Exhaust valve for internal combustion engine

Abstract

The invention relates to an exhaust valve for an internal combustion engine, comprising an exhaust valve stem having a shaft part and a disc part which are made integrally, the disc part having an end face facing away from the shaft part and a valve seat region on the upper side of the disc part, the exhaust valve stem comprising a deposited alloy forming a layer on at least a portion of the disc part, the layer comprises, in mass percent (%): Cr: 32% to 50%, Al: 0.5% to 10.0%, and the remainder of optional elements, unavoidable impurities and Ni, and wherein the layer has a microstructure comprising less than 10% of a basket-like braided structure and some layered structures. The invention further relates to an internal combustion engine having an exhaust valve and to a treatment method for an exhaust valve of an internal combustion engine.

Description

Technical Field

[0001] The present invention relates to an exhaust valve of an internal combustion engine, an internal combustion engine having an exhaust valve, and a method for treating a valve seat region of an exhaust valve of an internal combustion engine. Background Art

[0002] US 2016 / 0215660 discloses an exhaust valve for a diesel engine on a large ship. The valve comprises an integral shaft portion and an umbrella portion, the shaft portion and the umbrella portion being made of a nickel-based aged precipitation alloy of the Ni-Cr-Al system. The exhaust valve has a layered structure with an overall hardness of 600 HV or less, and the layered structure includes a layer formed of an α-Cr phase having a thickness of not less than 150 nm and having been aged to a level exceeding its peak mechanical strength. Notably, US 2016 / 0215660 involves forging the material to produce the integral valve, resulting in high manufacturing costs.

[0003] In the prior art, how to improve the exhaust valve, especially to improve the exhaust valve's ability to withstand mechanical shock and high-temperature combustion gas while keeping the manufacturing cost low, remains a problem. Summary of the Invention

[0004] The present invention relates to an exhaust valve of an internal combustion engine, an internal combustion engine having an exhaust valve, and a method for treating a valve seat region of an exhaust valve of an internal combustion engine.

[0005] In internal combustion engines, the hot combustion gases can be highly corrosive, so exposed components need to be made of highly corrosion-resistant materials. The hot combustion gases pass through the valve seats of the exhaust valves at high gas velocities, especially when the valves are opening and closing. In order to obtain exhaust valves that can withstand the hot, corrosive combustion gases as well as the mechanical loads, the exhaust valves need to be made of expensive nickel-based materials in order to obtain the required mechanical properties, namely strength and ductility. The most advanced exhaust valves currently available are made of forged Ni-Cr-Al alloy, which achieves an optimal balance between strength (for example, hardness), ductility and corrosion resistance through a special microstructure that includes a thin layer of α-Cr. Until now, such exhaust valves have been entirely forged from DSA760 material to obtain optimal hardness, ductility and corrosion resistance.

[0006] The exhaust valve shaft must possess a certain strength to function in the engine. To ensure that the valve disc also performs its sealing function, the disc portion, known as the valve seat, must possess sufficient hardness. Therefore, both the shaft and disc portions of known exhaust valves are manufactured to high strength and hardness. However, to avoid the risk of brittle failure and allow for cold rolling of the valve seat, good ductility must be maintained. Attempts have been made to manufacture one portion of the exhaust valve from one material, for example by welding a layer onto the valve disc, and the other portion from another material. However, these attempts have failed to achieve the same attractive property combination as wrought DSA760. Specifically, the ductility of the welded material is significantly lower. This is because the added material has a similar chemical composition to DSA760 but lacks the same uniform microstructure, containing a large thin layer of α-Cr, as the wrought material. Typically, welded or otherwise added materials can experience a high degree of segregation (an uneven distribution of chemical elements) during the production process, leading to uneven response during heat treatment and a non-uniform microstructure with localized regions exhibiting varying microstructures and mechanical properties. When trying to manufacture exhaust valves where only the valve seat and the areas most exposed to the gas are made of very expensive materials that can withstand corrosive gases, or when applying new materials to forging spindles to extend service life, known heat treatment processes either require very high temperatures (solution treatment) or result in an inhomogeneous microstructure and poor ductility. Full solution treatment is undesirable from a cost perspective and from the dilution between the welded and forged materials. Especially for extending the service life of the valve, it is difficult to fully solution heat treat the valve disc without affecting the performance of the shaft and ensuring dimensional stability. In addition, even if solution treatment is performed, it may not be sufficient to eliminate segregation and uneven response to subsequent aging treatment.

[0007] Tests have shown that the material of the weld material exposed to direct heat treatment, such as known heat treatments at temperatures above 750°C, is not homogeneous. Figure 4A and Figure 4D As shown, some areas have a basket weave microstructure, similar to the morphology of the so-called topologically close packed (TCP) phase, which is known to be particularly detrimental to the ductility of nickel-based alloys. Figure 4B and Figure 4C Other regions are shown without internal structure, and finally other regions have a desired layered structure, each with significantly different mechanical properties. In the case of components such as exhaust valves, which are exposed to thermomechanical fatigue when loaded by severe dynamic forces and temperature fluctuations, heterogeneous material components with different mechanical properties are particularly disadvantageous. Therefore, for the mechanical performance of exhaust valves, it is important that the microstructure is homogeneous, and that areas with known detrimental microstructural morphologies are minimized.

[0008] The object of the present invention is to fully or partially alleviate the above-mentioned disadvantages and drawbacks of the prior art. More specifically, the object of the present invention is to provide an improved exhaust valve in which the areas of the exhaust valve exposed to mechanical impacts (e.g., the valve seat, the valve head, and the valve bottom) are made of a homogeneous material and have high ductility and hardness without the entire exhaust valve having to be made of the same material.

[0009] The above objects and many other objects, advantages and features will become apparent from the following description and are achieved by an exhaust valve for an internal combustion engine according to the invention, the exhaust valve comprising:

[0010] An exhaust valve stem having an integral shaft portion and a disc portion, the disc portion having an end surface facing away from the shaft portion and a valve seat region located on an upper side of the disc portion, and the exhaust valve stem comprising a deposited alloy forming a layer on at least a portion of the disc portion, wherein the layer comprises, in mass percentage (%):

[0011] -Cr: 32% to 50%,

[0012] -Al: 0.5% to 10.0%, and

[0013] - The remaining optional elements, unavoidable impurities and Ni,

[0014] wherein the layer has a microstructure comprising less than 10% basket weave structure and some lamellar structure.

[0015] Furthermore, the layer may be a monolithic layer, and thus the layer structure may differ from that observed under a scanning electron microscope.

[0016] Furthermore, the layer may have a microstructure comprising at least 25% lamellar structures.

[0017] The microstructure is studied by taking electron micrographs, more specifically, by taking cross-sectional images of the structure using a scanning electron microscope. From the micrographs, it is possible to assess how much of the surface area is covered by the basket weave and / or layered structure. The ISO-9042:1988 standard can be used to quantify how much of the surface area is covered by the basket weave and / or layered structure.

[0018] Since this alloy contains Cr, Al and Ni, it is a Ni-Cr-Al system. Since the rest is Ni (and inevitable impurities), this alloy can be expressed as a Ni-based alloy.

[0019] In one embodiment, the alloy may be age-precipitated.

[0020] The layer may have a microstructure comprising less than 5% basket weave structure, and preferably the layer may have a microstructure comprising substantially no basket weave structure.

[0021] Exhaust valves are particularly suitable for large vessels, such as vessels having two-stroke marine engines producing more than 10,000 horsepower, more than 20,000 horsepower, or more than 30,000 horsepower, because the demands placed on such exhaust valves are high.

[0022] In one embodiment, the portion of the disk-shaped portion may be at least the end surface and / or the valve seat area.

[0023] In another embodiment, the alloy may include a composition comprising 0-4% W by mass (%).

[0024] Furthermore, by providing the exhaust valve stem with a layer having a microstructure comprising less than 10% basket-weave structure and some lamellar structure, the shaft portion of the exhaust valve stem can be made of different materials, for example, less expensive materials, while still maintaining the necessary ductility and hardness, as well as the desired level of corrosion resistance. Materials with high corrosion resistance, sufficient hardness, and the required ductility are expensive, so by forming only a thin layer of the more expensive material, costs can be reduced. Furthermore, worn exhaust valve stems can be reconditioned and reused, providing an environmentally friendly and cost-effective solution. While the shaft portion is exposed to harmful exhaust gases, the disc portion is more susceptible to corrosion because the velocity of harmful exhaust gases and the temperature of the disc's end face are higher than when gases pass through the shaft portion. Furthermore, since the disc portion is required to form a seal against the corresponding base member on the stationary component of the internal combustion engine, the material properties of the valve seat area and the disc's end face must have a predetermined level of ductility, hardness, and high corrosion resistance for proper function in the valve seat area. Excessively low ductility also creates the risk of unintended cracks propagating faster than in more ductile materials. The necessary ductility of the layer means an elongation higher than 10%, and the ductility can be measured using the standard ISO6892-1:2019. The necessary hardness of the layer refers to a hardness of 350-550 HV, and the hardness can be measured using the standard ISO6507-1:2019. When having an alloy layer containing more than 32% (mass percentage) chromium, the layer has sufficient corrosion resistance. For Ni-based superalloys, how to obtain high strength and good ductility at a high chromium content has historically been a long-standing unsolved problem. However, by adopting a microstructure layer containing less than 10% of a basket weave structure and some lamellar structure, a high chromium content can also provide high strength and good ductility.

[0025] Known forged exhaust valves cannot be refinished with layers having a layered microstructure because an important part of obtaining a homogeneous material with the required corrosion resistance, ductility and hardness is that the material is forged. Forging just one layer is not possible, so the entire exhaust valve must be displaced.

[0026] When having a microstructure comprising less than 10% basket weave structure and some lamellar structure, the material is sufficiently homogeneous to withstand severe dynamic forces, such as those experienced by exhaust valves.

[0027] Furthermore, in known solutions, the layer is welded to the disc-shaped portion of the exhaust valve stem. However, using known processes, it is impossible to obtain a homogeneous material that undergoes a lamellar transformation and possesses the necessary corrosion resistance and ductility without reducing hardness or ductility below the required levels. However, by heating the welded layer to a relatively low first temperature of 550-700°C for at least 12 hours, and then heating the layer to a higher second temperature of 750-1000°C for at least 1 hour, the layer develops a uniform microstructure containing at least 90% lamellar structure at the first temperature. At the second temperature, the structure in the layer is coarsened to achieve the desired combination of ductility and hardness, with less than 10% basket-weave structure and some lamellar structure. The low temperature of the first step is critical to allow only so-called boundary diffusion to occur. When the temperature is too low for volume diffusion to occur, the basket-weave structure is suppressed, and despite the presence of segregation, the formation of α-Cr lamellae will predominate in all areas. After sufficient time has passed to achieve a lamellar transformation completion of more than 90% (according to a TTT (time-temperature transformation) diagram), the temperature is raised in a second heating step to promote volume diffusion, coarsening the lamellar structure, thereby reducing hardness and increasing ductility until the desired combination of the two is achieved. Since the transformation to a lamellar structure has already occurred in the first step, no further transformation is possible, and the formation of a basket-like structure or precipitation-free zone is effectively limited to less than 10%. Coarsening can continue until the lamellar structure is partially destroyed while still maintaining the desired hardness and ductility range.

[0028] Furthermore, the alloy may be deposited on at least the end surface of the disc-shaped portion and the valve seat region by depositing powder, 3D printing, welding, or the like.

[0029] In one embodiment, the alloy is Ni-38Cr-3.8Al.

[0030] Furthermore, in one embodiment, the alloy is the alloy sold by Daido Steel Corporation under the trade name DSA760.

[0031] In addition, the composition may include the following optional elements in terms of mass percentage (%):

[0032] Fe: 0.1% to 20.0%,

[0033] Si: 5% or less,

[0034] B: 0.01% or less,

[0035] C: 0.1% or less,

[0036] Cu: 5% or less,

[0037] Ti: 0.1% or less,

[0038] Nb: 0.1% or less,

[0039] Ta: 0.1% or less, and

[0040] V: 0.1% or less,

[0041] The condition is that Ti+Nb+Ta+V is 0.1% or less, and the remainder is unavoidable impurities and Ni.

[0042] Furthermore, the layer may have a hardness of 350-550 HV and a ductility of at least 10% elongation.

[0043] Furthermore, the layer may have a microstructure comprising less than 5% basket weave structure.

[0044] Furthermore, the invention relates to an internal combustion engine having an exhaust valve.

[0045] Furthermore, the present invention relates to a method for treating a valve seat region of an exhaust valve for an internal combustion engine, the exhaust valve comprising a disk-shaped portion having an upper side and an end face, a shaft portion extending from the upper side of the disk-shaped portion, and a valve seat region located on the upper side of the disk-shaped portion, wherein at least the following steps are performed:

[0046] a) providing a disk-shaped portion having a valve seat area and an end face,

[0047] b) forming a layer on at least a portion of the disc-shaped portion by depositing an alloy,

[0048] Wherein, the layer comprises by mass percentage (%):

[0049] -Cr: 32% to 50%,

[0050] -Al: 0.5% to 10.0%, and

[0051] -The rest of you,

[0052] c) heating the layer to a first temperature of 550-700° C. for at least 12 hours, and

[0053] d) heating the layer to a second temperature of 750-1000° C. for at least 1 hour.

[0054] In one embodiment, the deposited alloy may be age-precipitated.

[0055] Furthermore, the portion of the disk-shaped portion may be at least the end face and / or the valve seat region.

[0056] By exposing the Ni-Cr-Al system, the Ni-based alloy, to a first heat treatment not exceeding 700°C and then to a second heat treatment above 750°C, a first region with a fine lamellar structure is formed during the first heat treatment, resulting in a uniform structure and high hardness and low ductility, and then coarsening of the lamellar structure occurs during the second heat treatment, the hardness decreases and the ductility increases, so as to achieve the combination of ductility, hardness and corrosion resistance required for the layer of the disk-shaped portion.

[0057] Additionally, heating the layer to a first temperature of 550-700° C. for at least 12 hours may be a first heat treatment.

[0058] Additionally, heating the layer to a second temperature of 750-1000° C. for at least 1 hour may be a second heat treatment.

[0059] Furthermore, the layer may be formed on at least a portion of the disk portion by welding.

[0060] Furthermore, no heat treatment may be performed between the forming step b) and the heating step c).

[0061] Furthermore, heat treatment above 750° C. may not be performed between the forming step b) and the heating step c).

[0062] Furthermore, during the heating step c), the layer may undergo a lamellar transformation without forming more than 10% of a basket weave structure.

[0063] Furthermore, in the heating step c), the duration of heating at the first temperature may be at least 12 hours, preferably at least 24 hours.

[0064] Furthermore, in the heating step c), the heating at the first temperature may be preferably performed at a first temperature of 600-650° C. for at least 12 hours, more preferably at a first temperature of 600-650° C. for at least 24 hours.

[0065] Furthermore, in the heating step d), the heating at the second temperature may have a duration of at least 2 hours.

[0066] Furthermore, in the heating step d), the heating at the second temperature may preferably be performed at a second temperature of 800-950° C. for at least 1 hour, more preferably at a second temperature of 850-925° C. for at least 1 hour, and even more preferably at a second temperature of 850-925° C. for at least 2 hours.

[0067] Furthermore, in the heating step d), the heating at the second temperature may be preferably performed at a second temperature of 850° C. for 10-24 hours, or at a second temperature of 900° C. for 2-10 hours.

[0068] Furthermore, during the heating step c), the layer may form a microstructure comprising at least 90% lamellar structures.

[0069] Furthermore, in the heating step d), a layer hardness of 350-550 HV can be achieved.

[0070] Furthermore, during the heating step d), the layer may form a microstructure comprising less than 10% basket weave structure and some lamellar structure, preferably less than 5% basket weave structure.

[0071] Furthermore, during the heating step d), the layer may form a microstructure comprising less than 10% basket weave structure and at least 25% lamellar structure, preferably at least 50% lamellar structure.

[0072] Furthermore, in step a), existing layers can be removed at least on the end face and the valve seat region.

[0073] Furthermore, the method may further comprise a step e) of heating the layer to a third temperature of 650-750° C. for at least 4 hours after step d).

[0074] The valve seat may be cold rolled between step d) and step e).

[0075] Additionally, heating the layer to a third temperature of 650-750° C. for at least 4 hours may be a third heat treatment.

[0076] Furthermore, the heating step c) of heating the layer to the first temperature may be performed only on a portion of the exhaust valve and not on the shaft portion.

[0077] Furthermore, in step d), a ductility of the layer with an elongation of at least 10% can be achieved.

[0078] Finally, the heating step d) of heating the layer to the second temperature may be performed only on a portion of the exhaust valve and not on the shaft portion. BRIEF DESCRIPTION OF THE DRAWINGS

[0079] The invention and its many advantages will be described in more detail below with reference to the accompanying drawings, which show some non-limiting embodiments for illustrative purposes, in which:

[0080] Figure 1 A view showing a portion of an internal combustion engine;

[0081] Figure 2A view showing an exhaust valve according to the present invention;

[0082] FIG3 shows a cross-sectional structural photograph (scanning electron microscope: SEM) of an exhaust valve in the prior art when exhibiting peak mechanical strength;

[0083] Figure 4A A cross-sectional structural photograph (scanning electron microscope: SEM) of an exhaust valve having a basket weave structure is shown;

[0084] Figure 4B A cross-sectional structural photograph (SEM) of a first region of a welded DSA760 alloy heat treated at 900° C. for 16 hours is shown;

[0085] Figure 4C Shown as Figure 4B A cross-sectional structural photograph (SEM) of the second region of the welded DSA760 alloy heat treated at 900°C for 16 hours is shown;

[0086] Figure 4D Shown as Figure 4B and Figure 4C A cross-sectional structural photograph (SEM) of the third region of the welded DSA760 alloy heat treated at 900°C for 16 hours is shown;

[0087] Figure 5 is a diagram showing the first and second heat treatments that the coating undergoes after being deposited on the end face of the disc portion and the valve seat area;

[0088] Figure 6A is a table of estimated percentages of non-lamellar areas in photographs of cross-sectional structures of exhaust valves subjected to a first heat treatment at different first temperatures and for different time periods (24 hours, 48 ​​hours, or 96 hours);

[0089] Figure 6B is a table showing the hardness measured after different first and second heat treatments.

[0090] Figure 7 shows a photograph (scanning electron microscope: SEM) of a cross-sectional structure of an exhaust valve, wherein the layers have undergone a first heat treatment; and

[0091] Figure 8 Shown Figure 7 Cross-sectional structural photograph of the exhaust valve after the layer has undergone the second heat treatment (scanning electron microscope: SEM).

[0092] All the figures are highly schematic and not necessarily to scale, and they show only parts which are necessary in order to elucidate the invention, other parts being omitted or merely suggested. DETAILED DESCRIPTION

[0093] Figure 1 A portion of an internal combustion engine 100 is shown with an exhaust valve 1 in an open position allowing intake air to be supplied from a turbocharger 20 to a scavenge air receiver 23 via a scavenge air cooler 24 and a water mist trap 25. From the scavenge air receiver 23, the intake and scavenge air flow through a row of scavenge air ports 26 in the lower end of a cylinder 27, upward through the cylinder 27 and upward toward the upper portion of the cylinder 27, while high-temperature combustion gases are injected into an exhaust receiver 28 via the open exhaust valve 1.

[0094] The exhaust valve 1 of the internal combustion engine 100 comprises an exhaust valve stem 2 having a shaft portion 3 and a disk portion 4. The disk portion 4 has an end face 5, also referred to as the combustion face, and a valve seat region 6 on the upper side 7 of the disk portion 4, which abuts a corresponding valve seat 21 on a stationary component of the internal combustion engine 100, also referred to as the bottom part of a valve housing 29. The exhaust valve 1 is mounted in an exhaust valve housing 29, which is fixed to a cylinder head 30 located at the top of the cylinder 27. In the closed position of the exhaust valve 1, the valve seat region 6 on the upper side 7 of the disk portion 4 abuts a corresponding valve seat 21 on a stationary component of the internal combustion engine 100.

[0095] The exhaust valve stem 2 also includes a deposited alloy forming a layer 8 on at least a portion of the disc portion 4, such as the end face 5 and / or the valve seat region 6 of the disc portion 4, wherein the layer 8 comprises, by mass percentage (%): 32% to 50% Cr (chromium), 0.5% to 10.0% Al (aluminum), and the remainder Ni (nickel), wherein the layer 8 has a microstructure comprising less than 10% of a basket weave structure and some lamellar structure. The composition may also include 0-4% W (tungsten).

[0096] The high temperature combustion gases, which can reach temperatures of 700°C or higher, can be very corrosive, so exposed components need to be made of very corrosion-resistant materials. The high temperature combustion gases also pass through the valve seat NR of the exhaust valve 1 at high gas velocities, particularly when the exhaust valve 1 is opened and closed.

[0097] By providing deposited layer 8 on end surface 5, the corrosion resistance of disc portion 4 at high temperatures can be improved through a combination of a first heat treatment T1 for forming a uniform layered structure with less than 10% basket weave structure and a second heat treatment T2 for achieving the desired hardness. As a result, the sealing performance of exhaust valve stem 2 is improved, and material uniformity can be achieved while still maintaining high corrosion resistance and hardness.

[0098] The term "basket weave structure" refers to the microstructure of the material of layer 8, in which the Ni-based alloy forms "plate-like or needle-like α-Cr precipitates" that appear as a substantially straight network forming a basket weave / cross pattern. The term "lamellar structure" refers to the microstructure of the material of layer 8, in which the Ni-based alloy provides a layered structure (lamellar structure) by discontinuous precipitation, the lamellar structure comprising layers 8 formed of α-Cr phase in the grains by a given aging heat treatment. This microstructure in the form of a lamellar structure is shown in Figures 3 and 4. Figure 8 FIG3 and FIG40 are shown in FIG3 and FIG40. Figure 8 The microstructure shown also has other areas in the form of darker coherent spots 43, which are α-Cr phases without a layered structure. Figure 3 shows the microstructure of an existing forged exhaust valve with the desired mechanical properties, but the forged material cannot be used to apply the layer 8 only to the most exposed areas of the disc.

[0099] Attempts have been made to manufacture a portion of the exhaust valve from one material and then weld a layer of another material onto the disc portion of the exhaust valve, but these attempts have failed to provide the same attractive combination of properties as the wrought DSA760 material. In particular, the ductility of the welded material is much lower. The reason for this is that the added welded material has a chemical composition similar to that of the nickel-based alloy (DSA760), but does not have the same uniform microstructure containing a large number of α-Cr laminae as the wrought material. Heat treatment of the welded DSA760 at 900°C for 16 hours produces the following: Figures 4B-4D The non-uniform microstructure shown. Figures 4B-4D The images in are taken from the same weld sample but a few millimeters apart and show how inhomogeneous the weld layer can be after a known / conventional heat treatment. Figure 4B 、 Figure 4C and Figure 4D shows the microstructure of a Ni-based alloy welded into layers, where Figure 4B Large globular α-Cr phase (dark spots), γ phase (light grey, no structure) and lamellar structure are shown. Figure 4C A spherical α-Cr phase (dark spots), a γ phase (light grey structureless), a lamellar structure, and a basket weave structure are disclosed. Figure 4D Spherical α-Cr phase (dark spots), lamellar structure and basket weave structure are disclosed. When comparing these closely located areas, it is clear that heat treating welded DSA760 at 900°C for 16 hours produces a very inhomogeneous microstructure.

[0100] Figure 7 The lamellar structure after the first heat treatment T1 at the first temperature is shown, with small α-Cr phase spots. Figure 8As shown, the second heat treatment T2 performed at the second temperature leads to the coarsening of the lamellar phase and the growth of spherical α-Cr domains. Figure 8 A homogeneous material without a basket weave structure is disclosed, having both a lamellar structure and small α-Cr regions, which is very similar in mechanical properties to the known wrought material of DSA760. Figure 7 The microstructure of FIG3 is more uniform than that of the forged material, and therefore the mechanical properties throughout the material are more uniform, which is particularly desirable for components exposed to high dynamic forces, such as exhaust valves.

[0101] By providing the exhaust valve stem 2 with a layer 8 having a microstructure consisting of less than 10% basket weave and some lamellar structure, preferably less than 5% basket weave, the shaft portion 3 of the exhaust valve stem 2 can be made of a different material, such as a less expensive material, while the layer 8 possesses the necessary ductility and hardness, as well as the desired degree of corrosion resistance. Materials with high corrosion resistance, sufficient hardness, and the required ductility are expensive, so by manufacturing only a thin layer 8 of the more expensive material, costs can be saved. Furthermore, worn exhaust valve stems 2 can be repaired and reused, providing an environmentally friendly and cost-effective solution. The shaft portion 3 is exposed to harmful exhaust gases, but the disc portion 4 is more susceptible to corrosion because the velocity of harmful exhaust gases at the end surface NR of the disc portion 4 is higher than the velocity of gases passing through the shaft portion 3. Furthermore, since the disc portion 4 needs to form a seal against the corresponding valve seat 21 on the stationary component of the internal combustion engine 100, the shaft portion 3 does not require the same high ductility as the disc portion 4. Therefore, the material properties of the valve seat area 6 and the end face 5 of the disk 4 must have predetermined ductility and hardness, and also high corrosion resistance, to allow the valve seat area 6 to function properly. Too low ductility also creates the risk of unexpected crack growth faster than in a more ductile material.

[0102] Layer 8 may have a hardness of 350-550 HV and a ductility of at least 10%, preferably at least 15%. The shaft portion 3 of the exhaust valve 1 requires a certain degree of hardness during engine installation. Therefore, conventional exhaust valves are manufactured with both the shaft portion and the disc portion having high hardness. However, in order for the disc portion to also function as a seal, a portion of the disc portion needs to have a certain degree of ductility, rather than the high hardness of the shaft portion. Since the exhaust valve 1 is exposed to highly corrosive, high-temperature gases, the material of the exhaust valve 1 also needs to be highly corrosion-resistant.

[0103] The disc portion 4 has a first side, namely an upper side 7 facing upward toward the exhaust passage 31, and a second side, namely an end face 5 facing downward toward the combustion chamber 22 in the cylinder 27. Figure 1As shown. The shaft portion 3 extends centrally from the upper side 7 and has a section 19 for mounting a valve rotor (not shown), a bearing region 18 for positioning in a fixed valve guide in the valve housing 29, a recess 41 for mounting an air spring piston or a spring end support, and an upper end region 42 for mounting on an actuator piston in a hydraulic valve actuator. The upper end of the shaft portion 3 can alternatively be actuated by a cam in a conventional manner.

[0104] In the installed position of the exhaust valve 1, the shaft portion 3 extends upward from the disc portion 4, passes through the exhaust passage 31, and continues upward through the valve guide and upward into the exhaust valve actuator (not shown). The piston of an air spring (not shown) is mounted on the outside of the shaft portion 3. The hydraulic actuator piston at the top of the shaft portion 3 can actuate the exhaust valve 1 downward, and when the pressure in the hydraulic actuator is released, the air spring acts in the opposite direction and closes the exhaust valve 1. Figure 2 The embodiment shown is an exhaust valve 1 for an ME and MC engine of the applicant's brand, but it can also be used for other engine types of the applicant's brand. The exhaust valve 1 can also be mechanically actuated in a well-known manner and a mechanical return spring can also be used.

[0105] The exhaust valve 1 can be used in a four-stroke internal combustion engine 100, or it can be used in a two-stroke internal combustion engine 100, preferably a large two-stroke crosshead engine, which can have a cylinder diameter in the range of 250 to 1100 mm. When the valve 1 is used in such a large two-stroke engine, the outer diameter of the disc 4 is in the range of 100 mm to 600 mm, depending on the cylinder bore. The internal combustion engine 100 using the exhaust valve stem 2 can be manufactured by MAN Energy Solutions, such as the MC or ME type, or by Wärtsilä. Or manufactured by Sulzer Diesel, such as the RTA type or RTA-flex type, or manufactured by Mitsubishi. When the exhaust valve 1 is used in a four-stroke engine, the outer diameter of the disc portion 4 is generally in the range of 50 mm to 300 mm. The valve seat area 6 is generally annular and conical and is located near the outer end portion of the disc portion 4.

[0106] The alloy can be deposited on at least the end face 5 and the valve seat region 6 of the disc 4 by depositing powder, 3D printing, welding or similar methods. When the powder is deposited, the layer 8 and the disc 4 are formed as a whole during heat treatment, for example, heat treatment to a first temperature also forms at least 90% of the layered structure.

[0107] The alloy is a Ni-based alloy of the Ni-Cr-Al system, which may be Ni-38Cr-3.8Al. The alloy may be an alloy sold by DAIDO Steel under the trade name DSA760. The composition may optionally contain the following elements, in % by mass: Fe: 0.1 to 20.0%, Si: 5% or less, B: 0.01% or less, C: 0.1% or less, Cu: 5% or less, Ti: 0.1% or less, Nb: 0.1% or less, Ta: 0.1% or less, and V: 0.1% or less, provided that Ti+Nb+Ta+V is 0.1% or less, and the remainder is unavoidable impurities and Ni.

[0108] The present invention also relates to a method for treating a portion of a disc portion 4 of an exhaust valve 1 for an internal combustion engine 100. The exhaust valve 1 includes a disc portion 4 having an upper side 7 and an end face 5, a shaft portion 3 extending from the upper side 7 of the disc portion 4, and a valve seat region 6 located on the upper side 7 of the disc portion 4. The method comprises a) providing the disc portion 4 having the valve seat region 6 and the end face 5, b) forming a layer 8 on at least a portion of the disc portion (e.g., the end face 5 of the disc portion 4 and the valve seat region 6) by depositing a Ni-Cr-Al-based Ni-based alloy, wherein the layer 8 comprises, by mass percentage (%), Cr: 32% to 50%, Al: 0.5% to 10.0%, and the remainder being optional elements, unavoidable impurities, and Ni. The method also comprises c) heating the layer 8 to a first temperature of 550-700°C for at least 16 hours, and d) heating the layer 8 to a second temperature of 750-1000°C for at least 1 hour.

[0109] Heating layer 8 to a first temperature of 550-700°C for at least 16 hours is a first heat treatment T1, and heating layer 8 to a second temperature of 750-1000°C for at least 1 hour is a second heat treatment T2. The method includes not performing a heat treatment above 750°C before the first heat treatment, and thus no heat treatment occurs between forming step b) and heating step c).

[0110] Tests have shown that forming a lamellar structure and a basket-weave structure of less than 10%, such that layer 8 has a microstructure consisting of at least 90% lamellar structure after the first heat treatment, is crucial for achieving the ductility required for the exhaust valve 1 to function adequately and avoid cracking. By forming the lamellar structure before any conventional heat treatment, layer 8 is ensured to achieve the desired ductility, and the material of layer 8 can then be heated to above 750°C to achieve the desired hardness of layer 8. However, tests have shown that heating the alloy layer 8 to above 750°C as the first heat treatment hinders the ability of the layer to subsequently undergo the lamellar transformation required to form the lamellar structure, such as by heating layer 8 to conventional temperatures of 750-1000°C, which is standard in known exhaust valve applications. Therefore, the lamellar structure must first be formed, so that a basket-weave structure of less than 10% is formed, to achieve the desired ductility, after which the material can be heated to achieve the desired hardness.

[0111] By subjecting a Ni-Cr-Al system, Ni-based aged precipitation alloy to a first heat treatment T1 of not more than 700°C and then to a second heat treatment T2 of more than 750°C, a layered structure 40 having very few α-Cr regions is first formed during the first heat treatment T1, as shown in FIG. Figure 7 As shown, a more homogeneous material is then formed during the second heat treatment T2, which has both a lamellar structure and a basket-weave structure of less than 10% and an α-Cr region, thereby producing the desired ductility, hardness, and corrosion resistance for the layer 8 of the disk component 4. Therefore, the first heat treatment is performed at a temperature below 700°C for at least 16 hours. The layer 8 is then heated at a first temperature below 700°C, for example, 550-700°C, preferably at a first temperature of 600-650°C for at least 24 hours, more preferably at a first temperature of 600-650°C for at least 36 hours, so that after the first heat treatment, the layer 8 forms a microstructure composed of at least 90% of a lamellar structure. Since the hardness is too high after the first heat treatment, a second heat treatment is performed to obtain the desired hardness by heating at a second temperature of 750-1000°C for at least 1 hour, preferably at a second temperature of 800-950°C for at least 1 hour, and more preferably at a second temperature of 875-925°C for at least 2 hours to obtain a hardness of 350-550 HV for layer 8.

[0112] like Figure 6BAs shown, heating a Ni-Cr-Al system layer at a first temperature of 600°C for 96 hours and then at a second temperature of 800°C for 24 hours resulted in a hardness of 497, the highest hardness value in this range. Furthermore, heating a Ni-Cr-Al system layer at a first temperature of 625°C for 48 hours and then at a second temperature of 900°C for 6 hours resulted in a hardness of 404, the lowest hardness value in this range. Furthermore, heating a Ni-Cr-Al system layer at a first temperature of 625°C for 48 hours and then at a second temperature of 900°C for only 2 hours resulted in a hardness of 442, which is within this range. Tests have shown that a hardness of approximately 400 HV is preferred for layers made of DSA760 material. Therefore, heating the layer at a first temperature of 625°C for 48 hours and then at a second temperature of 900°C for 6 hours, or heating it at a first temperature of 650°C for 48 hours and then at a second temperature of 900°C for 16 hours, can obtain the preferred hardness, and then the final heat treatment (H) is preferred because it requires the least energy compared to the heat treatment (D).

[0113] Figure 6A Disclosed are tables of estimated percentages of non-laminar structural areas for samples exposed to a first temperature of 600° C., 625° C., 650° C., or 700° C. for 24 hours, 48 ​​hours, or 96 hours. Figure 6A A first heat treatment is disclosed at different first temperatures for different time periods. When the non-lamellar area is estimated to be 1%, the material has approximately a 99% lamellar structure and no basket weave structure. Therefore, at a temperature of 600°C, the sample requires at least 48 hours to obtain at least a 90% lamellar structure, and optimally between 48 and 96 hours to obtain a 95% lamellar structure. When the first temperature is increased to 625°C, the sample only needs to be heated to this temperature for 48 hours to obtain a 99% lamellar structure, and only between 24 hours and 48 hours to obtain a 90-95% lamellar structure. If the first temperature is further increased to 650°C, a 96% lamellar structure is already obtained after 24 hours of heat treatment. However, if the temperature is further increased to 700°C, the sample needs to undergo a first heat treatment at the first temperature for more than 96 hours to obtain a lamellar structure of approximately 98%. For the manufacturer, the optimal first temperature is of course as low as possible, or applied for a shorter time; however, as Figure 6A As shown, the lower the temperature, the longer the exposure time, or if the temperature is around 700° C., the exposure time is also long. Therefore, the optimal temperature is about 625-650° C., at which the duration of the first heat treatment is the shortest. Figure 7 A cross-sectional structural photograph (Scanning Electron Microscope: SEM) of a sample subjected to a first heat treatment at a first temperature of 625° C. for 48 hours is disclosed, and Figure 7It is clearly shown in Figure 1 that this first heat treatment results in a 99% lamellar structure which is a very homogeneous material. Figure 7 There are only very few α-Cr areas in the MgO.

[0114] While the shaft portion 3 is exposed to harmful exhaust gases, the disc portion 4 is more susceptible to corrosion because the velocity of harmful exhaust gases at the end face 5 of the disc portion 4 is higher than the velocity of the gases as they pass through the shaft portion 3. Therefore, a worn exhaust valve stem 2 can be repaired by removing the previously worn, integral layer 8, at least on the end face 5 and the valve seat area 63. A new layer 8 is then applied and heat-treated at a first temperature of 550-700°C for at least 16 hours to achieve the desired ductility, followed by a second heat-treatment at a temperature of 750-1000°C for at least 1 hour to achieve the desired hardness. This allows a large portion of the exhaust valve 1 to be reused, an environmentally friendly and cost-effective solution.

[0115] Heating the layer 8 to the first temperature may be performed only on a portion of the exhaust valve 1, but not on the shaft portion 3, to prevent changes in the characteristics of the shaft portion 3. Therefore, heating the layer 8 to the second temperature may also be performed only on a portion of the exhaust valve 1, but not on the shaft portion 3.

[0116] For further age hardening or stress relief during welding, the method may further include e) heating the layer 8 to a third temperature of 650-750° C. for at least 4 hours as a third heat treatment. The third heat treatment may also be performed at a higher or lower temperature for other purposes.

[0117] Although the invention has been described above in conjunction with preferred embodiments thereof, it is obvious to a person skilled in the art that several modifications are conceivable without departing from the definition given in the following claims.

Claims

1. An exhaust valve (1) for an internal combustion engine (100), comprising: - an exhaust valve stem (2) comprising a shaft portion (3) and a disk portion (4) manufactured as one piece, the disk portion (4) having an end face (5) facing away from the shaft portion (3) and a valve seat region (6) located on an upper side (7) of the disk portion (4), and the exhaust valve stem (2) comprising a deposited alloy forming a layer (8) on at least a portion of the disk portion (4), wherein the layer (8) comprises, in percentage by mass (%): -Cr: 32% to 50%, -Al: 0.5% to 10.0%, and - The remaining optional elements, unavoidable impurities and Ni, Characterized in that the layer (8) has a microstructure comprising less than 10% of a basket weave structure and some lamellar structure, quantified using standard ISO-9042:1988.

2. The exhaust valve (1) according to claim 1, wherein The layer (8) comprises Ni-38Cr-3.8Al.

3. The exhaust valve (1) according to claim 1 or 2, wherein: The layer (8) comprises the following optional elements in mass percentage (%): Fe: 0.1% to 20.0%, Si: 5% or less, B: 0.01% or less, C: 0.1% or less, Cu: 5% or less, Ti: 0.1% or less, Nb: 0.1% or less, Ta: 0.1% or less, and V: 0.1% or less, The condition is that Ti+Nb+Ta+V is 0.1% or less, and the remainder is inevitable impurities and Ni.

4. Exhaust valve (1) according to any one of the preceding claims, wherein The layer (8) has a hardness of 350-550 HV measured using standard ISO 6507-1:2019 and a ductility of at least 10% elongation measured using standard ISO 6892-1:2019.

5. Exhaust valve (1) according to any one of the preceding claims, wherein The layer (8) has a microstructure comprising less than 5% of a basket weave structure quantified using standard ISO-9042:1988.

6. An internal combustion engine (100) having an exhaust valve (1) according to any one of the preceding claims.

7. A method for treating an exhaust valve (1) for an internal combustion engine (100), the exhaust valve (1) comprising a disc-shaped portion (4) having an upper side (7) and an end face (5), a shaft portion (3) extending from the upper side (7) of the disc-shaped portion (4), and a valve seat region (6) located on the upper side (7) of the disc-shaped portion (4), wherein the method comprises performing at least the following steps: a) providing a disk-shaped portion (4) having a valve seat area (6) and an end face (5), b) forming a layer (8) by depositing an alloy on at least a portion of the disk-shaped portion (4), The layer (8) comprises, by mass percentage (%): -Cr: 32% to 50%, -Al: 0.5% to 10.0%, and - The remaining optional elements, unavoidable impurities and Ni, c) heating the layer (8), characterized in that The layer (8) is heated to a first temperature of 550-700°C for at least 16 hours and to a second temperature of 750-1000°C for at least 1 hour.

8. The processing method according to claim 7, wherein: There is no heat treatment between the forming step b) and the heating step c).

9. The processing method according to claim 7 or 8, wherein: Upon heating to said first temperature, said layer (8) undergoes a lamellar transformation without forming more than 10% of a basket weave structure quantified using standard ISO-9042:1988.

10. The processing method according to any one of claims 7 to 9, wherein: When heated to the second temperature, a hardness of the layer (8) of 350-550 HV is achieved.

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