Method for producing hot-forged material

Abstract

When hot-forging a to-be-hot-forged-material that has a heat-resistant heat-insulation material bonded to a surface thereof, the present invention inhibits flaws caused by bonding of the heat-resistant heat-insulation material from occurring on a surface of the obtained hot-forged material. This method for producing a hot-forged material includes: a heating step in which a pre-heating material to be hot-forged is heated to a hot forging temperature in a heating furnace to obtain a heated material; a heat-resistant heat-insulation material bonding step in which a heat-resistant heat-insulation material having inorganic fibers is bonded to at least a portion of a surface of the heated material taken out from the heating furnace to form a to-be-hot-forged-material; and a hot-forging step in which a portion or the whole of the to-be-hot-forged-material is compressed by using one of a die, an anvil, and a tool to mold the material into a prescribed shape. A surface, which is to be bonded to the heated material, of the heat-resistant heat-insulation material having the inorganic fibers is subjected to application thereon of a liquid glass lubricant, which does not include a styrene acrylic resin or includes a styrene acrylic resin in an amount of less than 5 mass%, and to drying, and is then bonded to the heated material.

Description

Manufacturing method of hot forged materials

[0001] The present invention relates to a method for producing a hot-forged material, and more particularly to a method for producing a hot-forged material made of a difficult-to-work alloy.

[0002] When hot forging a hot forging material heated to the hot forging temperature, there is a problem of a decrease in hot workability due to a decrease in the temperature of the hot forging material. Therefore, various methods for preventing the temperature decrease have been proposed. For example, a method has been proposed in which an inorganic fiber heat-resistant insulating material is adhered to the surface of the heated material removed from a heating furnace to prepare the hot forging material, and then hot forging is performed on the material (Patent Document 1). It has also been proposed to apply a glass lubricant containing glass particles to the bonding surface of the heat-resistant insulating material with the heated material (including spray application), and then dry the heat-resistant insulating material with the glass particles attached. The glass lubricant containing glass particles can be, for example, SiO ２ Or B ２ O ３ A typical example is a "liquid glass lubricant" containing glass particles, a binder component (resin component), and water (Patent Document 2). In addition, a lubricant for hot working to which various trace components have been added has been proposed (Patent Document 3).

[0003] Regarding adhering the heat-resistant insulating material coated with the above-mentioned liquid glass lubricant to the surface of the heated material, it has been proposed to adhere the heat-resistant insulating material to the surface of the heated material after removing it from the heating furnace, and then return it to the heating furnace and "reheat" it, thereby increasing the adhesion of the heat-resistant insulating material to the surface of the heated material (Patent Document 4).

[0004] International Publication No. 2021 / 182606 Pamphlet Japanese Patent Application Publication No. 2016-215275 Japanese Patent Application Publication No. 2005-343948 Chinese Patent Application Publication No. 105478643 Specification

[0005] The technique of Patent Document 1 is an effective technique for suppressing the temperature drop of a hot forging material during hot forging. The surface of the hot forged material obtained after hot forging is generally smoothed by machining, such as grinding and polishing. When a hot forging material with a heat-resistant insulating material attached to its surface is hot forged, some of the heat-resistant insulating material may peel off or be worn away upon completion of the hot forging. Furthermore, "special defects" that occur due to a different mechanism from normal forging defects may be observed on the surface of the hot forged material to which the heat-resistant insulating material was attached. If these special defects are significant, it may be necessary to increase the grinding allowance.

[0006] The present invention aims to suppress defects that occur on the surface of the obtained hot forged material due to the adhesion of the heat-resistant insulating material in a method for manufacturing hot forged material in which a hot forging material having a heat-resistant insulating material adhered to the surface is hot forged.

[0007] That is, the present invention is a method for producing a hot-forged material, comprising: a heating step in which a pre-heated material to be hot-forged is heated to a hot forging temperature in a heating furnace to produce a heated material; a heat-resistant insulation bonding step in which an inorganic fiber heat-resistant insulation material is bonded to at least a portion of the surface of the heated material removed from the heating furnace to produce a hot-forged material; and a hot forging step in which a part or all of the hot-forged material is compressed into a predetermined shape using a die, anvil, or tool. The surface of the inorganic fiber heat-resistant insulation material to be bonded to the heated material is coated with a liquid glass lubricant that does not contain styrene-acrylic resin or contains less than 5% by mass of styrene-acrylic resin, dried, and then bonded to the heated material. Note that during the heating step, the oxygen concentration in the heating furnace can be set to 3.0% by volume or more.

[0008] The present invention also provides a method for producing the above-mentioned hot-forged material, in which the hot forging step is free forging, and in the heat-resistant insulating material bonding step, the above-mentioned heat-resistant insulating material is bonded to at least a portion of the surface of the freely deformable portion of the heated material that does not come into contact with any of the mold, anvil, or tool during this free forging.

[0009] According to the present invention, in a method for manufacturing hot forged material in which a hot forging material having a heat-resistant insulating material adhered to its surface is hot forged, it is possible to suppress defects that occur on the surface of the obtained hot forged material due to the adhesion of the heat-resistant insulating material.

[0010] 1 is a photograph showing an example of defects caused by glass corrosion on the surface of a hot forged material, and FIG. 2 is a photograph showing an example of the surface of a hot forged material obtained by the present invention.

[0011] The present invention will be described below step by step. Note that, hereinafter, "raw material before heating" refers to a material before being charged into a heating furnace, "raw material after heating" refers to a material heated to a hot forging temperature in a heating furnace, "raw material for hot forging" refers to a material in which a heat-resistant insulating material has been adhered to a predetermined portion and which is ready for hot forging, and "hot-forged material" refers to a material formed into a predetermined shape by a hot forging device.

[0012] <Heating Process> First, in the present invention, the pre-heated material to be hot forged is heated to the hot forging temperature in a heating furnace. The pre-heated material may be an ingot, billet, rough forge, powder compact, or the like, but the effects of the present invention are best achieved with ingots and billets that are formed into the desired shape by open forging. This pre-heated material is then heated to the hot forging temperature in a heating furnace to produce the post-heated material. The hot forging temperature varies depending on the material of the pre-heated material, but is easily determined as appropriate. For example, for nickel-based alloys, which are known to be difficult-to-work alloys, the temperature can be 950 to 1180°C, and if the alloy contains 20% or more by volume of gamma prime (γ') phase, the temperature can be 1010 to 1180°C. For titanium alloys, the temperature can be 900 to 1180°C.

[0013] However, in the present invention, the heat-resistant insulating material bonding process, which will be described later, is performed after this heating process. In the heat-resistant insulating material bonding process, a heat-resistant insulating material is bonded to the heated material removed from the heating furnace. While it would be preferable if there was no temperature drop in the heated material until the heat-resistant insulating material was bonded, in reality, the temperature drops by a significant amount. Therefore, when heating the pre-heated material in the heating process, the pre-heated material may be heated to a temperature that is about 5 to 100°C higher than the forging temperature (forging start temperature) at the start of hot forging (for example, a temperature obtained by adding about 5 to 100°C to the above-mentioned hot forging temperature). This prevents the temperature drop and allows the temperature to be maintained high during hot forging, even if the temperature of the heated material after removal from the heating furnace would drop by more than 100°C below the forging start temperature if the heat-resistant insulating material bonding process were not performed.

[0014] The surface roughness of the pre-heated workpiece is preferably rougher than a standard finish. This allows for a small space to form between the heat-resistant insulating material and the post-heated workpiece when the heat-resistant insulating material is bonded to the surface in the subsequent heat-resistant insulating material bonding process, and the air in this space is expected to function as an insulating layer. The glass particles attached to the heat-resistant insulating material are then more likely to adhere to the irregularities on the surface of the post-heated workpiece. While the as-cast or as-processed surface is acceptable, in the case of difficult-to-process alloys, cracks and other defects may occur on the surface due to the influence of additive elements. Therefore, it is advisable to remove these surface defects, which could cause cracks during hot forging, by machining, such as grinding or polishing. Even if no cracks or other defects are observed, it is preferable to machine the surface of the pre-heated workpiece to a roughness of at least a standard finish in the area where the heat-resistant insulating material will be bonded to the surface in the subsequent heat-resistant insulating material bonding process (i.e., the area where the glass lubricant will be bonded).

[0015] <Heat-Resistant Insulating Material Bonding Process> The pre-heated material is heated to a hot forging temperature, and a heat-resistant insulating material is bonded to at least a predetermined portion of the surface of the heated material removed from the heating furnace to prepare a hot forging material. First, the heat-resistant insulating material is inorganic fiber. In the present invention, "inorganic fiber" includes glass fiber, ceramic fiber, etc., and it is preferable to select ceramic fiber, which has excellent insulating properties. Among ceramic fibers, for example, KAOWOOL (registered trademark; hereinafter referred to as "KAOWOOL") is particularly preferred due to its ease of availability and low cost. Even if the surface of the heated material is somewhat rough, an inorganic fiber heat-resistant insulating material can be easily bonded to the surface shape, thanks to the adhesive effect of the glass lubricant applied to it. Furthermore, the fibers easily catch on the irregularities on the surface of the heated material. Furthermore, due to their lightweight nature, they can be easily bonded to the side of the heated material, for example.

[0016] Furthermore, as in the present invention, by adhering a heat-resistant insulating material to at least a portion of the surface of the heated material removed from the heating furnace, the heat-resistant insulating material remains intact on the surface of the heated material at the beginning of hot forging, thereby suppressing the temperature drop of the hot forging material during hot forging. If the heat-resistant insulating material is placed on the surface of the pre-heated material before it is loaded into the heating furnace, depending on the relationship between temperature and time, the material may be easily shattered during transportation for hot forging, making it difficult to suppress the temperature drop. Furthermore, by the end of hot forging, the peak temperature at which the temperature drop needs to be suppressed has passed, but with the present invention, some of the heat-resistant insulating material has peeled off or worn away, eliminating the need to remove the heat-resistant insulating material when machining the surface of the hot forged material obtained after hot forging is completed. Furthermore, by the end of hot forging, some of the heat-resistant insulating material has peeled off or worn away, thereby suppressing excessive build-up (processing heat) of the hot forging material. In order to obtain such an effect, it is effective to use glass fibers or ceramic fibers as the inorganic fibers of the heat-resistant heat insulating material.

[0017] In the heat-resistant insulating material bonding process described above, a known method for easily and quickly bonding the heat-resistant insulating material involves placing a glass lubricant between the heat-resistant insulating material and the bonding surface of the heated workpiece to which it is to be bonded. Specifically, glass particles are attached to the surface of the heat-resistant insulating material that will be bonded to the heated workpiece, and the heat-resistant insulating material is then bonded to the predetermined location on the heated workpiece. This method bonds the heat-resistant insulating material to the heated workpiece by softening the glass particles in the glass lubricant due to the heat retained on the surface of the heated workpiece. Therefore, this method is effective for hot forging of nickel-based superalloys, which have high hot forging temperatures. Methods for attaching glass particles to the heat-resistant insulating material include, for example, applying a liquid glass lubricant containing glass particles to the surface of the heat-resistant insulating material that will be bonded to the heated workpiece by brushing or spraying (atomizing or scattering). Spray application is preferred because it allows for uniform adhesion of glass particles to the surface of the heat-resistant insulating material that will be bonded to the heated workpiece. After applying the glass lubricant, it is preferable to dry the heat-resistant insulating material to which the glass particles have been attached, in order to ensure sufficient adhesion of the glass particles and to prevent, for example, glass corrosion of the base material, as described below. Furthermore, by drying the heat-resistant insulating material to which the glass particles have been attached, it is possible to prevent the rapid evaporation of volatile components, such as binders, contained in the glass lubricant when the heat-resistant insulating material is adhered to the surface of the base material after heating. In this regard, applying the glass lubricant directly to the surface of the base material after heating may cause the rapid evaporation of the volatile components.

[0018] However, according to the present invention, even if the temperature drop of the hot forging material can be suppressed by adhering the above-mentioned heat-resistant insulating material to the surface of the material after heating, it has been found that when the hot forged material is observed after hot forging is completed, "special defects" that occur by a different mechanism from normal forging defects are found on the surface where the heat-resistant insulating material has been adhered. After extensive research into the mechanism of occurrence of these special defects, it has been discovered that they are caused by "glass corrosion" that occurs at the three-phase interface between the material, glass lubricant, and forging atmosphere during forging.

[0019] First, when observing the surface of the hot-forged material immediately after the completion of hot forging, in some cases, there were no defects (i.e., normal forging defects) even on the surface where the heat-resistant insulation material was not attached, but there were defects (i.e., special defects) on the surface where the heat-resistant insulation material was attached (circled area in Figure 1). This result indicates that the special defects are not forging cracks caused by normal temperature drops. After investigating the details of these special defects, we found that they were caused by the glass corrosion mentioned above, and therefore that the occurrence of the glass corrosion mentioned above can be suppressed by specifying the type of glass lubricant applied to the heat-resistant insulation material.

[0020] That is, the glass lubricant is in a liquid state when applied to the heat-resistant insulating material, and is, for example, SiO ２ Or B ２ O ３ The glass lubricant contains glass particles composed of glass particles, a binder component (resin component), and water. When a heat-resistant insulating material is applied to the surface of a heated workpiece, the glass lubricant must be dried. However, even if the water has been removed from the dried glass lubricant, if the remaining resin component is flammable, it may come into contact with the workpiece heated to the hot forging temperature and react (burn) with oxygen in the environment before and during forging, potentially causing glass corrosion on the workpiece. After investigating various glass lubricants, we found that the "styrene-acrylic resin (copolymer)" contained therein is highly flammable, and limiting its content is effective in suppressing the progression of glass corrosion. That is, the glass lubricant of the present invention does not contain styrene-acrylic resin in its liquid state before application, or if it does contain it, it contains less than 5% by mass. Preferably, it is less than 4% by mass, and more preferably less than 2% by mass.

[0021] Furthermore, even if the glass lubricant according to the present invention has a sufficiently limited content of styrene-acrylic resin, it may still contain a significant amount of other flammable components. Therefore, once the heat-resistant insulating material coated with this glass lubricant is adhered to the surface of the heated workpiece, it should not be maintained in this state for a long time. In other words, prolonged contact of the glass lubricant with the surface of the heated workpiece at the hot forging temperature may still cause glass corrosion. Therefore, in order to improve the adhesion of the heat-resistant insulating material to the heated workpiece, it is preferable to omit coating the surface of the pre-heated workpiece with glass lubricant in the heating process, even if the oxygen concentration in the heating furnace is reduced. Furthermore, after adhering the heat-resistant insulating material to the surface of the heated workpiece in the heat-resistant insulating material adhering process, it is preferable to proceed promptly to the hot forging process described below without reheating the material.

[0022] Furthermore, in the present invention, since the coating of glass lubricant on the surface of the pre-heated material can be omitted in the heating process, the oxygen concentration in the heating furnace can be kept relatively high to prevent glass corrosion. For example, the oxygen concentration in the heating furnace can be set to 3.0% or more, 4.0% or more, or even 5.0% or more by volume. The upper limit can be, for example, 15.0% or less, 10.0% or less, or 8.0% or less. Regarding the oxygen concentration in the heating furnace, for example, if the pre-heated material is a nickel-based superalloy, most alloys contain 10 to 35% by mass of Cr. In this case, regulating the oxygen concentration in the heating furnace is effective in suppressing the reaction between the oxygen in the heating furnace and the Cr in the alloy during the heating process.

[0023] In this heat-resistant insulating material bonding process, the portion of the heated material to which the heat-resistant insulating material is bonded may be a portion or the entire surface. The portion to which the heat-resistant insulating material is bonded may be selected from the following two methods, taking into consideration the material and shape of the pre-heated material. The first method prioritizes preventing a temperature drop in the portion where forging cracks are expected. If the process of bonding the heat-resistant insulating material to the heated material takes too long, the temperature of the heated material may drop, potentially deteriorating its hot forgeability. Therefore, it is preferable to bond the heat-resistant insulating material to the surface of the material to the minimum necessary extent within a time that does not impair its hot forgeability. For example, when a hot forging material is placed in a hot forging apparatus, if there is a concern about heat transfer to the lower die (lower anvil or lower tool), the heat-resistant insulating material may be bonded to the surface that comes into contact with the lower die (lower anvil or lower tool). If the shape of the material is polygonal and cylindrical, the heat-resistant insulating material may be bonded to the edge portion or to the side surface of the cylindrical material. In other words, it is advisable to bond the heat-resistant insulating material to areas where defects such as cracks are likely to occur due to hot forging.

[0024] The second method is to adhere a heat-resistant insulating material to at least a part of the surface of the free-forming part of the material after heating. This method is mainly intended to reduce the temperature drop in the part that is not in contact with the upper die (upper anvil or upper tool) or lower die (lower anvil or lower tool) when the hot forging is free forging, for example, as it is left to cool in the air.

[0025] The above two methods are particularly effective for nickel-based alloys, which are known to be difficult to work, especially nickel-based alloys containing 20% ​​or more of the γ' phase by volume. In other words, the adhesion of the heat-resistant insulating material can reduce the precipitation of fine γ' phase that occurs when the temperature of the hot forging material is reduced, and can also promote recrystallization of the surface layer of the hot forging material, thereby reducing defects such as forging cracks. Furthermore, in nickel-based alloys, such as 718 alloy and Waspaloy alloy, which have a wide temperature range in which hot forging is possible, the ability to maintain the heating temperature can contribute to reducing forging defects (cracks).

[0026] <Hot Forging Process> Using the hot forging material prepared through the above-described processes, a part or the whole of the hot forging material is compressed into a predetermined shape using a die, anvil, or tool. The forging device used is preferably a large hot forging device with a forging load of several thousand tons or more, which is capable of forming a predetermined shape even for a difficult-to-process alloy. Furthermore, in the present invention, the above-described hot forging process is preferably free forging. The hot forging material used in free forging is heavy, has a large area for heat dissipation into the atmosphere, and requires a large amount of processing. Therefore, adhering a heat-resistant insulating material to the hot forging material effectively suppresses temperature drop. In this case, as described above, when hot forging a common nickel-based alloy, such as 718 alloy or Waspaloy alloy, which has a relatively wide temperature range in which hot forging is possible, it is preferable to adhere the heat-resistant insulating material of the present invention to at least a portion of the surface of the free-form portion of the heated material that does not come into contact with the die, anvil, or tool during free forging.

[0027] A commonly known liquid glass lubricant (water containing approximately 50% by mass of SiO ２ Or B ２ O ３ The binder component (styrene acrylic resin component) contained therein was adjusted using a glass lubricant containing glass particles (so-called borosilicate glass particles) composed of, etc., to prepare liquid glass lubricants 1 to 3 shown in Table 1. Then, liquid glass lubricants 1 to 3 shown in Table 1 were applied to the surface of Kaowool that would be bonded to the material after heating, and then dried to prepare inorganic fiber heat-resistant insulating materials.

[0028]

[0029] (Example 1) In Example 1, as a preliminary experiment, the influence of the following conditions A and B, which are related to the hot forging process, on the surface of the heated material was evaluated. Condition A: In the heating process, the surface of the material before heating is coated with a glass lubricant. Condition B: In the heat-resistant insulating material bonding process, a heat-resistant insulating material is bonded to the surface of the heated material removed from the heating furnace, and then the material is reheated.

[0030] First, the peripheral surface of a Waspaloy cylindrical billet (cross-sectional diameter 355 mm, height 800 mm) was turned to a standard finish to prepare a pre-heated material. Liquid glass lubricants 1 to 3 listed in Table 1 were then brushed onto a portion of the peripheral surface of the pre-heated material, which was then dried and placed in a heating furnace (furnace oxygen concentration: approximately 9.0% by volume). After the entire pre-heated material reached the hot forging temperature of 1,080°C, heating was maintained for at least four hours (Condition A). Next, the pre-heated material maintained at this heating temperature was temporarily removed from the heating furnace, and a heat-resistant insulating material coated with the glass lubricant was attached to the peripheral surface that was not coated with the liquid glass lubricant. The material was then returned to the heating furnace and heated for at least 30 minutes (Condition B). The reheated pre-heated material was then removed from the heating furnace as a post-heated material or a material for hot forging, and after air-cooling (natural cooling), its surface was observed.

[0031] As a result of the observation, first, when evaluating condition A, clear discoloration was observed on the peripheral surfaces of the material after heating where glass lubricants 1 and 2 were applied, indicating glass corrosion. Slight discoloration was also observed on the peripheral surface where glass lubricant 3 was applied, indicating localized glass corrosion. Then, when evaluating condition B, clear discoloration was observed on all peripheral surfaces where the heat-resistant insulating material was adhered after heating, indicating glass corrosion regardless of the types of glass lubricants 1 to 3. Therefore, in order to suppress glass corrosion, it is preferable to avoid having the glass lubricant in contact with the surface of the material for a long period of time at the hot forging temperature, as in the embodiments of conditions A and B.

[0032] In Example 2, based on the results of Example 1, in the heating step, the surface of the material before heating was not coated with a glass lubricant, and in the heat-resistant insulating material bonding step, the hot forging step was carried out without reheating the material for hot forging after the heat-resistant insulating material was bonded. Then, the state of defects on the surface of the hot forged material obtained by hot forging was confirmed.

[0033] First, the peripheral surface of a Waspaloy rectangular billet (490 mm square cross section, 1300 mm height) was ground to a regular finish using a grinder (abrasive grain size #16), and this was used as a pre-heated material. This pre-heated material was placed in a heating furnace (oxygen concentration in the furnace: approximately 9.0% by volume). After the entire pre-heated material reached the hot forging temperature of 1080°C, heating was maintained for more than four hours, and then the pre-heated material was removed from the heating furnace as a post-heated material. Then, a heat-resistant insulating material that had been applied and dried with each of the glass lubricants 1 to 3 was attached to a portion of the peripheral surface of this post-heated material. This was used as a hot forging material, and free forging was performed by pressing a die against this peripheral surface. The forging temperature (surface temperature) during hot forging was approximately 850°C to 1000°C. In both cases, the desired free forging was completed. The surface of the hot forged material was then observed immediately after free forging.

[0034] In the above hot-forged material, the heat-resistant insulating material adhered to its peripheral surface had worn away or peeled off, exposing the surface of the hot-forged material. While no flaws (i.e., normal forging flaws) were observed on the surface of the hot-forged material where the heat-resistant insulating material was not adhered, discoloration was observed in the exposed surface where the heat-resistant insulating material coated with glass lubricants 1 and 2 had been adhered, confirming the presence of flaws (circled areas in Figure 1). In this case, the flaws observed in the case of glass lubricant 1 were approximately 5 mm deep. In the case of glass lubricant 2, the flaws were approximately 2 mm deep, significantly reducing the severity of the flaws.

[0035] On the other hand, no discoloration or defects were observed in the area where the heat-resistant insulating material coated with the glass lubricant 3 was attached. The area where no discoloration or defects were observed was then heated again to the hot forging temperature of 1080°C while still at a high temperature after hot forging, and a heat-resistant insulating material coated with the glass lubricant 3 was attached, followed by repeated free forging. The forging temperature (surface temperature) during hot forging was approximately 850°C to 1000°C. Repeated free forgings revealed normal forging defects, but no defects due to glass corrosion (Figure 2 shows the surface of this area after the hot-forged material was air-cooled (naturally cooled)).

Claims

1. A method for manufacturing hot forged material, comprising: a heating step of heating a pre-heated material to be hot forged to a hot forging temperature in a heating furnace to form a heated material; a heat-resistant insulation bonding step of bonding an inorganic fiber heat-resistant insulation material to at least a portion of the surface of the heated material removed from the heating furnace to form the hot forging material; and a hot forging step of compressing a portion or the whole of the hot forging material into a predetermined shape using a die, anvil or tool, wherein a liquid glass lubricant which does not contain styrene-acrylic resin or contains less than 5 mass% of styrene-acrylic resin is applied to the surface of the inorganic fiber heat-resistant insulation material which will be bonded to the heated material, and then dried before bonding to the heated material.

2. The method for producing hot forged material according to claim 1, wherein the oxygen concentration in the heating furnace is 3.0% by volume or more in the heating step.

3. A method for manufacturing hot forged material according to claim 1, wherein the hot forging process is free forging, and the heat-resistant insulating material adhering process adheres the inorganic fiber heat-resistant insulating material to at least a portion of the surface of a freely deforming portion of the heated material that does not come into contact with any of the mold, anvil, or tool during free forging.

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