Method for manufacturing a hot working tool

Abstract

To provide a manufacturing method of tool for hot working that optimizes a chemical composition of weld metal used for a build-up layer, and has excellent abrasion resistance when being used in the build-up layer of a work surface of the tool for hot working without degrading weldability in build-up welding.SOLUTION: There is provided a manufacturing method of tool for hot working, the tool comprising a base material made of an alloy tool steel, a build-up layer made of a nickel (Ni) group alloy on a work surface, and an intermediate layer made of a solid-solution strengthened type Ni alloy in the middle between the base material and the build-up layer. The method comprises: forming the intermediate layer made of the solid-solution strengthened type Ni alloy on the base material made of the alloy tool steel; and sequentially repeating build-up welding for stacking to form the build-up layer, a composition of which contains, by mass%, 0.010-0.030% of C, 13.0-18.0% of Cr, 3.0-5.0% of W, 5.0-7.0% of Mo, 12.0-15.0% of Co, 1.0-2.8% of Al, 2.0-3.7% of Ti, 0.5-1.5% of Nb, 0.0010-0.0070% of B, and a balance Ni with inevitable impurities.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a hot working tool used for hot plastic working such as hot forging.

Background Art

[0002] Conventionally, hot working tools for plastically working a workpiece at high temperature have been used. For example, a hot forging die used for hot forging is used by overlay welding an alloy having high high-temperature strength and excellent wear resistance to a portion constituting the striking surface (hereinafter referred to as the striking surface or the working surface) for the purpose of extending the service life of the die. For example, the applicant of the present application proposed a hot forging die in Japanese Patent Application Laid-Open No. 2001-71086 (Patent Document 1) in which an alloy equivalent to JIS-SKT4 was used as a base material of the die, and a precipitation-strengthened Ni-based alloy was overlay welded on the base material to form a striking surface. Further, Japanese Patent Application Laid-Open No. 2001-62541 (Patent Document 2) proposes a method of making the entire die including the base material into a precipitation-strengthened Ni-based alloy, or using a base material made of a Ni-based alloy and overlay welding a precipitation-strengthened Ni-based alloy to form a striking surface. In the hot forging dies proposed in Patent Document 1 and Patent Document 2, examples of the overlay welding metal include precipitation-strengthened Ni-based alloys such as Alloy713C, Alloy718, and Alloy U520. By the way, Ni-based alloys having further increased high-temperature strength by increasing precipitation strengthening elements such as Al and Ti more than the above-described precipitation-strengthened Ni-based alloys have been developed as heat-resistant materials for aircraft engines and heat-resistant materials for gas turbines. However, in the case of Ni-based alloys, it is said that the higher the precipitation strengthening element content, the more likely high-temperature cracking and reheat cracking occur during overlay welding (Non-Patent Document 1), and from the viewpoint of weldability, Ni-based alloys containing an excessive amount of precipitation strengthening elements are generally not suitable as overlay welding metals for hot forging dies.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

[0004] [Non-Patent Document 1 Edited by the Welding Research Committee for Special Materials of the Japan Welding Society, "Welding of Superalloys (Welding Guidebook for Heat-Resistant and Corrosion-Resistant Alloys)", Sangyo Shuppan, p. 36, p. 46-48 [Summary of the Invention [Problems to be Solved by the Invention

[0005] Although the hot forging die formed by overlay welding a precipitation-strengthened Ni-based alloy such as Alloy U520 described above has excellent high-temperature strength, when the material to be forged is a difficult-to-work material such as a Ni-based alloy, the wear resistance of the striking surface is insufficient, and there is a problem that the striking surface plastically deforms and wears during forging, and the service life of the die is significantly shortened. When the hot forging die with a deformed striking surface is used, the shape accuracy of the material to be forged deteriorates extremely. Therefore, repair work of the plastically deformed striking surface is required, resulting in a large amount of man-hours and costs, and deterioration of productivity. In addition, when an alloy with an increased precipitation strengthening element is used as the overlay welding metal for the purpose of improving the wear resistance of the striking surface, although the high-temperature strength increases as described above, there is a problem that the weldability deteriorates and it is not suitable as the overlay welding metal. In the case of Ni-based alloys, when an excessive amount of precipitation strengthening element is contained, the occurrence of hot cracks and reheat cracks becomes remarkable during overlay welding. Reheat cracks are cracks that occur during post-weld heat treatment or during use, but even in the case of overlay welding metal, reheat cracks may occur because the lower overlay layer is affected by heat during overlay welding. When hot forging is performed using a hot forging die with even a slight crack in the striking surface, stress concentrates on the crack during forging, and the crack progresses from the striking surface of the die to the base material. Depending on the degree of the progressed crack, it becomes necessary to replace the entire die including the base material. From the above viewpoints, it has been difficult in the prior art to achieve both good weldability and wear resistance with respect to the build-up layer using the build-up welding metal for the striking surface of the hot forging die. An object of the present invention is to optimize the chemical composition of the build-up welding metal constituting the build-up layer, and to provide a method for manufacturing a hot working tool having excellent wear resistance when used for the build-up layer on the working surface of a hot forging anvil or a hot working tool for a hot forging die, without deteriorating the weldability during build-up welding.

Means for Solving the Problems

[0006] The inventor of the present invention studied the problem of insufficient wear resistance of the build-up layer and deterioration of weldability due to improvement of wear resistance when using a build-up layer on the working surface of a hot working tool such as the striking surface of a hot forging die or a hot forging anvil. Then, precipitation strengthening elements were added to the alloy within a range that does not deteriorate the weldability of the build-up alloy powder, and furthermore, by containing more solid solution strengthening elements in the alloy than in the conventional build-up welding metal, it was found that the wear resistance of the striking surface can be greatly improved, and the present invention was thus achieved. That is, the present invention provides a method for manufacturing a hot working tool having a base material made of alloy tool steel, a build-up layer made of an Ni-based alloy on the working surface, and an intermediate layer made of a solid solution strengthening type Ni-based alloy between the base material and the build-up layer. An intermediate layer made of a solid solution strengthening type Ni-based alloy is formed on the base material made of the alloy tool steel, and build-up welding is sequentially repeated and laminated on the intermediate layer to form a build-up layer. The composition of the build-up layer is, by mass%, C: 0.010 to 0.030%, Cr: 13.0 to 18.0%, W: 3.0 to 5.0%, Mo: 5.0 to 7.0%, Co: 12.0 to 15.0%, Al: 1.0 to 2.8%, Ti: 2.0 to 3.7%, Nb: 0.5 to 1.5%, B: 0.0010 to 0.0070%, and the balance is Ni and inevitable impurities. It is preferable that the build-up layer is made of an alloy powder obtained by mixing two types of alloy powders having different compositions as the build-up welding metal, and it is also preferable that the hot working tool is a hot forging die or a hot forging anvil.

Effects of the Invention

[0007] The alloy composition of the build-up layer used for the working surface of the hot working tool of the present invention is excellent in weldability and wear resistance, so that it is possible to suppress the occurrence of cracks when build-up welding is performed on the working surface of the hot working tool. Further, a hot working tool having a build-up layer made of this alloy composition has the effect of reducing the wear amount of the working surface when hot working a difficult-to-machine material and extending the service life of the hot working tool.

Brief Description of the Drawings

[0008]

Figure 1

Modes for Carrying Out the Invention

[0009] As described above, an important feature of the present invention is that an alloy composition is adopted which does not deteriorate weldability and has excellent wear resistance when used for the working surface of a hot forging die or a hot forging bolster as a build-up alloy. Specifically, by containing a solid solution strengthening element that does not extremely deteriorate weldability as an alloy element, it has been found that the strength of the build-up layer constituting the striking surface can be increased over a wide temperature range, and as a result, the wear resistance of the striking surface of the hot working tool can be improved. For example, among hot working tools, a hot forging die (bolster) requires high wear resistance. In the heat saturation state during hot forging, when the material to be forged is about 1000 °C, the striking surface of the hot forging die is used in a high temperature range with a maximum temperature of about 800 °C. However, the temperature of the striking surface at the initial stage of forging is not so high, and as the forging time elapses, the temperature gradually rises from the initial temperature to a high temperature and reaches a heat saturation state. Also, even at heat saturation, the entire area of the build-up layer constituting the striking surface does not reach the maximum temperature. In regions with less contact with the material to be forged, regions on the outside of the die where heat dissipation is easy, and regions inside from the surface, the build-up layer constituting the striking surface is maintained at a temperature lower than the maximum temperature reached. Therefore, it can be said that in order to improve the wear resistance of the striking surface of a hot forging die, it is necessary to increase the strength not only at high temperatures but also over a wide temperature range. Therefore, in the present invention, solid solution strengthening elements such as Mo and W that can increase strength in a wide temperature range are contained in the build-up alloy powder in a larger amount than in the conventional build-up welding metal, and the wear resistance of the build-up layer constituting the striking surface is improved. Compared with precipitation strengthening elements, solid solution strengthening elements do not contribute to reheat cracking caused by strain due to the precipitation of the gamma prime phase (hereinafter referred to as the γ' phase). Also, since the solidification temperature range is not extremely expanded, it is difficult to contribute to hot cracking. Therefore, if a larger amount of solid solution strengthening elements is contained than in the conventional precipitation strengthening type Ni-based alloy build-up welding metal, it is possible to improve the wear resistance of the striking surface without deteriorating the weldability.

[0010] The reasons for defining the additive elements and the ranges of the respective additive elements in the build-up layer used for the working surface of the present invention are as follows. Unless otherwise specified, it is expressed as mass%. <C: 0.010 to 0.030%> C forms carbides at the grain boundaries in the build-up layer, has the effect of strengthening the grain boundaries, increasing the strength of the build-up layer, and improving the wear resistance. To obtain this effect, it is necessary to set C in the range of 0.010 to 0.030%. If C is less than 0.010%, the strength of the build-up layer is insufficient and sufficient wear resistance cannot be obtained. On the other hand, if it exceeds 0.030%, coarse carbides are formed, so the build-up layer becomes brittle and the ductility decreases. Therefore, C is set in the range of 0.010 to 0.030%. The preferable upper limit of C is 0.025%, and the preferable lower limit is 0.015%. <Cr: 13.0 to 18.0%> Cr forms a Cr2O3 film on the surface of the build-up layer to improve oxidation resistance. It also dissolves in the austenite phase (hereinafter referred to as the γ-phase) that serves as the base of the build-up layer, enhancing the strength of the build-up layer over a wide temperature range through solid solution strengthening and improving wear resistance. To obtain this effect, it is necessary to set Cr in the range of 13.0 - 18.0%. If Cr is less than 13.0%, sufficient oxidation resistance cannot be obtained. On the other hand, if it exceeds 18.0%, it forms the sigma phase (hereinafter referred to as the σ-phase), which is a brittle phase, or forms coarsened Cr-based carbides, reducing the ductility of the build-up layer. Therefore, Cr is set in the range of 13.0 - 18.0%. The preferred upper limit of Cr is 17.0%, and the preferred lower limit is 14.0%.

[0011] <W: 3.0 - 5.0%> W dissolves in the γ-phase that serves as the base of the build-up layer, enhancing the strength of the build-up layer over a wide temperature range through solid solution strengthening and improving wear resistance. To obtain this effect, it is necessary to set W in the range of 3.0 - 5.0%. If W is less than 3.0%, the strength of the build-up layer is insufficient and sufficient wear resistance cannot be obtained. On the other hand, if it exceeds 5.0%, it forms the mu phase (hereinafter referred to as the μ-phase), which is a brittle phase, reducing the ductility of the build-up layer. Therefore, W is set in the range of 3.0 - 5.0%. The preferred upper limit of W is 4.5%, and the preferred lower limit is 3.5%. <Mo: 5.0 - 7.0%> Mo dissolves in the γ-phase, so it enhances the strength of the build-up layer over a wide temperature range through solid solution strengthening and improves wear resistance. To obtain this effect, it is necessary to set Mo in the range of 5.0 - 7.0%. If Mo is less than 5.0%, the strength of the build-up layer is insufficient and sufficient wear resistance cannot be obtained. On the other hand, if it exceeds 7.0%, it forms the μ-phase or σ-phase, which are brittle phases, reducing the ductility of the build-up layer. Therefore, Mo is set in the range of 5.0 - 7.0%. The preferred upper limit of Mo is 6.5%, and the preferred lower limit is 5.5%.

[0012] <Co: 12.0 - 15.0%> Co dissolves in the γ phase, so it has the effect of increasing the strength of the build-up layer over a wide temperature range through solid solution strengthening and improving wear resistance. To obtain this effect, Co needs to be in the range of 12.0 - 15.0%. If Co is less than 12.0%, the strength of the build-up layer is insufficient and sufficient wear resistance cannot be obtained. On the other hand, if it exceeds 15.0%, μ phase and σ phase, which are brittle phases, are formed and the ductility of the build-up layer decreases. Also, since Co is an expensive element, the manufacturing cost of the alloy powder for build-up increases. Therefore, Co is in the range of 12.0 - 15.0%. The preferred upper limit of Co is 14.0% and the preferred lower limit is 13.0%. <Al: 1.0 - 2.8%> Al combines with Ni to coherently precipitate the γ' phase composed of Ni3Al, and has the effect of increasing strength particularly in the high temperature range through precipitation strengthening and improving wear resistance. To obtain this effect, Al needs to be in the range of 1.0 - 2.8%. If Al is less than 1.0%, the strength of the build-up layer is insufficient in the high temperature range and sufficient wear resistance cannot be obtained. On the other hand, if it exceeds 2.8%, the susceptibility to hot cracking during build-up welding and reheat cracking on the lower layer side during build-up welding increases and the weldability deteriorates. Therefore, Al is in the range of 1.0 - 2.8%. The preferred upper limit of Al is 2.5% and the preferred lower limit is 2.0%.

[0013] <Ti: 2.0 - 3.7%> Ti substitutes and solid-solves in the Al site of the γ' phase of Ni3Al, and has the effect of increasing strength particularly in the high temperature range through precipitation strengthening and improving wear resistance. To obtain this effect, Ti needs to be in the range of 2.0 - 3.7%. If Ti is less than 2.0%, the strength of the build-up layer is insufficient in the high temperature range and sufficient wear resistance cannot be obtained. On the other hand, if it exceeds 3.7%, the susceptibility to hot cracking during build-up welding and reheat cracking on the lower layer side during build-up welding increases and the weldability deteriorates. Also, the eta phase (η phase) precipitates and reduces the strength of the build-up layer. Therefore, Ti is in the range of 2.0 - 3.7%. The preferred upper limit of Ti is 3.5% and the preferred lower limit is 3.0%. <Nb: 0.5 - 1.5%> Nb substitutes and solid-solves in the Al sites of the γ' phase of Ni3Al, and has the effect of increasing the strength especially in the high-temperature range and improving the wear resistance by precipitation strengthening. To obtain this effect, it is necessary to set Nb in the range of 0.5 to 1.5%. If Nb is less than 0.5%, the strength of the build-up layer is insufficient in the high-temperature range and sufficient wear resistance cannot be obtained. On the other hand, if it exceeds 1.5%, it segregates significantly during solidification in build-up welding, the susceptibility to hot cracking increases, and the weldability deteriorates. Also, the delta phase (δ phase) precipitates and reduces the strength of the build-up layer. Therefore, Nb is set in the range of 0.5 to 1.5%. The preferable upper limit of Nb is 1.3%, and the preferable lower limit is 0.8%.

[0014] <B: 0.0010 to 0.0070%> B has the effect of strengthening the grain boundaries and increasing the strength and ductility at high temperatures. To obtain this effect, it is necessary to set B in the range of 0.0010 to 0.0070%. If B is less than 0.0010%, the sufficient effect of grain boundary strengthening cannot be obtained. On the other hand, if it exceeds 0.0070%, during solidification in build-up welding, it segregates excessively at the grain boundaries to form low-melting-point compounds, the susceptibility to hot cracking increases, and the weldability deteriorates. Therefore, B is set in the range of 0.0010 to 0.0070%. The preferable upper limit of B is 0.0050%, and the preferable lower limit is 0.0020%. <Balance: Ni and unavoidable impurities> The balance is substantially Ni, but contains impurities inevitably mixed in during manufacturing. In particular, Si, Mn, P, S, Zr deteriorate the susceptibility to hot cracking, Fe reduces the precipitation amount of the γ' phase, O causes the entrainment of oxidation scale during build-up welding, and N causes blowholes. It is preferable that these impurity elements are less. The preferable allowable ranges of the above-mentioned impurity elements and other typical unavoidable impurity elements are described below. Si ≤ 1.0%, Mn ≤ 1.0%, P ≤ 0.01%, S ≤ 0.01%, Fe ≤ 1.0%, Zr ≤ 0.03%, Mg ≤ 0.005%, O ≤ 0.03%, N ≤ 0.05%.

[0015] The build-up layer used on the working surface of the hot working tool used in the present invention described above has effects such as good weldability during build-up welding due to its alloy composition and good wear resistance as a build-up metal. Therefore, it is suitable for the welding metal for build-up welding on the working surface of the hot working tool. In addition, the "hot working tool" referred to in the present invention is a tool used when pressing against a workpiece heated to a hot working temperature in contact with the workpiece to plastically deform the workpiece, and various rolls used for rolling, stretching, etc. are also included therein. And, as a preferred form of the hot working tool of the present invention, there are mentioned a hot forging anvil used for free forging or radial forging, a jig for partially pressing a workpiece, a hot forging die used for hot die forging, and the like. And, the alloy composition of the above build-up layer according to the present invention is particularly suitable for the build-up welding metal for build-up welding on the portion constituting the striking surface of the hot forging die or the hot forging anvil.

[0016] <Manufacturing method of hot working tool> Next, an example for forming a build-up layer having the above-described composition into a hot working tool will be described. The following is for a hot forging die or a hot forging anvil. The hot forging die and the hot forging anvil are composed of a base material and a build-up layer constituting the striking surface. The base material, which is most of the hot forging die, is an alloy tool steel having high rigidity, and as its material, for example, an alloy for "hot die" described in JIS-G4404 or its improved alloy may be used. The alloy tool steel serving as the base material is preferably subjected to quenching at, for example, 800 to 1100 °C and tempering at 500 to 700 °C, and adjusted to a hardness of usually about 330 to 380 HBW. In the present invention, one or two solution-strengthened Ni-based alloy intermediate layers can be provided between a base material made of alloy tool steel and the build-up layer having the above composition. By means of this intermediate layer, the weldability can be improved because the alloy composition and mechanical properties of each layer do not change abruptly compared to directly forming the build-up layer (build-up welding) on the base material. Furthermore, an effect of relaxing the stress generated between the base material and the build-up layer constituting the striking surface can be obtained, and the service life of the hot forging die can be further improved. This intermediate layer may be laminated by a general welding method such as the MIG welding method.

[0017] In the present invention, there are several methods for forming a build-up layer on the striking surface. As the first method, two types of alloy powders having different compositions are prepared, and while mixing the different alloy powders or using the mixed alloy powder as a build-up welding metal, build-up welding is performed, and the build-up layer is formed while adjusting the composition of the build-up layer to the above composition. In this first method, any one type of alloy powder can be used as the alloy powder of an existing alloy that is available, and it is possible to adjust the composition after build-up welding by adjusting the composition and blending ratio of the other alloy powder. In terms of using the alloy powder of the existing alloy, the raw material cost can be reduced. Also, when two types of alloy powders having different compositions are used, a synergistic effect of the characteristics of each alloy powder can be expected. Also, as the second method, alloy powder whose components are adjusted within the above chemical composition range is prepared and used as a build-up welding metal, and build-up welding is performed. It is preferable to appropriately select the first or second method to form the build-up layer.

[0018] In the present invention, to form a build-up layer on the striking surface, build-up welding is sequentially repeated and laminated to a predetermined thickness. As a build-up welding method therefor, known techniques such as the powder plasma welding method can be employed. The powder plasma welding method enables high-speed welding compared to the TIG welding method and the gas welding method, and the welded structure becomes fine and dense. Therefore, it is suitable as a welding method for precipitation-strengthened Ni-based alloys such as the present invention, which are prone to welding problems. Further, although precipitation-strengthened Ni-based alloys are excellent in high-temperature strength but are difficult-to-machine materials, it is difficult to manufacture welding materials such as wires. However, if in powder form, there is an advantage that it can be manufactured relatively easily by the gas atomization method. A hot forging die or hot forging blank having a build-up layer formed on the striking surface (working surface) by build-up welding can be directly used for hot forging. However, when reducing the stress generated during build-up welding, it is also possible to reduce the stress by performing a heat treatment at 500 to 620 ° C for 1 to 10 hours. Also, during build-up welding, if the temperature is lower than the annealing temperature of the base material, build-up welding may be performed on the preheated base material for the purpose of preventing heat cracks.

Example

[0019] (Example 1) The present invention will be described in more detail in the following examples. In Example 1, the weldability will be described. An alloy equivalent to JIS-SKT4 was prepared as a base material for a hot forging blank for radial forging and heat-treated to a hardness of about 330 to 380 HBW. Next, an alloy equivalent to Hastelloy (registered trademark) C of a solid-solution strengthened Ni-based alloy was build-up welded on the base material by the MIG welding method as an intermediate layer. At this time, the oxide scale on the surface of the intermediate layer was removed with a grinder for the purpose of improving the weldability between the build-up layer, which will be the subsequent striking surface (working surface), and the intermediate layer. As alloy powders of build-up welding metal for build-up welding on the intermediate layer, a mixed powder of two types of dissimilar alloy powders was prepared and their respective blending ratios were varied. The prepared powders were a commercially available alloy equivalent to Alloy U520 (C 0.016%, Cr 19.2%, W 1.0%, Mo 6.0%, Co 12.3%, Al 2.0%, Ti 3.2%, Nb less than 0.01%, B 0.0067%, the balance being Ni and unavoidable impurities), and Alloy A (C 0.024%, Cr 13.5%, W 5.8%, Mo 6.0%, Co 14.2%, Al 2.2%, Ti 3.4%, Nb 1.9%, B 0.0005%, the balance being Ni and unavoidable impurities).

[0020] By the gas atomization method, a build-up layer having the chemical composition shown in Table 1 was laminated on the intermediate layer by build-up welding using the powder plasma welding method, with a thickness of 2 mm for each layer to form a build-up layer constituting the striking surface. The alloy powder produced by the gas atomization method was classified to have a particle size suitable for the powder plasma welding method, and the particle size range was 63 - 250 μm. The build-up layer on the intermediate layer was build-up welded so as to laminate three layers (thickness approximately 6 mm). For each build-up welding of one layer, in order to prevent cracks and the inclusion of oxidation scale, the oxidation scale on the surface of the build-up layer was ground off with a grinder. In this example, when welding the build-up welding metal up to the third layer by the powder plasma welding method, if even one crack visible to the naked eye was confirmed on the surface of each layer, the build-up welding metal was determined to have poor weldability (symbol "×" in the table). On the other hand, if no crack could be confirmed up to the third layer, the weldability of the build-up welding metal was determined to be good (symbol "〇" in the table).

[0021] Table 2 shows the determination results of the weldability of the present invention examples, conventional examples, and comparative examples. No cracks were confirmed on the surfaces of the build-up layers of the present invention examples and the conventional examples, and the weldability of the present invention examples and the conventional examples was determined to be good. On the other hand, in the comparative examples, cracks were confirmed in the second or third layer, and the weldability was determined to be poor. In the case of the comparative examples, it is considered that some elements that deteriorate the weldability are contained in the alloy beyond the range of the chemical composition defined in the present invention, resulting in the occurrence of hot cracks or reheat cracks during build-up welding. From the above results, it was found that the weldability during build-up welding is good when using the build-up welding metal that becomes the build-up layer of the present invention example.

[0022]

Table 1

[0023]

Table 2

[0024] (Example 2) In Example 2, the wear resistance will be described. In Example 1, build-up welding layers having the compositions of the present invention examples No. 1 and 2 and the conventional example No. 11, which were determined to have good weldability, were build-up welded onto the striking surface (working surface) to produce a hot forging anvil, and hot forging was carried out using this anvil. The method for producing the hot forging anvil was the same as in Example 1, but the number of build-up welding layers was two layers. At this time, the thickness of the build-up layer on the striking surface was about 4 mm. Hot forging was carried out using a high-speed four-sided forging machine (radial forging machine). High-speed four-sided forging is a forging method in which the material to be forged is intermittently rotated, and forged simultaneously from four directions by four anvils arranged at X-shaped positions and relatively moved, so as to stretch the material only in the axial direction. First, the wear resistance of the hot forging anvils build-up welded with the build-up welding metals of the present invention example No. 1 and the conventional example No. 11 was evaluated. In order to accurately compare the wear resistance between the present invention example and the conventional example, among the four hot forging dies arranged in the high-speed four-sided forging machine, anvils having the build-up layer of the present invention example No. 1 were arranged in two diagonal dies (die 1 and die 2), and anvils having the build-up layer of the conventional example No. 11 were arranged in the other two dies (die 3 and die 4). The wear resistance was evaluated by measuring the wear amount of the striking surface of the hot forging anvil. At the most worn position of the striking surface at this time, the wear depth was measured with a depth gauge based on the surface position of the striking surface before wear, and defined as the wear amount. As for the forging conditions, four pieces of an alloy equivalent to Alloy 718, a precipitation-strengthened Ni-based alloy, were forged as the material to be forged, and then 15 pieces of an alloy equivalent to Alloy U520, the same alloy as in the conventional example, were forged. The forging load during forging was 700 to 750 tons. Also, the temperature of the striking surface of the hot forging die rose from the initial temperature to a maximum of about 800 °C during forging. Table 3 shows the results of the wear amount of each hot forging die after forging. Note that a total of 19 pieces of the precipitation-strengthened Ni-based alloy as the material to be forged were forged, and the wear amount per piece of forging in the present invention example and the conventional example was obtained by dividing the average wear amount by the total number of forging pieces.

[0025]

Table 3

[0026] The comparison between the present invention example of No. 2 and the conventional example of No. 11 was also carried out in the same manner. The manufacturing method of the hot forging die and the evaluation method of the wear amount were exactly the same as the above-mentioned methods. On the other hand, the forging conditions were different. In this case, there was one type of material to be forged, and 13 pieces of Alloy 718, a precipitation-strengthened Ni-based alloy, were used. The forging load during forging was 700 to 750 tons. Also, the temperature of the striking surface of the hot forging die rose from the initial temperature to a maximum of about 800 °C during forging. Table 4 shows the results of the wear amount of each hot forging die after forging.

[0027]

Table 4

[0028] From the results of Table 3 and Table 4, it can be seen that the wear amount per piece of forging in the present invention example is reduced compared with the conventional example, and the wear resistance of the present invention example is good. Considering that the wear amount is proportional to the number of forging pieces, it can be said that the service life of the hot forging die is greatly improved. Also, when comparing the present invention examples of No. 1 and No. 2, the wear amount per piece of forging is slightly better in No. 2, and it is considered that the fact that the contents of the solid solution strengthening element and the precipitation strengthening element in No. 1 are higher than those in No. 2 has an influence.

[0029] (Example 3) In Example 3, the strength at each temperature of the inventive example and the conventional example will be described. Among the chemical compositions shown in Table 1, the inventive example No. 1 and the conventional example No. 11 were build-up welded to a thickness of 20 mm in the same manner as in Example 1. A plurality of tensile test pieces were taken from this build-up layer, and tensile tests were carried out at each temperature from room temperature (22°C) to high temperature (900°C). The results of the 0.2% proof stress at each temperature by the tensile test are shown in FIG. 1. Compared with the conventional example, it can be seen that the 0.2% proof stress of the inventive example is higher at any temperature, and the strength of the inventive example increases over a wide temperature range. Since the Alloy U520 equivalent alloy of the conventional example also contains a large amount of Mo which is a solid solution strengthening element, it has high strength from the low temperature range although it does not have a strength peak in the high temperature range, but the inventive example exceeds its strength at all measurement temperatures. As described above, it can be understood that the die for hot forging and the padding for hot forging need to impart high strength not only in the high temperature range at thermal saturation but also in the low temperature range until thermal saturation, and this can be achieved by using the inventive example.

[0030] From the results of the above examples, it was confirmed that no hot cracking or reheat cracking was observed during build-up welding in the inventive example and the weldability was good. Also, when the build-up welding metal of the inventive example was used as the striking surface of the die for hot forging, it was confirmed that the wear resistance was superior to that of the Alloy U520 equivalent alloy of the conventional example. Furthermore, from the results of the tensile test, it was found that the strength of the inventive example was higher over a wide temperature range compared with the Alloy U520 equivalent alloy of the conventional example. Therefore, when the build-up welding using the build-up welding metal of the inventive example is applied to the working surface, it can be said that the weldability during build-up welding is not deteriorated, the strength of the build-up metal is increased over a wide temperature range to improve the wear resistance, and the service life of various hot working tools such as the die for hot forging can be extended.

Claims

1. In a method for manufacturing a hot working tool having a base material made of alloy tool steel, a build-up layer made of a Ni-based alloy on the working surface, and an intermediate layer made of a solution-strengthened Ni-based alloy between the base material and the build-up layer, an intermediate layer made of a solution-strengthened Ni-based alloy is formed on the base material made of the alloy tool steel, and build-up welding is sequentially repeated and laminated on the intermediate layer to form a build-up layer, wherein the composition of the build-up layer is, by mass%, C: 0.010 to 0.030%, Cr: 13.0 to 18.0%, W: 3.0 to 5.0%, Mo: 5.0 to 7.0%, Co: 12.0 to 15.0%, Al: 1.0 to 2.8%, Ti: 2.0 to 3.7%, Nb: 0.5 to 1.5%, B: 0.0010 to 0.0070%, and the balance is Ni and unavoidable impurities, a method for manufacturing a hot working tool.

2. The method for manufacturing a hot working tool according to claim 1, wherein the build-up layer is formed by using an alloy powder obtained by mixing two types of alloy powders having different compositions as a build-up welding metal.

3. The method for manufacturing a hot working tool according to claim 1 or 2, wherein the hot working tool is a hot forging die or a hot forging pad.

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