Surface treatment method for carburized material and carburized material

Abstract

The present invention provides a surface treatment method for carburized materials that can improve the seizure resistance of carburized materials using a simpler surface treatment method than sulfurization, and a carburized material with improved seizure resistance. [Solution] A nitrided layer is formed on the surface of a carburized material by shot peening with microparticles that satisfy the following conditions: (1) at least the surface of the microparticles is nitrided, (2) the Vickers hardness of the material inside the microparticles is greater than the Vickers hardness of the carburized material, and (3) the particle size of the microparticles is 53 μm or less.

Description

Technical Field

[0001] The present invention relates to a surface treatment method for a carburized material and a carburized material.

Background Art

[0002] In transportation equipment, mechanical components such as gears and axles are used to transmit the driving force from an internal combustion engine. These mechanical components generate friction due to contact with other members, and wear occurs at the sliding parts. Furthermore, it is required that seizure does not occur even under severe sliding conditions. In order to improve the friction and wear characteristics, particularly the seizure resistance, of such mechanical components, a carburizing treatment is generally performed to harden the surface by means of carburizing and quenching or the like.

[0003] And, in order to improve the seizure resistance of a carburized material that has been carburized and has carbon penetrated near the surface, a sulfurizing treatment is known as a surface treatment used in combination with the carburizing treatment. The sulfurizing treatment is a surface treatment technique for modifying the surface of a carburized material by forming a sulfide layer on the surface of the carburized material to improve the sliding characteristics. In the conventional sulfurizing treatment, in some of its steps, since a molten salt or an aqueous solution containing a sulfide is used, waste liquid treatment after the treatment is required.

[0004] On the other hand, as a surface treatment method that does not use an aqueous solution, Non-Patent Document 1 has been proposed. Non-Patent Document 1 proposes a surface treatment method in which fine particles of industrial pure titanium having a nitride layer on the surface are projected onto the surface of a Ti-6Al-4V alloy at room temperature.

Prior Art Documents

Patent Documents

[0005]

Non-Patent Document 1

[0006] Conventional surface treatment methods for carburized materials, such as sulfurization, utilize molten salts or aqueous solutions containing sulfides in some steps. Therefore, sulfurization requires wastewater treatment after the process, posing an environmental challenge. Consequently, there is a need for a simpler method than sulfurization to improve the seizure resistance of carburized materials. While Non-Patent Document 1 discloses a surface treatment method for titanium alloys that does not use aqueous solutions, it neither discloses nor suggests any improvement in the seizure resistance of carburized materials.

[0007] The present invention aims to provide a surface treatment method for carburized materials that can improve the seizure resistance of carburized materials using a simpler surface treatment method than sulfurization, and to provide a carburized material with improved seizure resistance. [Means for solving the problem]

[0008] A surface treatment method for a carburized material according to one embodiment of the present invention is: The method is characterized by shot peening the carburized area, where carbon has penetrated and formed near the surface of a carburized material, with fine particles that satisfy the following conditions (1), (2), and (3). (1) At least the surface of the fine particles is nitrided. (2) The Vickers hardness of the material inside the fine particles is greater than the Vickers hardness of the carburized treatment. (3) The particle size of the fine particles is 53 μm or less.

[0009] According to this method, the carbon-penetrated carburized area near the surface of a carburized material is shot-peened with fine particles. Here, the fine particles satisfy the following conditions: (1) at least the surface is nitrided, (2) the Vickers hardness of the internal material is greater than the Vickers hardness of the carburized area, and (3) the particle size is 53 μm or less. (1) By using fine particles with a nitrided surface, the formation of a nitrided layer on the surface of the carburized area of ​​the carburized material is promoted during shot peening. Also, (2) the Vickers hardness of the internal material of the fine particles is greater than that of the carburized area, which allows for the effective application of compressive residual stress. Furthermore, (3) the fine particle size of 53 μm or less allows for the formation of a uniform nitrided layer while suppressing an increase in surface roughness. Thus, the nitrided layer formed by shot peening with fine particles that satisfy the above conditions (1) to (3) has good adhesion to the carburized area and has compressive residual stress applied to its surface. The synergistic effect of the nitrided layer and compressive residual stress improves both the wear resistance of the surface and the fatigue strength. Furthermore, the formation of the nitrided layer by shot peening is simpler in process compared to conventional sulfurization treatment and reduces the environmental impact. Therefore, a surface treatment method using shot peening with fine particles that satisfy the above conditions (1) to (3) provides nitriding and compressive residual stress to a carburized material with a single surface treatment method, and as a result, it is possible to improve the seizure resistance of the carburized material with a simpler surface treatment method than sulfurization treatment. Therefore, according to the surface treatment method for a carburized material according to one embodiment of the present invention, it is possible to provide a surface treatment method for a carburized material that can improve the seizure resistance of the carburized material with a simpler surface treatment method than sulfurization treatment.

[0010] In a surface treatment method for a carburized material according to one embodiment of the present invention, When shot peening the carburizing treatment section with the fine particles, the distance between the tip of the nozzle that projects the fine particles and the surface of the carburizing treatment material may be 30 mm or less.

[0011] According to this method, by keeping the distance between the nozzle tip and the carburized material 30 mm or less when shot peening fine particles into the carburized section, the loss of kinetic energy of the fine particles can be suppressed, enabling more effective nitriding and the application of compressive residual stress.

[0012] In a surface treatment method for a carburized material according to one embodiment of the present invention, The aforementioned fine particles may further satisfy the following condition (4). (4) The surface and interior of the fine particles are made of a single material.

[0013] This method ensures that both the surface and interior of the microparticles are composed of a single material. In other words, the surface and interior of the microparticles are made of the same material. This uniformity of the microparticles' mechanical properties during shot peening allows for stable formation of the nitrided layer and the application of compressive residual stress. Furthermore, the single material composition of the microparticles facilitates quality control and enables more reliable surface treatment.

[0014] In a surface treatment method for a carburized material according to one embodiment of the present invention, The aforementioned fine particles may further satisfy the following condition (5). (5) The material inside the fine particles is TiN.

[0015] In this method, the internal material of the microparticles is TiN, which has high hardness and chemical stability. By using high-hardness and chemically stable microparticles in shot peening, it is possible to achieve more effective compression residual stress and excellent seizure resistance.

[0016] In the surface treatment method of the carburized material according to an embodiment of the present invention, The material inside the carburized material may be a steel material. Furthermore, the steel material may be chromium molybdenum steel.

[0017] According to this method, the material inside the carburized material is a steel material that is easy to adjust the balance between strength and toughness, and particularly chromium molybdenum steel with a good balance between strength and toughness. The steel material has a high affinity with nitrogen by being carburized, and as a result, the adhesion with nitrogen is improved. And while ensuring the balance between the strength and toughness inside the carburized material, a carburized material excellent in the adhesion between the nitrided layer formed on the surface of the carburized material and the carburized portion formed near the surface of the carburized material can be obtained. As a result, the compatibility between the mechanical properties of the carburized material and the nitrided layer is good, and a carburized material having excellent durability can be obtained.

[0018] The carburized material according to an embodiment of the present invention is characterized in that it has a nitrided layer composed of a metal and nitrogen different from the material inside the carburized material on the surface of the carburized portion where carbon has penetrated, which is formed near the surface of the carburized carburized material.

[0019] According to this configuration, the carburized material has a nitrided layer composed of a metal and nitrogen different from the material inside the carburized material on the surface of the carburized material. The nitrided layer can be formed, for example, in a surface treatment step of shot peening fine particles at least the surface of which is nitrided on the carburized material. The nitrided layer improves the hardness of the surface of the carburized portion, so that the wear resistance of the surface of the carburized material is improved and the fatigue strength is also improved. As a result, the seizure resistance of the carburized material can be improved. Therefore, according to the carburized material according to an embodiment of the present invention, a carburized material with improved seizure resistance can be provided.

[0020] The carburized material according to an embodiment of the present invention The Vickers hardness of the material of the nitrided layer may be greater than the Vickers hardness of the carburized portion.

[0021] According to this configuration, since the Vickers hardness of the material of the nitrided layer is greater than the Vickers hardness of the carburized portion, the hardness of the surface of the carburized material can be further improved. And, while further improving the wear resistance of the surface of the carburized material, the fatigue strength is also further improved, and as a result, the seizure resistance of the carburized material can be further improved.

[0022] The carburized material according to an embodiment of the present invention is The material of the nitrided layer may be TiN.

[0023] According to this configuration, the material of the nitrided layer is TiN. Since TiN has high hardness, the hardness of the surface of the carburized material can be further improved. And, while further improving the wear resistance of the surface of the carburized material, the fatigue strength is also further improved, and as a result, the seizure resistance of the carburized material can be further improved.

[0024] The carburized material according to an embodiment of the present invention is The material inside the carburized material may be a steel material. Furthermore, the steel material may be chrome molybdenum steel.

[0025] According to this configuration, the internal material of the carburized material is a steel material that easily adjusts the balance between strength and toughness, and particularly chrome molybdenum steel having a good balance between strength and toughness. Thereby, the compatibility between the mechanical properties of the carburized material and the nitrided layer is good, and the durability of the carburized material can be further improved.

[0026] In the present invention and its embodiments, "carburized material" refers to a base material such as a machine part that has been carburized. Furthermore, in the present invention and its embodiments, "carburized section" refers to a part of the carburized material, specifically the portion of the carburized layer formed by the penetration of carbon near the surface of the carburized material. Moreover, in the present invention and its embodiments, "interior of the carburized material" refers to the portion of the carburized material excluding the carburized layer (carburized section) formed near the surface of the carburized material.The material of the interior of the carburized material refers to the material of the base material such as a machine part before carburization.

[0027] In the present invention and its embodiments, the statement that the surface and interior of the microparticles are composed of a single material means that the surface and interior of the microparticles are composed of the same material, and that not only the surface of the microparticles, but also the surface and interior of the microparticles are composed of nitrided material.

[0028] In the present invention and its embodiments, "at least one of the multiple options" includes all possible combinations of the multiple options. "At least one of the multiple options" may be any one of the multiple options, or all of the multiple options. For example, "at least one of A, B, and C" may be A only, B only, C only, A and B, A and C, B and C, or A, B, and C.

[0029] In the present invention and its embodiments, the words including, comprising, having, and their derivatives are used with the intention of encompassing additional items in addition to the listed items and their equivalents.

[0030] Unless otherwise defined, all terms used herein and in the claims (including technical and scientific terms) have the same meaning as commonly understood by those skilled in the art to which the invention belongs. Terms such as those defined in commonly used dictionaries should be construed to have the meaning consistent with their meaning in the context of the relevant art and this disclosure, and not to be construed in an idealized or overly formal sense.

[0031] In this specification, the term “may be” is nonexclusive. “May be” means “may be, but not limited to.” In this specification, “may be” implicitly includes the possibility of “not being.” In this specification, configurations described as “may be” produce at least the effects obtained by the configuration of claim 1.

[0032] Before describing embodiments of the present invention in detail, it should be understood that the present invention is not limited to the details of the configuration and arrangement of components described in the following description or illustrated in the drawings. The present invention can also be made in embodiments other than those described below. The present invention can also be made in embodiments that have been modified in various ways from the embodiments described below. [Effects of the Invention]

[0033] According to the present invention, a surface treatment method for a carburized material that can improve the seizure resistance of the carburized material using a simpler surface treatment method than sulfurization treatment, and a carburized material with improved seizure resistance can be provided. [Brief explanation of the drawing]

[0034] [Figure 1] This figure illustrates a surface treatment apparatus and a carburized material according to an embodiment of the present invention. [Figure 2] This figure illustrates a ball-on-plate type reciprocating sliding test method, which is an evaluation test of seizure resistance performed on each test piece of the embodiment and comparative example of the present invention. [Figure 3]The results of the seizure resistance evaluation tests for each test specimen of the three examples and four comparative examples are shown. [Figure 4] The following shows an example of the change in the coefficient of friction over time during the seizure resistance evaluation test, and the results of the wear mark observation after the test, for each test specimen (untreated material and sulfur-treated material) of the example and comparative example. (a) is the result for the test specimen of the example that underwent shot peening using TiN fine particles (nozzle distance 30 mm). (b) is the result for the test specimen (untreated material) of the comparative example that did not undergo either surface treatment by fine particle shot peening or sulfur treatment. (c) is the result for the test specimen (sulfur-treated material) of the comparative example that underwent sulfur treatment. [Figure 5] The FE-EPMA-based elemental analysis images and secondary electron images of Ti in cross-sections of specimens from three examples of shot peening using TiN nanoparticles at different nozzle distances are shown. [Figure 6] Elemental analysis images of Ti and N obtained by FE-EPMA of cross-sections of test specimens from three examples are shown. [Modes for carrying out the invention]

[0035] The surface treatment method and carburized material of the embodiment of the present invention will be described below with reference to Figure 1. As shown in Figure 1, the carburized material 1 has been pre-carburized to form a carburized section 11 where carbon has penetrated near the surface. Furthermore, the material of the interior of the carburized material 1, excluding the carburized section 11, is preferably a steel material, particularly chromium-molybdenum steel. In other words, the material of the base material of the carburized material 1 before carburizing is preferably a steel material, particularly chromium-molybdenum steel. SCM420 is preferred for the chromium-molybdenum steel. Although the carburized material 1 of this embodiment shown in Figure 1 has a flat plate shape, the carburized material 1 of the present invention may have any shape.

[0036] In the surface treatment method of this embodiment, the carburized section 11 of the carburized material 1 is shot peened with fine particles 2 that satisfy the following conditions (1), (2), and (3). Shot peening is performed at room temperature and atmospheric pressure. As a result, the fine particles 2 are transferred to the surface of the carburized section 11 of the carburized material 1, and a nitrided layer 21 is formed as a transfer layer. The surface of the carburized section 11 of the carburized material 1 is then modified, resulting in improved hardness of the treated surface, generation of compressive residual stress, and refinement of crystal grains. Here, when shot peening the carburized section 11 with fine particles 2, the distance h between the tip of the nozzle 3 that projects the fine particles 2 and the surface of the carburized material 1 is preferably 30 mm or less. Also, the projection pressure is preferably 0.5 MPa or higher.

[0037] The conditions (1), (2), and (3) that particle 2 satisfies are as follows: (1) At least the surface of the fine particles 2 is nitrided. (2) The Vickers hardness of the material inside the fine particles 2 is greater than the Vickers hardness of the carburized treatment 11. (3) The particle size of particle 2 is 53 μm or less. Here, under condition (1), it is sufficient that at least the surface of the fine particles 2 is nitrided, and the surface and interior of the fine particles 2 may also be nitrided. Furthermore, under condition (2), it is preferable that the material inside the fine particles 2 contains a different metal element than the material inside the carburized material 1. Furthermore, under condition (3), the maximum particle size of the multiple fine particles 2 used in shot peening is 53 μm or less. In addition, the average particle size of the multiple fine particles 2 used in shot peening may be 45 μm or less.

[0038] The fine particle 2 may further satisfy the following condition (4). (4) The surface and interior of the fine particles 2 are composed of a single material. Here, under condition (4), the surface and interior of the fine particle 2 are composed of the same material. That is, the surface and interior of the fine particle 2 are composed of a metallic nitride.

[0039] The fine particles 2 may further satisfy the following condition (5). (5) The material inside the fine particle 2 is TiN (titanium nitride). Here, under condition (5), the material of the surface of the fine particles 2, as well as the interior of the fine particles 2, may be TiN.

[0040] The carburized material 1 of this embodiment has a nitrided layer 21 on the surface of the carburized section 11. The nitrided layer 21 is composed of a different metal and nitrogen from the material inside the carburized material 1. The nitrided layer 21 may be formed over the entire surface of the carburized section 11, or it may be formed on a part of the surface of the carburized section 11. Furthermore, the thickness of the nitrided layer 21 does not have to be uniform and may vary depending on the part of the carburized material 1.

[0041] Here, it is preferable that the Vickers hardness of the nitrided layer 21 is greater than that of the carburized section 11. Furthermore, the material of the nitrided layer 21 may be TiN. [Examples]

[0042] The carburized materials of this embodiment and the carburized materials of the comparative example, as well as their surface treatment methods, will be described below with reference to Figures 2 to 6. In this embodiment and comparative example, carburized materials, which are chromium-molybdenum steel that have been carburized, quenched, and tempered, were subjected to surface treatment by fine particle shot peening (FPP) under different conditions, and the seizure resistance of these carburized materials was evaluated using a vibration friction wear tester (SRV tester). In some of the comparative examples, the carburized materials underwent both surface treatment by fine particle shot peening and sulfurization. In addition, the seizure resistance of carburized materials that underwent neither surface treatment by fine particle shot peening nor sulfurization, and carburized materials that underwent sulfurization without surface treatment by fine particle shot peening, were also evaluated.

[0043] Before the carburizing treatment, a plate-shaped test specimen (20 mm wide, 12 mm long, 4 mm thick) made of chromium-molybdenum steel (SCM420) having the chemical composition shown in Table 1 was used as the base material for the carburized material. After carburizing, quenching, and tempering (surface hardness 710 HV, effective hardened layer depth 0.6 mm) of this plate-shaped test specimen, both sides of a 20 mm x 12 mm area were machined by 50 μm to remove distortion, resulting in a surface roughness Rz 0.7, and a carburized treatment area was formed near the surface.

[0044] [Table 1]

[0045] In the example, titanium nitride nanoparticles (manufactured by Kojun Chemical Laboratory, purity 99% or higher) were used for shot peening. In the comparative example, three types of nanoparticles were used for shot peening: iron sulfide (FeS) nanoparticles (manufactured by Nippon Seikou, having the chemical composition (mass%) shown in Table 2 below), tin (Sn) nanoparticles (manufactured by Fukuda Metal Foil Industry, purity 99.5% or higher). The average particle size of each nanoparticle was 45 μm or less, the maximum particle size of the FeS nanoparticles was 45 μm or less, and the maximum particle size of the TiN nanoparticles was 53 μm or less.

[0046] [Table 2]

[0047] A Shinto Kogyo MY-30 blasting system was used for shot peening. The shot peening conditions were room temperature and atmospheric environment. The projection pressure was fixed at 0.5 MPa, the projection time at 90 s, and the particle supply rate at 0.6 g / s. In Example 1, Comparative Examples 2 and 3 using FeS fine particles, and Comparative Example 4 using Sn fine particles, the distance between the nozzle tip and the test specimen (nozzle distance) during shot peening was set to 30 mm. For Comparative Example 2 using FeS fine particles, the test specimen was subjected to sulfurization treatment before shot peening at a nozzle distance of 30 mm. In addition, to investigate the effect of nozzle distance on the amount of TiN fine particles transferred and the resistance to seizing, shot peening was performed using TiN fine particles at nozzle distances of 60 mm and 90 mm in Examples 2 and 3.

[0048] For each test specimen in this embodiment and comparative example, an SRV testing machine (manufactured by Optimol, Germany) was used to evaluate seizure resistance using the ball-on-plate reciprocating sliding test method shown in Figure 2. For the upper movable ball test specimen, bearing steel balls (SUJ2, 3 / 8 inch diameter) were used, and mineral oil with a viscosity of 10W-40 was used as the lubricant, with an initial drop of 20 μL. The test conditions were a constant load of 600 N (initial application speed of 300 N / s), a temperature of 130°C, a frequency of 50 Hz, a stroke of 2 mm, and a continuous test time of 1 hour. The coefficient of friction was measured during the test, and the test was terminated when the coefficient of friction exceeded 0.25, excluding the initial break-in period, indicating seizure. The same test was performed 5 or 10 times for each test specimen, and the mean time to failure (MTTF) was compared. After evaluating the seizure resistance using an SRV testing machine, surface observation was performed using a stereomicroscope (Olympus SZX16).

[0049] Figure 3 shows the Mean Time to Failure (MTTF), an evaluation index for seizure resistance, for each test specimen in this example and comparative example. In Figure 3, "Surfurizing" refers to the test specimen of the sulfurized material treated with sulfurization without shot peening, as in Comparative Example 1. In Figure 3, "TiN-FPP" refers to the test specimens of Examples 1 and 3 that underwent shot peening using TiN fine particles. To distinguish between the TiN-FPP test specimens of Examples 1 and 3, the nozzle distance is indicated in parentheses. In Figure 3, "Surfurizing + FeS-FPP" refers to the test specimen of Comparative Example 2 that underwent shot peening using FeS fine particles after sulfurization. In Figure 3, "FeS-FPP" refers to the test specimen of Comparative Example 3 that underwent shot peening using FeS fine particles. In Figure 3, "Sn-FPP" refers to the test specimen of Comparative Example 4 that underwent shot peening using Sn fine particles. In Figure 3, "As machined" refers to a test specimen of untreated material from Comparative Example 5, which was not subjected to either surface treatment by shot peening or sulfurization. Among the test specimens treated with surface treatment by fine particle shot peening, the mean time to failure (MTTF) of the TiN-FPP (nozzle distance 30 mm) material of Example 1 was the longest, and the seizure resistance of the TiN-FPP (nozzle distance 30 mm) material of Example 1 was improved compared to the test specimen of Comparative Example 5 (untreated material), which was not subjected to either surface treatment by fine particle shot peening or sulfurization. Furthermore, the TiN-FPP (nozzle distance 90 mm) material of Example 3 had a longer mean time to failure (MTTF) compared to the TiN-FPP (nozzle distance 30 mm) material of Example 1.

[0050] Figure 4 shows an example of the change in the coefficient of friction over time during the seizure resistance evaluation test, and the results of the wear mark observation after the test, for the test specimen of this embodiment, the untreated material of Comparative Example 5, and the sulfur-treated material of Comparative Example 1. Figure 4(a) shows the results for the test specimen of Example 1 (TiN-FPP (nozzle distance 30 mm) material). Figure 4(b) shows the results for the test specimen of Comparative Example 5 (untreated material). Figure 4(c) shows the results for the test specimen of Comparative Example 1 (sulfur-treated material). The wear marks on the TiN-FPP (nozzle distance 30 mm) material of Example 1 were smaller than those on the untreated material of Comparative Example 5 and showed a similar trend to that of the sulfur-treated material of Comparative Example 1.

[0051] Furthermore, for detailed examination, cross-sectional analysis was performed on test specimens from this example and comparative example, each of which was subjected to FPP with different nozzle distances using TiN nanoparticles. A field emission electron probe microanalyzer (FE-EPMA, Shimadzu EPMA-8050G) was used for the cross-sectional analysis. Figure 5 shows the elemental analysis images and secondary electron images (SE images) of titanium (Ti) from the cross-sections of each test specimen (TiN-FPP material) from Examples 1-3, each of which was subjected to FPP with different nozzle distances using TiN nanoparticles, as measured by FE-EPMA. The conditions for the cross-sectional elemental analysis of titanium (Ti) using FE-EPMA were: air pressure 5 × 10⁻⁶ -8 Pa, acceleration voltage 15kV, irradiation current 5×10⁻⁶ -8 This was designated as A. Furthermore, using image analysis software (WinROOF2021, manufactured by Mitani Corporation) on the cross-sectional elemental analysis image of titanium (Ti) in Figure 5, the equivalent thickness of the transfer layer formed by the transfer of TiN nanoparticles was calculated by dividing the detected area of ​​Ti elements by the analysis width. The calculated equivalent thicknesses of the transfer layer were 1.4 μm for the specimen of Example 1 with a nozzle distance of 30 mm shown in Figure 5(a), 1.0 μm for the specimen of Example 2 with a nozzle distance of 60 mm shown in Figure 5(b), and 0.8 μm for the specimen of Example 3 with a nozzle distance of 90 mm shown in Figure 5(c). It can be seen that the most TiN nanoparticles were transferred to the specimen of Example 1 with a nozzle distance of 30 mm. Figures 5(d), (e), and (f) are secondary electron images (SE images) showing the surface properties, including surface roughness and shape, of each specimen of Examples 1 to 3 with nozzle distances of 30 mm, 60 mm, and 90 mm, respectively. Furthermore, since delamination of the TiN layer was observed in the wear mark observation results of the TiN-FPP material of Example 1 with a nozzle distance of 30 mm in Figure 4(a), it is presumed that in the seizure resistance evaluation tests of the TiN-FPP materials of Examples 2 and 3 with nozzle distances of 60 mm and 90 mm, wear of the TiN layer progressed during the test, causing seizure due to the exposure of the chromium-molybdenum steel. Therefore, it is presumed that the nozzle distance affected the mean time to failure (MTTF) due to the thickness of the TiN layer.

[0052] Furthermore, Figure 6 shows the elemental analysis images of titanium (Ti) and nitrogen (N) of the cross-sections of the TiN-FPP material specimens with nozzle distances of 30 mm, 60 mm, and 90 mm, which are the test specimens for Examples 1 to 3, as measured by FE-EPMA. Figure 6(a) shows the results of the cross-sectional elemental analysis of nitrogen (N) in the test specimen of Example 1, and Figure 6(b) shows the results of the cross-sectional elemental analysis of titanium (Ti) in the test specimen of Example 1. Figure 6(c) shows the results of the cross-sectional elemental analysis of nitrogen (N) in the test specimen of Example 2, and Figure 6(d) shows the results of the cross-sectional elemental analysis of titanium (Ti) in the test specimen of Example 2. Figure 6(e) shows the results of the cross-sectional elemental analysis of nitrogen (N) in the test specimen of Example 3, and Figure 6(f) shows the results of the cross-sectional elemental analysis of titanium (Ti) in the test specimen of Example 3. Note that Figure 6(b) is the same as Figure 5(a), Figure 6(d) is the same as Figure 5(b), and Figure 6(f) is the same as Figure 5(c). The conditions for the cross-sectional elemental analysis of nitrogen (N) by FE-EPMA are the same as those for the cross-sectional elemental analysis of titanium (Ti). As shown in Figure 6, the presence of Ti and N in the specimen of this embodiment indicates that the transfer layer formed on the surface is a nitride layer.

[0053] In this example and comparative example, carburized chromium-molybdenum steel, which is a carburized, quenched, and tempered material, was subjected to shot peening with various fine particles, and its seizure resistance was compared and evaluated by SRV testing. The results are shown below. (1) In the test specimens of this embodiment, shot peening was performed using TiN fine particles at a nozzle distance of 30 mm, resulting in improved seizure resistance compared to the untreated test specimens of the comparative example. (2) The nozzle distance during shot peening affects the equivalent thickness of the TiN nanoparticle transfer layer (nitride layer), with the thickness increasing in the order of 90 mm, 60 mm, and 30 mm. In the seizure resistance evaluation by SRV test, the test specimen of the example with a nozzle distance of 30 mm showed superior seizure resistance. Furthermore, it is considered important to increase the thickness of the transfer layer to further improve seizure resistance. [Explanation of Symbols]

[0054] 1: Carburized material, 2: Fine particles, 3: Nozzle, 11: Carburized treatment section, 12: Inside of the carburized material, 21: Nitrided layer

Claims

1. A surface treatment method for a carburized material, characterized by shot peening the carburized area, where carbon has penetrated and formed near the surface of the carburized material, with fine particles that satisfy the following conditions (1), (2), and (3). (1) At least the surface of the fine particles is nitrided. (2) The Vickers hardness of the material inside the fine particles is greater than the Vickers hardness of the carburized treatment area. (3) The particle size of the fine particles is 53 μm or less.

2. A surface treatment method for a carburized material according to claim 1, A method for surface treatment of a carburized material, characterized in that, when shot peening the carburized material with the fine particles, the distance between the tip of the nozzle that projects the fine particles and the surface of the carburized material is 30 mm or less.

3. A surface treatment method for a carburized material according to claim 1 or 2, A surface treatment method for a carburized material, characterized in that the fine particles further satisfy the following condition (4). (4) The surface and interior of the fine particles are made of a single material.

4. A surface treatment method for a carburized material according to claim 1 or 2, A surface treatment method for a carburized material, characterized in that the fine particles further satisfy the following condition (5). (5) The material inside the fine particles is TiN.

5. A surface treatment method for a carburized material according to claim 1 or 2, A method for surface treatment of a carburized material, characterized in that the material inside the carburized material is a steel material.

6. A surface treatment method for a carburized material according to claim 5, A surface treatment method for a carburized material, characterized in that the steel material is chromium-molybdenum steel.

7. A carburized material characterized by having a nitride layer composed of a different metal and nitrogen from the material inside the carburized material on the surface of the carburized treatment area where carbon has penetrated, formed near the surface of the carburized material.

8. The carburized material according to claim 7, A carburized material characterized in that the Vickers hardness of the nitrided layer is greater than the Vickers hardness of the carburized section.

9. A carburized material according to claim 7 or 8, A carburized material characterized in that the material of the nitrided layer is TiN.

10. A carburized material according to claim 7 or 8, A carburized material characterized in that the material inside the carburized material is a steel material.

11. A carburized material according to claim 10, A carburized material characterized in that the steel material is chromium-molybdenum steel.

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