A method for preparing ultra-hard and high-strength M42 high-speed steel by electron beam powder bed fusion

Abstract

The present invention relates to the technical field of additive manufacturing of high-speed steel, and specifically to a method for preparing ultra-hard and high-strength M42 high-speed steel by electron beam powder bed melting. The present invention uses an electron beam as a heat source and adopts a powder bed strengthening and preheating adjustment process in the additive manufacturing process of M42 high-speed steel. The method can effectively avoid defects caused by powder splashing during the printing process, thereby reducing the porosity of the formed part and improving the density of the sample. The M42 high-speed steel additively manufactured by the method of the present invention is free of cracks, has high density, good surface quality and uniform structure, and can increase the precipitation of nano-micrometer-level carbides in the crystal.

Description

Technical Field

[0001] The present invention relates to the technical field of additive manufacturing of high-speed steel, and in particular to a method for preparing ultra-hard and high-strength M42 high-speed steel by electron beam powder bed fusion. Background Art

[0002] As a key component of industrial machine tools, the development level of cutting tools directly determines the core competitiveness of the manufacturing industry. High-speed steel (HSS) is a key material for cutting tools due to its high hardness and wear resistance.

[0003] M42 (W2Mo9Cr4VCo8) is a representative of high performance in high-speed steel due to its high carbon (1-1.15wt.%) and high alloy (>20wt.%) characteristics. However, due to the slow cooling rate, the cast structure of traditional cast M42 steel is prone to form segregation characterized by coarse, network-like eutectic carbides, which not only makes the subsequent forging and rolling hot processing processes difficult, but also significantly damages the mechanical properties.

[0004] Additive manufacturing technology is based on the principle of "discrete-accumulation" and manufactures physical parts in a point-by-point / line-by-line / layer-by-layer accumulation manner. Electron beam powder bed fusion is a technology that has developed rapidly in recent years. Due to its small molten pool (0.2mm~1mm), high cooling rate (10 6 Ks -1 ~10 8 Ks -1 ) properties can effectively control its microstructure, resulting in excellent performance. High energy density, high efficiency, and impurity-free vacuum melting can produce high-performance M42 high-speed steel. However, existing methods are prone to defects caused by powder splashing during the printing process, resulting in increased porosity and uneven microstructure in the specimens. Summary of the Invention

[0005] In view of the above-mentioned deficiencies in the prior art, the purpose of the present invention is to provide a method for preparing ultra-hard and high-strength M42 high-speed steel by electron beam powder bed fusion. The present invention strengthens the powder bed through unique powder bed strengthening and thermal compensation before and after powder spreading. The whole process is in a vacuum environment and continuously filled with protective gas helium to eliminate the interference of harmful elements such as oxygen / nitrogen. The electron beam powder bed fusion printing process parameters are regulated to obtain excellent performance M42 high-speed steel that does not require subsequent heat treatment. At the same time, the residual stress is reduced through a high preheating temperature to obtain M42 high-speed steel with excellent performance / good quality.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0007] A method for preparing ultra-hard and high-strength M42 high-speed steel by electron beam powder bed fusion comprises the following steps:

[0008] M42 steel powder was prepared by gas atomization and pretreated before printing to obtain pretreated M42 steel powder;

[0009] A three-dimensional model of M42 steel was constructed, sliced ​​along the height direction, and scanning parameters were set to obtain slice scanning data, which were then imported into the electron beam powder bed fusion equipment.

[0010] The pretreated M42 steel powder is spread flat on the powder bed on the substrate, thermal compensation is performed before spreading, and the powder bed is strengthened after spreading. Under a protective atmosphere, when the substrate preheating temperature reaches 820℃~850℃, the pretreated M42 steel powder is melted, formed and printed layer by layer using an electron beam to obtain ultra-hard and high-strength M42 high-speed steel.

[0011] The present invention uses an electron beam as a heat source, and adopts powder bed reinforcement and thermal compensation before and after powder spreading to strengthen the powder bed during the additive manufacturing process of M42 high-speed steel. The electron beam powder bed fusion printing process parameters are regulated to obtain excellent performance M42 high-speed steel without subsequent heat treatment. At the same time, residual stress is reduced through high preheating temperature to obtain M42 high-speed steel with excellent performance and good quality. This method can effectively avoid defects caused by powder splashing during the printing process, thereby reducing the porosity of the formed part, improving the density of the sample, and making the structure more uniform.

[0012] In a preferred embodiment of the present invention, the powder in the forming area is reinforced for the second time by powder bed reinforcement, and the powder bed reinforcement parameters are: beam spot defocus 0V~0.5V, rated current 20mA~26mA, scanning speed 5m / s~8m / s, starting current 12mA~14mA, average current 14mA~16mA, current step increase: 3 levels, powder solidification times 1~2 times.

[0013] In a preferred embodiment of the present invention, the electron beam current is 12 mA to 18 mA, the scanning speed is 5 m / s to 6.5 m / s, the beam spot diameter is 0.1 mm to 0.2 mm, the defocus amount is 0 V to 0.5 V, and the powder amount is 95 μm to 105 μm.

[0014] In a preferred embodiment of the present invention, the thermal compensation parameters before powder spreading are: beam spot defocus: 0V to 0.5V, rated current 10mA, time: 10s to 12s.

[0015] In a preferred embodiment of the present invention, the powder bed is preheated for micro-sintering after spreading the powder, and the sintering powder bed parameters are: scanning speed: 12m / s~14m / s, scanning spacing: 1mm~1.5mm, beam spot defocus: 0V~0.5V, rated current: 35mA, time: 10s~12s.

[0016] The powder bed is strengthened after printing and spreading, and the powder bed is micro-sintered under a smaller preheating condition, thereby improving the conductivity of the powder bed and reducing the risk of "powder blowing" and "powder pushing" to improve the forming quality. Step preheating can reduce the impact of the electron beam on the powder bed, thereby reducing powder blowing during the printing process.

[0017] In a preferred embodiment of the present invention, the powder bed preheating method is step-up preheating, the preheating is divided into 3 levels, the preheating current is gradually increased, the maximum preheating current is 35mA~40mA, and the starting current is 10mA~12mA.

[0018] In a preferred embodiment of the present invention, the scanning mode is set to unidirectional scanning, and the scanning parameters are: the rotation angle between layers is 90°, the scanning interval is 80 μm, and the layer thickness is 50 μm.

[0019] Furthermore, the printing parameters were set, the powder amount was set to 95 μm to 105 μm, and the pre-powder thickness was 50 μm.

[0020] In a preferred embodiment of the present invention, after the M42 steel powder is loaded into the powder cylinder, vacuum is drawn until the vacuum degree of the gun chamber reaches 10 -3 After the order of Pa, the forming chamber is filled with helium protective atmosphere to ensure that the oxygen content of the air in the powder cylinder does not exceed 0.5%.

[0021] In a preferred embodiment of the present invention, the substrate is preheated manually, and the temperature of the preheated forming substrate is 820°C to 850°C.

[0022] The specific preheating is divided into three stages. The base plate is heated to 400℃, 700℃, and 850℃. The preheating current is 5mA~10mA and the scanning speed is 5m / s~10m / s to heat it to 350℃~400℃, the preheating current is 15mA~20mA and the scanning speed is 10m / s~20m / s to heat it to 600℃~700℃, and the preheating current is 25mA~30mA and the scanning speed is 10m / s~20m / s to heat it to 850℃ and keep them warm for 5 minutes~10 minutes, 5 minutes~10 minutes, and 10 minutes~15 minutes respectively. The substrate material is 316L stainless steel substrate. The preheating process adopts a step-by-step heating method, gradually increasing the current and scanning speed to make the substrate temperature rise steadily. In the insulation stage, the temperature should be prevented from rising too fast and heating unevenly.

[0023] In a preferred embodiment of the present invention, helium as a protective gas is continuously introduced during the entire printing process.

[0024] In a preferred embodiment of the present invention, the particle size of the M42 steel powder is 53 μm to 120 μm, and the composition of the M42 steel powder includes, by mass percentage, C: 1.05% to 1.15%, Cr: 3.50% to 4.25%, W: 1.15% to 1.85%, Mo: 9.0% to 10.0%, V: 0.95% to 1.35%, Co: 7.75% to 8.75%, Mn: ≤0.40%, Si: ≤0.65%, S: ≤0.03%, P: ≤0.03%, and the balance is Fe.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] 1. The present invention uses an electron beam as a heat source, and adopts powder bed reinforcement and thermal compensation before and after powder spreading to strengthen the powder bed during the additive manufacturing process of M42 high-speed steel. The electron beam powder bed fusion printing process parameters are regulated to obtain excellent performance M42 high-speed steel without subsequent heat treatment. At the same time, the high preheating temperature is used to reduce residual stress to obtain M42 high-speed steel with excellent performance and good quality. This method can effectively avoid defects caused by powder splashing during the printing process, thereby reducing the porosity of the formed part, improving the density of the sample, and making the structure more uniform.

[0027] 2. The M42 high-speed steel formed by the additive manufacturing method of the present invention is crack-free, has high density, good surface quality and uniform structure, and can increase the precipitation of nano-micrometer-level carbides in the crystal. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a macroscopic image of M42 high-speed steel prepared by electron beam powder bed fusion in the present invention.

[0029] Figure 2 This is the surface quality image of M42 high-speed steel prepared by electron beam powder bed fusion in the present invention.

[0030] Figure 3 This is the SEM uniform structure diagram of the M42 high-speed steel prepared in Example 16 of the present invention. DETAILED DESCRIPTION

[0031] The following is a detailed description of the technical solutions in the embodiments of the present invention, using preferred embodiments and accompanying drawings in conjunction with the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0032] It should be noted that all professional terms used in the present invention are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention. Unless otherwise specified, the various raw materials, reagents, instruments and equipment used in the following embodiments of the present invention can be purchased from the market or prepared by existing methods.

[0033] Example 1

[0034] A method for preparing ultra-hard and high-strength M42 high-speed steel by electron beam powder bed fusion comprises the following steps:

[0035] (1) M42 steel powder was prepared by gas atomization. The powder particle size range was 53 μm to 120 μm. The powder composition by mass percentage included: C: 1.05-1.15%, Cr: 3.50-4.25%, W: 1.15-1.85%, Mo: 9.0-10.0%, V: 0.95-1.35%, Co: 7.75-8.75%, Mn: ≤0.40%, Si: ≤0.65%, S: ≤0.03%, P: ≤0.03%, and the balance was Fe. The powder was pretreated before printing and dried in a vacuum drying oven at 80 °C for 2 h to ensure that the powder had good fluidity. The powder was sieved with a 160-mesh sieve and collected for later use.

[0036] (2) A three-dimensional model of M42 steel was constructed and sliced ​​along the height direction. The construction size was 15 mm × 15 mm × 10 mm. The slicing software used was SL-EBM BuildPrepare. The scanning mode was set to unidirectional scanning, with each layer rotated 90° between layers, a scanning spacing of 80 μm, and a layer thickness of 50 μm. The obtained layer scanning data was imported into the electron beam powder bed melting equipment.

[0037] (3) Preparation of M42 high-speed steel by electron beam powder bed fusion

[0038] Thermal compensation was performed before powder spreading, and the thermal compensation parameters were: beam defocus: 0.2V, rated current 10mA, time: 11s. After powder spreading, the powder bed was strengthened. First, the powder bed was micro-sintered. The micro-sintering powder bed parameters were: scanning speed: 13.6m / s, scanning spacing: 1mm, beam defocus: 0.2V, rated current: 35mA, time: 11s. The powder in the forming area was then reinforced for the second time through powder bed reinforcement. The powder bed reinforcement parameters were: electron beam current 15mA, rated current 26mA, scanning speed 8m / s, starting current 16mA, current step increase: 3 levels, beam defocus 0.2V, and powder solidification times 1 time.

[0039] Put the dried powder into the powder cylinder of the forming chamber, level the forming area substrate, the substrate material is 316L stainless steel substrate, fill the M42 steel powder into the powder cylinder, and then draw the vacuum until the vacuum degree of the gun chamber reaches 10 -3 After the temperature reaches the Pa level, the forming chamber is filled with a helium protective atmosphere to ensure that the oxygen content of the air in the powder cylinder does not exceed 0.5%. Then the substrate is preheated manually. The temperature of the preheated forming substrate is 850℃. The preheating adopts a step-by-step heating method. The substrate preheating is divided into three stages. The preheating current is 10mA, the scanning speed is 10m / s, heated to 400℃ and kept warm for 5 minutes, 18mA, 20m / s heated to 700℃ and kept warm for 5 minutes, 25mA, 20m / s heated to 850℃ and kept warm for 10 minutes. When the power supply voltage rises to 60kV, the forming chamber is vacuumed and filled with protective gas helium. When the preheated substrate reaches 850℃, printing begins. The electron beam scans according to the predetermined slicing trajectory. The printing parameters are: electron beam current size is 18mA, scanning speed is 5m / s, beam spot diameter is 0.1mm, defocus size is 0V, and powder amount is 0.95mm.

[0040] The substrate is lowered by one powder bed layer thickness, thermal compensation is performed before powder spreading, powder is spread again, and the powder bed is strengthened. The scanning process is layered and stacked layer by layer to obtain ultra-hard and high-strength M42 high-speed steel.

[0041] Example 2

[0042] The preparation method is the same as that of Example 1, except that the printing parameters are set as follows: the electron beam current is 12 mA, the scanning speed is 5 m / s, the beam spot diameter is 0.1 mm, the defocus amount is 0 V, and the powder amount is 1.05 mm.

[0043] Example 3

[0044] The preparation method is the same as that of Example 1, except that the printing parameters are set as follows: the electron beam current is 14 mA, the scanning speed is 5 m / s, the beam spot diameter is 0.1 mm, the defocus amount is 0 V, and the powder amount is 1.0 mm.

[0045] Example 4

[0046] The preparation method is the same as that of Example 1, except that the printing parameters are set as follows: the electron beam current is 16 mA, the scanning speed is 5 m / s, the beam spot diameter is 0.1 mm, the defocus amount is 0 V, and the powder amount is 0.95 mm.

[0047] Example 5

[0048] The preparation method is the same as that of Example 1, except that the printing parameters are set as follows: electron beam current is 18 mA, scanning speed is 5.5 m / s, beam spot diameter is 0.1 mm, defocusing amount is 0.1 V, and powder taking amount is 0.95 mm.

[0049] Example 6

[0050] The preparation method is the same as that of Example 1, except that the printing parameters are set as follows: electron beam current is 18 mA, scanning speed is 6 m / s, beam spot diameter is 0.1 mm, defocusing amount is 0.2 V, and powder taking amount is 0.95 mm.

[0051] Example 7

[0052] The preparation method is the same as that of Example 1, except that the printing parameters are set as follows: electron beam current is 18 mA, scanning speed is 6.5 m / s, beam spot diameter is 0.1 mm, defocusing amount is 0.3 V, and powder taking amount is 0.95 mm.

[0053] Example 8

[0054] The preparation method is the same as that of Example 1, except that the printing parameters are set as follows: the electron beam current is 18 mA, the scanning speed is 5 m / s, the beam spot diameter is 0.1 mm, the defocus amount is 0.1 V, and the powder amount is 0.95 mm.

[0055] Example 9

[0056] The preparation method is the same as that of Example 1, except that the printing parameters are set as follows: electron beam current is 18 mA, scanning speed is 5 m / s, beam spot diameter is 0.1 mm, defocusing amount is 0.2 V, and powder taking amount is 0.95 mm.

[0057] Example 10

[0058] The preparation method is the same as that of Example 1, except that the printing parameters are set as follows: the electron beam current is 18 mA, the scanning speed is 0.1 to 5 m / s, the beam spot diameter is 0.1 mm, the defocus amount is 0.3 V, and the powder amount is 0.95 mm.

[0059] Example 11

[0060] The preparation method is the same as that of Example 1, except that the printing parameters are set as follows: the electron beam current is 18 mA, the scanning speed is 5 m / s, the beam spot diameter is 0.1 mm, the defocus amount is 0.4 V, and the powder amount is 0.95 mm.

[0061] Example 12

[0062] The preparation method is the same as that of Example 1, except that the printing parameters are set as follows: the electron beam current is 18 mA, the scanning speed is 5 m / s, the beam spot diameter is 0.1 mm, the defocus amount is 0.5 V, and the powder amount is 0.95 mm.

[0063] Example 13

[0064] The preparation method is the same as that of Example 1, except that the powder bed strengthening parameters are: the number of powder solidification is 1, the electron beam current is 14 mA, and the beam spot defocus is 0.2V.

[0065] Example 14

[0066] The preparation method is the same as that of Example 1, except that, in the powder bed strengthening parameters, the number of powder solidification is 1 and the electron beam current is 15 mA.

[0067] Example 15

[0068] The preparation method is the same as that of Example 1, except that, in the powder bed strengthening parameters, the number of powder solidification is 1 and the electron beam current is 16 mA.

[0069] Example 16

[0070] The preparation method is the same as that of Example 1, except that, in the powder bed strengthening parameters, the number of powder solidification is 2 times, and the electron beam current is 18 mA.

[0071] Example 17

[0072] The preparation method is the same as that in Example 1, with the only difference being that, in the powder bed strengthening parameters, the beam spot defocus is 0.3 V, the rated current is 20 mA, the scanning speed is 5 m / s, and the starting current is 12 mA; in the printing parameters, the electron beam current is 12 m / s, the scanning speed is 6.5 m / s, the beam spot diameter is 0.2 mm, the defocus is 3 V, and the powder amount is 100 μm; in the thermal compensation parameters before powder spreading, the beam spot defocus is 3 V, and the time is 10 s.

[0073] Example 18

[0074] The preparation method is the same as that in Example 1, with the only difference being that, in the powder bed strengthening parameters, the beam spot defocus is 0.1 V, the rated current is 22 mA, the scanning speed is 7 m / s, and the starting current is 13 mA; in the printing parameters, the electron beam current is 15 m / s, the scanning speed is 5.5 m / s, the beam spot diameter is 0.15 mm, the defocus is 0.5 V, and the powder amount is 105 μm; in the thermal compensation parameters before powder spreading, the beam spot defocus is 5 V and the time is 12 s.

[0075] Result Analysis

[0076] For the M42 high-speed steel prepared in the examples, the performance of each example was tested, and each sample was tested five times to obtain the average value. The results are as follows:

[0077] Table 1 Performance test results of Examples 1 to 6

[0078] Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 density 99.6％ 98.5％ 98.9％ 99.2％ 99.1％ 99.2％ Hardness / HV 842.2 774.5 785.5 801.6 807.3 822.9

[0079] Table 2 Performance test results of Examples 7 to 11

[0080] Example 7 Example 8 Example 9 Example 10 Example 11 density 98.4％ 99.4％ 98.0％ 99.0％ 98.5％ Hardness / HV 785.7 821.7 763.1 801.4 779.5

[0081] Table 3 Performance test results of Examples 12 to 16

[0082] Example 12 Example 13 Example 14 Example 15 Example 16 density 99.0％ 99.3％ 99.2％ 99.1％ 99.5％ Hardness / HV 809.5 825.5 830.1 821.3 834.3

[0083] Tables 1-3 show the performance test results of M42 high-speed steel prepared in Examples 1-16. It can be seen that the printing parameters have a significant impact on the formed samples. Samples 1-4 show that when the scanning speed is constant, the density gradually increases with increasing electron beam current. Samples 1, 5, 6, and 7 show that when the electron beam current is constant, defects such as pores and lack of fusion in the samples gradually decrease as the scanning speed increases, and the density significantly increases. This is because the scanning speed can regulate the solidification and melting rates. Different scanning speeds mean different heat inputs, which in turn changes the temperature and heat input of the molten pool.

[0084] At the same time, the quality of the powder bed during printing affects microstructure evolution and the final build quality. Increasing the number of powder bed consolidation cycles effectively strengthens the bed's stability, tightly bonding the powder together. This reduces the risk of powder blowoff caused by excessive electron beam energy, improves print quality, and results in higher density and greater hardness.

[0085] Figure 1 This is a macroscopic image of M42 high-speed steel prepared by electron beam powder bed fusion in the present invention. As can be seen from the image, the high-speed steel sample prepared by the method of the present invention has no obvious defects and good forming quality.

[0086] Figure 2 This is the surface quality image of M42 high-speed steel prepared by electron beam powder bed fusion in this invention. The surface quality of the sample is good and very smooth. Especially after powder bed strengthening, that is, the increase in the number of powder solidification times, the melting line on the surface can be clearly seen.

[0087] Figure 3This is an SEM image of the uniform microstructure of M42 high-speed steel prepared in Example 16 of the present invention. The parameters used were: electron beam current of 18 mA, scanning speed of 5 m / s, defocusing distance of 0 V, and two powder consolidation cycles. It can be seen that a large amount of carbides precipitated in the sample, reflecting its high hardness. Furthermore, fine nano- and micron-sized carbides are evenly dispersed within the grains, contributing to the sample's toughness.

[0088] In summary, the present invention uses an electron beam as a heat source, adopts powder bed reinforcement and thermal compensation before and after powder spreading to strengthen the powder bed in the additive manufacturing process of M42 high-speed steel, and regulates the electron beam powder bed fusion printing process parameters to obtain excellent performance M42 high-speed steel without subsequent heat treatment. At the same time, the residual stress is reduced by a high preheating temperature to obtain M42 high-speed steel with excellent performance / good quality. This method can effectively avoid defects caused by powder splashing during the printing process, thereby reducing the porosity of the formed part and improving the density of the sample.

[0089] It should be noted that when numerical ranges are mentioned in the present invention, it should be understood that both endpoints of each numerical range and any value between the two endpoints may be selected. Since the steps and methods used are the same as those in the embodiments, in order to avoid redundancy, the present invention describes preferred embodiments. Although preferred embodiments of the present invention have been described, those skilled in the art may make additional changes and modifications to these embodiments once they understand the basic inventive concepts. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0090] Obviously, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if such changes and modifications fall within the scope of the claims and their equivalents, the present invention is intended to include such changes and modifications.

Claims

1. A method for preparing ultra-hard and high-strength M42 high-speed steel by electron beam powder bed fusion, characterized in that: The following steps are involved: M42 steel powder was prepared by gas atomization and pretreated before printing to obtain pretreated M42 steel powder; Construct a 3D model of M42 steel, slice it along the height direction, set scanning parameters, obtain slice scanning data, and import the obtained slice scanning data into the electron beam powder bed fusion equipment; The pre-treated M42 steel powder is spread flat on the powder bed on the substrate. Thermal compensation is performed before spreading the powder, and the powder bed is strengthened after spreading the powder. Under a protective atmosphere, when the substrate preheating temperature reaches 820℃~850℃, the pre-treated M42 steel powder is melted and formed layer by layer using an electron beam to print, thereby obtaining ultra-hard and high-strength M42 high-speed steel. The powder bed strengthening parameters are as follows: beam defocus 0 V~0.5 V, rated current 20 mA~26 mA, scanning speed 5 m / s~8 m / s, starting current 12 mA~14 mA, average current 14 mA~16 mA, current step increase: 3 levels, powder consolidation times 1~2 times; Thermal compensation parameters before powder coating are: beam defocus: 0-0.5 V, rated current 10 mA, time: 10 s-12 s; After spreading the powder, the powder bed is preheated for sintering. The sintering parameters of the powder bed are: scanning speed 12 m / s~14 m / s, scanning spacing: 1 mm~1.5 mm, beam spot defocus: 0 V~0.5 V, rated current: 35 mA, and time: 10 s~12 s.

2. The method for preparing ultra-hard and high-strength M42 high-speed steel by electron beam powder bed fusion according to claim 1, characterized in that: The printing parameters are: electron beam current 12 mA~18 mA, scanning speed 5 m / s~6.5 m / s, beam spot diameter 0.1 mm~0.2 mm, defocus size 0 V~0.5 V, and powder size 95 μm~105 μm.

3. The method for preparing ultra-hard and high-strength M42 high-speed steel by electron beam powder bed fusion according to claim 1, characterized in that: The powder bed preheating method is step-up preheating. The preheating is divided into 3 levels, and the preheating current is gradually increased. The maximum preheating current is 35mA~40mA, and the starting current is 10mA~12mA.

4. The method for preparing ultra-hard and high-strength M42 high-speed steel by electron beam powder bed fusion according to claim 1, characterized in that: The scanning mode is unidirectional scanning, and the scanning parameters are: the rotation angle between layers is 90°, the scanning interval is 80 μm, and the powder thickness is 50 μm.

5. The method for preparing superhard and high-strength M42 high-speed steel by electron beam powder bed fusion according to claim 1, characterized in that: The substrate was preheated manually and was divided into three stages: preheating current of 5 mA~10 mA and scanning speed of 5 m / s~10 m / s to heat to 350℃~400℃, preheating current of 15 mA~20 mA and scanning speed of 10 m / s~20 m / s to heat to 600℃~700℃, preheating current of 25 mA~30 mA and scanning speed of 10 m / s~20 m / s to heat to 850℃ and keeping warm for 5 minutes~10 minutes, 5 minutes~10 minutes and 10 minutes~15 minutes respectively.

6. The method for preparing ultra-hard and high-strength M42 high-speed steel by electron beam powder bed fusion according to claim 1, characterized in that: Helium is the protective gas that is continuously introduced during the entire printing process.

7. The method for preparing ultra-hard and high-strength M42 high-speed steel by electron beam powder bed fusion according to claim 1, characterized in that: The particle size of M42 steel powder is 53 μm~120 μm. The composition of M42 steel powder includes, by mass percentage: C: 1.05%~1.15%, Cr: 3.50%~4.25%, W: 1.15%~1.85%, Mo: 9.0%~10.0%, V: 0.95%~1.35%, Co: 7.75%~8.75%, Mn: ≤0.40%, Si: ≤0.65%, S: ≤0.03%, P: ≤0.03%, and the balance is Fe.

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