Gray cast iron cylinder sleeve with low residual stress as well as preparation method and application of gray cast iron cylinder sleeve

Abstract

The invention discloses a gray cast iron cylinder sleeve with low residual stress and a preparation method and application thereof, and belongs to the field of metal material processing. The preparation method of the gray cast iron air cylinder sleeve with the low residual stress comprises the following steps that (1) the gray cast iron air cylinder sleeve with large internal stress after rough machining is placed in heat treatment equipment, heating and heat preservation are conducted, and the heat preservation time is determined according to the heating temperature and the thickness of the gray cast iron air cylinder sleeve; and (2) the material prepared in the step (1) is cooled in heat treatment equipment, then taken out and air-cooled to the room temperature, and the gray cast iron air cylinder sleeve with the low residual stress is obtained. According to the method, the gray cast iron cylinder sleeve with large internal stress after rough machining is subjected to heat treatment, precise control of greatly reducing the residual stress of the gray cast iron cylinder sleeve is achieved through temperature and time regulation and control, the performance of a finished product is stable and reliable, and the method is suitable for commercial application.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal material processing, and particularly relates to a gray cast iron cylinder liner with low residual stress, a preparation method thereof, and an application thereof. Background Art

[0002] The cylinder liner is one of the important parts of a diesel engine. Its working conditions are very harsh and it needs to bear the action of various complex stresses such as high temperature, high pressure, friction, and corrosion. The inner surface of the cylinder liner is directly impacted by high-temperature and high-pressure combustion gas, and high-speed sliding friction occurs between the piston ring and the inner wall of the cylinder liner, which will cause a certain amount of wear to the cylinder liner. The inner surface of the cylinder liner is directly affected by high-temperature combustion gas, while the outside is cooled by coolant or other means. Therefore, there is a large temperature difference between the inside and outside of the cylinder wall. This temperature difference causes thermal stress to be generated on the inner side of the cylinder wall. At high temperatures, compressive stress is generated on its inner surface due to thermal expansion, while tensile stress is generated on the cooling surface due to contraction. The working condition of the cylinder liner has a direct impact on the working efficiency and reliability of the engine body. Gray cast iron is the preferred material for cylinder liner production. During production and use, uneven internal stress may occur, affecting subsequent processing or service performance. Summary of the Invention

[0003] Object of the Invention: To solve the problems existing in the prior art, the first object of the present invention is to provide a gray cast iron cylinder liner with low residual stress, the second object of the present invention is to provide a preparation method of the above-mentioned gray cast iron cylinder liner with low residual stress, and the third object of the present invention is to provide an application of the above-mentioned gray cast iron cylinder liner with low residual stress.

[0004] Technical Solution: The preparation method of the gray cast iron cylinder liner with low residual stress provided by the present invention includes the following steps:

[0005] (1) Place the gray cast iron cylinder liner with large internal stress after rough machining in a heat treatment device, heat it and keep it warm. The holding time is determined according to the heating temperature and the thickness of the gray cast iron cylinder liner. The calculation formula is as follows:

[0006] t = 26 - T / 24 + t 透 (1),

[0007] t 透 = 0.04D + t’(2),

[0008] wherein, t represents the holding time / h, T represents the heating temperature / °C, t 透 represents the heat penetration time / h of the gray cast iron cylinder liner, D represents the thickness of the cylinder liner / mm, and t’ represents the time / h for the heat treatment device to rise to the set temperature;

[0009] (2) Cool the material obtained in step (1) in a heat treatment equipment, and then take it out and air-cool it to room temperature to obtain a gray cast iron cylinder liner with low residual stress.

[0010] Further, in step (1), the heating temperature is 550 - 600 °C.

[0011] Further, in step (1), the microstructure of the gray cast iron cylinder liner with large internal stress is ferrite + flake graphite; the chemical composition of the gray cast iron cylinder liner, by mass percentage, includes the following components: C: 2.5 - 3.5%, Si: 1.5 - 2.5%, Mn: 0.5 - 1.0%, Cr: 0.2 - 0.6%, P: ≤0.3%, S: ≤0.1%, and the rest is Fe and unavoidable impurities.

[0012] Further, in step (1), the gray cast iron cylinder liner is melted by an intermediate frequency induction furnace, the melting temperature is 1500 - 1550 °C, the molten iron is left standing and insulated for 10 min after pouring out of the furnace, and then it is formed by centrifugal casting.

[0013] Further, the thickness of the gray cast iron cylinder liner is 1 - 8 mm.

[0014] Further, in step (2), the cooling rate is 60 ± 5 °C / h.

[0015] Further, in step (2), it is cooled to 200 ± 20 °C in the heat treatment equipment.

[0016] The present invention provides a gray cast iron cylinder liner with low residual stress prepared by the above preparation method, and the residual stress is (25 - 30) ± 20 MPa.

[0017] The present invention also provides the application of the above gray cast iron cylinder liner with low residual stress in a sleeve piston diesel engine.

[0018] Beneficial effects: Compared with the prior art, the present invention has the following remarkable advantages: For a gray cast iron cylinder liner with large internal stress after rough machining, the present invention can greatly reduce the residual stress by precisely controlling the heat treatment parameters, thereby improving its machining performance. The control of the heating temperature is to improve the diffusion ability and effect of elements, and then determine the holding time according to the heating temperature and the thickness of the cylinder liner to precisely control the diffusion degree of elements, and finally achieve the precise control of greatly reducing the residual stress of the gray cast iron cylinder liner. The performance of the finished product is stable and reliable, and it is suitable for commercial applications. Description of the Drawings

[0019] Figure 1 It is a microstructural diagram of a large internal stress area of an untreated gray cast iron cylinder liner;

[0020] Figure 2 The microstructural diagram after holding at 550 °C for 4.5 h and then furnace cooling in step (1) of Example 1;

[0021] Figure 3 The microstructural diagram after holding at 550 °C for 2.5 h and then furnace cooling in step (1) of Comparative Example 1;

[0022] Figure 4 The microstructural diagram after holding at 600 °C for 2.5 h and then furnace cooling in step (1) of Example 2;

[0023] Figure 5 The microstructural diagram after holding at 600 °C for 4.5 h and then furnace cooling in step (1) of Comparative Example 2. Detailed implementation manners

[0024] Next, the present invention will be further described in conjunction with specific embodiments and the accompanying drawings.

[0025] The chemical composition of the gray cast iron cylinder liner used in the following examples, by mass percentage, includes the following components: C: 3.1%, Si: 2.0%, Mn: 0.85%, Cr: 0.4%, P: ≤0.3%, S: ≤0.1%, and the rest are Fe and inevitable impurities. The gray cast iron cylinder liner is melted by an intermediate frequency induction furnace, the melting temperature is 1500 - 1550 °C, the molten iron is left to stand and heat-insulated for 10 min after being taken out of the furnace, and then formed by centrifugal casting. Subsequently, a sample with large internal stress appears after rough machining and annealing processes.

[0026] Example 1: The preparation method of the gray cast iron cylinder liner with low residual stress provided in this example includes the following steps:

[0027] (1) Select the area of the gray cast iron cylinder liner with large internal stress after rough machining, and its microstructural morphology is as Figure 1 shown. It can be seen from Figure 1 that the graphite morphology in the cylinder liner sample is mainly in the form of flaky chrysanthemum-like graphite, and there is also a small amount of dendritic flaky graphite. The residual stress value measured by the indentation method is about 100 ± 60 MPa. Put the sample into a high-precision heat treatment heating furnace, set the heating temperature to 550 °C, and the furnace temperature difference is not more than ±3 °C. Then carry out heat preservation. According to the sample thickness (4 mm) and heating temperature, calculate the heat preservation time through the following formula:

[0028] t = 26 - T / 24 + t 透 (1),

[0029] t 透 = 0.04D + t’(2),

[0030] where t represents the heat preservation time / h, T represents the heating temperature / °C, t 透Let \(t\) represent the heat penetration time of the gray cast iron cylinder liner in hours, \(D\) represent the thickness of the cylinder liner in millimeters, and \(t'\) represent the time for the heat treatment equipment to reach the set temperature in hours. The holding time is calculated and determined to be \(4.5\) h, where the heat penetration time is \(0.5\) h (the heat penetration time depends on the actual situation of the furnace).

[0031] (2) Cool the sample processed in step (1) in the furnace at a cooling rate of \(60^{\circ}C / h\) to below \(200^{\circ}C\), and then take it out of the furnace and cool it to room temperature. The microstructure morphology of the processed sample is as Figure 2 shown. As can be seen from Figure 2 , the eutectic cementite decomposes, forming flocculent graphite evenly distributed in the matrix, and the microstructure distribution is relatively uniform. At the same time, at high temperatures, the high internal stress area reduces the internal stress through micro-deformation of the microstructure. At this time, the measured residual stress value is about \(30\pm20\) MPa.

[0032] Comparative Example 1: The difference from Example 1 is that in step (1), instead of holding at the time according to the formula, the holding time is set to \(2.5\) h, where the heat penetration time is \(0.5\) h.

[0033] The microstructure morphology of the processed sample is as Figure 3 shown. As can be seen from Figure 3 , the eutectic cementite begins to decompose, but due to insufficient holding time, the number of flocculent graphite formed is less than Figure 2 , the uniformity of the microstructure distribution is low, and the measured residual stress value is about \(55\pm40\) MPa at this time.

[0034] Example 2: The preparation method of the gray cast iron cylinder liner with low residual stress provided in this example includes the following steps:

[0035] (1) Select the area of the gray cast iron cylinder liner with large internal stress after rough machining. Its microstructure morphology is as Figure 1 shown. The residual stress value is measured to be about \(100\pm60\) MPa by the indentation method. Put the specimen into a high-precision heat treatment heating furnace, set the heating temperature to \(600^{\circ}C\), and the furnace temperature difference is not more than \(\pm3^{\circ}C\). Then carry out holding. According to the specimen thickness (\(4\) mm) and heating temperature, calculate and determine the holding time to be \(2.5\) h through formulas (1) and (2), where the heat penetration time is \(2.5\) h.

[0036] (2) Cool the sample processed in step (1) in the furnace at a cooling rate of \(60^{\circ}C / h\) to below \(200^{\circ}C\), and then take it out of the furnace and cool it to room temperature. The microstructure morphology of the processed sample is as Figure 4 shown. As can be seen from Figure 4 , the eutectic cementite decomposes, forming flocculent graphite evenly distributed in the matrix, and the microstructure distribution is relatively uniform. At this time, the measured residual stress value is about \(25\pm20\) MPa.

[0037] Comparative Example 2: The difference from Example 2 is that in step (1), instead of insulating according to the formula time, the insulation time is set to 4.5 h, and the heat penetration time is 0.5 h.

[0038] The microstructure of the treated sample is as Figure 5 shown. It can be seen from Figure 5 that eutectic cementite decomposes into free carbon. Due to the long-term insulation at high temperature, these free carbons do not form flocculent graphite in the vicinity, but diffuse to the flaky graphite at a farther distance and precipitate by enrichment, thus vacating a relatively large area of ferrite, resulting in uneven distribution of the microstructure. When these different microstructures are cooled, new residual stresses will be generated due to the volume change differences. At this time, the measured residual stress value is about 50 ± 40 MPa.

[0039] From the above examples and comparative examples, it can be seen that the present invention determines the insulation time according to the heating temperature and the thickness of the cylinder liner, accurately controls the degree of element diffusion, and then greatly reduces the accurate control of the residual stress of the gray iron cylinder liner. The finished product has stable and reliable performance and is suitable for commercial applications.

Claims

1. A preparation method of a gray cast iron cylinder liner with low residual stress, characterized in that, It includes the following steps: (1) Place the gray iron cylinder liner with large internal stress after rough machining in a heat treatment equipment, heat it and keep it warm. The holding time is determined according to the heating temperature and the thickness of the gray iron cylinder liner. The calculation formula is as follows: t = 26 - T / 24 + t 透 (1), t 透 = 0.04D + t’(2), Among them, t represents the heat preservation time / h, T represents the heating temperature / ℃, t 透 represents the heat penetration time / h of the gray iron cylinder liner, D represents the thickness / mm of the cylinder liner, and t’ represents the time / h for the heat treatment equipment to reach the set temperature; (2) Cool the material obtained in step (1) in the heat treatment equipment, and then take it out and air-cool it to room temperature to obtain a gray iron cylinder liner with low residual stress.

2. The preparation method according to claim 1, characterized in that, In step (1), the heating temperature is 550 - 600 °C.

3. The preparation method according to claim 1, wherein In step (1), the microstructure of the gray iron cylinder liner with large internal stress is ferrite + flake graphite.

4. The preparation method according to claim 3, characterized in that, In step (1), the chemical composition of the gray iron cylinder liner, by mass percentage, includes the following components: C: 2.5 - 3.5%, Si: 1.5 - 2.5%, Mn: 0.5 - 1.0%, Cr: 0.2 - 0.6%, P: ≤0.3%, S: ≤0.1%, and the rest is Fe and unavoidable impurities.

5. The preparation method according to claim 1, characterized in that, In step (1), the gray iron cylinder liner is melted by an intermediate frequency induction furnace. The melting temperature is 1500 - 1550 °C. After the molten iron is out of the furnace, it is kept warm for 10 min, and then formed by centrifugal casting.

6. The preparation method according to claim 1, wherein In step (2), the cooling rate is 60 ± 5 °C / h.

7. The preparation method according to claim 1, characterized in that, In step (2), it is cooled to 200 ± 20 °C in the heat treatment equipment.

8. A gray iron cylinder liner with low residual stress prepared by the preparation method according to any one of claims 1 - 7.

9. The gray cast iron cylinder liner with low residual stress according to claim 8, characterized in that The residual stress is (25 - 30) ± 20 MPa.

10. An application of the gray iron cylinder liner with low residual stress according to any one of claims 8 - 9 in a sleeve piston diesel engine.

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