Preparation process of injection-molded high-density alloy steel

Abstract

The preparation process comprises the steps of proper raw material selection, feed preparation, injection molding, thermal desintering, heat treatment and the like, carbonyl iron powder (D90 is smaller than or equal to 12 microns) with finer particle size and a proper alloy ratio are selected, and a segmented thermal degreasing and argon protection sintering process is combined, so that the carbon content is accurately controlled, and the high-density alloy steel is obtained. The carbon content is stabilized to be 0.38-0.43 wt%, the fluctuation ratio is smaller than + / -0.02%, the sintering compactness is improved, and the sintering density is larger than or equal to 7.65 g / cm < 3 > and is close to the level of forged parts; a thermal dewaxing binder (MCW / PW / EVA / PP / SA) is used for replacing a traditional plastic-based binder for feeding, the catalytic degreasing link of the MIM technology is reduced, the technology efficiency is improved, the degreasing oxidation rusting risk of an alloy steel injection blank is reduced, the production cost is reduced, and the prepared alloy steel is excellent in mechanical property, the hardness is 47-49 HRC, the tensile strength is larger than or equal to 1400 MPa, the yield strength is larger than or equal to 1250 MPa, and the service life of the alloy steel is prolonged. The ductility is greater than or equal to 4%, and the comprehensive performance is close to that of a 4340 rolled plate.

Description

Technical Field

[0001] The invention relates to the technical field of metal injection molding, in particular to a preparation process of injection-molded high-density alloy steel. Background Art

[0002] Injection molded high-density alloy steel is a high-strength, high-density alloy steel material manufactured by the Metal Injection Molding (MIM) process. Its preparation process mainly includes the following steps: mixing fine alloy steel powder with thermoplastic plastic (binder) to form a granular feed; injecting the above feed into the mold under heating to form the so-called "green part"; removing the binder in the green part to leave a porous metal skeleton; sintering at high temperature and protective atmosphere to fuse and shrink the metal particles to form a high-density, high-strength final product. This process combines the advantages of plastic injection molding and powder metallurgy, and can produce small parts with complex shapes, high precision and excellent mechanical properties. Injection molded high-density alloy steel is widely used in medical devices, electronic products, automotive industry, aerospace and other fields due to its excellent mechanical properties and complex formability.

[0003] 4340 alloy steel is widely used in high-load parts such as aerospace and automobiles due to its high strength, high toughness and excellent heat treatment performance. The traditional MIM process for preparing 4340 alloy steel has problems such as sintering decarburization and large fluctuations in carbon content. In addition, the use of plastic-based binders for feeding can easily cause oxidation and rust of the 4340 alloy steel injection blank during the catalytic degreasing process, resulting in a low sintering density of 4340 alloy steel (usually ≤7.6g / cm 3 ), and its mechanical properties are unstable, which limits its application in high-precision scenarios. Therefore, a new MIM process that can stabilize the carbon content and increase the density of 4340 alloy steel is urgently needed. Summary of the invention

[0004] In order to solve the above problems, the present invention provides a process for preparing high-density alloy steel by injection molding, comprising the following steps:

[0005] Select raw materials: alloy steel powder includes 0.6-0.9% chromium, 1.65-2.0% nickel, 0.2-0.3% molybdenum, 0.38-0.43% carbon, and the balance is iron, and the iron powder is carbonyl iron powder;

[0006] Preparation of feed: alloy steel powder and thermal dewaxing binder are mixed in a weight percentage of 55.6-57.1%: 42.9-44.4%, and kneaded at 150-180°C for 60-90min. The kneading temperature should not be too high or the kneading time should not be too long, otherwise the thermal dewaxing binder is easy to decompose. The kneading temperature should not be too low or the kneading time should not be too short, otherwise the alloy steel powder and the thermal dewaxing binder will be unevenly mixed or the thermal dewaxing binder will not be completely melted, resulting in poor consistency of the finished product and loose internal structure. The feed is extruded by an extruder and cut into 1-5mm cylindrical particles to obtain the feed;

[0007] Injection molding: feed the material into the injection molding machine and perform injection molding at 150-180°C, and then demould after cooling for 5-15 seconds to obtain the injection blank;

[0008] Thermal desintering: The injection blank is heated and degreased in stages under nitrogen protection, with 180℃ for 40-50min, 300℃ for 55-65min, 450℃ for 85-95min, and 650℃ for 55-65min, and then heated to 1280-1320℃ in argon atmosphere and sintered for 3-4h;

[0009] Heat treatment: After the sintered parts are oil quenched at 810-845℃, they are tempered at 200-650℃ and cooled in air to obtain the finished products.

[0010] Preferably, in the raw material selection step, the particle size distribution of the carbonyl iron powder is D10=3-4 μm.

[0011] Preferably, in the raw material selection step, the particle size distribution of the carbonyl iron powder is D50=4.5-6.5 μm.

[0012] Preferably, in the raw material selection step, the particle size distribution of the carbonyl iron powder is D90=9-12 μm.

[0013] Preferably, in the step of selecting raw materials, the alloy steel powder comprises, by weight percentage, 0.6% chromium, 1.65% nickel, 0.2% molybdenum, 0.38% carbon, and 97.17% carbonyl iron powder.

[0014] Preferably, in the step of selecting raw materials, the alloy steel powder comprises, by weight percentage, 0.9% chromium, 2.0% nickel, 0.3% molybdenum, 0.43% carbon, and 96.37% carbonyl iron powder.

[0015] Preferably, in the step of selecting raw materials, the alloy steel powder includes, by weight percentage, 0.75% chromium, 1.8% nickel, 0.25% molybdenum, 0.4% carbon, and 96.8% carbonyl iron powder.

[0016] Preferably, in the step of preparing the feed material, the thermal dewaxing-based binder is composed of the following components by weight: PW 1100-1500 parts, MCW 210-250 parts, PP 130-170 parts, EVA 90-110 parts, and SA 60-100 parts. The components and proportions of the thermal dewaxing-based binder directly affect the mixing uniformity, fluidity, and formability of the feed material, as well as the subsequent dewaxing and sintering processes. If the proportion of paraffin wax PW is too high, although the fluidity of the binder can be improved, it may lead to insufficient mechanical strength of the binder and affect the molding quality of the injection blank; if the proportion of paraffin wax PW is too low, the fluidity will deteriorate, which may lead to insufficient filling of the feed during the injection process, resulting in pores or defects; increasing the proportion of polypropylene PP can significantly improve the mechanical strength and heat resistance of the binder, but it may also reduce the fluidity and increase the difficulty of injection; however, if the proportion of polypropylene PP is too low, the mechanical strength of the binder is insufficient, which may cause the injection blank to deform or break during the processing; increasing the proportion of microcrystalline wax can improve the flexibility and temperature resistance of the binder, but it will also affect the fluidity of the binder; and if the proportion of microcrystalline wax is too low, the flexibility of the binder is insufficient, which may cause the feed to break or crack during the processing. In addition, the appropriate ratio of paraffin wax and microcrystalline wax helps to improve the dewaxing performance, ensure that the binder can be quickly and completely removed during the subsequent heating process, reduce residues, and improve the purity of the blank. In practical applications, the binder formula needs to be adjusted according to the characteristics of the alloy steel and the specific process, so that the decomposition temperature of the binder matches the preparation temperature of the alloy steel, in order to obtain alloy steel with excellent comprehensive performance.

[0017] Preferably, in the thermal desintering step, the injection blank is heated and degreased in stages under nitrogen protection, keeping at 180°C for 45 minutes, 300°C for 60 minutes, 450°C for 90 minutes, and 650°C for 60 minutes, then heated to 1000°C in an argon atmosphere for 30 minutes, and then continued to be heated to 1280°C and sintered for 3 hours.

[0018] The beneficial effects are:

[0019] This application uses finer carbonyl iron powder (D90≤12μm) and appropriate alloy ratio, and combines segmented thermal degreasing with argon protection sintering process to accurately control the carbon content, so that the carbon content is stabilized at 0.38-0.43wt%, the fluctuation rate is less than ±0.02%, and the sintering density is improved, and the sintering density is greater than or equal to 7.65g / cm 3 , close to the level of forgings;

[0020] Thermal dewaxing-based binders (MCW / PW / EVA / PP / SA) are used instead of traditional plastic-based binders for feeding, which reduces the catalytic degreasing step of the MIM process, improves process efficiency, reduces the risk of degreasing, oxidation and rusting of alloy steel injection blanks, and reduces production costs. The prepared alloy steel has excellent mechanical properties, a hardness of 47-49HRC, a tensile strength of ≥1400MPa, a yield strength of ≥1250MPa, an elongation of ≥4%, and its comprehensive performance is close to that of 4340 rolled plates. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The drawings described herein are used to provide a further understanding of the present application and constitute a part of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0022] Figure 1 This is a tensile stress-strain curve of the alloy steel powder of Example 2 after heat treatment. DETAILED DESCRIPTION

[0023] The present invention is further described in detail below in conjunction with specific embodiments so that those skilled in the art can understand the present invention more clearly.

[0024] The following embodiments are only used to illustrate the present invention, but are not limited to the scope of the present invention. Based on the specific embodiments of the present invention, all other embodiments obtained by ordinary technicians in the field without creative work belong to the protection scope of the present invention.

[0025] In the examples of the present invention, unless otherwise specified, all raw material components are commercially available products well known to those skilled in the art; in the examples of the present invention, unless otherwise specified, the technical means used are conventional means well known to those skilled in the art.

[0026] Example 1

[0027] This embodiment provides a process for preparing high-density alloy steel by injection molding, comprising the following steps:

[0028] Select raw materials: alloy steel powder includes 0.6% chromium, 1.65% nickel, 0.2% molybdenum, 0.38% carbon, and 97.17% carbonyl iron powder by weight, and the particle size distribution of carbonyl iron powder is D10 = 3-4 μm;

[0029] Prepare feed material: mix the alloy steel powder and the thermal dewaxing base binder in a weight percentage of 57.1%:42.9%, knead at 150° C. for 60 min, extrude through an extruder and cut into 1 mm cylindrical particles to obtain the feed material, wherein the thermal dewaxing base binder is composed of the following components in weight parts: paraffin wax PW 1500 parts, microcrystalline wax MCW 210 parts, polypropylene PP 130 parts, ethylene-vinyl acetate copolymer EVA 90 parts, stearic acid SA 60 parts;

[0030] Injection molding: the feed material is put into an injection molding machine for injection molding at 150° C., and demoulding is performed after cooling for 5 seconds to obtain an injection blank;

[0031] Thermal desintering: the injection blank is degreased in stages under nitrogen protection, at 180°C for 40 min, 300°C for 55 min, 450°C for 85 min, 650°C for 55 min, then heated to 1000°C in an argon atmosphere for 30 min, and then heated to 1280°C for sintering for 3 h;

[0032] Heat treatment: The sintered parts are oil quenched at 810℃, tempered at 200℃, and cooled in air to obtain the finished product.

[0033] Example 2

[0034] This embodiment provides a process for preparing high-density alloy steel by injection molding, comprising the following steps:

[0035] Select raw materials: alloy steel powder includes 0.9% chromium, 2.0% nickel, 0.3% molybdenum, 0.43% carbon, and 96.37% carbonyl iron powder by weight, and the particle size distribution of carbonyl iron powder is D50=4.5-6.5μm;

[0036] Prepare feed material: mix the alloy steel powder and the thermal dewaxing base binder in a weight percentage of 55.6%:42.9%, knead at 165° C. for 75 min, extrude through an extruder and cut into 3 mm cylindrical particles to obtain the feed material, wherein the thermal dewaxing base binder is composed of the following components in weight parts: paraffin wax PW 1300 parts, microcrystalline wax MCW 230 parts, polypropylene PP 150 parts, ethylene-vinyl acetate copolymer EVA 100 parts, stearic acid SA 80 parts;

[0037] Injection molding: the feed material is put into an injection molding machine for injection molding at 165° C., and demoulding is performed after cooling for 10 seconds to obtain an injection blank;

[0038] Thermal desintering: the injection blank is degreased in stages under nitrogen protection, at 180°C for 45 min, 300°C for 60 min, 450°C for 90 min, and 650°C for 60 min, then heated to 1000°C in an argon atmosphere for 30 min, and then heated to 1300°C for sintering for 3.5 h;

[0039] Heat treatment: The sintered parts are oil quenched at 828℃, tempered at 425℃, and cooled in air to obtain the finished product.

[0040] Example 3

[0041] This embodiment provides a process for preparing high-density alloy steel by injection molding, comprising the following steps:

[0042] Select raw materials: alloy steel powder includes 0.75% chromium, 1.8% nickel, 0.25% molybdenum, 0.4% carbon, and 96.8% carbonyl iron powder by weight, and the particle size distribution of carbonyl iron powder is D90=9-12μm;

[0043] Prepare feed material: mix the alloy steel powder and the thermal dewaxing base binder in a weight percentage of 55.6%:44.4%, knead at 180° C. for 90 min, extrude through an extruder and cut into 5 mm cylindrical particles to obtain the feed material, wherein the thermal dewaxing base binder is composed of the following components in weight parts: paraffin wax PW 1100 parts, microcrystalline wax MCW 250 parts, polypropylene PP 170 parts, ethylene-vinyl acetate copolymer EVA 110 parts, stearic acid SA 100 parts;

[0044] Injection molding: the feed material is put into an injection molding machine for injection molding at 180° C., and demoulding is performed after cooling for 15 seconds to obtain an injection blank;

[0045] Thermal desintering: the injection blank is degreased in stages under nitrogen protection, at 180°C for 50 min, 300°C for 65 min, 450°C for 95 min, 650°C for 65 min, then heated to 1000°C in an argon atmosphere for 30 min, and then heated to 1320°C for sintering for 4 h;

[0046] Heat treatment: The sintered parts are oil quenched at 845℃, tempered at 650℃, and cooled in air to obtain the finished product.

[0047] Comparative Example 1

[0048] The difference between this comparative example and Example 2 is that reduced iron powder with the same particle size distribution is used instead of carbonyl iron powder, and the other components and process steps are the same as those in Example 2.

[0049] Comparative Example 2

[0050] This comparative example is different from Example 2 in that a plastic-based binder is used instead of a thermal dewaxing-based binder, wherein the plastic-based binder consists of 1860 parts of polyethylene PE, and the other ingredients and process steps are the same as those in Example 2.

[0051] Comparative Example 3

[0052] The difference between this comparative example and Example 2 is that in the step of preparing the feeding material: the banburying temperature is 145° C., the banburying time is 55 min, and the other ingredients and process steps are the same as those in Example 2.

[0053] Comparative Example 4

[0054] The difference between this comparative example and Example 2 is that in the step of preparing the feeding material: the banburying temperature is 185°C, the banburying time is 95 min, and the other ingredients and process steps are the same as those in Example 2.

[0055] The alloy steels prepared in Examples 1 to 3 and Comparative Examples 1 to 4 were tested.

[0056] Test method:

[0057] The sintering density is tested according to the method of "GB / T 4325-2016 Determination of density of powder metallurgy materials";

[0058] The sintered carbon content is tested according to the method of "GB / T 223.88-2000 Chemical Analysis Methods for Iron, Steel and Alloys Combustion-Volumetric Method for Determination of Total Carbon Content";

[0059] Heat treatment hardness is tested according to the method of "GB / T 230.1-2018 Rockwell Hardness Test for Metal Materials";

[0060] Mechanical properties tests of alloy steel such as tensile strength, yield strength and elongation are conducted in accordance with ASTM A370 "Mechanical Test Methods for Steel Products";

[0061] The test results are shown in Table 1.

[0062] Table 1 Alloy steel performance test results

[0063] Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 <![CDATA[Sintered density (g / cm 3 )]]> 7.67 7.67 7.66 5.82 6.03 6.14 6.23 Sintered carbon content (wt%) 0.38 0.42 0.41 0.32 0.56 0.36 0.52 Heat treatment hardness (HRC) 47 49 48 38 36 41 43 Tensile strength(MPa) 1487 1502 1514 1401 1385 1392 1415 Yield strength (MPa) 1280 1294 1306 1085 1007 1114 1067 Elongation(%) 4.2 4.4 4.5 3.6 3.4 3.8 3.7

[0064] Combined with the experimental data in Table 1 and the Figure 1It can be seen that the sintered density and sintered carbon content of the alloy steel prepared by the preparation process of the present application in each embodiment are relatively stable and have excellent comprehensive mechanical properties, and can be mass-produced. Continuing to analyze Table 1, it can be found that the sintered density, sintered carbon content, heat treatment hardness, tensile strength, yield strength and elongation of the alloy steel prepared by using reduced iron powder with the same particle size distribution instead of carbonyl iron powder in Comparative Example 1 have all decreased, especially the sintered density has decreased significantly. This may be because the reduced iron powder is not as pure as the carbonyl iron powder, and its particle shape is not as regular and fine as the carbonyl iron powder, resulting in poor density and uniformity of the sintered alloy steel, which in turn affects the hardness of the alloy steel and weakens the mechanical properties of the alloy steel; Comparative Example 2 uses a plastic-based binder instead of the present application The sintered density, heat treatment hardness, tensile strength, yield strength and elongation of the alloy steel prepared by the thermal dewaxing-based binder are all reduced, while the sintered carbon content is increased on the contrary. This may be because the decomposition is incomplete at high temperature, and carbon residues and other impurities are easily left, which directly increase the carbon content after sintering. The residues will further affect the effective contact and densification process between the various components of the alloy steel, thereby resulting in a decrease in the sintered density, heat treatment hardness, tensile strength, yield strength and elongation. In the preparation feeding step of Comparative Example 3, The banburying temperature was adjusted to 145°C and the banburying time was adjusted to 55min, resulting in a decrease in sintered density, sintered carbon content, heat treatment hardness, tensile strength, yield strength and elongation. This may be because the banburying temperature is too low and the banburying time is too short, the alloy steel powder and the thermal dewaxing base binder are not mixed evenly or the thermal dewaxing base binder is not completely melted, resulting in poor consistency and loose internal structure of the finished alloy steel; in the comparative example 4, the banburying temperature is adjusted to 185°C and the banburying time is adjusted to 95min in the preparation feeding step, resulting in a decrease in sintered density, heat treatment hardness, tensile strength, yield strength and elongation. The tensile strength, yield strength and elongation indicators also decreased, while the sintered carbon content increased slightly. This may be because the mixing temperature is too high and the mixing time is too long, resulting in the decomposition of the thermal dewaxing-based adhesive before entering the thermal desintering process. In this way, the binding force between the particles will be weakened, the fluidity and molding performance of the feed will deteriorate, and the density of the injection blank will decrease, which will lead to a decrease in sintering density. In addition, the premature decomposition of the binder will produce more carbon residues, which will not only increase the sintered carbon content but also increase the microstructural defects, thereby weakening the mechanical properties of the finished alloy steel products as a whole.

[0065] In summary, the present application provides a process for preparing high-density alloy steel by injection molding. By selecting finer carbonyl iron powder (D90≤12μm) and a suitable alloy ratio, and combining segmented thermal degreasing with argon protection sintering process, the carbon content is precisely controlled to stabilize the carbon content at 0.38-0.43wt%, with a fluctuation rate of less than ±0.02%, thereby improving the sintering density and achieving a sintering density greater than or equal to 7.65g / cm 3, close to the level of forged parts; hot dewaxing based binder (MCW / PW / EVA / PP / SA) is used to replace traditional plastic-based binder for feeding, reducing the catalytic degreasing link of MIM process, improving process efficiency, reducing the risk of degreasing, oxidation and rusting of alloy steel injection blanks, and reducing production costs. The prepared alloy steel has excellent mechanical properties, hardness of 47-49HRC, tensile strength ≥1400MPa, yield strength ≥1250MPa, elongation ≥4%, and comprehensive performance is close to 4340 rolled plate.

[0066] The above is only an embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A process for preparing high-density alloy steel by injection molding, characterized in that: The following steps are involved: Select raw materials: alloy steel powder includes 0.6-0.9% chromium, 1.65-2.0% nickel, 0.2-0.3% molybdenum, 0.38-0.43% carbon, and the balance is iron, and the iron powder is carbonyl iron powder; Prepare feed material: mix the alloy steel powder and the thermal dewaxing base binder in a weight percentage of 55.6-57.1%:42.9-44.4%, knead at 150-180° C. for 60-90 minutes, extrude through an extruder and cut into 1-5 mm cylindrical particles to obtain the feed material; Injection molding: the feed material is put into an injection molding machine for injection molding at 150-180° C., and demoulding is performed after cooling for 5-15 seconds to obtain an injection blank; Thermal desintering: the injection blank is degreased in stages under nitrogen protection, kept at 180°C for 40-50min, 300°C for 55-65min, 450°C for 85-95min, 650°C for 55-65min, and then heated to 1280-1320°C in an argon atmosphere for sintering for 3-4h; Heat treatment: After the sintered parts are oil quenched at 810-845℃, they are tempered at 200-650℃ and cooled in air to obtain the finished products.

2. The process for preparing high-density alloy steel by injection molding according to claim 1, characterized in that: In the raw material selection step, the particle size distribution of the carbonyl iron powder is D10=3-4 μm.

3. The process for preparing high-density alloy steel by injection molding according to claim 1, characterized in that: In the raw material selection step, the particle size distribution of the carbonyl iron powder is D50=4.5-6.5 μm.

4. The process for preparing high-density alloy steel by injection molding according to claim 1, characterized in that: In the raw material selection step, the particle size distribution of the carbonyl iron powder is D90=9-12 μm.

5. The process for preparing high-density alloy steel by injection molding according to claim 1, characterized in that: In the step of selecting raw materials, the alloy steel powder includes 0.6% chromium, 1.65% nickel, 0.2% molybdenum, 0.38% carbon, and 97.17% carbonyl iron powder by weight percentage.

6. The process for preparing high-density alloy steel by injection molding according to claim 1, characterized in that: In the step of selecting raw materials, the alloy steel powder includes 0.9% chromium, 2.0% nickel, 0.3% molybdenum, 0.43% carbon, and 96.37% carbonyl iron powder by weight percentage.

7. The process for preparing high-density alloy steel by injection molding according to claim 1, characterized in that: In the step of selecting raw materials, the alloy steel powder includes 0.75% chromium, 1.8% nickel, 0.25% molybdenum, 0.4% carbon, and 96.8% carbonyl iron powder by weight percentage.

8. The process for preparing high-density alloy steel by injection molding according to claim 1, characterized in that: In the step of preparing the feed material, the thermal dewaxing-based binder is composed of the following components in parts by weight: 1100-1500 parts of PW, 210-250 parts of MCW, 130-170 parts of PP, 90-110 parts of EVA, and 60-100 parts of SA.

9. The process for preparing high-density alloy steel by injection molding according to claim 1, characterized in that: In the thermal desintering step, the injection blank is degreased in stages under nitrogen protection, kept at 180°C for 45 minutes, 300°C for 60 minutes, 450°C for 90 minutes, and 650°C for 60 minutes, then heated to 1000°C in an argon atmosphere for 30 minutes, and then continued to be heated to 1280°C for sintering for 3 hours.

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