A process for preparing high-performance high-nitrogen steel by underwater laser cladding

Abstract

The present invention relates to a process method for preparing high-performance high-nitrogen steel by underwater laser cladding, comprising: preparing high-nitrogen steel powder, planning the scanning trajectory of the high-nitrogen steel powder; placing a substrate workpiece in an underwater environment, constructing a local dry area on the processing surface of the substrate workpiece, and the remaining surface of the substrate workpiece is in direct contact with the water environment, constructing a nitrogen atmosphere in the local dry area, and the cladding nozzle is at an initial cladding height, according to the scanning trajectory, cladding the high-nitrogen steel powder on the processing surface of the substrate workpiece to form a molten pool, promoting the nitriding of the gas-liquid interface near the molten pool by the nitrogen atmosphere to improve the solubility of nitrogen in the molten pool, and obtaining an initial high-nitrogen steel deposition sample on the processing surface of the substrate workpiece through the cooling effect of the water environment; performing the cladding operation again on the surface of the initial high-nitrogen steel deposition sample according to the scanning trajectory until the processing of the high-nitrogen steel sample of the set thickness is completed. The obtained product has uniform and fine structure, high dislocation density, high strength and high corrosion resistance.

Description

Technical Field

[0001] The invention relates to the technical field of high nitrogen steel preparation, and in particular to a process for preparing high-performance high nitrogen steel by underwater laser cladding. Background Art

[0002] Low alloy steel with excellent mechanical properties is a commonly used marine structural material, but it is prone to rust in the marine environment and requires frequent anti-corrosion treatment. Stainless steel with good corrosion resistance generally contains a large amount of nickel and is expensive. Nitrogen, as an important element in steel, can greatly improve the strength of steel without reducing toughness through the comprehensive effects of solid solution strengthening, fine grain strengthening and dislocation strengthening. Through the synergistic effect of nitrogen and other alloying elements (Cr, Mo, etc.), it can improve the stability of the passivation film and enhance the repassivation ability, thereby significantly improving the corrosion resistance of steel. Under normal pressure, the solubility of nitrogen in steel is low, and nitrogen is prone to nitrogen segregation during the solidification of molten steel, forming nitrogen pores and causing nitrogen escape, which seriously restricts the development and application of high nitrogen steel varieties. Therefore, pressurized metallurgy has become the main way to prepare high nitrogen steel, including pressurized induction melting, large molten pool method, pressurized electroslag remelting, etc. These methods have problems such as insufficient purity of ingots, multiple remelting required to improve uniformity, small tonnage of pressurized induction furnaces, and unfavorable for industrial production. In addition, the high nitrogen steel samples obtained by pressure casting have coarse structure and severe surface oxidation, resulting in problems such as waste of raw materials.

[0003] In the prior art, the Chinese patent with publication number CN114032461A discloses "a method for preparing high nitrogen steel by adjusting the element composition of the ingot", and its technical solution is to increase the solubility of nitrogen in the steel by increasing the element content of Cr and Mn in the ingot, but the ingot needs to be subsequently subjected to solid solution forging and hot rolling. Although this method does not require the use of pressurized equipment and is low in cost, the post-processing process is cumbersome and there are also problems such as coarse organization. The Chinese patent with authorization announcement number CN110560842B discloses "a nitrogen fixing device and method in welding based on side-axis wire feeding", and its technical solution is to add nitrides to the welding wire and use the metallurgical reaction of nitrides for nitriding during the welding process. Its disadvantage is that the melting point of nitrides is relatively high, and the welding heat source cannot completely melt the nitrides, and there will be a small amount of residual nitrides, which will cause segregation at the grain boundaries. Summary of the invention

[0004] In view of the deficiencies in the prior art, the present invention provides a process method for preparing high-performance high-nitrogen steel by underwater laser cladding, the purpose of which is to simplify the process flow and improve the performance of the resulting structure.

[0005] The technical solution adopted by the present invention is as follows:

[0006] The present application provides a process for preparing high-performance high-nitrogen steel by underwater laser cladding, comprising the following steps:

[0007] S1. Prepare high nitrogen steel powder and plan the scanning trajectory of the high nitrogen steel powder;

[0008] S2. Perform underwater laser cladding operation using laser cladding equipment: place the substrate workpiece in an underwater environment, construct a local dry area on the processing surface of the substrate workpiece, and the remaining surface of the substrate workpiece is in direct contact with the water environment, and a nitrogen atmosphere is constructed in the local dry area. The cladding nozzle is at an initial cladding height and, according to the scanning trajectory, clads high nitrogen steel powder on the processing surface of the substrate workpiece to form a molten pool, and the nitrogen atmosphere promotes nitriding of the gas-liquid interface near the molten pool to increase the solubility of nitrogen in the molten pool, and the initial high nitrogen steel deposition sample is obtained on the processing surface of the substrate workpiece through the cooling effect of the water environment;

[0009] S3, performing cladding operation again on the surface of the initial high nitrogen steel deposition sample according to the scanning trajectory, and repeating S2 and S3 until the processing of the high nitrogen steel sample with a set thickness is completed.

[0010] Further technical solutions are:

[0011] The components of the high nitrogen steel powder include: C: 0.02-0.1wt.%, Mn: 5-30wt.%, Cr: 18-30wt.%, Si<1wt.%, Mo: 0-6.0wt.%, N: 0.2-1.0wt.%, Ni: 0-25wt.%, S:<0.015wt.%, P:<0.05wt.%, and the balance is Fe.

[0012] The high nitrogen steel powder is spherical powder with a particle size ranging from 20 to 150 μm.

[0013] The process parameters used in underwater laser cladding operations are: laser spot diameter 2-4 mm, laser power 2000-5000 W, scanning line speed 800-1500 mm / min, powder flow rate 10-30 g / min, shielding gas flow rate 1.5-2.0 m 3 / h, the powder delivery gas flow rate is 0.7~1.5m 3 / h, the shielding gas pressure is 0.3-0.5MPa higher than the pressure corresponding to the working water depth, and the powder feeding gas pressure is 0.3-0.5MPa higher than the pressure corresponding to the working water depth. The shielding gas is nitrogen or a mixed gas mainly composed of nitrogen.

[0014] The overlap rate between tracks in the scanning trajectory is 30% to 60%.

[0015] The operating water depth is 30-100m.

[0016] The nitrogen atmosphere uses pure nitrogen or a mixed gas mainly composed of nitrogen.

[0017] The processed surface of the substrate workpiece is a smooth and pollution-free surface after machining.

[0018] The local dry zone is achieved by continuously introducing drainage gas to form an exhaust water curtain around the cladding nozzle.

[0019] The beneficial effects of the present invention are as follows:

[0020] The present invention realizes efficient nitrogen alloying, and the nitrogen is evenly distributed under the action of Marangoni flow in the molten pool. The obtained high nitrogen steel sample has no pore defects, high nitrogen content, uniform and fine structure, high dislocation density, good flexibility, high tensile strength and high corrosion resistance, and can be widely used as marine structural materials. The present invention specifically includes the following advantages:

[0021] The external water pressure of the method can increase the solubility of nitrogen in the molten pool, nitriding into the molten pool under the nitrogen protective atmosphere in the local dry area, and making the nitrogen evenly distributed in the molten pool under the action of Marangoni flow, so that the prepared sample has uniform structure.

[0022] The cooling effect of the water environment of this method on the substrate workpiece can accelerate the solidification of the molten pool, induce grain refinement and increase dislocation density, improve nitrogen segregation, and combined with increased nitrogen solubility, avoid the generation of nitrogen pores and improve the strength and toughness of the sample.

[0023] This method is insensitive to process parameters and has a large process parameter window.

[0024] This method is simple to operate, safe and reliable, and has controllable precision. The processed samples do not require complicated post-processing processes and can be put into use after a little polishing.

[0025] Other features and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The process flow chart of the method of the present invention is shown in FIG.

[0027] Figure 2 Metallographic diagrams of high nitrogen steel samples obtained in Example 1 of the present invention and the comparative example.

[0028] Figure 3 The macroscopic fracture morphology and corresponding tensile strength results of the high nitrogen steel samples obtained in Example 1 of the present invention and the comparative example.

[0029] Figure 4 Polarization curves of high nitrogen steel samples obtained in Example 1 of the present invention and the comparative example. DETAILED DESCRIPTION

[0030] The specific implementation of the present invention is described below with reference to the accompanying drawings.

[0031] See also Figure 1 The embodiment of the present application provides a process for preparing high-performance high-nitrogen steel by underwater laser cladding, comprising the following steps:

[0032] S1. Prepare high nitrogen steel powder and plan the scanning trajectory of the high nitrogen steel powder;

[0033] S2. Perform underwater laser cladding operation using laser cladding equipment: place the substrate workpiece in an underwater environment, construct a local dry area on the processing surface of the substrate workpiece, and the remaining surface of the substrate workpiece is in direct contact with the water environment, and a nitrogen atmosphere is constructed in the local dry area. The cladding nozzle is at an initial cladding height and, according to the scanning trajectory, clads high nitrogen steel powder on the processing surface of the substrate workpiece to form a molten pool, and the nitrogen atmosphere promotes nitriding of the gas-liquid interface near the molten pool to increase the solubility of nitrogen in the molten pool, and the initial high nitrogen steel deposition sample is obtained on the processing surface of the substrate workpiece through the cooling effect of the water environment;

[0034] S3, performing cladding operation again on the surface of the initial high nitrogen steel deposition sample according to the scanning trajectory, and repeating S2 and S3 until the processing of the high nitrogen steel sample with a set thickness is completed.

[0035] Preferably, the high nitrogen steel powder is a spherical powder, and its components include: C: 0.02-0.1wt.%, Mn: 5-30wt.%, Cr: 18-30wt.%, Si<1wt.%, Mo: 0-6.0wt.%, N: 0.2-1.0wt.%, Ni: 0-25wt.%, S: <0.015wt.%, P: <0.05wt.%, and the balance is Fe.

[0036] Preferably, the particle size of the high nitrogen steel powder is in the range of 20 to 150 μm.

[0037] Preferably, the processing parameters used in underwater laser cladding operations are: laser spot diameter 2-4 mm, laser power 2000-5000 W, scanning line speed 800-1500 mm / min, powder feed flow rate 10-30 g / min, shielding gas flow rate 1.5-2.0 m 3 / h, the powder delivery gas flow rate is 0.7~1.5m 3 / h, the shielding gas pressure is 0.3-0.5MPa higher than the pressure corresponding to the working water depth, and the powder feeding gas pressure is 0.3-0.5MPa higher than the pressure corresponding to the working water depth. The shielding gas is nitrogen or a mixed gas mainly composed of nitrogen.

[0038] Preferably, the overlap rate between tracks in the scanning track is 30% to 60%.

[0039] Preferably, the operating water depth is 30-100m.

[0040] Specifically, the nitrogen atmosphere uses pure nitrogen or a mixed gas mainly composed of nitrogen.

[0041] Specifically, the processed surface of the substrate workpiece is a smooth and pollution-free surface after machining.

[0042] Specifically, the nitrogen atmosphere can increase the solubility of nitrogen in the molten pool. When the temperature is 1873K, the solubility of nitrogen in the molten pool is as shown in formula (1):

[0043]

[0044] In formula (1), P N2 is the nitrogen partial pressure, in MPa, and P0 is the atmospheric pressure, 0.101 MPa.

[0045] Specifically, the solubility of nitrogen in the molten pool is increased under the protective atmosphere of nitrogen, and the gas-liquid interface nitriding is completed. The nitriding reaction obeys the second-order rate equation, as shown in formula (2):

[0046]

[0047] In formula (2), A is the upper surface area of ​​the molten pool, in cm 2 , V is the volume of the molten pool, in cm 3 , [N] t , [N] e They respectively represent the nitrogen concentration in the molten pool and the solubility of nitrogen in the molten pool at a certain moment in the solidification process.

[0048] The nitrogen content in the cladding formed sample of this embodiment can be increased by 20%-50% compared with the raw material high nitrogen steel, which can effectively improve the strength and corrosion resistance of the high nitrogen steel.

[0049] Specifically, the molten pool and the deposited sample are in a dry environment in a local dry area during the operation. Preferably, an air inlet can be provided in the laser cladding equipment, and drainage gas (which can be compressed air, etc.) is continuously introduced into the air inlet, so that a drainage air curtain can be formed around the cladding nozzle and the water around the processing surface can be drained, thereby forming a local dry area on the processing surface.

[0050] The underwater laser cladding process of this embodiment is carried out in a local dry area under water with high pressure. The substrate is in contact with the water environment on all sides. The water has a strong chilling effect on the substrate, which can significantly increase the cooling rate of the molten pool, increase the supercooling inside the molten pool, increase the number of nucleation particles, refine the grains, and improve the strength and corrosion resistance of high nitrogen steel. A deposited sample with increased dislocation density is obtained. The accelerated cooling rate can increase the shrinkage deformation degree of the molten pool, increase the dislocation density of the sample, and further improve the strength and corrosion resistance of high nitrogen steel.

[0051] The following example 1 and comparative example further illustrate the process and effectiveness of the underwater laser cladding method for preparing high-performance high-nitrogen steel of the present application.

[0052] Example 1

[0053] On a smooth substrate surface, at a depth of 30 m underwater, a high-performance high-nitrogen steel sample was prepared using an underwater laser cladding device based on a local dry method. The following steps were used:

[0054] First, high nitrogen steel powder with a particle size distribution of 20 to 150 μm is prepared, and the high nitrogen steel powder is placed in a powder feeder. The element composition of the high nitrogen steel powder is shown in Table 1:

[0055] Table 1 Alloy element composition of high nitrogen steel powder (wt.%)

[0056] element C N Cr Ni Mn Mo Cu O Fe powder 0.035 0.42 18.96 0.16 12.60 2.97 0.35 0.07 Bal.

[0057] Plan the scanning trajectory of the single-layer high-nitrogen steel sample for underwater laser cladding and write the corresponding program. Use reciprocating scanning to clad the same layer of planned high-nitrogen steel powder scanning trajectory; prepare nitrogen with a purity greater than 99.99% as a protective gas, and set the underwater laser cladding processing parameters: laser power is 2500W, scanning rate is 1000mm / min, powder feeding rate is 30g / min, the overlap rate between the single-layer scanning track is 50%, and the protective gas flow rate is 1.2m 3 / h, powder delivery gas flow rate is 1.0m 3 / h, the pressure of the protective gas is 0.6MPa, and the pressure of the powder feeding gas is 0.7MPa.

[0058] Secondly, use underwater laser cladding equipment to perform underwater laser cladding operations on the substrate workpiece: open the drainage gas of the equipment to form a local dry area under the cladding nozzle, operate the underwater laser cladding equipment, move the cladding nozzle to the specified position, open the powder feeder, open the protective gas, and open the powder feeding gas. After the powder feeding is stable, execute the underwater laser cladding program and carry out underwater laser cladding operations. During the operation, under the nitrogen atmosphere, the cladding nozzle is at the initial cladding height and, according to the scanning trajectory, the high nitrogen steel powder is clad on the substrate workpiece to form a molten pool. Under the cooling effect of the surrounding water environment, the initial high nitrogen steel deposition sample is obtained on the substrate workpiece. The increased environmental pressure in the underwater environment can increase the solubility of nitrogen in the molten pool, and the nitrogen atmosphere is used to promote the nitriding of the gas-liquid interface near the molten pool. The nitrogen protective atmosphere will continuously infiltrate nitrogen into the molten pool, thereby increasing the nitrogen content in the sample. Raise the cladding height, and perform cladding operations again on the surface of the initial high nitrogen steel deposition sample according to the scanning trajectory until the high nitrogen steel sample of the set thickness is processed.

[0059] The high nitrogen steel samples after deposition were taken out and used to prepare metallographic samples, tensile samples, and samples for corrosion resistance tests.

[0060] Comparative Example

[0061] Based on the same process parameters as in Example 1, high nitrogen steel samples were prepared by conventional laser cladding method in an air environment rather than an underwater environment.

[0062] Table 2 shows the statistical results of dislocation density, grain size and nitrogen content of the deposited samples obtained in Example 1 and the comparative example.

[0063] Table 2 Dislocation density, grain size and nitrogen content of the deposited samples obtained in Example 1 and Comparative Example

[0064] name Example 1 Comparative Example <![CDATA[Misalignment density (10 14 / m 2 )]]> 1.43 1.11 Grain size (μm) 4.31 7.11 Nitrogen content (wt.%) 0.52 0.40

[0065] Depend on Figure 2 It can be seen from Table 2 that the high nitrogen steel deposited sample in Example 1 has a higher dislocation density and a finer microstructure than the high nitrogen steel deposited sample in the comparative example.

[0066] Among them, for the same initial high nitrogen steel powder with a nitrogen content of 0.42wt.%, the nitrogen content in the high nitrogen steel deposition sample prepared in Example 1 increased to 0.52wt.%, and the nitrogen content in the high nitrogen steel deposition sample prepared in the comparative example decreased to 0.40wt.%.

[0067] Depend on Figure 3 It can be seen that the fracture morphology of the deposited sample prepared in Example 1 has no pore defects, and the tensile strength reaches 1099.14 MPa. The fracture morphology of the deposited sample prepared in the comparative example has pore defects visible to the naked eye, and the tensile strength is 914.51 MPa.

[0068] Depend on Figure 4 It can be seen that because more nitrogen is infiltrated, the pitting potential of the deposited sample prepared in Example 1 is as high as 1112.82 mV, and the corrosion resistance is excellent. The pitting potential of the deposited sample prepared in the comparative example is only 493.62 mV.

[0069] It can be seen that the high nitrogen steel sample obtained by the method of the present application has no pore defects, high nitrogen content, uniform and fine structure, high dislocation density, high tensile strength and high corrosion resistance.

[0070] Those skilled in the art can understand that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention is described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions recorded in the aforementioned embodiments or replace some of the technical features therein by equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A process for preparing high-performance high-nitrogen steel by underwater laser cladding, characterized in that: The following steps are involved: S1. Prepare high nitrogen steel powder and plan the scanning trajectory of the high nitrogen steel powder; S2. Perform underwater laser cladding operation using laser cladding equipment: place the substrate workpiece in an underwater environment, construct a local dry area on the processing surface of the substrate workpiece, and the remaining surface of the substrate workpiece is in direct contact with the water environment, and a nitrogen atmosphere is constructed in the local dry area. The cladding nozzle is at an initial cladding height and, according to the scanning trajectory, clads high nitrogen steel powder on the processing surface of the substrate workpiece to form a molten pool, and the nitrogen atmosphere promotes nitriding of the gas-liquid interface near the molten pool to increase the solubility of nitrogen in the molten pool, and the initial high nitrogen steel deposition sample is obtained on the processing surface of the substrate workpiece through the cooling effect of the water environment; S3, performing cladding operation again on the surface of the initial high nitrogen steel deposition sample according to the scanning trajectory, repeating S2 and S3 until the high nitrogen steel sample processing of the set thickness is completed; The high nitrogen steel powder comprises: C: 0.02-0.1 wt.%, Mn: 5-30 wt.%, Cr: 18-30 wt.%, Si<1 wt.%, Mo: 0-6.0 wt.%, N: 0.2-1.0 wt.%, Ni: 0-25 wt.%, S: <0.015 wt.%, P: <0.05 wt.%, and the balance is Fe; The process parameters used in underwater laser cladding operations are: laser spot diameter 2-4 mm, laser power 2000-5000 W, scanning line speed 800-1500 mm / min, powder flow rate 10-30 g / min, shielding gas flow rate 1.5-2.0 m 3 / h, powder delivery gas flow rate is 0.7~1.5 m 3 / h, the shielding gas pressure is 0.3-0.5 MPa higher than the pressure corresponding to the working water depth, the powder feeding gas pressure is 0.3-0.5 MPa higher than the pressure corresponding to the working water depth, and the shielding gas is nitrogen or a mixed gas mainly composed of nitrogen.

2. The process for preparing high-performance high-nitrogen steel by underwater laser cladding according to claim 1 is characterized in that: The high nitrogen steel powder is spherical powder with a particle size ranging from 20 to 150 μm.

3. The process for preparing high-performance high-nitrogen steel by underwater laser cladding according to claim 1 is characterized in that: The overlap rate between tracks in the scanning trajectory is 30% to 60%.

4. The process for preparing high-performance high-nitrogen steel by underwater laser cladding according to claim 1 is characterized in that: The operating water depth is 30-100m.

5. The process for preparing high-performance high-nitrogen steel by underwater laser cladding according to claim 1, characterized in that: The nitrogen atmosphere uses pure nitrogen or a mixed gas mainly composed of nitrogen.

6. The process for preparing high-performance high-nitrogen steel by underwater laser cladding according to claim 1, characterized in that: The processed surface of the substrate workpiece is a smooth and pollution-free surface after machining.

7. The process for preparing high-performance high-nitrogen steel by underwater laser cladding according to claim 1, characterized in that: The local dry zone is achieved by continuously introducing drainage gas to form an exhaust water curtain around the cladding nozzle.

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