Exhaust valve spindle and manufacturing method of exhaust valve spindle

Abstract

According to exemplary embodiments of the present invention, an exhaust valve spindle with improved chemical and mechanical properties and a manufacturing method of the exhaust valve spindle can be provided.

Description

Exhaust valve spindle and method for manufacturing the exhaust valve spindle

[0001] The present invention relates to an exhaust valve spindle and a method for manufacturing an exhaust valve spindle.

[0002] Generally, the exhaust valve spindle of a diesel engine used in large ships serves to open and close the engine's exhaust port. During the explosion and combustion process, the exhaust valve spindle is exposed to pressures of over 700 bar and high temperatures of over 600°C. Furthermore, as combustion temperatures and engine output have recently increased, it must withstand even higher pressures and temperatures. Therefore, it is necessary to improve the chemical resistance and mechanical properties of the exhaust valve spindle.

[0003] Under operating conditions of the exhaust valve, the head portion of the exhaust valve is exposed to high-temperature exhaust gas approaching 500 to 600°C and is subject to thermal effects, whereas the stem portion is relatively unaffected by high-temperature thermal effects. Therefore, due to material characteristics, the head portion requires strength, corrosion resistance, and wear resistance in a high-temperature oxidizing atmosphere, while the stem portion requires wear resistance for the opening and closing sliding of the exhaust valve.

[0004] Accordingly, it is necessary to select materials suitable for specific characteristics by utilizing the differing operating environments of exhaust valves. In particular, there is a need to develop technologies that utilize these dissimilar metal materials to ensure high reliability of exhaust valve spindles and improve their chemical and mechanical properties.

[0005] (Patent Document 1) Republic of Korea Published Patent Application No. 10-2014-0047451

[0006] The problem that the technical concept of the present invention aims to solve is to provide an exhaust valve spindle with improved chemical and mechanical properties and a method for manufacturing the same.

[0007] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall details of the specification.

[0008] According to exemplary embodiments of the present invention for solving the above-described problem, a method for manufacturing an exhaust valve spindle is provided. The method comprises the steps of: preparing an exhaust valve spindle in which parts of dissimilar materials are friction-welded; performing a first heat treatment on the exhaust valve spindle in a range of 700 to 1100°C; and performing a second heat treatment on the exhaust valve spindle that has undergone the first heat treatment at a temperature lower than the temperature of the first heat treatment.

[0009] The above first heat treatment can be maintained for 2 to 12 hours.

[0010] The above secondary heat treatment can be performed in the range of 650 to 900°C.

[0011] The above second heat treatment can be maintained for a longer period than the holding time of the above first heat treatment.

[0012]

[0013] According to other exemplary embodiments of the present invention, an exhaust valve spindle is provided. The exhaust valve spindle comprises a stem; a head joined to one end of the stem and comprising a Ni-based heat-resistant alloy; and a friction welding portion comprising a joining surface between the stem and the head, wherein the friction welding portion comprises a first region located on the head side with respect to the joining surface and a second region located on the stem side with respect to the joining surface, and the first region comprises a spherical first α phase.

[0014] The first α phase may have a distribution in which the average size increases as it is spaced further from the bonding surface.

[0015] The first region above includes a spherical first γ' phase, and the average size of the first γ' phase may be 380 to 500 nm.

[0016] The above head may contain, in weight percent, carbon (C): 0.10% or less (excluding 0%), chromium (Cr): 35-45%, aluminum (Al): 3.0-5.0%, and the remainder being nickel (Ni) and unavoidable impurities.

[0017] The above head may contain less than 0.1% iron (Fe).

[0018] The above stem may contain, in weight percent, carbon (C): 0.10% or less (excluding 0%), chromium (Cr): 15.0~25.0%, aluminum (Al): 1.5~2.0%, iron (Fe): 1.5% or less (excluding 0%), and the remainder being nickel (Ni) and unavoidable impurities.

[0019] According to exemplary embodiments of the present invention, an exhaust valve spindle with improved chemical and mechanical properties and a method for manufacturing the same can be provided.

[0020] The various and beneficial advantages and effects of the present invention are not limited to those described above and will be more easily understood in the process of explaining specific embodiments of the present invention.

[0021] FIG. 1 is a drawing for illustrating an exhaust valve spindle (10) according to exemplary embodiments.

[0022] FIG. 2 is a flowchart for explaining a method for manufacturing an exhaust valve spindle according to exemplary embodiments.

[0023] FIG. 3 is a flowchart for explaining a method for manufacturing an exhaust valve spindle according to other exemplary embodiments.

[0024] FIG. 4 is a flowchart illustrating a method for manufacturing an exhaust valve spindle according to other exemplary embodiments.

[0025] FIG. 5 is a flowchart for illustrating a method for manufacturing an exhaust valve spindle according to other exemplary embodiments.

[0026] Figure 6 is an SEM-EDS analysis image observed at the 0.1 mm point.

[0027] Figure 7 is an SEM-EDS analysis image observed at the 0.3 mm point.

[0028] Figure 8 is an SEM-EDS analysis image observed at the 0.5 mm point.

[0029] Figure 9 is an SEM-EDS analysis image observed at the 1.0 mm point.

[0030] Figure 10 is an SEM-EDS analysis image observed at the 1.5 mm point.

[0031] Figure 11 is an SEM-EDS analysis image observed at the 2.0 mm point.

[0032] Figure 12 is an SEM-EDS analysis image observed at the 3.0 mm point.

[0033] Figure 13 is an SEM-EDS analysis image observed at the 5.0 mm point.

[0034] Figure 14 is a graph showing the results of measuring hardness in the micro-range of a friction weld.

[0035] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. Prior to this, terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings. Instead, based on the principle that the inventor can appropriately define the concepts of terms to best describe his invention, they should be interpreted in a meaning and concept consistent with the technical spirit of the present invention.

[0036] In the following descriptions with reference to the drawings, identical or corresponding components are assigned the same reference numerals, and redundant descriptions thereof will be omitted.

[0037] In the following embodiments, the terms first, second, etc. are used not in a limiting sense, but for the purpose of distinguishing one component from another component.

[0038] In the following embodiments, the singular expression includes the plural expression unless the context clearly indicates otherwise.

[0039] In the following embodiments, terms such as "include" or "have" mean that the features or components described in the specification are present, and do not preclude the possibility that one or more other features or components may be added.

[0040] In the drawings, the size of components may be exaggerated or reduced for convenience of explanation. For example, the size and thickness of each component shown in the drawings are depicted arbitrarily for convenience of explanation, so the present invention is not necessarily limited to what is illustrated.

[0041] Where an embodiment can be implemented differently, a specific process sequence may be performed differently from the order described. For example, two processes described consecutively may be performed substantially simultaneously or proceed in the reverse order of the description.

[0042] In addition, in describing the present invention, if it is determined that a detailed description of related known components or functions may obscure the essence of the invention, such detailed description is omitted.

[0043] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0044] The present invention will be described in detail below through examples. However, it should be noted that the following examples are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0045]

[0046] [Exhaust valve spindle]

[0047] Since the exhaust valve spindle is a component that continuously performs linear reciprocating motion in a high-temperature environment, it is necessary to secure a strength greater than a certain amount. According to exemplary embodiments, the tensile strength of the exhaust valve spindle (10) may be 1000 MPa or more. While it is preferable for the tensile strength of the exhaust valve spindle (10) to be higher, as a non-limiting example, the tensile strength of the exhaust valve spindle (10) may be 2000 MPa or less.

[0048] FIG. 1 is a drawing for illustrating an exhaust valve spindle (10) according to exemplary embodiments.

[0049] Referring to FIG. 1, the exhaust valve spindle (10) includes a stem (11), a head (12), and a friction weld (13).

[0050] The stem (11) supports the head (12) and can guide the linear reciprocating motion of the exhaust valve spindle (10).

[0051] According to exemplary embodiments, the stem may comprise, in weight percent, carbon (C): 0.10% or less (excluding 0%), chromium (Cr): 15.0 to 25.0%, aluminum (Al): 1.5 to 2.0%, iron (Fe): 1.5% or less (excluding 0%), and the remainder being nickel (Ni) and unavoidable impurities. This allows for the provision of a stem (11) with excellent wear resistance.

[0052] Carbon (C) can improve the wear resistance of the material by contributing to strength enhancement. However, if carbon is added in excess, it can form an excess amount of carbides and reduce the toughness of the material. Therefore, carbon may be included in an amount of less than 0.10% (excluding 0%). More specifically, carbon may be included in an amount of 0.04 to 0.08%.

[0053] Chromium (Cr) can contribute to improved wear resistance by increasing the hardness of the material. However, if chromium is added in excess, it can form chromium carbide (Cr3C2, Cr7C3, etc.) at the friction joint surface, thereby reducing the joint strength. Therefore, chromium may be included in an amount of 15.0 to 25.0%. More specifically, chromium may be included in an amount of 19.0 to 21.0%.

[0054] Aluminum (Al) can increase the wear resistance of the material by forming a γ' phase and improving strength. However, if aluminum is added in excess, γ' phase precipitates are excessively formed, and toughness and ductility may decrease. Therefore, aluminum may be included in an amount of 1.5 to 2.0%. More specifically, aluminum may be included in an amount of 1.5 to 1.8%.

[0055] Iron (Fe) can partially replace nickel to improve the strength and hardness of the material. However, if iron is added in excess, the corrosion resistance of the material may be weakened and durability may be reduced. As a result, the wear resistance of the material may be inferior. Therefore, iron may be included in an amount of 1.5% or less (excluding 0%). More specifically, iron may be included in an amount of 0.5% to 1.1%.

[0056] According to exemplary embodiments, the stem (11) may contain titanium (Ti) in an amount of 2.0 to 3.0%. Titanium forms a γ' phase like aluminum and can improve the wear resistance of the material. However, if titanium is added in excess, precipitates may be excessively formed and toughness and ductility may be reduced. Therefore, the titanium content may be controlled within the range described above. More specifically, titanium may be contained in an amount of 2.3 to 2.8%.

[0057] In addition, it may contain residual nickel (Ni) and unavoidable impurities. Unavoidable impurities refer to materials that are inevitably added during the manufacturing process of general nickel-based superalloys and cannot be completely excluded; therefore, a detailed explanation is omitted.

[0058] The head (12) is a part directly responsible for opening and closing the exhaust port, through which high-temperature, high-pressure gas discharged from inside the engine combustion chamber passes. Therefore, the head (12) may have a diameter corresponding to the diameter of the exhaust port to open and close the exhaust port. The head (12) may have an end having a diameter larger than the diameter of the stem (11). The head (12) is joined to one end of the stem (11).

[0059] The head (12) may include a Ni-based heat-resistant alloy. As a result, its structure and performance can be stably maintained even when exposed to a high-temperature gas environment.

[0060] The material of the head (12) is provided as a material different from the material of the stem (11). More specifically, the material of the head (12) may have a different content of chromium and aluminum than the material of the stem (11). More specifically, the head (12) may have a higher content of chromium and aluminum than the stem (11). Chromium and aluminum are components that are closely involved in the formation of the microstructure and physical properties of the Ni-based heat-resistant alloy. In this way, by including materials optimized to ensure microstructure and physical properties suitable for the usage environment of the head (12) and the stem (11), the structural stability and chemical stability of the exhaust valve spindle (10) can be optimized.

[0061] According to exemplary embodiments, the head may comprise, in weight percent, carbon (C): 0.10% or less (excluding 0%), chromium (Cr): 35-45%, aluminum (Al): 3.0-5.0%, and the remainder being nickel (Ni) and unavoidable impurities. This allows for the provision of a head (12) with excellent high-temperature properties and chemical resistance while maintaining sufficient strength.

[0062] Carbon (C) can contribute to strength improvement. However, if carbon is added in excess, it can form excessive carbides and reduce the toughness of the material. Therefore, carbon may be included in an amount of less than 0.10% (excluding 0%). More specifically, carbon may be included in an amount greater than 0 and less than or equal to 0.05%.

[0063] Chromium (Cr) can improve hardness by forming an α-Cr phase. Furthermore, it can prevent a decrease in strength and toughness by suppressing grain coarsening. Additionally, the α-Cr phase can improve the low-temperature corrosion characteristics of the material by forming a lamellar structure with other precipitates. However, if chromium is added excessively, it may reduce bond strength by forming chromium carbides (Cr3C2, Cr7C3, etc.) at the friction joint surface. Therefore, chromium may be included in an amount of 35–45%. More specifically, chromium may be included in an amount of 37–40%. More specifically, chromium may be included in an amount of 37–39%.

[0064] Aluminum (Al) can form a γ' phase and improve strength. Furthermore, it can contribute to improving high-temperature corrosion resistance. However, if aluminum is added in excess, γ' phase precipitates are excessively formed, and toughness and ductility may decrease. Therefore, aluminum may be included in an amount of 3.0 to 5.0%. More specifically, aluminum may be included in an amount of 3.0 to 4.0%.

[0065] In addition, the present invention is not necessarily limited to the alloy composition described above. According to exemplary embodiments, the head (12) may contain less than 0.1% (including 0%) iron (Fe). Iron can cause phase transformation in a friction welding environment and can degrade the high-temperature properties of the material. In addition, iron can degrade the corrosion resistance of the material. Therefore, it is desirable to substantially exclude iron from the head (12), and the head (12) may not substantially contain iron.

[0066] In addition, it may contain residual nickel (Ni) and unavoidable impurities. Unavoidable impurities refer to materials that are inevitably added during the manufacturing process of general nickel-based superalloys and cannot be completely excluded; therefore, a detailed explanation is omitted.

[0067] The friction weld (13) includes the joint surface (W) of the stem (11) and the head (12). More specifically, the friction weld (13) includes a first area (13_1) located on the head (12) side relative to the joint surface (W) and a second area (13_2) located on the stem (11) side relative to the joint surface (W). The friction weld (13) is the heat-affected zone of the exhaust valve spindle (10) formed by friction welding. Accordingly, the range of the first area (13_1) and the second area (13_2) may vary depending on the friction welding conditions, but as a non-limiting example, it may refer to an area up to a distance of about 1 to 2.5 mm from the joint surface (W).

[0068] According to exemplary embodiments, the first region (13_1) includes a spherical first α phase. The first α phase (α-Cr phase) is a phase that mainly contains Cr and is provided in a spherical shape. Thus, by providing a spherical α phase in the first region (13_1), which is the heat-affected zone on the head (12) side, a uniform hardness distribution can be provided despite the thermal deformation that occurs during friction welding. As a result, the hardness characteristics of the exhaust valve spindle (10) can be improved despite friction welding of dissimilar materials. The first α phase can be identified through Scanning Electron Microscopy (SEM)-Energy Dispersive X-ray Spectroscopy (EDS) analysis. In the first region (13_1), a region where Cr is dense can be identified as the α phase. Additionally, it can be identified by considering whether other metal components (particularly Ni, and Al) are deficient in that region. Subsequently, whether it is the first α phase can be determined by considering the shape of the region. At this time, if it does not form a lamellar structure in the form of a layered structure with other alloy phases, it can be determined to be the spherical first α phase of the present invention. As a non-limiting example, an α phase region with an aspect ratio of 2.5 or less can be determined to be the first α phase.

[0069] According to exemplary embodiments, the first α phase has a distribution in which the average size increases as it is separated from the bonding surface (W). As a result, the hardness of the first region (13_1) may decrease as it is separated from the bonding surface (W). Due to the high heat generated during the friction welding process, the existing microstructure in the friction weld (13) may collapse. In particular, in the head (12) with a relatively high chromium content, the α-Cr phase may collapse due to the high temperature and undergo fine recrystallization, which can increase the hardness of the bonding surface (W). This may cause a large difference in hardness with the base material (especially the head (12)), thereby causing material variation. According to exemplary embodiments, the hardness variation in the first region (13_1) can be minimized by providing the first α phase to gradually increase in average size.

[0070] The average size of the first α phase may be 240 to 680 nm. If the average size of the first α phase is excessively large, it may be difficult to obtain the target material properties. If the average size of the first α phase becomes excessively small, the hardness may increase excessively. In this way, if the average size of the first α phase deviates from the above-described range, material deviation may occur in the friction weld (13), and the structural stability of the exhaust valve spindle (10) may be reduced.

[0071] The average size of the first α phase can be measured through SEM-EDS analysis.

[0072] According to exemplary embodiments, the first region (13_1) may include a spherical first γ' phase. The first γ' phase (γ'-phase) is a phase mainly containing Al and is provided in a spherical shape. In this way, by providing a spherical γ' phase in the first region (13_1), which is the welding heat-affected zone on the head (12) side, a uniform hardness distribution can be provided despite thermal deformation during friction welding.

[0073] The first γ' phase can be identified through Scanning Electron Microscopy (SEM)-Energy Dispersive X-ray Spectroscopy (EDS) analysis. In the first region (13_1), the region where Al is densely concentrated can be identified as the γ' phase. Additionally, it can be identified by considering whether other metal components (especially Cr) are deficient in the region. Subsequently, whether it is the first γ' phase can be determined by considering the shape of the region. At this time, if it does not form a lamellar structure in the form of a layered structure with other alloy phases, it can be determined to be the spherical first γ' phase of the present invention. As a non-limiting example, a γ' phase region with an aspect ratio of 2.5 or less can be determined to be the first γ' phase.

[0074] The average size of the first γ' phase can be 380 to 500 nm. By controlling the average size of the first γ' phase within a predetermined range, material variation in the first region (13_1) can be reduced. The average size of the first γ' phase can be measured through SEM-EDS analysis.

[0075] In the head (12) portion other than the first region (13_1), a so-called lamellar structure may be provided in which a linear second α phase and a second γ' phase form a layered structure.

[0076] According to exemplary embodiments, the average grain size of the first region (13_1) may be 1 to 5 μm. This improves the high-temperature strength and heat resistance of the head (12). If the average grain size of the first region (13_1) is excessively small, the hardness of the head (12) increases excessively and brittleness may increase. If the average grain size of the first region (13_1) becomes excessively coarse, the high-temperature strength and heat resistance of the head (12) may be compromised. The average grain size can be measured using an SEM-EDS analysis method.

[0077] As a non-limiting example, the average grain size of the second region (13_2) may be 10 to 15 μm.

[0078] According to exemplary embodiments, the difference between the maximum hardness of the first region (13_1) and the minimum hardness of the second region (13_2) is 60 Hv 0.2 It may be less than or equal to the following. The maximum hardness of the first region (13_1) refers to the maximum value obtained when measuring Vickers hardness under a load condition of 0.2 kgf on the weld heat-affected zone from the joint surface (W) to the head (12) side. The minimum hardness of the second region (13_2) refers to the minimum value obtained when measuring Vickers hardness under a load condition of 0.2 kgf on the weld heat-affected zone from the joint surface (W) to the stem (11) side.

[0079] When friction welding dissimilar materials, a difference in hardness may occur on both sides of the joint surface (W). If this difference in hardness becomes excessively large, the joint between the two members may become unstable and easily break. Additionally, the material variation may become uneven, which may lead to a deterioration in the quality of the exhaust valve spindle (10). Meanwhile, a material with relatively high hardness tends to decrease in hardness to the joint surface (W) after reaching maximum hardness in the heat-affected zone. Conversely, a material with relatively low hardness tends to increase in hardness to the joint surface (W) after reaching minimum hardness in the heat-affected zone. Therefore, it is necessary to control the hardness distribution uniformly by minimizing the difference between maximum and minimum hardness in the heat-affected zone. In this regard, by minimizing the difference in hardness between the first region (13_1) and the second region (13_2), high-temperature toughness and wear resistance of the friction weld (13) can be secured. In this regard, more preferably, the difference between the maximum hardness of the first region (13_1) and the minimum hardness of the second region (13_2) is 50 Hv 0.2It may be less than or equal to. Also, as a non-limiting example, the difference between the maximum hardness of the first region (13_1) and the minimum hardness of the second region (13_2) is 1.0 Hv 0.2 It could be more than that.

[0080] According to exemplary embodiments, the maximum hardness of the first region (13_1) is 380 Hv 0.2 Exceeding 500Hv 0.2 It may be less than. If the maximum hardness of the first region (13_1) is excessively high, the brittleness of the first region (13_1) may increase in a high-temperature environment. If the maximum hardness of the first region (13_1) is excessively low, the toughness of the friction weld (13) may be reduced, and the durability may be reduced.

[0081] According to exemplary embodiments, the minimum hardness of the second region (13_2) is 365 Hv 0.2 ~ 430Hv 0.2 It may be less than. If the minimum hardness of the second region (13_2) is excessively low, wear resistance may be inferior. If the minimum hardness of the second region (13_2) is excessively high, impact resistance may be reduced in the usage environment of the exhaust valve spindle (10).

[0082]

[0083] [Method for Manufacturing Exhaust Valve Spindle]

[0084] FIG. 2 is a flowchart illustrating a method for manufacturing an exhaust valve spindle according to exemplary embodiments.

[0085] A method for manufacturing an exhaust valve spindle (10) comprises the steps of: preparing an exhaust valve spindle (P10) in which parts of different materials are friction-welded; performing a first heat treatment of the valve spindle in the range of 800 to 1100°C (P20); and performing a second heat treatment of the valve spindle that has undergone the first heat treatment at a temperature lower than the first heat treatment temperature (P30). In this way, by performing a plurality of heat treatments after forming the exhaust valve spindle (10), the metal crystal structures of the friction-welded interface can be stabilized. As a result, the chemical and mechanical properties of the exhaust valve spindle (10) can be improved.

[0086] In step P10, an exhaust valve spindle (10) can be prepared by friction welding two parts of different materials using a friction welding method. Friction welding is a welding method that simultaneously utilizes frictional heat generated by friction between the contact surfaces of two parts and plastic deformation of the parts at high temperatures. As a result, the joining can be performed by inducing thermal deformation only at localized joining sites, thereby minimizing thermal damage to the base material and enabling rapid joining.

[0087] FIG. 3 is a flowchart illustrating a method for manufacturing an exhaust valve spindle according to other exemplary embodiments.

[0088] Referring to FIG. 3, the step (P11) of preparing an exhaust valve spindle containing different materials using friction welding may include a first pressing step (P11_1) of pressing and moving a second material in the shape of a bar towards the first material with a pressing force of 20 to 215 bar while the first material in the shape of a bar or head is rotated, and a second pressing step (P11_2) of pressing the second material toward the first material with a pressing force higher than the pressing force of the first pressing and moving step (P11_1). More specifically, the first material and the second material may be friction welded through flywheel friction welding.

[0089] According to exemplary embodiments, the sum of the length change values ​​of the first material and the second material in step P11 may be 15 to 40 mm. The sum of the length change values ​​of the first material and the second material can be calculated by measuring the lengths of the first material and the second material before and after friction welding. If the sum of the length change values ​​of the first material and the second material is less than 15 mm, friction welding may not be performed sufficiently, and the joint strength may be inferior. If the sum of the length change values ​​of the first material and the second material exceeds 40 mm, the quality of the friction weld may be inferior due to excessive plastic deformation, and the manufacturing cost of the exhaust valve spindle (10) may increase due to excessive consumption of material.

[0090] In step P11_1, frictional heat may be generated at the joint surface between the first material and the second material due to the rotation of the first material and the pressurized movement of the second material. As a result, the first material and the second material may undergo plastic deformation at the joint surface and be joined to each other. At this time, the degree of friction welding can be controlled by controlling the rotational speed of the first material or the pressurized force of the second material.

[0091] If the pressure applied to the second material is less than 20 bar, sufficient frictional heat may not be generated at the bonding interface between the first material and the second material. As a result, the bonding strength between the first material and the second material may be reduced. On the other hand, if the pressure applied to the second material exceeds 215 bar, it may be difficult to precisely control the degree of friction welding due to a rapid temperature rise. Therefore, according to exemplary embodiments, the second material can be pressed and moved toward the first material with a pressure of 20 to 215 bar. More preferably, the pressure applied to the second material may be 30 to 200 bar.

[0092] As a non-limiting example, the rotational speed of the first material may be 250 to 550 RPM. In step P11_1, the first material may rotate by receiving rotational power from a rotary drive motor. If the rotational speed of the first material is excessively fast, excessive frictional heat may be generated, making friction control difficult. If the rotational speed of the first material is excessively slow, the bonding speed may be slowed down.

[0093] In step P11_2, the second material is pressed toward the first material with a higher pressure than that of step 11_1, thereby inducing plastic deformation of the first and second materials. As a result, sufficient bonding strength between the first and second materials can be secured. In addition, by applying a stronger pressure, heat loss that may occur during friction welding can be compensated for, and the heating state of the bonding surface can be effectively maintained.

[0094] As a non-limiting example, in step P11_2, the second material can be moved by pressure toward the first material with a pressure of 30 to 250 bar. More specifically, in step P11_2, the second material can be moved by pressure toward the first material with a pressure of 40 to 230 bar.

[0095] In step P11_2, the first material can rotate due to rotational inertia. That is, it can rotate due to the rotational inertia generated in step P11_1 without applying separate external power.

[0096] According to exemplary embodiments, the first material may comprise, in weight percent, carbon (C): 0.10% or less (excluding 0%), chromium (Cr): 35-45%, aluminum (Al): 3.0-5.0%, and the remainder being nickel (Ni) and unavoidable impurities.

[0097] The first material is a material to be included in the head (12) of the exhaust valve spindle (10), and must have excellent high-temperature properties and chemical resistance while maintaining sufficient strength. The difference in alloy composition between the first material and the head (12) may not be significant. However, due to friction welding and heat treatment, the crystal structure in the weld heat-affected zone (first region) of the head (12) may differ from that of the first material.

[0098] According to exemplary embodiments, the second material may comprise, in weight percent, carbon (C): 0.10% or less (excluding 0%), chromium (Cr): 15.0 to 25.0%, aluminum (Al): 1.5 to 2.0%, iron (Fe): 1.5% or less (excluding 0%), and the remainder being nickel (Ni) and unavoidable impurities.

[0099] The second material is a component to be the stem (11) of the exhaust valve spindle (10) and must have excellent wear resistance. The difference in alloy composition between the second material and the stem (11) may not be significant. However, due to friction welding and heat treatment, the crystal structure of the stem (11) in the weld heat-affected zone (second region) may differ from that of the first material.

[0100] Optionally, depending on the shape of the first material being friction-welded, a step of forming the bar-shaped first material into a head shape can be performed before the first pressurization step (P11_1) or after the second pressurization step (P11_2).

[0101] FIG. 4 is a flowchart illustrating a method for manufacturing an exhaust valve spindle according to other exemplary embodiments.

[0102] Referring to FIG. 4, the step (P12) of preparing an exhaust valve spindle by friction welding parts of different materials may include a bar-shaped stem (11), a step (P12_1) of obtaining a preliminary spindle by joining the stem and a tip of a different material having a bar shape through friction welding, and a step (P12_2) of obtaining an exhaust valve spindle (10) by forming and forging the tip of the preliminary spindle into a head (12). In this way, by first welding the round bar-shaped parts and then forming the head, the control of the friction welding process can be performed more easily. In addition, the quality of the friction weld can be provided consistently, and the loss of parts can be minimized.

[0103] According to exemplary embodiments, friction welding in step P12_1 can be performed as in step P11. Through this step, a preliminary spindle comprising a bar-shaped stem and a tip joined to one end of the stem can be obtained. The preliminary spindle may have a round bar shape overall.

[0104] According to exemplary embodiments, in step P12_2, a tip portion including the tip of a preliminary spindle may be upset molded at a temperature of 950 to 1100°C to form a preliminary shape of the head portion, and then a head (12) may be formed by forging at a temperature of 1000 to 1200°C. The tip portion of the preliminary spindle may include not only the tip but also a part of the stem portion. Through this step, an exhaust valve spindle (10) having the overall shape of the stem (11) and the head (12) can be provided.

[0105] FIG. 5 is a flowchart for explaining a method of manufacturing an exhaust valve spindle (10) according to other exemplary embodiments.

[0106] Referring to FIG. 5, the step (P13) of preparing an exhaust valve spindle made of friction-welded parts of different materials may include the step (P13_1) of forming and forging a bar-shaped preliminary head portion to process it into a head (12), and the step (P13_2) of joining the head and the bar-shaped stem through friction welding to obtain an exhaust valve spindle (10). In this way, by performing the forming and forging of the head first, the forming of the head can be performed more precisely. In addition, the stress that may be applied to the friction welded portion due to forming and forging can be minimized.

[0107] According to exemplary embodiments, in step P13_1, a bar-shaped preliminary head portion may be upset molded at a temperature of 950 to 1100°C to form a preliminary shape of the head portion, and then forged at a temperature of 1000 to 1200°C to provide the head (12). Through this step, the head (12) of the exhaust valve spindle (10) can be obtained.

[0108] According to exemplary embodiments, friction welding in step P13_2 can be performed as in step P11. Through this step, an exhaust valve spindle (10) having the overall shape of a stem portion (11) and a head portion (12) can be provided.

[0109] In this way, the forming sequence of the head (12) does not particularly limit the present invention. The important thing is to properly control the friction welded portion (13) of the exhaust valve spindle (10) by properly controlling the friction welding and, in particular, the heat treatment process.

[0110] In step 20, heat treatment may be performed at a temperature of 700 to 1100°C to control the shape and size of the crystal phases. More specifically, the first heat treatment temperature may be 800 to 1000°C. The first heat treatment temperature may be 800 to 900°C.

[0111] If the first heat treatment temperature is below 800°C, the shape and size of the crystal phases are not sufficiently controlled, which may degrade the properties of the friction weld. If the first heat treatment temperature exceeds 1100°C, the crystal phase precipitates and grains become coarse, which may reduce the hardness of the friction weld. In addition, chromium carbides may form, which may degrade the bonding properties of the friction weld.

[0112] According to exemplary embodiments, the first heat treatment may be maintained for 2 to 12 hours. More specifically, the first heat treatment may be maintained for 2 to 8 hours. The first heat treatment may be maintained for 2 to 5 hours.

[0113] If the holding time of the first heat treatment is less than 2 hours, the shape and size control of the crystal phases may not be sufficiently achieved. If the holding time of the first heat treatment exceeds 12 hours, the crystal phases may be formed coarsely and an excess amount of chromium carbide may precipitate.

[0114] Step P30 is performed at a temperature lower than the first heat treatment temperature. This allows for the stabilization of the microstructure of the friction weld. Additionally, the hardness of the friction weld can be improved, and mechanical properties can be enhanced. On the other hand, if the second heat treatment temperature is higher than the first heat treatment temperature, recrystallization of the crystal phases may be induced, which can alter the microstructure of the friction weld. Furthermore, an excessive amount of precipitates may precipitate, which can degrade the properties of the friction weld.

[0115] As a non-limiting example, the secondary heat treatment may be performed in the range of 650 to 900°C. More specifically, the secondary heat treatment may be performed in the range of 650 to 800°C. If the secondary heat treatment is performed at a temperature below 650°C, the exhaust valve spindle (10) may be rapidly cooled, and an excessive hard microstructure may be formed. In this case, the brittleness of the exhaust valve spindle (10) may increase.

[0116] According to exemplary embodiments, the second heat treatment can be maintained for a longer period than the first heat treatment. As a result, the microstructure of the friction weld can be sufficiently stabilized and the grains can be formed uniformly.

[0117] As a non-limiting example, the secondary heat treatment may be maintained for 18 to 30 hours. More specifically, the secondary heat treatment may be maintained for 18 to 25 hours. If the maintenance time of the secondary heat treatment exceeds 30 hours, excessive energy may be consumed during the secondary heat treatment. Additionally, the hardness of the friction weld may increase excessively, leading to increased brittleness.

[0118] [Test Example]

[0119] Test Example 1: Microstructural distribution in the first region

[0120] A first material (head) and a second material (stem) having alloy compositions according to Table 1 below were prepared. Then, friction welding was performed by controlling the friction speed to approximately 370 rpm, the friction travel distance to approximately 1.8 mm, and the forward pressure to approximately 50 bar. Afterward, the friction-welded samples were subjected to a first heat treatment at approximately 850°C for 4 hours, followed by a second heat treatment at approximately 750°C for 24 hours to produce samples of exhaust valve spindles.

[0121] Classification CCrAlTiFe Primary Material 0.05 38.5 3.57 -- Secondary Material 0.04 19.4 16 82.4 81.02 * Balance Ni and unavoidable impurities (Unit weight%)

[0122] Subsequently, the first α phase and the first γ' phase were observed at points approximately 0.1 mm, 0.3 mm, 0.5 mm, 1 mm, 2 mm, 3 mm, and 5 mm from the friction weld surface. In addition, the average size of the first α phase and the first γ' phase at each point was measured using the EBSD analysis method and is shown in Table 2 below.

[0123] Figure 6 is an SEM-EDS analysis image observed at a point of 0.1 mm.

[0124] Figure 7 is an SEM-EDS analysis image observed at the 0.3 mm point.

[0125] Figure 8 is an SEM-EDS analysis image observed at a point of 0.5 mm.

[0126] Figure 9 is an SEM-EDS analysis image observed at the 1.0 mm point.

[0127] Figure 10 is an SEM-EDS analysis image observed at the 1.5 mm point.

[0128] Figure 11 is an SEM-EDS analysis image observed at the 2.0 mm point.

[0129] Figure 12 is an SEM-EDS analysis image observed at the 3.0 mm point.

[0130] Figure 13 is an SEM-EDS analysis image observed at the 5.0 mm point.

[0131] Classification Average Size (nm) 0.1mm Point 0.3mm Point 0.5mm Point 1.0mm Point 1.5mm Point 2.0mm Point 3.0mm Point 5.0mm Point 1st α-phase 242347446532624658397422 1st γ'-phase 397424430453456479527623

[0132] Referring to Table 2 and Figures 6 to 13, it was confirmed that a first α phase (A), in which Cr is more concentrated and Al is deficient than in other regions, appeared starting from a point 0.1 mm closest to the contact surface (W). Additionally, it was confirmed that a spherical first γ' phase (B) appeared around it. In particular, it was confirmed that the average size of the first α phase increased as it moved further away from the contact surface. Outside of the first region (13_1), the average size of the first α phase decreased, and a second α phase (D) and a second γ' phase (E), forming a lamellar structure (C), began to appear.

[0133] Subsequently, hardness measurements were performed on the friction welded portion of the sample.

[0134] Figure 14 is a graph showing the results of measuring hardness in the micro-range of a friction weld.

[0135] Referring to Fig. 14, it was confirmed that the hardness gradually decreases as it moves away from the bonding surface in the first region.

[0136]

[0137] Test Example 2: Hardness test according to heat treatment conditions

[0138] To determine the effect of heat treatment conditions on the hardness of the friction weld, samples were prepared by heat treatment under the conditions shown in Table 3 below.

[0139] Classification 1 Heat Treatment 2 Heat Treatment Holding Temperature (°C) Holding Time (hr) Holding Temperature (°C) Holding Time (hr) Example 1 850 4750 24 Example 2 850 4800 24 Example 3 850 8750 24 Example 4 850 8800 24 Comparative Example 1 750 24 -- Comparative Example 2 800 24 -- Comparative Example 3 850 24 --

[0140] Subsequently, the hardness of each sample was measured and shown in Table 4. In addition, the area up to 2 mm from the bonding surface to the first material side (head side) was designated as the first area, and the area up to 2 mm to the second material side (stem side) was designated as the second area.

[0141] Section 1 Zone Maximum Hardness (Hv 0.2 ) Zone 2 Minimum Hardness (Hv 0.2 ) Difference between the maximum hardness of the first zone and the minimum hardness of the second zone (Hv 0.2 Example 145941247 Example 241537045 Example 344039545 Example 443038050 Comparative Example 152042595 Comparative Example 244537372 Comparative Example 340036040

[0142] Examples 1 to 4 were able to secure a uniform hardness distribution by satisfying the heat treatment method according to exemplary examples.

[0143] However, Comparative Examples 1 to 3 did not satisfy the heat treatment conditions, and it was confirmed that the hardness of the friction welded part was non-uniform.

[0144] Although the invention has been described with reference to the above embodiments, those skilled in the art will understand that various modifications and changes can be made to the invention without departing from the spirit and scope of the invention as described in the following claims.

[0145] [Explanation of the symbol]

[0146] 10: Exhaust valve spindle

[0147] 11: Stem

[0148] 12: Head

[0149] 13: Friction weld

[0150] 13_1: First Zone

[0151] 13_2: Second Area

[0152] W: Joint surface

Claims

1. A step of preparing an exhaust valve spindle in which parts of dissimilar materials are friction-welded; A step of performing a first heat treatment on the exhaust valve spindle in the range of 700 to 1100°C; and A method for manufacturing an exhaust valve spindle comprising the step of performing a second heat treatment on the exhaust valve spindle that has undergone a first heat treatment at a temperature lower than the first heat treatment temperature.

2. In Paragraph 1, A method for manufacturing an exhaust valve spindle in which the above first heat treatment is maintained for 2 to 12 hours.

3. In Paragraph 1, The above secondary heat treatment is a method for manufacturing an exhaust valve spindle, performed in the range of 650 to 900°C.

4. In Paragraph 1, A method for manufacturing an exhaust valve spindle in which the above-mentioned second heat treatment is maintained for a longer period than the holding time of the above-mentioned first heat treatment.

5. Stem; A head joined to one end of the above-mentioned stem and comprising a Ni-based heat-resistant alloy; and It includes a friction weld portion comprising the joining surface of the stem and the head, and The friction welded portion includes a first region located on the head side with respect to the joint surface and a second region located on the stem side with respect to the joint surface. The first region above is an exhaust valve spindle including a spherical first α phase.

6. In Paragraph 5, The above first α phase is, An exhaust valve spindle having a distribution in which the average size increases as it is spaced further from the above-mentioned joint surface.

7. In Paragraph 5, The above first region includes a spherical first γ' phase, and An exhaust valve spindle having an average size of the first γ' phase of 380 to 500 nm.

8. In Paragraph 5, The above head is, in weight percent, An exhaust valve spindle comprising carbon (C): 0.10% or less (excluding 0%), chromium (Cr): 35–45%, aluminum (Al): 3.0–5.0%, and the remainder being nickel (Ni) and unavoidable impurities.

9. In Paragraph 8, The above head is an exhaust valve spindle containing less than 0.1% iron (Fe).

10. In Paragraph 5, The above stem is, in weight %, An exhaust valve spindle comprising carbon (C): 0.10% or less (excluding 0%), chromium (Cr): 15.0–25.0%, aluminum (Al): 1.5–2.0%, iron (Fe): 1.5% or less (excluding 0%), and the remainder being nickel (Ni) and unavoidable impurities.

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