Engine valve and method for manufacturing the same
By integrating a nitrided martensitic steel shaft with an austenitic steel cap, the engine valve addresses galvanic corrosion and wear resistance issues in fuel-efficient engines, ensuring durability and longevity.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- SUZUKI MOTOR CORP
- Filing Date
- 2024-12-19
- Publication Date
- 2026-07-01
AI Technical Summary
Engines using CNG, LPG, or alcohol as fuel face significant corrosion issues due to the generation of moisture and acidic condensate, leading to galvanic corrosion between dissimilar metals, particularly affecting martensitic stainless steel engine valves.
The engine valve design incorporates a martensitic heat-resistant steel SUH11 shaft with a nitrided layer on its surface and an austenitic heat-resistant steel SUH35 cap, with the nitrided layer only formed on the shaft side to reduce potential difference and enhance wear resistance.
This configuration effectively suppresses galvanic corrosion and ensures adequate wear resistance by minimizing potential differences between the materials, thereby protecting the engine valve from corrosion and wear.
Smart Images
Figure 2026109263000001_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to an engine valve for an engine using compressed natural gas (CNG), liquefied petroleum gas (LPG), or alcohol as fuel, and a method for manufacturing the same.
Background Art
[0002] Generally, for the exhaust-side engine valve of an internal combustion engine, austenitic stainless steel (such as SUH35) with excellent heat resistance is adopted on the valve umbrella side where the temperature is high, and martensitic stainless steel (such as SUH3 and SUH11) is adopted on the valve shaft side, and it has a structure in which these two materials are joined. Also, in order to ensure wear resistance, it is common to use these materials after nitriding the surface (for example, Patent Document 1).
Prior Art Documents
Patent Documents
[0003]
Patent Document 1
Summary of the Invention
Problems to be Solved by the Invention
[0004] Engines using CNG, LPG, or alcohol as fuel have a problem that engine valves are easily corroded because, unlike gasoline engines, a large amount of moisture and acidic condensed water are generated in the exhaust. In particular, martensitic stainless steel has a lower natural potential than austenitic stainless steel, so in addition to corrosion by acidic condensed water, galvanic corrosion also occurs due to the effect of joining dissimilar metals, and there is a problem that corrosion is significantly promoted by these synergistic effects.
[0005] Therefore, in view of the above problems, the present invention aims to provide an engine valve and a method for manufacturing the same, which can suppress galvanic corrosion even when dissimilar metals are joined, and which has excellent wear resistance and corrosion resistance, for use in engines that use CNG, LPG, or alcohol as fuel. [Means for solving the problem]
[0006] To achieve the above objective, in one aspect of the present invention, an engine valve for an engine that uses compressed natural gas, liquefied petroleum gas, or alcohol as fuel is provided, wherein the engine valve comprises a shaft made of martensitic heat-resistant steel SUH11 and a cap made of austenitic heat-resistant steel SUH35 joined together, wherein a nitrided layer is formed on the surface of the base material on the shaft side from the joint portion between the shaft and the cap portion, and the surface of the base material is exposed on the cap portion side from the joint portion.
[0007] In another aspect, the present invention relates to a method for manufacturing an engine valve for an engine that uses compressed natural gas, liquefied petroleum gas, or alcohol as fuel, the method for manufacturing the engine valve comprising the step of forming a nitride layer only on the surface of the base material on the shaft side from the joint portion between the shaft portion and the umbrella portion of a joint member in which a shaft portion made of martensitic heat-resistant steel SUH11 and an umbrella portion made of austenitic heat-resistant steel SUH35 are joined. [Effects of the Invention]
[0008] Thus, according to the present invention, even when SUH11 and SUH35, which have a large potential difference in their natural potential, are used as the base materials for the shaft and umbrella portions of the engine valve, forming a nitride layer only on the surface of the base material on the shaft side from the joint reduces the potential difference between the shaft and umbrella portions, thereby suppressing the occurrence of galvanic corrosion. Furthermore, since a nitride layer is formed on the shaft portion, which requires high wear resistance, the wear resistance required for the entire engine valve can be ensured. [Brief explanation of the drawing]
[0009] [Figure 1]This is a schematic side view showing one embodiment of an engine valve according to the present invention. [Figure 2] This is a cross-sectional photograph of the nitrided SUH11 material used in the test example. [Figure 3] This is a cross-sectional photograph of the nitrided SUH35 material used in the test example. [Figure 4] This graph shows the hardness of the nitrided SUH11 and SUH35 materials used in the test examples, as a function of distance from the surface. [Modes for carrying out the invention]
[0010] Hereinafter, an embodiment of the engine valve and its manufacturing method according to the present invention will be described with reference to the attached drawings.
[0011] As shown in Figure 1, the engine valve 10 of this embodiment has a cylindrical shaft with a gradually expanding umbrella 11 formed at one end in the longitudinal direction. The umbrella portion 12, which is integrally formed with the umbrella 11 and a part of the shaft, is joined to the shaft portion 14, which consists of the remaining shaft, at the joint portion 13. An annular groove 15, for example, is formed at the end of the shaft portion 14 opposite to the umbrella 11, for fitting with a mechanism (not shown) that drives the engine valve 10 in the longitudinal direction.
[0012] The shaft portion 14 is made of martensitic heat-resistant steel SUH11 as the base material, and a nitrided layer (not shown) is formed on the surface of the base material. On the other hand, the umbrella portion 12 is made of austenitic heat-resistant steel SUH35 as the base material, and the surface of the base material is exposed without a nitrided layer. SUH11 has a lower natural potential than SUH35 and a very large potential difference, so in engines that use CNG, LPG, or alcohol as fuel, where a large amount of water and acidic condensate is generated in the exhaust, SUH11 is preferentially susceptible to corrosion due to the effects of galvanic corrosion.
[0013] According to the present invention, when stainless steel is nitrided, its natural potential changes toward the noble direction. Therefore, even if SUH11 and SUH35, which have significantly different natural potentials, are used as the base materials for the shaft portion 14 and the umbrella portion 12, respectively, forming a nitrided layer only on the surface of the SUH11 base material of the shaft portion 14 reduces the potential difference and suppresses galvanic corrosion. The specific potential difference between the umbrella portion 12 and the shaft portion 14 is not limited to this, but is preferably less than 100 mV, for example.
[0014] Although the substrate surface of the umbrella portion 12 made of SUH35 is not subjected to nitriding treatment, SUH35 is harder than SUH11, and since the shaft portion 14 of the engine valve 10 reciprocates at high speed along the valve guide, the shaft portion 14 requires high wear resistance, while the umbrella portion 12 does not require the same level of wear resistance. Therefore, sufficient wear resistance can be ensured for the entire engine valve 10. The specific hardness values are not limited to these, but for example, the Vickers hardness of the umbrella portion 12 with the substrate surface exposed is preferably 350 to 450 HV, and the Vickers hardness of the shaft portion 14 with a nitrided layer formed on the substrate surface is preferably 1000 to 1250 HV.
[0015] Next, an embodiment of a method for manufacturing an engine valve will be described. The method for manufacturing an engine valve according to this embodiment includes a joining step of joining a shaft portion 14 made of martensitic heat-resistant steel SUH11 and a umbrella portion 12 made of austenitic heat-resistant steel SUH35 to form a jointed member, and a nitriding treatment step of forming a nitrided layer only on the surface of the base material on the shaft portion 14 side from the joining portion 13 between the shaft portion 14 and the umbrella portion 12.
[0016] The joining process can employ the conventional method used in the manufacture of engine valves, which involves joining a shaft made of martensitic heat-resistant steel to a cap made of austenitic heat-resistant steel. A detailed explanation is omitted here.
[0017] The nitriding treatment process can also adopt the nitriding treatment in the manufacture of conventional engine valves. As specific nitriding treatments, for example, salt bath soft nitriding treatment, plasma nitriding treatment, gas soft nitriding treatment, etc. can be used.
[0018] Before the nitriding treatment process, a masking treatment process may be performed on the bonding member on the substrate surface from the bonding portion 13 to the umbrella portion 12 side. Thereby, in the nitriding treatment process, it is possible to prevent the formation of a nitride layer on the substrate surface of the umbrella portion 12. The masking treatment is preferably carried out particularly when performing plasma nitriding treatment or gas soft nitriding treatment. In the case of salt bath soft nitriding treatment, by immersing the shaft portion 14 side from the bonding portion 13 of the bonding member in the salt bath, a nitride layer can be formed only on the substrate surface of the shaft portion 14 side from the bonding portion 13 without performing the masking treatment.
Example
[0019] [Test Example 1] For test pieces of martensitic heat-resistant steel SUH11 (hereinafter also referred to as "SUH11 material") and austenitic heat-resistant steel SUH35 (hereinafter also referred to as "SUH35 material"), the surfaces were nitrided respectively, and the natural potentials before and after nitriding treatment were measured. The nitriding treatment was carried out as salt bath soft nitriding treatment by immersing each test piece in the salt bath.
[0020] The measurement of the natural potential was performed using a measurement device in which a test solution tank in which the working electrode was immersed and a reference solution tank in which the reference electrode was immersed were connected by a salt bridge, and the working electrode and the reference electrode were connected by an electrometer. As the test solution, exhaust condensate water actually recovered from a CNG engine, as the reference solution, a saturated potassium chloride aqueous solution, as the salt bridge, a saturated potassium chloride aqueous solution solidified with agar, as the reference electrode, a silver-silver chloride electrode, and as the electrometer, an electrochemical measurement device were used respectively. The results are shown in Table 1.
[0021]
Table 1
[0022] As shown in Table 1, when both conventional SUH11 and SUH35 materials were subjected to nitriding treatment, the natural potentials were -255mV for SUH11 and +230mV for SUH35, resulting in a very large potential difference of 485mV. With such a potential difference, the less noble SUH11 material would be susceptible to galvanic corrosion. On the other hand, when the SUH11 material of the present invention was subjected to nitriding treatment and the SUH35 material was not, the natural potential of the SUH35 material before nitriding treatment was -200mV, and the potential difference with the SUH11 material after nitriding treatment was significantly reduced to 55mV. Generally, it is said that galvanic corrosion is unlikely to occur if the potential difference between dissimilar metals is less than 100mV, so it has been confirmed that galvanic corrosion can be suppressed by the present invention.
[0023] [Test Example 2] Cross-sectional photographs were taken of both the SUH11 and SUH35 materials that underwent nitriding treatment as used in Test Example 1 above, and their surface hardness was measured. The cross-sectional photographs were taken using an optical microscope. Figure 2 shows a cross-sectional photograph of the SUH11 material that underwent nitriding treatment, and Figure 3 shows a cross-sectional photograph of the SUH35 material that underwent nitriding treatment. As shown in Figure 2, it was confirmed that a nitrided layer 22 was formed on the surface of the SUH11 material 21 in the SUH11 material 20 that underwent nitriding treatment. Similarly, as shown in Figure 3, it was confirmed that a nitrided layer 32 was formed on the surface of the SUH35 material 31 in the SUH35 material 30 that underwent nitriding treatment.
[0024] Surface hardness was measured using a Vickers hardness tester with a test force of 200 gf. As a result, both the nitrided SUH11 material and the nitrided SUH35 material had a surface Vickers hardness of 1000 to 1250 HV.
[0025] [Test Example 3] For SUH11 and SUH35 materials that had undergone nitriding treatment, the change in hardness in the depth direction from the treated surface was further measured. The treated surface was polished, and the hardness of the polished surface at each depth from the treated surface was measured using a Vickers hardness tester, as in Test Example 2. The results are shown in Figure 4. The initial polishing depth was 50 μm.
[0026] As shown in Figure 4, at a depth of 50 μm from the treated surface, the Vickers hardness of both the SUH11 and SUH35 materials decreased significantly, and the hardness remained almost the same at all depths up to 3.0 mm. As shown in the cross-sectional photographs in Figures 2 and 3, the thickness of the nitrided layer is less than 50 μm, so the hardness values measured in Figure 4 all represent the hardness of the SUH11 and SUH35 materials themselves. As shown in Figure 4, the Vickers hardness of the SUH11 material was in the range of 280-320 HV, and the Vickers hardness of the SUH35 material was in the range of 350-450 HV. [Explanation of Symbols]
[0027] 10 Engine Valves 11 Umbrella 12 Umbrella section 13 Joint site 14. Shaft section 15 groove 20 Nitrided SUH11 material 21 SUH11 material 22 Nitride layer 30 Nitrided SUH35 material 31 SUH35 material 32 Nitride layer
Claims
1. An engine valve for an engine that uses compressed natural gas, liquefied petroleum gas, or alcohol as fuel, The shaft portion is made of martensitic heat-resistant steel SUH11 as the base material, The umbrella portion is made of austenitic heat-resistant steel SUH35 as the base material. An engine valve comprising a shaft portion and an umbrella portion joined together, wherein a nitride layer is formed on the surface of the base material on the shaft portion side from the joint portion between the shaft portion and the umbrella portion, and the surface of the base material is exposed on the umbrella portion side from the joint portion.
2. The engine valve according to claim 1, wherein the difference in natural potential between the nitrided layer of the shaft portion and the exposed substrate of the umbrella portion is less than 100 mV.
3. The engine valve according to claim 1, wherein the Vickers hardness of the nitrided layer on the shaft portion is 1000 to 1250 HV, and the Vickers hardness of the exposed substrate on the umbrella portion is 350 to 450 HV.
4. A method for manufacturing engine valves for engines that use compressed natural gas, liquefied petroleum gas, or alcohol as fuel, A method for manufacturing an engine valve, comprising the step of forming a nitride layer only on the surface of the base material on the shaft side, from the joint between the shaft portion and the umbrella portion, on a joined member in which a shaft portion made of martensitic heat-resistant steel SUH11 and an umbrella portion made of austenitic heat-resistant steel SUH35 are joined.
5. The method for manufacturing an engine valve according to claim 4, wherein the difference in natural potential between the nitrided layer of the shaft portion and the exposed substrate of the umbrella portion is less than 100 mV.
6. The method for manufacturing an engine valve according to claim 4, wherein the Vickers hardness of the nitrided layer on the shaft portion is 1000 to 1250 HV, and the Vickers hardness of the exposed substrate on the umbrella portion is 350 to 450 HV.
7. The method for manufacturing an engine valve according to claim 4, further comprising the step of applying a masking treatment to the surface of the base material of the joining member from the joining portion to the umbrella portion side, prior to the step of forming the nitride layer.
8. The method for manufacturing an engine valve according to claim 4, wherein in the step of forming the nitride layer, a salt bath soft nitriding treatment is performed on the substrate surface on the shaft side from the joint portion.