CRANKSHAFT AND METHOD OF MANUFACTURING THE SAME.

Abstract

A crankshaft with improved seizing resistance is provided. A crankshaft having main journals 11 and connecting rod journals 12 includes a composite layer containing iron and nitrogen on its surface, wherein, in the composite layer, for both the main journals 11 and the connecting rod journals 12, the ratio of the porosity area of ​​the thinnest part of a region from the surface to a depth of 3.0 µm and a region along the full thickness of the composite layer is not greater than 10.0%, and both the main journals 11 and the connecting rod journals 12 have a surface geometry such that the arithmetic mean deviation of the primary profile, Pa, is not greater than 0.090 µm.

Description

CRANKSHAFT AND METHOD OF MANUFACTURING THE SAME TECHNICAL FIELD

[0001] The present invention relates to a crankshaft and a method of manufacturing said crankshaft. STATE OF THE ART

[0002] Some crankshafts are nitrided before use to improve fatigue and wear resistance. One nitriding process commonly used for crankshafts is gas nitriding, which offers high productivity.

[0003] Documents JP 2018-70928 A, WO 2018 / 066667 Al and JP 2013-221203 A each describe the control of the nitriding potential during a nitriding process to form a dense composite layer made of γ' phase on the surface of the steel, thereby providing both high fatigue strength and bend-straightening performance.

[0004] Japanese Patent No. 5898092 describes a method for manufacturing an impulse cam that includes performing a mild nitriding process on a sliding surface of an impulse cam to form a hardened layer and a composite layer, and then removing the composite layer so that the hardened layer is present on the surface of the sliding surface.

[0005] In addition to fatigue and wear resistance, crankshafts must be resistant to seizing. It has been proposed to improve seizing resistance by controlling the surface geometry of the friction parts.

[0006] Document JP 2017-218951 A states that the surface roughness Ra of a crankshaft for a refrigeration machine compressor must not exceed 0.05 pm. Document WO 2016 / 072305 Al states that, in a rotary sliding bearing composed of a bearing and a shaft, the surface roughness Ra of the shaft must not exceed 0.10 pm. Japanese Patent No. 5199728 states that the surface roughness of the martensite or nitride layer of a crankshaft must be less than the surface roughness of the associated main bearing. STATE OF THE ART DOCUMENTS PATENT DOCUMENTS

[0007] [Patent Document 1] JP 2018-70928 A [Patent Document 2] WO 2018 / 066667 Al [Patent Document 3] JP 2013-221203 A [Patent Document 4] Japanese Patent No. 5898092 [Patent Document 5] JP 2017-218951 A [Patent Document 6] WO 2016 / 072305 Al [Patent Document 7] Japanese Patent No. 5199728 Rrnn Ln / zznz / E / YiAi NON-PATENT DOCUMENTS

[0008] [Non-patent document 1] Dieter Liedtke et al., “Nitriding and nitrocarburizing in iron material”, AGNE Gijutsu Center Inc., pp. 23 and 72, 2011 SUMMARY OF THE INVENTION PROBLEMS THAT THIS INVENTION WILL SOLVE

[0009] In recent years, lubricating oils with lower viscosities and crankshafts with friction parts made up of thinner shafts have been developed to improve fuel efficiency, and consequently, a crankshaft with even better resistance to seizing is required.

[0010] An object of the present invention is to provide a crankshaft with improved seizing resistance and a method of manufacturing said crankshaft. MEANS TO SOLVE PROBLEMS

[0011] A crankshaft according to an embodiment of the present invention is a crankshaft having a main journal and a connecting rod journal, including a composite layer containing iron and nitrogen on a surface thereof, wherein, in the composite layer, for each of the main journals and connecting rod journals, a porosity area ratio of the thinnest part of a region from the surface to a depth of 3.0 µm and a region through the total thickness of the composite layer is not greater than 10.0%, and each of the main journals and connecting rod journals has a surface geometry such that an arithmetic mean deviation of a primary profile, Pa, is not greater than 0.090 pm.

[0012] A method of manufacturing a crankshaft according to an embodiment of the present invention is a crankshaft manufacturing method described above, comprising: an intermediate grinding step for grinding a main journal and a connecting rod journal of an intermediate crankshaft product; after the intermediate grinding step, an intermediate lapping step for lapping the main journal and connecting rod journal of the intermediate product; after the intermediate lapping step, a nitriding step for nitriding the intermediate product; after the nitriding step, a grinding step for grinding the main journal and connecting rod journal of the intermediate product; after the grinding step, a coarse lapping step for lapping the main journal and connecting rod journal of the intermediate product using a film coated with alumina abrasive grains;and after the rough lapping step, a finishing lapping step for the main journal and connecting rod journal of the intermediate product using a film coated with diamond abrasive grains.; EFFECTS OF THE INVENTION

[0013] The present invention provides a crankshaft with increased resistance to seizing. BRIEF DESCRIPTION OF THE FIGURES Rrnn Ln / zznz / E / YiAi

[0014] Figure 1 is a schematic cross-sectional view of a near-surface structure of a steel that has been subjected to a typical gas nitriding process. Figure 2 shows an exemplary primary profile. Figure 3 shows an exemplary roughness profile. Figure 4 is a schematic view of a crankshaft according to an embodiment of the present invention. Figure 5 is a flowchart illustrating an exemplary method of manufacturing the crankshaft of Figure 4. Figure 6 is a schematic view of the evaluation equipment used for the gripping tests. Figure 7 is a schematic view of a bearing and nearby parts of the evaluation equipment in Figure 6. Figure 8 schematically illustrates the changes over time of the surface pressure applied to the test tree. MODALITIES FOR CARRYING OUT THE INVENTION

[0015] A nitriding process for a crankshaft aims to improve wear resistance and / or fatigue strength. However, the relationship between a nitriding process and seizing resistance has not been sufficiently investigated. In particular, the relationship between the composite layer formed during nitriding and seizing resistance has not been systematically investigated, although it has been noted that a composite layer with a high porosity area ratio acts as an oil basin to contribute to improved seizing resistance (Dieter Liedtke, et al., Nitriding and Nitrocarburizing in Iron Materials, AGNE Gijutsu Center Inc., p. 27, 2011).

[0016] Figure 1 is a schematic cross-sectional view of a near-surface structure of a steel that has undergone a typical gas nitriding process. A composite layer 50, approximately tens of micrometers thick, forms on the steel surface. Below the composite layer 50, a nitrogen diffusion layer 60 forms by diffusion of nitrogen from the steel surface. The composite layer 50 includes a porous layer 51 located near the surface with a high porosity area ratio, and a dense layer 52 located between the porous layer 51 and the nitrogen diffusion layer 60, which has a low porosity area ratio.

[0017] The composite layer 50 is known to include ε phase (Fez^N), γ' phase (Fe4N), and α phase (aFe). In particular, a composite layer 50 including such a porous layer 51, as mentioned above, is known to be composed primarily of ε phase. As described in JP 2018-70928 A and WO 2018 / 066667 Al, Rrnn Ln / zznz / E / YiAi mentioned above, it is also known that controlling the nitriding potential during nitriding can form a less porous composite layer composed mainly of the γ' phase.

[0018] The present inventors carried out a detailed investigation into the relationship between the composite layer and the resistance to seizing. Specifically, they evaluated the resistance to seizing of (1) a steel with a composite layer, mainly composed of the ε phase and including a porous layer and a dense layer; (2) a steel with a composite layer composed mainly of the ε phase and including only a dense layer after the removal of the porous layer; (3) a steel with an exposed nitrogen diffusion layer after the removal of the composite layer; and (4) a steel including a layer composed mainly of the γ' phase.

[0019] The investigation showed that steels (2) and (4) had better resistance to seizing than steels (1) and (3). This demonstrates that the presence of a composite layer is advantageous for improving resistance to seizing and that a composite layer with a lower porosity area ratio is advantageous for improving resistance to seizing.

[0020] To improve resistance to seizing, the surface geometry of friction parts provided with a composite layer is also important. The present inventors have discovered that the resistance to seizing of a nitrided crankshaft can be significantly improved compared to conventional crankshafts if the porosity area ratio of the portion near the surface of the composite layer is no greater than 10.0% and the arithmetic mean deviation of the primary profile, Pa, is no greater than 0.090 pm.

[0021] JP 2017-218951 A and WO 2016 / 072305 A1, mentioned above, each specify a surface geometry using the arithmetic mean deviation of the roughness profile, Ra (hereafter referred to as mean roughness Ra). However, specifying a geometry using mean roughness Ra has the following problems.

[0022] Figures 2 and 3 show an exemplary primary profile and an exemplary roughness profile, respectively. The surface geometry of an industrial product, such as a crankshaft, includes not only short-period components (i.e., roughness) but also non-negligible levels of long-period components (i.e., waviness) caused, for example, by grinding machine vibrations. The average roughness Ra is based on the roughness profile (Figure 3), which results from removing the waviness components using a high-pass filter. Therefore, it cannot be stated without reservation that the value exactly reflects the actual surface geometry. Furthermore, the average roughness Ra value varies significantly depending on the cutoff value 7c of the high-pass filter used to obtain the roughness profile.In reality, even for the same average roughness level Ra, the resistance to seizure varies significantly depending on the magnitude of the waviness. Therefore, a parameter of. Rrnn Ln / zznz / E / YiAi evaluation that uses the primary profile (Figure 2) as the evaluated profile would be a more appropriate indication used to control resistance to seizure.

[0023] Typically, a nitriding process disadvantageously increases surface roughness by 1.5 to 2 times (Dieter Liedtke et al., Nitriding and Nitrocarburizing in Iron Materials, AGNE Gijutsu Center Inc., p. 72, 2011). Therefore, to reduce the arithmetic mean deviation of the primary profile Pa of a steel after nitriding, the steel must be polished after nitriding to provide a clean surface geometry. Meanwhile, the thickness of a composite layer that can be formed in an industrially realistic time is a few tens of micrometers. The small amount that can be polished means that, to reduce the arithmetic mean deviation of the primary profile Pa to 0.090 pm or less, while still leaving some composite layer, a sufficiently smooth surface geometry must be provided before polishing. This requires sufficient polishing before nitriding, not just after.

[0024] The present invention is based on the results described above. The embodiments of the present invention will now be described in detail with reference to the figures. Identical or corresponding elements in the drawings are labelled with the same reference characters, and their descriptions will not be repeated. The size relationships between the components shown in the figures do not necessarily indicate the actual size relationships.

[0025] [Crankshaft] Figure 4 is a schematic view of a crankshaft 10 according to an embodiment of the present invention. The crankshaft 10 includes main bearing journals 11, connecting rod journals 12, and crank arms 13.

[0026] The main bearing journals 11 are coupled to a cylinder block (not shown). The connecting rod journals 12 are coupled to the connecting rods (not shown). The crank arms 13 connect the main bearing journals 11 to the connecting rod journals 12.

[0027] For example, crankshaft 10 may be made of a structural machine steel. Although not limiting, crankshaft 10 may be made of a carbon structural machine steel according to JIS G 4051:2009; or an alloy structural machine steel according to JIS G 4053:2008, for example. S45C, S50C, and S53C steels according to JIS G 4051:2009 and SMn438 steel according to JIS G 4053:2008 are particularly suitable, and steel materials with added S to improve machinability are especially suitable.

[0028] Crankshaft 10 may have a chemical composition (i.e., the chemical composition of a base material, excluding the composite layer and nitrogen diffusion layer) that includes, for example, in % by mass: 0.30 to 0.60% C; 0.01 to 2.0% Si; 0.1 to 2.0% Mn; 0.01 to 0.50% Cr; 0.001 to 0.06% Al; 0.001 to 0.02% N; up to 0.03% P; and up to 0.20% S, in addition to Fe and impurities. The chemical composition of crankshaft 10 may include other elements. Rrnn Ln / zznz / E / YiAi of the crankshaft 10 may include, for example, in % by mass: 0 to 0.50% Mo; 0 to 0.50% Cu; 0 to 0.50% Ni; 0 to 0.050% Ti; 0 to 0.050% Nb; 0 to 0.005% Ca; 0 to 0.30% Bi; and 0 to 0.20% V.

[0029] A composite layer containing iron and nitrogen forms on the surface of crankshaft 10. The composite layer is mainly composed of an iron and nitrogen compound, but may contain small amounts of elements other than iron and nitrogen. Preferably, in the composite layer, the total content of elements other than iron and nitrogen does not exceed 10% by mass.

[0030] The composite coating typically covers the entire surface of the crankshaft 10. However, the composite coating should only cover the surfaces of the friction parts, i.e., the main bearing journals 11 and the connecting rod journals 12, and does not necessarily have to cover the entire surface of the crankshaft 10.

[0031] The composite layer may be composed primarily of the ε (Fe2-3N) phase, or it may be composed primarily of the γ' (Fe4N) phase. The composite layer may be a mixture of ε and γ' phases.

[0032] In one embodiment, the crankshaft 10 may include a composite layer in which the proportion of the ε phase, represented by a cross-sectional area ratio, is not less than 80%. The ε phase has a crystalline structure consisting of a tightly packed hexagonal lattice and has better fatigue strength and better wear resistance than the γ' phase, making it suitable for applications where mechanical strength properties are important. Furthermore, the self-diffusion coefficient of the ε phase is 10 times greater than that of the γ' phase under the same temperature conditions, meaning that the ε phase can be readily produced.Therefore, a crankshaft incorporating a composite layer with a high proportion of ε phase is more advantageous in manufacturing than a crankshaft incorporating a composite layer with a high proportion of γ' phase. The cross-sectional area ratio of the ε phase is preferably not less than 90%.

[0033] In another embodiment, crankshaft 10 may include a composite layer in which the proportion of the γ' phase, represented by a cross-sectional area ratio, is not less than 80%. The γ' phase has a crystal structure composed of a face-centered cubic lattice and has a coefficient of cubic expansion approximately 30% lower than that of the ε phase, making it suitable for applications where thermal stability, such as thermal shock resistance, is important. A cross-sectional area ratio of the γ' phase is preferable to not less than 90%.

[0034] The proportions of the ε, γ', and α phases in the composite layer can be determined by electron backscatter diffraction (EBSD). Specifically, an EBSD measurement is performed on a cross-section of the composite layer, followed by mapping of the ε, γ', and α phases to determine the area ratio of these phases. It is convenient for the EBSD measurements for about 10 fields of view to be performed at a magnification of approximately 4000x. Rrnn Ln / zznz / E / YiAi

[0035] In the composite layer, for both the main bearing journals 11 and the connecting rod journals 12, the porosity area ratio of the thinnest part of a region from the surface to a depth of 3.0 pm shall not exceed 10.0%. However, if the thickness of the composite layer is less than 3.0 pm, the porosity area ratio measured over the entire thickness shall not exceed 10.0%. The ratio of the porosity area of ​​a thinner region from the surface to a depth of 3.0 pm and a region throughout the entire thickness of a composite layer shall hereinafter be referred to as the layer-to-surface porosity area ratio of the composite layer.

[0036] Although the mechanism is not clear, the lower the ratio of porosity area of ​​the layer to surface area of ​​the composite layer, the better the resistance to seizing. The ratio of porosity area of ​​the layer to surface area of ​​the composite layer of the main bearing journals 11 and connecting rod journals 12 is preferably not greater than 5.0%, and more preferably not greater than 3.0%.

[0037] The layer-surface porosity area ratio can be measured as follows: A cross-section of the composite layer is photographed by scanning electron microscopy (SEM) at a magnification of about 5000 times; 12 lines are drawn 0.25 pm apart and parallel to the surface of the composite layer, together with 92 lines 0.25 pm apart and perpendicular to the surface of the composite layer; and the proportion of those intersections of said lines that lie in voids is treated as the layer-surface porosity area ratio.

[0038] The composite layer can have any layer-to-surface porosity area ratio for parts of the crankshaft other than the main bearing journals 11 and connecting rod journals 12. The composite layer over the whole crankshaft can have low surface layer-to-porosity area ratios, or only portions of the layer over the main bearing journals 11 and connecting rod journals 12 can have low surface layer-to-porosity area ratios.

[0039] The composite layer thickness for the main bearing journals 11 and connecting rod journals 12 is preferably from 1.0 to 50 pm. A lower limit for the composite layer thickness for the main bearing journals 11 and connecting rod journals 12 is more preferably 2.0 pm, and even more preferably 3.0 pm. An upper limit for the composite layer thickness for the main bearing journals 11 and connecting rod journals 12 is more preferably 30 pm, and even more preferably 20 pm, and even more preferably 8 pm. The composite layer may have any thickness for crankshaft parts other than the main bearing journals 11 and connecting rod journals 12.

[0040] The thickness of the composite layer can be measured as follows: A cross-section of the composite layer is polished, etched with a nital solution, and observed using optical microscopy. The composite layer appears as a white, uncorroded layer. Five fields of view are observed on photographs of the microstructure taken by optical microscopy at 500x magnification. For each field of view, the thickness of the composite layer is measured at four horizontally arranged points. Rrnn Ln / zznz / E / YiAi and separated from each other by 30 pm. The average of the thickness values ​​at the 20 measured points is treated as the thickness of the composite layer.

[0041] The hardness of the composite coating for the main bearing journals 11 and connecting rod journals 12 is preferably from HV500 to HV1000. A lower limit for the hardness of the composite coating for the main bearing journals 11 and connecting rod journals 12 is more preferably HV700, and even more preferably HV800. The composite coating may have any hardness for parts of the crankshaft other than the main bearing journals 11 and connecting rod journals 12.

[0042] The main bearing journals 11, as well as the connecting rod journals 12, have a surface geometry such that the arithmetic mean deviation of the primary profile Pa is not greater than 0.090 pm. As used herein, the arithmetic mean deviation of a primary profile Pa is that defined by JIS B 0601:2001.

[0043] More specifically, the arithmetic mean deviation of the primary profile Pa is measured as follows: Test samples are taken from the crankshaft 10 at the measurement locations (i.e., on the main bearing journals 11 and the connecting rod journals 12), and a contact roughness tester is used to obtain a measured primary profile. The contact roughness tester used has a stylus with a tip radius of 2 mm and a cone with a taper angle of 60°. The scanning speed is 0.5 mm / s or less, and the length for measurement is 5 mm or more.

[0044] A low-pass filter with a cutoff value Xs is applied to the measured primary profile to obtain a primary profile. As shown in Figure 2, using the primary profile as the evaluated profile, the average of the absolute values ​​of Z(x) is calculated for an evaluation length of 1, which is treated as the arithmetic mean deviation of the primary profile Pa. Here, Z(x) is the vertical coordinate at a location x; the cutoff value ks is 2.5 pm, and the evaluation length is 5 mm.

[0045] Because the layer-to-surface area ratio of the composite layer is not greater than 10.0% and the arithmetic mean deviation of the primary Pa profile is not greater than 0.090 pm, the resistance to cracking will be significantly improved compared to conventional techniques. The arithmetic mean deviation of the primary Pa profile is preferably not greater than 0.080 pm.

[0046] [Crankshaft manufacturing method] An exemplary method of manufacturing crankshaft 10 will be described below. The manufacturing method described below is merely illustrative and in no way limits the manufacturing method of crankshaft 10.

[0047] Figure 5 is a flow diagram illustrating an exemplary method for manufacturing crankshaft 10. The manufacturing method includes a material preparation step (step S1), a hot forging step (step S2), a heat treatment step (step S3), a machining step (step S4), an intermediate grinding step (step S5), an intermediate lapping step (step S6), and a nitriding step (step Rrnn Ln / zznz / E / YiAi S7), a grinding step (step S8), a rough lapping step (step S9), and a finishing lapping step (step S10). These steps will now be described in detail.

[0048] A crankshaft material is prepared (SI pitch). The crankshaft material is not limited to any particular chemical composition and may be a structural machine steel mentioned above, for example. The material may be produced, for example, by continuous casting or roughing of a molten steel having a chemical composition as specified above.

[0049] The material is hot forged to form an approximately shaped crankshaft (step S2). The hot forging process can be divided into rough forging and final forging.

[0050] The roughly shaped (rough) crankshaft produced by hot forging may be subjected to heat treatment, such as quenching, tempering, and / or normalizing, as required (step S3). The heat treatment step (step S3) is optional and may be omitted depending on the required crankshaft properties or other factors.

[0051] The roughly shaped (rough) crankshaft product is machined (step S4). Machining processes include cutting, grinding, and drilling. This step results in an intermediate crankshaft product that has a shape similar to the final product.

[0052] The main and connecting rod journals of the intermediate crankshaft product undergo intermediate grinding and lapping (steps S5 and S6). As discussed above, in the crankshaft conforming to the present embodiment, the arithmetic mean deviation of the primary profile Pa is no more than 0.090 pm while any composite layer is left. This requires that the arithmetic mean deviation of the primary profile Pa of the main and connecting rod journals be reduced before the nitriding stage (step S7). Preferably, the intermediate grinding and lapping reduce the arithmetic mean deviation of the primary profile Pa of both the main and connecting rod journals to 0.15 pm or less.

[0053] The intermediate crankshaft product subjected to intermediate grinding and intermediate lapping is nitrided (step S7). The nitriding process is carried out in an atmosphere containing NH3, H2, and N2, for example. The nitriding process can be carried out in an atmosphere containing CO2 in addition to NH3, H2, and N2. The process temperature is 550 to 620 °C, for example. The process time is 1.5 to 10 hours, for example.

[0054] During this time, the nitriding potential Kn = Pnh3 / (Ph2)3 / 2 can be controlled to control the proportions of the ε and γ' phases in the composite layer. Pnh3 and Ph2 denote the partial pressures of NH3 and H2, respectively. Specifically, increasing the nitriding potential Kn increases the proportion of the ε phase, while decreasing the nitriding potential Kn increases the proportion of the γ' phase. Rrnn Ln / zznz / E / YiAi

[0055] After nitriding, the main and connecting rod journals are ground again to obtain a clean surface geometry (step S8). If the composite layer formed during nitriding includes a porous layer (denoted by number 51 in Figure 1), this grinding step removes the porous layer.

[0056] Next, the main and connecting rod journals are lapped (steps S9 and S10). This lapping process is divided into a rough lapping step and a finishing lapping step, where the rough lapping step uses a lapping film coated with alumina abrasive grains, while the finishing lapping step uses a lapping film coated with diamond abrasive grains. This reduces the arithmetic mean deviation of the primary profile Pa to 0.090 pm or less, while leaving some composite layer. It is difficult at this point to reduce the arithmetic mean deviation of the primary profile Pa to 0.090 pm or less while leaving some composite layer if the aforementioned intermediate grinding and intermediate lapping are insufficient.

[0057] To reduce the arithmetic mean deviation of the primary profile Pa to 0.090 pm or less, it is necessary to reduce both the roughness and the waviness during the intermediate grinding pass (step S5) and the grinding pass (step S8). Roughness, in particular, depends on the size of the abrasive grains used for grinding. Therefore, it is preferable to use the smallest possible abrasive grains for grinding.

[0058] On the main journals and connecting rod journals, there are wavinesses with periods ranging from several hundred micrometers to several millimeters, caused by the tool feed and vibration during the machining pass (step S4). Even if the surface roughness is sufficiently reduced (i.e., the arithmetic mean deviation of the surface roughness profile Ra is sufficiently reduced), the arithmetic mean deviation of the primary profile Pa does not decrease if the wavinesses remain. Therefore, in the intermediate grinding pass (step S5) and the final grinding pass (step S8), the wavinesses must be sufficiently removed by continuing grinding even after the deviation Ra has decreased.

[0059] In addition, in the lapping steps (steps S6, S9, and S10), it is preferable to perform the following processes, (1) to (4), to avoid the formation of a centrally recessed shape with a concave central portion: (1) polishing is performed while feeding a lapping film with a small width in the axial direction, which facilitates the lubricating oil reaching the central portion of the lapping film; (2) abrasive grains of as small a size as possible are used, resulting in a small depth of cut, mitigating over-grinding; (3) the rotational speed of the workpiece is increased while reducing the pressure force, which increases the thickness of the oil (or water) film between the lapping film and the workpiece; and (4) the amount of lubricating oil (or water) is increased, which increases the thickness of the oil (or water) film between the lapping film and the workpiece. Rrnn Ln / zznz / E / YiAi

[0060] Furthermore, in each of the intermediate lapping (step S6), rough lapping (step S9), and finishing lapping (step S10) stages, the speed of advance of the lapping film in the axial direction of the crankshaft must be as low as possible. This eliminates small undulations and further reduces the arithmetic mean deviation of the primary profile Pa.

[0061] An exemplary construction of the crankshaft 10 in accordance with an embodiment of the present invention and an exemplary method of manufacturing the same have been described. The embodiments provide a crankshaft with improved resistance to seizure. Rrnn Ln / zznz / E / YiAi EXAMPLES

[0062] The present invention will now be described more specifically by means of examples. The present invention is not limited to these examples.

[0063] A steel having the chemical composition shown in Table 1 was used as material to manufacture a plurality of test trees for the jamming tests.

[0064] Table 1 TABLE 1 Chemical composition (in % by mass, the remainder is Fe impurities) C Si Mn P s Cr Al N 0.53 0.25 0.80 0.015 0.028 0.07 0.035 0.0050

[0065] Specifically, the material was heated to 1250 °C for one hour and then hot forged at about 1150 °C; after forging was completed, the material was air-cooled to room temperature. The material was then machined (i.e., ground) to an outside diameter of about 53 mm.

[0066] After machining, intermediate grinding and lapping were performed to adjust the arithmetic mean deviation of the primary profile Pa to 0.15 pm or less. A lapping film coated with alumina abrasive grains with grain diameters of 9 to 15 pm was used for the intermediate lapping. For some test shafts, the intermediate grinding and lapping were omitted, and the next step, i.e., nitriding, was performed for comparison.

[0067] The nitriding process performed was (A) a nitriding process to form a layer composed mainly of ε phase, or (B) a nitriding process to form a layer composed mainly of γ' phase. In both cases, the nitriding process was carried out in an atmosphere containing ammonia, hydrogen, and CO2. In process (A), the nitriding potential Knse was adjusted to a range of 1 to 10, and the temperature was maintained at 570 °C for 3 hours, followed by oil quenching. In process (B), the nitriding potential Knse was adjusted to a range of 0.3 to 0.5, and the temperature was maintained at 570 °C for 8 hours, followed by oil quenching.

[0068] EBSD measurements showed that, in a test tree subjected to process (A), a composite layer was formed in which the proportion of phase ε represented by a cross-sectional area ratio was not less than 90%, and, in a test sample subjected to process (B), a composite layer was formed in which the proportion of phase γ' represented by a cross-sectional area ratio was not less than 90%.

[0069] The nitrided test shaft was subjected to grinding, rough lapping, and finish lapping. The grinding step was performed to leave a composite layer. A test shaft with a composite layer, primarily composed of ε-phase, was ground to remove the porous layer and leave the dense layer. In the rough lapping, a lapping film coated with alumina abrasive grains with grain diameters of 9 to 15 µm was used, while in the finish lapping, a lapping film coated with diamond abrasive grains with grain diameters of 1 to 3 µm (#8000 to #4000) was used. The outside diameter of the test shaft was adjusted so that the clearance with a bearing to be used for the seizure tests, discussed later, was approximately 0.080 mm.Test shafts were also manufactured with the porous layer, test shafts with the entire composite layer removed, and test shafts subjected to a simplified grinding step in which only the outside diameter was adjusted, to facilitate comparison.

[0070] The surface geometry, layer-to-surface porosity area ratio, and hardness of each fabricated test shaft were measured. Surface geometry was measured using a contact-type roughness gauge (Mitutoyo Corporation SJ-412). Hardness was measured using a nanoindenter.

[0071] Seizure tests were performed on each fabricated test shaft. A schematic view of the evaluation equipment 20 used for the seizure test is shown in Figure 6. The test shaft TP was inserted through the test bearings 21 and clamping bearings 22 and rotated by a motor (not shown) at a circumferential speed of 20 m / s while the test bearings 21 were oiled. As shown in Figure 7, a wedge 25 with a thickness of 30 pm was inserted between the housing of each test bearing 21 and the tester to force friction from one side.The bearing metal used was a Bi / Cu alloy. The lubricating oil was VG22, the temperature of the supplied oil was 100 °C, and the oil supply rate was 150 ml / min.

[0072] After 50 minutes of break-in, a load was applied to the test bearings 21, and operation continued while the surface pressure applied to the test shaft TP was gradually increased until seizure occurred. Figure 8 schematically illustrates the changes over time in the surface pressure applied to the test shaft TP. The time during which the surface pressure remained at the same level was 10 minutes, and the increase in surface pressure for each step was 5 MPa. Seizure was determined to occur when the temperature at the back of the bearing rose to 230 °C or higher, or when the belt slipped due to torsional variations. Rrnn Ln / zznz / E / YiAi

[0073] The manufacturing conditions and test results for the various test trees are shown in Table 2. The entries in the column labeled Layer-to-Surface Porosity Area Ratio for test tree brands KI, K2, and K3 show the porosity area ratios in a region from the surface of the nitrogen diffusion layer to a depth of 3.0 pm.

[0074] Table 2 TABLE 2 Rrnn Ln / zznz / E / YiAi Test tree mark Intermediate grinding, intermediate lapping Nitriding Grinding, rough lapping, finish lapping Pa (pm) Porosity area ratio of the layer to the surface (%) Hardness (HV) Composite layer thickness (pm) Seizing surface pressure (MPa) Category C1Z Yes A Yes 0.059 1.0 970 3.0 115 e.g. of invention CIA Yes A Yes 0.061 1.3 980 3.5 110 e.g. of invention G1A Yes B Yes 0.081 1.1 940 3.5 115 e.g. of invention C1B Yes A Yes 0.085 2.0 870 4.1 105 e.g. of invention C1C yes A YES 0.087 2.1 860 5.1 100 example of invention CID yes A yes 0.089 2.4 840 7.3 100 example of invention G1B yes B yes 0.088 1.2 930 7.3 105 example of invention C2 no A yes 0.21 1.5 970 3.4 90 comparative example G2 no B yes 0.30 1.5 970 4.5 90 comparative example P1 yes A yes 0.079 12 920 15 90 comparative example P2 yes A no 0.39 12 840 13 80 comparative example P3 yes A no 0.49 14 900 16 55 example comparative K1 no A yes 0.079 0.1 420 0 85 example. comparative K2 no A yes 0.072 0.1 410 0 80 example.comparative K3 no A yes 0.069 0.1 410 0 85 comparative example.

[0075] For each of the test trees labeled with the test tree markings C1Z, CIA, G1A, C1B, C1C, CID, and G1B, the layer-to-surface porosity area ratio of the composite layer was not greater than 10.0%, and its surface geometry was such that the arithmetic mean deviation of the primary Pa profile was not greater than 0.090 pm. These test trees had surface sticking pressures not less than 100 MPa, showing good sticking resistance.

[0076] The test trees labeled C2 and G2 had surface seizure pressures below 100 MPa. This is presumably because the arithmetic mean deviation of the primary profile Pa was high in these test trees. These test trees, after nitriding, had been ground, rough lapped, and finish lapped, but not intermediate ground and intermediate lapped, and therefore did not sufficiently reduce the arithmetic mean deviation of the primary profile Pa.

[0077] The test trees labeled with test tree marks Pl, P2, and P3 had the porous layer remaining on the outermost surface of the composite layer. These test trees had surface seizure pressures below 100 MPa.

[0078] The test trees labeled with test tree markings KI, K2, and K3 had their nitrogen diffusion layer exposed, with the composite layer completely removed. These test trees had surface sticking pressures below 100 MPa.

[0079] Although embodiments of the present invention have been described, the embodiments described above are merely illustrative examples useful for carrying out the present invention. Therefore, the present invention is not limited to the embodiments described above, and the embodiments described above, when carried out, may be modified as appropriate without departing from the scope of the invention.

Claims

1. A crankshaft having a main journal and a connecting rod journal, including a composite layer containing iron and nitrogen on a surface thereof, wherein, in the composite layer, for each of the main journals and connecting rod journals, a porosity area ratio of the thinnest part of a region from the surface to a depth of 3.0 pm and a region along the entire thickness of the composite layer is not greater than 10.0%, and each of the main journals and connecting rod journals has a surface geometry such that an arithmetic mean deviation of a primary profile, Pa, is not greater than 0.090 pm.

2. The crankshaft according to claim 1, wherein the composite layer has a hardness of HV500 to HV1000.

3. The crankshaft according to claim 1 or 2, wherein, for each of the main bearing journals and connecting rod journals, the composite layer has a thickness of 1.0 to 50 pm.

4. The crankshaft according to any one of claims 1 to 3, wherein a phase ratio ε in the composite layer, represented by a cross-sectional area ratio, is not less than 80%.

5. The crankshaft according to any one of claims 1 to 3, wherein a phase ratio γ' in the composite layer, represented by a cross-sectional area ratio, is not less than 80%.

6. A method of manufacturing a crankshaft according to any one of claims 1 to 5, comprising: an intermediate grinding step for grinding a main journal and a connecting rod journal of an intermediate crankshaft product; after the intermediate grinding step, an intermediate lapping step for lapping the main journal and a connecting rod journal of the intermediate product; after the intermediate lapping step, a nitriding step for nitriding the intermediate product; after the nitriding step, a grinding step for grinding the main journal and a connecting rod journal of the intermediate product; after the grinding step, a coarse lapping step for lapping the main journal and a connecting rod journal of the intermediate product using a film coated with alumina abrasive grains;and after the rough lapping stage, a finishing lapping step to lap the main journal and connecting rod journal of the intermediate product using a film coated with diamond abrasive grains.

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