Manufacturing method for exhaust valve spindle of marine engin

Abstract

A method for manufacturing an exhaust valve spindle for a marine engine having a head portion and a stem portion is provided, comprising the steps of: forming a preliminary head portion at the tip of a billet in the form of a round bar having a predetermined length; processing the surface of the preliminary head portion before welding; welding a first alloy portion containing nickel to the preliminary head portion; forming the preliminary head portion into a shape corresponding to the head portion by processing the preliminary head portion with the first alloy portion welded thereto through a hot forging method using a forging die; processing the seat surface of the preliminary head portion processed by the hot forging method before welding; forming a valve seat by welding a second alloy portion containing nickel to the seat surface of the preliminary head portion; and completing the head portion by machining the preliminary head portion with the formed valve seat.

Description

Technology Field

[0001] The embodiments disclosed in this specification relate to a method for manufacturing an exhaust valve spindle for a marine engine, and more specifically, to a method for manufacturing a spindle made of a high heat-resistant alloy and used as an exhaust valve for a marine engine. Background Technology

[0003] The exhaust valve spindle, which is one of the engine parts generally used in large vessels such as container ships, tankers, or bulk carriers, is a part designed to open and close the exhaust port of an engine. It functions to maintain airtightness of the cylinder by moving upward to close the exhaust port when the mixed fuel explodes inside the engine cylinder, and to discharge combustion gases from the cylinder into the exhaust port by moving downward after the explosion to open the exhaust port.

[0004] Such exhaust valve spindles consist of a head section that opens and closes the exhaust port and a stem section that guides the reciprocating motion of the head section.

[0005] Here, the exhaust valve spindle operates at a pressure of over 700 bar and a high temperature of over 600°C during the explosion and combustion strokes inside the engine, which causes serious damage to the cylinder head. In some cases, if it is damaged inside the engine, the explosion pressure decreases and incomplete combustion occurs, leading to a problem where the engine's output decreases.

[0006] Meanwhile, recently, exhaust valve spindles have been manufactured using Nimonic 80A, a nickel (Ni) alloy with excellent heat resistance, but this material contains more than 70% expensive nickel, so it is very expensive and takes a long time to procure the material.

[0007] Accordingly, recently, as disclosed in Korean Patent Publication No. 10-2014-0047451, products are being manufactured in which a nickel alloy is welded to the bottom surface of the head section and the seat surface, which are areas where damage due to high heat occurs.

[0008] This conventional technology is configured to form a head portion by die-forging a base material and then welding an alloy to the formed head portion to form a welded portion.

[0009] However, the aforementioned conventional technology has a limitation in that the weld must be performed as a multi-layered, thick weld to prevent defects from occurring in the weld area. Consequently, there are problems in production due to strict standards regarding defects such as porosity that may occur during welding.

[0010] Therefore, technology was needed to solve the aforementioned problems.

[0011] Meanwhile, the aforementioned background technology is technical information that the inventor possessed for the derivation of the present invention or acquired during the process of deriving the present invention, and it cannot be considered as prior art disclosed to the general public prior to the filing of the present invention. The problem to be solved

[0013] The embodiments disclosed in this specification aim to provide a method for manufacturing an exhaust valve spindle for a marine engine that can secure heat resistance, corrosion resistance, and wear resistance by welding an alloy part to the head part.

[0014] In addition, the embodiments disclosed in this specification aim to provide a method for manufacturing an exhaust valve spindle for a marine engine that can secure mechanical properties such as corrosion resistance, heat resistance, and wear resistance while minimizing the occurrence of defects such as porosity by welding the alloy part before forming the head part through hot forging and then performing hot forging.

[0015] Furthermore, the embodiments disclosed in this specification aim to provide a method for manufacturing an exhaust valve spindle for a marine engine that simplifies the manufacturing process and minimizes welding defects by forming a pre-head portion by producing a pre-formed product for the head portion in a base material area through an electric upset process, welding an alloy portion to the pre-head portion, and then performing hot forging of the pre-head portion with the welded alloy portion using a forging die.

[0016] In addition, the embodiments disclosed in this specification aim to provide a method for manufacturing an exhaust valve spindle for a marine engine in which the alloy part can be formed in an accurate ratio on the surface of the pre-head part by ensuring that the base material and the alloy part forming the pre-head part each have an accurate volume.

[0017] In addition, the embodiments disclosed in this specification aim to provide a method for manufacturing an exhaust valve spindle for a marine engine, wherein the alloy portion welded to the pre-head portion can be welded in a shape that allows for even plastic deformation during forging.

[0018] In addition, the embodiments disclosed in this specification aim to provide a method for manufacturing an exhaust valve spindle for a marine engine, wherein the alloy part welded to the pre-head portion can be heated in a short time within a range where no degradation of physical properties occurs during forging. means of solving the problem

[0020] A method for manufacturing an exhaust valve spindle for a marine engine having a head portion and a stem portion according to one embodiment as a technical means for achieving the above-described technical problem may include: a step of forming a preliminary head portion at the tip of a billet in the form of a round bar having a predetermined length; a step of processing the surface of the preliminary head portion before welding; a step of welding a first alloy portion containing nickel to the preliminary head portion; a step of forming the preliminary head portion into a shape corresponding to the head portion while processing the preliminary head portion with the first alloy portion welded thereto by a hot forging method through a forging die; a step of processing the seat surface of the preliminary head portion processed by the hot forging method before welding; a step of forming a valve seat by welding a second alloy portion containing nickel to the seat surface of the preliminary head portion; and a step of completing the head portion by machining the preliminary head portion with the valve seat formed thereon.

[0021] In addition, the step of forming the preliminary head portion may be performed by processing the billet with an electric upsetting device to expand the outer diameter of the leading edge of the billet and forming the preliminary head portion at the leading edge of the billet.

[0022] In addition, the step of forming the preliminary head portion may expand the outer diameter of the billet so that the volume of the leading edge of the billet corresponds to the volume of the area occupied by the base material forming the billet in the head portion.

[0023] Additionally, the step of welding the first alloy part may include: a step of welding a bottom alloy part that forms a part of the first alloy part and wraps around the bottom surface of the pre-head part; and a step of welding a side alloy part that forms an onion shape together with the bottom alloy part and wraps around a part of the side of the pre-head part.

[0024] In addition, the bottom alloy part can be welded to form a cross-sectional area that is relatively thicker than the side alloy part.

[0025] In addition, the bottom alloy part and the side alloy part may be formed with different nickel contents.

[0026] In addition, the bottom alloy part may contain a relatively larger amount of nickel than the side alloy part.

[0027] Additionally, the step of processing by the hot forging method may include: a step of preheating the pre-head portion, on which the first alloy portion is welded, at a temperature of 1100 to 1150°C for a period of time within 1 hour; a step of forging the preheated pre-head portion using the forging die; and a step of cooling the forged pre-head portion. Effects of the invention

[0029] According to any one of the aforementioned means for solving the problem, a method for manufacturing an exhaust valve spindle for a marine engine can be provided, which can secure heat resistance, corrosion resistance, and wear resistance by welding a first alloy part to the head part.

[0030] In addition, according to any one of the aforementioned means for solving the problem, a method for manufacturing an exhaust valve spindle for a marine engine can be presented in which a first alloy part is welded before the head part is formed by hot forging, and then hot forging is performed, thereby minimizing the occurrence of defects such as pores while securing mechanical properties such as corrosion resistance, heat resistance, and wear resistance.

[0031] In addition, according to any one of the aforementioned means for solving the problem, a pre-head portion is formed in a base material area through an electric upset process, a first alloy portion is welded to the pre-head portion, and then hot forging is performed on the pre-head portion with the first alloy portion welded through a forging die, thereby simplifying the manufacturing process and minimizing welding defects.

[0032] In addition, according to any one of the aforementioned means for solving the problem, the base material forming the preliminary head part and the first alloy part each form an accurate volume, so that the first alloy part can be formed on the surface of the preliminary head part in an accurate ratio.

[0033] In addition, according to any one of the aforementioned means for solving the problem, since the first alloy part welded to the pre-head part forms an onion-shaped cross-section and is welded in a manner that surrounds the pre-head part, the effect of even plastic deformation during hot forging can be expected.

[0034] In addition, according to any one of the aforementioned means for solving the problem, the bottom alloy part forming the first alloy part is formed with a cross-sectional area that is relatively thicker than the side alloy part and contains a relatively larger amount of nickel than the side alloy part, so the bottom surface to which high heat and high pressure are applied can be formed more robustly.

[0035] In addition, according to any one of the aforementioned means for solving the problem, the first alloy part to be welded to the pre-head part is preheated to a temperature of about 1150°C during forging and then processed, so it can be heated in a short time within a range where no deterioration of the physical properties of the welded part occurs.

[0036] The effects obtainable from the disclosed embodiments are not limited to those mentioned above, and other unmentioned effects will be clearly understood by those skilled in the art to which the disclosed embodiments belong from the description below. Brief explanation of the drawing

[0038] FIG. 1 is a flowchart illustrating a method for manufacturing an exhaust valve spindle for a ship engine according to one embodiment. FIG. 2 is a schematic diagram showing a method for manufacturing an exhaust valve spindle for a ship engine according to one embodiment. FIG. 3 is a schematic diagram showing the step of forming a preliminary head portion at the leading end of a billet in a method for manufacturing an exhaust valve spindle for a ship engine according to one embodiment. FIG. 4 is a longitudinal cross-sectional view showing a pre-head portion with the first alloy portion welded thereto. FIG. 5 is a flowchart showing the hot forging step in a method for manufacturing an exhaust valve spindle for a ship engine according to one embodiment. Specific details for implementing the invention

[0039] Various embodiments are described in detail below with reference to the attached drawings. The embodiments described below may be implemented in various different forms. In order to explain the features of the embodiments more clearly, detailed descriptions of matters widely known to those skilled in the art to which the following embodiments belong have been omitted. Additionally, parts of the drawings unrelated to the description of the embodiments have been omitted, and similar parts throughout the specification have been given similar reference numerals.

[0040] Throughout the specification, when a configuration is described as being "connected" to another configuration, this includes not only cases where they are "directly connected," but also cases where they are "connected with another configuration in between." Furthermore, when a configuration is described as "including" another configuration, this means that, unless specifically stated otherwise, it does not exclude other configurations but may include additional configurations.

[0041] The embodiments will be described in detail below with reference to the attached drawings.

[0042] FIG. 1 is a flowchart showing a method for manufacturing an exhaust valve spindle for a ship engine according to one embodiment, FIG. 2 is a schematic diagram showing a method for manufacturing an exhaust valve spindle for a ship engine according to one embodiment, FIG. 3 is a schematic diagram showing a step of forming a pre-head portion at the leading edge of a billet in a method for manufacturing an exhaust valve spindle for a ship engine according to one embodiment. FIG. 4 is a longitudinal cross-sectional view showing a pre-head portion welded with a first alloy portion, and FIG. 5 is a flowchart showing a step of hot forging in a method for manufacturing an exhaust valve spindle for a ship engine according to one embodiment.

[0043] A method for manufacturing an exhaust valve spindle for a ship engine according to one embodiment is a method for manufacturing an exhaust valve spindle to be installed in the engine of a large ship such as a container ship, a tanker, or a bulk carrier to open and close an exhaust port according to the stroke of an engine piston.

[0044] Here, the exhaust valve spindle may be configured to include a head portion (10) and a stem portion (20) as shown in S700 of FIG. 2. During the combustion stroke of the engine, the head portion (10), which forms an umbrella shape, rises to shield the exhaust port through the seat surface (upper surface), and after the combustion stroke, the head portion (10) descends to open the exhaust port, and the stem portion (20) can guide the movement of the head portion (10).

[0045] A method for manufacturing an exhaust valve spindle for a ship engine (S10) according to one embodiment may be performed by including the steps of forming a preliminary head portion at the leading edge of a billet (S100), processing the surface of the preliminary head portion (S200), welding a first alloy portion to the preliminary head portion (S300), forming the preliminary head portion by hot forging (S400), processing the seat surface of the preliminary head portion (S500), welding a second alloy portion to the seat surface of the preliminary head portion (S600), and completing the head portion (S700), as illustrated in FIGS. 1 and 2.

[0046] The step (S100) of forming a preliminary head portion at the leading edge of a billet is a step for forming a preliminary head portion (100) at the leading edge of a billet (1) having a predetermined length.

[0047] The billet (1) can be manufactured by processing the base material forming the valve spindle into a round bar shape and then cutting it to a predetermined length. Here, the base material constituting the billet (1) can be composed of an alloy material that does not contain expensive nickel, for example, a stainless steel alloy material.

[0048] The step (S100) of forming a preliminary head portion can form the front end of the billet (1) into a lump shape by processing the front end of the billet (1) through an electric upsetting device (50) as shown in FIG. 3 to expand the outer diameter of the front end of the billet (1).

[0049] Here, electric upsetting is a process that induces plastic deformation by heating to the recrystallization temperature and applying pressure. It is a technique that utilizes the principle of bulging, where cross-sectional shape expands due to lateral flow, and can ultimately be formed into a cylinder shaped like an onion or a door handle. This electric upsetting process enables continuous forming operations by using round bars, minimizes scale formation due to heating, allows for precise control of the volume of the formed part, maintains the continuity of the material's grain flow, and results in fine grains, making it a forming method with excellent fatigue strength and mechanical properties for valve spindles.

[0050] At this time, the step (S100) of forming the pre-head portion can be performed using an electric upsetting device (50) as shown in FIG. 3, and the electric upsetting device (50) can be configured to include a clamp electrode (51), an anvil (52), a pressure cylinder (53), and a retraction cylinder (54) as shown in FIG. 3.

[0051] The clamp electrode (51) can perform the function of one of the electrodes for heating the billet (1) while movably supporting the billet (1) for an upset operation. Specifically, the clamp electrode (51) is formed with left and right divisions to guide the movement of the billet (1) while holding the billet (1) so that it comes into contact with the anvil (52) described later, and can be manufactured mainly using copper alloys such as phosphor bronze and beryllium copper to ensure high electrical conductivity and tensile strength.

[0052] The anvil (52) can support the tip of the billet (1) while performing the function of an electrode together with the clamp electrode (51). This anvil (52) can be manufactured using a material that has low thermal conductivity, excellent electrical conductivity, and minimal deformation at high temperature and high pressure. Since the surface of the anvil is damaged when subjected to high temperature and high pressure, the damaged area can be reprocessed and reused after a certain period of use.

[0053] The pressure cylinder (53) is a component that presses the billet (1) toward the anvil (52), and is movably positioned toward the rear end of the billet (1) to support the rear end of the billet (1), and can press the billet (1) while moving toward the anvil (52).

[0054] The retraction cylinder (54) is a component that moves together with the pressure cylinder (53) while movably supporting the anvil (52) to process the leading edge of the billet (1). Specifically, the retraction cylinder (54) moves in the same direction as the pressure cylinder (53) while supporting the anvil (52), but moves at a relatively slower speed than the pressure cylinder (53), thereby expanding the leading edge diameter of the billet (1) through the speed difference.

[0055] Specifically, the step of forming a preliminary head (S100) can fix a billet (1) to a clamp electrode (51) constituting an electric upsetting device (50) as shown in FIG. 3 (S110).

[0056] At this time, in step S110, after fixing the billet (1) to the clamp electrode (51), the pressure cylinder (53) can be operated to bring the leading edge of the billet (1) into close contact with the anvil (52), and the retraction cylinder (54) can be brought into close contact with the anvil (52).

[0057] And after step S110 is performed, the billet (1) can be heated by applying current to the clamp electrode (51) and the anvil (52), respectively (S120).

[0058] Accordingly, the portion between the clamp electrode (51) and the anvil (52) can be heated to a high temperature by supplying current to the clamp electrode (51) and the anvil (52) of the billet (1).

[0059] At this time, in step S120, the current can be controlled by providing current to the anvil (52) and the clamp electrode (51), and the current can be uniformly controlled by detecting changes in the current according to changes in the distance between the clamp electrode (51) and the anvil (52).

[0060] And, after step S120 is performed, the pressure cylinder (53) can be operated to move the billet (1) toward the anvil (52) while pressing the billet (1) toward the anvil (52) and moving the retraction cylinder (54) (S130).

[0061] At this time, in step S130, the pressurizing cylinder (53) can pressurize the billet (1) while moving toward the anvil (52), and the retraction cylinder (54) can move the anvil (52) by moving in the same direction as the pressurizing cylinder (53) but at a slower speed than the pressurizing cylinder (53).

[0062] Accordingly, the billet (1) is continuously heated by the current of the anvil (52) and clamp electrode (51) and pressed by the retraction cylinder (54), so that the outer diameter of the tip gradually expands and can be formed into a lump.

[0063] More specifically, the billet (1) is heated by electrical resistance heat generated between the clamp electrode (51) and the anvil (52) forming the + and - electrodes. As the flow stress of the heated billet (1) rapidly decreases, the outer diameter of the leading edge of the billet (1) expands due to the speed difference between the pressurizing cylinder (53) and the retraction cylinder (54), thereby forming a preliminary head portion (100).

[0064] Meanwhile, in the step (S100) of forming the preliminary head portion, the outer diameter of the front portion of the billet (1) can be expanded so that the volume of the front portion of the billet (1) being expanded corresponds to the volume of the area occupied by the base material in the head portion (10) of the exhaust valve spindle.

[0065] That is, the step (S100) of forming the preliminary head portion can first calculate the volume of the area occupied by the base material in the head portion (10), and then form the preliminary head portion (100) by expanding the outer diameter of the leading edge of the billet (1), so that the outer diameter and length of the preliminary head portion (100) correspond to the calculated volume. To this end, the volume of the area occupied by the base material and the alloy portion in the head portion (10) can be calculated by utilizing modeling data for the head portion (10) or modeling data for the forging die (30).

[0066] The step (S200) of processing the surface of the pre-head portion can be performed by cutting the surface of the pre-head portion (100) before welding the alloy portion (200) to the pre-head portion (100).

[0067] The step of welding the first alloy part to the pre-head portion (S300) is a step of welding the alloy part prior to the hot forging process (S400) by welding the first alloy part (200) containing nickel to the pre-head portion (100).

[0068] Nickel (Ni) improves high-temperature strength and corrosion resistance, and its content can be 100% by weight together with the aforementioned components and other unavoidable impurities. Since such a nickel (Ni) alloy has relatively high corrosion resistance and wear resistance, the durability of the exhaust valve spindle can be improved.

[0069] Here, the alloy material forming the first alloy part (200) is Ni known as Nimonic 80A. Cr20 Ni, known as TiAl nickel alloy or Inconel 625 Cr22 Ni, known as Mo9Nb or Inconel 718Cr19 It can be manufactured from Fe18Nb nickel alloy and is called Ni DSA760 Cr38 It may also be composed of Al4 nickel alloy.

[0070] In the step (S300) of welding the first alloy part to the preliminary head part, the first alloy part (200) can be welded to form an onion-shaped cross-section while wrapping around the bottom surface and a portion of the side of the preliminary head part (100).

[0071] Here, the first alloy part (200) can be formed so that the outer diameter and length of the pre-head part (100) correspond to the calculated volume. To this end, the volume of the area occupied by the base material and the first alloy part (200) in the head part (10) can be calculated by utilizing modeling data for the head part (10) or modeling data for the forging die (30).

[0072] Specifically, when welding the first alloy part (200) to the preliminary head part (100), the volume of the area occupied by the base material and the alloy part (200) in the head part (10) can be calculated, and then the first alloy part (200) corresponding to the calculated volume can be welded to the preliminary head part (100). In addition, when calculating the volume of the area occupied by the base material and the first alloy part (200) in the head part (10), the volume of the area occupied by the base material and the first alloy part (200) in the head part (10) can be calculated by utilizing modeling data for the head part (10) or modeling data for the forging die (30).

[0073] Meanwhile, in the step (S300) of welding the first alloy part to the pre-head part, as shown in FIG. 4, a bottom alloy part (210) forming a shape that wraps around the bottom surface of the pre-head part (100) can be welded, and a side alloy part (220) forming a shape that wraps around a part of the side of the pre-head part (100) can be welded.

[0074] Here, the side alloy part (220) is welded to the pre-head part (100) together with the bottom alloy part (210) to form an onion shape, so that when hot forging is performed by the forging die (30), the bottom alloy part (210) and the side alloy part (220) undergo even plastic deformation and can be formed into a shape that completely surrounds the pre-head part (100).

[0075] Meanwhile, the exhaust valve spindle is subjected to a relatively high temperature and pressure on the surface (bottom) of the head portion (10) facing the cylinder of the ship engine, and a relatively low temperature and pressure on the seat surface (top) facing the opposite side of the cylinder.

[0076] Here, the bottom alloy part (210) can be formed with a cross-sectional area that is relatively thicker than the thickness of the side alloy part (220) as shown in FIG. 4, and accordingly, by reinforcing the bottom surface of the head part (10), which is the part where high heat and high pressure are applied when applied to the exhaust valve spindle, the mechanical properties can be improved while reducing welding bonding.

[0077] Additionally, the bottom alloy part (210) may be formed with a nickel content different from that of the side alloy part (220), and according to the embodiment, the bottom alloy part (210) may contain a relatively larger amount of nickel than the side alloy part (220).

[0078] That is, the bottom alloy part (210), to which a relatively high temperature and pressure are applied, is composed of an alloy containing a relatively larger amount of nickel than the side alloy part (220), thereby providing relatively higher corrosion resistance and wear resistance compared to the side alloy part (220), so that mechanical properties can be improved while reducing welding bonding by reinforcing.

[0079] The step (S400) of forming the preliminary head portion by hot forging is a process of forming the preliminary head portion (100) into a shape corresponding to the head portion (10) by processing the preliminary head portion (100), on which the first alloy portion (200) is welded as shown in FIG. 2, through a forging die (30) by forging.

[0080] The step (S400) of forming a pre-head portion by hot forging can be performed by including a preheating step (S410), a forging step (S420), and a cooling step (S430), as shown in FIG. 5.

[0081] In the preheating step (S410), the pre-head part (100) with the first alloy part (200) welded through step S300 can be placed into a heating furnace and heated at a temperature of 1100 to 1150°C for a period of time within 1 hour.

[0082] In the forging step (S320), the preheated pre-head portion (100) is processed into a shape corresponding to the head portion (10). As shown in FIG. 2, the preheated pre-head portion (100) with the first alloy portion (200) welded is loaded into a forging die (30) and forged at a predetermined temperature to form the pre-head portion (100).

[0083] At this time, in step S320, a release agent for hot forging may be applied inside the forging die (30), or an insulating material may be used to reduce the temperature difference between the die and the material to perform forging.

[0084] In the cooling step (S330), the pre-head portion (100) hot-forged according to the aforementioned S320 step is cooled. Since the surface of the billet (1) cools faster than the core during hot forging and the core cools relatively slower, causing grain growth, the pre-head portion (100) can be air-cooled or water-cooled at room temperature.

[0085] The step (S500) of processing the seat surface of the pre-head portion is for forming a valve seat (300) on the seat surface (upper surface) of the pre-head portion (100) that has been hot-forged, and can form a welding groove (310) by cutting a part of the upper surface of the pre-head portion (100).

[0086] The step (S600) of welding a second alloy part to the seat surface of the pre-head part (100) can form a valve seat by welding a second alloy part (300) containing nickel into the welding groove (310) of the seat surface processed through step S500.

[0087] That is, by welding a second alloy part (300) containing nickel into a welding groove (310) formed on the seat surface of the pre-head part (100) to form a valve seat, the durability of the valve seat, which is the part that comes into close contact with the exhaust port of the engine, can be improved.

[0088] The step of completing the head portion (S700) can be formed into a head portion (10) by cutting the pre-head portion (100) on which a valve seat is formed on the seat surface.

[0089] According to the embodiment, in the step (S700) of completing the head portion, roughing is performed on the preliminary head portion (100) to cut the preliminary head portion (100) first, and then the preliminary head portion (100) can be cut again through rolling and finishing processes to form the head portion (10).

[0090] As described above, the billet (1) with the head portion (10) formed can be manufactured into a valve spindle by processing the shaft portion and processing the stem portion (20).

[0091] As described above, according to the method for manufacturing an exhaust valve spindle for a ship engine according to one embodiment, by welding the first alloy part (200) before the head part (10) is formed through hot forging and then performing hot forging, it is possible to secure mechanical properties such as corrosion resistance, heat resistance, and wear resistance while minimizing the occurrence of defects such as pores.

[0092] The embodiments described above are for illustrative purposes only, and those skilled in the art will understand that the embodiments described above can be easily modified into other specific forms without altering the technical concept or essential features of the embodiments described above. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive. For example, each component described as a single unit may be implemented in a distributed manner, and components described as distributed may likewise be implemented in a combined form.

[0093] The scope of protection sought through this specification is defined by the claims set forth below rather than by the detailed description above, and should be interpreted to include all modifications or variations derived from the meaning and scope of the claims and the concept of equivalents. Explanation of the symbols

[0095] 1 : Billet 10: Head section 20: Stem part 30 : Forging die 50 : Electric upsetting device 51 : Electrode clamp 52 : Anvil 53 : Pressurized cylinder 54 : Retraction cylinder 100 : Spare head section 200: 1st alloy part 210 : Bottom alloy part 220 : Side alloy part 300: Second alloy part 310 : Seat Home

Claims

Claim 1 A method for manufacturing an exhaust valve spindle for a marine engine having a head portion and a stem portion, comprising the steps of: forming a preliminary head portion at the tip of a billet in the form of a round bar having a predetermined length; processing the surface of the preliminary head portion before welding; welding a first alloy portion containing nickel to the preliminary head portion; forming the preliminary head portion into a shape corresponding to the head portion while processing the preliminary head portion with the first alloy portion welded thereon by hot forging through a forging die; processing the seat surface of the preliminary head portion processed by hot forging before welding; forming a valve seat by welding a second alloy portion containing nickel to the seat surface of the preliminary head portion; and completing the head portion by machining the preliminary head portion with the formed valve seat. Claim 2 A method for manufacturing an exhaust valve spindle for a ship engine, wherein the step of forming the pre-head portion in claim 1 is to form the pre-head portion at the leading edge of the billet while expanding the outer diameter of the leading edge of the billet by processing the billet with an electric upsetting device. Claim 3 A method for manufacturing an exhaust valve spindle for a marine engine, wherein the step of forming the preliminary head portion involves expanding the outer diameter of the billet so that the volume of the leading edge portion of the billet corresponds to the volume of the area occupied by the base material forming the billet in the head portion. Claim 4 A method for manufacturing an exhaust valve spindle for a ship engine, wherein the step of welding the first alloy part comprises: a step of welding a bottom alloy part that forms a part of the first alloy part and surrounds the bottom surface of the pre-head part; and a step of welding a side alloy part that forms an onion shape together with the bottom alloy part and surrounds a part of the side of the pre-head part. Claim 5 A method for manufacturing a valve spindle for a ship engine according to claim 4, characterized in that the bottom alloy part is welded to form a cross-sectional area relatively thicker than the side alloy part. Claim 6 A method for manufacturing a valve spindle for a ship engine according to claim 4, wherein the bottom alloy part and the side alloy part are formed with different nickel contents. Claim 7 A method for manufacturing a valve spindle for a ship engine according to claim 6, wherein the bottom alloy part contains a relatively larger amount of nickel than the side alloy part. Claim 8 A method for manufacturing a valve spindle for a ship engine according to claim 1, wherein the step of processing by the hot forging method comprises: a step of preheating the pre-head portion, on which the first alloy portion is welded, at a temperature of 1100 to 1150°C for a period of time within 1 hour; a step of forging the preheated pre-head portion using the forging die; and a step of cooling the forged pre-head portion.

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