Manufacturing method of hot forged materials

Abstract

To suppress defects caused by adhesion of a heat-resistant heat insulating material, which occur on the surface of a hot forged material obtained when a hot forging material having a heat-resistant heat insulating material adhered to the surface is hot forged. The method includes 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 insulating material bonding step of bonding a heat-resistant insulating material made of inorganic fiber to at least a part of the surface of the heated material removed from the heating furnace to form a hot forging material; and a hot forging step of compressing a part or the whole of the hot forging material using a die, anvil, or tool to form it into a predetermined shape. A method for manufacturing hot forged materials, in which a liquid glass lubricant that 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 that will be bonded to the post-heating material, and then dried, and the surface is bonded to the post-heating material.

Description

[Technical Field]

[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. [Background technology]

[0002] When a hot forging material heated to a hot forging temperature is hot forged, 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 a heat-resistant insulating material made of inorganic fibers is bonded to the surface of a heated material removed from a heating furnace, which is then used as a 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 to the heated material (including spray application), and then dry the heat-resistant insulating material with the glass particles attached. A typical glass lubricant containing glass particles is a "liquid glass lubricant" containing glass particles composed of, for example, SiO2 or BO3, a binder component (resin component), and water (Patent Document 2). Furthermore, lubricants for hot working to which various trace components have been added have 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 it has been removed 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). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2021 / 182606 Brochure [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-215275 [Patent Document 3] Japanese Patent Application Laid-Open No. 2005-343948 [Patent Document 4] Chinese Patent Application Publication No. 105478643 Summary of the Invention [Problem to be solved by the invention]

[0005] The technique of Patent Document 1 is an effective technique for suppressing a 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 after hot forging is completed. Furthermore, "special defects" that occur due to a different mechanism from normal forging defects may be found 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. [Means for solving the problem]

[0007] That is, the present invention is a heating step of heating the pre-heated material to be hot forged in a heating furnace to a hot forging temperature to form a post-heated material; a heat-resistant insulating material bonding step of bonding an inorganic fiber heat-resistant insulating material to at least a part of the surface of the heated material removed from the heating furnace to prepare a material for hot forging; A hot forging process in which a part or the whole of the hot forging material is compressed and formed into a predetermined shape using a die, anvil, or tool; Including, This is a method for producing hot forged material, in which a liquid glass lubricant that does not contain styrene-acrylic resin or contains less than 5 mass% of styrene-acrylic resin is applied to the surface of the above-mentioned inorganic fiber heat-resistant insulation material that will be bonded to the post-heating material, and then dried, and the surface is bonded to the above-mentioned post-heating material. In this case, the oxygen concentration in the heating furnace can be set to 3.0% by volume or more in the heating step.

[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. [Effects of the Invention]

[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. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a photograph showing an example of a defect caused by glass corrosion on the surface of a hot forged material. [Figure 2] FIG. 2 is a photograph showing an example of the surface of a hot forged material obtained according to the present invention. DETAILED DESCRIPTION OF THE 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, a pre-heated material to be hot forged is heated to a 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 most pronounced with ingots and billets to be formed into a desired shape by open forging. Then, this pre-heated material is heated to the hot forging temperature in a heating furnace to produce a post-heated material. The hot forging temperature varies depending on the material before heating, but it can be easily determined appropriately. For example, for nickel-based alloys, which are known to be difficult to process, the temperature can be 950 to 1180°C. 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. It is preferable that the temperature of the heated material does not decrease at all until the heat-resistant insulating material is bonded, but in reality, the temperature decreases to a certain extent. 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 from decreasing by more than 100°C from the forging start temperature after removal from the heating furnace if the heat-resistant insulating material bonding process is not performed, and the temperature during hot forging can be maintained high.

[0014] It is better for the surface roughness of the pre-heated material to be rougher than a standard finish. This ensures that when the heat-resistant insulation material is bonded to the surface in the subsequent heat-resistant insulation bonding process, a small space is formed between the heat-resistant insulation material and the post-heated material, and the air in this space is expected to function as an insulating layer. This also makes it easier for the glass particles attached to the heat-resistant insulation material to fuse to the uneven surface of the post-heated material. Of course, the surface may be as cast or as plastically processed, but in the case of difficult-to-process alloys, cracks may occur on the surface due to the influence of added elements, etc., so it is advisable to remove surface defects that could cause cracks during hot forging by machining such as grinding or polishing. Even if no cracks are observed, it is preferable to machine the surface of the material before heating to a roughness of at least average 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 deposited).

[0015] <Heat-resistant insulation material bonding process> The pre-heated material is heated to a hot forging temperature, and after removal from the heating furnace, a heat-resistant heat insulating material is adhered to at least a predetermined portion of the surface of the post-heated material to prepare a material for hot forging. 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 preferable due to its ease of availability and low cost. Even if the surface of the material is somewhat rough after heating, an inorganic fiber heat-resistant insulating material can be easily adhered to the surface shape, thanks to the adhesive effect of the glass lubricant applied to it. The fibers easily catch on the unevenness of the surface of the material after heating, and because they are lightweight, they can be easily adhered to the side of the material after heating, 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 bonding the heat-resistant insulating material easily and in a short time is to have a glass lubricant between the heat-resistant insulating material and the bonding surface of the heated material to which it is to be bonded. In other words, glass particles are attached to the surface of the heat-resistant insulating material that will be bonded to the heated material, and the heat-resistant insulating material is then bonded to a predetermined location on the heated material. This technique adheres 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, it is effective for hot forging of nickel-based superalloys, which have high hot forging temperatures. For example, a method for adhering glass particles to the heat-resistant insulating material involves 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 the glass particles to be uniformly adhered 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 hot-forged material immediately after hot forging, in some cases, there were no defects (i.e., normal forging defects) 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 discovered 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 identifying the type of glass lubricant applied to the heat-resistant insulation material.

[0020] Specifically, when applied to a heat-resistant insulating material, the glass lubricant is in liquid form and contains glass particles composed of, for example, SiO2 or BO3, a binder component (resin component), and water. However, before adhering the applied heat-resistant insulating material to the surface of the 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 promoting glass corrosion on the workpiece. Therefore, 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. Specifically, the glass lubricant of the present invention, in its liquid state before application, does not contain styrene-acrylic resin, or if it does contain it, it contains less than 5% by mass. Preferably, it contains 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 raw material before heating 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 regulation of the oxygen concentration in the heating furnace, for example, if the material before heating is a nickel-based heat-resistant superalloy, most alloys contain Cr in the range of 10 to 35 mass %, so in this case, it is effective to suppress the reaction between oxygen in the heating furnace and 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 part of its surface or the entire surface. The portion to which the heat-resistant insulating material is bonded should be selected from the following two methods, taking into consideration the material and shape of the material before heating. The first method prioritizes preventing a temperature drop in areas where forging cracks are expected. If the time required to bond the heat-resistant insulating material to the material after heating is long, the temperature of the material after heating may decrease, potentially deteriorating its hot forgeability. Therefore, it is preferable to bond the heat-resistant insulating material to the surface of the material within the minimum necessary range for a time that does not impair hot forgeability. For example, when a material for hot forging 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). In the case of a polygonal columnar shape, the heat-resistant insulating material may be bonded to an area including the edge portion. In the case of a cylindrical shape, the heat-resistant insulating material may also be bonded to the side surface. In other words, it is preferable to bond the heat-resistant insulating material to areas where defects such as cracks are likely to occur during 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 those containing 20% ​​or more by volume of the γ' phase. In other words, by bonding the heat-resistant insulating material, it is possible to reduce the precipitation of fine γ' phase that occurs when the temperature of the hot forging material decreases, and it is also possible to promote recrystallization in the surface layer of the hot forging material, thereby reducing the occurrence of defects such as forging cracks. Furthermore, among nickel-based alloys, alloys with a wide temperature range that can be hot forged, such as 718 alloy and Waspaloy alloy, can maintain the heating temperature for a long period of time, which can contribute to reducing forging defects (cracks).

[0026] <Hot forging process> The hot forging material prepared by the above-mentioned steps is compressed in part or in whole 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 even difficult-to-work alloys into a predetermined shape. In the present invention, the hot forging process is preferably free forging. The material for hot forging during free forging is heavy, has a large heat dissipation area into the atmosphere, and requires a large amount of processing. Therefore, adhering a heat-resistant insulating material to the material for hot forging effectively suppresses the temperature drop of the material. 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 for hot forging, it is preferable to adhere the heat-resistant insulating material according to the present invention to at least a portion of the surface of the free-form portion of the material after heating that does not come into contact with the die, anvil, or tool during free forging. [Example]

[0027] A commonly known liquid glass lubricant (a glass lubricant containing approximately 50% by mass of glass particles (so-called borosilicate glass particles) composed of SiO2, B2O3, etc. in water) was used, and the binder component (styrene acrylic resin component) contained therein was adjusted to prepare liquid glass lubricants 1 to 3 shown in Table 1. Then, as a heat-resistant heat insulating material of inorganic fiber, liquid glass lubricants 1 to 3 in Table 1 were applied to the surface of Kaowool that would be bonded to the material after heating, and then dried to prepare the material.

[0028] [Table 1]

[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 glass corrosion on the surface of the material after heating was evaluated. Condition A: In the heating process, the surface of the material is coated with glass lubricant before heating. Condition B: In the heat-resistant insulation bonding process, the heat-resistant insulation material is bonded to the surface of the heated material removed from the heating furnace, and then 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 this 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 another 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 where glass lubricants 1 and 2 were applied after heating, 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] Example 2 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 in cross section, 1300 mm high) was ground to a medium finish using a grinder (abrasive grain size #16), which served as a pre-heating material. This pre-heating material was placed in a heating furnace (oxygen concentration in the furnace: approximately 9.0% by volume). After the entire pre-heating material reached the hot forging temperature of 1080°C, heating was maintained for at least four hours, and then the pre-heating material was removed from the heating furnace as a post-heating material. A heat-resistant insulating material, which had been coated with one of the glass lubricants 1 to 3 and dried, was attached to a portion of the peripheral surface of this post-heating material. This material was used as a hot forging material, and free forging was performed by pressing a die against the 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 successfully completed. The surface of the hot-forged material was then observed immediately after free forging.

[0034] In the hot-forged material, the heat-resistant insulation material that had been attached to the peripheral surface had worn away or peeled off, exposing the surface of the hot-forged material. While no flaws (i.e., typical forging flaws) were observed on the surface of the hot-forged material where the heat-resistant insulation material had not been attached, discoloration was observed in the exposed surface where the heat-resistant insulation material coated with glass lubricants 1 and 2 had been attached, confirming the presence of flaws (circled area in Figure 1). In the case of glass lubricant 1, the flaws observed 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 flaws were observed in the area where the heat-resistant insulating material coated with glass lubricant 3 was attached. The area where no discoloration or flaws were observed was then reheated to the hot forging temperature of 1080°C while still at a high temperature after hot forging. A heat-resistant insulating material coated with glass lubricant 3 was then attached, and the same free forging was repeated. The forging temperature (surface temperature) during hot forging was approximately 850°C to 1000°C. Repeating the above free forging process revealed normal forging flaws, but no flaws due to glass corrosion. (Figure 2 shows the surface of this area after the hot-forged material was air-cooled.)

Claims

1. a heating step of heating a pre-heated material of a nickel-based superalloy to be hot forged to a hot forging temperature in a heating furnace to form a post-heated material; a heat-resistant insulating material bonding step of bonding an inorganic fiber heat-resistant insulating material to at least a part of the surface of the heated material removed from the heating furnace to prepare a hot forging material; A hot forging process in which a part or the whole of the hot forging material is compressed and formed into a predetermined shape using a die, an anvil, or a tool; Including, A liquid glass lubricant containing no styrene-acrylic resin or containing less than 5% by mass of styrene-acrylic resin is applied to the surface of the inorganic fiber heat-resistant insulating material to be bonded to the post-heating material, and the surface is dried before being bonded to the post-heating material. Manufacturing method for hot forged materials.

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

3. The hot forging process is free forging, In the heat-resistant heat insulating material bonding step, the inorganic fiber heat-resistant heat insulating material is bonded to at least a part of the surface of a freely deformable portion of the heated material that does not come into contact with any of the die, anvil, and tool during the free forging. A method for producing a hot forged material according to claim 1.

4. In the heating step, the oxygen concentration in the heating furnace is 15.0% by volume or less. A method for producing a hot forged material according to claim 1.

5. The nickel-based superalloy contains 10 to 35 mass% of Cr. A method for producing a hot forged material according to claim 1.

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