JP4202000B2 - Mandrel for ring rolling - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a ring rolling mandrel whose life is improved by surface treatment. The mandrel of the present invention can be applied to a ring rolling that forms a ring having a large diameter by rotating and forging a ring-shaped rough material together with a forming roll in a cold, warm or hot condition.
[0002]
[Prior art]
In the case of forging a ring having a large diameter by reducing the radial thickness or cross-sectional area of the ring-shaped rough material, a ring rolling which is a kind of rotary forging is used.
[0003]
In this ring rolling, a mandrel is inserted into the hole of the ring-shaped coarse material, a forming roll is disposed on the outer surface of the ring-shaped rough material, and the ring-shaped rough material is sandwiched between the mandrel and the forming roll, By rotating the mandrel and the forming roll in opposite directions, the ring-shaped coarse material is formed to be stretched in the circumferential direction. As a result, the radial thickness or the cross-sectional area of the ring-shaped coarse material is reduced, and a ring having a large diameter is formed.
[0004]
[Problems to be solved by the invention]
By the way, in the above-mentioned ring rolling, since the mandrel is inserted into the hole of the ring-shaped coarse material, there is a limit to increase the mandrel itself by increasing the outer diameter of the mandrel.
[0005]
For this reason, especially when trying to form a ring-shaped rough material having a small diameter, the early breakage of the mandrel due to lack of rigidity or strength becomes a problem. In particular, in cold forging, the molding load is relatively large and the frictional force generated between the forging die and the material is large, so the problem of early breakage of the mandrel becomes significant.
[0006]
The cause of the early breakage of the mandrel is considered to be the occurrence of scratches and wear due to contact with the material and insufficient fatigue strength due to repeated stress increase.
[0007]
Therefore, in order to improve the fatigue strength and hardness of the mandrel, it may be considered to be a mandrel made of powder high speed, but the powder high speed contributes to the improvement of fatigue strength and hardness, but on the other hand, the toughness is insufficient and is once small. It is not an effective measure because it easily breaks when a scratch occurs.
[0008]
The present invention has been made in view of the above circumstances, and a technical problem to be solved is to improve the mandrel life by suppressing the early breakage of the ring rolling mandrel by using an inexpensive and simple surface treatment. Is.
[0009]
[Means for Solving the Problems]
  The ring rolling mandrel of the present invention that solves the above problems is a ring rolling mandrel that is inserted into a hole of a ring-shaped coarse material and rotationally forging the coarse material with a forming roll disposed on the outer surface of the coarse material. In this case, a nitriding layer is formed on the surface of the molding part that is in contact with the coarse material.In addition, at least a hard coating film having a hardness higher than that of the nitriding layer is formed on the surface of the nitriding layer, and the white layer occupies in a range from the outermost surface of the nitriding layer to a depth of 200 μm. The area ratio is 5.0% or less, and the depth of the nitriding layer is 75 μm or less from the outermost surface of the nitriding layer.It is characterized by this.
[0011]
  In a preferred embodiment, the ring rolling mandrel of the present invention has a depth of the nitriding layer.Of the nitriding layerFrom the top surfaceLess than 50μmIt is.
[0012]
  In a preferred embodiment, the ring rolling mandrel of the present invention is:The white layer portion is not recognized in the nitriding layer by visual observation at an optical microscope level (400 times)..
[0013]
DETAILED DESCRIPTION OF THE INVENTION
In the ring rolling mandrel of the present invention, a nitriding treatment layer is formed on the surface of a molding part in contact with a rough material by a nitriding treatment which is an inexpensive and simple surface treatment, and the surface hardness is improved. Further, this nitriding layer has a compressive residual stress based on strain at the time of forming the nitriding layer, and the tensile strength is improved accordingly. For this reason, fatigue strength and wear resistance are improved, and consequently the mandrel life is improved.
[0014]
The shape and size of the ring rolling mandrel of the present invention are not particularly limited. For example, the ring rolling mandrel has an annular engagement recess (impression portion) or engagement protrusion at a molding portion that contacts the ring-shaped coarse material. be able to. The engagement recesses and the engagement projections are engaged with the engaged portions of the rough material that can be engaged with the engagement recesses or the engagement projections, so that the mandrel and the coarse material in the axial direction during forging. It is for restricting the relative movement of. Moreover, when it is set as an engagement recessed part, it can shape | mold, restrict | limiting the axial direction length of this coarse material by accommodating a coarse material in this engagement recessed part. The shape and size of the engagement concave portion and the engagement convex portion are not particularly limited.
[0015]
The material of the mandrel base material of the mandrel for ring rolling according to the present invention is not particularly limited, and can be cold die steel or the like as in the past, but particularly high toughness SKD11 improved steel or matrix high speed is used. It is preferable. This is because if the toughness of the mandrel base material is high, it is more advantageous for suppressing damage. In the present invention, since the surface is hardened by forming a nitriding layer, a mandrel member to which at least one nitriding accelerating element selected from Al, Cr, Mo, Ti, V, etc. is added so as to be hardened by nitriding treatment Must be used.
[0016]
The hardness of the mandrel base material is preferably 50 to 67 HRC (Rockwell C scale hardness), particularly preferably 55 to 62 HRC. If the hardness of the mandrel base material is too low, the wear resistance is reduced, and scratches due to contact with the coarse material are likely to occur. On the other hand, if the hardness is too high, the mandrel base material becomes brittle due to insufficient toughness.
[0017]
  In the mandrel for ring rolling according to the present invention, a nitriding layer is formed on the surface of the molding part of the mandrel base material that comes into contact with the rough material.In addition, a hard coating film that is at least harder than the nitriding layer is formed on the surface of the molding site where the nitriding layer is formed.ing.
[0018]
  In general, the presence of a white layer portion is observed in the nitriding layer. The white layer portion is a nitrogen compound (ε-Fe2-3 (CN), γ-Fe4 N) that is harder and more brittle than the other portions. For this reason, if a white layer portion is present in the nitriding layer, cracks are likely to be generated from the white layer portion, which is disadvantageous in suppressing crack propagation. Therefore, it is preferable that the amount of the white layer portion present in the nitriding layer is as small as possible, and it is more preferable that there is little or no white layer portion. Specifically, it is preferable that no white layer portion is observed in the nitriding layer by visual observation at an optical microscope level (400 times). More specifically,The ring rolling mandrel of the present invention isIn the range from the outermost surface of the nitriding layer to a depth of 200 μm, the area ratio occupied by the white layer portion is 5.0% or less..More preferably, it is 3.0% or less.
[0019]
  Also,When a hard coating film is formed on the surface of the molding part where the nitriding layer is formedThe depth of the nitriding layer is 10 to 10 from the outermost surface of the mandrel base material.100 μmPreferably,25-75 μmMore preferredYes.If the depth of the nitriding layer is too shallow, it becomes difficult to produce a substantially uniform nitriding layer due to variations in processing. On the other hand, since the nitriding layer has a higher crack propagation rate than the mandrel base material, once a crack occurs in the nitriding layer, the crack is likely to propagate throughout the depth direction of the nitriding layer. For this reason, if the nitriding layer is too deep, once a crack occurs in the nitriding layer, it will develop into a larger (deeper) crack due to crack propagation. Therefore, from the viewpoint of suppressing crack propagationThe ring rolling mandrel of the present inventionNitrided layer depthThe nitriding layerFrom the top surface75μm or less. Less than 50μmMore preferredYes.
[0020]
Furthermore, the hardness of the nitriding layer is not particularly limited, but can be about 600 to 1300 HV (Vickers hardness). If the hardness of the nitriding layer is too low, the effect of improving the fatigue strength and wear resistance by forming the nitriding layer cannot be expected, while the hardness of the nitriding layer is increased even more than necessary. The improvement of the effect cannot be expected, and crack generation and propagation are promoted. Therefore, the hardness of the nitriding layer is preferably 700 to 1200 HV, and more preferably 800 to 1100 HV.
[0021]
The nitriding method for forming the nitriding layer on the surface portion of the mandrel base material is not particularly limited, and NH_ThreeIn addition to the usual nitriding treatment using gas, N₂And H₂Ion nitriding using NH, NH_ThreeGas and H₂A radical nitriding treatment using a gas or a salt bath nitriding treatment using a cyan base bath can be used.
[0022]
Moreover, in order to reduce or eliminate the white layer part in the nitriding layer, for example, it is possible to appropriately adjust the gas mixture ratio and the heating temperature / time. For example, NH_ThreeIn the case of a normal nitriding treatment using a gas, the processing temperature can be set to about 400 to 500 ° C., the processing time can be set to about 1 to 5 hours, and in the case of ion nitriding, the processing temperature can be set to about 400 to 500 ° C. Time: about 1 to 5 hours, and in the case of radical nitriding treatment, treatment temperature: about 400 to 500 ° C., treatment time: about 2 to 10 hours.
[0023]
As described above, the tensile strength of the nitriding layer is improved by the presence of compressive residual stress, but since the crack propagation rate is faster than that of the mandrel base material, once a crack occurs in the nitriding layer. Cracks tend to propagate throughout the depth of the nitriding layer. For this reason, the ring rolling mandrel with the nitriding layer formed on the surface tends to have a low fatigue strength once a crack occurs in the nitriding layer due to contact with the coarse material.
[0024]
  Therefore, a hard coating film having a hardness higher than at least the nitriding layer is formed on the surface of the molding portion of the mandrel base material on which the nitriding layer is formed.The
[0025]
This hard coating film can effectively suppress the occurrence of cracks in the nitriding layer by functioning as a protective film for the nitriding layer. Further, since the compressive residual stress based on strain is applied to the nitriding layer during the formation of the hard coating film, the tensile strength of the nitriding layer is further improved. Furthermore, since the hard coating film itself has a high hardness, the occurrence of cracks in the hard coating film itself due to contact with the coarse material is also effectively suppressed. Therefore, in a ring rolling mandrel in which a hard coating film is further formed on the surface of the molding site where the nitriding layer is formed, a crack occurs in either the hard coating film or the nitriding layer due to contact with the rough material. Therefore, it is possible to effectively suppress a decrease in fatigue strength due to the occurrence of such cracks.
[0026]
The hard coating film should be at least harder than the nitriding layer, but can effectively function as a protective film for the nitriding layer and effectively suppress the occurrence of cracks in the hard coating film itself. As is possible, the hardness is preferably about 1000 to 2500 HV, more preferably 1300 to 2200 HV, while the hardness is higher than that of the nitriding layer. In addition, if the difference between the hardness of the hard coating film and the hardness of the nitriding layer is too large, cracks are likely to occur at the interface between them, leading to a decrease in fatigue strength. It is preferable to set it to about 1200 HV.
[0027]
Moreover, when the thickness of the hard coating film is too thin, the function as a protective film of the nitriding layer cannot be effectively exhibited. On the other hand, if the thickness of the hard coating film is too thick, it becomes easy to peel off, and cracks are likely to occur at the interface between the hard coating film and the nitriding layer, leading to a decrease in fatigue strength. Therefore, the thickness of the hard coating film is preferably 2 to 8 μm, and more preferably 3 to 5 μm.
[0028]
The kind of the hard coating film is not particularly limited as long as it is harder than the nitriding layer. For example, TiN (hardness: 1700 to 2400 HV), CrN (hardness: 1700 to 2200 HV), TiAlN (hardness: 2100 to 2900 HV) and TiCrN (hardness: 1800 to 2100 HV) can be employed.
[0029]
This hard coating film can be formed by a PVD (physical vapor deposition) method. In the CVD method involving high-temperature treatment, the nitriding layer is softened at a high temperature, so that the CVD method cannot be adopted as a method for forming the hard coating film.
[0031]
Here, without forming a nitriding layer on the surface of the molding part of the mandrel base material, a hard coating film is formed as it is, and a hard coating film is further formed on the surface of the molding part where the nitriding layer is formed The difference in effect between the case and the case will be described below.
[0032]
First, when a hard coating film is formed on the surface of the molding part of the mandrel base material on which the nitriding layer is formed, the amount of deformation with respect to the external stress of the film is small, and a hard coating film is formed on the surface of the untreated molding part Compared to the case, the peel resistance of the hard coating film is improved. That is, the nitriding layer plays the role of an underlayer that improves the bondability between the hard coating film and the mandrel base material.
[0033]
Further, when a hard coating film is formed on an untreated mandrel base material, the hardness difference between the hard coating film and the mandrel base material is large, and cracks are likely to occur at the interface between the two. For this reason, as shown in the Example mentioned later, the bending fatigue strength in the case where there is no contact with a rough material becomes lower than an untreated mandrel in which neither a hard coating film nor a nitriding layer is formed. In contrast, when a hard coating film is formed on the surface of the mandrel base material on which the nitriding layer is formed, the hardness difference between the hard coating film and the mandrel base material is reduced by the increase in hardness due to the nitriding layer. Therefore, the generation of cracks at the interface can be suppressed accordingly. For this reason, it becomes possible to suppress the fall of the said bending fatigue strength accompanying formation of a hard coating film rather than the case where a hard coating film is formed in an untreated mandrel base material.
[0034]
Also, regarding the bending fatigue strength when there is contact with the rough material, when the hard coating film is formed on the untreated mandrel base material, the bending fatigue strength is slightly higher than when there is no contact with the rough material. On the other hand, when a hard coating film is formed on the mandrel base material on which the nitriding layer is formed, a value approximately equal to the bending fatigue strength when there is no contact with the coarse material is maintained. This difference occurs when a hard coating film is formed on an untreated mandrel base material, whereas the hard coating film is divided or peeled off due to the effect of contact with the rough material, whereas the nitriding layer It is considered that when the hard coating film is formed on the mandrel base material on which the film is formed, the destruction of the film is suppressed.
[0035]
The ring rolling mandrel of the present invention can be applied to any of cold, warm or hot forging, but is particularly effective for cold forging where the forming load is relatively large and the mandrel is likely to be damaged. Can be applied.
[0036]
[Reference exampleAnd examples]
  Hereinafter, the ring rolling mandrel of the present inventionReference examples andEmbodiments will be specifically described with reference to the drawings.
[0037]
  (Reference example)
  BookReference exampleAs shown in FIG. 1, the ring-shaped coarse material 4 is ring-rolled using a mandrel 1, a forming roll 2 and a pair of receiving rolls 3 and 3.
[0038]
The mandrel 1 includes a shaft portion 11 and a cylindrical portion 12 having an outer diameter of 60 mm, and a mandrel base material constituting the cylindrical portion 12 is made of cold die steel. This cold die steel is 8% Cr-based SKD11 improved steel (hardness: 60 HRC).
[0039]
An impression portion (annular engagement recess) 13 having a depth of 7 mm and an axial length of 30 mm is provided in a substantially central portion in the axial direction of the cylindrical portion 12.
[0040]
Further, a nitriding layer 14 is formed on the surface portion of the entire outer surface of the cylindrical portion 12 (see FIG. 2). The nitriding layer 14 has a depth of 50 μm from the outermost surface, and the area ratio occupied by a white layer portion (not shown) is 0% in the range from the outermost surface of the nitriding layer 14 to a depth of 200 μm. It is.
[0041]
The depth of the nitriding layer 14 is measured by measuring the hardness from the outermost surface of the nitriding layer 14 using a micro-Vickers hardness measuring machine, and nitriding a portion having the same hardness as the mandrel base material. This is obtained as the depth of the layer 14.
[0042]
In addition, the area ratio is measured at 400 times the optical microscope in the measuring range of 200 μm × 200 μm including the outermost surface (it is possible to discriminate the white layer portion by etching with Biclar on observation). This was done by determining the area ratio occupied by the white layer using a processing apparatus.
[0043]
  The nitriding layer 14 was formed by a radical nitriding apparatus using NH3 gas and H2 gas. The processing conditions at this time are as follows: temperature: 400 to 500 ° C.Reference example440 ° C) and time: 2-10 hours (bookReference example4 hours).
[0044]
The forming roll 2 has a cylindrical shape with an outer diameter of 200 mm and is made of cold die steel. An impression portion 21 similar to the above-described impression portion 13 of the mandrel 1 is also recessed in a substantially central portion in the axial direction of the forming roll 2.
[0045]
Each of the receiving rolls 3 and 3 is for receiving a load acting on the mandrel 1 at the time of molding, and is disposed on both ends of the mandrel 1 on the side opposite to the molding roll 2.
[0046]
The forming roll 2 can be driven to rotate by a rotation driving means (not shown), and the mandrel 1 can be reciprocated in a direction approaching and separating from the forming roll 2 by a driving means (not shown).
[0047]
Hereinafter, the state of ring rolling will be described.
[0048]
First, a ring-shaped rough material 4 was formed by forging from a steel material (S45) into a ring shape having an outer diameter of 115 mm, an inner diameter of 70 mm, and an axial length of 30 mm, and then subjected to normalization.
[0049]
And while inserting the mandrel 1 in the hole of this ring-shaped rough material 4, the shaping | molding roll 2 was arrange | positioned on the outer side of the ring-shaped rough material 4. FIG. At this time, the ring-shaped coarse material 4 is engaged with the impression portions 13 and 21 of the mandrel 1 and the molding roll 2, and the relative movement in the axial direction between the ring-shaped coarse material 4, the mandrel 1 and the molding roll 2 is restricted. It was to so. In addition, receiving rolls 3 and 3 are disposed at both ends of the cylindrical portion 12 of the mandrel 1 at positions opposite to the forming roll 2.
[0050]
In this state, the mandrel 1 and the forming roll 2 were rotated in directions opposite to each other by a rotation driving means (not shown), and the mandrel 1 was moved in a direction approaching the forming roll 2 by a driving means (not shown). Thus, while forming the ring-shaped coarse material 4 to stretch in the circumferential direction, the ring-shaped coarse material 4 was formed into a ring with a large diameter by reducing the radial thickness or the cross-sectional area.
[0051]
The forging conditions were a forming roll rotation speed: 150 to 200 rpm, a rolling amount: 0.1 to 0.3 mm / rev, and an outer diameter tube expansion ratio: about 1.4.
[0052]
(Evaluation)
The life of the ring rolling mandrel 1 was evaluated. This was done by examining the number of rings produced (mandrel life) until the mandrel 1 was broken.
[0053]
  For comparison, the above except that the nitriding layer 14 is not formed.Reference exampleThe same evaluation was performed for the same comparative example.
[0054]
  As a result, in the comparative example in which the nitriding layer 14 was not formed on the mandrel surface, the mandrel life number was about 12,000, whereas the nitriding layer 14 having a predetermined depth and white layer area ratio was formed. BookReference exampleThen the mandrel lifespan increased to about 60000.
[0055]
  (Relationship between area ratio of white layer and mandrel life)
  the aboveReference exampleIn this example, an ion nitriding apparatus is used in place of the radical nitriding apparatus, and the N2: H2 ratio is changed from 1: 1 to 1: 4, the processing temperature is set to 400 to 550 ° C., and the processing time is changed to 1 to 30 hours. Thus, the area ratio occupied by the white layer portion was variously changed from the outermost surface of the nitriding layer 14 to a depth of 200 μm, and the relationship between the area ratio and the mandrel life number was examined. At this time, the depth from the outermost surface of the nitriding layer 14 is 50 μm.
[0056]
As shown in FIG. 3, the area ratio of the white layer portion from the outermost surface to a depth of 200 μm is set to 5% or less, so that the mandrel life number (12,000) in the comparative example in which the nitriding layer 14 is not formed is formed. If the area ratio exceeds 3% or less, the lifetime is remarkably improved.
[0057]
Note that when the area ratio of the white layer portion exceeds 5%, the life is reduced compared to the comparative example in which the nitriding layer 14 is not formed even though the nitriding layer 14 is formed. This is probably because cracks were generated from the layer part and crack propagation was promoted in the white layer part.
[0058]
  (Relationship between nitriding layer depth and mandrel life)
  the aboveReference exampleIn this, the processing conditions of the radical nitriding treatment are variously changed in the respective ranges of temperature: 400 to 550 ° C. and time: 2 to 20 hours, so that the depth from the outermost surface of the nitriding treatment layer 14 is variously changed. The relationship between the depth of the nitriding layer 14 and the mandrel life number was examined.
[0059]
As shown in FIG. 4, the depth of the nitriding layer 14 is set to 170 μm or less, thereby exceeding the mandrel life number (about 12,000) in the comparative example in which the nitriding layer 14 is not formed, It can be seen that the mandrel life is remarkably improved in the range of 20 to 120 μm, and the mandrel life is almost the maximum in the range of 50 to 100 μm.
[0060]
In addition, when the depth of the nitriding layer 14 exceeds about 160 μm, the lifetime is reduced as compared with the comparative example in which the nitriding layer 14 is not formed although the nitriding layer 14 is formed. This is considered to be due to the effect of promoting crack propagation in the nitriding layer 14 which is harder and more brittle.
[0061]
  (Example)
  In this embodiment, as shown in FIG.Reference exampleA hard coating film 15 is further formed on the mandrel 1.
[0062]
That is, in the mandrel 1, a hard coating film 15 is further formed on the entire outer surface of the cylindrical portion 12 in which the nitriding layer 14 is formed on the surface portion.
[0063]
The hard coating film 15 is a 3 μm-thick film made of TiN (hardness: 2000 HV) and formed by the PVD method. The processing conditions by the PVD method were set at 460 ° C. for 2 hours in a vacuum chamber.
[0064]
  Other configurations are described above.Reference exampleIt is the same.
[0065]
Therefore, since the hard coating film 15 is further formed on the nitriding layer 14 in the mandrel 1 of this embodiment, the life of the mandrel 1 is greatly improved. Further, the presence of the hard coating film 15 reduces the wear of the mandrel 1 and reduces the friction between the mandrel 1 and the ring-shaped coarse material 4, so that the quality of the ring molded product can be improved.
[0066]
(Evaluation of fatigue strength when there is no workpiece contact)
After processing the powder high-speed steel into a test piece shape having a parallel part φ6 mm, heat treatment was performed to obtain a base material hardness of 61 to 63HRC. Then, finish processing was performed, and the sample No. 1 test piece.
[0067]
Sample No. above. Sample No. 1 in which a hard coating film made of TiN and having a thickness of 3 μm was formed on the surface of the test piece 1 by the PVD method (treatment conditions: 460 ° C., held for 2 hours) to form only the hard coating film. 2 specimens were obtained.
[0068]
Sample No. above. 1 on the surface of the test piece_ThreeGas and H₂Sample No. in which only a nitriding layer was formed by forming a nitriding layer without a white layer having a depth of 65 μm using a radical nitriding apparatus using a gas (processing temperature: 480 ° C., processing time: 6 hours) . 3 test pieces were obtained.
[0069]
Sample No. above. Sample No. 3 in which a hard coating film made of TiN and having a thickness of 3 μm was formed on the surface of the test piece 3 by the PVD method to form a nitriding layer and a hard coating. 4 test pieces were obtained.
[0070]
And said sample No. About each test piece of 1-4, the bending fatigue strength in the case where there is no workpiece | work contact was evaluated. This is done using a rotating bending fatigue tester with a rotational speed of 1500 rpm.⁶This was done by measuring the fatigue strength after the time. The result is shown in FIG.
[0071]
As can be seen from FIG. It can be seen that the fatigue strength is reduced by forming a hard coating film on one test piece.
[0072]
In addition, sample No. without surface treatment. No. 1 in which the nitriding layer was formed, the fatigue strength was improved, while the sample No. 1 in which the nitriding layer was formed was formed. It can be seen that even if a hard coating film is formed on the test piece 3, the fatigue strength decreases only slightly.
[0073]
(Evaluation of fatigue strength when there is workpiece contact)
Sample No. About each test piece of 1-4, the bending fatigue strength in the case where there exists a workpiece | work contact was evaluated. This is done using a rotating bending fatigue tester with a rotational speed of 1500 rpm, while pressing the S30C plate against the test piece.⁶This was done by measuring the fatigue strength after the time. The result is shown in FIG.
[0074]
As can be seen from FIG. The test piece 1 was greatly affected by the workpiece contact, and was significantly lower than the fatigue strength when there was no workpiece contact.
[0075]
In addition, sample No. 1 in which only a hard coating film was formed. The test piece of No. 2 is an untreated sample No. 2. Although it was not about 1, it was affected by the workpiece contact, which was lower than the fatigue strength when there was no workpiece contact.
[0076]
In contrast, Sample No. with a nitriding layer and a hard coating film formed thereon. The test piece No. 4 had almost no influence by the workpiece contact, and the fatigue strength was almost the same as the fatigue strength when there was no workpiece contact.
[0077]
(Relationship between nitriding layer depth and fatigue strength when hard coating film is formed)
Sample No. 4, that is, when preparing a test piece on which a nitriding layer and a hard coating film are formed, the processing conditions of radical nitriding treatment at the time of forming the nitriding layer are 400 to 550 ° C., and each time range is 2 to 20 hours. The depth of the nitriding layer from the outermost surface was changed variously, and the relationship between the depth of the nitriding layer and the fatigue strength when a hard coating film was formed was investigated.
[0078]
The test method is the same as the test method of fatigue strength when there is a workpiece contact as described above.
[0079]
As shown in FIG. 8, when the nitriding layer and the hard coating film are formed, the fatigue strength decreases when the nitriding layer is too deep. This is presumably because when the nitriding layer becomes deeper, cracks are once generated at the interface between the nitriding layer and the hard coating film, thereby developing into larger cracks.
[0080]
Further, FIG. 8 shows that the depth of the nitriding layer is preferably 100 μm and more preferably 75 μm.
[0081]
  (Evaluation of mandrel number)
  SaidReference exampleThe mandrel 1 having only the nitriding layer 14 formed thereon,ExampleThe lifespan of the nitriding layer 14 and the mandrel 1 on which the hard coating film 15 was formed was comparatively evaluated. This was done by examining the number of rings produced (mandrel life) until the mandrel 1 was broken.
[0082]
  As a result, only the nitriding layer 14 was formed.Reference exampleThe mandrel 1 had a mandrel life of 60,000, whereas the nitriding layer 14 and the hard coating film 15 were formed.ExampleIn the mandrel 1, the mandrel lifespan increased to about 400,000.
[0083]
【The invention's effect】
As described above in detail, the ring rolling mandrel of the present invention has a nitriding layer formed by a nitriding treatment that is an inexpensive and simple surface treatment, so that the mandrel life can be improved while suppressing an increase in cost. .
[0084]
  In particular, the area ratio and depth of the white layer portion in the nitriding layer shouldBecauseThe mandrel life can be significantly improved.
[0085]
  In addition, a hard coating film is further formed on the nitriding layer.Because is formedIn addition to significantly improving the mandrel life, it can also contribute to improving the quality of the ring molded product.
[Brief description of the drawings]
[Figure 1]Reference exampleIt is sectional drawing which shows typically a mode that ring rolling is performed using this mandrel.
[Figure 2]Reference exampleIt is a fragmentary sectional view which shows typically the principal part of this mandrel.
FIG. 3 is a graph showing the relationship between the area ratio occupied by the white layer portion and the mandrel lifetime from the outermost surface of the nitriding layer to a depth of 200 μm.
FIG. 4 is a graph showing the relationship between the depth of a nitriding layer and the mandrel lifetime.
[Figure 5] BookExampleIt is a fragmentary sectional view which shows typically the principal part of this mandrel.
FIG. 6 is a diagram showing an evaluation result of fatigue strength when there is no workpiece contact.
FIG. 7 is a diagram showing an evaluation result of fatigue strength when there is a workpiece contact.
FIG. 8 is a graph showing the relationship between the depth of a nitriding layer and the fatigue strength when a hard coating film is formed.
[Explanation of symbols]
1 ... Mandrel
2 ... Forming roll
3 ... Receiving roll
4 ... Ring-like rough material
13 ... Impression part (engagement recess)
14 ... Nitrided layer
15 ... Hard coating film

Claims (3)

In a ring rolling mandrel that is inserted into the hole of the ring-shaped coarse material and rotationally forging the coarse material with a forming roll disposed on the outer surface of the coarse material,
The coarse material nitrided layer is formed on the surface of the molding portion which contacts the Rutotomoni, the surface of the nitride treated layer, and the hard coating film higher hardness than at least nitride treated layer is formed,
In the range from the outermost surface of the nitriding layer to a depth of 200 μm, the area ratio occupied by the white layer portion is 5.0% or less,
A ring rolling mandrel, wherein the depth of the nitriding layer is 75 μm or less from the outermost surface of the nitriding layer . Claim 1 ring rolling mandrel of, wherein the depth of the nitrided layer is less than 50μm from the outermost surface of the nitride treated layer. 3. The ring rolling mandrel according to claim 1 , wherein the white layer portion is not recognized in the nitriding layer by visual observation at an optical microscope level (400 times) . 4.

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