Heat treatment method of nodular cast iron crankshaft

Abstract

The invention discloses a heat treatment method of a nodular cast iron crankshaft and relates to the technical field of heat treatment processes. The heat treatment method specifically comprises the following steps that the nodular cast iron crankshaft is hoisted and then placed in a heating device, and temperature rise heating and heat preservation are conducted; a circulating cooling device is started in advance, cooling liquid is conveyed in a circulating mode, the spheroidal graphite cast iron crankshaft obtained after normalizing treatment is placed in the circulating cooling device containing the cooling liquid to be cooled, and then the spheroidal graphite cast iron crankshaft is taken out and subjected to standing cooling; the cooled nodular cast iron crankshaft is hoisted and then placed in a tempering device, and temperature rising, heating and heat preservation are conducted; and cooling the tempered nodular cast iron crankshaft to obtain the heat-treated nodular cast iron crankshaft. In the cooling process, open fire and oil smoke are not generated, a sample does not need to be washed after being cooled, the cooling efficiency is improved by being matched with a circulating cooling device, it is guaranteed that the use temperature of cooling liquid is within the target range, and the device is suitable for batch production.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of nodular cast iron crankshaft heat treatment method, belong to heat treatment process technical field. BACKGROUND

[0002] At present, the work of improving the mechanical properties of nodular cast iron is mainly through conventional heat treatment to adjust its matrix structure. Under this background, isothermal quenching nodular cast iron realizes high strength and good ductility due to its unique microstructure, but the common problem of isothermal quenching nodular cast iron nodular cast iron is that the production cost is high, the production cycle is long, and the economic benefit is low, and when the tensile strength exceeds 1100MPa, the elongation decreases sharply, usually not more than 5%. How to improve the strength and toughness of nodular cast iron under the condition of low cost needs to be solved. Normalizing can improve the matrix structure of nodular cast iron crankshaft, and balance the strength and toughness.

[0003] In recent years, for nodular cast iron normalizing heat treatment process, mechanical atomization nozzle or ultrasonic atomizer, water-based organic quenching liquid and quenching oil are used as cooling medium. But the common problems are: (1) water mist cooling is not uniform. (2) The nozzle and atomizing piece are often maintained and replaced to prevent the nozzle or atomizing piece from being blocked. (3) The cooling speed is too fast in the high temperature zone, the temperature difference between the surface and the core of the casting is large, and the temperature distribution is uneven, which leads to casting cracking, reduces the qualified rate of casting and causes resource waste. (4) Using oil as cooling liquid also has the risk of fire and subsequent flushing process, which is dangerous and time-consuming and labor-intensive.

[0004] At present, when using organic solvent as cooling liquid, the use temperature of organic solvent is usually required, which also affects the final heat treatment effect. If the temperature is too high, the cooling effect will be greatly reduced, and under high temperature conditions, the evaporation speed of organic solvent will increase, and the loss of cooling liquid will increase. Therefore, the present application provides a nodular cast iron crankshaft heat treatment process, which cooperates with the cooling liquid and the circulating cooling device to significantly improve the mechanical properties of the nodular cast iron crankshaft. SUMMARY

[0005] One of the purposes of the present application is to provide a nodular cast iron crankshaft heat treatment method, which specifically comprises the following steps: (1) normalizing treatment: after hoisting the nodular cast iron crankshaft, place it in the heating device (preferably high temperature furnace), heat and keep warm.

[0006] (2) cooling treatment: start circulating cooling device in advance, circulating conveying cooling liquid, place the nodular cast iron crankshaft after normalizing treatment in the circulating cooling device filled with cooling liquid, and then take it out.

[0007] (3) tempering treatment: hoisting the cooled nodular cast iron crankshaft and placing it in a tempering device (preferably a tempering furnace), heating and keeping warm.

[0008] (4) air cooling treatment: cooling the tempered nodular cast iron crankshaft to obtain the heat-treated nodular cast iron crankshaft.

[0009] Preferably, the chemical composition of the nodular cast iron crankshaft comprises, by mass percentage, 3.5% of C, 2.4% of Si, 0.5% of Mn, 0.9% of Cu, 0.32% of S, and the balance of Fe.

[0010] Preferably, the heating device in step (1) is heated to 880-900℃ and kept warm for 2-2.5h.

[0011] Preferably, in step (2), the circulating cooling device is started in advance, and the pressure is set to 45-60kgf / cm 2 , the screw rod rotation speed is set to 30-60rpm; when the temperature of the nodular cast iron crankshaft in the circulating cooling device is <200℃, the nodular cast iron crankshaft is taken out; and the cooling liquid is used at a temperature of 20-40℃.

[0012] Preferably, the circulating cooling device in step (2) comprises a reverse screw rod I1, a forward screw rod II2, a sample box 3, a residue filter net 4, a transparent PVC pipeline 5, a cooling liquid cooling box 6, a positive displacement pump 7, an inlet 8, an outlet 9, and a sample rack 10; the sample box 3 is located above the cooling liquid cooling box 6, the outlet 9 of the positive displacement pump 7 is in communication with one side of the sample box 3, the inlet 8 is in communication with one side of the cooling liquid cooling box 6; the other side of the sample box 3 is provided with the transparent PVC pipeline 5, and the transparent PVC pipeline 5 is internally provided with the residue filter net 4; the two sides of the sample box 3 are respectively provided with the reverse screw rod I1 and the forward screw rod II2; and the inside of the sample box 3 is provided with the sample rack 10.

[0013] Preferably, the coolant in step (2) is prepared by PAG polymer, polyether defoaming agent and water in a certain proportion, and the preparation method is as follows: PAG polymer and water are mixed uniformly in a volume ratio of 1:5-7 to form a dispersion liquid, then the polyether defoaming agent is added into the dispersion liquid and mixed uniformly to obtain the coolant, and the mass percentage concentration of the polyether defoaming agent in the obtained coolant is 0.15-0.25%.

[0014] Preferably, in step (3), the tempering device is heated to 500-520℃ and kept warm for 2-2.5h.

[0015] Another object of the present application is to provide a nodular cast iron crankshaft treated by the heat treatment method.

[0016] Advantages of the present application: (1) The cooling liquid of the present application, the organic solvent is easy to evaporate at high temperature, forming a gas film, which will cut off the direct contact between the metal and the cooling liquid, thereby reducing the cooling efficiency, making the casting uniform cooling, effectively reducing the surface and internal temperature difference of the casting at high temperature, avoiding the cracking of the casting. While at low temperature, the evaporation rate of the organic solvent is reduced, the gas film effect is reduced, and the liquid is in direct contact with the casting, which can effectively take away the heat.

[0017] (2) The cooling liquid of the present application, the cooling effect can reach 7-15℃ / s, the cooling capacity is good, and is suitable for the normalizing heat treatment process of nodular cast iron.

[0018] (3) In order to improve the cooling efficiency and use efficiency of the organic solvent and the stability of the final heat treatment effect of the nodular cast iron, the cooling circulating device is designed, which can ensure that the use temperature of the organic solvent is always in the target range. The positive displacement pump is designed, which can realize the flow of the cooling liquid by adjusting the pressure, promote the circulation of the cooling liquid. A pair of spiral propelling rods are designed, which can change the flow rate of the cooling liquid by adjusting the rotating speed, thereby improving the cooling capacity and promoting the circulation of the cooling liquid, and adjusting the real-time temperature of the cooling liquid. The slag filter net can effectively collect the oxide skin falling off during the cooling process of the casting, ensure the cleanliness of the cooling liquid and prolong the service life.

[0019] (4) The present application realizes the performance optimization of nodular cast iron through the synergistic effect of normalizing treatment, circulating device cooling treatment, tempering treatment and cooling liquid, forms dual-phase (pearlite + tempered martensite) nodular cast iron, and the metallographic structure and mechanical performance index of the nodular cast iron after heat treatment reach or even exceed the national standard, and can achieve the synchronous improvement of strength and toughness.

[0020] (5) The present application can be used for the heat treatment of other parts of nodular cast iron material, and has wide application scenarios. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 It is a schematic diagram of the circulating cooling device in the present application.

[0022] Figure 2 It is a metallographic graph of example 1 of the present application.

[0023] Figure 3 It is a scanning electron microscope graph of example 1 of the present application.

[0024] Figure 4 It is a metallographic graph of example 2 of the present application.

[0025] Figure 5 It is a scanning electron microscope graph of example 2 of the present application.

[0026] Figure 1In the middle: 1-Reverse screw rod I; 2-Forward screw rod II; 3-Sample box; 4-Filter screen; 5-Transparent PVC pipe; 6-Coolant cooling tank; 7-Positive displacement pump; 8-Inlet; 9-Outlet; 10-Sample placement rack; 11:Sample. Detailed Implementation

[0027] To better illustrate the purpose, technical solution, and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0028] The schematic diagrams of Embodiments 1-4 of the present invention and the circulating cooling device for batch processing ductile iron crankshafts are shown below. Figure 1 As shown, the circulating cooling device includes a reverse-rotating screw rod I1; a forward-rotating screw rod II2; a sample box 3; a filter screen 4; a transparent PVC pipe 5; a coolant cooling tank 6; a positive displacement pump 7; an inlet 8; an outlet 9; and a sample placement rack 10. The sample box 3 is located above the coolant cooling tank 6. The outlet 9 of the positive displacement pump 7 is connected to one side of the sample box 3, and the inlet 8 is connected to one side of the coolant cooling tank 6. A transparent PVC pipe 5 is installed on the other side of the sample box 3, and a filter screen 4 is installed inside the transparent PVC pipe 5. The reverse-rotating screw rod I1 and the forward-rotating screw rod II2 are respectively installed on both sides of the sample box 3. A sample placement rack 10 is set inside the sample box 3. The circulating cooling device delivers coolant from the coolant cooling tank to the sample box through the positive displacement pump. Then, the reverse-rotating screw rod and the forward-rotating screw rod are activated, and the sample is placed on the sample placement rack of the test box and cooled by the coolant. The coolant is discharged from the transparent PVC pipe with the filter screen installed inside.

[0029] The PAG polymer with a mass percentage concentration of 85-90% used in the embodiments and comparative examples of this invention was purchased from Hefei Hastai Lubricating Oil Co., Ltd., and the polyether defoamer (LJ-619) with a mass percentage concentration of 99% was purchased from Beijing Greelong Technology Co., Ltd. The chemical composition of the ductile iron crankshaft used in this invention, by mass percentage, includes: 3.5% C, 2.4% Si, 0.5% Mn, 0.9% Cu, 0.32% S, and the balance being Fe.

[0030] Example 1 A heat treatment method for ductile iron crankshaft specifically includes the following steps: (1) Normalizing treatment: After hoisting the ductile iron crankshaft, place it in a high-temperature furnace and heat it to 900℃ at a heating rate of 60℃ / h and hold it for 2h.

[0031] (2) Cooling treatment: Turn on the positive displacement pump of the circulating cooling device in advance and set the pressure to 45 kgf / cm. 2The forward and reverse screw speeds were set to 30 rpm, and the coolant was circulated. The normalized ductile iron crankshaft was placed in a circulating cooling device containing coolant (the operating temperature of the coolant was 20℃), and allowed to cool statically on a sample rack. When the temperature of the ductile iron crankshaft reached 100℃, the ductile iron crankshaft was removed. The coolant was prepared by mixing PAG polymer and water in a volume ratio of 1:5 to form a dispersion. Then, a polyether defoamer was added to the dispersion and mixed evenly to obtain the coolant. The mass percentage concentration of the polyether defoamer in the obtained coolant was 0.15%.

[0032] (3) Tempering treatment: After cooling, the ductile iron crankshaft is hoisted and placed in a tempering furnace, heated to 520°C at a heating rate of 60°C / h and held for 2 hours.

[0033] (4) Air cooling treatment: The tempered ductile iron crankshaft is cooled to room temperature on refractory bricks to obtain the heat-treated ductile iron crankshaft.

[0034] The ductile iron crankshaft treated in Example 1 was subjected to hardness, tensile strength, and elongation testing, metallographic observation, and scanning electron microscopy. The specific methods for testing hardness, tensile strength, and elongation were as follows: Hardness testing: The hardness of the heat-treated ductile iron crankshaft was tested using a Brinell hardness tester (model: HBS-3000ET), with a test pressure set at 6.25 kgf / cm². 2 The pressure holding time is 15 seconds.

[0035] Tensile strength and elongation testing: The tensile strength and elongation of the heat-treated ductile iron crankshaft were tested using a universal testing machine (model: AG-X) with a strain rate of 1 mm / min.

[0036] Tests showed that the ductile iron crankshaft of this embodiment has a hardness of 300HB, a tensile strength of 1150MPa, and an elongation of 6.3%. Figure 2 The microstructure shown is a mixture of pearlite and tempered sorbite, with each comprising approximately half. Figure 3 Scanning electron microscopy (SEM) images show that the interlamellar spacing of the pearlite is approximately 250 nm, and the granular cementite is arranged in lines or layers with slight aggregation at grain boundaries. The heat-treated ductile iron crankshaft of this embodiment exhibits superior hardness and application performance, thanks to the synergistic effect of the heat treatment process, the circulating cooling device, and the coolant.

[0037] Example 2 A heat treatment method for ductile iron crankshaft specifically includes the following steps: (1) Normalizing treatment: After hoisting the ductile iron crankshaft, place it in a high-temperature furnace and heat it to 880℃ at a heating rate of 60℃ / h and hold it for 2.25h.

[0038] (2) Cooling treatment: Turn on the positive displacement pump of the circulating cooling device in advance and set the pressure to 50 kgf / cm. 2 The forward and reverse screw speeds were set to 50 rpm, and the coolant was circulated. The normalized ductile iron crankshaft was placed in a circulating cooling device containing coolant (the operating temperature of the coolant was 40℃), and allowed to cool statically on a sample rack. When the temperature of the ductile iron crankshaft reached 100℃, the ductile iron crankshaft was removed. The coolant was prepared by mixing PAG polymer and water in a volume ratio of 1:7 to form a dispersion. Then, a polyether defoamer was added to the dispersion and mixed evenly to obtain the coolant. The mass percentage concentration of the polyether defoamer in the obtained coolant was 0.25%.

[0039] (3) Tempering treatment: After cooling, the ductile iron crankshaft is hoisted and placed in a tempering furnace, heated to 500℃ at a heating rate of 60℃ / h and held for 2.25h.

[0040] (4) Air cooling treatment: The tempered ductile iron crankshaft is cooled to room temperature on refractory bricks to obtain the heat-treated ductile iron crankshaft.

[0041] The ductile iron crankshaft treated in Example 2 was subjected to hardness, tensile strength, elongation testing, metallographic observation, and scanning electron microscopy observation in the same manner as in Example 1. The tests showed that the hardness of the ductile iron crankshaft in this example was 320 HB, the tensile strength was 1136 MPa, and the elongation was 6.7%. Figure 4 The microstructure shown is a mixture of pearlite and tempered sorbite, with a volume ratio of tempered sorbite to pearlite of approximately 3:2. Figure 5 Scanning electron microscopy (SEM) images show that the interlamellar spacing of pearlite is approximately 250 nm, and the granular cementite is arranged in lines or layers with slight aggregation at grain boundaries. The heat-treated ductile iron crankshaft of this embodiment exhibits superior hardness and application performance, thanks to the synergistic effect of the heat treatment process, the circulating cooling device, and the coolant.

[0042] Example 3 A heat treatment method for ductile iron crankshaft specifically includes the following steps: (1) Normalizing treatment: After hoisting the ductile iron crankshaft, place it in a high-temperature furnace and heat it to 890℃ at a heating rate of 60℃ / h and hold it for 2.5h.

[0043] (2) Cooling treatment: The positive displacement pump of the circulating cooling device was started in advance, the pressure was set to 60 kgf / cm 2 , the rotating speed of the positive screw and the reverse screw was set to 60 rpm, the cooling liquid was circulated, the normalized nodular cast iron crankshaft was placed in the circulating cooling device containing the cooling liquid (the use temperature of the cooling liquid was 30℃), and the normalized nodular cast iron crankshaft was cooled on the sample rack, when the temperature of the nodular cast iron crankshaft was 100℃, the nodular cast iron crankshaft was taken out, wherein the preparation method of the cooling liquid was as follows: PAG polymer and water were mixed uniformly according to the volume ratio of 1:6 to form a dispersion liquid, then the polyether defoaming agent was added into the dispersion liquid and mixed uniformly to obtain the coolant, and the mass percentage concentration of the polyether defoaming agent in the obtained coolant was 0.2%.

[0044] (3) Tempering treatment: The normalized nodular cast iron crankshaft was hoisted and placed in the tempering furnace, heated to 510℃ at a heating rate of 60℃ / h and kept for 2.5h.

[0045] (4) Air cooling treatment: The normalized nodular cast iron crankshaft was cooled to room temperature on the refractory brick to obtain the heat-treated nodular cast iron crankshaft.

[0046] The hardness, tensile strength and elongation of the nodular cast iron crankshaft treated in Example 3 were detected in the same way as in Example 1, and the hardness, tensile strength and elongation of the nodular cast iron crankshaft in this example were all excellent. The heat-treated nodular cast iron crankshaft in this example has excellent hardness and application performance, which is due to the synergistic effect of the heat treatment process, the circulating cooling device and the cooling liquid.

[0047] Example 4 A heat treatment method of a nodular cast iron crankshaft, specifically comprising the following steps: (1) Normalizing treatment: The nodular cast iron crankshaft was hoisted and placed in the high-temperature furnace, heated to 880℃ at a heating rate of 60℃ / h and kept for 2h.

[0048] (2) Cooling treatment: The positive displacement pump of the circulating cooling device was started in advance, the pressure was set to 60 kgf / cm 2, the rotating speed of the forward screw and the reverse screw is set to 60 rpm, the cooling liquid is circulated, the spheroidal graphite cast iron crankshaft after the normalizing treatment is placed in the circulating cooling device filled with the cooling liquid (the service temperature of the cooling liquid is 30℃), and the spheroidal graphite cast iron crankshaft is cooled on the sample placing rack, and when the temperature of the spheroidal graphite cast iron crankshaft is 100℃, the spheroidal graphite cast iron crankshaft is taken out, wherein the preparation method of the cooling liquid is as follows: the PAG polymer and water are uniformly mixed according to the volume ratio of 1:6 to form a dispersion liquid, and then the polyether antifoaming agent is added into the dispersion liquid and uniformly mixed to obtain the coolant, and the mass percentage concentration of the polyether antifoaming agent in the obtained coolant is 0.2%.

[0049] (3) tempering treatment: the spheroidal graphite cast iron crankshaft after the cooling treatment is hung and placed in a tempering furnace, heated to 500℃ at a heating rate of 60℃ / h and kept for 2h.

[0050] (4) air cooling treatment: the spheroidal graphite cast iron crankshaft after the tempering treatment is cooled to room temperature on the refractory bricks to obtain the spheroidal graphite cast iron crankshaft after the heat treatment.

[0051] The spheroidal graphite cast iron crankshaft after the treatment of Example 4 is detected for hardness, tensile strength and elongation in the same manner as Example 1, and it is found through the test that the hardness, tensile strength and elongation of the spheroidal graphite cast iron crankshaft of the present example are all excellent. The spheroidal graphite cast iron crankshaft after the heat treatment of the present example has excellent hardness and application performance, which is due to the synergistic effect of the heat treatment process, the circulating cooling device and the cooling liquid.

[0052] Comparative Example 1 A heat treatment method of a spheroidal graphite cast iron crankshaft, specifically comprising the following steps: (1) normalizing treatment: the spheroidal graphite cast iron crankshaft is hung and placed in a high-temperature furnace, heated to 880℃ at a heating rate of 60℃ / h and kept for 2h.

[0053] (2) cooling treatment: the spheroidal graphite cast iron crankshaft after the normalizing treatment is cooled by water mist, and the specific method is as follows: a mechanical atomizing nozzle is used, a booster pump pressure of about 45kPa is set, the nozzle is arranged around the spheroidal graphite cast iron crankshaft with an interval of about 10cm, and the spheroidal graphite cast iron crankshaft after the normalizing treatment is cooled, and when the temperature of the spheroidal graphite cast iron crankshaft is 100℃, the spheroidal graphite cast iron crankshaft is taken out.

[0054] (3) tempering treatment: the spheroidal graphite cast iron crankshaft after the cooling treatment is hung and placed in a tempering furnace, heated to 500℃ at a heating rate of 60℃ / h and kept for 2h.

[0055] (4) air cooling treatment: the spheroidal graphite cast iron crankshaft after the tempering treatment is cooled to room temperature on the refractory bricks to obtain the spheroidal graphite cast iron crankshaft after the heat treatment.

[0056] The hardness, tensile strength, elongation, and metallographic observation of the nodular cast iron crankshaft after the treatment of Comparative Example 1 were detected in the same manner as in Example 1. The hardness of the nodular cast iron crankshaft was 280 HB, the tensile strength was 962 MPa, and the elongation was 6.3%. The metallographic observation showed that the nodular cast iron crankshaft was entirely pearlite. This is because the nodular cast iron crankshaft was not subjected to the cooling treatment of the circulating cooling device and the cooling liquid, resulting in a large temperature difference between the surface and the center of the nodular cast iron crankshaft casting, thereby causing cracking of the casting and poor application performance.

[0057] Batch heat treatment of nodular cast iron crankshaft A batch heat treatment method of a nodular cast iron crankshaft, specifically comprising the following steps: (1) Normalizing treatment: After hoisting, the nodular cast iron crankshaft was placed in a high-temperature furnace and heated to 880℃ at a heating rate of 60℃ / h and maintained for 2h.

[0058] (2) Cooling treatment: The positive displacement pump of the circulating cooling device was started in advance, the pressure was set to 60kgf / cm 2 , the screw rod rotation speed was set to 60rpm, and the cooling liquid was circulated. The nodular cast iron crankshaft after normalizing treatment was placed in the circulating cooling device containing the cooling liquid, and was cooled on the sample placing rack. When the temperature of the nodular cast iron crankshaft was 100℃, the nodular cast iron crankshaft was taken out. The cooling liquid was prepared by mixing PAG polymer and water in a volume ratio of 1:6 to form a dispersion liquid, and then adding a polyether defoaming agent to the dispersion liquid to obtain a coolant. The mass percentage concentration of the polyether defoaming agent in the obtained coolant was 0.2%.

[0059] (3) Tempering treatment: After the cooling treatment, the nodular cast iron crankshaft was hoisted and placed in a tempering furnace and heated to 500℃ at a heating rate of 60℃ / h and maintained for 2h.

[0060] After 2min of cooling of the first sample, the temperature of the cooling liquid in the sample box was measured with a thermometer, and the temperature was 26℃. The second sample was placed on the sample placing rack in the circulating cooling device containing the cooling liquid for cooling. When the temperature of the nodular cast iron crankshaft was 100℃, the nodular cast iron crankshaft was taken out. After cooling, the tempering step was performed again.

[0061] After 2min of cooling of the second sample, the temperature of the cooling liquid in the sample box was measured with a thermometer, and the temperature was 28℃. The third sample was placed on the sample placing rack in the circulating cooling device containing the cooling liquid for cooling. When the temperature of the nodular cast iron crankshaft was 100℃, the nodular cast iron crankshaft was taken out. After cooling, the tempering step and the air cooling step were performed again.

[0062] The fourth sample was cooled for 2 minutes, and the temperature of the cooling liquid in the sample box was measured by a thermometer, and the temperature was 30℃. The fifth sample was placed on the sample rack in the circulating cooling device filled with the cooling liquid, and cooled. When the temperature of the ductile iron crankshaft was 100℃, the ductile iron crankshaft was taken out. After cooling, the tempering step and the air cooling step were performed again.

[0063] The fourth sample was cooled for 2 minutes, and the temperature of the cooling liquid in the sample box was measured by a thermometer, and the temperature was 30℃. The fifth sample was placed on the sample rack in the circulating cooling device filled with the cooling liquid, and cooled. When the temperature of the ductile iron crankshaft was 100℃, the ductile iron crankshaft was taken out. After cooling, the tempering step and the air cooling step were performed again.

[0064] The air cooling step: the sample was placed on the refractory brick and cooled to room temperature.

[0065] Finally, product inspection was performed, and the ductile iron crankshaft after heat treatment in this batch was tested for performance in the same manner as in Example 1. The test results showed that the hardness of the ductile iron crankshaft after batch heat treatment by the heat treatment method of the application was 300-310HB, the tensile strength was 1100-1125MPa, the elongation was 6.5-6.7%, and the microstructure was a mixed structure of pearlite and tempered sorbite. The specific performance parameters tested are shown in Table 1.

[0066] Table 1 From Table 1, it can be seen that through the synergistic effect of normalizing treatment, circulating device cooling treatment, tempering treatment and cooling liquid, the microstructure and mechanical properties of the ductile iron crankshaft obtained by batch heat treatment are very stable. In particular, the combined use of the cooling liquid and the circulating cooling device in the application makes the cooling speed of the sample moderate during batch heat treatment, the initial temperature of the cooling liquid moderate and maintained within a certain temperature range, and the cooling effect of different samples consistent. In the continuous cooling process of the ductile iron austenite, a mixed structure of pearlite and tempered sorbite can be stably obtained. In summary, the heat treatment method of the application can be used for batch treatment of the ductile iron crankshaft.

[0067] The above has described various embodiments of the application, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes are obvious to those skilled in the art without departing from the scope and spirit of the described embodiments. The selection of terms used herein is intended to best explain the principles, practical applications or improvements to the technology in the market of the embodiments, or to enable other ordinary skilled persons in the art to understand the embodiments disclosed herein.

Claims

1. A heat treatment method of a spheroidal graphite cast iron crankshaft, characterized by, The heat treatment method specifically comprises the following steps: (1) normalizing treatment: hoisting the nodular cast iron crankshaft and placing it in a heating device, heating and keeping warm; (2) cooling treatment: starting the circulating cooling device in advance, circulating and conveying the cooling liquid, placing the nodular cast iron crankshaft after the normalizing treatment in the circulating cooling device containing the cooling liquid, and then taking it out; (3) tempering treatment: hoisting the nodular cast iron crankshaft after the cooling treatment and placing it in a tempering device, heating and keeping warm; (4) air cooling treatment: cooling the nodular cast iron crankshaft after the tempering treatment to obtain the nodular cast iron crankshaft after the heat treatment.

2. The heat treatment method of the nodular cast iron crankshaft according to claim 1, characterized by, The chemical composition of the nodular cast iron crankshaft comprises, by mass percentage, 3.5% of C, 2.4% of Si, 0.5% of Mn, 0.9% of Cu, 0.32% of S, and the balance of Fe.

3. The heat treatment method of the nodular cast iron crankshaft according to claim 1, characterized by, In the step (1), the heating device is heated to 880-900℃ and kept warm for 2-2.5h.

4. The heat treatment method of the nodular cast iron crankshaft according to claim 1, characterized by, The step (2) is to start the circulating cooling device in advance and set the pressure to 45-60 kgf / cm 2 , the screw rod rotation speed is set to 30-60 rpm; when the temperature of the nodular cast iron crankshaft in the circulating cooling device is <200℃, the nodular cast iron crankshaft is taken out from the circulating cooling device; the use temperature of the cooling liquid is 20~40℃.

5. The heat treatment method of the nodular cast iron crankshaft according to claim 1, characterized by, The circulating cooling device in the step (2) comprises a reverse screw rod I (1), a forward screw rod II (2), a sample box (3), a residue filter net (4), a transparent PVC pipeline (5), a cooling liquid cooling box (6), a positive displacement pump (7), a liquid inlet (8), a liquid outlet (9), and a sample placing rack (10). The sample box (3) is located above the cooling liquid cooling box (6), the liquid outlet (9) of the positive displacement pump (7) is communicated with one side of the sample box (3), the liquid inlet (8) is communicated with one side of the cooling liquid cooling box (6), the other side of the sample box (3) is installed with the transparent PVC pipeline (5), the transparent PVC pipeline (5) is internally installed with the residue filter net (4), the reverse screw rod I (1) and the forward screw rod II (2) are respectively installed on the two sides of the sample box (3), and the sample placing rack (10) is arranged in the sample box (3).

6. The heat treatment method of the nodular cast iron crankshaft according to claim 1, characterized by, The cooling agent in the step (2) is prepared by PAG polymer, polyether defoaming agent and water in a certain proportion, and the preparation method is as follows: the PAG polymer and water are mixed uniformly in a proportion of 1:5-7 in volume ratio to form a dispersion liquid, then the polyether defoaming agent is added into the dispersion liquid to mix uniformly to obtain the cooling agent, and the mass percentage concentration of the polyether defoaming agent in the obtained cooling agent is 0.15-0.25%.

7. The heat treatment method of the nodular cast iron crankshaft according to claim 1, characterized by, In the step (3), the tempering device is heated to 500-520℃ and kept warm for 2-2.5h.

8. The nodular cast iron crankshaft obtained by the heat treatment method in any one of claims 1-7.

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