Exhaust valves of internal combustion engines

Abstract

To provide an improved exhaust valve made of a homogeneous material having high ductility and high hardness without requiring the whole exhaust valve to be made of the same material.SOLUTION: The present invention relates to an exhaust valve for an internal combustion engine, comprising an exhaust valve spindle having a shaft portion and a disk portion made as a whole, in which the disk portion has an end face facing away from the shaft portion and a valve seat area above the disk portion. The exhaust valve spindle includes a deposited alloy forming a layer on at least a portion of the disk portion, in which the layer includes, in mass percent (%), Cr: 32% to 50%, Al: 0.5% to 10.0%, and the balance optionally including any elements, as well as unavoidable impurities and Ni, and the layer has a microstructure including less than 10% of a basketweave structure and some lamellar structure. Additionally, the present invention relates to an internal combustion engine having an exhaust valve and a method for treating an exhaust valve for an internal combustion engine.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an exhaust valve for an internal combustion engine. The present invention also relates to an internal combustion engine having an exhaust valve, and to a method for treating the valve seat area of ​​an exhaust valve for an internal combustion engine. [Background technology]

[0002] U.S. Patent Application Publication No. 2016 / 0215660 discloses an exhaust valve for a marine diesel engine, including a shaft portion and an umbrella portion integral with each other, made of a NiCrAl-based Ni-based age-hardened alloy, the exhaust valve having a layered structure and an overall hardness of 600 HV or less, the layered structure including a layer formed of an α-Cr phase having a thickness of 150 nm or more that has been age-hardened beyond its peak mechanical strength. It is important to note that U.S. Patent Application Publication No. 2016 / 0215660 relates to forging of the material to produce the one-piece valve, which imposes high manufacturing costs. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] US Patent Application Publication No. 2016 / 0215660 Summary of the Invention

[0004] It remains a problem in the prior art to improve exhaust valves, particularly their ability to withstand mechanical shock and hot combustion gases, while keeping manufacturing costs down.

[0005] The present invention relates to an exhaust valve for an internal combustion engine. The present invention also relates to an internal combustion engine having an exhaust valve, and to a method for treating the valve seat area of ​​an exhaust valve for an internal combustion engine.

[0006] In internal combustion engines, hot combustion gases can be very aggressive, requiring exposed components to be made of highly corrosion-resistant materials. These gases pass through the exhaust valve seat at high gas velocities, especially when the valve opens and closes. To ensure exhaust valves can withstand the hot, aggressive combustion gases as well as the mechanical loads, the valves must be made of expensive Ni-based materials to achieve the necessary mechanical properties, namely, strength and ductility. State-of-the-art exhaust valves are made of forged Ni-Cr-Al alloys, which provide an optimal balance between strength (exemplified by hardness), ductility, and corrosion resistance through a special microstructure containing alpha-Cr lamellae. Until now, such exhaust valves have been fully forged in DSA760 material to achieve optimal hardness, ductility, and corrosion resistance.

[0007] The shaft portion of an exhaust valve must have a certain strength to function within the engine. For the disk portion of the exhaust valve to also function as a seal, the portion of the disk, known as the seat, must have sufficient hardness. Therefore, known exhaust valves are manufactured with high strength and hardness in both the shaft and disk portions. However, good ductility must be maintained to avoid the risk of brittle fracture and to allow for cold rolling of the seat. Attempts have been made to make parts of the exhaust valve one material and parts of another, for example by welding layers onto the disk portion of the exhaust valve. However, these have not been successful in providing the same attractive combination of properties as forged DSA760 material; specifically, the ductility of the welded material is much lower. The reason for this is that additive materials with a similar chemical composition to DSA760 material do not have the same homogeneous microstructure as the forged material, which contains a large proportion of alpha-Cr lamellae. Generally, materials added by welding or other methods can experience high levels of segregation (uneven distribution of chemical elements) during manufacturing, resulting in uneven response during heat treatment and a heterogeneous microstructure with localized regions having different microstructures and mechanical properties. When creating exhaust valves where only the seat and most exposed areas are made of very expensive materials capable of withstanding aggressive gases, or when adding new material to forged spindles for extended service life, known heat treatment processes either require very high temperatures (solution treatment) or result in a heterogeneous microstructure and poor ductility. Full solution treatment is undesirable due to cost and dilution between the welded and forged materials. It is difficult to fully solution heat treat the disk without affecting the shaft's properties while ensuring dimensional stability, especially for valve service life extension. Furthermore, even if solution treatment is performed, it may not be sufficient to eliminate segregation and the uneven response to subsequent age-hardening treatment.

[0008] Testing has shown that the material of weld materials exposed to direct heat treatments, such as known heat treatments at temperatures above 750°C, is not homogeneous. As shown in Figures 4A and 4D, some regions have a basketweave microstructure resembling the so-called topologically close-packed (TCP) phase morphology, which is known to be particularly detrimental to the ductility of Ni-based alloys. Other regions, shown in Figures 4B and 4C, have no internal structure. Finally, others have the desired lamellar structure, with each structure having substantially different mechanical properties. Having components of non-homogeneous materials with different mechanical properties is particularly undesirable for components such as exhaust valves, which are subjected to thermomechanical fatigue when loaded by severe dynamic forces and temperature fluctuations. Therefore, it is important for the mechanical properties of exhaust valves that the microstructure is homogeneous and that regions with known detrimental microstructural morphologies are minimized.

[0009] The object of the present invention is to alleviate, in whole or in part, the above-mentioned drawbacks and disadvantages of the prior art, and more particularly to provide an improved exhaust valve in which the areas thereof exposed to mechanical shock, such as the seat, and the hot combustion gases (e.g., the seat, nose, and bottom) are made of a homogeneous material that also has high ductility and high hardness, without the need to make the entire exhaust valve out of the same material.

[0010] The above objects, together with many other objects, advantages, and features which will become apparent from the following description, are achieved by providing an exhaust valve for an internal combustion engine, comprising: An exhaust valve spindle having a shaft portion and a disk portion fabricated as a single monolithic whole, the disk portion having an end face facing away from the shaft portion and a valve seat area above the disk portion, the exhaust valve spindle including a deposited alloy forming a layer on at least a portion of the disk portion, the layer (8) comprising, in mass percent (%): -Cr: 32%~50%, -Al: 0.5% to 10.0%, and The balance is made up of optional elements, inevitable impurities and Ni. The solution is achieved by an exhaust valve of an internal combustion engine, comprising an exhaust valve spindle including: a layer having a microstructure including less than 10% basketweave structure and some lamellar structure.

[0011] Additionally, the layer may be a monolithic layer, which differs from the layer structure seen under a scanning electron microscope.

[0012] The layer may also have a microstructure that is at least 25% lamellar.

[0013] The microstructure is studied by taking electron micrographs, more specifically cross-sectional structural photographs using a scanning electron microscope. From the micrographs, the extent of the surface area covered by the basketweave and / or lamellar structure can be assessed. The ISO-9042:1988 standard can be used for such quantification of the extent of the surface area covered by the basketweave and / or lamellar structure.

[0014] The alloy contains Cr, Al, and Ni, so it is a Ni-Cr-Al system. The alloy can be described as Ni-based because the balance is Ni (and unavoidable impurities).

[0015] The alloy may be age precipitated in one embodiment.

[0016] The layer may have a microstructure consisting of less than 5% basketweave structure, and preferably the layer may have a microstructure that is substantially free of basketweave structure.

[0017] The exhaust valve is particularly suitable for large ships such as ships having two-stroke marine engines producing more than 10,000 Hp (horsepower), more than 20,000 Hp or more than 30,000 Hp due to the high requirements of such exhaust valves.

[0018] In one embodiment, a portion of the disc portion may be at least the end face and / or the valve seat area.

[0019] In another embodiment, the alloy may have a composition comprising, in mass percent (%), 0-4% W.

[0020] Furthermore, by having an exhaust valve spindle with a layer having a microstructure that includes less than 10% basketweave and some lamellar structure, the shaft portion of the exhaust valve spindle can be made of a different material, e.g., a cheaper material, while the layer has the necessary ductility and hardness, as well as the necessary degree of corrosion resistance. Materials with high corrosion resistance, sufficient hardness, and the necessary ductility are expensive, so costs can be reduced by simply creating a small layer of a more expensive material. Furthermore, worn exhaust valve spindles can be repaired and therefore reused, which is an environmentally friendly and cost-saving solution. While the shaft portion is exposed to harmful exhaust gases, the disk portion is more exposed to corrosion because the velocity and temperature of harmful exhaust gases are higher at the end face of the disk portion than when the gases pass through the shaft portion. Furthermore, because the disk portion must form a seal with the corresponding bottom component on the stationary part of the internal combustion engine, the material properties of the valve seat area and the end face of the disk portion must be characterized by both predetermined ductility and hardness for the valve seat area to function properly, as well as high corrosion resistance. Too little ductility also risks unintended crack propagation faster than in a more ductile material. The required ductility of the layer implies an elongation of more than 10%, and ductility can be measured using standard ISO 6892-1:2019. The required hardness of the layer implies a hardness of 350 to 550 HV, and hardness can be measured using standard ISO 6507-1:2019. Having a layer of alloy containing more than 32% chromium (by mass) provides sufficient corrosion resistance. Obtaining high strength and good ductility with a high chromium content has been a historical and long-standing challenge for Ni-base superalloys. However, by having a layer with a microstructure consisting of less than 10% basketweave and some lamellar structure, a high chromium content can also provide high strength and good ductility.

[0021] Known forged exhaust valves cannot be repaired with layers having a lamellar microstructure because the key to obtaining a homogeneous material with the necessary corrosion resistance, ductility, and hardness is the fact that the material is forged. Since forging only layers is not possible, the entire exhaust valve must be replaced.

[0022] With a microstructure consisting of less than 10% basketweave and some lamellar structure, the material is sufficiently homogeneous to withstand severe dynamic forces such as those to which an exhaust valve is exposed.

[0023] Furthermore, in known solutions, layers are welded to the disk portion of exhaust valve spindles, but this process does not allow for lamellar transformation to occur, resulting in a homogeneous material with the required corrosion resistance and ductility without reducing hardness or ductility below the required level. However, by heating the welded layer to a relatively low first temperature of 550-700°C for at least 12 hours, and then to a higher second temperature of 750-1000°C for at least 1 hour, the layer at the first temperature forms a homogeneous microstructure consisting of at least 90% lamellar structure. At the second temperature, the structure within the layer is coarsened to obtain the desired combination of ductility and hardness, with less than 10% basketweave structure and some lamellar structure. The low temperature of the first step is important to allow only so-called boundary diffusion to occur. If the temperature is too low and volume diffusion does not occur, the basketweave structure is suppressed, and the formation of α-Cr lamellae becomes dominant in all regions, despite the presence of segregation. After sufficient time has passed for the lamellar transformation to reach greater than 90% completion (according to the TTT (time-temperature transformation) diagram), the temperature is increased in a second heating step to promote volume diffusion, coarsening the lamellar structure and decreasing hardness while increasing ductility until the desired combination of hardness and ductility is reached. Because the transformation to a lamellar structure has already occurred in the first step, no further transformation is possible, effectively limiting the area free of basketweave structure formation or precipitation to less than 10%. Coarsening can continue until a portion of the lamellar structure is destroyed, while still maintaining the desired range of hardness and ductility.

[0024] Additionally, the alloy may be deposited on at least the end face and seat area of ​​the disc portion by powder deposition, 3D printing, welding, or the like.

[0025] In one embodiment, the alloy is Ni-38Cr-3.8Al.

[0026] Additionally, in one embodiment, the alloy is the alloy sold by DAIDO under the trade name DSA760.

[0027] Additionally, the composition may comprise, in weight percent (%): 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, V: May contain 0.1% or less of optional elements; However, Ti+Nb+Ta+V is 0.1% or less, and the remainder is unavoidable impurities and Ni.

[0028] Additionally, the layer may have a hardness of 350-550 HV and a ductility of at least 10% elongation.

[0029] The layers may also have a microstructure consisting of less than 5% basketweave structure.

[0030] Additionally, the present invention relates to an internal combustion engine having an exhaust valve.

[0031] Additionally, the present invention also provides a method for treating a valve seat area of ​​an exhaust valve for an internal combustion engine, comprising the steps of: The exhaust valve includes a disk portion having an upper side and an end surface, a shaft portion extending from the upper side of the disk portion, and a valve seat area located above the disk portion, and is manufactured by at least the following steps: a) providing a disk portion having a valve seat area and an end face; b) forming a layer on at least a part of the disk portion by depositing an alloy as a layer 8, Layer 8 has, in mass percent (%): -Cr: 32%~50%, -Al: 0.5% to 10.0%, and - the remainder comprising Ni; c) heating the layer to a first temperature of 550-700°C for a duration of at least 12 hours; d) heating the layer to a second temperature of 750-1000°C for a duration of at least 1 hour; The present invention relates to a method, comprising:

[0032] In one embodiment, the deposited alloy may be age precipitated.

[0033] Additionally, a portion of the disc portion may be at least an end face and / or a valve seat area.

[0034] By subjecting a Ni-Cr-Al, Ni-based alloy to a first heat treatment below 700°C and then a second heat treatment above 750°C, a first region with a fine lamellar structure is formed in the first heat treatment, resulting in a homogeneous structure and high hardness and low ductility, and then in the second heat treatment, coarsening of the lamellar structure occurs, reducing hardness and increasing ductility until the required combination of ductility, hardness and corrosion resistance for the disk portion layer is achieved.

[0035] Additionally, heating the layer to a first temperature of 550-700° C. for at least 12 hours can be the first heat treatment.

[0036] Further, heating the layer to a second temperature of 750-1000° C. for at least 1 hour can be the second heat treatment.

[0037] Also, forming the layer on at least a portion of the disk portion may be performed by welding.

[0038] Additionally, there may be no heat treatment between the forming step b) and the heating step c).

[0039] Furthermore, there may be no need to perform a heat treatment above 750° C. between the forming step b) and the heating step c).

[0040] Furthermore, in heating step c), the layer may undergo a lamellar transformation without forming more than 10% of a basketweave texture.

[0041] Additionally, the heating at the first temperature in heating step c) may have a duration of at least 12 hours, preferably at least 24 hours.

[0042] Furthermore, the heating at the first temperature in heating step c) may be carried out at a first temperature of 600-650°C for a duration of preferably at least 12 hours, more preferably at a first temperature of 600-650°C for at least 24 hours.

[0043] Also, the heating at the second temperature in heating step d) may have a duration of at least 2 hours.

[0044] Additionally, the heating at the second temperature in heating step d) may be carried out preferably at a second temperature of 800-950°C for a duration of at least 1 hour, more preferably at a second temperature of 850-925°C for a duration of at least 1 hour, and even more preferably at a second temperature of 850-925°C for a duration of at least 2 hours.

[0045] Furthermore, the heating at the second temperature in heating step d) may preferably be carried out at a second temperature of 850°C for a duration of 10 to 24 hours, or at a second temperature of 900°C for a duration of 2 to 10 hours.

[0046] Furthermore, in the heating step c), the layer may form a microstructure consisting of at least 90% lamellar structures.

[0047] Additionally, in the heating step d), a layer hardness of 350 to 550 HV may be obtained.

[0048] Also, in the heating step d), the layer may form a microstructure consisting of less than 10% basketweave structure and some lamellar structure, preferably less than 5% basketweave structure.

[0049] Furthermore, in the heating step d), the layer may form a microstructure consisting of less than 10% basketweave structure and at least 25% lamellar structure, preferably at least 50% lamellar structure.

[0050] Furthermore, in step a) the previous layer on at least the end face and the valve seat area may be removed.

[0051] The method may also further comprise the step e) of heating the layer after step d) to a third temperature of 650-750° C. for a duration of at least 4 hours.

[0052] The valve seat may be cold rolled between steps d) and e).

[0053] Additionally, heating the layer to a third temperature of 650-750° C. for a duration of at least 4 hours may be the third heat treatment.

[0054] 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.

[0055] Furthermore, in step d), a ductility of the layer of at least 10% elongation may be obtained.

[0056] 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.

[0057] The invention and its many advantages will be explained in more detail below with reference to the accompanying schematic drawings, which show, by way of example, several non-limiting embodiments. [Brief explanation of the drawings]

[0058] [Figure 1] 1 is a diagram of a portion of an internal combustion engine. [Figure 2] 1 is a diagram of an exhaust valve according to the present invention; [Figure 3] 1 is a photograph (scanning electron microscope: SEM) of the cross-sectional structure of a prior art exhaust valve when showing peak mechanical strength. [Figure 4A] This is a photograph (scanning electron microscope: SEM) of the cross-sectional structure of an exhaust valve with a basket weave structure. [Figure 4B] This is a cross-sectional structure photograph (scanning electron microscope: SEM) of the first region of welded DSA760 that was heat treated at 900°C for 16 hours. [Figure 4C] FIG. 4C is a cross-sectional structural photograph (scanning electron microscope: SEM) of the second region of the welded DSA760, which was heat-treated at 900° C. for 16 hours as shown in FIG. 4B. [Figure 4D] 4B and 4C are cross-sectional structural photographs (scanning electron microscope: SEM) of a third region of the welded DSA760 that was heat treated at 900° C. for 16 hours. [Figure 5] 10A and 10B illustrate first and second heat treatments applied after deposition on the end face and valve seat area of ​​the disk portion. [Figure 6A] 1 is a table of estimated percentages of non-lamellar regions in cross-sectional structural photographs of exhaust valves subjected to first heat treatments at various first temperatures and for various durations of 24 hours, 48 ​​hours, or 96 hours. [Figure 6B] 1 is a table of measured hardness after different first and second heat treatments. [Figure 7] 1 is a cross-sectional structural photograph (scanning electron microscope: SEM) of an exhaust valve whose layer has undergone a first heat treatment. [Figure 8] 8 is a photograph (Scanning Electron Microscope: SEM) of the cross-sectional structure of the exhaust valve of FIG. 7 after the layer has also been subjected to a second heat treatment. DETAILED DESCRIPTION OF THE INVENTION

[0059] All figures are highly schematic and not necessarily to scale, showing only those parts necessary to elucidate the invention, other parts being omitted or merely suggested.

[0060] 1 shows a portion of an internal combustion engine 100 with an exhaust valve 1 in an open position allowing intake air to be supplied from a turbocharger 20 through a scavenge air cooler 24 and a water mist catcher 25 to a scavenge air receiver 23. The intake and scavenge air flows from the scavenge air receiver 23 through a row of scavenge air ports 26 at the bottom end of a cylinder 27, upward through the cylinder 27 towards the top of the cylinder 27, and simultaneously expelling hot combustion gases through the open exhaust valve 1 into an exhaust gas receiver 28.

[0061] The exhaust valve 1 of the internal combustion engine 100 comprises an exhaust valve spindle 2 having a shaft portion 3 and a disc portion 4. The disc portion 4 has an end face 5, also referred to as the combustion face, and a valve seat area 6 on an upper side 7 of the disc portion 4 that abuts against a corresponding valve seat 21 on a stationary portion of the internal combustion engine 100, also referred to as the bottom part of a valve housing 29. The exhaust valve 1 is mounted on an exhaust valve housing 29 that is fixed to a cylinder cover 30 on top of a cylinder 27. In the closed position of the exhaust valve 1, the valve seat area 6 on the upper side 7 of the disc portion 4 abuts against the corresponding valve seat 21 on the stationary portion of the internal combustion engine 100.

[0062] The exhaust valve spindle 2 further 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, the layer 8 including, in mass percent (%), 32% to 50% Cr (chromium), 0.5% to 10.0% Al (aluminum), and the balance Ni (nickel), the layer 8 having a microstructure consisting of less than 10% basketweave structure and some lamellar structure. Potentially, the component composition further includes 0 to 4% W (tungsten).

[0063] Hot combustion gases, at temperatures of up to 700°C or more, can be very aggressive and therefore exposed components need to be made of highly corrosion-resistant materials. The hot combustion gases also pass through the seat NR of the exhaust valve 1 at high gas velocities, particularly as the valve 1 opens and closes.

[0064] The deposition layer 8 on the end surface 5, combined with the first heat treatment T1 to form a homogeneous lamellar structure with less than 10% basketweave and the second heat treatment T2 to form the required hardness, improves the corrosion resistance of the disk portion 4 at high temperatures. As a result, the sealing performance of the exhaust valve spindle 2 can be improved, and material homogeneity can be achieved while maintaining high corrosion resistance and hardness.

[0065] The term "basketweave structure" refers to the microstructure of the material of layer 8 in which the Ni-based alloy forms "plate-like or needle-like alpha-Cr precipitates" that appear in a substantially linear network that forms a basketweave / cross pattern. The term "lamellar structure" refers to the microstructure of the material of layer 8 in which the Ni-based alloy provides a lamellar structure (lamellar structure) containing layers 8 formed of the α-Cr phase within the grains through discontinuous precipitation after a given age-hardening heat treatment. Such a microstructure in the form of a lamellar structure is shown in region 40 of Figures 3 and 8. The microstructures shown in Figures 3 and 8 also have other regions in the form of black coherent spots 43, which are the α-Cr phase without a lamellar structure. Figure 3 shows the microstructure of a known forged exhaust valve with the required mechanical properties, but it is not possible to apply layer 8 only to the most exposed areas of the disk section using forged material.

[0066] Attempts have been made to fabricate a portion of the exhaust valve from one material and then weld a layer of another material onto the exhaust valve disc portion, but these attempts have not been successful in providing the same attractive combination of properties as the forged DSA760 material. Specifically, the ductility of the welded material is much lower. This is because the added weld material, which has a similar chemical composition to the Ni-based alloy (DSA760) material, does not have the same homogeneous microstructure with a large proportion of alpha-Cr lamellae as the forged material. Heat treating the welded DSA760 at 900°C for 16 hours results in a heterogeneous microstructure, as shown in Figures 4B-4D. The images in Figures 4B-4D were taken from the same weld specimen but a few millimeters apart, and they demonstrate the extent to which the weld layer is heterogeneous after known / conventional heat treatments. Figures 4B, 4C, and 4D show the microstructure of a Ni-based alloy welded in layers. Figure 4B shows large globular α-Cr phases (dark spots), γ phases (light gray with no structure), and lamellar structures. Figure 4C shows globular α-Cr phases (dark spots), γ phases (light gray with no structure), lamellar structures, and basketweave structures. Figure 4D shows globular α-Cr phases (dark spots), lamellar structures, and basketweave structures. Comparing these very closely spaced regions, it is clear that heat treatment of welded DSA760 at 900°C for 16 hours results in a very inhomogeneous microstructure.

[0067] Figure 7 shows a lamellar structure after a first heat treatment (T1) at a first temperature, resulting in fewer spots of α-Cr phase. As shown in Figure 8, a second heat treatment (T2) at a second temperature results in coarsening of the lamellar phase and the growth of globular α-Cr domains. Figure 8 shows a homogeneous material without a basketweave structure, but with both a lamellar structure and smaller α-Cr domains, and its mechanical properties are very similar to those of the known forged material DSA760. The microstructure of Figure 7 is more homogeneous than that of the forged material of Figure 3, and therefore the mechanical properties are also more uniform throughout the material, which is particularly necessary for components exposed to high dynamic forces, such as exhaust valves.

[0068] By having an exhaust valve spindle 2 with a layer 8 having a microstructure consisting of less than 10% basketweave and some lamellar structure, preferably less than 5% basketweave, the shaft portion 3 of the exhaust valve spindle 2 can be made of a different material, e.g., a cheaper material, while the layer 8 has the required degree of ductility and hardness as well as corrosion resistance. Materials with high corrosion resistance, sufficient hardness, and the required ductility are expensive, so creating only a small layer 8 of the more expensive material can save costs. Furthermore, worn exhaust valve spindles 2 can be repaired and therefore reused, which is an environmentally friendly and cost-saving solution. While the shaft portion 3 is exposed to harmful exhaust gases, the disk portion 4 is more exposed to corrosion because the velocity of the harmful exhaust gases is higher at the end face NR of the disk portion 4 than when the gases pass through the shaft portion 3. Furthermore, the shaft portion 3 does not require as much ductility as the disk portion 4 because the disk portion 4 must form a seal with a corresponding valve seat 21 on a stationary part of the internal combustion engine 100. Therefore, the material properties of the seat area 6 and the end face 5 of the disc portion 4 must be characterized by both a certain ductility and hardness in order for the seat area 6 to function properly, but also by high corrosion resistance. Too low ductility also creates the risk of unintended cracks propagating faster than in a more ductile material.

[0069] Layer 8 can have a hardness of 350-550 HV and a ductility of at least 10% elongation, preferably at least 15% elongation. Because the shaft portion 3 of the exhaust valve 1 requires a certain hardness during the engine installation process, known exhaust valves are manufactured with both the shaft portion and the disk portion having high hardness. However, in order for the disk portion to also function as a seal, a portion of the disk portion must have a certain ductility, and does not necessarily have the same hardness as the shaft portion. Because the exhaust valve 1 is exposed to very aggressive high-temperature gases, the material of the exhaust valve 1 must also be highly corrosion-resistant.

[0070] 1, the disk portion 4 has a first side, which is the top side 7, facing upward toward the exhaust channel 31, and a second side, which is the end surface 5, facing downward toward the combustion chamber 22 in the cylinder 27. The shaft portion 3 extends centrally from the top side 7 and has a portion 19 for mounting a valve rotor (not shown), a bearing area 18 for positioning within a stationary valve guide in the valve housing 29, a groove 41 for mounting an air spring piston or spring end support, and an upper end area 42 for mounting to an actuator piston of a hydraulic valve actuator. Alternatively, the upper end of the shaft portion 3 may be cam actuated in a conventional manner.

[0071] In the installed position of the exhaust valve 1, the shaft portion 3 extends upward from the disc portion 4, passes through the exhaust channel 31, continues upward through the valve guide, and enters the exhaust valve actuator (not shown). A pneumatic spring piston (not shown) is attached to the outside of the shaft portion 3. The pneumatic actuator piston on top of the shaft portion 3 can actuate the exhaust valve 1 for downward movement, and the pneumatic spring acts in the opposite direction, closing the exhaust valve 1 when pressure in the pneumatic actuator is released. The embodiment shown in FIG. 2 is an exhaust valve 1 for one of the applicant's brand types ME and MC engines, but may also be used for other engine types of the applicant's brand. The exhaust valve 1 can also be mechanically actuated in a known manner, or a mechanical return spring can be used.

[0072] The exhaust valve 1 may be for a four-stroke internal combustion engine 100 or a two-stroke internal combustion engine 100, preferably a large two-stroke crosshead engine, which may have a cylinder diameter ranging from 250 to 1100 mm. The outer diameter of the disk portion 4 ranges from 100 mm to 600 mm depending on the cylinder bore when the valve 1 is for such a large two-stroke engine. The internal combustion engine 100 utilizing the exhaust valve spindle 2 may be manufactured by MAN Energy Solutions, such as the MC or ME type, Wartsla or Sulzer Diesel, such as the RTA Flex type, or Mitsubishi. When the exhaust valve 1 is for a four-stroke engine, the outer diameter of the disk portion 4 typically ranges from 50 mm to 300 mm. The valve seat area 6 is generally annular and conical and is located adjacent to the outer end of the disk portion 4.

[0073] The alloy may be deposited on at least the end face 5 and the valve seat region 6 of the disc portion 4 by powder deposition, 3D printing, welding or similar methods. If powder is deposited, the layer 8 and the disc portion 4 as a monolithic whole are formed during heat treatment, for example, to a first temperature that also forms at least 90% lamellar structures.

[0074] The alloy may be a Ni-Cr-Al, Ni-based alloy, such as Ni-38Cr-3.8Al. The alloy may be the alloy sold by DAIDO Steel under the trade name DSA760. The composition may include, in mass percent (%), optional elements: Fe: 0.1-20.0%, 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, with the proviso that Ti + Nb + Ta + V is 0.1% or less, and the balance is unavoidable impurities and Ni.

[0075] The present invention also relates to a method for treating a portion of a disk portion 4 of an exhaust valve 1 for an internal combustion engine 100, the exhaust valve 1 comprising: a disk portion 4 having an upper side 7 and an end face 5; a shaft portion 3 extending from the upper side 7 of the disk portion 4; and a valve seat area 6 located on the upper side 7 of the disk portion 4. The method includes the steps of: a) providing the disk portion 4 with the valve seat area 6 and the end face 5; and b) forming a layer 8 on at least a portion of the disk portion, such as the end face 5 and the valve seat area 6 of the disk portion 4, by depositing a Ni-Cr-Al based Ni-based alloy, the layer 8 containing, in mass percent (%), 32% to 50% Cr, 0.5% to 10.0% Al, and the remainder being optional elements, unavoidable impurities, and Ni. The method further includes the steps of c) heating layer 8 to a first temperature of 550-700°C for a duration of at least 16 hours, and d) heating layer 8 to a second temperature of 750-1000°C for a duration of at least 1 hour.

[0076] Heating layer 8 to a first temperature of 550-700°C for a duration of at least 16 hours is the first heat treatment T1, and heating layer 8 to a second temperature of 750-1000°C for a duration of at least 1 hour is the second heat treatment T2. Because the method does not include a heat treatment above 750°C prior to the first heat treatment, there is no heat treatment between forming step b) and heating step c).

[0077] Testing has shown that forming a lamellar structure and less than 10% basketweave structure, so that layer 8 has a microstructure consisting of at least 90% lamellar structure after the first heat treatment, is important for the exhaust valve 1 to obtain the ductility required to function well and avoid cracking. Forming the lamellar structure before any conventional heat treatment is performed ensures that layer 8 obtains the required ductility, and the material of layer 8 can then be heated to above 750°C to obtain the required hardness of layer 8. However, testing has shown that heating layer 8 of this alloy above 750°C as a first treatment prevents the layer from subsequently undergoing lamellar transformation to form a lamellar structure, for example, by heating layer 8 to conventional temperatures of 750-1000°C, which is the standard applied in known exhaust valves. Therefore, the lamellar structure must first be formed so that less than 10% basketweave structure is formed to obtain the required ductility, and the material can then be heated to obtain the required hardness.

[0078] By subjecting a Ni-Cr-Al system, i.e., Ni-based aging-precipitation alloy, to a first heat treatment T1 at or below 700°C and then to a second heat treatment T2 above 750°C, a lamellar structure 40 with very little α-Cr domains is first formed in the first heat treatment T1, as shown in FIG. 7, and then in the second heat treatment T2, a more homogeneous material is formed with both a lamellar structure and less than 10% basketweave structure, as well as α-Cr domains that produce the ductility, hardness, and corrosion resistance required for layer 8 of disk portion 4.

[0079] Thus, the first heat treatment is carried out at a temperature below 700°C for a duration of at least 16 hours. Heating layer 8 at a first temperature below 700°C, e.g., 550-700°C, may be carried out at a first temperature of 600-650°C for a duration of at least 24 hours, more preferably at a first temperature of 600-650°C for a duration of at least 36 hours, so that layer 8 forms a microstructure consisting of at least 90% lamellar structures after the first heat treatment. A second heat treatment to obtain the required hardness, which is too high after the first treatment, is carried out by heating at a second temperature of 750-1000°C for a duration of at least 1 hour, preferably at a second temperature of 800-950°C for a duration of at least 1 hour, more preferably at a second temperature of 875-925°C for a duration of at least 2 hours, to obtain a hardness of layer 8 of 350-550 HV.

[0080] As can be seen in Figure 6B, heating the Ni-Cr-Al-based layer at a first temperature of 600°C for 96 hours and a second temperature of 800°C for 24 hours results in a hardness of 497, a hardness at the high end of the interval. Furthermore, heating the Ni-Cr-Al-based layer at a first temperature of 625°C for 48 hours and a second temperature of 900°C for 6 hours results in a hardness of 404, a hardness at the low end of the interval. Additionally, heating the Ni-Cr-Al-based layer at a first temperature of 625°C for 48 hours and a second temperature of 900°C for only 2 hours results in a hardness of 442, a hardness within the interval. Tests have shown that for a layer of DSA760 material, a hardness of about 400 HV is preferred, and therefore heating the layer for only 48 hours at a first temperature of 625°C and 6 hours at a second temperature of 900°C, or for 48 hours at a first temperature of 650°C and 16 hours at a second temperature of 900°C, will result in the preferred hardness, with the final heat treatment (H) then being preferred as it requires the least energy compared to heat treatment (D).

[0081] Figure 6A shows a table of estimated percentages of non-lamellar structured regions for specimens exposed to first temperatures of 600°C, 625°C, 650°C, or 700°C for durations of 24, 48, or 96 hours. Figure 6A also shows different first temperatures for various durations of the first heat treatment. With an estimated non-lamellar region of 1%, the material has approximately 99% lamellar structure and no basketweave texture. Thus, at a temperature of 600°C, specimens require at least 48 hours to achieve at least 90% lamellar structure, and optimally, 48 to 96 hours to achieve 95% lamellar structure. If the first temperature is increased to 625°C, specimens only need to be heated to that temperature for 48 hours to achieve 99% lamellar structure, and only 24 to 48 hours to achieve 90-95% lamellar structure. Further increasing the first temperature to 650°C already results in a 96% lamellar structure after 24 hours of heat treatment. However, further increasing the temperature to 700°C requires a first heat treatment at the first temperature for longer than 96 hours to achieve approximately 98% lamellar structure. The optimal first temperature for a manufacturer is, of course, the lowest possible or shortest possible exposure time, as shown in Figure 6A. However, as temperatures approach 700°C, the exposure time also increases. Therefore, the optimal temperature is approximately 625-650°C, since this provides the shortest duration of the first heat treatment. Figure 7 shows a cross-sectional structural photograph (scanning electron microscope: SEM) of a specimen subjected to a first heat treatment at 625°C for 48 hours. Figure 7 clearly shows that this first treatment results in a highly homogeneous material with a 99% lamellar structure. Only a small amount of α-Cr regions is present in Figure 7.

[0082] The shaft portion 3 is exposed to harmful exhaust gases, but the disk portion 4 is more susceptible to corrosion because the velocity of the harmful exhaust gases is higher at the end face 5 of the disk portion 4 than when the gases pass through the shaft portion 3. Therefore, a worn exhaust valve spindle 2 can be repaired by removing the previously worn monolithic layer 8 on at least the end face 5 and the valve seat area 6, then applying a new layer 8 and heat-treating the layer 8 at a first temperature of 550-700°C for a duration of at least 16 hours to obtain the required ductility, and then at a second temperature of 750-1000°C for a duration of at least 1 hour to obtain the required hardness. In this way, most of the exhaust valve 1 can be reused, which is an environmentally friendly and cost-saving solution.

[0083] The step of heating layer 8 to a first temperature may be performed on only a portion of exhaust valve 1 and not on shaft portion 3 to prevent changes in the properties of shaft portion 3. Accordingly, heating layer 8 to a second temperature may also be performed on only a portion of exhaust valve 1 and not on shaft portion 3.

[0084] For purposes of further precipitation hardening or annealing of stresses from the welding process, the method may further include step e) heating layer 8 to a third temperature of 650-750°C for a duration of at least 4 hours as a third heat treatment. The third heat treatment may be performed at a higher or lower temperature for other purposes.

[0085] While the present invention has been described above with reference to preferred embodiments thereof, it will be apparent to those skilled in the art that several modifications can be made without departing from the invention as defined by the claims that follow. [Explanation of symbols]

[0086] 1 Exhaust valve 2 exhaust valve spindle 3 Shaft part 4 Disc part 5 End face 6 Valve seat area 7 Upper side of the disk 8 Monolithic Layer 18 Bearing area 19 Part for attaching the valve rotor 20 Turbocharger 21 Valve seat 22 Combustion chamber 23 Scavenging air receiving part 24 Scavenging air cooler 25 Water Mist Catcher 26 Scavenging port 27 cylinders 28 Exhaust gas receiving part 29 Exhaust valve housing 30 Cylinder cover 31 Exhaust channel 40 Lamellar structure, region 41 Groove 42 Top area 43 Coherent Spot 100 Internal combustion engine T1 First heat treatment T2 Second heat treatment

Claims

1. In an exhaust valve (1) of an internal combustion engine (100), 1. An exhaust valve spindle (2) having a shaft portion (3) and a disk portion (4) made as a whole monolithically, the disk portion (4) having an end face (5) facing away from the shaft portion (3) and a valve seat area (6) on an upper side (7) of the disk portion (4), the exhaust valve spindle (2) comprising a deposited alloy forming a layer (8) on at least a portion of the disk portion (4), the layer (8) comprising, in mass percent (%): Cr: 32% to 50%, Al: 0.5% to 10.0%, and The balance is made up of optional elements, inevitable impurities and Ni. An exhaust valve (1) comprising an exhaust valve spindle (2) comprising: The layer (8) is characterized in that it has a microstructure that includes less than 10% basketweave structure and lamellar structure, as quantified using the ISO-9042:1988 standard.

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

3. The layer (8) comprises, in mass percent (%): Fe: 0.1-20.0%, 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, V: 0.1% or less of any optional element; 3. The exhaust valve (1) according to claim 1 or 2, wherein Ti+Nb+Ta+V is 0.1% or less, and the balance is unavoidable impurities and Ni.

4. 3. The exhaust valve (1) according to claim 1 or 2, wherein the layer (8) has a hardness of 350 to 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. 3. The exhaust valve (1) according to claim 1 or 2, wherein the layer (8) has a microstructure that includes less than 5% basketweave structure as quantified using the ISO-9042:1988 standard.

6. An internal combustion engine (100) having an exhaust valve (1) according to claim 1 or 2.

7. 1. A method for treating an exhaust valve (1) for an internal combustion engine (100), the exhaust valve (1) comprising 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 area (6) located on the upper side (7) of the disc portion (4), comprising: At least the following steps: a) providing said disc portion (4) having said valve seat area (6) and said end face (5); b) forming a layer (8) on at least a portion of said disk portion (4) by depositing an alloy, The layer (8) comprises, in mass percent (%): Cr: 32% to 50%, Al: 0.5% to 10.0%, and The balance is made up of optional elements, inevitable impurities and Ni. and c) heating said layer (8), wherein said layer (8) is heated to a first temperature of 550-700°C for a duration of at least 16 hours and to a second temperature of 750-1000°C for a duration of at least 1 hour; A processing method comprising:

8. The method of claim 7, wherein there is no heat treatment between the forming step b) and the heating step c).

9. 9. The method of claim 7 or 8, wherein upon heating to the first temperature, the layer (8) undergoes a lamellar transformation without forming more than 10% of a basketweave structure, as quantified using the ISO-9042:1988 standard.

10. A treatment method according to claim 7 or 8, wherein heating to said second temperature results in a hardness of said layer (8) of 350 to 550 HV.

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