7 0.19 0.06 0.04
200 0.6-0.8 0.14 0.05 0.03
235 0.6-0.8 0.10  0.05 0.03
23 30-40
200 0.6-0.8 0.14 0.02 0.01
235 0.6-0.8 0.10  0.02 0.01

Influence of part structure on internal spline deformation

Part structure has a great influence on heat treatment deformation. For induction hardened parts of internal spline, its wall thickness is one of the most important influencing factors. The thinner the wall thickness, the greater the taper and roundness of internal spline. For the internal splines of the two structures in this paper, the length is long. During induction quenching, the cooling speed of the orifice position relative to the central position is fast and the shrinkage is large. Therefore, there is a certain taper in the whole internal spline length.
Under the background of the same hardening layer depth, after comparing the induction quenching of 12mm and 23mm wall thickness, it is found that the taper of internal spline with 23mm wall thickness is small, only 0.02mm. During process preparation, the outer circle diameter of 12mm wall thickness is increased from 78mm to 100mm, and the outer circle diameter is processed to the required size after induction quenching of the inner spline.
After verification, the taper of internal spline with 12mm wall thickness is also stably controlled within 0.02mm by increasing the excircle turning allowance before heat treatment and machining to the excircle size after heat treatment.

Influence of machining method on internal spline deformation

During induction quenching, the clamping accuracy and cylindrical runout of parts also have a certain impact on the deformation. The greater the clamping runout, the greater the roundness and taper of the internal spline after induction quenching. The induction quenching processing method and process control shall be strictly required, and the outer circle runout of parts shall be controlled within 0.2mm.

Effect of material on internal spline deformation

The deformation of internal splines made of different materials such as steel parts and cast iron parts is also different after induction quenching. The inner spline of planet carrier made of QT700-2 processed by Xi’an shuangte intelligent transmission Co., Ltd. has relatively large deformation for the parts with poor spheroidization rate and graphite morphology in the matrix structure.
Therefore, for induction quenching, it is necessary to stabilize the material structure and state, so as to ensure that the size shrinkage of the machined internal spline is consistent.

Conclusion

From the summary of test data, the deformation trend of internal spline after induction quenching is reduced. The deformation data are different with different hardening layer depth, heating power frequency, internal spline wall thickness and machining process control. In order to reduce other distortions caused by excessive shrinkage, such as taper and roundness, it is recommended to select a heating power supply with higher frequency for induction quenching of internal spline.
During the development of induction hardening parts for internal splines, process tests should be carried out for the selected induction hardening equipment to obtain stable deformation data. On this basis, the process parameters and equipment are fixed, and the fixed pre shrinkage is reserved in the internal spline processing before heat treatment, which can ensure the stable and qualified internal spline size after induction quenching.

Authors: Yan Ke, Li Jinghua, Jia Wu, Liu Guangzong

Source: Network Arrangement – 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 sales@epowermetals.com

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