A preparation method of a new type of austempered ductile iron with both high strength, toughness and high wear resistance

Abstract

The present invention discloses a preparation method of a new type of austempered ductile iron with both high strength, toughness and high wear resistance, belonging to the technical field of the preparation of ductile iron, and comprising the following steps: (1) preparation of a nano-ceramic particle reinforcing agent; (2) preparation of nano-ceramic particle-reinforced ductile iron; (3) preparation of the new type of austempered ductile iron; The preparation method of the present invention has the characteristics of simple operation, no need to change the existing casting and heat treatment process flow and equipment of cast iron, low cost, environmental protection, etc.; through the gradual dispersion of nano-scale ceramic particles into the molten iron as the outer nickel foil and aluminum foil of the reinforcing agent melt, and then through hierarchical austempering treatment, the synergistic regulation of the nano-particles and heat treatment on the strength, toughness and wear resistance of ductile iron is realized, so that the tensile strength, elongation, impact toughness and wear resistance of the ductile iron are all improved.

Description

Technical Field

[0001] The present invention relates to the field of preparation of new high-performance steel materials, and particularly to a method for preparing a new austempered ductile iron with both high strength and toughness and high wear resistance. Background Art

[0002] With the rapid development of the R & D technology of high-power density diesel engines in China, higher requirements are put forward for the materials of their key castings such as cylinder blocks and cylinder heads. Among many metal materials, ductile iron combines the excellent comprehensive properties of gray iron and nodular iron, making it an ideal material for producing the above components. However, traditional ductile iron can no longer meet the urgent needs of designers and customers for high-performance materials. And conventional means for improving the properties of steel materials, including alloy strengthening, heat treatment strengthening, and precipitation strengthening, have all encountered bottlenecks and it is difficult to make greater breakthroughs in a short time. Therefore, we urgently need to propose a brand-new, flexible and practical, low-cost method that can significantly improve the properties of ductile iron. The successful research and development of new high-performance ductile iron can not only greatly improve the working life and reliability of heavy-duty mechanical internal combustion engines, but also meet the requirements of thin-walling and lightweight of important components, achieving the goals of high efficiency, energy saving, and emission reduction.

[0003] After more than 40 years of development, China has accumulated certain theoretical and practical experience in the research of ductile iron, and in some aspects has caught up with the developed countries. However, there are still many production technology problems, especially the optimization technology of the structure and properties of ductile iron is difficult to be actually applied in factories. In recent years, automobile manufacturers in many industrialized countries such as Audi, Volkswagen, and General Motors have begun to massively research and develop high-performance ductile iron and apply it to the production of key components such as diesel engine cylinder blocks, cylinder heads, piston rings, and exhaust pipes. In order to keep up with the world and continuously promote the relevant research work of ductile iron, academic organizations such as the China Foundry Association, the China Materials Conference, and the China Rare Earth Society have held seminars many times to guide more scientific researchers to pay attention to and track the development of ductile iron. China also officially released and implemented national standards such as "Ductile Iron Castings" and "Metallographic Standard of Ductile Iron" in March 2012 to further promote the development of relevant research on ductile iron.

[0004] How to improve the properties of ductile iron such as tensile strength, elongation, impact toughness, and wear resistance is the key point generally concerned by the industry. Summary of the Invention

[0005] In view of the above problems, the present invention provides a method for preparing a new austempered ductile iron with both high strength and toughness and high wear resistance. The new austempered ductile iron prepared by this method has been improved to a certain extent in terms of tensile strength, elongation, impact toughness, and wear resistance.

[0006] The technical solution adopted by the present invention is as follows:

[0007] A preparation method of a new type of austempered ductile iron with both high strength and toughness and high wear resistance, characterized by including the following steps:

[0008] (1) Preparation of nano-ceramic particle reinforcing agent:

[0009] 1a. Weigh a certain amount of TiC nano-ceramic particles;

[0010] 1b. Put the TiC nano-ceramic particles into a high-energy ball mill for ball milling activation;

[0011] 1c. Take out the TiC nano-ceramic particles from the ball mill, first wrap the nano-ceramic particles with aluminum foil, and then wrap them with nickel foil;

[0012] 1d. Use a small hydraulic press to cold-press the wrapped nano-ceramic particles into blocks;

[0013] (2) Preparation of nano-ceramic particle-reinforced ductile iron:

[0014] 2a. Use an intermediate frequency induction furnace to melt the cast iron raw materials;

[0015] 2b. Preheat the ladle, then add the vermiculizing agent and the inoculant into the ladle pit in sequence, and finally spread half of the total amount of the nano-ceramic particle blocks on the inoculant;

[0016] 2c. Pour the molten iron into the ladle, and add the other half of the nano-ceramic particle blocks into the molten iron along with the flow from above the ladle during the pouring process. After the molten iron level in the ladle is calm, pour the molten iron into a pre-prepared sand mold, and cool to obtain a casting sample of nano-ceramic particle-reinforced ductile iron;

[0017] (3) Preparation of the new type of austempered ductile iron:

[0018] 3a. Heat the ductile iron containing nano-ceramic particles prepared in step (2) to 900 - 940 °C for austenitizing treatment, and the holding time is 60 - 80 min;

[0019] 3b. Transfer the fully austenitized ductile iron into a low-temperature salt bath furnace for the first-stage austempering heat treatment, the austempering temperature is 220 - 250 °C, and the holding time is 3 - 5 min;

[0020] 3c. Then transfer the ductile iron into a high-temperature salt bath furnace for the second-stage austempering heat treatment, the austempering temperature is 420 - 450 °C, and the holding time is 80 - 100 min, and then water-cool to room temperature;

[0021] 3d. Then, the austempered ductile iron is put into a box furnace for tempering heat treatment. The tempering temperature is 200 - 240°C, and the tempering time is 30 - 45 min. Finally, it is air-cooled to room temperature to obtain the new type of austempered ductile iron.

[0022] The further setting is as follows:

[0023] In step (1):

[0024] The average particle size of the TiC nano-ceramic particles is preferably 50 nm.

[0025] The TiC nano-ceramic particles are put into a high-energy ball mill and ball-milled and activated at a speed of 30 rpm for 10 hr. The reverse and forward conversion time of the ball mill is 5 min.

[0026] The TiC nano-ceramic particles are taken out from the ball mill. First, the nano-ceramic particles are coated with an aluminum foil with a thickness of 50 μm, and then coated with a nickel foil with a thickness of 30 μm. The purity of the aluminum foil and the nickel foil is preferably 99.999%.

[0027] The coated nano-ceramic particles are cold-pressed into blocks using a small hydraulic press. The pressure is 50 KN, the diameter of the pressed block is 30 mm, and the height is 15 mm.

[0028] In step (2):

[0029] The cast iron raw materials are melted using an intermediate frequency induction furnace. The melting temperature is 1550°C, and the cast iron raw materials are pig iron, ferromanganese, and scrap steel.

[0030] The vermiculizing agent is a rare earth magnesium alloy, and the addition amount is 0.5 - 0.8 wt%. The rare earth magnesium alloy with a particle size of 8 - 15 mm is preferably used; the inoculant is 75 ferrosilicon, and the addition amount is 0.3 - 0.5 wt%. The 75 ferrosilicon with a particle size of 2 - 5 mm is preferably used.

[0031] In step (3):

[0032] The ductile iron containing nano-ceramic particles prepared in step (2) is heated to 920°C for austenitizing treatment, and the holding time is 70 min.

[0033] The fully austenitized ductile iron is transferred into a low-temperature salt bath furnace within 30 sec for the first-stage austempering heat treatment. The isothermal temperature is 220°C, and the holding time is 3 min.

[0034] The salt bath medium of the low-temperature salt bath furnace is preferably 50% - 55% KNO3 and 45% - 50% NaNO2 (by weight).

[0035] Transfer the vermicular graphite cast iron after the isothermal quenching heat treatment in the first stage into a high-temperature salt bath furnace for the isothermal quenching heat treatment in the second stage. The isothermal temperature is 440 °C, and the holding time is 90 min. Then, cool it to room temperature by water quenching.

[0036] The salt bath medium of the high-temperature salt bath furnace is preferably 5%-10% NaNO2 and 90%-95% NaNO3 (by weight).

[0037] Put the vermicular graphite cast iron after the isothermal quenching in the second stage into a box furnace for tempering heat treatment. The tempering temperature is 220 °C, and the tempering time is 30 min; finally, air-cool it to room temperature to obtain the new type of isothermal quenched vermicular graphite cast iron.

[0038] The technical effects of the present invention are as follows:

[0039] 1. The present invention combines the technology of regulating the structure and properties by nano-ceramic particles with the step-by-step isothermal quenching heat treatment technology. Using nano-scale ceramic particles as the reinforcing agent for vermicular graphite cast iron, during the casting process, as the outer nickel foil and aluminum foil melt, the nano-ceramic particles gradually disperse into the molten iron to prepare vermicular graphite cast iron containing nano-ceramic particles, and then through step-by-step isothermal quenching heat treatment, finally obtain the strengthened new type of vermicular graphite cast iron. Due to the addition of nano-ceramic particles, the nano-particles can serve as heterogeneous nucleation cores to refine the microstructure of vermicular graphite cast iron during the liquid-solid phase transformation process. The refined solidification structure and as-cast structure of vermicular graphite cast iron can further affect the solid-state phase transformation during the subsequent step-by-step isothermal quenching heat treatment to form a fine heat treatment structure. The refinement of the microstructure can synergistically improve the mechanical properties of vermicular graphite cast iron. The present invention combines the addition of nano-ceramic particles and step-by-step isothermal quenching heat treatment, so that the product is improved to a certain extent in terms of tensile strength, elongation, impact toughness and wear resistance.

[0040] 2. The successful development of the new type of isothermal quenched vermicular graphite cast iron in the present invention provides new ideas and technical means for the high-performance and lightweight of key components such as diesel engine cylinder blocks, cylinder heads, brake discs, and piston rings, and has important practical application value. Specific Embodiments

[0041] In order to make the technical means, creative features, achieved purposes and effects of the present invention easy to understand, the present invention will be further described below in conjunction with specific embodiments.

[0042] Example 1:

[0043] A preparation method of a new type of isothermal quenched vermicular graphite cast iron with both high strength, toughness and high wear resistance, including the following steps:

[0044] (1). Preparation of nano-ceramic particle reinforcing agent:

[0045] 1a. Weigh 600 g of TiC nano-ceramic particles with an average particle size of about 50 nm;

[0046] 1b. Put the TiC nano-ceramic particles into a high-energy ball mill and ball-mill and activate them at a speed of 30 rpm for 10 hr. The forward and reverse conversion time of the ball mill is 5 min;

[0047] 1c. Take out the TiC nano-ceramic particles from the ball mill. First, coat the nano-ceramic particles with an aluminum foil with a thickness of 50 μm, and then coat them with a nickel foil with a thickness of 30 μm;

[0048] 1d. Use a small hydraulic press to cold-press the coated nano-ceramic particles into blocks. The pressure is 50 KN, the diameter of the pressed block is 30 mm, and the height is 15 mm.

[0049] (2) Preparation of nano-ceramic particle-reinforced vermicular graphite cast iron:

[0050] 2a. Use an intermediate frequency induction furnace to melt the cast iron raw materials. The melting temperature is 1550 °C. The cast iron raw materials are pig iron, ferromanganese and scrap steel. The selection and ratio of the cast iron raw materials are determined according to the product grade. In this embodiment, RuT450 vermicular graphite cast iron is prepared. If vermicular graphite cast iron of other grades needs to be manufactured, the cast iron raw materials can be selected according to the corresponding grades.

[0051] 2b. Preheat a 500 kg ladle, then add the vermiculizing agent and the inoculant into the ladle pit in sequence, and then spread 300 g of the nano-ceramic particle pressed blocks on the inoculant. The vermiculizing agent is rare earth magnesium alloy (Si 41.30%, Mg 8.27%, Ce 6.38%, Ca 1.85%, Fe 42.2%). The addition amount of the rare earth magnesium alloy is 2400 g, and the inoculant is 75 ferrosilicon. The addition amount of 75 ferrosilicon is 1500 g.

[0052] 2c. Pour the molten iron into the ladle. During the pouring process, add the remaining 300 g of the nano-ceramic particle pressed blocks into the molten iron from above the ladle along with the flow. After the molten iron level in the ladle is calm, pour the molten iron into a pre-prepared sand mold. After cooling, a casting sample of nano-ceramic particle-reinforced vermicular graphite cast iron is obtained.

[0053] In this embodiment, the molten iron pouring temperature is 1450 °C, the molten iron output is 300 kg, and the total addition amount of the nano-ceramic particles is 0.2 wt% of the molten iron.

[0054] (3) Preparation of new austempered vermicular graphite cast iron:

[0055] 3a. Heat the nano-ceramic particle-reinforced vermicular graphite cast iron prepared in step (2) to 920 °C for austenitizing treatment, and the holding time is 70 min;

[0056] 3b. Transfer the fully austenitized vermicular graphite cast iron into a low-temperature salt bath furnace within 30 seconds for the first-stage isothermal quenching heat treatment. The isothermal temperature is 220 °C and the holding time is 3 minutes. The salt bath medium of the low-temperature salt bath furnace is 50% KNO3 and 50% NaNO2.

[0057] 3c. Then transfer the vermicular graphite cast iron into a high-temperature salt bath furnace for the second-stage isothermal quenching heat treatment. The isothermal temperature is 440 °C and the holding time is 90 minutes, and then quench it to room temperature. The salt bath medium of the high-temperature salt bath furnace is 5% NaNO2 and 95% NaNO3.

[0058] 3d. Put the isothermally quenched vermicular graphite cast iron into a box furnace for tempering heat treatment. The tempering temperature is 220 °C and the tempering time is 30 minutes. Finally, air cool it to room temperature to obtain a new type of isothermally quenched vermicular graphite cast iron (marked as sample 1).

[0059] Comparative Example 1:

[0060] The preparation method is the same as that of Example 1, except that: in step (2), nano-ceramic particles are not added, and the step (3) of stepwise isothermal quenching heat treatment is not carried out, and the obtained is the casting sample treated in step 2c.

[0061] Comparative Example 2:

[0062] The preparation method is the same as that of Example 1, except that: nano-ceramic particles are added in step (2), but the step (3) of stepwise isothermal quenching heat treatment is not carried out, and the obtained is the casting sample treated in step 2c.

[0063] Comparative Example 3:

[0064] The preparation method is the same as that of Example 1, except that: nano-ceramic particles are not added in step (2), and then the step (3) of stepwise isothermal quenching heat treatment is carried out.

[0065] Performance test:

[0066] Carry out mechanical property and tribological property tests on the new type of isothermally quenched vermicular graphite cast iron (sample 1) prepared in Example 1 and the vermicular graphite cast iron products prepared in Comparative Examples 1-3 respectively, where:

[0067] The tensile property is tested according to the test standard of GB / T228.1-2010;

[0068] The impact property is tested according to the test standard of GB / T229-2007;

[0069] The tribological performance test was carried out on a multi-functional friction and wear testing machine, using a ball-disk rotational sliding wear test module. The upper sample was a Gcr15 bearing steel ball with a diameter of 8 mm, a hardness of 60 HRC, and a surface roughness of 50 ± 10 nm. The lower sample was vermicular graphite cast iron with dimensions of 20 mm (length) × 20 mm (width) × 6 mm (height), a surface roughness of 300 ± 50 nm, an external load of 200 N, a movement speed of 1 m / s, a test time of 15 min, and a test temperature of 20 °C.

[0070] The test results are shown in Table 1:

[0071] Table 1. Mechanical properties and wear resistance of vermicular graphite cast iron under different process conditions

[0072] Name UTS (MPa) <![CDATA[ε f (%)]]> <![CDATA[a k (J / cm 2 )]]> <![CDATA[WVL(mm 3 )]]> Example 1 984 4.4 48.2 1.9 Comparative Example 1 501 6.2 57.1 9.7 Comparative Example 2 558 6.3 60.5 7.2 Comparative Example 3 725 4.2 45.0 4.7

[0073] In the table: UTS is the tensile strength, ε f is the elongation, a k is the impact toughness, and WVL is the wear volume

[0074] Analysis: As shown in Table 1:

[0075] Combined with Comparative Example 2 and Comparative Example 1, the addition of nano-ceramic particles can significantly improve the wear resistance of vermicular graphite cast iron, and the mechanical properties of vermicular graphite cast iron also increase, but not significantly.

[0076] Combined with Comparative Example 3 and Comparative Example 1, stepwise isothermal quenching heat treatment can significantly improve the tensile strength and wear resistance of vermicular graphite cast iron, but the elongation and impact toughness decrease.

[0077] By comparing Example 1 with Comparative Examples 1-3, it can be seen that the new type of isothermally quenched vermicular graphite cast iron prepared by "adding nano-ceramic particles + stepwise isothermal quenching heat treatment" has greatly improved both mechanical properties and wear resistance. It is speculated that the reason is that nano-particles can act as heterogeneous nucleation cores to refine the microstructure of vermicular graphite cast iron during the liquid-solid phase transformation process. The refined solidification structure and as-cast structure of vermicular graphite cast iron can further affect the solid-state phase transformation during the subsequent stepwise isothermal quenching heat treatment, forming a fine heat treatment structure. The refinement of the microstructure can synergistically improve the mechanical properties of vermicular graphite cast iron. In addition, the nano-particles present in the structure can further improve the strength and toughness of vermicular graphite cast iron by pinning grain boundaries, thermal misfit, crack scattering, etc. In terms of tribological performance, the improvement in wear resistance is mainly attributed to the increase in the strength of vermicular graphite cast iron and the fact that the nano-particles present in the structure can separate the contact surface during the relative movement of the friction pair, reducing adhesive wear. Through comparative examples, it is proved that using "nano-ceramic particles + stepwise isothermal quenching heat treatment" has a significant effect improvement compared with using only "nano-ceramic particles" or only "stepwise isothermal quenching heat treatment" alone, indicating that the two play a synergistic role.

[0078] Example 2:

[0079] The preparation method is the same as that of Example 1, except that in step (2), the total addition amount of nano-ceramic particles is 0.1 wt% of the molten iron.

[0080] Example 3:

[0081] The preparation method is the same as that of Example 1, except that in step (2), the total addition amount of nano-ceramic particles is 0.3 wt% of the molten iron.

[0082] Example 4:

[0083] The preparation method is the same as that of Example 1, except that in step (2), the total addition amount of nano-ceramic particles is 0.4 wt% of the molten iron.

[0084] Performance test:

[0085] The new austempered ductile iron prepared in Example 1 (Sample 1) and the new austempered ductile iron products prepared in Examples 2-4 were respectively subjected to mechanical property and tribological property tests. Among them: the tensile property was tested according to the test standard of GB / T228.1-2010; the impact property was tested according to the test standard of GB / T229-2007; the tribological property test was carried out on a multi-functional friction and wear testing machine, using a ball-disk rotary sliding wear test module. The upper sample was a Gcr15 bearing steel ball with a diameter of 8 mm, a hardness of 60 HRC, and a surface roughness of 50±10 nm. The lower sample was ductile iron with dimensions of 20 mm (length) × 20 mm (width) × 6 mm (height), a surface roughness of 300±50 nm, an external load of 200 N, a movement speed of 1 m / s, a test time of 15 min, and a test temperature of 20°C.

[0086] The test results are shown in Table 2:

[0087] Table 2. Mechanical properties and wear resistance of austempered ductile iron prepared with different nano-ceramic particle contents

[0088] Name UTS (MPa) <![CDATA[ε f (%)]]> <![CDATA[a k (J / cm 2 )]]> <![CDATA[WVL(mm 3 )]]> Example 1 984 4.4 48.2 1.9 Example 2 948 4.7 50.4 2.4 Example 3 980 3.9 45.1 1.9 Example 4 964 3.2 41.8 2.2

[0089] In the table: UTS is the tensile strength, ε f is the elongation, a k is the impact toughness, and WVL is the wear volume

[0090] Analysis:

[0091] As shown in Table 2, the content of nano-ceramic particles has a great influence on the mechanical properties and wear resistance of the austempered ductile iron prepared. With the increase of the content of nano-ceramic particles, the tensile strength and wear resistance first increase and then decrease, the elongation and impact toughness gradually decrease. The comprehensive best embodiment is that the addition amount of nano-ceramic particles is 0.2wt% of the molten iron.

[0092] Example 5:

[0093] The preparation method is the same as that of Example 1, except that in step (3), the process conditions of the step-by-step austempering heat treatment are as follows:

[0094] 3a. Heat the ductile iron containing nano-ceramic particles prepared in step (2) to 920°C for austenitizing treatment, and the holding time is 70 min;

[0095] 3b. Transfer the fully austenitized ductile iron into a low-temperature salt bath furnace within 30 s for the first-stage austempering heat treatment, the austempering temperature is 240°C, and the holding time is 5 min;

[0096] 3c. Then transfer the ductile iron into a high-temperature salt bath furnace for the second-stage austempering heat treatment, the austempering temperature is 440°C, and the holding time is 90 min, and then quench to room temperature;

[0097] 3d. Put the austempered ductile iron into a box furnace for tempering heat treatment, the tempering temperature is 220°C, and the tempering time is 30 min; finally air-cool to room temperature to obtain a new type of austempered ductile iron sample.

[0098] Example 6:

[0099] The preparation method is the same as that of Example 1, except that in step (3), the process conditions of the step-by-step austempering heat treatment are as follows:

[0100] 3a. Heat the ductile iron containing nano-ceramic particles prepared in step (2) to 920°C for austenitizing treatment, and the holding time is 70 min;

[0101] 3b. Transfer the fully austenitized ductile iron into a low-temperature salt bath furnace within 30 s for the first-stage austempering heat treatment, the austempering temperature is 220°C, and the holding time is 3 min;

[0102] 3c. Then transfer the ductile iron into a high-temperature salt bath furnace for the second-stage austempering heat treatment, the austempering temperature is 420°C, and the holding time is 80 min, and then quench to room temperature;

[0103] 3d. Then put the austempered vermicular graphite cast iron into a box furnace for tempering heat treatment. The tempering temperature is 220 °C and the tempering time is 30 min. Finally, air cool it to room temperature to obtain a new type of austempered vermicular graphite cast iron sample.

[0104] Example 7:

[0105] The preparation method is the same as that of Example 1, except that in step (3), the process conditions of the step-by-step austempering heat treatment are as follows:

[0106] 3a. Heat the vermicular graphite cast iron containing nano-ceramic particles prepared in step (2) to 920 °C for austenitizing treatment, and the holding time is 70 min.

[0107] 3b. Transfer the fully austenitized vermicular graphite cast iron into a low-temperature salt bath furnace within 30 s for the first-stage austempering heat treatment. The austempering temperature is 220 °C and the holding time is 3 min.

[0108] 3c. Then transfer the vermicular graphite cast iron into a high-temperature salt bath furnace for the second-stage austempering heat treatment. The austempering temperature is 450 °C and the holding time is 100 min, and then quench it to room temperature.

[0109] 3d. Then put the austempered vermicular graphite cast iron into a box furnace for tempering heat treatment. The tempering temperature is 220 °C and the tempering time is 30 min. Finally, air cool it to room temperature to obtain a new type of austempered vermicular graphite cast iron sample.

[0110] Example 8:

[0111] The preparation method is the same as that of Example 1, except that in step (3), the process conditions of the step-by-step austempering heat treatment are as follows:

[0112] 3a. Heat the vermicular graphite cast iron containing nano-ceramic particles prepared in step (2) to 920 °C for austenitizing treatment, and the holding time is 70 min.

[0113] 3b. Transfer the fully austenitized vermicular graphite cast iron into a low-temperature salt bath furnace within 30 s for the first-stage austempering heat treatment. The austempering temperature is 220 °C and the holding time is 3 min.

[0114] 3c. Then transfer the vermicular graphite cast iron into a high-temperature salt bath furnace for the second-stage austempering heat treatment. The austempering temperature is 440 °C and the holding time is 90 min, and then quench it to room temperature.

[0115] 3d. Then put the austempered vermicular graphite cast iron into a box furnace for tempering heat treatment. The tempering temperature is 200 °C and the tempering time is 30 min. Finally, air cool it to room temperature to obtain a new type of austempered vermicular graphite cast iron sample.

[0116] Example 9:

[0117] The preparation method is the same as that of Example 1, except that in step (3), the process conditions of the grading isothermal quenching heat treatment are as follows:

[0118] 3a. Heat the vermicular graphite cast iron containing nano-ceramic particles prepared in step (2) to 920 °C for austenitizing treatment, and the holding time is 70 min;

[0119] 3b. Transfer the fully austenitized vermicular graphite cast iron into a low-temperature salt bath furnace within 30 seconds for the first-stage isothermal quenching heat treatment. The isothermal temperature is 220 °C, and the holding time is 3 min;

[0120] 3c. Then transfer the vermicular graphite cast iron into a high-temperature salt bath furnace for the second-stage isothermal quenching heat treatment. The isothermal temperature is 440 °C, and the holding time is 90 min, and then quench to room temperature;

[0121] 3d. Put the isothermally quenched vermicular graphite cast iron into a box furnace for tempering heat treatment. The tempering temperature is 240 °C, and the tempering time is 30 min; finally, air-cool to room temperature to obtain a new type of isothermally quenched vermicular graphite cast iron sample.

[0122] Performance test:

[0123] Perform mechanical property and tribological property tests on the new type of isothermally quenched vermicular graphite cast iron (sample 1) prepared in Example 1 and the new type of isothermally quenched vermicular graphite cast iron products prepared in Examples 5-9 respectively. Among them: The tensile property is tested according to the GB / T228.1-2010 test standard; the impact property is tested according to the GB / T229-2007 test standard; the tribological property test is carried out on a multi-functional friction and wear testing machine, using a ball-disk rotational sliding wear test module. The upper sample is a Gcr15 bearing steel ball with a diameter of 8 mm, a hardness of 60 HRC, and a surface roughness of 50 ± 10 nm. The lower sample is vermicular graphite cast iron with dimensions of 20 mm (length) × 20 mm (width) × 6 mm (height), a surface roughness of 300 ± 50 nm, an external load of 200 N, a movement speed of 1 m / s, a test time of 15 min, and a test temperature of 20 °C.

[0124] The test results are shown in Table 3:

[0125] Table 3. Mechanical properties and wear resistance of vermicular graphite cast iron products prepared under different isothermal quenching heat treatment process conditions

[0126] Name UTS (MPa) <![CDATA[ε f (%)]]> <![CDATA[a k (J / cm 2 )]]> <![CDATA[WVL(mm 3 )]]> Example 1 984 4.4 48.2 1.9 Example 5 967 4.5 46.7 2.8 Example 6 987 4.0 42.5 2.0 Example 7 950 5.1 52.4 3.9 Example 8 990 3.7 40.3 1.7 Example 9 978 4.4 48.5 2.1

[0127] In the table: UTS is the tensile strength, ε f is the elongation, a kwhere AKV is the impact toughness and WVL is the wear volume.

[0128] Analysis:

[0129] As shown in Table 3, with the increase of the isothermal temperature and the prolongation of the holding time in the first stage, the tensile strength, impact toughness and wear resistance of vermicular graphite cast iron decrease, and the elongation remains basically unchanged; with the decrease of the isothermal temperature and the shortening of the holding time in the second stage, the tensile strength of vermicular graphite cast iron slightly increases, the elongation and impact toughness decrease, and the wear resistance remains basically unchanged; with the increase of the isothermal temperature and the prolongation of the holding time in the second stage, the tensile strength and wear resistance of vermicular graphite cast iron decrease, and the elongation and impact toughness increase; with the decrease of the tempering temperature, the tensile strength and wear resistance of vermicular graphite cast iron increase, and the elongation and impact toughness decrease; with the increase of the tempering temperature, the tensile strength and wear resistance of vermicular graphite cast iron decrease, and the elongation and impact toughness remain basically unchanged.

Claims

1. A preparation method of a new type of austempered ductile iron with both high strength and toughness and high wear resistance, characterized in that, It includes the following steps: (1) Preparation of nano-ceramic particle reinforcement: 1a. Weigh a certain amount of TiC nano-ceramic particles; 1b. Put the TiC nano-ceramic particles into a high-energy ball mill for ball milling activation; 1c. Take out the TiC nano-ceramic particles from the ball mill, first wrap the nano-ceramic particles with aluminum foil, and then wrap them with nickel foil; 1d. Use a small hydraulic press to cold-press the wrapped nano-ceramic particles into blocks; (2) Preparation of nano-ceramic particle-reinforced vermicular graphite cast iron: 2a. Use an intermediate frequency induction furnace to melt the cast iron raw materials; 2b. Preheat the ladle, then add the vermiculizing agent and inoculant into the ladle pit in sequence, and finally sprinkle half of the total amount of nano-ceramic particle blocks on the inoculant; 2c. Pour the molten iron into the ladle, and add the other half of the nano-ceramic particle blocks into the molten iron along with the flow from above the ladle during the process of the molten iron flowing out. After the molten iron level in the ladle becomes calm, pour the molten iron into a pre-prepared sand mold. After cooling, a casting sample of nano-ceramic particle-reinforced vermicular graphite cast iron is obtained; The addition amount of the nano-ceramic particles is 0.2wt% of the molten iron; (3) Preparation of new austempered vermicular graphite cast iron: 3a. Heat the vermicular graphite cast iron containing nano-ceramic particles prepared in step (2) to 900 - 940°C for austenitizing treatment, and the holding time is 60 - 80 min; 3b. Transfer the fully austenitized vermicular graphite cast iron into a low-temperature salt bath furnace for the first-stage austempering heat treatment. The austempering temperature is 220 - 250°C, and the holding time is 3 - 5 min; 3c. Then transfer the vermicular graphite cast iron after the first-stage austempering heat treatment into a high-temperature salt bath furnace for the second-stage austempering heat treatment. The austempering temperature is 420 - 450°C, and the holding time is 80 - 100 min, and then water-cool to room temperature; 3d. Put the vermicular graphite cast iron after the second-stage austempering heat treatment into a box furnace for tempering heat treatment. The tempering temperature is 200 - 240°C, and the tempering time is 30 - 45 min; finally air-cool to room temperature to obtain new austempered vermicular graphite cast iron.

2. A preparation method of a new type of austempered ductile iron with both high strength and toughness and high wear resistance according to claim 1, characterized in that: In step (1), the average particle size of the TiC nano-ceramic particles is 50 nm.

3. The preparation method of a new type of austempered ductile iron with both high strength and toughness and high wear resistance according to claim 1, which is characterized in that: In step (1), put the TiC nano-ceramic particles into a high-energy ball mill and ball mill and activate them at a speed of 30 rpm for 10 hr. The reverse and forward conversion time of the ball mill is 5 min.

4. A method for preparing a new type of austempered vermicular graphite cast iron with both high strength and toughness and high wear resistance according to claim 1, characterized in that: In step (1), take out the TiC nano-ceramic particles from the ball mill, first wrap the nano-ceramic particles with aluminum foil with a thickness of 50 μm, and then wrap them with nickel foil with a thickness of 30 μm. The purity of the aluminum foil and nickel foil is 99.999%.

5. A method for preparing a new type of austempered ductile iron with both high strength and toughness and high wear resistance according to claim 1, characterized in that: In step (1), use a small hydraulic press to cold-press the wrapped nano-ceramic particles into blocks. The pressure is 50 KN, the diameter of the block is 30 mm, and the height is 15 mm.

6. A method for preparing a new type of austempered ductile iron with both high strength and toughness and high wear resistance according to claim 1, characterized in that: In step (2), use an intermediate frequency induction furnace to melt the cast iron raw materials. The melting temperature is 1550°C, and the cast iron raw materials are pig iron, ferromanganese and scrap steel.

7. A method for preparing a new type of austempered ductile iron with both high strength and toughness and high wear resistance according to claim 1, characterized in that: In step (2), the vermicularizing agent is rare earth magnesium alloy, and the addition amount is 0.5 - 0.8 wt%, and the inoculant is 75 ferrosilicon, and the addition amount is 0.3 - 0.5 wt%.

8. A method for preparing a new type of austempered ductile iron with both high strength and toughness and high wear resistance according to claim 1, characterized in that: In step (3), the vermicular graphite cast iron containing nano-ceramic particles prepared in step (2) is heated to 920 °C for austenitizing treatment, and the holding time is 70 min; The fully austenitized vermicular graphite cast iron is transferred into a low-temperature salt bath furnace within 30 sec for the first-stage isothermal quenching heat treatment, the isothermal temperature is 220 °C, and the holding time is 3 min; The vermicular graphite cast iron after the first-stage isothermal quenching heat treatment is transferred into a high-temperature salt bath furnace for the second-stage isothermal quenching heat treatment, the isothermal temperature is 440 °C, and the holding time is 90 min, and then water-cooled to room temperature; The vermicular graphite cast iron after the second-stage isothermal quenching is put into a box furnace for tempering heat treatment, the tempering temperature is 220 °C, and the tempering time is 30 min; finally, it is air-cooled to room temperature to obtain a new type of isothermal quenched vermicular graphite cast iron.

9. A method for preparing a new type of austempered ductile iron with both high strength and toughness and high wear resistance according to claim 1, characterized in that: In step (3), the salt bath medium of the low-temperature salt bath furnace is 50% - 55% KNO3 and 45% - 50% NaNO2.

10. A preparation method of a novel austempered ductile iron with both high strength and toughness and high wear resistance according to claim 1, characterized in that: In step (3), the salt bath medium of the high-temperature salt bath furnace is 5% - 10% NaNO2 and 90% - 95% NaNO3.