A thin-walled gear is shown in Fig. 1.
The material is 20Cr2Ni4A steel, and its chemical composition is shown in Table 1.
Technical requirements: carburized layer depth is 1.3~1.6mm, surface hardness ≥ 60HRC, central hardness 35~49HRC, end face warpage ≤ 0.25mm, inner hole roundness ≤ 0.25mm.
Fig. 1 Thin-walled gear
Table 1 Chemical Composition of 20Cr2Ni4A Steel Thin walled Gear
(Mass fraction) (%)
In the original quenching process plan, although the actual problem that the parts are prone to warp due to their shape characteristics was considered, the thin-wall characteristics of the parts were ignored, and only the quenching mandrel was added to the inner hole to control the roundness change of the inner hole.
As the gear belongs to thin-walled parts, the thin-wall of the parts can not be effectively controlled, resulting in the increase of the size of the thin-walled part, that is, the size of the outermost end of the thin-walled part increases by 0.8~1.0mm, and even the size of individual parts increases by more than 1.5mm.
From the perspective of appearance size, the part presents a “bell mouth” shape, and the roundness reaches more than 1.0mm, which seriously affects the subsequent processing of the part, and even directly scrapped due to size changes.
1. Cause analysis
Due to the above deformation of thin-walled gear parts, it brings great difficulties to the next step of machining.
Because of the change of the benchmark, it is difficult to finish the final finish machining of the tooth.
For this kind of deformation, we analyze that there are two main reasons:
1) Although the roundness of the inner hole is controlled under the control of the quenching mandrel, the thin-wall of the part is not limited.
During quenching, distortion occurs due to the effect of organizational stress and thermal stress.
2) Because of the distortion at the thin wall, the end warpage of the part also increases.
Therefore, in order to control the distortion of the thin-walled part and the change of the end face warpage, it is necessary to limit the thin-walled part and the end face during the quenching process to ensure that the deformation of the part during the quenching process is within the range of the process requirements.
Through the specific analysis of the part deformation, the reason for the increase of the size of the thin-walled part and the excessive end warping is that effective measures were not taken during the quenching process, and the parts prone to deformation were not limited.
According to experience, such parts need to be put into the quenching oil flat during quenching to reduce the deformation of parts.
However, there is an inevitable factor, that is, when the part is placed in the quenching oil horizontally, due to the size difference of each section of the part, it is bound to lead to uneven cooling of each part, which will lead to the disharmony of thermal expansion and cold contraction.
As a result, the size of the thin wall of the part will increase, and improper operation and control methods will cause warping deformation of the part.
If necessary control methods are not taken, the warpage of parts will increase, which will affect product quality and cause losses.
2. Improvement and effect of heat treatment process and tooling
To sum up, the thermal expansion and cold contraction of the parts are not coordinated, causing the size of the thin-walled parts to expand.
In order to effectively solve such problems, we will improve the process and tooling, that is, according to the structural characteristics of the parts and the quenching equipment used in actual production, we will design the quenching tooling as shown in Fig. 2, limit the parts that are prone to deformation during the whole quenching process, effectively control the cooling speed of each part, ensure the cooling uniformity of the parts, so as to 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
In order to solve the distortion of the part, the quenching die was redesigned according to the shape characteristics of the part.
This set of quenching die is mainly composed of external die, internal die, lower die, 6 sliding blocks, mandrel, etc., as shown in Fig. 3~Fig. 7.
Fig. 3 External Die
Fig. 4 Internal Pressure Die
Fig. 5 Quenching lower die
Fig. 6 Slider
Fig. 7 Mandrel
The slider in the quenching die is connected to the external die by bolts, and the external die is connected to the quenching press by screws.
Because the external die is connected to the quenching press, the sliding block moves along the 15 ° inclined plane of the internal wall of the external die during the falling process of the external die.
When the internal die is pressed on the surface of the part, the sliding block just completes the movement and fits with the outer circle of the part, fixing the outer circle size of the part, thus controlling the size change during the quenching process of the part, and realizing the purpose of controlling the deformation of the part with the quenching die.
Specific working steps of quenching die:
1) Connect the sliding block to the external pressure die, and keep the sliding block always in a free state without clamping stagnation.
2) Connect the external pressure die and internal pressure die on the quenching press, lift them to the specified height, and turn the control button of the quenching press to the automatic position.
3) Install the lower die and mandrel on the workbench of the quenching press.
4) Position with mandrel, and place the parts on the lower die.
5) Press the control button on the quenching press, and the internal and external pressure dies fall at the same time.
When the internal pressure die falls on the surface of the part, the slider installed on the external pressure die just fits with the external surface of the part, and the external pressure die falls on the workbench.
At this time, the external pressure die and the part form a closed space.
At this time, the oil spray valve works, and the closed space is filled with quenching oil, and the part finishes quenching.
2.2 Use effect of quenching die
When quenching is carried out on the quenching press, the pressure of the quenching press can be adjusted timely according to different conditions during quenching, so that the parts are always in the state of pressure quenching during quenching.
Because the shape and size of the part are limited by the quenching die, the warpage of the part during quenching is restricted, thus ensuring that the warpage of the part can be controlled within the required range.
Because this quenching die is used in the quenching process, the size change of the original thin wall is controlled.
The roundness can be controlled at about 0.25mm by measuring, and the “bell mouth” phenomenon does not appear again;
Due to the effect of mandrel, the roundness of φ115mm inner hole can also be controlled at about 0.2mm, which greatly improves the quenching qualification rate of parts.
3. Process analysis and experience
Thin wall parts have large section differences, and their deformation laws are shrinkage of inner holes and end face warping deformation, which are the result of the combined effect of various complex stresses.
Therefore, it is necessary to take effective control measures to limit or reduce the size change of parts, so that they can meet the machining requirements of the next step.
Through the quenching production practice of thin-walled parts, the following experience is obtained:
1) To solve the size change of thin-walled parts, it is necessary to limit the size change of thin-walled parts.
According to the shape of the parts and the characteristics of our quenching equipment, the quenching tooling is improved.
The inner wall of the external pressure die is designed as a 15 ° inclined plane.
2) According to the characteristics of quenching equipment, one end of the sliding block is designed as a 15 ° inclined plane, which is consistent with the inner wall size of the external pressure die.
3) When machining the 15 ° inclined plane, the surface roughness of the part shall be controlled at a relatively high machining accuracy, such as Ra=0.4~0.8μm, so that the sliding block will not be stuck when falling along the inner wall of the external pressure die.
4) 36 grooves are machined on the end face of the quenching lower die, and 6 holes are machined on the side face of the slider.
This design is conducive to the circulation of quenching oil during the quenching process of the part, thus enhancing the cooling performance of the quenching oil, improving the cooling capacity of the quenching oil, and also ensuring that the quenching hardness of the part surface can meet the technical requirements of the drawing.
5) Process the sliding block into 6 pieces and install them evenly on the external pressure die.
In this way, in the process of falling, the external pressing die can avoid the clamping phenomenon when the sliding block moves, and ensure that the sliding block fully fits on the outer circle of the part, thus limiting the change of the outer circle size of the part.
The main reason for part deformation is that the size of the thin wall of the part expands due to the combined effect of section difference, thermal stress and organizational stress during cooling, presenting a “bell mouth” phenomenon.
In the original quenching process plan, the various stresses generated in the quenching process of the parts could not be effectively limited, so the technical requirements of the process could not be met.
When the quenching die is used for quenching, the deformation of the part can be limited to the maximum extent due to the pressure of the quenching press.
This kind of force can make the mutual stress tend to balance, and limit the occurrence of the warping deformation of parts.
In addition, the change of the carbon concentration gradient of the carburized layer, the metallographic structure, and the smoothness of the operation will affect the deformation of the part.
In actual production, it is also necessary to strictly control the flow of various penetrants, so that the concentration gradient of the carburizing layer can transition to the center more smoothly, and combine with the original tissue in the center more gently, so as to avoid the occurrence of too steep carbon concentration gradient, which will affect the use performance of the product.
When conducting heat treatment (especially during quenching) for thin-walled parts, it is necessary to carefully analyze and demonstrate the parts that are prone to deformation according to the structural characteristics of the parts and drawing requirements, and take effective process methods and protective measures according to the actual production situation to control the shape distortion and size change of the parts within the range of process requirements, so as to better ensure the smooth production.