Graduated form

Abstract

Graduated mold (1) for powder forming of metal powder, comprising: an inner ring (2) formed from a sintered hard alloy and having a cylindrical shape, and an outer ring (3) having a cylindrical shape, which is attached to an outer circumference of the inner ring (2) by shrink fit, wherein a recessed section (4) for forming, comprising a step section (7), is formed on an inner side of the inner ring (2), wherein a flange section (6) which engages in a punch plate (5) is formed on an outer circumference of the outer ring (3), wherein only the flange section (6) of the stepped shape (1) is held by the punch plate (5), while a lower surface of the stepped shape (1) is not held by any further element, and wherein a shrinkage fit ratio of the outer ring (3) to the inner ring (2) is set to a value which falls within a range of 0.12% to 0.25%.

Description

Technical area

[0001] The present invention relates to a stepped shape. In particular, the present invention relates to a stepped shape in which an outer ring is attached to an outer circumference of an inner ring by a shrink fit. State of the art

[0002] In powder forming, there may be a case where a mold, referred to as a stepped mold, is used, for example, to form an outer circumferential side of a part 31 with a step 30 on an outer circumference, as in Fig. 8 shown, in use. Fig. Figure 9 shows a top view of an example of such a stepped form 21, and Fig. Figure 10 shows a cross-sectional view of the stepped shape 21.

[0003] The stepped form 21 comprises an inner ring 22 with a cylindrical shape and an outer ring 23 with a cylindrical shape, which is fitted to an outer circumference of the inner ring 22 by a shrink fit, and a recessed section 24 for forming, which is formed on an inner surface of the inner ring 22. The recessed section 24 has a stepped section 25, which corresponds to a step 30 of a part 31. As in Fig. As shown in Figure 9, the step section 25 has a rectangular shape in a top view. A flange section 27, which engages in a punch plate 26, is formed on an outer circumference of the outer ring 23.

[0004] During forming, part 31 uses the aforementioned stepped mold 21, and after forming, part 31 is removed from the stepped mold 21 in such a way that the stepped mold 21, together with the die plate 26, is lowered, so that part 31 is pushed upwards in a fixed position relative to the stepped mold 21 by a lower die 28. Accordingly, a support holding the stepped mold cannot be arranged in a space S below the stepped mold 21, since the support would obstruct the lowering of the stepped mold 21. In view of this, pressing of the powder is carried out using an upper surface 28a of the lower punch 28 and an upper surface 25a of the stepped section 25 as pressure surfaces in a state in which only a flange section 27 formed on an outer circumference of the stepped shape 21 is held, but a lower surface of the stepped shape 21 is not held.

[0005] In such a pressure application method, a pressure applied to the step section 25 is absorbed by an edge section or a corner section of the step section 25, and thus a bending stress is concentrated on the corner section, which can cause a crack C to form (see Fig. 11) The occurrence of crack C may not only lead to a break in the stepped form 21, but may also impair the accuracy of a manufactured part 31.

[0006] In view of the above, to prevent cracking by reducing the stress concentration in the corner section of the stepped section of the stepped mold, a method was proposed in which a ring is attached to an outer circumference of a mold section on which a bending stress acts due to a tight fit (see patent literature 1). Citation list

[0007] [Patent literature] Japanese utility model publication no. JP H03 - 59 329 U

[0008] Document JP 2012-136760 A discloses a mold consisting of an upper and a lower mold, wherein the lower mold comprises an inner and an outer part connected by a shrink fit, the outer part reinforcing the inner part which defines the mold opening.

[0009] Document JP 2006-305626 A describes a combined die with a ceramic inner sleeve inserted into a metallic outer sleeve, with a groove and a cavity provided at their interface to control the stress distribution during the pressing process.

[0010] Document JP 2001-138002 A describes a die insert with a sintered reinforcing ring that is shrink-fitted around its outer circumference to introduce a circumferential compressive stress into the inner mold cavity. Summary of the invention: Technical problem

[0011] However, in the method described in patent literature 1, it is necessary to produce an additional part, which is referred to as a ring next to the mold, and the method therefore also requires a step of attaching the ring to the outer circumference of the mold by fitting it tightly.

[0012] In view of this, it can be assumed that residual compressive stress around a corner section of a step section is generated by setting a slightly larger shrink-fit ratio or shrink-fit extent at the time of tightening an outer ring to an outer circumference of an inner ring.

[0013] However, even if the shrink-fit ratio is only increased, a residual compressive stress sufficient to counteract the bending stress generated in the corner section of the inner ring's step section during compression molding cannot be maintained, and thus cracking can occur. Furthermore, the method has the inherent disadvantage that, at the time of shrinkage, an excessively high stress is generated in a different section of the inner ring's step section than the corner section, which can lead to cracking.

[0014] The present invention was conceived in view of such circumstances, and it is an object of the present invention to create a stepped shape which can prevent the occurrence of a crack in a corner section of a step section without increasing the number of components and the number of working hours. Solution to the problem

[0015] A stepped mold according to the present invention comprises a stepped mold for powder forming a metal powder, having an inner ring formed from a sintered hard alloy and having a cylindrical shape, and an outer ring having a cylindrical shape, which is attached to an outer circumference of the inner ring by a shrink fit, wherein a recessed section for forming, comprising a step section, is formed on an inner surface of the inner ring, wherein a flange section engaging in a die plate is formed on an outer circumference of the outer ring, wherein only the flange section of the stepped mold is held by the die plate, while a lower surface of the stepped mold is not held by any other element, and wherein a shrink fit ratio of the outer ring to the inner ring is set to a value falling in the range of 0.12% to 0.25%.

[0016] According to the stepped form of the present invention, a shrinkage fit ratio of the outer ring to the inner ring is set to a value that falls in the range of 0.12% to 0.25%, and therefore a corresponding compressive stress can be applied to a corner section of the stepped section of the recessed section for forming, making it possible to prevent the occurrence of a crack in the corner section that may be caused by a bending stress concentrated on the corner section at the time of compression forming. Advantageous effects of the invention

[0017] According to the graduated form of the present invention, it is possible to prevent the occurrence of a crack in a corner section of a step section without increasing the number of components and the number of working hours. Brief description of the drawings Fig. Fig. Figure 1 shows a top view of a stepped shape according to an embodiment of the present invention. Fig. Fig. Figure 2 shows a cross-sectional view of the stepped shape in Fig. 1. Fig. Fig. Figure 3 shows an explanatory, perspective view of an inner ring of the stepped form in Fig. 1. Fig. Fig. Figure 4 shows a diagram illustrating a relationship between the strength ratio of a stepped corner radius section and the shrinkage fit ratio. Fig. Fig. Figure 5 shows a diagram illustrating a relationship between the strength ratio of the stepped corner radius section and the inner ring ratio. Fig. Fig. Figure 6 shows a diagram illustrating a relationship between a compressive strength ratio and a wall thickness. Fig. Fig. Figure 7 shows a diagram illustrating a relationship between a compressive strength ratio and an inner ring ratio. Fig. Fig. Figure 8 shows a perspective view illustrating an example of a powdered product that has a stepped section on its outer surface. Fig. Fig. Figure 9 shows a top view, which is an example of a stepped shape. Fig. Fig. Figure 10 shows a cross-sectional view of the stepped shape in Fig. 9. Fig. Fig. Figure 11 shows a photograph depicting a crack created in a corner section of a step section. Description of the embodiments

[0018] A stepped form of the present invention comprises an inner ring formed from a sintered hard alloy and having a cylindrical shape, and an outer ring with an annular shape, which is fitted to an outer circumference of the inner ring by a shrink fit, and a recessed section for forming, which has a stepped section and is formed on an inner surface of the inner ring. A flanged section engaging with a die plate is formed on an outer circumference of the outer ring. Only the flanged section of the stepped form is held by the die plate, while a lower surface of the stepped form is not held by any other element. A shrink fit ratio of the outer ring to the inner ring is set to a value in the range of 0.12% to 0.25%.

[0019] Preferably, the ratio between the outer diameter of the inner ring and the diameter of a largest imaginary circle, which is an imaginary circle centered on a central axis of the inner ring and passing through a corner section of the step section furthest from the center in a radially outward direction, is set to 1.4 or greater. In this case, specifying a certain wall thickness for the inner ring can increase its resistance to residual compressive stress applied to the inner ring due to the shrink fit of the outer ring.

[0020] Furthermore, the ratio is set to 2.0 or less. In this case, by limiting the wall thickness of the inner ring to a predetermined value or less, an enlargement of the inner ring and ultimately the enlargement of the stepped shape can be suppressed, while maintaining resistance of the inner ring to residual compressive stress.

[0021] Preferably, a wall thickness, which is the difference between the outer diameter of the inner ring and the diameter of the largest imaginary circle (an imaginary circle centered on the central axis of the inner ring and passing through a corner section of the step section furthest from the center in a radially outward direction), is set to 5 mm or more. In this case, specifying a certain wall thickness for the inner ring can increase its resistance to residual compressive stress applied to it by the shrink fit of the outer ring.

[0022] The inner ring material can be a sintered hard alloy, and the outer ring material can be hardened steel. In this case, the required compressive strength and fatigue strength for the inner ring can be ensured.

[0023] In the following, a graduated form according to an embodiment of the present invention is described in detail with reference to the accompanying drawings. Fig. Figure 1 shows a top view of a stepped shape 1 according to an embodiment of the present invention, and Fig. Figure 2 shows a cross-sectional view of the stepped shape 1 of the Fig. 1.

[0024] The stepped mold 1 according to the present embodiment is a mold used in the metallurgical production of a green compact formed by pressing a powder. The stepped mold 1 comprises an inner ring 2 and an outer ring 3, which is fitted to an outer circumference of the inner ring 2 by a shrink fit. A recessed section 4 for compression forming is provided on an inner surface of the inner ring 2.

[0025] The inner ring 2 has a cylindrical shape and can be manufactured, for example, using a sintered hard alloy such as a WC-Co alloy or a WC-TiC-Co alloy. The outer ring 3 also has a cylindrical shape and can be manufactured using ordinary hardened steel. A flange section 6, which engages with a die plate 5, is formed on an outer circumference of the outer ring 3 over its entire circumference.

[0026] The recessed section 4 has on one upper surface side (upper side in Fig. 2) the inner ring 2 has a rectangular shape in plan view, and has a lower surface side (lower side in Fig. 2) of the inner ring 2 has a circular shape in plan view. A step section 7 is provided at a boundary section between an upper recessed section with a rectangular shape in plan view and a lower recessed section with a circular shape in plan view. The step section 7 is a section corresponding to a step of a formed product (see Fig. 8), which is formed by shapes using the graduated shape 1.

[0027] In this embodiment, the outer diameter of the inner ring 2 and the inner diameter of the outer ring 3 are set such that a shrink fit ratio or shrink fit extent, expressed by the following formula (1) (hereinafter referred to as ‘shrink fit ratio’), has a value that falls within a range of 0.25% to 0.12%. Shrinkage ratio (%) = {1 − (inner diameter of outer ring / outer diameter of inner ring)} × 100

[0028] If the shrinkage fit ratio (%) is less than 0.12%, there is a possibility that the residual compressive stress will be insufficient, leading to cracking during forming. Conversely, if the shrinkage fit ratio (%) is greater than 0.25%, cracking may occur during the shrinkage fit. From the standpoint of reliably preventing cracking and minimizing the enlargement of the inner ring, the shrinkage fit ratio (%) is preferably set to a value within the range of 0.15% to 0.20%.

[0029] In this embodiment, the ratio between an outer diameter d1 of the inner ring 2 and a diameter d2 of an imaginary circle P, which is an imaginary circle centered on a central axis O of the inner ring 2 and passing through a corner section 7a of the step section 7 furthest from the center O in a radially outward direction, is set to 1.4 or greater (hereinafter, the imaginary circle is also referred to as the "largest imaginary circle"). This ratio is also referred to as the "inner ring ratio." If the inner ring ratio is less than 1.4, a crack may occur in a thin-walled section of the inner ring 2 as a result of residual compressive stress 2 generated in the inner ring by the shrink fit of the outer ring 3 to the outer circumference of the inner ring 2.On the other hand, if the inner ring ratio is set to 1.4 or higher, the aforementioned disadvantage cannot occur. However, if the inner ring ratio is excessively large, inner ring 2 and ultimately the stepped form 1 will also become large, and therefore the inner ring ratio is set to 2.0 or lower.

[0030] Furthermore, in this embodiment, for the same reason as previously described with regard to the inner ring ratio, the wall thickness, which has a value obtained by dividing the difference between the outer diameter d1 of the inner ring and a diameter d2 of the aforementioned largest imaginary circle by 2, is set to 5 mm or more. If the wall thickness is less than 5 mm, there is a possibility that a crack may occur in a thin wall section of the inner ring 2 due to residual compressive stress generated in the inner ring 2 by the shrink fit of the outer ring 3 to the outer circumference of the inner ring 2. On the other hand, if the wall thickness is 5 mm or more, the aforementioned disadvantage cannot occur. However, if the wall thickness is too large, the inner ring 2 and ultimately also the stepped form 1 become large, so the wall thickness is preferably set to 40 mm or less. <Testbeispiel 1>

[0031] Green compacts were produced by compression molding, in which a metal powder was filled into the recessed section for forming and a mold pressure of 10t / cm² was applied. 2 was press-molded, while, as shown in Table 1, an inner ring diameter, an inner ring ratio, a wall thickness (a value obtained by dividing by 2 a difference between an outer diameter of the inner ring and a diameter of the largest imaginary circle, as previously described) and a shrink-fit ratio (see formula (1)) in a stepped form that the in Fig. 1 and Fig. The configuration and shape shown in section 2 have been modified in various ways.

[0032] A height h of the stepped form (see Fig. 2) was set to 40 mm. The length w1 of a long side of the rectangular section of the recessed part for forming was set to 21 mm, the length w2 of the short side of the rectangular section to 16 mm, and the diameter d3 of a circular column section of the recessed part to 10 mm. Furthermore, a WC-Co-based sintered hard metal was used as the material for the inner ring, and hot die steel was used as the material for the outer ring.

[0033] Table 1 shows a comparative stress σaeq of the stepped corner radius section when the diameter, inner ring ratio, wall thickness, and shrink-fit ratio of each inner ring have been changed differently. As in Fig. As shown in Figure 3, the term "stepped corner radius section" refers to an edge section of the short side 7b of the step section 7 with a rectangular shape as seen in plan view, and the term "side surface corner section" in Tables 3 and 4 refers, as described below, to a boundary section between two adjacent surfaces of the inner surfaces of the inner ring, which are opposite the recessed section 4 with a rectangular shape as seen in plan view, and is the same section as the corner section 7a described above.

[0034] The equivalent voltage σaeq is a value that is calculated by the following formula (2). σaeq=σa / (1−σm / σB)

[0035] In formula (2), σa is a stress amplitude generated by pressure forming at the time of compression forming of the metal powder, and σm represents an average stress. Bσ is a tensile strength value inherent to the material. In the present test example 1, a WC-Co sintered cemented carbide was used as the material for the inner ring, so that the value of σ B 1,600 MPa.

[0036] Table 2 shows a strength ratio (fatigue strength / σaeq) calculated based on the equivalent stress σaeq in Table 1 and a fatigue strength that is a value inherent to the material. In the present test example 1, a WC-Co sintered cemented carbide was used as the material for the inner ring, resulting in a fatigue strength of 700 MPa.

[0037] Fig. Figure 4 shows the result shown in Table 2 in graphical form for the respective inner ring ratios, and Fig. Figure 5 shows the result shown in Table 2 in graphical form for the respective shrinkage fit ratios. Fig. Figure 4 shows the strength ratio of the stepped corner radius segment plotted on the y-axis, and a shrinkage fit ratio (%) is plotted on the x-axis. Furthermore, in Fig. 5. A strength ratio of the stepped corner radius section is plotted on the y-axis, and an inner ring ratio is plotted on the x-axis.

[0038] Out of Fig. As shown in Figure 4, it is evident that the strength ratio of the stepped corner radius section can be fixed in a substantially stable manner if the shrinkage fit ratio (%) falls within a range of 0.12 to 0.25. It is also evident from Fig. 5 shows that the strength ratio of the stepped corner radius section plotted on the x-axis assumes a substantially constant value when the inner ring ratio exceeds 2.0.

[0039] In test example 1, it was confirmed (visually observed) that a crack had formed in the sample (strength ratio: 1.06) where a shrinkage fit ratio was set to 0.35% and an inner ring ratio to 2.4. Conversely, no crack formation could be confirmed in the sample (strength ratio: 1.11) where a shrinkage fit ratio was set to 0.15% and an inner ring ratio to 1.6. [Test example 2]

[0040] A compressive stress was generated in a side-face corner section of the step section of the inner ring (the section defined by “7a” in Fig. 3 is characterized as described above) obtained, while, as shown in Table 3, an inner ring diameter, an inner ring ratio, a wall thickness (a value obtained by dividing a difference between an outer diameter of the inner ring and a diameter of the largest imaginary circle described above by 2) and a shrink-fit ratio (see formula (1)) in a stepped die with the in Fig. 1 and Fig. The configuration shown in section 2 has been modified differently.

[0041] A height h of the stepped stamp (see Fig. 2) was set to 40 mm. The length w1 of a long side of the rectangular section of the recessed part for forming was set to 21 mm, the length w2 of the short side of the rectangular section was set to 16 mm, and the diameter d3 of a circular column section of the recessed part was set to 10 mm. WC-Co sintered hard metal was used as the material for the inner ring, and hot die steel was used as the material for the outer ring.

[0042] Table 4 shows a compressive strength ratio (compressive strength / generated compressive stress) calculated based on the generated compressive stress shown in Table 3 and the compressive strength, which has a unique value assigned to the material. In the present test example 2, a WC-Co sintered cemented carbide was used as the material for the inner ring, resulting in a compressive strength of 4,000 MPa.

[0043] Fig. Figure 6 shows the result in Table 4 in graphical form for the case where a shrinkage fit ratio (%) was set to 0.15%. Fig. In figure 6, a compressive strength ratio was plotted on the x-axis, and a wall thickness (mm) was plotted on the y-axis. Also Fig. Figure 7 shows the result in Table 4 in graphical form for the same case. Fig. In Figure 7, a compressive strength ratio was plotted on the x-axis, and an inner ring ratio was plotted on the y-axis.

[0044] Out of Fig. As can be seen in Figure 6, with a wall thickness of approximately 5 mm as a limiting factor, the way in which a compressive strength ratio changes differs considerably between the case where the wall thickness is less than the limiting factor and the case where the wall thickness is greater than the limiting factor. More precisely, a relationship between a compressive strength ratio and a wall thickness in three test examples where the wall thickness was set to 5 mm or less can be expressed by y = 0.94x + 0.65 (R² = 0.96), and a relationship between a compressive strength ratio and a wall thickness in seven test examples where the wall thickness was set to 5 mm or more can be expressed by y = 0.13x + 5.08 (R² = 0.94). It is understood that the slope of a regression line changes significantly above and below a value of 5 mm, which serves as the limiting factor.

[0045] Out of Fig.As can be seen in Figure 7, with an inner ring ratio set approximately to 1.4 as the limit, the way in which a compressive strength ratio changes differs considerably between the case where the inner ring ratio is less than the limit and the case where the inner ring ratio is greater than the limit. More precisely, a relationship between a compressive strength ratio and an inner ring ratio in three test examples where the inner ring ratio is set to 1.4 or less can be expressed by y = 12.02x + 11.39 (R² = 0.99), and a relationship between a compressive strength ratio and an inner ring ratio in seven test examples where the inner ring ratio is set to 1.4 or more can be expressed by y = 1.65x + 3.44 (R² = 0.94). It is understood that the slope of a regression line above and below a value of 1.4, which serves as the limit, changes significantly.

[0046] From the results of test example 1 and test example 2, it is evident that a shrinkage fit ratio (%) is preferably set to a value within the range of 0.12 to 0.25%, as this allows for a substantially defined strength ratio of the stepped corner radius section. It is also understood that an inner ring ratio is preferably set to 1.4 or higher. It is also understood that a wall thickness is preferably set to 5 mm or higher. On the other hand, it is also understood that an upper limit for an inner ring ratio is preferably set to 2.0 or lower. [Further modifications]

[0047] It should be understood that the embodiments serve only for illustrative purposes and are in no way to be considered limiting. The scope of protection of the present invention is not to be determined by the meaning disclosed in the embodiments, but rather the present invention is to encompass all modifications described in the claims that fall within the meaning and scope corresponding to the meaning and scope of the claims.

[0048] For example, in the embodiment mentioned above, the recessed section for forming has a rectangular shape in a top view. However, the shape and size of the recessed section can be selected appropriately according to a formed product, and the recessed section can, for example, be formed in the shape of a circle or a polygon in a top view. Reference symbol list 1. Graduated form 2 inner ring 3 Outer ring 4 in-depth section 5 stamp plates 6 Flange section 7th stage section 7A Corner section 7B stepped corner radius section 21 Graduated form 22 inner ring 23 Outer ring 24 in-depth section 25th stage section 26 stamp plates 27 Flange section 28 lower stamp Level 30 Part 31 O Central axis C jump P imaginary circle S lower room d1 Outer diameter of the inner ring d2 Diameter of the largest imaginary circle d3 Diameter of the recessed section w1 long side of the recessed section w2 short side of the recessed section h Height of the stepped shape

Claims

[1] Graduated mold (1) for powder forming of metal powder, comprising: an inner ring (2) formed from a sintered hard alloy and having a cylindrical shape, and an outer ring (3) having a cylindrical shape, which is attached to an outer circumference of the inner ring (2) by shrink fit, wherein a recessed section (4) for forming, comprising a step section (7), is formed on an inner side of the inner ring (2), wherein a flange section (6) which engages in a punch plate (5) is formed on an outer circumference of the outer ring (3), wherein only the flange section (6) of the stepped shape (1) is held by the punch plate (5), while a lower surface of the stepped shape (1) is not held by any further element, and wherein a shrinkage fit ratio of the outer ring (3) to the inner ring (2) is set to a value which falls within a range of 0.12% to 0.25%. [2] Stepped shape (1) according to claim 1, wherein a ratio between an outer diameter (d1) of the inner ring and a diameter (d2) of a largest imaginary circle, which is an imaginary circle having a center point on a central axis (O) of the inner ring (2) and passing through a corner section (7A) of the step section (7) that is furthest outwards from the center in a radial direction, is set to 1.4 or more. [3] Stepped shape (1) according to claim 2, wherein the ratio is set to 2.0 or less. [4] Stepped shape (1) according to claim 1, wherein a wall thickness having a value obtained by dividing a difference between an outer diameter (d1) of the inner ring and a diameter (d2) of a largest imaginary circle by 2, which is an imaginary circle having a center point on a central axis (O) of the inner ring (2) and passing through a corner section (7A) of the step section (7) that is furthest outwards from the center in a radial direction, is set to 5 mm or more. [5] Stepped shape (1) according to any one of claims 1 to 4, wherein a material of the inner ring (2) is a sintered hard alloy and a material of the outer ring (3) is hardened steel. [6] Stepped shape (1) according to any one of claims 1 to 5, wherein the shrink fit ratio of the outer ring (3) to the inner ring (2) is set to a value which falls within a range of 0.15% to 0.20%.

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