Method for improving impact toughness of electric arc additive manufacturing martensitic stainless steel through nitrogen alloying mode

Abstract

The invention relates to a method for improving the impact toughness of electric arc additive manufactured martensitic stainless steel in a nitrogen alloying mode, which comprises the following steps of: placing dried martensitic stainless steel powder in a mixed gas atmosphere consisting of nitrogen and argon, melting the martensitic stainless steel powder by using an electric arc as a heat source, and then carrying out electric arc additive manufacturing, after additive manufacturing is completed, cooling to room temperature in a mixed gas atmosphere to obtain an additive manufacturing component; wherein in the electric arc additive manufacturing process, the interlayer temperature is kept between 200 DEG C and 300 DEG C. According to the method, nitrogen can be effectively used as an alloy element to be added into an additive structure, the grain size can be effectively reduced, the proportion of austenite can be regulated and controlled through adding of the nitrogen element, the content of ferrite is greatly reduced, and the impact toughness of the component is improved. Especially, the ferrite serves as a key phase for reducing the impact toughness of the additive structure, and the toughness can be greatly improved by reducing the content of the ferrite.

Description

Technical Field

[0001] The invention belongs to the technical field of metal additive manufacturing and welding, and in particular relates to a method for improving the impact toughness of martensitic stainless steel manufactured by arc additive manufacturing by nitrogen alloying. Background Art

[0002] Martensitic stainless steel is known for its high strength, high toughness and good corrosion resistance, but when it is used in additive manufacturing, its toughness, especially impact toughness, is greatly reduced, making it difficult to meet service requirements.

[0003] Normally, martensitic stainless steel is improved in impact toughness by rolling annealing. Taking common martensitic precipitation hardened stainless steel as an example, the impact energy of the rolled annealed base material is 60J to 110J, while the impact energy of the additively manufactured base material is generally less than 50J. The impact energy of additively manufactured martensitic precipitation hardened stainless steel is significantly lower than that of the base material, which will lead to brittle fracture and failure of the material during use. Therefore, how to improve the impact toughness of martensitic stainless steel additive organization is a key step in promoting the industrial application of martensitic steel additive manufacturing technology. Summary of the invention

[0004] In view of the deficiencies of the above-mentioned prior art, the object of the present invention is to provide a method for improving the impact toughness of martensitic stainless steel manufactured by arc additive manufacturing by nitrogen alloying, wherein nitrogen can be effectively added to the additive structure as an alloying element. The addition of nitrogen can effectively reduce the grain size, regulate the proportion of austenite, and significantly reduce the ferrite content. In particular, ferrite is a key phase for reducing the impact toughness of the additive structure, and reducing its content will significantly improve the toughness.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0006] The first implementation scheme provided by the present invention is to provide a method for improving the impact toughness of martensitic stainless steel manufactured by arc additive manufacturing by nitrogen alloying, comprising the following steps:

[0007] The dried martensitic stainless steel powder is placed in a mixed gas atmosphere composed of nitrogen and argon, and the martensitic stainless steel powder is melted by an electric arc as a heat source. The melted martensitic stainless steel powder is subjected to multi-layer and multi-pass arc additive manufacturing. After the additive manufacturing is completed, it is cooled to room temperature in the mixed gas atmosphere to obtain an additively manufactured component. During the arc additive manufacturing process, the interlayer temperature is maintained at 200°C to 300°C.

[0008] Preferably, the purity of the nitrogen and argon are both greater than or equal to 99.99%, and the volume percentage of nitrogen in the mixed gas composed of nitrogen and argon is 12.5% ​​to 37.5%.

[0009] Preferably, before additive manufacturing, the time for filling the mixed gas is greater than or equal to 5 minutes, and during the cooling process, the time for filling the mixed gas is greater than or equal to 5 minutes.

[0010] Preferably, the martensitic stainless steel powder is dried in a vacuum environment, wherein the vacuum degree is maintained at 10 -3 Pa or less, the drying temperature is 200℃~500℃, and the drying time is greater than or equal to 8 hours.

[0011] Preferably, the additive manufacturing method is any one of plasma arc welding, tungsten inert gas welding or metal inert gas shielded welding.

[0012] Preferably, the martensitic stainless steel powder material is any one of 17-4PH stainless steel, 15-4PH stainless steel, 4Cr13 stainless steel or 0Cr18Ni9 stainless steel.

[0013] Preferably, the process parameters of arc additive manufacturing are: current of 85A, printing speed of 2.57mm / s, powder feeding speed of 6g / min, shielding gas flow of 10L / min, ion gas of 1.2L / min, wherein both shielding gas and ion gas are argon. Ion gas is argon gas used to generate plasma arc.

[0014] The second implementation scheme provided by the present invention is to provide a method for improving the impact toughness of arc additively manufactured martensitic stainless steel by nitrogen alloying to prepare a martensitic stainless steel component with impact toughness.

[0015] Preferably, an impact-resistant V-shaped cut is cut in the middle of the top surface of the martensitic stainless steel component, the radius of the V-shaped cut is 0.25 mm, and then the V-shaped cut is polished until the surface is smooth to prepare the experimental sample.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] 1. The method for improving the impact toughness of martensitic stainless steel produced by arc additive manufacturing by nitrogen alloying provided by the present invention can improve the impact toughness of martensitic stainless steel additive structure. This is because the martensitic stainless steel is produced by arc additive manufacturing in an atmosphere of a mixed gas of nitrogen and argon to achieve the goal of adding nitrogen to the additive component. The nitrogen element entering the component exists in the form of nitrogen atoms and nitrogen compounds. First, nitrogen, as an austenite (γ) forming element, can effectively promote the formation of austenite and reduce the amount of high-temperature ferrite (δ) phase, while excessive δ will be distributed in a network along the original γ grain boundary, significantly reducing the impact toughness of the component; secondly, the nitride (TiN) present in the component can significantly refine the grains, reduce the tendency of the δ phase to be distributed in a network, and improve the toughness.

[0018] After testing, it was found that the room temperature impact toughness of the additively manufactured component prepared by the present invention is greater than the impact toughness value of the base material at room temperature, and the impact toughness value of the additively manufactured component prepared by the present invention at room temperature is increased by 47% compared with the impact toughness value of the base material at room temperature, thereby overcoming the problem in the prior art that the impact energy of the additively manufactured martensitic precipitation hardened stainless steel is significantly lower than the impact energy of the base material, which may lead to brittle fracture and failure of the material during use.

[0019] 2. The content of nitrogen in the additively manufactured component obtained by the present invention is 0.2wt% to 1.5wt%. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of protective tooling for arc additive manufacturing and the additive manufacturing process.

[0021] Figure 2 Dimensions of impact specimens for arc additively manufactured components Figure 1 .

[0022] Figure 3 Dimensions of impact specimens for arc additively manufactured components Figure 2 .

[0023] Figure 4 This is a scanning electron microscope secondary electron image of the impact fracture of the martensitic stainless steel additively manufactured component in Example 1.

[0024] Figure 5 This is a scanning electron microscope secondary electron image of the impact fracture of the martensitic stainless steel additively manufactured component in Example 2.

[0025] Figure 6 This is the secondary electron scanning electron microscope image of the impact fracture of the martensitic stainless steel additively manufactured component in Control Example 1.

[0026] Figure 7 The nitrogen oxygen analyzer in Example 1 measures the nitrogen content in the additively manufactured component.

[0027] Figure 8 The nitrogen oxygen analyzer in Example 2 measures the nitrogen content in the additively manufactured component.

[0028] Fig. 9 The nitrogen oxygen analyzer in Control Example 1 measures the nitrogen content in the additively manufactured component.

[0029] Fig.10 Impact toughness of arc additively manufactured martensitic stainless steel components and the nitrogen-argon volume ratio in the protective atmosphere.

[0030] Reference numerals

[0031] 1. Plasma arc welding gun; 2. Sealing protection tooling; 3. Pressure reducing valve; 4. Ventilation pipe; 5. Mixed gas storage device; 6. Components; 7. Base plate. DETAILED DESCRIPTION

[0032] The specific embodiments of the present invention are described in detail below, but it should be understood that the protection scope of the present invention is not limited by the specific embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work belong to the protection scope of the present invention. The experimental methods described in the embodiments of the present invention are conventional methods unless otherwise specified.

[0033] The inventors have found that the performance of martensitic precipitation hardening stainless steel can be greatly improved when a certain amount of nitrogen is contained in it. In addition, the nitrogen in the stainless steel can partially or even completely replace the expensive nickel, which has significant economic benefits. At the same time, the nitrogen content in the steel is relatively easy to accurately control.

[0034] The technical solution of the present invention is further illustrated in the form of specific examples. It is found through experiments that any martensitic stainless steel powder of 17-4PH stainless steel, 15-4PH stainless steel, 4Cr13 stainless steel or 0Cr18Ni9 stainless steel and any additive method of plasma arc welding, tungsten inert gas welding or metal inert gas shielded welding can be applied to the method of improving the impact toughness of arc additive manufacturing martensitic stainless steel by nitrogen alloying to prepare martensitic stainless steel components with impact toughness. The following is an example of 17-4PH martensitic stainless steel powder and plasma arc welding additive method.

[0035] The equipment used for the experiment is Figure 1 As shown, a device for improving the impact toughness of martensitic stainless steel manufactured by plasma arc additive manufacturing is provided, the device comprises a sealing protective tool 2, an arc plasma welding gun 1 for heating martensitic stainless steel powder, and a mixed gas storage device 5, wherein the martensitic stainless steel powder is added to the powder container of the welding equipment, sprayed out from the welding gun through the powder delivery gas, and melted by the arc to form a deposited body for accumulation; the mixed gas storage device 5 is used to store a mixed gas of nitrogen and argon, and the mixed gas storage device is connected to the sealing protective tool 2 through a pipe 4 and a pressure reducing valve 3; the substrate 7 is horizontally arranged in the sealing protective tool 2, and its top surface is used to receive the component 6 manufactured by the arc additive manufacturing. Before additive manufacturing, the surface of the substrate 7 is polished by a grinder to make the oxide layer of the substrate 7 fall off and expose the metallic luster, and the substrate 7 is preheated at the same time.

[0036] Example 1

[0037] Arc additive manufacturing of components using 17-4PH martensitic stainless steel powder material includes the following steps:

[0038] 1) Place the powder in a vacuum drying oven and keep the vacuum degree of the drying oven at 10 -3 Pa, the drying temperature was set to 400°C, and the drying time was 8 hours.

[0039] 2) Place a substrate 7 made of 45 steel into a sealing protective tooling 2. Before the start of additive manufacturing, a mixed gas is introduced into the sealing protective tooling 2 for 5 minutes. The mixed gas in the mixed gas storage device 5 is prepared in advance. The volume ratio of nitrogen to argon in the mixed gas is 1:7, that is, the percentage of nitrogen in the mixed gas is 12.5%.

[0040] 3) Before additive manufacturing, a grinder is used to grind the surface of the substrate 7 to remove the oxide layer and expose the metallic luster. The substrate 7 is preheated. The process parameters of arc additive manufacturing are: current 85A, printing speed 2.57mm / s, powder feeding speed 6g / min, shielding gas flow 10L / min, ion gas 1.2L / min.

[0041] 4) During the additive manufacturing process, the arc is used as a heat source to melt the 17-4PH martensitic stainless steel powder. The melted 17-4PH martensitic stainless steel powder is subjected to plasma arc additive manufacturing. The interlayer temperature needs to be maintained at 200°C, that is, when starting a new layer of additive manufacturing, the temperature of the formed layer should be maintained at 200°C.

[0042] 5) A plasma arc welding gun 1 is used to perform additive manufacturing at the position of the substrate 7. The mixed gas is continuously introduced during the additive manufacturing process. After the additive manufacturing is completed, the mixed gas is continued to be introduced for 5 minutes to obtain additive manufacturing 6.

[0043] Embodiment 2,

[0044] Arc additive manufacturing of components using 17-4PH martensitic stainless steel powder material includes the following steps:

[0045] 1) Place the powder in a vacuum drying oven and keep the vacuum degree of the drying oven at 10 -3 Pa, the drying temperature was set to 400°C and the drying time was 8 hours.

[0046] 2) Place a substrate 7 made of 45 steel into a sealing protective tooling 2. Before the start of additive manufacturing, a mixed gas is introduced into the sealing protective tooling 2 for no less than 5 minutes. The mixed gas in the mixed gas storage device 5 is prepared in advance. The volume ratio of nitrogen to argon in the mixed gas is 3:5, that is, the percentage of nitrogen in the mixed gas is 37.5%.

[0047] 3) Before additive manufacturing, a grinder is used to grind the surface of the substrate 7 to remove the oxide layer and expose the metallic luster. The substrate 7 is preheated. The process parameters of arc additive manufacturing are: current 85A, printing speed 2.57mm / s, powder feeding speed 6g / min, shielding gas flow 10L / min, ion gas 1.2L / min.

[0048] 4) During the additive manufacturing process, the arc is used as a heat source to melt the 17-4PH martensitic stainless steel powder. The melted 17-4PH martensitic stainless steel powder is subjected to plasma arc additive manufacturing. The interlayer temperature needs to be maintained at 200°C, that is, when starting a new layer of additive manufacturing, the temperature of the formed layer should be maintained at 200°C.

[0049] 5) A plasma arc welding gun 1 is used to perform additive manufacturing at the position of the substrate 7. The mixed gas is continuously introduced during the additive manufacturing process. After the additive manufacturing is completed, the mixed gas is continuously introduced for no less than 5 minutes to obtain an additive component 6.

[0050] Comparison Example

[0051] The method for improving the impact toughness of a component is basically the same as that given in Example 1, except that the mixed gas is changed to pure argon gas.

[0052] The additive component 6 obtained in the above-mentioned Examples 1 to 2 and Comparative Example 1 was subjected to data analysis.

[0053] Before conducting experimental analysis, it is necessary to make experimental samples. That is, cut an impact-resistant V-shaped cut in the middle of the top surface of all components, and the radius of the V-shaped cut is 0.25mm. Figure 2-3 As shown, then use 240#, 400#, 800#, and 1000# sandpaper to grind the V-shaped cut until the surface is smooth.

[0054] 1) Morphology analysis

[0055] like Figure 4 , Figure 5 and Figure 6 The following are the secondary electron images of the impact fracture of the 17-4PH stainless steel components prepared in Example 1, Example 2 and Comparative Example 1 of the present invention. Figure 3 to Figure 5 It was found that the fracture surface of the stainless steel component prepared by adding the mixed gas containing nitrogen is still ductile fracture, but with the increase of nitrogen content, the dimple size increases significantly and becomes shallower. In addition, by observing the second phase particles in the dimple, it is found that the second phase particles are more dispersed and finer after adding nitrogen than before alloying. It shows that after adding nitrogen, there is still obvious ductile fracture, and the dimple is large and deep, which are all manifestations of good impact toughness.

[0056] 2) Nitrogen and oxygen analyzer analysis

[0057] Depend on Figure 7 , Figure 8 and Fig. 9 Shown are the nitrogen content determination diagrams of 17-4PH stainless steel components prepared in Example 1, Example 2 and Control Example 1, respectively. By comparison, it is found that when the percentage of nitrogen in the mixed gas is 37.5%, the average content of nitrogen in the component is 0.205wt%, and when the percentage of nitrogen in the mixed gas is 12.5%, the average content of nitrogen in the component is 0.0703wt%, which are both higher than the average content of nitrogen in the component obtained under a pure argon atmosphere, i.e., 0.0641wt%. Since the value of 0.0641wt% is very small, it can be approximately equal to 0. Therefore, the nitrogen content in the additively manufactured component determined by a nitrogen oxygen analyzer can prove that the plasma arc additive manufacturing method of 17-4PH stainless steel components in a nitrogen-containing atmosphere can achieve the purpose of adding nitrogen to the component.

[0058] The components obtained in the nitrogen atmosphere were further impacted, and the impact results were as follows: Fig.10 It can be seen that when the current is 85A, the impact toughness value before alloying is 30.965J. After different proportions of nitrogen-argon mixed gas are introduced into the atmosphere hood, it is found that the impact toughness of the 17-4PH stainless steel component is improved to varying degrees. When the nitrogen-argon ratio is 1:7 (that is, the percentage of nitrogen in the mixed gas is 12.5%), the impact toughness is increased to 35.9J; when the nitrogen-argon mixed gas is 3:5 (the percentage of nitrogen in the mixed gas is 37.5%), the impact toughness of the 17-4PH stainless steel component in the Z direction is maximized, the impact toughness reaches 45.765J, and the toughness is increased by 47%. In summary, it can be seen that the present invention can significantly improve the room temperature impact toughness of 17-4PH stainless steel through nitrogen alloying.

[0059] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A method for improving the impact toughness of martensitic stainless steel manufactured by arc additive manufacturing by nitrogen alloying, characterized in that: The steps include: The dried martensitic stainless steel powder is placed in a mixed gas atmosphere composed of nitrogen and argon, and the martensitic stainless steel powder is melted by an electric arc as a heat source. The melted martensitic stainless steel powder is subjected to multi-layer and multi-pass arc additive manufacturing. After the additive manufacturing is completed, it is cooled to room temperature in the mixed gas atmosphere to obtain an additively manufactured component. During the arc additive manufacturing process, the interlayer temperature is maintained at 200°C to 300°C.

2. The method for improving the impact toughness of martensitic stainless steel produced by arc additive manufacturing by nitrogen alloying as claimed in claim 1, characterized in that: The purity of the nitrogen and argon are both greater than or equal to 99.99%, and the volume percentage of nitrogen in the mixed gas composed of nitrogen and argon is 12.5% ​​to 37.5%.

3. The method for improving the impact toughness of martensitic stainless steel produced by arc additive manufacturing by nitrogen alloying as claimed in claim 1, characterized in that: Before additive manufacturing, the time for filling the mixed gas is greater than or equal to 5 minutes, and during the cooling process, the time for filling the mixed gas is greater than or equal to 5 minutes.

4. The method for improving the impact toughness of martensitic stainless steel produced by arc additive manufacturing by nitrogen alloying as claimed in claim 1, characterized in that: The martensitic stainless steel powder is dried in a vacuum environment, where the vacuum degree is maintained at 10 -3 Pa or less, the drying temperature is 200℃~500℃, and the drying time is greater than or equal to 8 hours.

5. The method for improving the impact toughness of martensitic stainless steel produced by arc additive manufacturing by nitrogen alloying as claimed in claim 1, characterized in that: The additive manufacturing method is any one of plasma arc welding, argon tungsten arc welding or metal inert gas shielded welding.

6. The method for improving the impact toughness of martensitic stainless steel produced by arc additive manufacturing by nitrogen alloying as claimed in claim 1, characterized in that: The martensitic stainless steel powder material is any one of 17-4PH stainless steel, 15-4PH stainless steel, 4Cr13 stainless steel or 0Cr18Ni9 stainless steel.

7. The method for improving the impact toughness of martensitic stainless steel produced by arc additive manufacturing by nitrogen alloying as claimed in claim 1, characterized in that: The process parameters of arc additive manufacturing are: current of 85A, printing speed of 2.57mm / s, powder feeding speed of 6g / min, shielding gas flow rate of 10L / min, and ion gas of 1.2L / min, where both shielding gas and ion gas are argon.

8. The method for improving the impact toughness of martensitic stainless steel produced by arc additive manufacturing by nitrogen alloying as claimed in any one of claims 1 to 7, wherein a martensitic stainless steel component having impact toughness is prepared.

9. The martensitic stainless steel component having impact resistance toughness according to claim 8, characterized in that: An impact-resistant V-shaped cutout was cut in the middle of the top surface of the martensitic stainless steel component, the radius of the V-shaped cutout was 0.25 mm, and then the V-shaped cutout was polished until the surface was smooth to prepare an experimental sample.

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