A preparation method for improving the strength and toughness of additively manufactured 17-4PH stainless steel

Abstract

The present invention discloses a preparation method for improving the strength and toughness of additively manufactured 17‑4PH stainless steel in the field of metal material heat treatment technology; the preparation method comprises: forming a 17‑4PH stainless steel component by an additive manufacturing process; and heat treating the 17‑4PH stainless steel component obtained by additive manufacturing, wherein the heat treatment process sequentially comprises direct aging treatment, cryogenic treatment, and tempering treatment. The present invention utilizes a laser additive manufacturing process to form a 17‑4PH stainless steel component, and then performs a specific heat treatment on the component to optimize and improve the strength and toughness of the component. The method is simple in process and can significantly overcome the strength-toughness mismatch problem and the risk of deformation and cracking caused by traditional high-temperature solid solution plus aging heat treatment, thereby significantly improving the strength and toughness of laser additively manufactured 17‑4PH stainless steel and ensuring the service stability and safety of laser additively manufactured 17‑4PH stainless steel components.

Description

Technical Field

[0001] The present invention belongs to the technical field of heat treatment of metal materials, and more specifically, relates to a preparation method for improving the strength and toughness of additively manufactured 17-4PH stainless steel. Background Art

[0002] 17-4PH martensitic precipitation-hardening stainless steel, with its excellent mechanical properties and corrosion resistance, has been widely used in aerospace, offshore platforms, petrochemicals, and the nuclear energy industry. Traditional casting, forging, and powder metallurgy technologies, due to inherent flaws such as complex processes, high costs, and low material utilization, struggle to meet the high-precision, high-reliability manufacturing requirements of modern complex components. Laser additive manufacturing technology, with its high degree of forming freedom, short manufacturing cycles, and the absence of molds, enables the integrated production of complex structural parts. It has been successfully applied to the printing and production of components from a variety of metal materials, including 17-4PH stainless steel. Unlike 17-4PH stainless steel produced by traditional casting / forging processes, 17-4PH stainless steel produced by additive manufacturing is primarily composed of δ-ferrite, austenite, and martensite, and the ratios of these phases can be adjusted through process parameters. However, the main strengthening mechanism of this stainless steel comes from the precipitation hardening effect produced during the aging process. For 17-4PH stainless steel manufactured by laser additive manufacturing, both the traditional solid solution plus aging heat treatment process and the direct aging process after printing result in an increase in material strength while significantly reducing its toughness, seriously affecting the service safety and stability of the formed components.

[0003] To address the inversion of strength and toughness in laser additively manufactured 17-4PH stainless steel, researchers have employed methods such as optimizing laser process parameters, regulating alloy composition, and improving heat treatment regimes, aiming to alter the alloy's strength and toughness by controlling the alloy phase content. However, most laser additively manufactured 17-4PH stainless steels processed using these methods require peak aging heat treatment to increase their strength, often accompanied by a significant decrease in the material's toughness. The mismatch between strength and toughness remains a significant challenge for laser additively manufactured 17-4PH stainless steel. Summary of the Invention

[0004] The purpose of the present invention is to address the above shortcomings and provide a preparation method and 17-4PH stainless steel for improving the strength and toughness of 17-4PH stainless steel. For 17-4PH stainless steel formed by laser additive manufacturing, the strength and toughness mismatch problem caused by traditional heat treatment is overcome, and the strength and toughness of 17-4PH stainless steel formed by laser additive manufacturing can be greatly improved, thereby ensuring the service stability and safety of 17-4PH stainless steel components formed by laser additive manufacturing; and the heat treatment process is simplified, avoiding the structural deformation and cracking problems caused by traditional high-temperature solution treatment.

[0005] To achieve the above object, the present invention is implemented through the following technical solutions:

[0006] In a first aspect, the present invention provides a preparation method for improving the strength and toughness of additively manufactured 17-4PH stainless steel, comprising:

[0007] 17-4PH stainless steel components formed by additive manufacturing;

[0008] The 17-4PH stainless steel component obtained by additive manufacturing is subjected to heat treatment, and the heat treatment process sequentially includes direct aging treatment, deep cooling treatment and tempering treatment; wherein the direct aging treatment includes: heating the 17-4PH stainless steel component to 250°C~550°C, holding the temperature for 0.01 h~100 h, and then air cooling to room temperature.

[0009] Preferably, the additive manufacturing process includes a selective laser melting process, a laser melting deposition process or an electron beam selective melting process.

[0010] Preferably, the parameters of the laser additive manufacturing process include: laser power 100W~1000W, scanning speed 100mm / s~2000 mm / s, powder layer thickness 0.02mm~0.15mm, scanning spacing 0.01mm~0.15mm, layer-by-layer remelting power 100W~1000W, scanning speed 100 mm / s~2000mm / s, scanning spacing 0.01mm~0.15mm; the preferred parameters are: laser power 250W, scanning speed 800mm / s, powder layer thickness 0.04mm, scanning spacing 0.1mm, layer-by-layer remelting power 250W, layer-by-layer remelting scanning speed 800mm / s, and layer-by-layer remelting scanning spacing 0.1mm.

[0011] Preferably, the density of the 17-4PH stainless steel component formed by the laser additive manufacturing process is above 99%.

[0012] The direct aging treatment described above allows the precipitation of nanoscale copper-rich phases in the matrix, retaining a small amount of austenite and fine δ-ferrite. This significantly increases the material's yield strength while also improving tensile strength and ductility, thereby improving the strength-ductility trade-off. The critical phase transition temperature for 17-4PH stainless steel from face-centered cubic (FCC) to body-centered cubic (BCC) structure is reached at 550°C, and temperatures exceeding 550°C easily induce matrix phase transformation.

[0013] More preferably, the temperature of the direct aging treatment is 300° C. to 482° C., and the holding time is 0.1 h to 5 h.

[0014] Preferably, the cryogenic treatment comprises: placing the 17-4PH stainless steel component in a cryogenic environment, keeping it warm for more than 1 hour, and returning it to room temperature in the air; the temperature of the cryogenic environment is ≤-50°C.

[0015] Cryogenic treatment can regulate the kinetics of austenite-martensite phase transformation, and by increasing the kinetics of the phase transformation zone, it can promote the continued transformation of retained austenite in steel into martensite.

[0016] Preferably, the cryogenic environment is a liquid nitrogen environment, and the preferred temperature is -196°C.

[0017] Preferably, the tempering treatment includes heating the 17-4PH stainless steel component to 100°C to 300°C, holding the temperature for 0.1 to 24 hours, and then air-cooling it to room temperature. Tempering reduces residual stress, retains high hardness, eliminates some brittleness, promotes the decomposition of retained austenite into a stable phase, and adjusts the balance between hardness and toughness.

[0018] In a second aspect, the present invention provides a 17-4PH stainless steel, which is prepared by the preparation method for improving the strength and toughness of 17-4PH stainless steel described in the first aspect.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] The present invention utilizes a laser additive manufacturing process to form 17-4PH stainless steel components, and then performs a specific heat treatment on the components. The two synergistically optimize the strength and toughness of the components. This method is simple in process and can significantly overcome the strength-toughness mismatch problem and the risk of deformation and cracking caused by traditional solid solution plus aging heat treatment. It greatly improves the strength and toughness of 17-4PH stainless steel manufactured by laser additive manufacturing, and ensures the service stability and safety of 17-4PH stainless steel components formed by laser additive manufacturing.

[0021] The heat treatment method provided by the present invention suppresses the rapid hardening and embrittlement tendency of the copper-rich phase in the martensite matrix at the peak aging temperature by regulating the precipitation kinetics through low-temperature aging in the direct aging stage, and can also achieve high-density dispersed precipitation of the strengthening phase. Subsequently, deep cryogenic treatment is combined to induce metastable austenite phase transformation to promote the formation of precipitation-free martensite, and subsequent tempering treatment is combined to reduce residual stress. Finally, by constructing multiple heterogeneous structures, the synergistic optimization of the strength and toughness of the component is promoted. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 A diagram of the heat treatment scheme provided by the present invention;

[0023] Figure 2 Comparison of scanning electron micrographs of the printed sample without heat treatment and the aged sample in Example 3; (a) is the printed sample; (b) is the aged sample in Example 3;

[0024] Figure 3Comparison of stress-strain curves and strength-impact toughness synergistic analysis diagrams of Example 1 and 17-4PH stainless steel in the forged solid solution-aged state, additive solid solution-aged state, and additive printing state;

[0025] Figure 4 Comparison of stress-strain curves and strength-impact toughness synergistic analysis diagrams of Example 2 and 17-4PH stainless steel in the forged solid solution-aged state, additive solid solution-aged state, and additive printing state;

[0026] Figure 5 Comparison of stress-strain curves and strength-impact toughness synergistic analysis diagrams of Example 3 and 17-4PH stainless steel in the forged solid solution aging state, additive solid solution aging state, and additive printing state. DETAILED DESCRIPTION

[0027] The preferred embodiments of the present invention will be described in more detail below with reference to the accompanying drawings and specific examples.

[0028] The present invention provides a preparation method for improving the strength and toughness of additively manufactured 17-4PH stainless steel, comprising: forming a 17-4PH stainless steel component through an additive manufacturing process; and heat treating the obtained 17-4PH stainless steel component.

[0029] like Figure 1 As shown, the heat treatment process of the present invention comprises the following steps in sequence:

[0030] Direct aging treatment: heat the 17-4PH stainless steel component to 250℃~550℃, keep it warm for 0.01h~100h, and then air cool it to room temperature;

[0031] Cryogenic treatment: Place the 17-4PH stainless steel components in a cryogenic environment (≤-50°C), keep them warm for more than 1 hour, and then return them to room temperature in the air;

[0032] Tempering treatment: Heat the 17-4PH stainless steel components to 100℃~300℃, keep warm for 0.1h~24h, and then air cool to room temperature.

[0033] Example 1

[0034] This embodiment provides an additively manufactured 17-4PH stainless steel, wherein the preparation method comprises the following steps:

[0035] S1: 17-4PH stainless steel component samples were formed using the selective laser melting process. The laser power was set to 250W, the scanning speed was 800mm / s, the powder layer thickness was 0.04mm, the scanning spacing was 0.1mm, the layer-by-layer remelting power was 250W, the layer-by-layer remelting scanning speed was 800mm / s, and the layer-by-layer remelting scanning spacing was 0.1mm. The density of the formed sample was ensured to be above 99%.

[0036] S2: Place the sample in a low-temperature heating furnace, heat to 350°C and keep it there for 1 hour, then take out the sample and air-cool it to room temperature;

[0037] S3: Place the sample in liquid nitrogen (-196°C) for 12 hours and return it to room temperature in air.

[0038] S4: Place the sample in the low-temperature heating furnace again, heat it to 200°C and keep it there for 1 hour, then take out the sample and air-cool it to room temperature.

[0039] The samples after heat treatment were stretched at room temperature, and the yield strength was measured to be 1032 MPa, the tensile strength was 1264 MPa, and the elongation at break was 16%. The Charpy impact test of the samples after heat treatment was performed, and the impact absorption energy was measured to be 45 J.

[0040] Example 2

[0041] This embodiment provides an additively manufactured 17-4PH stainless steel, wherein the preparation method comprises the following steps:

[0042] S1: 17-4PH stainless steel was formed using the selective laser melting process. The laser power was set to 250W, the scanning speed was 800mm / s, the powder layer thickness was 0.04mm, the scanning spacing was 0.1mm, the layer-by-layer remelting power was 250W, the layer-by-layer remelting scanning speed was 800mm / s, and the layer-by-layer remelting scanning spacing was 0.1mm. The density of the formed sample was ensured to be above 99%.

[0043] S2: Place the sample in a low-temperature heating furnace, heat to 380°C and keep warm for 1 hour, then take out the sample and air cool it to room temperature;

[0044] S3: Place the sample in liquid nitrogen (-196°C) for 12 hours and return it to room temperature in air.

[0045] S4: Place the sample in the low-temperature heating furnace again, heat it to 200°C and keep it there for 1 hour, then take out the sample and air-cool it to room temperature.

[0046] The samples after heat treatment were stretched at room temperature, and the yield strength was measured to be 1108 MPa, the tensile strength was 1338 MPa, and the elongation at break was 12%. The Charpy impact test of the samples after heat treatment was performed, and the impact absorption energy was measured to be 30 J.

[0047] Example 3

[0048] This embodiment provides an additively manufactured 17-4PH stainless steel, the preparation method of which includes the following steps:

[0049] S1: 17-4PH stainless steel component samples were formed using the selective laser melting process. The laser power was set to 250W, the scanning speed was 800mm / s, the powder layer thickness was 0.04mm, the scanning spacing was 0.1mm, the layer-by-layer remelting power was 250W, the layer-by-layer remelting scanning speed was 800mm / s, and the layer-by-layer remelting scanning spacing was 0.1mm. The density of the formed sample was ensured to be above 99.5%.

[0050] S2: Place the sample in a low-temperature heating furnace, heat to 365°C and keep it there for 1 hour, then take out the sample and air-cool it to room temperature; the sample that has completed the treatment at this stage is recorded as the aged sample;

[0051] S3: Place the sample in liquid nitrogen (-196°C) for 12 hours and return it to room temperature in air.

[0052] S4: Place the sample in the low-temperature heating furnace again, heat it to 200°C and keep it there for 1 hour, then take out the sample and air-cool it to room temperature.

[0053] The samples after the heat treatment were stretched at room temperature, and the yield strength was measured to be 1047 MPa, the tensile strength was 1319 MPa, and the elongation at break was 15.2%. The Charpy impact test of the samples after the heat treatment was performed, and the impact absorption energy was measured to be 38 J.

[0054] The present application used SEM to characterize the phase distribution and crystallographic characteristics of the microstructure morphology of the additively printed sample without heat treatment and the aged sample in Example 3, as shown in FIG. Figure 2 As shown in the figure, copper-rich precipitates are present in the aged sample. The enhanced thermodynamic driving force causes large-scale segregation of copper within the matrix, forming a high-density copper-rich precipitate. Copper-rich precipitates typically form during the aging process. While heat transfer induced by laser scanning during the selective laser melting process produces a slight aging effect, during rapid heating and cooling, copper can be incorporated into the matrix but is difficult to precipitate in large quantities. Therefore, no obvious copper-rich precipitates were observed in the as-printed matrix.

[0055] Figure 3 Comparison of stress-strain curves and strength-impact toughness synergy analysis diagrams of Example 1 (additive new heat treatment state) and prior art 17-4PH stainless steel in the forged solid solution aging state, additive solid solution aging state, and additive printing state; Figure 4 Comparison of stress-strain curves and strength-impact toughness synergy analysis diagrams of Example 2 (additive new heat treatment state) and prior art 17-4PH stainless steel in the forged solid solution aging state, additive solid solution aging state, and additive printing state; Figure 5The stress-strain curves and strength-impact toughness synergistic analysis diagrams of Example 3 (additive new heat treatment state) and the prior art 17-4PH stainless steel in the forged solid solution aging state, additive solid solution aging state, and additive printing state are compared.

[0056] It can be seen from the above three figures that the strength of the additive printing state is generally lower than that of the other heat-treated states, and its yield strength is low, but the elongation is good, and the impact energy absorption is excellent; the forged sample solid solution aging state and the additive solid solution aging state have the highest yield and tensile strength, but their elongation is very poor, less than 6%. At this time, the impact toughness of the two heat-treated samples is very low, and the impact energy absorption is less than 15J; the additive new heat-treated state obtained in each embodiment of the present invention has excellent comprehensive mechanical properties, and its yield strength is only about 100MPa lower than that of the forging and additive solid solution aging states, but it has the best elongation, reaching 16%, and each embodiment has good impact energy absorption, about 30-50J, indicating that after the new heat treatment, the material has excellent yield strength and is relatively stable in the plastic deformation stage, which effectively improves the comprehensive performance of material strength and impact resistance and optimizes the matching of material mechanical properties.

[0057] The above characterization results demonstrate that the material produced by the treatment method described herein exhibits high tensile strength and excellent plasticity, a well-balanced strength-ductility ratio, more balanced deformation and load-bearing capacity, strong strength retention during impact resistance, an optimal strength-toughness ratio, and good adaptability to complex stress-bearing scenarios. This simple and efficient method not only addresses the shortcomings of additive manufacturing but also adapts to complex working conditions, effectively improving the strength and toughness of additively manufactured 17-4PH stainless steel components and broadening their application scenarios.

[0058] While various embodiments of the present invention have been described above, the foregoing description is intended to be illustrative and non-exhaustive, and is not intended to be limiting of the disclosed embodiments. Numerous modifications and variations will be apparent to those skilled in the art without departing from the scope and technical principles of the described embodiments, and such modifications and variations should be considered within the scope of the present invention.

Claims

1. A preparation method for improving the strength and toughness of additively manufactured 17-4PH stainless steel, characterized in that: include: 17-4PH stainless steel components formed by additive manufacturing; The 17-4PH stainless steel component obtained by additive manufacturing is subjected to heat treatment, and the heat treatment process sequentially includes direct aging treatment, deep cooling treatment and tempering treatment; wherein the direct aging treatment includes: heating the 17-4PH stainless steel component to 300°C~380°C, keeping the temperature for 0.1h~5h, and then air cooling to room temperature.

2. The preparation method for improving the strength and toughness of 17-4PH stainless steel by additive manufacturing according to claim 1, characterized in that: The additive manufacturing process includes a selective laser melting process, a laser melting deposition process or an electron beam selective melting process.

3. The method for improving the strength and toughness of additively manufactured 17-4PH stainless steel according to claim 2, characterized in that: The parameters of the additive manufacturing process include: laser power of 100W~1000W, scanning speed of 100mm / s~2000 mm / s, powder layer thickness of 0.02mm~0.15mm, scanning spacing of 0.01mm~0.15mm, layer-by-layer remelting power of 100W~1000W, scanning speed of 100mm / s~2000mm / s, and scanning spacing of 0.01mm~0.15mm.

4. The preparation method for improving the strength and toughness of 17-4PH stainless steel by additive manufacturing according to claim 2, characterized in that: The density of the 17-4PH stainless steel component formed by the additive manufacturing process is above 99%.

5. The preparation method for improving the strength and toughness of 17-4PH stainless steel by additive manufacturing according to claim 1, characterized in that: The cryogenic treatment includes: placing the 17-4PH stainless steel component in a cryogenic environment, keeping it warm for more than 1 hour, and returning it to room temperature in the air; the temperature of the cryogenic environment is ≤-50°C.

6. The preparation method for improving the strength and toughness of additively manufactured 17-4PH stainless steel according to claim 5, characterized in that: The cryogenic environment is a liquid nitrogen environment.

7. The method for improving the strength and toughness of 17-4PH stainless steel by additive manufacturing according to claim 1, characterized in that: The tempering treatment includes: heating the 17-4PH stainless steel component to 100° C. to 300° C., keeping the temperature for 0.1 hour to 24 hours, and then air cooling to room temperature.

8. The method for improving the strength and toughness of additively manufactured 17-4PH stainless steel according to claim 7, characterized in that: The tempering treatment includes: heating the 17-4PH stainless steel component to 100° C. to 300° C., keeping the temperature for 0.1 h to 10 h, and then air cooling to room temperature.

Images