阅读说明：本技术 一种激光增材制造24CrNiMo合金钢的热等静压方法 (Hot isostatic pressing method for manufacturing 24CrNiMo alloy steel by laser additive manufacturing ) 是由 梁京 林子杨 刘常升 陈岁元 郭洋 张宏伟 汤广全 田坤 赵宇辉 于 2021-08-13 设计创作，主要内容包括：本发明提供一种激光增材制造24CrNiMo合金钢的热等静压方法,具体步骤如下：(1)选区激光熔化：根据成形状态、样品表面平整度、气孔和裂纹对选区激光熔化技术进行工艺优化,确定工艺参数后使用粒径20-80μm的球形24CrNiMo合金钢粉末进行选区激光熔化,制得成形态SLM 24CrNiMo合金钢样品；(2)热等静压：确定成形态SLM 24CrNiMo合金钢样品的相转变点温度、缺陷问题和致密度,确定热等静压工艺参数将成形态SLM 24CrNiMo合金钢样品进行热等静压处理,所述工艺参数为：780-830℃保温1-4h,压力100-200MPa,缓冷5-15℃/min。利用激光增材制造的制备周期短、生产成本低、便于制备梯度成分样品、易于提升材料性能等优势制备24CrNiMo高性能合金钢的成形态样品,并通过热等静压后处理改善组织、致密度和韧性。(The invention provides a hot isostatic pressing method for manufacturing 24CrNiMo alloy steel by laser additive manufacturing, which comprises the following specific steps: (1) selective laser melting: carrying out process optimization on the selective laser melting technology according to the forming state, the surface flatness of the sample, air holes and cracks, and carrying out selective laser melting on spherical 24CrNiMo alloy steel powder with the particle size of 20-80 mu m after determining process parameters to prepare a shaped SLM 24CrNiMo alloy steel sample; (2) hot isostatic pressing: determining the phase transition point temperature, defect problems and density of the morphic SLM 24CrNiMo alloy steel sample, determining hot isostatic pressing process parameters, and carrying out hot isostatic pressing treatment on the morphic SLM 24CrNiMo alloy steel sample, wherein the process parameters are as follows: 780 and 830 ℃ for 1-4h, the pressure is 100 and 200MPa, and the temperature is slowly cooled for 5-15 ℃/min. The method has the advantages of short preparation period, low production cost, convenience in preparing gradient component samples, easiness in improving material performance and the like in laser additive manufacturing, is used for preparing the forming state samples of the 24CrNiMo high-performance alloy steel, and improves the structure, the density and the toughness through hot isostatic pressing post-treatment.)

1. A hot isostatic pressing method for manufacturing 24CrNiMo alloy steel by laser additive manufacturing is characterized in that: the method comprises the following specific steps:

(1) selective laser melting: carrying out process optimization on the selective laser melting technology according to the forming state, the surface flatness of the sample, air holes and cracks, and carrying out selective laser melting on spherical 24CrNiMo alloy steel powder with the particle size of 20-80 mu m after determining process parameters to prepare a shaped SLM 24CrNiMo alloy steel sample;

(2) hot isostatic pressing: determining the phase transition point temperature, defect problems and density of the shaped SLM 24CrNiMo alloy steel sample, and determining hot isostatic pressing process parameters according to the conditions to perform hot isostatic pressing treatment on the shaped SLM 24CrNiMo alloy steel sample, wherein the process parameters are as follows: 780 and 830 ℃ for 1-4h, the pressure is 100 and 200MPa, and the temperature is slowly cooled for 5-15 ℃/min.

2. The method for hot isostatic pressing of laser additive manufacturing of 24CrNiMo alloy steel according to claim 1, wherein: the 24CrNiMo alloy powder is 24CrNiMo alloy powder with the average grain diameter of 46.3 mu m prepared by using an aerosol method.

3. The method for hot isostatic pressing of laser additive manufacturing of 24CrNiMo alloy steel according to claim 1, wherein: the selective laser melting process comprises the following steps: the laser power is 300-.

4. The method for hot isostatic pressing of laser additive manufacturing of 24CrNiMo alloy steel according to claim 1, wherein: the selective laser melting adopts a laser, the laser adopts an EOS M290 metal material 3D printer, the laser power is 320W, the laser scanning speed is 950mm/s, the scanning interval is 110M, the laser spot diameter is 75M, the argon protection of a forming chamber is adopted, and the oxygen content is 0.05%.

5. The method for hot isostatic pressing of laser additive manufacturing of 24CrNiMo alloy steel according to claim 1, wherein: the sample size of the morphological SLM 24CrNiMo alloy steel is 100mm multiplied by 20mm multiplied by 15mm, and the compactness is more than 98%.

6. The method for hot isostatic pressing of laser additive manufacturing of 24CrNiMo alloy steel according to claim 1, wherein: the microstructure of the morphological SLM 24CrNiMo alloy steel sample is as follows: 1-5 μm lath bainite, granular bainite and 100-300nm lath tempered martensite in the HAZ region, and 100-300nm acicular bainite and less than 1 μm lath martensite in the AS region.

7. The method for hot isostatic pressing of laser additive manufacturing of 24CrNiMo alloy steel according to claim 1, wherein: the compactness of the morphological SLM 24CrNiMo alloy steel sample is 98.9%, the average microhardness is 458.2HV, the tensile strength is 1370MPa, the elongation is 11.2%, the yield ratio is 93.4%, the microstructure is divided into an AS area and an HAZ area, and the AS area is mainly 200nm needle-shaped bainite and lath martensite; the HAZ region is mainly composed of 1 μm lath bainite, granular bainite and 200nm lath tempered martensite.

8. The method for hot isostatic pressing of laser additive manufacturing of 24CrNiMo alloy steel according to claim 1, wherein: the hot isostatic pressing process parameters are as follows: keeping the temperature at 800 ℃ for 2h, keeping the pressure at 120MPa, and slowly cooling the mixture at 10 ℃/min.

9. The method for hot isostatic pressing of laser additive manufacturing of 24CrNiMo alloy steel according to claim 1, wherein: the compactness of a 24CrNiMo alloy steel material sample subjected to hot isostatic pressing post-treatment is more than 98 percent, and the sample structure is polygonal ferrite of 5-10 mu m, wherein spherical carbide of less than 100nm, granular pearlite with carbide of 100-300nm particles, massive bainite of 3-5 mu m and residual austenite exist.

10. The method for hot isostatic pressing of laser additive manufacturing of 24CrNiMo alloy steel according to claim 1, wherein: after hot isostatic pressing, the density of a 24CrNiMo alloy steel material sample is 99.5%, the average microhardness is 385.5HV, the tensile strength is 1100MPa, the elongation is 15.3%, and the yield ratio is 67.6%. The microstructure is 5-10 μm polygonal ferrite, wherein spherical carbide with the size of about 100nm exists, granular pearlite with the size of 100-300nm discontinuous grain carbide exists, 3-5 μm blocky bainite and residual austenite.

Technical Field

The invention relates to the technical field of additive manufacturing, in particular to a hot isostatic pressing method for manufacturing 24CrNiMo alloy steel by laser additive manufacturing.

Background

The running speed and the load of the train are continuously increased day before, the current running speed can reach 380km/h, and the load is more than 60 tons, so higher requirements on the stable organization and the toughness of the train brake disc are provided. The brake disc is used as a key part for safety braking of a high-speed train, the core radiating rib of the brake disc needs high toughness, and the surface part of the brake disc needs high hardness and wear resistance. The traditional manufacturing processes such as casting, forging and the like have the problems of complex preparation process, low material utilization rate and the like; the workpiece is easy to deform under the action of external force and heat treatment residual stress in the machining process; cracking and failure easily occur in severe service environment of high and cold areas. The traditional manufacturing process has high difficulty and high cost when preparing materials with higher requirements on precision and shape, so the hot isostatic pressing method for manufacturing 24CrNiMo alloy steel by laser additive manufacturing is developed to overcome the problems.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides a hot isostatic pressing method for manufacturing a 24CrNiMo high-performance alloy steel material by laser additive manufacturing. The method has the advantages of short preparation period, low production cost, convenience in preparing gradient component samples, easiness in improving material performance and the like in laser additive manufacturing, is used for preparing the forming state samples of the 24CrNiMo high-performance alloy steel, and improves the structure, the density and the toughness through hot isostatic pressing post-treatment.

The technical scheme of the invention is realized as follows:

a hot isostatic pressing method for manufacturing 24CrNiMo alloy steel by laser additive manufacturing comprises the following specific steps:

(1) selective laser melting: carrying out process optimization on the selective laser melting technology according to the forming state, the surface flatness of the sample, air holes and cracks, and carrying out selective laser melting on spherical 24CrNiMo alloy steel powder with the particle size of 20-80 mu m after determining process parameters to prepare a shaped SLM 24CrNiMo alloy steel sample;

(2) hot isostatic pressing: determining the phase transition point temperature, defect problems and density of the shaped SLM 24CrNiMo alloy steel sample, and determining hot isostatic pressing process parameters according to the conditions to perform hot isostatic pressing treatment on the shaped SLM 24CrNiMo alloy steel sample, wherein the process parameters are as follows: 780 and 830 ℃ for 1-4h, the pressure is 100 and 200MPa, and the temperature is slowly cooled for 5-15 ℃/min.

Preferably, the 24CrNiMo alloy powder is 24CrNiMo alloy powder with the average grain diameter of 46.3 μm prepared by using an aerosol method.

Preferably, the selective laser melting process is as follows: the laser power is 300-.

Preferably, the selective laser melting adopts a laser, the laser adopts an EOS M290 metal material 3D printer, the laser power is 320W, the laser scanning speed is 950mm/s, the scanning interval is 110M, the laser spot diameter is 75M, the forming chamber is protected by argon, and the oxygen content is 0.05%.

Preferably, the sample size of the SLM 24CrNiMo alloy steel is 100mm multiplied by 20mm multiplied by 15mm, and the compactness is more than 98%.

Preferably, the microstructure of the morphic SLM 24CrNiMo alloy steel sample is: 1-5 μm lath bainite, granular bainite and 100-300nm lath tempered martensite in the HAZ region, and 100-300nm acicular bainite and less than 1 μm lath martensite in the AS region.

Preferably, the compactness of the shaped SLM 24CrNiMo alloy steel sample is 98.9%, the average microhardness is 458.2HV, the tensile strength is 1370MPa, the elongation is 11.2%, the yield ratio is 93.4%, the microstructure is divided into an AS area and an HAZ area, and the AS area is mainly 200nm needle-shaped bainite and lath martensite; the HAZ region is mainly composed of 1 μm lath bainite, granular bainite and 200nm lath tempered martensite.

Preferably, the hot isostatic pressing process parameters are: keeping the temperature at 800 ℃ for 2h, keeping the pressure at 120MPa, and slowly cooling the mixture at 10 ℃/min.

Preferably, the density of a 24CrNiMo alloy steel material sample subjected to hot isostatic pressing post-treatment is more than 98%, and the sample structure is polygonal ferrite of 5-10 μm, wherein spherical carbides of less than 100nm, granular pearlite with 100-300nm granular carbides, massive bainite of 3-5 μm and retained austenite are present.

Preferably, after hot isostatic pressing, the density of a 24CrNiMo alloy steel material sample is 99.5%, the average microhardness is 385.5HV, the tensile strength is 1100MPa, the elongation is 15.3%, and the yield ratio is 67.6%. The microstructure is 5-10 μm polygonal ferrite, wherein spherical carbide with the size of about 100nm exists, granular pearlite with the size of 100-300nm discontinuous grain carbide exists, 3-5 μm blocky bainite and residual austenite.

The invention has the following beneficial effects: the selective laser melting technology has the advantages of one-time near-net shaping, preparation of a sample with gradient components, adjustability of the components and the like, so that the production period and the research and development time of a new product can be reduced, and the performance of the product can be improved. Because the selective laser melting technology has the capability of processing complex structural parts, the selective laser melting technology is more suitable for preparing high-speed train brake discs with supporting structures required inside and high requirements on dimensional accuracy. However, in the application process of the selective laser melting technology, defects are easily generated at the lap joints between powder layers of prepared samples and between laser beam melting channels. The hot isostatic pressing can enable defects such as air holes, porosity and the like to form metallurgical closure after the action of high temperature and high pressure, the structure is uniform and compact, the comprehensive mechanical property of the structure is effectively improved, the production period is shortened, and the cost is saved. According to the invention, hot isostatic pressing treatment is adopted after 24CrNiMo alloy steel is prepared by selective laser melting, the machining of the high-speed train brake disc with a complex structure is realized, the subsequent machining allowance is small, the period is short, the material is saved, and the cost is reduced. The hot isostatic pressing treatment is carried out on the basis of preparing the 24CrNiMo alloy steel sample by selective laser melting, so that the defect mainly comprising air holes is overcome, the density and the toughness of the material are improved, the yield ratio is reduced, the material obtains excellent comprehensive performance, and the material is better applied to a brake disc of a high-speed train. According to the invention, solid phase change of the 24CrNiMo alloy steel manufactured by laser additive manufacturing is researched, the influence of the grain size on the microstructure and toughness is researched, the temperature, pressure, time, cooling speed and the like of hot isostatic pressing are adjusted, and hot isostatic pressing process parameters are provided to balance the alloy steel structure, improve the toughness and improve the comprehensive performance.

The 24CrNiMo alloy steel prepared by selective laser melting is subjected to subsequent hot isostatic pressing treatment to obtain good comprehensive performance. The morphology of the 24CrNiMo alloy steel (SLM 24CrNiMo) prepared by selective laser melting and the structure and performance obtained after hot isostatic pressing are greatly different, so that the hot isostatic pressing post-treatment process parameters of the 24CrNiMo alloy steel prepared by the SLM are optimized by controlling the phase composition, the microstructure and the density of a 24CrNiMo alloy steel hot isostatic pressing state sample prepared by the SLM, and an excellent 24CrNiMo alloy steel selective laser melting and hot isostatic pressing process flow is provided for preparing a high-performance high-speed train brake disc core material.

The selective laser melting method comprises the following steps: spherical 24CrNiMo alloy steel powder with the particle size of 20-80 mu M is used, argon is used as protective gas on an EOS M290 metal material 3D printer, and zone selection laser melting is carried out by using laser power of 300-340W, laser scanning speed of 900-1000mm/s, scanning interval of 100-120 mu M and laser spot diameter of 70-80 mu M to prepare a sample. The density of the SLM 24CrNiMo alloy steel is more than 98 percent, and the microstructure thereof is as follows: 1-5 μm lath bainite, granular bainite and 100-plus 300nm lath tempered martensite in the HAZ region, and 100-plus 300nm needle bainite and lath martensite less than 1 μm in the AS region have finer structures, which is greatly helpful for improving the performance.

The hot isostatic pressing process for the SLM 24CrNiMo alloy steel sample after hot isostatic pressing under different parameters is finally determined to be the following steps: the working pressure is 100-200MPa, the pressure medium is high-purity argon, the temperature is kept for 1-4h at 780-830 ℃, and the pressure is released after the temperature is cooled to 200-300 ℃ at the speed of 5-15 ℃/min. The microstructure of the SLM 24CrNiMo alloy steel after the hot isostatic pressing treatment is as follows: 5-10 μm polygonal ferrite in which spherical carbides < 100nm, granular pearlite in which 100-300nm granular carbides are present, massive bainite in 3-5 μm and retained austenite are present. The density is more than 99 percent, the tensile strength reaches 1100MPa level, the elongation reaches 15 to 20 percent, and the yield ratio is reduced to 60 to 70 percent. In the material structure prepared by the process, two nano-scale carbides exist, and one of the nano-scale carbides exists in granular pearlite with better performance. The nano-scale carbide has obvious effect on the strength and hardness of alloy steel, and the granular pearlite can better improve the toughness of the material and is an indispensible special structure in the alloy steel. The nano carbide capable of remarkably improving the performance is obtained by selecting appropriate parameters for hot isostatic pressing after 24CrNiMo alloy steel is prepared by selective laser melting, wherein the preparation process and the post-treatment process are both indispensable. The alloy steel with proper components needs to be screened in material selection, and the parameters of the material prepared by selective laser melting are difficult to adjust technically so as to obtain bainite and martensite with proper shapes and sizes. In the aspect of hot isostatic pressing post-treatment, the cooling temperature and time have great influence on the formation of nano-scale carbides and other structures, and the parameter needs to be obtained after a plurality of times of debugging and is a technical difficulty to be overcome.

Drawings

FIG. 1 is a photograph of the microstructure of a selective laser melting 24 CrNiMo; (a) HAZ and AS zone tissues, (b) HAZ zone micro-zone tissue morphology, and (c) AS zone micro-zone tissue morphology.

FIG. 2 is a SEM microstructure picture after hot isostatic pressing of selected area laser additive manufacturing 24 CrNiMo; (a) x 5000, (b) x 50000.

Detailed Description

For a more clear understanding of the technical features, objects and advantages of the present invention, reference is now made to the following detailed description of the embodiments of the present invention taken in conjunction with the accompanying drawings, which are included to illustrate and not to limit the scope of the present invention.

Example 1

The invention discloses a hot isostatic pressing method for manufacturing 24CrNiMo alloy steel by laser additive manufacturing, which comprises the following steps:

step 1: selective laser melting:

according to the grain diameter of alloy steel powder, a single-pass optimized preparation forming process is adopted, and the technological parameters of selective laser melting are determined to prepare a sample. 24CrNiMo alloy powder with the average grain diameter of 46.3 mu m prepared by an aerosol method is subjected to selective laser melting, and argon is used as protective gas. The selective laser melting process of the embodiment comprises the following steps: the laser power is 320W, the laser scanning speed is 950mm/s, the scanning interval is 110m, the laser spot diameter is 75m, and the forming chamber is protected by argon (the oxygen content is 0.05%). The laser uses an EOS M290 metallic material 3D printer.

Through detection, the density of a formed SLM 24CrNiMo alloy steel sample is 98.9%, the average microhardness is 458.2HV, the tensile strength is 1370MPa, the elongation is 11.2%, the yield ratio is 93.4%, the microstructure is divided into an AS area and an HAZ area, and the AS area is mainly 200nm needle-shaped bainite and lath martensite; the HAZ region is mainly composed of 1 μm lath bainite, granular bainite and 200nm lath tempered martensite.

Step 2: hot isostatic pressing:

and determining hot isostatic pressing process parameters including heating temperature, holding time, pressure and cooling speed according to the compactness, microcrack and grain size of the formed sample. When the hot isostatic pressing temperature is selected, the alloy steel is heated at a temperature appropriate for the transformation point or higher, depending on the microstructure of the alloy steel in which the temperature is integrated into a morphology. The pearlite-austenite critical transformation temperature Ac1 point of the 24CrNiMo alloy steel is 740 ℃; the ferrite-austenite critical transformation temperature Ac3 point is 770 ℃, and the temperature is selected to be 780-830 ℃. And reasonably selecting the heat preservation time according to the size of the sample and the size of the formed crystal grains, so that the phase change is completed and the crystal grains are not excessively grown. The influence of the pressure on the tissue is not obvious, and the cooling speed is determined to be 5-15 ℃/min within the range of 100-200 MPa.

The hot isostatic pressing process described in this example was: keeping the temperature at 800 ℃ for 2 h; the pressure is 120 MPa; slowly cooling at 10 deg.C/min.

Through detection, the density of a sample subjected to hot isostatic pressing is 99.5%, the average microhardness is 385.5HV, the tensile strength is 1100MPa, the elongation is 15.3%, and the yield ratio is 67.6%. The microstructure is 5-10 μm polygonal ferrite, wherein spherical carbide with the size of about 100nm exists, granular pearlite with the size of 100-300nm discontinuous grain carbide exists, 3-5 μm blocky bainite and residual austenite. The yield ratio of the 24CrNiMo alloy steel after the hot isostatic pressing treatment is reduced by 25.8%, the compactness is improved by 0.6%, the elongation is improved by 4.1%, and the 24CrNiMo alloy steel has good comprehensive mechanical properties such as strength, plasticity and toughness and the like.

From FIG. 1 it can be seen that the microstructure of the AS-formed sample of the selected region laser melted 24CrNiMo consists of 1-5 μm lath bainite in the HAZ region, granular bainite and 100-300nm lath tempered martensite in the 100-300nm lath region and 100-300nm needle bainite in the AS region and lath martensite < 1 μm. As can be seen from FIG. 2, the microstructure of the sample after the 24CrNiMo hot isostatic pressing treatment in the laser additive manufacturing process is 3-5 μm blocky bainite, granular pearlite and 5-10 μm polygonal ferrite, wherein spherical carbide precipitated phases with the size of about 100nm exist in the polygonal ferrite, the carbide in the granular pearlite structure is in an intermittent granular shape, and the size of the carbide is 100-300 nm. Therefore, the hot isostatic pressing treatment is carried out on the basis of preparing the 24CrNiMo alloy steel sample by selective laser melting, so that the defect mainly comprising air holes is overcome, the density and the toughness of the material are improved, the yield ratio is reduced, the material obtains excellent comprehensive performance, and the material is better applied to the brake disc of a high-speed train.

The above embodiments are merely provided to help understand the method and core principle of the present invention, and the main steps and embodiments of the present invention are described in detail by using specific examples. To those skilled in the art, the various conditions and parameters may be varied as desired in a particular implementation in accordance with the principles of the invention, and in view of the foregoing, the description is not to be taken as limiting the invention.