JP7508873B2 - Hot working tools - Google Patents

Description

The present invention relates to a hot working tool used for hot plastic processing such as hot forging.

Conventionally, hot working tools are used for hot plastic working of workpieces. For example, hot forging dies used in hot forging are used with an alloy having high high temperature strength and excellent wear resistance welded overlay on the part constituting the striking surface (hereinafter referred to as the striking surface or the working surface) in order to extend the service life of the dies.
For example, in Japanese Patent Application Laid-Open No. 2001-71086 (Patent Document 1), the applicant of the present application has proposed a hot forging die in which an alloy equivalent to JIS-SKT4 is used as the base material of the die, and a precipitation-strengthened Ni-based alloy is overlaid and welded onto the base material to form a striking surface. In addition, in Japanese Patent Application Laid-Open No. 2001-62541 (Patent Document 2), a method has been proposed in which the entire die including the base material is made of a precipitation-strengthened Ni-based alloy, or a base material made of a Ni-based alloy is used and a precipitation-strengthened Ni-based alloy is overlaid and welded to form a striking surface. In the hot forging die proposed in Patent Document 1 and Patent Document 2, precipitation-strengthened Ni-based alloys such as Alloy 713C, Alloy 718, and Alloy U520 are listed as overlaid weld metals.
Incidentally, Ni-based alloys in which the precipitation strengthening elements such as Al and Ti are increased to further improve high-temperature strength compared to the above-mentioned 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 more the precipitation strengthening elements are increased, the more likely hot cracking and reheat cracking are to occur during overlay welding (Non-Patent Document 1), and from the viewpoint of weldability, Ni-based alloys containing excessive precipitation strengthening elements are generally not suitable as overlay weld metals for hot forging dies.

JP 2001-71086 A JP 2001-62541 A

"Welding of Superalloys (Guidebook for Welding of Heat-Resistant and Corrosion-Resistant Alloys)" edited by the Special Materials Welding Research Committee of the Japan Welding Society, Sanpo Publishing, pp. 36, pp. 46-48

Although the hot forging die having a striking surface formed by overlay welding of a precipitation-strengthened Ni-based alloy such as the above-mentioned Alloy U520 has excellent high-temperature strength, when the material to be forged is a difficult-to-process material such as a Ni-based alloy, the abrasion resistance of the striking surface is insufficient, and the striking surface is plastically deformed and worn during forging, resulting in a significantly shortened service life of the die. If the hot forging die is used with the striking surface deformed, the shape accuracy of the material to be forged is extremely deteriorated. Therefore, repair work for the plastically deformed striking surface is required, which requires a large number of steps and costs and reduces productivity.
In addition, when an alloy with increased precipitation strengthening elements is used as a weld metal for overlay welding in order to improve the wear resistance of the impact surface, the high-temperature strength is increased as described above, but the weldability is deteriorated, making it unsuitable as a weld metal for overlay welding. In the case of Ni-based alloys, if the alloy contains an excessive amount of precipitation strengthening elements, the occurrence of high-temperature cracking and reheat cracking during overlay welding becomes prominent. Reheat cracking is a type of cracking that occurs during heat treatment after welding or during use, but even in the case of weld metal for overlay welding, reheat cracking may occur because the underlying overlay layer is thermally affected during overlay welding.
If hot forging is performed using a hot forging die that contains even a small crack on its striking surface, stress will concentrate in the cracked area during forging, and the crack will propagate from the striking surface of the die to the base material. Depending on the extent of the propagated crack, it may become 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 in an overlay layer using an overlay weld metal on the striking surface of a hot forging die.
An object of the present invention is to provide a hot working tool that has excellent wear resistance when used in an overlay layer on the working surface of a hot working tool such as an anvil or die for hot forging, by optimizing the chemical composition of the overlay weld metal that constitutes the overlay, without deteriorating the weldability during overlay welding.

The present inventors have investigated the problem of insufficient wear resistance of the build-up layer and the deterioration of weldability caused by improving the wear resistance when the build-up layer is used on the working surface of a hot working tool such as the striking surface of a hot forging die or a hot forging anvil, and have found that the wear resistance of the striking surface can be greatly improved by adding precipitation strengthening elements to the alloy within a range that does not deteriorate the weldability of the build-up alloy powder, and further by making the alloy contain more solid solution strengthening elements than conventional build-up weld metals, thereby arriving at the present invention.
That is, the present invention relates to a hot working tool having an overlay made of a Ni-based alloy on its working surface, the overlay having a composition, in mass %, of C: 0.010-0.030%, Cr: 13.0-18.0%, W: 3.0-5.0%, Mo: 5.0-7.0%, Co: 12.0-15.0%, Al: 1.0-2.8%, Ti: 2.0-3.7%, Nb: 0.5-1.5%, B: 0.0010-0.0070%, and the balance being Ni and unavoidable impurities.
The hot working tool is preferably a hot forging die or a hot forging anvil.

The alloy composition of the build-up layer used on the working surface of the hot working tool of the present invention has excellent weldability and wear resistance properties, making it possible to suppress the occurrence of cracks when build-up welding is performed on the working surface of the hot working tool. In addition, a hot working tool having a build-up layer made of this alloy composition reduces the amount of wear on the working surface when hot working difficult-to-work materials, thereby extending the service life of the hot working tool.

1 shows the results of a tensile test showing 0.2% yield strength at various temperatures for comparing the strength of an example of the present invention with that of a conventional example.

As described above, an important feature of the present invention is the adoption of an alloy composition that does not deteriorate weldability and has excellent wear resistance when used as an overlay alloy on the working surfaces of hot forging dies and hot forging anvils.
Specifically, it has been discovered that by including, as an alloy element, a solid-solution strengthening element that does not excessively deteriorate weldability, it is possible to increase the strength of the build-up layer that constitutes the striking surface over a wide temperature range, thereby improving the wear resistance of the striking surface of a hot working tool.
For example, among hot working tools, hot forging dies (anvils) are required to have high wear resistance, and in the thermally saturated 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 beginning of forging is not so high, and as the forging time progresses, the temperature gradually rises from the initial temperature to a high temperature and reaches a thermally saturated state. In addition, even at the time of thermal saturation, the entire area of the buildup layer constituting the striking surface does not reach the maximum temperature, and in areas with few opportunities for contact with the material to be forged, areas outside the die where heat is easily dissipated, and areas inside the surface, the buildup layer constituting the striking surface is maintained at a temperature lower than the maximum temperature. Therefore, in order to improve the wear resistance of the striking surface of the hot forging die, it can be said that it is necessary to increase not only the high temperature strength but also the strength over a wide temperature range.
Therefore, in the present invention, the alloy powder for overlay contains more solid solution strengthening elements such as Mo and W, which can increase the strength in a wide temperature range, than the conventional overlay weld metal, thereby improving the wear resistance of the overlay layer that constitutes the impact surface. Compared to precipitation strengthening elements, solid solution strengthening elements do not contribute to reheat cracking caused by strain due to the precipitation of gamma prime phase (hereinafter referred to as γ' phase). In addition, since they do not significantly expand the solidification temperature range, they are unlikely to contribute to high-temperature cracking. Therefore, if the alloy powder for overlay contains more solid solution strengthening elements than the conventional precipitation strengthened Ni-based alloy for overlay weld metal, it is possible to improve the wear resistance of the impact surface without deteriorating the weldability.

In the build-up layer used on the working surface of the present invention, the reasons for specifying the additive elements and the ranges of each additive element are as follows. Note that unless otherwise specified, the amounts are expressed as mass %.
<C: 0.010 to 0.030%>
C has the effect of forming carbides at grain boundaries in the build-up layer, strengthening the grain boundaries, increasing the strength of the build-up layer, and improving the wear resistance. To obtain this effect, C needs to be 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 C exceeds 0.030%, coarse carbides are formed, making the build-up layer embrittled and reducing toughness and ductility. Therefore, C is set to 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 Cr ₂ O ₃ film on the surface of the build-up layer to improve oxidation resistance, and further dissolves in the austenite phase (hereinafter referred to as γ phase) which is the base of the build-up layer to increase the strength of the build-up layer in a wide temperature range by solid solution strengthening, and has the effect of improving wear resistance. In order to obtain this effect, it is necessary to set Cr in the range of 13.0 to 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 a sigma phase (hereinafter referred to as σ phase), which is an embrittlement phase, or forms coarse Cr-based carbides, which reduce the toughness and ductility of the build-up layer. Therefore, Cr is set in the range of 13.0 to 18.0%. The preferable upper limit of Cr is 17.0%, and the preferable lower limit is 14.0%.

<W: 3.0 to 5.0%>
W has the effect of increasing the strength of the build-up layer over a wide temperature range by dissolving in the γ phase, which is the base of the build-up layer, and improving the wear resistance through solid solution strengthening. To obtain this effect, W needs to be in the range of 3.0 to 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 W exceeds 5.0%, a brittle phase, the mu phase (hereinafter referred to as μ phase), is formed, and the toughness and ductility of the build-up layer are reduced. Therefore, W is set to the range of 3.0 to 5.0%. The preferable upper limit of W is 4.5%, and the preferable lower limit is 3.5%.
<Mo: 5.0 to 7.0%>
Mo has the effect of increasing the strength of the build-up layer over a wide temperature range by solid solution strengthening due to its solid solution dissolution in the γ phase, and improving the wear resistance. To obtain this effect, Mo needs to be in the range of 5.0 to 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 Mo exceeds 7.0%, the build-up layer forms brittle phases such as μ phase and σ phase, which reduce the toughness and ductility of the build-up layer. Therefore, Mo is set to the range of 5.0 to 7.0%. The preferred upper limit of Mo is 6.5%, and the preferred lower limit is 5.5%.

<Co: 12.0 to 15.0%>
Co is dissolved in the γ phase, and thus has the effect of increasing the strength of the build-up layer in a wide temperature range by solid solution strengthening, and improving the wear resistance. To obtain this effect, it is necessary to set Co in the range of 12.0 to 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 Co exceeds 15.0%, the build-up layer is formed with brittle phases such as μ phase and σ phase, and the toughness and ductility of the build-up layer are reduced. In addition, since Co is an expensive element, the manufacturing cost of the alloy powder for build-up increases. Therefore, Co is set in the range of 12.0 to 15.0%. The preferable upper limit of Co is 14.0%, and the preferable lower limit is 13.0%.
<Al: 1.0 to 2.8%>
Al combines with Ni to form a coherent precipitation of the γ' phase consisting of Ni ₃ Al, and has the effect of increasing strength and improving wear resistance, particularly in high temperature regions, through precipitation strengthening. To obtain this effect, it is necessary to set Al in the range of 1.0 to 2.8%. If Al is less than 1.0%, the strength of the overlay layer is insufficient in high temperature regions, and sufficient wear resistance cannot be obtained. On the other hand, if it exceeds 2.8%, the susceptibility to high temperature cracking during overlay welding and reheat cracking on the lower layer side during overlay welding increases, and weldability deteriorates. Therefore, Al is set in the range of 1.0 to 2.8%. The preferable upper limit of Al is 2.5%, and the preferable lower limit is 2.0%.

<Ti: 2.0 to 3.7%>
Ti has the effect of increasing the strength and improving the wear resistance, particularly in the high temperature range, by substitutional solid solution in the Al site of the γ' phase of Ni ₃ Al and precipitation strengthening. To obtain this effect, Ti needs to be in the range of 2.0 to 3.7%. If Ti is less than 2.0%, the strength of the overlay layer is insufficient in the high temperature range and sufficient wear resistance cannot be obtained. On the other hand, if Ti exceeds 3.7%, the susceptibility to high temperature cracking during overlay welding and reheat cracking on the lower layer side during overlay welding increases, and weldability deteriorates. In addition, eta phase (η phase) is precipitated to reduce the strength of the overlay layer. Therefore, Ti is set to the range of 2.0 to 3.7%. The preferable upper limit of Ti is 3.5%, and the preferable lower limit is 3.0%.
<Nb: 0.5 to 1.5%>
Nb dissolves in the Al site of the γ' phase of Ni ₃ Al, and has the effect of increasing the strength and improving the wear resistance, particularly in the high temperature range, 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 overlay layer is insufficient in the high temperature range, and sufficient wear resistance cannot be obtained. On the other hand, if Nb exceeds 1.5%, it segregates significantly during solidification during overlay welding, increasing the sensitivity to hot cracking and deteriorating the weldability. In addition, it precipitates delta phase (δ phase) and reduces the strength of the overlay 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%.

<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, B needs to be in the range of 0.0010 to 0.0070%. If B is less than 0.0010%, sufficient grain boundary strengthening effect cannot be obtained. On the other hand, if B exceeds 0.0070%, it will excessively segregate at the grain boundaries during solidification during overlay welding to form low-melting point compounds, which will increase the susceptibility to hot cracking and deteriorate the weldability. Therefore, B is set in the range of 0.0010 to 0.0070%. The preferred upper limit of B is 0.0050%, and the preferred lower limit is 0.0020%.
<Balance: Ni and inevitable impurities>
The balance is essentially Ni, but contains impurities that are inevitably mixed in during manufacturing. In particular, Si, Mn, P, S, and Zr increase the susceptibility to hot cracking, Fe reduces the amount of γ' phase precipitation, O causes oxide scale to be rolled in during overlay welding, and N causes blowholes, so it is preferable that the amount of these impurity elements is small. Below, the preferred allowable ranges of the above impurity elements and other representative unavoidable impurity elements are described.
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%.

The build-up layer used on the working surface of the hot working tool used in the present invention described above has an alloy composition that provides good weldability during build-up welding, and also has the effect of providing good wear resistance as an overlay metal, making it suitable as a weld metal to be overlay welded onto the working surface of a hot working tool.
In the present invention, the term "hot working tool" refers to a tool used to contact and press a workpiece heated to a hot working temperature to plastically deform the workpiece, and includes various rolls used for rolling, drawing, etc. Preferred forms of the hot working tool of the present invention include a hot forging die used in free forging or radial forging, a jig for partially pressing a workpiece, and a hot forging die used in hot die forging. The alloy composition of the buildup layer according to the present invention is particularly suitable for the buildup weld metal to be built up and welded to the part constituting the striking surface of a hot forging die or a hot forging die.

<Method of manufacturing hot working tools>
Next, an example of forming a build-up layer having the above-mentioned composition to produce a hot working tool will be described. The following is for producing a hot forging die or hot forging anvil.
Hot forging dies and hot forging anvils are composed of a base material and a build-up layer that constitutes the striking surface. The base material, which is the majority of the hot forging die, is an alloy tool steel having high rigidity, and its material may be, for example, an alloy for "hot die" as described in JIS-G4404 or an improved alloy thereof. The alloy tool steel that is the base material is, for example, quenched at 800 to 1100°C and tempered at 500 to 700°C, and is usually adjusted to a hardness of about 330 to 380 HBW.
In addition, in the present invention, one or two types of solid solution strengthened Ni-based alloy intermediate layers can be provided between the base material made of alloy tool steel and the buildup layer having the above composition. By using this intermediate layer, the alloy composition and mechanical properties of each layer do not change abruptly compared to forming the buildup layer directly on the base material (buildup welding), so that weldability can be improved. Furthermore, the effect of mitigating the stress generated between the base material and the buildup layer constituting the striking surface can be obtained, and the service life of the hot forging die can be further improved. This intermediate layer can be laminated by a general welding method such as MIG welding.

In the present invention, there are several methods for forming an overlay on the striking surface. The first method is a method in which two types of alloy powders with different compositions are prepared, and the different types of alloy powders are mixed or the mixed alloy powder is used as a weld metal for overlay welding, and the overlay is formed while adjusting the composition of the overlay. In this first method, one type of alloy powder is an alloy powder of an available existing alloy, and the composition after overlay welding can be adjusted by adjusting the composition and blending ratio of the other type of alloy powder, and the raw material cost can be reduced by using the alloy powder of an existing alloy. In addition, by using two types of alloy powders with different compositions, a synergistic effect of the properties of each alloy powder can be expected.
The second method is to prepare an alloy powder having a chemical composition within the above range, and use the alloy powder as a weld metal for overlay welding. The overlay layer can be formed by appropriately selecting the first or second method.

In the present invention, to form an overlay layer on the striking surface, overlay welding is sequentially repeated to build up a predetermined thickness. As the overlay welding method for this purpose, a known technique such as powder plasma welding can be adopted. Powder plasma welding allows for faster welding than TIG welding or gas welding, and the weld structure becomes fine and dense, so it is suitable as a welding method for precipitation hardened Ni-based alloys such as those of the present invention, whose weldability is likely to be a problem, since the powder plasma welding method allows for faster welding than TIG welding or gas welding, and the weld structure becomes fine and dense. In addition, although the precipitation hardened Ni-based alloy has excellent high-temperature strength, it is a difficult-to-process material, so it is difficult to manufacture welding materials such as wires. However, if it is in powder form, it has the advantage that it can be manufactured relatively easily by gas atomization.
The hot forging die or hot forging anvil with a buildup layer formed on the striking surface (working surface) by buildup welding can be used for hot forging as is. However, if stress generated during buildup welding is to be reduced, it is also possible to reduce the stress by performing heat treatment at 500 to 620°C for 1 to 10 hours.
In addition, in order to prevent heat cracks, overlay welding may be performed in a state where the base material is preheated, so long as the temperature is lower than the annealing temperature of the base material.

Example 1
The present invention will be described in more detail in the following examples. Example 1 illustrates weldability.
An alloy equivalent to JIS-SKT4 was prepared as the base material for a hot forging anvil for radial forging, and was heat-treated to have a hardness of about 330 to 380 HBW.
Next, an intermediate layer was formed on the substrate by MIG welding using a solid-solution strengthened Ni-based alloy equivalent to Hastelloy (registered trademark) C. At this time, the oxide scale on the surface of the intermediate layer was removed with a grinder in order to improve the weldability between the intermediate layer and the buildup layer, which will later become the striking surface (working surface).
As alloy powders for overlay welding on the intermediate layer, mixtures of two different alloy powders were prepared with different mixing ratios: 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%, balance 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%, balance Ni and unavoidable impurities).

A build-up layer having the chemical composition shown in Table 1 was formed by gas atomization on the intermediate layer by powder plasma welding, with each layer being 2 mm thick, to form the build-up layer that constitutes the striking surface. The alloy powder produced by gas atomization was classified to have a particle size suitable for powder plasma welding, with a particle size range of 63 to 250 μm. The build-up layer on the intermediate layer was build-up welded so that three layers were stacked (thickness: approximately 6 mm), and after each layer, the oxide scale on the surface of the build-up layer was ground off with a grinder to prevent cracking and inclusion of oxide scale.
In this example, when the overlay weld metal was welded up to the third layer by the powder plasma welding method, if even one visually visible crack was found on the surface of each layer, the overlay weld metal was judged to have poor weldability (symbol "x" in the table). On the other hand, if no crack was found up to the third layer, the overlay weld metal was judged to have good weldability (symbol "o" in the table).

The results of the evaluation of the weldability of the inventive example, conventional example, and comparative example are shown in Table 2. No cracks were found on the surface of the overlay layer of the inventive example and conventional example, and the weldability of the inventive example and conventional example was evaluated to be good. On the other hand, cracks were found in the second or third layer of the comparative example, and the weldability was evaluated to be poor. In the case of the comparative example, it is believed that high-temperature cracking or reheat cracking occurred during overlay welding because some elements that deteriorate weldability are contained in the alloy in excess of the range of the chemical composition specified in the present invention.
From the above results, it was found that the use of the build-up weld metal for the build-up layer of the invention example provided good weldability during build-up welding.

Example 2
In Example 2, the wear resistance will be described.
In Example 1, an anvil for hot forging was prepared by overlay welding an overlay having the composition of the inventive examples No. 1 and 2 and the conventional example No. 11, which were judged to have good weldability, onto the striking surface (working surface), and hot forging was carried out using this anvil. The method of preparing the anvil for hot forging was the same as in Example 1, but the number of welded layers was two. The thickness of the overlay on the striking surface in this case was about 4 mm.
Hot forging was performed 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 simultaneously forged from four directions with four anvils arranged in an X-shape while rotating intermittently, and the material is stretched only in the axial direction by moving the anvils relative to one another.
First, the wear resistance of hot forging anvils with overlay welded overlay weld metals of No. 1, an example of the present invention, and No. 11, a conventional example, was evaluated. In order to accurately compare the wear resistance of the examples of the present invention and the conventional examples, anvils with overlay layers of No. 1, an example of the present invention, were placed in two dies (dies 1 and 2) diagonally out of four hot forging dies arranged in a high-speed four-sided forging machine, and anvils with overlay layers of No. 11, a conventional example, were placed in the other two dies (dies 3 and 4).
The wear resistance was evaluated by measuring the amount of wear on the striking surface of the hot forging anvil. At the most worn position on the striking surface, the wear depth was measured with a depth gauge based on the surface position of the striking surface before wear, and this was defined as the amount of wear.
As for the forging conditions, four pieces of alloy equivalent to Alloy 718, a precipitation-strengthened Ni-based alloy, were forged as the forged material, followed by 15 pieces of alloy equivalent to Alloy U520, which is the same alloy as the conventional example. The forging load during forging was 700 to 750 tons. The temperature of the striking surface of the hot forging anvil rose from the initial temperature to a maximum of approximately 800°C during forging.
The results of the wear amount of each hot forging anvil after forging are shown in Table 3. A total of 19 pieces of precipitation hardened Ni-based alloy were forged as the forged material, and the wear amount per forging of the inventive example and the conventional example was calculated by dividing the average wear amount by the total number of forgings.

A comparison was also made between the example of the present invention (No. 2) and the conventional example (No. 11) in the same manner. The method of making the hot forging anvil and the method of evaluating the wear amount were exactly the same as those described above. However, the forging conditions were different, and in this case, only one type of material was forged, 13 pieces of Alloy 718, a precipitation-strengthened Ni-based alloy. The forging load during forging was 700 to 750 tons. In addition, the temperature of the impact surface of the hot forging anvil rose from the initial temperature to a maximum of approximately 800°C during forging. The results of the wear amount of each hot forging anvil after forging are shown in Table 4.

From the results of Tables 3 and 4, it is clear that the wear amount per forging of the examples of the present invention is smaller than that of the conventional examples, and that the wear resistance of the examples of the present invention is good. Considering that the wear amount is proportional to the number of forgings, it can be said that the service life of the hot forging anvil is greatly improved.
In addition, when comparing No. 1 and No. 2, which are examples of the present invention, the wear amount per forging is slightly better in No. 2, which is thought to be influenced by the fact that No. 1 contains more solid solution strengthening elements and precipitation strengthening elements than No. 2.

Example 3
In Example 3, the strength at each temperature of an example of the present invention and a conventional example will be described.
Of the chemical compositions shown in Table 1, No. 1, the inventive example, and No. 11, the conventional example, were overlay welded to a thickness of 20 mm in the same manner as in Example 1. A number of tensile test pieces were taken from the overlay layer, and tensile tests were carried out at various temperatures from room temperature (22°C) to high temperature (900°C). The results of the 0.2% proof stress at each temperature from the tensile test are shown in Figure 1.
Compared to the conventional example, the 0.2% proof stress of the inventive example is higher at all temperatures, and it is clear that the inventive example has increased strength over a wide temperature range. The conventional alloy equivalent to Alloy U520 also contains a large amount of Mo, which is a solid solution strengthening element, so it does not have a strength peak in the high temperature range and has high strength from the low temperature range, but the inventive example exceeds that strength at all measurement temperatures.
As mentioned above, hot forging dies and hot forging anvils need to provide high strength not only in the high temperature range at the time of thermal saturation, but also in the low temperature range up to the time of thermal saturation, and it can be seen that this can be achieved by using the examples of the present invention.

From the results of the above examples, it was confirmed that the examples of the present invention had good weldability with no hot cracking or reheat cracking observed during overlay welding. In addition, it was confirmed that when the overlay weld metal of the examples of the present invention was used as the striking surface of a hot forging die, it had better wear resistance than the alloy equivalent to Alloy U520 of the conventional example. Furthermore, the results of the tensile test showed that the strength of the examples of the present invention was higher over a wide temperature range than that of the alloy equivalent to Alloy U520 of the conventional example.
Therefore, when the overlay welding using the overlay weld metal of the present invention is applied to a work surface, it can be said that the weldability during overlay welding is not deteriorated, the strength of the overlay metal is increased over a wide temperature range, and the wear resistance is improved, thereby extending the service life of various hot working tools, including hot forging dies.

Claims (2)

A hot working tool having a build-up layer made of a Ni-based alloy on the working surface, the build-up layer having a composition, in mass %, of C: 0.010-0.030%, Cr: 13.0-18.0%, W: 3.0-5.0%, Mo: 5.0-7.0%, Co: 12.0-15.0%, Al: 1.0-2.8%, Ti: 2.0-3.7%, Nb: 0.5-1.5%, B: 0.0010-0.0070%, with the balance being Ni and unavoidable impurities. 2. The hot working tool according to claim 1, wherein the hot working tool is a hot forging die or a hot forging anvil.