A method to improve the unstable fatigue life of springs

Abstract

The invention relates to the technical field of spring processing, and discloses a processing method for improving the instability of spring fatigue life, comprising the following steps: melting and pouring raw material components to obtain a casting; forging the casting to obtain a forging blank; drawing, winding, heat treating and shot peening the forging blank to obtain a finished spring; performing a pressurized nitriding treatment after the heat treatment and before shot peening; washing with a high-pressure water flow and drying before shot peening; and using a steel ball with a diameter not greater than 0.5 mm for the shot peening. The method of the invention can significantly reduce the difference in fatigue life between the same batch of springs, and can effectively reduce the surface microscopic defects of the treated springs through the combined effect of multiple treatments, thereby avoiding the problem that the fatigue life of some springs is too different from that of other springs due to the presence of surface microscopic defects.

Description

Technical Field

[0001] The invention relates to the technical field of spring processing, in particular to a processing method for improving the instability of spring fatigue life. Background Art

[0002] A spring is a mechanical part that uses elasticity to work. Parts made of elastic materials deform under external force and return to their original shape after the external force is removed. They are also called "springs".

[0003] Springs are generally made of spring steel. There are many types of springs, and according to their shapes, they mainly include helical springs, scroll springs, leaf springs, special-shaped springs, etc.

[0004] Then, when the prior art conducts fatigue tests on springs made of 60Si2CrVAT spring steel, in the same batch of springs, the fatigue life of some springs made of 60Si2CrVAT spring steel can meet the standard requirements, while some cannot meet the standard requirements. Among them, the highest fatigue life can reach millions of times, but the lowest fatigue life is only about 200,000 times, which makes the fatigue life stability of springs made of the same batch of 60Si2CrVAT spring steel poor, greatly limiting its use.

[0005] Based on this, we proposed a method to improve the instability of spring fatigue life, hoping to solve the shortcomings of the existing technology. Summary of the invention

[0006] 1. Technical issues to be resolved

[0007] In view of the deficiencies in the prior art, the present invention provides a method for improving the instability of spring fatigue life.

[0008] (II) Technical solution

[0009] To achieve the above-mentioned purpose, the present invention provides the following technical solutions:

[0010] A method for improving the instability of spring fatigue life comprises the following steps:

[0011] The raw material components are melted and poured to obtain a casting;

[0012] Forging the casting to obtain a forging blank;

[0013] Drawing, winding, heat treating and shot peening the forged blank to obtain a finished spring;

[0014] After the heat treatment, a pressurized nitriding treatment is performed before shot peening;

[0015] The shot blasting is preceded by flushing with high pressure water and drying;

[0016] The shot peening treatment is performed using steel balls with a diameter not greater than 0.5 mm.

[0017] As a further technical solution, the finished spring is made of 60Si2CrVAT spring steel material.

[0018] As a further technical solution, the initial forging temperature of the forging process is 1100-1130°C, and the final forging temperature is 910-930°C;

[0019] The forging ratio is 7.2-7.6.

[0020] As a further technical solution: the difference between the initial forging temperature and the final forging temperature is fixed at 200°C, that is, the initial forging temperature minus the final forging temperature is 200°C.

[0021] As a further technical solution, the pressurized nitriding treatment is:

[0022] Place the test piece in a resistance furnace, introduce nitrogen-containing gas, exhaust the air in the resistance furnace, then increase the pressure to 0.31-0.35MPa, adjust the temperature to 550℃, keep warm for 30min, then increase the temperature by 50℃ every 20min and increase the pressure by 0.1MPa. After 100min, keep warm and pressure for 3-4 hours, then cool with the furnace.

[0023] As a further technical solution, the nitrogen-containing gas is any one of nitrogen and ammonia.

[0024] As a further technical solution: the high-pressure water flow is used for flushing:

[0025] The pressure of the high-pressure water flow is 0.8-0.85MPa.

[0026] As a further technical solution: the drying process is carried out in a drying chamber;

[0027] The drying temperature in the drying chamber is 70-75℃.

[0028] (III) Beneficial effects

[0029] Compared with the prior art, the present invention provides a method for improving the instability of spring fatigue life, which has the following beneficial effects:

[0030] The method of the present invention can significantly reduce the difference in fatigue life between the same batch of springs. Through the combined effect of multiple treatments, the surface micro defects of the treated springs can be effectively reduced, thereby avoiding the problem that the fatigue life of some springs is too different from that of other springs due to the presence of surface micro defects. The stability of the fatigue life of springs produced in the same batch is guaranteed, and the problem that the fatigue life of some springs is greatly different after the springs are installed and used, affecting the normal operation of the equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 To compare the effects of different shot peening times on spring fatigue life. DETAILED DESCRIPTION

[0032] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0033] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0034] A method for improving the instability of spring fatigue life comprises the following steps:

[0035] The raw material components are melted and poured in a 50 kg vacuum induction furnace to obtain a casting. The pouring temperature is controlled at 1450°C. After pouring, water cooling is performed. The casting is a cast structure, containing large columnar crystals and loose centers. Therefore, it is necessary to perform a large plastic deformation to break the columnar crystals into fine grains and compact the looseness, thereby improving the structure and performance of the casting.

[0036] The casting is forged to obtain a forging blank, the initial forging temperature of the forging treatment is 1100-1130°C, and the final forging temperature is 910-930°C;

[0037] The forging ratio is 7.2-7.6.

[0038] The difference between the initial forging temperature and the final forging temperature is fixed at 200°C, that is, the initial forging temperature minus the final forging temperature is 200°C; in order to effectively prevent the formation of dendritic structure in steel, it is necessary to increase the equiaxed crystal area and reduce the columnar crystal area as much as possible. The present invention has debugged the initial forging temperature and the final forging temperature through a large number of experimental studies, and found that when the forging ratio is in the range of 7.2-7.6, the difference between the initial forging temperature and the final forging temperature is fixed at 200°C, which can cause the dendritic structure formed in the molten steel that has just begun to solidify in the early stage to be broken, and these broken dendrites can become new crystallization cores, thereby forming fine equiaxed crystals in a larger area within the specimen, thereby expanding the core equiaxed crystal area, greatly reducing the columnar crystal area, thereby greatly improving the organizational properties of the specimen, making all aspects of time performance more stable, thereby reducing the difference in fatigue resistance of each specimen.

[0039] The forging blank is subjected to drawing, spring winding, heat treatment and shot peening to obtain a finished spring; a pressurized nitriding treatment is performed after the heat treatment and before the shot peening; a high-pressure water flow is used for flushing and a drying treatment is performed before the shot peening; and a steel ball with a diameter not greater than 0.5 mm is used for the shot peening.

[0040] The present invention preferably uses 0.35mm steel balls for shot peening. Although the larger the diameter of the steel ball, the greater its strength, it will lead to a lower coverage rate of the steel ball. The lower coverage rate will lead to more defects on the surface of the specimen after shot peening. These defects will gradually develop into larger cracks in the future until the spring specimen breaks. Therefore, after a large number of experimental observations and comparisons, the present invention uses steel balls with a diameter of no more than 0.5mm for treatment, which can increase the coverage rate of the steel enterprise as much as possible and reduce surface defects after shot peening. At the same time, the optimal steel ball diameter is 0.35mm, which is the choice to maximize the coverage rate of the steel ball while ensuring the required steel ball strength.

[0041] The hardness of the steel ball is HRC55, the shot peening speed is 1500r / min, and the shot peening time is 250s.

[0042] Defects and even microscopic defects on the surface of spring specimens will significantly reduce the fatigue life of the spring. After shot peening, the non-uniform plastic deformation inner surface of the spring specimen produces residual tensile stress and the outer surface produces residual compressive stress. The stability of the spring organization is further improved, the surface roughness changes to a certain extent, and the microstructure is modified at the same time. The surface microscopic defects are greatly reduced. These changes can improve the fatigue fracture resistance of the spring specimen, prevent fatigue failure, brittle fracture and plastic deformation, and thus improve the fatigue life of the workpiece. At the same time, the comprehensive performance of each spring specimen is further homogenized, the difference between them is greatly reduced, and the fatigue life tends to be stable.

[0043] The finished spring is made of 60Si2CrVAT spring steel material.

[0044] The pressurized nitriding treatment is:

[0045] Place the specimen in a resistance furnace, introduce nitrogen-containing gas, exhaust the air in the resistance furnace, then increase the pressure to 0.31-0.35MPa, adjust the temperature to 550℃, keep warm for 30min, then increase the temperature by 50℃ every 20min, and increase the pressure by 0.1MPa. After 100min, keep warm and pressure for 3-4 hours, then cool with the furnace. The nitrogen-containing gas is either nitrogen or ammonia. When the specimen is nitrided at a pressure below 0.5MPa, the surface of the specimen is mainly composed of γ'-Fe4N phase and ε-Fe 2-3 N phase, but does not contain the more stable FeN. Therefore, the present invention gradually promotes the γ'-Fe4N phase and ε-Fe by increasing the temperature and pressure every 20 minutes. 2-3 The N phase decomposes and transforms into the FeN phase, and finally all the γ'-Fe4N phase and ε-Fe 2-3 N phase is replaced by FeN phase. At the same time, due to the subsequent increase in temperature and pressure, the γ'-Fe4N phase and ε-Fe 2-3 The N atoms produced by the decomposition of the N phase diffuse into the interior of the specimen material, further improving the specimen microstructure and performance. After being completely replaced by FeN in the surface tissue of the specimen, no further decomposition will occur in the subsequent treatment, and a denser and more uniform surface tissue will eventually be formed.

[0046] Flushing with high pressure water flow is as follows:

[0047] The pressure of high-pressure water flow is 0.8-0.85MPa. Flushing with high-pressure water flow can effectively remove scales, iron oxide, etc., greatly improving the surface finish of the specimen, thereby preventing the specimen from becoming uneven on the surface during subsequent shot peening, which affects the fatigue resistance of the specimen.

[0048] The drying process is to carry out drying process in a drying room;

[0049] The drying temperature in the drying chamber is 70-75℃.

[0050] The following are specific embodiments:

[0051] Example 1

[0052] A method for improving the instability of spring fatigue life comprises the following steps:

[0053] The raw material components are melted and poured in a 50 kg vacuum induction furnace to obtain a casting, and the pouring temperature is controlled at 1450°C. After the pouring is completed, water cooling is performed; the casting is forged to obtain a forging blank, and the initial forging temperature of the forging treatment is 1100°C, and the final forging temperature is 910°C; the forging ratio is 7.2. The difference between the initial forging temperature and the final forging temperature is fixed at 200°C, that is, the initial forging temperature minus the final forging temperature is 200°C; the forging blank is drawn, coiled, heat treated, and shot peened to obtain a finished spring; after the heat treatment, pressurized nitriding treatment is performed before shot peening; before the shot peening, high-pressure water flow is used for flushing, and the pressure of the high-pressure water flow is 0.8MPa. The drying treatment is to dry the drying room; wherein the drying temperature in the drying room is 70°C, and drying treatment is performed; 0.35mm steel balls are used for shot peening; the hardness of the steel balls is HRC55, the shot peening speed is 1500r / min, and the shot peening time is 250s. The finished spring is made of 60Si2CrVAT spring steel. The pressurized nitriding treatment is as follows: place the test piece in a resistance furnace, introduce nitrogen, exhaust the air in the resistance furnace, then pressurize to 0.31MPa, adjust the temperature to 550℃, keep warm for 30min, then increase the temperature by 50℃ every 20min, and increase the pressure by 0.1MPa, after 100min, keep warm and pressure for 3 hours, then cool with the furnace.

[0054] Example 2

[0055] A method for improving the instability of spring fatigue life comprises the following steps:

[0056] The raw material components are melted and poured in a 50 kg vacuum induction furnace to obtain a casting, and the pouring temperature is controlled at 1450°C. After the pouring is completed, water cooling is performed; the casting is forged to obtain a forging blank, and the initial forging temperature of the forging treatment is 1120°C, and the final forging temperature is 920°C; the forging ratio is 7.4. The difference between the initial forging temperature and the final forging temperature is fixed at 200°C, that is, the initial forging temperature minus the final forging temperature is 200°C; the forging blank is drawn, coiled, heat treated, and shot peened to obtain a finished spring; after the heat treatment, pressurized nitriding treatment is performed before shot peening; before the shot peening, high-pressure water flow is used for flushing, and the pressure of the high-pressure water flow is 0.82MPa. The drying treatment is to dry the drying room; wherein the drying temperature in the drying room is 73°C, and drying treatment is performed; 0.35mm steel balls are used for shot peening; the hardness of the steel balls is HRC55, the shot peening speed is 1500r / min, and the shot peening time is 250s. The finished spring is made of 60Si2CrVAT spring steel. The pressurized nitriding treatment is as follows: place the specimen in a resistance furnace, introduce nitrogen, exhaust the air in the resistance furnace, then pressurize to 0.33MPa, adjust the temperature to 550℃, keep warm for 30min, then increase the temperature by 50℃ every 20min, and increase the pressure by 0.1MPa, after 100min, keep warm and pressure for 4 hours, then cool with the furnace.

[0057] Example 3

[0058] A method for improving the instability of spring fatigue life comprises the following steps:

[0059] The raw material components are melted and poured in a 50 kg vacuum induction furnace to obtain a casting, and the pouring temperature is controlled at 1450°C. After the pouring is completed, water cooling is performed; the casting is forged to obtain a forging blank, and the initial forging temperature of the forging treatment is 1120°C, and the final forging temperature is 920°C; the forging ratio is 7.4. The difference between the initial forging temperature and the final forging temperature is fixed at 200°C, that is, the initial forging temperature minus the final forging temperature is 200°C; the forging blank is drawn, coiled, heat treated, and shot peened to obtain a finished spring; after the heat treatment, pressurized nitriding treatment is performed before shot peening; before the shot peening, high-pressure water flow is used for flushing, and the pressure of the high-pressure water flow is 0.84MPa. The drying treatment is to dry the drying room; wherein the drying temperature in the drying room is 73°C, and drying treatment is performed; 0.35mm steel balls are used for shot peening; the hardness of the steel balls is HRC55, the shot peening speed is 1500r / min, and the shot peening time is 250s. The finished spring is made of 60Si2CrVAT spring steel. The pressurized nitriding treatment is as follows: place the test piece in a resistance furnace, introduce nitrogen, exhaust the air in the resistance furnace, then pressurize to 0.34MPa, adjust the temperature to 550℃, keep warm for 30min, then increase the temperature by 50℃ every 20min, and increase the pressure by 0.1MPa, after 100min, keep warm and pressure for 3.5 hours, then cool with the furnace.

[0060] Example 4

[0061] A method for improving the instability of spring fatigue life comprises the following steps:

[0062] The raw material components are melted and poured in a 50 kg vacuum induction furnace to obtain a casting, and the pouring temperature is controlled at 1450°C. After the pouring is completed, water cooling is performed; the casting is forged to obtain a forging blank, and the initial forging temperature of the forging treatment is 1130°C, and the final forging temperature is 930°C; the forging ratio is 7.6. The difference between the initial forging temperature and the final forging temperature is fixed at 200°C, that is, the initial forging temperature minus the final forging temperature is 200°C; the forging blank is drawn, coiled, heat treated, and shot peened to obtain a finished spring; after the heat treatment, pressurized nitriding treatment is performed before shot peening; before the shot peening, high-pressure water flow is used for flushing, and the pressure of the high-pressure water flow is 0.85MPa. The drying treatment is to dry the drying room; wherein the drying temperature in the drying room is 75°C, and drying treatment is performed; 0.35mm steel balls are used for shot peening; the hardness of the steel balls is HRC55, the shot peening speed is 1500r / min, and the shot peening time is 250s. The finished spring is made of 60Si2CrVAT spring steel. The pressurized nitriding treatment is as follows: place the test piece in a resistance furnace, introduce nitrogen, exhaust the air in the resistance furnace, then pressurize to 0.35MPa, adjust the temperature to 550℃, keep warm for 30min, then increase the temperature by 50℃ every 20min, and increase the pressure by 0.1MPa, after 100min, keep warm and pressure for 4 hours, then cool with the furnace.

[0063] The parameters of the spring in the present invention are:

[0064] Table 1

[0065] The fatigue life of the same batch of 60Si2CrVAT springs treated by the embodiment method was tested, referring to GB / T 16947-2009, and compared:

[0066] Table 2

[0067]

[0068] It can be seen from Table 2 that the fatigue life difference of the same batch of springs treated by the method of the present invention is greatly reduced, which effectively improves the problem of poor stability of fatigue life of the same batch of springs.

[0069] Using the treatment method of Example 1, the effects of different shot peening times on the fatigue life of the spring are compared. Figure 1 ,Depend on Figure 1 It can be seen that with the increase of shot peening time, the fatigue life of the spring first increases and then decreases.

[0070] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for improving the instability of spring fatigue life, characterized in that: The following steps are involved: The raw material components are melted and poured to obtain a casting; The casting is forged to obtain a forging blank; the initial forging temperature of the forging treatment is 1100-1130°C, the final forging temperature is 910-930°C; the forging ratio is 7.2-7.6; the forging blank is drawn, coiled, heat treated, and shot peened to obtain a finished spring, the difference between the initial forging temperature and the final forging temperature is fixed at 200°C, that is, the initial forging temperature minus the final forging temperature is 200°C; After the heat treatment, a pressurized nitriding treatment is performed before shot peening. The pressurized nitriding treatment is as follows: the test piece is placed in a resistance furnace, a nitrogen-containing gas is introduced, the air in the resistance furnace is exhausted, and then the pressure is increased to 0.31-0.35MPa, the temperature is adjusted to 550°C, and the temperature is kept at 0.1MPa for 30 minutes, and then the temperature is increased by 50°C every 20 minutes, and the pressure is increased by 0.1MPa. After 100 minutes, the temperature is kept at 0.5-100°C for 3-4 hours, and then the test piece is cooled with the furnace. Before shot peening, the test piece is rinsed with a high-pressure water flow and dried. During the shot peening treatment, a steel ball with a diameter not greater than 0.5mm is used for treatment.

2. A method for improving the instability of spring fatigue life according to claim 1, characterized in that: The finished spring is made of 60Si2CrVAT spring steel material.

3. A method for improving the instability of spring fatigue life according to claim 1, characterized in that: The nitrogen-containing gas is any one of nitrogen and ammonia.

4. The method for improving the instability of spring fatigue life according to claim 1, characterized in that: The high-pressure water flow is used for flushing: the pressure of the high-pressure water flow is 0.8-0.85MPa.

5. The method for improving the instability of spring fatigue life according to claim 1, characterized in that: The drying process is to carry out drying process in a drying chamber; The drying temperature in the drying chamber is 70-75℃.

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