The material used is 20Cr2Ni4A steel, and its chemical composition is presented in Table 1.
The technical requirements for this gear are as follows: the carburized layer depth should be 1.3-1.6 mm, the surface hardness should be equal or higher than 60HRC, the central hardness should range from 35 to 49HRC, the end face warpage should not exceed 0.25mm, and the inner hole roundness should be within 0.25mm.
Fig. 1 Thin-walled gear
Table 1 Chemical Composition of 20Cr2Ni4A Steel Thin walled Gear
(Mass fraction) (%)
The original quenching process plan took into account the problem of parts warping due to their shape characteristics. However, the plan overlooked the thin-wall characteristics of the parts and only added a quenching mandrel to control the roundness change of the inner hole.
Since gears are thin-walled parts, the thin-wall characteristics cannot be effectively controlled, resulting in an increase in the size of the outermost end of the thin-walled part. In some cases, individual parts have increased by more than 1.5mm.
In terms of appearance size, the part presents a “bell mouth” shape, and the roundness exceeds 1.0mm, seriously affecting subsequent processing and potentially resulting in direct scrapping due to size changes.
1. Cause analysis
The deformation of thin-walled gear parts mentioned above poses great difficulties for the next machining step, especially for finishing the final tooth machining due to changes in the benchmark.
We have identified two main reasons for this kind of deformation:
1）The roundness of the inner hole is controlled during the quenching process, but the thin-walled part is not, resulting in distortion due to organizational and thermal stress.
2）Distortion at the thin wall causes an increase in end warpage.
To control this deformation and warpage, it is necessary to limit the thin-walled part and the end face during quenching. By doing so, we can ensure that the deformation of the part during the quenching process falls within the range of process requirements.
Based on our analysis, the increase in thin-walled part size and excessive end warping is a result of ineffective measures during the quenching process and the lack of restrictions on parts prone to deformation.
To reduce part deformation, such parts should be placed flat in the quenching oil during the quenching process. However, this approach may result in uneven cooling of each part due to the size differences between each section. This, in turn, can lead to disharmony between thermal expansion and contraction, causing an increase in the thin wall’s size and warping deformation of the part.
If proper control measures are not implemented, part warpage will continue to increase, affecting product quality and resulting in losses.
2. Improvement and effect of heat treatment process and tooling
In summary, the thermal expansion and cold contraction of the parts are not in sync, causing thin-walled parts to expand in size.
To effectively address such issues, we will enhance the process and tooling. Specifically, we will design the quenching tooling based on the structural characteristics of the parts and the quenching equipment used in actual production, as shown in Fig. 2. We will also limit the parts that are prone to deformation during the entire quenching process, effectively regulate the cooling speed of each part, and ensure uniform cooling of the parts. These measures will improve the quenching qualification rate of the parts.
Fig. 2 Working Diagram of Quenching Die
- 1. Mandrel;
- 2. Internal pressure die;
- 3. Lower mold;
- 4. Sliding block;
- 5. External pressing die;
- 6. Washer;
- 7. Hexagon head bolt.
2.1 Design of quenching die
To address the part distortion issue, the quenching die underwent redesigning based on the shape characteristics of the part.
This particular quenching die comprises an external die, internal die, lower die, six sliding blocks, mandrel, and more, as depicted in Figures 3 through 7.
Fig. 3 External Die
Fig. 4 Internal Pressure Die
Fig. 5 Quenching lower die
Fig. 6 Slider
Fig. 7 Mandrel
The quenching die’s slider is connected to the external die using bolts, and the external die is linked to the quenching press using screws.
During the falling process of the external die, the sliding block moves along the 15° inclined plane of the external die’s internal wall since the external die is attached to the quenching press.
When the internal die presses against the part’s surface, the sliding block completes its movement and fits with the part’s outer circle. This fixing of the outer circle size controls the part’s size changes during the quenching process and achieves the quenching die’s purpose of controlling the part’s deformation.
Specific working steps of quenching die:
1）Connect the sliding block to the external pressure die, ensuring that the sliding block remains in a free state without clamping stagnation.
2）Connect the external and internal pressure dies to the quenching press, raise them to the specified height, and set the quenching press control button to the automatic position.
3）Install the lower die and mandrel on the workbench of the quenching press.
4）Position the parts using the mandrel and place them on the lower die.
5）Press the control button on the quenching press, causing the internal and external pressure dies to fall simultaneously.
As the internal pressure die reaches the surface of the part, the slider attached to the external pressure die fits perfectly onto the external surface of the part, and the external pressure die falls onto the workbench, creating a closed space between it and the part.
At this point, the oil spray valve activates, filling the closed space with quenching oil, and completing the quenching process for the part.
2.2 Use effect of quenching die
During the quenching process carried out on a quenching press, the press’s pressure can be adjusted promptly based on various conditions, ensuring that the parts are continually subjected to pressure quenching. The shape and size of the parts are limited by the quenching die, which restricts the warpage of the parts, thereby ensuring that the warpage remains within the required range.
By using this quenching die during the quenching process, the size change of the original thin wall can be controlled. The roundness of the parts can be measured and controlled at approximately 0.25mm, with no occurrence of the “bell mouth” phenomenon.
Additionally, the mandrel’s effect can control the roundness of the φ115mm inner hole at approximately 0.2mm, significantly enhancing the quenching qualification rate of parts.
3. Process analysis and experience
Thin-walled parts exhibit significant sectional differences and are susceptible to deformation, including shrinkage of inner holes and warping deformation of end faces. These deformations are the result of various complex stresses acting in combination.
Therefore, it is crucial to implement effective control measures to limit or reduce the size change of parts, ensuring they meet the machining requirements of subsequent steps.
Through the quenching production practice of thin-walled parts, the following experience is obtained:
1. In order to address the size change of thin-walled parts, it is necessary to limit the extent of this change by improving the quenching tooling in accordance with the shape of the parts and the characteristics of our quenching equipment. Specifically, the inner wall of the external pressure die has been designed with a 15° inclined plane.
2. The design of the sliding block at one end has also been updated to align with the size of the inner wall of the external pressure die, based on the characteristics of the quenching equipment.
3. During the machining process of the 15° inclined plane, it is important to control the surface roughness of the part with a relatively high machining accuracy, such as Ra=0.4~0.8μm, to prevent the sliding block from becoming stuck along the inner wall of the external pressure die.
4. The quenching lower die has been designed with 36 grooves on the end face and 6 holes on the side face of the slider. This design facilitates the circulation of quenching oil during the quenching process of the part, leading to enhanced cooling performance and capacity of the quenching oil. Moreover, this ensures that the quenching hardness of the part surface can meet the technical requirements specified in the drawing.
5. The sliding block has been processed into six pieces and installed evenly on the external pressure die. This ensures that the external pressing die can avoid clamping during the falling process of the sliding block, thereby allowing the sliding block to fit fully on the outer circle of the part and limit the size change of the outer circle of the part.
6. The main reason for part deformation is the expansion of the size of the thin wall of the part due to the combined effect of section difference, thermal stress, and organizational stress during cooling, which can present a “bell mouth” phenomenon.
7. The original quenching process plan failed to effectively limit the various stresses generated during the quenching process of the parts, making it difficult to meet the technical requirements of the process.
8. However, the use of the quenching die during the quenching process can limit the deformation of the part to the maximum extent due to the pressure of the quenching press, which helps to balance mutual stress and prevent warping deformation of parts.
9. In addition, the change of the carbon concentration gradient of the carburized layer, the metallographic structure, and the smoothness of the operation will also affect the deformation of the part.
10. Therefore, in actual production, it is necessary to strictly control the flow of various penetrants to ensure the smooth transition of the concentration gradient of the carburizing layer to the center and gentle combination with the original tissue in the center. This will avoid the occurrence of too steep carbon concentration gradient, which could affect the product’s performance.
When conducting heat treatment, especially during quenching, for thin-walled parts, it is necessary to carefully analyze and identify the parts that are prone to deformation based on their structural characteristics and drawing requirements.
Effective process methods and protective measures should be taken based on the actual production situation to control the shape distortion and size change of the parts within the range of process requirements. This will ensure smooth production and meet the necessary standards.