A cold-upsetting method for manufacturing a mandrel

Abstract

The application is a cold upsetting method for manufacturing a mandrel, comprising the following steps: cutting a cold upsetting steel cylinder; downwardly cold upsetting a first positioning hole on the upper end surface of the cold upsetting steel cylinder and upwardly cold upsetting a second positioning hole on the lower end surface of the cold upsetting steel cylinder to form a first cold upsetting embryo; downwardly cold upsetting along the first positioning hole of the first cold upsetting embryo and upwardly cold upsetting along the second positioning hole to form a second cold upsetting embryo; downwardly cold upsetting along a first chamfer cavity of the second cold upsetting embryo to form a third cold upsetting embryo; downwardly cold upsetting along a second chamfer cavity of the third cold upsetting embryo to form a fourth cold upsetting embryo; downwardly cold upsetting along a second step cavity of the fourth cold upsetting embryo to make the second step cavity through the third positioning hole to form a mandrel blank; during the cold upsetting process, the outer diameter of the cold upsetting steel cylinder does not change or only slightly changes during the forming of the mandrel blank, the cold upsetting forming process reduces the local extrusion accumulation of the decarburized layer, ensures that the decarburized layer can be removed in subsequent processing, and avoids the occurrence of soft spots.

Description

Technical Field

[0001] This invention relates to the field of mandrel manufacturing, and in particular to a cold heading method for manufacturing mandrels. Background Technology

[0002] The mandrel is a key component of the pulley. The mandrel is usually made by drilling, turning, boring and face-turning bearing steel bars to obtain the mandrel blank. However, the above steps are numerous and the process is complex, resulting in low utilization of the bar stock and thus high production cost of the mandrel.

[0003] To reduce production costs and improve the strength and wear resistance of the mandrel, the applicant has experimented with various manufacturing processes. The applicant attempted to use cold heading to prepare the mandrel. In conventional cold heading, the upsetting ratio is set at approximately 1.2. When forming a mandrel blank with an outer diameter of approximately 29.4 mm, a standard cold-headed cylinder 01 with an outer diameter of 24 mm is used. During the upsetting process of the cold-headed cylinder, the decarburized layer 001 on the outer peripheral surface of the cold-headed cylinder is locally quite deep, such as... Figure 13 As shown. After cold-forging the mandrel blank 5 with a diameter of 29.4 mm, it undergoes tempering, grinding, and high-frequency quenching. During the grinding process, the decarburized layer 001 on the surface of the mandrel blank 5 cannot be completely removed, resulting in soft spots and affecting the surface hardness of the material. A larger chip allowance is required for the mandrel blank 5 to completely remove the decarburized layer 001, which increases grinding time and production costs and reduces material utilization. In addition, when using a cold-forged cylinder 01 with an outer diameter of 24 mm to form the mandrel blank 5 with a diameter of about 29.4 mm, cracks appear on the surface of the formed mandrel blank due to the fact that the mandrel blank is usually formed by cold forging and the axial compression and radial outward extension during the cold forging process are relatively large. Although a spheroidizing pretreatment process is used to remedy this before cold forging, cracks are still easily generated on the surface of the mandrel blank 5, resulting in a low yield.

[0004] In summary, how to achieve cold heading of mandrels, improve yield, reduce production costs, and enhance the mechanical properties of mandrels has become an urgent problem for researchers in this field. Summary of the Invention

[0005] The technical problem this invention aims to solve is how to address the difficulty in cold heading of mandrels made of bearing steel. To this end, this invention provides a cold heading method for manufacturing mandrels that avoids localized accumulation of decarburized layers during the cold heading process of mandrels made of bearing steel. This method suppresses soft spots that occur during heat treatment due to localized accumulation of decarburized layers after cold heading, thereby improving yield, reducing grinding allowance, lowering production costs, and enhancing the mechanical properties of the mandrel.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0007] This invention relates to a cold heading method for manufacturing mandrels, comprising the following steps: Step S1: Selecting bearing steel suitable for cold heading, and cutting a cold heading steel cylinder of appropriate length according to the mandrel specifications after spheroidization; Step S2: Cold heading a first positioning hole downward on the upper end face of the cold heading steel cylinder, and cold heading a second positioning hole upward on the lower end face of the cold heading steel cylinder, forming a first cold heading blank; Step S3: Cold heading downward along the first positioning hole of the first cold heading blank and cold heading upward along the second positioning hole, forming a second cold heading blank having a first chamfer cavity and a third positioning hole axially; Step S4: Cold heading downward along the first chamfer cavity of the second cold heading blank, forming a third cold heading blank having a first chamfer cavity, a first step cavity, a second chamfer cavity, and a third positioning hole axially; Step S5: Step S6: Cold heading downwards along the second chamfer cavity of the third cold heading blank to form a fourth cold heading blank having a first chamfer cavity, a first step cavity, a second chamfer cavity, a second step cavity, and a third positioning hole in the axial direction; Step S6: Cold heading downwards along the second step cavity of the fourth cold heading blank, breaking off the core material between the second step cavity and the third positioning hole, and then connecting the second step cavity and the third positioning hole to form a mandrel blank having a first chamfer cavity, a first step cavity, a second chamfer cavity, and a through cavity in the axial direction; wherein, the outer diameter of the cold heading steel cylinder is 0.3~0.6mm smaller than that of the mandrel blank.

[0008] Furthermore, in step S1, the axial height of the cold-headed steel cylinder is less than the axial height of the mandrel blank.

[0009] Furthermore, in step S2, after the first cold-forged blank is formed by cold forging, a first rounded corner is formed at the bottom outer diameter;

[0010] Furthermore, after the second cold-forged blank is formed by cold forging, a second rounded corner is formed at its bottom outer diameter, wherein the radius of the second rounded corner is smaller than the radius of the first rounded corner; the bottom outer diameter of the third cold-forged blank, the fourth cold-forged blank, and the mandrel blank all have a second rounded corner; in step S5, after the fourth cold-forged blank is formed by cold forging, a third rounded corner is formed at its top outer diameter; the top outer diameter of the mandrel blank has a third rounded corner.

[0011] Furthermore, in step S2, the height of the cold-heading steel cylinder is greater than the height of the first cold-heading blank, and the outer diameter of the cold-heading steel cylinder is smaller than the outer diameter of the first cold-heading blank.

[0012] Furthermore, in step S3, the height of the first cold heading blank is greater than the height of the second cold heading blank, and the outer diameter of the first cold heading blank is smaller than the outer diameter of the second cold heading blank; the diameter of the first positioning hole is greater than the diameter of the first chamfering cavity, and the height of the first positioning hole is smaller than the height of the first chamfering cavity; the diameter of the second positioning hole is greater than the diameter of the third positioning hole, and the height of the second positioning hole is smaller than the height of the third positioning hole.

[0013] Furthermore, in step S4, the height of the second cold heading blank is less than the height of the third cold heading blank, and the outer diameter of the second cold heading blank is less than the outer diameter of the third cold heading blank; the height and diameter of the third positioning hole of the second cold heading blank are the same as those of the third positioning hole of the third cold heading blank; and the height of the first chamfer cavity of the second cold heading blank is less than the height of the first step cavity.

[0014] Furthermore, in step S5, the height of the third cold heading blank is less than the height of the fourth cold heading blank, and the outer diameter of the third cold heading blank is less than the outer diameter of the fourth cold heading blank; the diameter and height of the first chamfer cavity, the first step cavity, and the second chamfer cavity in the third cold heading blank are the same as the diameter and height of the first chamfer cavity, the first step cavity, and the second chamfer cavity in the fourth cold heading blank.

[0015] Further, in step S6, the height of the fourth cold heading blank is equal to the height of the mandrel blank, and the outer diameter of the fourth cold heading blank is equal to the outer diameter of the mandrel blank; the diameter and height of the first chamfer cavity, the first step cavity, and the second chamfer cavity in the fourth cold heading blank are consistent with the diameter and height of the first chamfer cavity, the first step cavity, and the second chamfer cavity in the mandrel blank; the sum of the heights of the second step cavity and the third positioning hole in the fourth cold heading blank is less than the height of the through cavity in the mandrel blank.

[0016] The beneficial effects of this invention are as follows: This invention provides a cold heading method for manufacturing mandrels. The outer diameter of the cold-headed steel cylinder is 0.3mm to 0.6mm smaller than that of the mandrel blank. This ensures that the outer diameter of the cold-headed steel cylinder changes only slightly during the cold heading process, thus suppressing the risk of surface cracking of the mandrel blank. The entire cold heading process reduces localized extrusion and accumulation of the decarburized layer. Subsequent machining of the mandrel blank removes the decarburized layer through grinding, resulting in less cutting allowance for the mandrel blank. Furthermore, the mandrel blank does not exhibit soft spots after heat treatment, improving yield and reducing production costs. Compared to the drilling process of bar stock, the cold heading method for forming the mandrel blank produces less waste and improves the density of the microstructure.

[0017] Furthermore, the slight change in the outer diameter of the cold-heading steel cylinder or blank during the cold-heading process allows for a small radial gap between the cylinder or blank and the corresponding mold cavity. This prevents scratches on the inner wall of the mold when the cylinder or blank is placed inside. Moreover, in the cold-heading mandrel forming process, compared to the constant outer diameter of the cylinder or blank during the standard cold-heading process, this method only slightly changes the outer diameter, compensating for the poor plasticity of the bearing steel mandrel and the resulting surface cracking. More importantly, the radial gap before cold-heading allows the mandrel to extend radially in addition to axially, preventing surface cracks caused by axial extension alone. Attached Figure Description

[0018] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0019] Figure 1 This is a sectional view of a cold-headed steel cylinder;

[0020] Figure 2 This is a cross-sectional view of the first cold heading blank;

[0021] Figure 3 This is a cross-sectional view of the second cold heading blank;

[0022] Figure 4 This is a cross-sectional view of the third cold heading blank;

[0023] Figure 5 This is a cross-sectional view of the fourth cold heading blank;

[0024] Figure 6 This is a sectional view of the mandrel blank;

[0025] Figure 7 This is a cross-sectional view of the cutting die;

[0026] Figure 8 This is a cross-sectional view of the first cold heading die;

[0027] Figure 9 This is a cross-sectional view of the second cold heading die;

[0028] Figure 10 This is a sectional view of the third cold heading die;

[0029] Figure 11 This is a sectional view of the fourth cold heading die;

[0030] Figure 12 This is a sectional view of the fourth cold heading die;

[0031] Figure 13 This is a schematic diagram of a standard cold-headed steel cylindrical mandrel blank.

[0032] Figure 14 This is a microstructure diagram of the mandrel after it has been formed using a conventional cold heading process;

[0033] Figure 15 This is a microstructure diagram of the mandrel after it has been formed using the cold heading process of this invention;

[0034] Figure 16 This is a process flow diagram for the cold heading of mandrel blanks. Detailed Implementation

[0035] The present invention will now be described in further detail with reference to the accompanying drawings. These drawings are simplified schematic diagrams, illustrating only the basic structure of the invention, and therefore only show the components relevant to the invention.

[0036] See Figure 6 , Figure 6 The figure shows a cross-sectional view of the mandrel blank 5. From top to bottom, the middle part of the mandrel blank 5 is provided with a first chamfered cavity 21, a first stepped cavity 31, a second chamfered cavity 32, and a through cavity 52. ​​The above four are arranged to run through the middle part of the mandrel blank 5 along its axis. The inner diameter of the first stepped cavity 31 is larger than the inner diameter of the through cavity 52. ​​The first chamfered cavity 21 connects the top of the mandrel blank 5 and the top of the first stepped cavity 31, serving as a transition. The second chamfered cavity 32 connects the bottom of the first stepped cavity 31 and the top of the through cavity 52, also serving as a transition. A third fillet 42 is provided at the outer diameter of the top of the mandrel blank 5, and a second fillet 23 is provided at the outer diameter of the bottom of the mandrel blank 5. The curvature of the second fillet 23 and the third fillet 42 is the same.

[0037] In order to form cold heading Figure 6 The mandrel blank 5 in the process of cold heading cylinder 01, and to avoid excessive local extrusion of the decarburized layer during the upsetting process, which would lead to soft spots in subsequent heat treatment, the present application provides the following embodiments. Example

[0038] See Figure 1 , Figure 1 This is a cross-sectional view of the cold-heading steel cylinder 01. The cylinder 01 is formed by cutting bearing steel after spheroidizing. The outer diameter of the cold-heading steel cylinder 01 is less than the outer diameter of the mandrel blank 5 by 0.3-0.6 mm. This 0.3-0.6 mm represents the range of outer diameter variation as the cold-heading steel cylinder 01 is gradually upset to the outer diameter of the mandrel blank 5. Figure 6 The outer diameter of the central shaft blank 5 is 29.4mm, and the axial length is 39mm. Figure 1The cold-heading steel cylinder 01 with an outer diameter of 29mm and an axial length of 32.5mm can be used. The weight of the cold-heading steel cylinder 01 needs to be greater than the weight of the mandrel blank 5, so as to give the mandrel blank 5 a margin for the core 51 to be formed. The core 51 will be broken by the corresponding punch in step S6. The greater the weight of the cold-heading steel cylinder 01, the more margin the core 51 has, and the greater the impact force on the punch. Therefore, it is necessary to select a cold-heading steel cylinder 01 with appropriate weight and axial length.

[0039] See Figure 7 The figure shows a cross-sectional view of a cutting die used to cut steel into cold-headed steel cylinders 01. The end of the steel is fixed to the cutting die 03 of the cutting die 02. The cutting blade 04 moves to cut the steel into cold-headed steel cylinders 01.

[0040] See Figure 2 , Figure 2 The first cold heading blank 1 is shown in cross-sectional view. In step S2, the upper end face of the cold heading steel cylinder 01 is cold-headed downward by a punch to form a first positioning hole 11, and the lower end face of the cold heading steel cylinder 01 is cold-headed upward by another punch to form a second positioning hole 12. Thus, the first cold heading blank 1 has the first positioning hole 11 and the second positioning hole 12. Since the two punches act on the upper and lower end faces of the cold heading steel cylinder 01 respectively, the axial height of the first cold heading blank 1 will be shorter than that of the cold heading steel cylinder 01. However, due to the limitation of the inner diameter of the cold heading mold, the outer diameter of the first cold heading blank 1 is slightly expanded compared to the outer diameter of the cold heading steel cylinder 01. That is, the outer diameter of the cold heading steel cylinder 01 expands from 29mm to 29.1mm.

[0041] In addition, during the cold heading process, the outer diameter of the bottom wall of the first cold heading blank 1 is affected by the corresponding cold heading mold, forming the first rounded corner 13.

[0042] See Figure 8 The figure shows a cross-sectional view of the first cold heading mold 1-1 for forming the first cold heading blank 1. In the figure, the cold heading steel cylinder 01 is placed in the first cavity 1-2 of the first cold heading mold 1-1. The first upper punch 1-3 above the cold heading steel cylinder 01 moves downward and acts on the upper end face of the cold heading steel cylinder 01 to form the first positioning hole 11. The first lower punch 1-4 below the cold heading steel cylinder 01 moves upward and acts on the lower end face of the cold heading steel cylinder 01 to form the second positioning hole 12. The cold heading steel cylinder 01 is cold-headed to form the first cold heading blank 1.

[0043] See Figure 3 , Figure 3A cross-sectional view of the second cold heading blank 2. In step S3, the first positioning hole 11 of the first cold heading blank 1 is cold-headed downward by a punch to form a first chamfer cavity 21, and the second positioning hole 12 is cold-headed upward by another punch to form a third positioning hole 22. Thus, the second cold heading blank 2 has a first chamfer cavity 21 and a third positioning hole 22. The function of the third positioning hole 22 is to enable the third cold heading blank 3 and the fourth cold heading blank 4 to achieve positioning with the corresponding cold heading mold in steps S4 and S5, and to ensure the coaxiality between the corresponding punch and the third cold heading blank 3 and the fourth cold heading blank 4. Figure 3 The height of the first chamfered cavity 21 is greater than Figure 2 The height of the first positioning hole 11 in the middle, Figure 3 The height of the third positioning hole 22 is greater than Figure 2 The height of the second positioning hole 12; similarly, since the two punches act on the upper and lower end faces of the second cold heading blank 2 respectively, the axial length of the third cold heading blank 3 will be shorter than that of the second cold heading blank 2. However, due to the limitation of the inner diameter of the cold heading die, the outer diameter of the second cold heading blank 2 is slightly expanded compared to the outer diameter of the first cold heading blank 1, that is, the outer diameter of the first cold heading blank 1 (29.1 mm) expands to the outer diameter of the second cold heading blank 2 (29.15 mm).

[0044] In addition, during the cold heading process, the outer diameter of the bottom wall of the second cold heading blank 2 is affected by the corresponding cold heading mold, and the first rounded corner 13 is formed into the second rounded corner 23.

[0045] See Figure 9 The figure shows a cross-sectional view of the second cold heading mold 2-1 for forming the second cold heading blank 2. The first cold heading blank 1 is placed in the second cavity 2-2 of the second cold heading mold 2-1. The second upper punch 2-3 above the first cold heading blank 1 moves downward to upset the first positioning hole 11 of the first cold heading blank 1 to form a first chamfer cavity 21. The diameter of the body of the second upper punch 2-3 matches the diameter of the first cold heading blank 1. The second lower punch 2-4 below the first cold heading blank 1 moves upward to upset the second positioning hole 12 of the first cold heading blank 1 to form a third positioning hole 22. The diameter of the second lower punch 2-4 is smaller than the diameter of the first cold heading blank 1. In this way, the first cold heading blank 1 is cold-headed to form the second cold heading blank 2.

[0046] See Figure 4 , Figure 4This is a cross-sectional view of the third cold-forged blank 3. In step S4, the lower part of the second cold-forged blank 2 remains unchanged, that is, the third positioning hole 22 and the second fillet 23 remain unchanged. The first chamfer cavity 21 of the second cold-forged blank 2 is cold-forged downward by a punch to form a first step cavity 31 and a second chamfer cavity 32. Thus, the third cold-forged blank 3, from top to bottom, consists of the first chamfer cavity 21, the first step cavity 31, the second chamfer cavity 32, and the third positioning hole 22. Hole 22 is not connected to the second chamfering cavity 32; because the punch deepens the bottom of the first chamfering cavity 21, the height of the third cold heading blank 3 after forming will increase compared to the second cold heading blank 2. Due to the limitation of the corresponding cold heading mold, the outer diameter of the third cold heading blank 3 will be slightly expanded compared to the outer diameter of the second cold heading blank 2, that is, the outer diameter of the second cold heading blank 2 (29.15 mm) will expand to the outer diameter of the third cold heading blank 3 (29.2 mm).

[0047] See Figure 10 The figure shows a cross-sectional view of the third cold heading mold 3-1 for forming the third cold heading blank 3. In the figure, the second cold heading blank 2 is placed in the third cavity 3-2. The position of the third lower punch 3-4 below the second cold heading blank 2 remains unchanged and is matched and positioned with the third positioning hole 22. The third upper pressure head 3-5 is adjusted to be above the second cold heading blank 2. The third upper punch 3-3 above the second cold heading blank 2 moves downward to form the first step cavity 31 and the second chamfer cavity 32 by increasing the upsetting depth of the first chamfer cavity 21. The diameter of the third upper punch 3-3 is smaller than the outer diameter of the second cold heading blank. Therefore, during the process of forming the second cold heading blank 2 into the third cold heading blank 3, the axial height will increase until the top of the second cold heading blank 2 is in contact with the third upper pressure head 3-5.

[0048] See Figure 5 , Figure 5 This is a cross-sectional view of the fourth cold heading blank 4. In step S5, keeping the lower part of the third cold heading blank 3 unchanged, i.e., the third positioning hole 22 and the second fillet 23 unchanged, the second chamfer cavity 32 is upset downwards using a punch to form the second stepped cavity 41. Thus, the fourth cold heading blank 4 formed in this way consists of, from top to bottom, the first chamfer cavity 21, the first stepped cavity 31, the second chamfer cavity 32, the second stepped cavity 41, and the third positioning hole 22; the third positioning hole 22 and the second stepped cavity 41 are not connected; the second stepped cavity 41... The inner diameter of the first stepped cavity 31 is smaller than the inner diameter of the first stepped cavity 31, and the height of the first stepped cavity 31 is smaller than the height of the second stepped cavity 41. As the punch deepens the bottom of the second chamfered cavity 32, the height of the fourth cold heading blank 4 after forming will increase compared to the third cold heading blank 3. Due to the limitation of the corresponding cold heading mold, the outer diameter of the fourth cold heading blank 4 is slightly expanded compared to the outer diameter of the third cold heading blank 3, that is, the outer diameter of the third cold heading blank 3 (29.2 mm) expands to the outer diameter of the fourth cold heading blank 4 (29.4 mm).

[0049] In addition, during the cold heading process, the outer diameter of the top wall of the fourth cold heading blank 4 is affected by the corresponding cold heading mold, and a third rounded corner 42 is formed.

[0050] See Figure 11 The figure shows a cross-sectional view of the fourth cold heading mold 4-1 for forming the fourth cold heading blank 4. In the figure, the third cold heading blank 3 is placed in the fourth cavity 4-2. The position of the fourth lower punch 4-4 below the third cold heading blank 3 remains unchanged and is matched and positioned with the third positioning hole 22. The fourth upper pressure head 4-5 is adjusted to be above the third cold heading blank 3. The fourth upper punch 4-3 above the third cold heading blank 3 moves downward to form the second step cavity 41 for the second chamfer cavity 32. The diameter of the fourth upper punch 4-3 is smaller than the diameter of the third upper punch 3-3. Therefore, during the process of forming the third cold heading blank 3 into the fourth cold heading blank 4, the axial height will increase until the top of the third cold heading blank 3 is in contact with the fourth upper pressure head 4-5.

[0051] See Figure 6 , Figure 6 The diagram shows a cross-sectional view of the mandrel blank 5. The fourth cold heading blank 4 is placed in the corresponding cold heading mold. The upper and lower positions of the fourth cold heading blank 4 are limited by two pressure heads. The second step cavity 41 is then upset downwards by a punch until the second step cavity 41 and the second positioning hole 12 are connected to form a through cavity 52. ​​The inner diameter of the through cavity 52 is the same as the inner diameter of the second step cavity 41. The excess material of the core 51 will fall from the bottom of the through cavity 52. ​​Thus, the middle part of the mandrel blank 5 is arranged from top to bottom as the first chamfer cavity 21, the first step cavity 31, the second chamfer cavity 32, and the through cavity 52. ​​During this process, since the axial height of the fourth cold heading blank 4 is limited by the pressure heads, the punch plays a breaking role. Therefore, the outer contour of the fourth cold heading blank 4 does not change with the outer contour of the mandrel blank 5, that is, the outer diameter of the fifth mandrel blank 5 is 29.4 mm.

[0052] See Figure 12 The figure shows a cross-sectional view of the fifth cold heading mold 5-1 for forming the mandrel blank 5. The fourth cold heading blank 4 is placed in the fifth cavity 5-2. The bottom of the fifth cavity 5-2 is open and has a discharge channel 5-4. The fifth upper pressure head 5-5 is adjusted to abut and limit the top of the fourth cold heading blank 4. The fifth upper punch 5-3 above the fourth cold heading blank 4 moves downward and passes axially through the mandrel blank 5, thus forging through the space between the second stepped cavity 41 and the third positioning hole 22. The core 51 between the second stepped cavity 41 and the third positioning hole 22 falls from the discharge channel 5-4. Therefore, the axial height will not increase during the process of forming the fourth cold heading blank 4 into the mandrel blank 5.

[0053] See Figure 16In the above process, that is, during the formation of the cold-headed steel cylinder 01 into the mandrel blank 5, the outer diameter changes only slightly. The depth of local extrusion of the decarburized layer is relatively shallow during the entire cold-heading process (unlike the 24mm cold-headed cylinder 01 in the background technology which is cold-headed to form the mandrel blank 5 by axial compression and radial expansion). After the decarburized layer 001 on the outer surface of the mandrel blank 5 is ground, no soft spots appear due to the presence of the decarburized layer 001 after heat treatment. For example, to form a 29.2mm mandrel, the 29mm cold-headed cylinder 01 is cold-headed to a 29.4mm mandrel cold-headed part. During the cold-heading process, the outer diameter of the mandrel only expands slightly and the axial height extends, reducing the degree of local extrusion of the decarburized layer. The outer surface of the 29.4mm mandrel cold-headed part is ground by 0.2mm. At this time, the decarburized layer is removed to avoid soft spots appearing on the surface of the mandrel after heat treatment.

[0054] It should be noted that if the outer diameter of the cold-forged steel cylinder 01 is 0.3mm smaller than the outer diameter of the mandrel blank 5, for example, if a 29.2mm cold-forged cylinder 01 is used to cold-forge a 29.4mm mandrel blank 5, during the cutting process, the uneven cut surface of the cold-forged cylinder 01 will produce flash. This flash protrudes from the end of the cold-forged cylinder, causing damage to the inner wall of the mold or preventing it from being installed in the mold. Furthermore, even if the end of the cold-forged cylinder 01 is flat and free of flash, and the cold-forged cylinder 01 can be installed in the mold, the diameter difference between the cold-forged cylinder 01 and the mold will be significant. The small gap between the cold and hot sections leads to greater axial flow in the cold-forged cylinder 01 during the cold-forging process. For bearing steel with poor plasticity, this excessive axial flow can cause surface cracking, thus affecting the quality of the finished product. Therefore, the application proposes a gradual upsetting process from the cold-forged steel cylinder to the mandrel blank, which can reduce the manufacturing cost of the mandrel and improve material utilization. The outer diameter of the cold-forged steel cylinder is 0.3mm to 0.6mm smaller than that of the mandrel blank, which reduces metal flow, avoids cracking, and prevents local accumulation of decarburized layers. This also reduces the amount of chips in the later stages, while improving the density of the microstructure and enhancing the mechanical properties of the mandrel.

[0055] It should also be noted that if the first positioning hole 11 is directly cold-forged into the first stepped cavity 31, that is, if step S5 is executed directly after step S3 and step S4 is skipped, the punch is easily damaged due to the large impact force required for cold forging. In addition, cracks are easily generated during the process of directly forming the fourth cold forging blank 2 from the second cold forging blank 4. Therefore, adding step S3 between steps S2 and S4 can improve the service life of the punch and make the second cold forging blank 2 less prone to cracking.

[0056] See Figure 14Microstructure of mandrel blank 5 with diameter cold-forged from φ24mm to φ29.4mm. The decarburized layer 001 in the image is darker, indicating a larger local accumulation of decarburized layer 001. Soft spots appeared on the high-frequency quenched surface due to the decarburized layer 001 (20 pieces sampled). See also... Figure 15 The microstructure of mandrel blank 5, with a diameter from φ29mm to φ29.4mm after cold heading, shows that the decarburized layer 001 is lighter in color, indicating less local accumulation of decarburized layer 001. No soft spots appeared on the surface after high-frequency quenching (20 pieces were sampled). The above comparative example shows that the greater the diameter change after cold heading, the more local accumulation of decarburized layer occurs, leading to soft spots on the surface. Subsequent comparison of products formed by the two processes showed consistent test results, further indicating that the greater the diameter change, the more local accumulation of decarburized layer occurs, making high-frequency soft spots more likely.

[0057] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A cold heading method for manufacturing mandrels, characterized in that, Includes the following steps: Step S1: Select bearing steel suitable for cold heading, and cut cold heading steel cylinders of appropriate length according to the specifications of the mandrel after spheroidization; Step S2: Cold-forge the first positioning hole downward on the upper end face of the cold-forged steel cylinder, and cold-forge the second positioning hole upward on the lower end face of the cold-forged steel cylinder to form the first cold-forged blank; Step S3: Cold heading downwards along the first positioning hole of the first cold heading blank and cold heading upwards along the second positioning hole to form a second cold heading blank having a first chamfer cavity and a third positioning hole in the axial direction; Step S4: Cold-forging downwards along the first chamfer cavity of the second cold-forging blank to form a third cold-forging blank having a first chamfer cavity, a first step cavity, a second chamfer cavity, and a third positioning hole in the axial direction; Step S5: Cold-forging downwards along the second chamfer cavity of the third cold-forging blank to form a fourth cold-forging blank having a first chamfer cavity, a first step cavity, a second chamfer cavity, a second step cavity, and a third positioning hole in the axial direction; Step S6: Cold-forge downwards along the second step cavity of the fourth cold-forging blank, and after breaking the core material between the second step cavity and the third positioning hole, the second step cavity and the third positioning hole are connected to form a mandrel blank with a first chamfer cavity, a first step cavity, a second chamfer cavity, and a through cavity in the axial direction; The outer diameter of the cold-headed steel cylinder is 0.3~0.6mm smaller than the outer diameter of the mandrel blank.

2. The cold heading method for manufacturing a mandrel according to claim 1, characterized in that, In step S1, the axial height of the cold-headed steel cylinder is less than the axial height of the mandrel blank.

3. The cold heading method for manufacturing a mandrel according to claim 1, characterized in that, In step S2, after the first cold heading blank is formed by cold heading, a first rounded corner is formed at the bottom outer diameter; In step S3, after the cold heading forms the second cold heading blank, a second rounded corner is formed at the bottom outer diameter, wherein the radius of the second rounded corner is smaller than the radius of the first rounded corner; The third cold heading blank, the fourth cold heading blank, and the mandrel blank all have a second rounded corner at the bottom outer diameter; In step S5, after the fourth cold-forged blank is formed by cold heading, a third fillet is formed at the outer diameter of its top. The mandrel blank has a third fillet at the top outer diameter.

4. The cold heading method for manufacturing a mandrel according to claim 1, characterized in that, In step S2, the height of the cold heading steel cylinder is greater than the height of the first cold heading blank, and the outer diameter of the cold heading steel cylinder is smaller than the outer diameter of the first cold heading blank.

5. The cold heading method for manufacturing a mandrel according to claim 1, characterized in that, In step S3, the height of the first cold heading blank is greater than the height of the second cold heading blank, and the outer diameter of the first cold heading blank is smaller than the outer diameter of the second cold heading blank; the diameter of the first positioning hole is greater than the diameter of the first chamfering cavity, and the height of the first positioning hole is smaller than the height of the first chamfering cavity; the diameter of the second positioning hole is greater than the diameter of the third positioning hole, and the height of the second positioning hole is smaller than the height of the third positioning hole.

6. The cold heading method for manufacturing a mandrel according to claim 1, characterized in that, In step S4, the height of the second cold heading blank is less than the height of the third cold heading blank, and the outer diameter of the second cold heading blank is less than the outer diameter of the third cold heading blank; the height and diameter of the third positioning hole of the second cold heading blank are the same as those of the third positioning hole of the third cold heading blank; the height of the first chamfer cavity of the second cold heading blank is less than the height of the first step cavity.

7. The cold heading method for manufacturing a mandrel according to claim 1, characterized in that, In step S5, the height of the third cold heading body is less than the height of the fourth cold heading body, and the outer diameter of the third cold heading body is less than the outer diameter of the fourth cold heading body; the diameter and height of the first chamfer cavity, the first step cavity, and the second chamfer cavity in the third cold heading body are the same as the diameter and height of the first chamfer cavity, the first step cavity, and the second chamfer cavity in the fourth cold heading body.

8. The cold heading method for manufacturing a mandrel according to claim 1, characterized in that, In step S6, the height of the fourth cold heading blank is equal to the height of the mandrel blank, and the outer diameter of the fourth cold heading blank is equal to the outer diameter of the mandrel blank; the diameter and height of the first chamfer cavity, the first step cavity, and the second chamfer cavity in the fourth cold heading blank are consistent with the diameter and height of the first chamfer cavity, the first step cavity, and the second chamfer cavity in the mandrel blank; the sum of the heights of the second step cavity and the third positioning hole in the fourth cold heading blank is less than the height of the through cavity in the mandrel blank.

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