Method for manufacturing hot-forged materials

Abstract

This invention suppresses surface defects that occur when a hot-forging material, which has a heat-resistant insulating material bonded to its surface, is hot-forged. The process includes: a heating step of heating the raw material to be hot forged to a hot forging temperature in a heating furnace to obtain the raw material after heating; a heat-resistant insulating material bonding step of bonding an inorganic fiber heat-resistant insulating material to at least a portion of the surface of the raw material after heating, which has been removed from the heating furnace, to obtain the raw material for hot forging; and a hot forging step of compressing a portion or all of the raw material for hot forging into a predetermined shape using a mold, anvil, or tool. A method for manufacturing a hot-forged material, comprising applying a liquid glass lubricant, which does not contain styrene-acrylic resin, or contains less than 5% by mass of styrene-acrylic resin, and does not contain sodium salt, or contains less than 0.5% by mass of sodium salt, to the surface of the above-mentioned inorganic fiber heat-resistant insulating material that will be bonded to the material after heating, allowing it to dry, and then bonding it to the above-mentioned material after heating.

Description

Technical Field

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

Background Art

[0002] When hot-forging a hot-forging material heated to the hot-forging temperature, there is a problem of deterioration of hot workability due to temperature drop of the hot-forging material. Therefore, various proposals for preventing temperature drop have been made conventionally. For example, a method has been proposed in which a heat-resistant insulation material of inorganic fiber is adhered to the surface of a heated material taken out from a heating furnace, and then this is used as a hot-forging material and hot-forged (Patent Document 1). And at this time, after applying a glass lubricant containing glass particles to the adhesion surface of the heat-resistant insulation material and the heated material (including spray coating by spraying), it has been proposed to dry the heat-resistant insulation material to which these glass particles are attached. Regarding the glass lubricant containing these glass particles, for example, a "liquid glass lubricant" containing glass particles composed of SiO2, B2O3, etc., a binder component (resin component), and water is common (Patent Document 2). And in addition, hot-working lubricants to which various trace components are added have been proposed (Patent Documents 3 and 4).

[0003] Regarding adhering the heat-resistant insulation material coated with the above liquid glass lubricant to the surface of the heated material, after adhering this heat-resistant insulation material to the surface of the heated material taken out from the heating furnace, it has been proposed to return this to the heating furnace and "reheat" it to enhance the adhesion of the heat-resistant insulation material to the surface of the heated material (Patent Document 5).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

[0005] The method described in Patent Document 1 is an effective method for suppressing the temperature drop of the material used for hot forging during hot forging. Incidentally, after hot forging is complete, the surface of the hot-forged material is smoothed to a nearly even state by machining processes such as grinding and polishing. However, when a material for hot forging with a heat-resistant insulating material bonded to its surface is hot-forged, some of the heat-resistant insulating material may peel off or wear away after the hot forging is complete. In addition, the surface of the hot-forged material to which the heat-resistant insulating material was bonded may exhibit "special defects" that differ in their mechanism of occurrence from ordinary forging defects. If these special defects are significantly large on the surface of the hot-forged material, it may be necessary to increase the grinding allowance as described above.

[0006] The present invention relates to a method for manufacturing a hot-forged material, which involves hot-forging a material for hot-forging on which a heat-resistant insulating material has been bonded to the surface, and aims to suppress defects that occur on the surface of the obtained hot-forged material due to the bonding of the heat-resistant insulating material. [Means for solving the problem]

[0007] In other words, the present invention is A heating process in which the material to be hot forged is heated to the hot forging temperature in a heating furnace to become the material after heating, A heat-resistant insulating material bonding process is performed to bond an inorganic fiber heat-resistant insulating material to at least a portion of the surface of the heated material removed from the above heating furnace to create a material for hot forging. A hot forging process in which a part or all of the above-mentioned hot forging material is compressed and shaped into a predetermined form using a mold, anvil, or tool, Includes, This is a method for manufacturing a hot-forged material, in which a liquid glass lubricant that does not contain styrene-acrylic resin, or contains less than 5% by mass of styrene-acrylic resin, and does not contain sodium salt, or contains less than 0.5% by mass of sodium salt, is applied to the surface of the above-mentioned inorganic fiber heat-resistant insulating material that will be bonded to the material after heating, and the lubricant is dried before being bonded to the above-mentioned material after heating. Furthermore, during the heating process described above, the oxygen concentration in the heating furnace can be set to 3.0% by volume or higher. Alternatively, the oxygen concentration can be set to 15.0% by volume or lower.

[0008] Furthermore, the present invention relates to a method for manufacturing hot-forged materials, wherein the hot-forging process is free forging, and in the heat-resistant insulating material bonding process, the heat-resistant insulating material is bonded to at least a portion of the surface of the freely deformed portion of the heated material that does not come into contact with the mold, anvil, or tool during the free forging process. The material of the pre-heating material described above can be a nickel-based alloy. Furthermore, this nickel-based alloy may contain Cr in the range of 10 to 35% by mass. [Effects of the Invention]

[0009] According to the present invention, in a method for manufacturing a hot-forged material in which a material for hot forging has a heat-resistant insulating material bonded 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 drawing]

[0010] [Figure 1] This is a photographic representation of an example of a defect on the surface of a hot-forged material caused by glass corrosion. [Figure 2] This is a photographic image showing an example of a defect caused by glass corrosion on the surface of another hot-forged material. [Figure 3]This is an external photograph showing an example of the surface of a hot-forged material obtained according to the present invention. [Figure 4] This is an external photograph showing an example of the surface of a hot-forged material obtained according to the present invention. [Modes for carrying out the invention]

[0011] The present invention will be described below step by step. In the following, "material before heating" refers to the material before it is placed in the heating furnace, "material after heating" refers to the material heated to the hot forging temperature in the heating furnace, "material for hot forging" refers to the material to which a heat-resistant insulating material has been bonded to a predetermined part, making it ready for hot forging, and "hot forged material" refers to the molded material formed into a predetermined shape by a hot forging device. Furthermore, "hot forging temperature" refers to the heating temperature of the material before heating to be hot forged.

[0012] <Heating process> First, in this invention, the raw material to be hot forged is heated to the hot forging temperature in a heating furnace. The raw material to be hot forged is not particularly limited to ingots, billets, rough materials, powder molded bodies, etc., but the effects of this invention can be best demonstrated with ingots, billets, etc., which are formed into a desired shape by free forging. Then, this raw material to be hot forged is heated to the hot forging temperature in a heating furnace to become the heated material. The hot forging temperature varies depending on the material of the raw material before heating, but it can be easily determined as needed. For example, for nickel-based alloys, which are known to be difficult to process, the temperature can be set to 950-1180°C. Furthermore, if the composition includes gamma-prime (γ') phase or gamma-double-prime (γ'') phase, and the total volume percentage of the γ' and γ'' phases is, for example, 10% or more by volume, 15% or more by volume, or 20% or more by volume, the temperature can be set to 1010-1180°C. For titanium alloys, the temperature can be set to 900-1180°C.

[0013] However, in the present invention, after this heating step, a heat-resistant heat-insulating material bonding step described below will be performed. In the heat-resistant heat-insulating material bonding step, a heat-resistant heat-insulating material is bonded to the heated material taken out from the heating furnace. It is preferable that the temperature drop of the heated material is zero until the heat-resistant heat-insulating material is bonded, but in reality, the temperature drops at least a little. Therefore, when heating the pre-heated material in the heating step, the pre-heated material may be heated to a temperature about 5 to 100 °C higher than the forging temperature (forging start temperature) when starting hot forging (for example, a temperature obtained by adding about 5 to 100 °C to the above numerical value of the hot forging temperature). By doing so, even if the temperature of the heated material after being taken out from the heating furnace drops by more than 100 °C with respect to the forging start temperature when the heat-resistant heat-insulating material bonding step is not performed, the temperature drop can be suppressed, and the forging temperature during hot forging can be kept high.

[0014] The surface roughness of the pre-heated material should be rougher than the finish machining. By this, when the heat-resistant heat-insulating material is bonded to its surface in the next heat-resistant heat-insulating material bonding step, a slight space is formed between the heat-resistant heat-insulating material and the heated material, and it can be expected that the air in this space functions as a heat-insulating layer. And the glass particles attached to the heat-resistant heat-insulating material side are likely to weld to the unevenness of the surface of the heated material. Of course, the surface finish as-cast or as-plastically-worked may also be used. However, in the case of a difficult-to-machine alloy, cracks or the like may occur on the surface due to the influence of added elements, etc. Therefore, surface defects that cause cracking during hot forging should be removed by machining such as grinding or polishing. Even when no cracks or the like are observed, in the portion where the heat-resistant heat-insulating material is bonded to its surface in the next heat-resistant heat-insulating material bonding step (that is, the portion where the glass lubricant is welded), it is preferable to make the surface of the pre-heated material rougher than the finish machining by machining.

[0015] <Heat-resistant heat-insulating material bonding step> The pre-heated material is heated to the hot forging temperature, and a heat-resistant heat-insulating material is bonded to at least a predetermined portion of the surface of the heated material taken out from the heating furnace to obtain a material for hot forging. First, the above heat-resistant and heat-insulating material is made of inorganic fibers. The "inorganic fibers" referred to in the present invention include glass fibers, ceramic fibers, etc., and it is preferable to select ceramic fibers with excellent heat-insulating properties. Among ceramic fibers, for example, KAOWOOL (registered trademark: hereinafter referred to as "Kaowool") is particularly preferable because it is easily available and inexpensive. As long as it is a heat-resistant and heat-insulating material made of inorganic fibers, even if the surface roughness of the material after heating is somewhat rough, combined with the effect of the adhesive by the glass lubricant applied thereto, it becomes easy to adhere along the surface shape, and the fibers are likely to catch on the unevenness of the surface of the material after heating, and since it is lightweight, for example, it is also easy to adhere to the side surface of the material after heating.

[0016] Also, as in the present invention, when the heat-resistant and heat-insulating material is adhered to at least a part of the surface of the material after heating taken out from the heating furnace, at the initial stage of hot forging, the heat-resistant and heat-insulating material is maintained on the surface of the material after heating in its complete state, and the temperature drop of the material for hot forging during hot forging can be suppressed. If the heat-resistant and heat-insulating material is arranged on the surface of the material before heating, which is before loading into the heating furnace, depending on the relationship between temperature and time, it will be in a state where it is easily crushed during transportation for hot forging, and it is difficult to suppress the above-mentioned temperature drop. And at the final stage of hot forging, when the peak for suppressing the above-mentioned temperature drop has passed, in the present invention, a part of the heat-resistant and heat-insulating material has peeled off or worn away, and when machining the surface of the hot-forged material obtained after completing hot forging, the labor for removing the heat-resistant and heat-insulating material can be saved. Also, at the final stage of hot forging, since a part of the above heat-resistant and heat-insulating material has peeled off or worn away, it is also possible to suppress excessive build-up (processing heat generation) of the material for hot forging. And in order to obtain such an effect, it is effective to use inorganic fibers of the above heat-resistant and heat-insulating material, particularly glass fibers or ceramic fibers.

[0017] Furthermore, in the heat-resistant insulation bonding process described above, a known method for easily and quickly bonding the heat-resistant insulation is to place a glass lubricant between the heat-resistant insulation 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 insulation that will be bonded to the heated material, and the heat-resistant insulation is then bonded to the designated location on the heated material. This method works by allowing glass particles in the glass lubricant to soften due to the heat retained on the material surface after heating, thereby bonding the heat-resistant insulating material to the heated material. Therefore, it is effective for hot forging of nickel-based superalloys and other materials that require high hot forging temperatures. One method for attaching glass particles to the heat-resistant insulating material is to apply a liquid glass lubricant containing glass particles to the surface of the heat-resistant insulating material that will adhere to the heated material, either by brushing or spraying. Spraying is preferable because it allows for uniform adhesion of glass particles to the surface of the heat-resistant insulating material that will adhere to the heated material. Furthermore, after applying the glass lubricant, it is preferable to allow the heat-resistant insulating material with the glass particles attached to it to dry, both in terms of ensuring sufficient adhesion of the glass particles and preventing, for example, glass corrosion of the material described later. Also, by allowing the heat-resistant insulating material with the glass particles attached to dry, when the heat-resistant insulating material is bonded to the surface of the material after heating, the rapid evaporation of volatile components such as binders contained in the glass lubricant can be suppressed. In this respect, directly applying the glass lubricant to the surface of the material after heating may lead to the rapid evaporation of the volatile components mentioned above.

[0018] However, according to the present invention, even if the temperature drop of the material for hot forging can be suppressed by bonding the above-mentioned heat-resistant insulating material to the surface of the material after heating, it has been found that when observing the hot-forged material after hot forging is completed, a "special defect" with a different mechanism of occurrence than ordinary forging defects can be observed on the surface to which the heat-resistant insulating material was bonded. Through diligent research into the mechanism of occurrence of this special defect, it was discovered that it is caused by "glass corrosion" occurring at the three-phase interface between the material, the glass lubricant, and the forging atmosphere during forging.

[0019] First, when observing the surface of hot-forged materials after they had been air-cooled (allowed to cool), it was found that in some cases, no defects (i.e., normal forging defects) were present on surfaces where the heat-resistant insulating material was not adhered, while defects (i.e., special defects) were present on surfaces where the heat-resistant insulating material was adhered (circled area in Figure 1). This result indicates that the special defects were not forging cracks caused by normal temperature drops. Further investigation into these special defects revealed that they were caused by the aforementioned glass corrosion, and therefore, it was determined that the occurrence of glass corrosion could be suppressed by identifying the type of glass lubricant applied to the heat-resistant insulating material.

[0020] In other words, the glass lubricant applied to the heat-resistant insulating material is in liquid form and contains, for example, glass particles composed of SiO2 or B2O3, a binder component (resin component), and water. When the heat-resistant insulating material, after being coated with this glass lubricant, is to be bonded to the surface of the material after heating, the glass lubricant should be allowed to dry. However, even if the water is removed from the dried glass lubricant, if the remaining resin component is flammable, it is conceivable that it could come into contact with the material heated to the hot forging temperature and react (burn) with oxygen in the environment before and during forging, causing glass corrosion to progress on the material. Therefore, after investigating various glass lubricants, it was found that the "styrene-acrylic resin (polymer)" contained therein is highly flammable, and that limiting its content is effective in suppressing the progression of the above-mentioned glass corrosion. That is, the glass lubricant according to the present invention, in its liquid state before application, does not contain styrene-acrylic resin, or if it does, 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, if the dried glass lubricant contains a large amount of sodium salt, it is conceivable that this will decompose when heated to the hot forging temperature, forming a corrosive atmosphere and promoting corrosion of the material surface after heating. Therefore, after investigating various glass lubricants, it was found that limiting the amount of "sodium salt" contained in them is also effective in suppressing the progression of the above-mentioned glass corrosion. In other words, the glass lubricant according to the present invention, in its liquid state before application, does not contain sodium salt, or if it does, contains less than 0.5% by mass. Preferably it is less than 0.4% by mass, and more preferably less than 0.3% by mass. Typical sodium salts include sodium salt of carboxymethylcellulose.

[0022] Furthermore, even if the glass lubricant according to the present invention has sufficiently limited content of the styrene-acrylic resin and sodium salt mentioned above, it is assumed that it may still contain a considerable amount of other flammable and decomposable components. Therefore, after the heat-resistant insulating material coated with this glass lubricant is adhered to the surface of the material after heating, it should not be maintained in this state for an extended period. For example, the next hot forging process can be initiated within approximately 5 minutes of adhering the heat-resistant insulating material to the surface of the material after heating. In other words, if the glass lubricant is in contact with the surface of the material after heating at the hot forging temperature for an extended period, it is conceivable that this could lead to glass corrosion. Therefore, in order to improve the adhesion of the heat-resistant insulating material to the material after heating, it is preferable to omit the pre-coating of the surface of the material before heating in the heating process, even if the oxygen concentration in the heating furnace is reduced. Also, after adhering the heat-resistant insulating material to the surface of the material after heating in the heat-resistant insulating material bonding process, it is preferable to proceed quickly to the hot forging process described later without reheating.

[0023] Furthermore, with the present invention, since the coating of the material surface with glass lubricant before heating can be omitted in the heating process described above, the oxygen concentration in the heating furnace can be kept relatively high, as glass corrosion cannot occur. For example, the oxygen concentration in the heating furnace can be set to 3.0% or higher, 4.0% or higher, or even 5.0% or higher by volume. The upper limit can be, for example, 15.0% or lower, 10.0% or lower, or 8.0% or lower. As a result, the heating furnace does not need to be equipped with special equipment for, for example, creating a vacuum inside the heating furnace or replacing it with an inert gas atmosphere. Regarding the regulation of oxygen concentration in the heating furnace, for example, if the material of the raw material before heating is a nickel-based alloy, and especially if it is a nickel-based superalloy with a component composition containing γ' or γ'' phases, then it is likely that most alloys contain Cr in the range of 10 to 35 mass%. In this case, the Cr in the nickel-based alloy reacts with oxygen to form a passivation film, which is expected to suppress the oxidation of the alloy. However, if the oxygen concentration in the heating furnace is too high, the formation of the above-mentioned passivation film may not keep up. From this point of view as well, the upper limit of the oxygen concentration in the heating furnace can be set to the value mentioned above.

[0024] In this heat-resistant insulation bonding process, the portion of the heated material to which the heat-resistant insulation is bonded may be only a part of its surface or the entire surface. Furthermore, the bonding state between the heat-resistant insulation and the heated material does not necessarily have to be continuous across the entire bonding surface. Even if there are areas of partial bonding (localized adhesion or welding), it is sufficient as long as the adhesion is such that the heat-resistant insulation does not peel off significantly during use and the heat insulation function and temperature retention function during hot forging are not substantially impaired (for example, if approximately 50% or more of the area of ​​the bonding portion between the heat-resistant insulation and the heated material is in close contact), as this will be bonded during the initial forging process when free forging is started. When deciding which part to bond this heat-resistant insulation material to, it is best to choose from the following two methods, taking into consideration the material and shape of the material before heating. The first method is to prioritize preventing temperature drops in areas where forging cracks are expected. If the time spent bonding the heat-resistant insulation material to the material after heating is too long, the temperature of the material may drop after heating, potentially degrading its hot forgeability. Therefore, it is preferable to bond the heat-resistant insulation material to the surface of the material in the minimum necessary area within a time that does not impair its hot forgeability. For example, when a material for hot forging is placed in a hot forging machine, if there is concern about heat dissipation to the lower die (lower metal plate or lower tool), the heat-resistant insulation material may be bonded to the surface in contact with the lower die (lower metal plate or lower tool), or, if it is a polygonal columnar shape, it may be bonded to the area including the edges. If it is cylindrical, it may be bonded to its sides. In short, it is best to bond the heat-resistant insulation material to areas that are prone to defects such as cracks during hot forging.

[0025] The second method involves bonding a heat-resistant insulating material to at least a portion of the surface of the freely deformable parts of the material after heating. This method primarily aims to reduce the temperature drop in parts that are not in contact with the upper die (upper anvil or upper tool) or lower die (lower anvil or lower tool), which are cooled in the atmosphere, for example, when hot forging is free forging.

[0026] The two methods described above are particularly effective for nickel-based alloys known as difficult-to-process alloys, such as nickel-based superalloys with a composition containing γ' and γ'' phases. They are especially effective for nickel-based superalloys with a composition in which the total volume percentage of γ' and γ'' phases is 10 vol% or more, 15 vol% or more, or 20 vol% or more. In other words, by bonding the heat-resistant insulating material, it is possible to reduce the precipitation of fine γ' and γ'' phases as the temperature of the hot forging material decreases, and to promote the recrystallization of the surface layer of the hot forging material, thereby reducing the occurrence of defects such as forging cracks. Furthermore, among nickel-based alloys, such as 718 alloy and Waspaloy alloy, which have a wide temperature range in which hot forging is possible, this technology can maintain the heating temperature, thereby contributing to the reduction of forging defects (cracking).

[0027] <Hot forging process> Using the hot forging material prepared through the above-described processes, a part or all of this hot forging material is compressed and shaped into a predetermined form using a mold, anvil, or tool. The forging apparatus used is preferably a large hot forging apparatus with a forging load of several thousand tons or more that can form even difficult-to-process alloys into a predetermined shape. Furthermore, in the present invention, it is preferable that the hot forging process described above is free forging. When free forging is performed, the material used for hot forging is heavy, has a large surface area for heat dissipation into the atmosphere, and involves a large amount of processing. Therefore, bonding a heat-resistant insulating material is highly effective in suppressing the temperature drop of the material used for hot forging. In this case, as mentioned above, if hot forging is performed on a general nickel-based alloy with a relatively wide temperature range in which hot forging is possible, such as 718 alloy or Waspaloy alloy, it is preferable to bond the heat-resistant insulating material according to the present invention to at least a part of the surface of the freely deformable portion of the heated material that does not come into contact with any of the molds, anvils, or tools during free forging.

[0028] After the hot forging described above, the heat-resistant insulating material that was adhered to the surface of the obtained hot-forged material may be worn away or peeled off. In such cases, the hot-forged material can be reheated to the hot-forging temperature while it is still at a high temperature after the hot forging, and then a heat-resistant insulating material coated with the glass lubricant according to the present invention can be attached, and an additional hot forging can be performed. When the hot-forged material is reheated to the hot-forging temperature, even if some glass lubricant remains on the surface of the hot-forged material, this does not hinder the achievement of the effects of the present invention, as it significantly suppresses the occurrence of defects that may be a problem in the initial (main) hot forging. [Examples]

[0029] Using a commonly known liquid glass lubricant (a glass lubricant containing approximately 50% by mass of glass particles composed of SiO2, B2O3, etc. (so-called borosilicate glass particles) in water), the binder component (styrene-acrylic resin component) and sodium salt (sodium salt of carboxymethylcellulose) contained therein were adjusted to prepare liquid glass lubricants 1 to 3 shown in Table 1. Then, as a heat-resistant insulating material made of inorganic fibers, we prepared a material by applying liquid glass lubricants 1-3 from Table 1 to the surface of the KaoWool that would be bonded to the material after heating, and then drying it.

[0030] [Table 1]

[0031] (Example 1) In Example 1, as a preliminary experiment, the effect of the following conditions A and B on glass corrosion on the heated material surface before the hot forging process was evaluated. Condition A: In the heating process, the surface of the material before heating is coated with glass lubricant. Condition B: In the heat-resistant insulation bonding process, after the heat-resistant insulation material is bonded to the surface of the heated material removed from the heating furnace, it is reheated.

[0032] First, a cylindrical billet made of Waspaloy (355 mm cross-sectional diameter, 800 mm height) was turned to a smooth finish, and this was used as the raw material before heating. Then, liquid glass lubricants 1 to 3 from Table 1 were brushed onto a portion of the surface of this raw material before heating, and after drying, it was placed in a heating furnace (oxygen concentration in the furnace: approximately 9.0 vol%). The entire raw material before heating reached a hot forging temperature of 1080°C and was heated for more than 4 hours (Condition A). Next, the raw material maintained at this heating temperature was removed from the heating furnace, and heat-resistant insulating material coated with the above-mentioned glass lubricant was attached (adhered) to the surface of the raw material that did not have the liquid glass lubricant applied. Then, it was returned to the heating furnace and heated for another 30 minutes or more (Condition B). Finally, the reheated raw material was removed from the heating furnace as the post-heated material or material for hot forging, air-cooled, and its surface was observed.

[0033] Observations revealed that, first, under condition A, clear discoloration was observed on the circumferential surfaces of the heated material where glass lubricants 1 and 2 were applied, indicating glass corrosion. Slight discoloration was also observed on the circumferential surface where glass lubricant 3 was applied, indicating localized glass corrosion. Next, under condition B, clear discoloration was observed on all circumferential surfaces of the heated material where the heat-resistant insulating material was adhered, indicating glass corrosion regardless of the type of glass lubricant used (1-3). Therefore, as in the embodiments of conditions A and B, it is preferable to avoid prolonged contact of the glass lubricant with the material surface at hot forging temperatures, in order to suppress glass corrosion.

[0034] (Example 2) In Example 2, based on the results of Example 1, the hot forging process was carried out without coating the surface of the material before heating with glass lubricant during the heating process, and without reheating the material for hot forging after the heat-resistant insulating material was bonded during the heat-resistant insulating material bonding process. The type of defects on the surface of the hot forged material obtained by hot forging was then examined.

[0035] First, the circumferential surface of a Waspaloy rectangular billet (490mm square cross-section, 1300mm height) was polished to a standard finish using a grinder (abrasive grit size #16), and this was used as the material before heating. This material before heating was placed in a heating furnace (oxygen concentration inside the furnace: approximately 9.0 vol%), and after the entire material before heating reached the hot forging temperature of 1080°C, it was heated for more than 4 hours, and then removed from the heating furnace as the material after heating. Then, heat-resistant insulating material, which had been coated with each of the above glass lubricants 1 to 3 and dried, was attached (bonded) to a part of the circumferential surface of this material for hot forging, and free forging was performed by pressing a die against this circumferential surface. When the above heat-resistant insulating material was attached to a part of the circumferential surface of the material after heating, there was no significant peeling of the heat-resistant insulating material, and the time from attaching the heat-resistant insulating material to proceeding to free forging was approximately 4 minutes. The forging temperature (surface temperature) during hot forging was approximately 850°C to 1000°C. In all cases, the predetermined free forging process was completed. The surface of the hot-forged material after this free forging was then observed.

[0036] In the hot-forged material described above, the heat-resistant insulating material that was adhered to its circumferential surface had worn away or peeled off, exposing the surface of the hot-forged material. No defects (i.e., normal forging defects) were observed on the surface of the hot-forged material where the heat-resistant insulating material was not adhered. However, on the exposed surface, discoloration was observed in the areas where the heat-resistant insulating material coated with glass lubricants 1 and 2 had been adhered, indicating defects. Figures 1 and 2 show examples of defects observed on the surface of the hot-forged material after the hot forging process was completed and it was air-cooled (allowed to cool) for each case of glass lubricant 1 and 2 (circled areas). In the case of glass lubricant 1, the defects observed were approximately 6% of the area per unit surface area on the surface of the hot-forged material to which the heat-resistant insulating material was attached, due to the high content of styrene-acrylic resin in glass lubricant 1. Furthermore, the defects found in glass lubricant 2 were due to the fact that, although glass lubricant 2 had reduced amounts of styrene-acrylic resin, it still contained a considerable amount of sodium salt, resulting in defects of approximately 9 area percentage per unit surface area on the surface to which the heat-resistant insulation material was attached to the hot-forged material.

[0037] On the other hand, in the case of glass lubricant 3 without added styrene-acrylic resin and sodium salt, no discoloration was observed in the area where the heat-resistant insulation material coated with glass lubricant 3 was adhered, and no major defects caused by glass corrosion were observed (Figure 3 shows the surface of this area after the hot-forged material has been air-cooled). For the areas where no discoloration or defects were observed, the material was reheated to a hot forging temperature of 1080°C while still at a high temperature after the hot forging process. A heat-resistant insulating material coated with glass lubricant 3 was then attached, and the same free forging process was repeated. The forging temperature (surface temperature) during hot forging was approximately 850°C to 1000°C. Even after repeating the free forging process, while normal forging defects were observed, no major defects caused by glass corrosion were found (Figure 4 shows the surface of this area after the hot forged material has been air-cooled).

Claims

1. A heating process in which the material to be hot forged is heated to the hot forging temperature in a heating furnace to become the material after heating, A heat-resistant insulating material bonding step is performed to bond an inorganic fiber heat-resistant insulating material to at least a portion of the surface of the heated material removed from the heating furnace to create a material for hot forging. A hot forging process in which a part or all of the material for hot forging is compressed and shaped into a predetermined shape using a mold, an anvil, or tool, Includes, The surface of the inorganic fiber heat-resistant insulating material that adheres 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, and does not contain sodium salt, or contains less than 0.5% by mass of sodium salt, and is allowed to dry before being bonded to the heated material. A method for manufacturing hot-forged materials.

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

3. In the heating step, the oxygen concentration in the heating furnace is 15.0% by volume or less. The method for manufacturing a hot-forged material according to claim 2.

4. The aforementioned hot forging process is free forging, In the heat-resistant insulating material bonding step, the inorganic fiber heat-resistant insulating material is bonded to at least a portion of the surface of the freely deformed portion of the heated material that does not come into contact with the mold, anvil, or tool during the free forging process. A method for manufacturing a hot-forged material according to claim 1.

5. The material of the material before heating is a nickel-based alloy. A method for manufacturing a hot-forged material according to claim 1.

6. The nickel-based alloy contains Cr in the range of 10 to 35% by mass. The method for manufacturing a hot-forged material according to claim 5.

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