Method for homogenizing microstructure of low-temperature high manganese steel welding heat affected zone by pulse current

Abstract

The invention discloses a method for homogenizing a microstructure of a low-temperature high manganese steel welding heat affected zone by using pulse current, and belongs to the technical field of welding and postweld heat treatment of high manganese steel for liquefied natural gas (LNG) storage and transportation. The treatment method comprises the following steps: (1) sampling a heat affected zone of the low-temperature high-manganese steel welded joint with microstructure gradient distribution; (2) the low-temperature high manganese steel welding heat affected zone sample is subjected to pulse current treatment at the room temperature, the pulse frequency ranges from 0.6 kHz to 6 kHz, the pulse width ranges from 0.2 ms to 6 ms, the current density ranges from 20 A / mm < 2 > to 200 A / mm < 2 >, and the treatment time ranges from 5 min to 120 min; and (3) grinding and polishing the sample, observing and analyzing the element distribution state in the heat affected zone, and quantitatively representing the grain boundary structure characteristics. According to the method, atomic diffusion, dislocation slippage and grain boundary migration of the low-temperature high-manganese steel welding heat affected zone can be promoted by applying the pulse current, redistribution of segregated atoms and dislocations in grains and at grain boundaries is rapidly achieved, and then the purposes of improving the microstructure uniformity of the low-temperature high-manganese steel welding heat affected zone and improving the impact toughness of the low-temperature high-manganese steel welding heat affected zone are achieved. The method is easy to operate, short in treatment time, environmentally friendly and particularly suitable for postweld treatment of low-temperature high manganese steel parts.

Description

Technical Field

[0001] The present invention relates to the technical field of high manganese steel welding and post-weld heat treatment for liquefied natural gas (LNG) storage and utilization, and in particular to a method for rapidly homogenizing the microstructure of a heat-affected zone of low-temperature high manganese steel welding by using pulse current. Background Art

[0002] Low temperature high manganese steel has become the most promising candidate structural material for key equipment for storage and transportation of environmentally friendly alternative fuels such as liquefied natural gas (LNG), liquid ammonia, and liquid hydrogen due to its high strength, excellent low temperature toughness, and low cost. LNG ships and storage tanks, as components directly facing ultra-low temperature liquids, face problems such as random loads from ocean waves and seawater corrosion. The low temperature high manganese steel welded structures used in such liquid fuel storage and transportation equipment should meet extremely complex and severe environmental requirements.

[0003] However, the study found that the microstructure gradient of the heat-affected zone of low-temperature high-manganese steel after welding is unevenly distributed, resulting in the degradation of its low-temperature mechanical properties (especially low-temperature toughness), which is significantly weaker than the parent material. This restricts the engineering application of low-temperature high-manganese steel in liquid fuel storage and transportation equipment and polar ships. At present, some researchers have integrated the control of the uneven distribution of microstructures affected by welding heat and improved its low-temperature mechanical properties through material alloying design and post-weld heat treatment of joints. Chinese patent CN116083813A discloses a N microalloyed high manganese steel and its heat treatment method and application, which utilizes the microalloying of the N element to inhibit the segregation of carbon atoms at the grain boundaries and the precipitation of carbides during the welding process of low-temperature high manganese steel to achieve the uniformity of element distribution and organization in the welding heat-affected zone; Chinese patent CN118814102A discloses a microalloyed high manganese steel and its preparation method, which is to form fine rare earth inclusions (Ce2O3, Ce2O2S) during welding by adding La-Ce rare earth elements to inhibit heat-affected grain coarsening and grain boundary carbide precipitation; Literature [Chen J., Ren JK, Liu ZY, et al. Interpretation of significant decrease in cryogenic-temperature Charpy impact toughness in a high manganese steel. Materials Science & Engineering A, 737 (2018) 158-165] reported a high-temperature heat treatment process for high-manganese LNG tank steel, which expanded the solution treatment window of high-manganese steel and achieved relatively uniform distribution of elements within the grain and at the grain boundary to a certain extent; Chinese patent CN108356417A also disclosed a heat treatment method for improving the plasticity of high-strength and high-ductility medium manganese steel laser welded joints, which can homogenize the alloy elements and grain boundary structure of low-temperature high-manganese steel welded joints. Although material alloying design and joint heat treatment can achieve the purpose of homogenizing the microstructure of the heat-affected zone, they both have their specific limitations; for example, material microalloying has little effect on the characteristics of the grain boundary structure and may cause the material to deteriorate in weldability; secondly, the addition of rare earth elements leads to increased costs, which is contrary to the original intention of introducing low-temperature high-manganese steel to replace 9Ni steel; finally, conventional heat treatment requires higher temperatures and longer times, and the process is complicated and cumbersome, costly, and easily causes grain coarsening.

[0004] At present, the use of high-energy external fields to control the microstructure of metal structural materials and improve their comprehensive performance is one of the hot topics in materials science research. Chinese patent CN113774192A discloses a method of using pulse current to control the network carbide in bearing steel. By dissolving and re-precipitating carbides with pulse current, the morphology and distribution of carbides are regulated, and the performance of metal materials can be improved. In summary, conventional material alloying design and post-weld heat treatment technology are difficult to completely eliminate the gradient inhomogeneity of the microstructure of the heat-affected zone of low-temperature high-manganese steel welding, and the purpose and effect of the existing pulse current treatment technology do not involve the content of the present invention. In view of this, how to achieve precise control and homogenization of the microstructure of the heat-affected zone of low-temperature high-manganese steel welding by pulse current is one of the key problems to be solved in this field, and it is also the goal that the present invention is committed to solving. Summary of the invention

[0005] The purpose of the present invention is to provide a non-traditional, efficient, green and environmentally friendly post-processing method to address the problems of high cost, long cycle and low effect of existing technical means such as material alloying design and post-weld heat treatment, that is, a method for homogenizing the microstructure of the heat affected zone of low-temperature high manganese steel welding by using pulse current, so as to alleviate the segregation of grain boundary elements, optimize the grain boundary structure, and improve the impact toughness of the heat affected zone of welding.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] The present invention provides a method for homogenizing the microstructure of the heat-affected zone of low-temperature high-manganese steel welding by using pulse current, and the method significantly improves the uniformity of the microstructure of the heat-affected zone of low-temperature high-manganese steel welding and improves its impact toughness; the method comprises the following steps:

[0008] (1) Preparation of low temperature high manganese steel welded joints by fusion welding;

[0009] (2) Sampling of the heat affected zone of welding where there is uneven distribution of microstructure;

[0010] (3) subjecting the obtained low-temperature high manganese steel welding heat-affected zone specimen to pulse current treatment at room temperature;

[0011] (4) grinding and polishing, chemical etching and vibration polishing the sample treated in step (3);

[0012] (5) Characterizing the element distribution state and quantifying the grain boundary structure characteristics of the sample after being processed in step (4).

[0013] Preferably, the melting welding method in step (1) is metal inert gas shielded welding.

[0014] Preferably, the thickness of the low-temperature high manganese steel plate in step (1) is 8 to 20 mm; the welding voltage of the metal inert gas shielded welding is 20 to 24 V, the welding current is 140 to 220 A, the inert gas is argon, the purity of the argon is ≥99.99%, and the gas flow rate is 14 to 20 L / min.

[0015] Preferably, in step (2), the heat affected zone of the low-temperature high manganese steel weld with microstructural inhomogeneity is sampled by using electric spark wire cutting to extract the heat affected zone sample by local sectioning of the low-temperature high manganese steel weld joint obtained in step (1).

[0016] Preferably, the sampling of the heat affected zone of the low-temperature high manganese steel weld with microstructural inhomogeneity in step (2) includes determining the sample size and preliminary grinding of the sample surface to ensure good contact during pulse current treatment.

[0017] Preferably, in step (3), the extracted low-temperature high-manganese steel welding heat-affected zone sample is subjected to pulse current treatment at room temperature, and the determined pulse current parameter range is: pulse frequency 0.6-6kHz, pulse width 0.2-6ms, current density 20-200A / mm 2 , processing time 5 to 120 minutes.

[0018] Preferably, in step (4), the heat-affected zone sample after the pulse current treatment is subjected to grinding and polishing, chemical etching and vibration polishing.

[0019] Preferably, the surface of the sample after the pulse current treatment is polished with 200-400#, 800-1000#, 1500-2000# water-abrasive sandpaper in sequence until only scratches in a single direction exist, and the surface of the sample is polished to a scratch-free mirror surface with a diamond grinding paste with a particle size of 0.5-2.5 μm.

[0020] Preferably, the sample after pulse current treatment and grinding and polishing is chemically etched at room temperature, the etching solution consists of 2-4g picric acid, 4-6mL hydrochloric acid and 90-100mL anhydrous ethanol, and the etching time is 2-4min.

[0021] Preferably, the sample after pulse current treatment and grinding and polishing is subjected to vibration polishing at room temperature, the vibration polishing frequency is 5400-7200 r / min, the vibration polishing liquid is 0.02-0.05 μm non-crystalline silicon dioxide polishing liquid, and the vibration polishing time is 60-120 min.

[0022] Preferably, the element distribution state of the sample in step (5) is characterized by performing line scanning analysis on the intracrystalline and grain boundary element distribution characteristics of no less than 8 positions of the same sample using electron probe X-ray microanalysis technology (EPMA) to evaluate the uniformity of the element distribution of the sample after pulse current treatment.

[0023] Preferably, the quantification of the grain boundary structure characteristics of the sample in step (5) is performed by analyzing the number and distribution characteristics of large and small angle grain boundaries at no less than 6 locations of the same sample using electron backscatter diffraction technology (EBSD), and quantitatively analyzing and statistically analyzing the Channel5 software to evaluate the uniformity of the grain boundary structure distribution of the sample after pulse current treatment.

[0024] Preferably, according to the traditional post-weld heat treatment process for low-temperature marine steel, a heating furnace is used to carry out a conventional post-weld heat treatment comparison experiment on the heat-affected zone of low-temperature high-manganese steel welding. The heat treatment temperature is 500°C and the heat treatment time is 1h. The electron backscatter diffraction technology is also used to quantitatively characterize the grain boundary structure of the sample after heat treatment.

[0025] Preferably, compared with conventional post-weld heat treatment, the proportion of small-angle grain boundaries in the heat-affected zone after pulse current treatment is significantly reduced, the uniformity of grain boundary structure distribution is increased by 2 times, and the impact toughness of the sample is increased by 20%.

[0026] The principle of using pulse current treatment to achieve uniform distribution of elements and grain boundary structure in the heat affected zone of low temperature high manganese steel welding:

[0027] The present invention mainly targets the microstructural inhomogeneity caused by the segregation of grain boundary elements and the aggregation of small-angle grain boundaries in the heat-affected zone of low-temperature high-manganese steel after welding, which seriously deteriorates its impact toughness. The proposed pulse current treatment is mainly based on the difference in electrical properties between the distorted microstructure and the matrix at the element segregation and small-angle grain boundary aggregation, which causes the uneven distribution of current density during the pulse current treatment process. The current density near the distorted microstructure under the action of the electric field is relatively high. The energy difference between the matrix and the microstructure caused by this current with uneven distribution will promote the rapid diffusion of segregated elements at the grain boundary and the slip of stacked dislocations, so as to achieve element diffusion homogenization and redistribution of grain boundary structure. At the same time, compared with traditional post-weld heat treatment, pulse current treatment is an instantaneous high-energy current treatment method with the advantages of simple operation, green environmental protection, and short time consumption. Therefore, it is feasible to use pulse current treatment to quickly homogenize the microstructure of the heat-affected zone of low-temperature high-manganese steel welding and regulate its impact toughness.

[0028] Therefore, compared with the prior art, the beneficial effects of the present invention are:

[0029] (1) The present invention promotes the diffusion of segregated elements and the slip of stacked dislocations by applying pulse current treatment to the heat affected zone of low-temperature high-manganese steel welding, and can quickly and accurately achieve the uniform distribution of intracrystalline and grain boundary elements and grain boundary structure, thereby improving the impact toughness of the heat affected zone of low-temperature high-manganese steel welding.

[0030] (2) The method of pulse current treatment for regulating the microstructure and properties of the heat affected zone of low-temperature high manganese steel welding can be carried out at room temperature without the need for an additional auxiliary heat source. It is simple to operate, short in time, highly efficient, and energy-saving. It has broad application prospects and application value in regulating the microstructure and properties of the heat affected zone of welding. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is the microstructure diagram of the untreated low-temperature high-manganese steel base material and the welding heat-affected zone of the present invention.

[0032] Figure 2 This is the grain boundary structure distribution diagram of the heat affected zone of low temperature high manganese steel welding without treatment in this invention

[0033] Figure 3 This is the element line scanning distribution diagram of the heat affected zone after pulse current treatment in Example 1 of the present invention

[0034] Figure 4 This is a comparison diagram of the grain boundary structure of the heat affected zone after pulse current treatment in Example 1 of the present invention

[0035] Figure 5 This is the element line scan distribution diagram of the heat affected zone after pulse current treatment in Example 2 of the present invention

[0036] Figure 6 This is a comparison diagram of the grain boundary structure of the heat affected zone after pulse current treatment in Example 2 of the present invention DETAILED DESCRIPTION

[0037] In order to better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. The specific embodiments of the present invention are described in detail as follows:

[0038] In the following embodiments, the devices used for pulse current treatment are all pulse power supplies.

[0039] The low-temperature high-manganese steel used in the following examples is obtained by conventional smelting, hot rolling and water toughening processes. The mass percentages of the raw material components are: C: 0.45%; Mn: 22.85%; Cr: 3.32%; Al: 1.47%; the rest are Fe and impurity elements. The low-temperature high-manganese steel plate is subjected to melting extremely inert gas shielded welding, the welding voltage is 21V, the welding current is 190A, the inert gas is argon, the argon purity is ≥99.99%, and the gas flow rate is 18L / min. The low-temperature high-manganese steel base material and the welding heat-affected zone are observed by SEM, and the results are as follows Figure 1 As shown in the figure, the low temperature high manganese steel is composed of single-phase austenite, and there is obvious grain coarsening in the welding heat affected zone, but no phase change occurs.

[0040] The sandpaper, chemical reagents, etc. used in the following examples are all reagents and materials obtained from commercial sources.

[0041] Example 1

[0042] In this example, pulse current treatment is performed on a small-sized low-temperature high-manganese steel weld heat-affected zone sample with a size of 25×10×4mm. 3 , including the following steps:

[0043] (1) The heat affected zone of the low temperature high manganese steel welded joint with microstructural inhomogeneity was sampled by electric spark wire cutting, and a 25×10×4mm 3 Heat affected zone specimen.

[0044] (2) Set the parameters of pulse current treatment, and determine the pulse current parameters as follows: pulse frequency 2kHz, pulse width 2ms, current density 100A / mm 2 , processing time 60min; according to the set pulse current parameters, the small-sized sample is fixed at the output end of the pulse power supply and pulse current treatment is carried out at room temperature.

[0045] (3) The samples treated with pulse current were polished and chemically etched. The sample surface was polished with 400#, 800#, 1500# and 2000# water-abrasive sandpaper until only scratches in a single direction were left. The surface of the sample was polished to a scratch-free mirror surface with 1.5μm diamond paste. To ensure that the grain boundaries could be clearly distinguished under the electron microscope, the polished sample surface was chemically etched at room temperature. The etching solution consisted of 3g picric acid, 5mL hydrochloric acid and 100mL anhydrous ethanol, and the etching time was 3min.

[0046] (4) Use electron probe X-ray analysis equipment to perform element analysis on no less than 8 locations of the same sample to evaluate the uniformity of element distribution in the sample after pulse current treatment, and characterize the element distribution state within the grain and grain boundary of the heat affected zone sample after pulse current treatment, such as Figure 3After the heat affected zone is treated by the pulse current of this embodiment, alloy elements such as Mn, C, Al, and Cr are evenly distributed in the grains and at the grain boundaries, and the grain boundary segregation phenomenon is basically eliminated.

[0047] (5) After the pulse current treatment, the sample was mechanically polished and then vibrated at room temperature to remove the deformation layer on the sample surface. The vibration polishing frequency was 7200 r / min, the polishing liquid was 0.02 μm amorphous silicon dioxide, and the vibration polishing time was 80 min. The distribution characteristics of the grain boundaries in the sample after the pulse current treatment were characterized using a field emission scanning electron microscope, such as Figure 4 As shown, and compared with the grain boundary structure distribution of the pulse current treated sample, Figure 2 After the pulse current treatment method of this embodiment is adopted, the number of small-angle grain boundaries in the original sample is significantly reduced due to the pulse current promoting dislocation movement, and the sample basically presents the characteristic of uniform distribution of large-angle grain boundaries.

[0048] Example 2

[0049] A method for homogenizing the microstructure of the heat-affected zone of low-temperature high-manganese steel welding by using pulse current, which is different from Example 1 in that the pulse current parameters determined in step (2) are: pulse frequency 3kHz, pulse width 2ms, current density 100A / mm 2 , processing time 60min; other uniformity is the same as Example 1.

[0050] like Figure 5 and 6 The distribution status of elements within the grain and at the grain boundaries of the heat-affected zone sample after pulse current treatment, as well as the distribution characteristics of the grain boundary structure, are shown. It can be seen that the segregation of grain boundary elements in the heat-affected zone sample is improved after pulse current treatment, and the small-angle grain boundaries (dislocation walls) caused by welding thermal cycles are basically eliminated, and the organizational uniformity is better.

[0051] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the protection scope of the present invention. In the present invention, the pulse frequency is 0.6-6kHz, the pulse width is 0.2-6ms, and the current density is 20-200A / mm 2 , The treatment time of 5 to 120 minutes is for laboratory small-sized low-temperature high-manganese steel welding heat-affected zone specimens. When applied to actual production, the corresponding pulse current treatment parameter range should be obtained according to the specific part size. The larger the specimen size, the lower the pulse frequency, the larger the pulse width, the greater the current density, and the longer the action time.

[0052] The protection scope of the present invention is not limited to the above-mentioned embodiments. Those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents (including similar materials, equipment, etc.) without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A method for homogenizing the microstructure of the heat-affected zone of low-temperature high-manganese steel welding by using pulse current, characterized in that: Pulse current treatment of the heat affected zone of low temperature high manganese steel welding with uneven microstructure gradient can significantly improve the impact toughness of the heat affected zone; comprising the following steps: (1) Preparation of low temperature high manganese steel welded joints by fusion welding; (2) Sampling of the heat affected zone of welding where there is uneven distribution of microstructure; (3) subjecting the obtained low-temperature high manganese steel welding heat-affected zone specimen to pulse current treatment at room temperature; (4) grinding and polishing, chemical etching and vibration polishing the sample treated in step (3); (5) Characterizing the element distribution state and quantifying the grain boundary structure characteristics of the sample after being processed in step (4).

2. The method for rapidly homogenizing the microstructure of the heat-affected zone of low-temperature high-manganese steel welding by using pulse current according to claim 1, characterized in that: In step (1), the preparation method of the low-temperature high manganese steel welded joint is melting extremely inert gas shielded welding, the thickness of the low-temperature high manganese steel workpiece to be welded is 8-20 mm, the welding voltage is 20-24 V, the welding current is 140-220 A, the inert gas is argon, the argon purity is ≥99.99%, and the gas flow rate is 14-20 L / min.

3. The method for rapidly homogenizing the microstructure of the heat-affected zone of low-temperature high-manganese steel welding by using pulse current according to claim 1, characterized in that: In step (2), a sample is taken from the welding heat affected zone where the microstructure is unevenly distributed. The method is to extract the welding heat affected zone sample by local sectioning of the welding joint obtained in step (1) using electric spark wire cutting.

4. The method for rapidly homogenizing the microstructure of the heat-affected zone of low-temperature high-manganese steel welding by using pulse current according to claim 1, characterized in that: The pulse current parameters applied in step (3) are as follows: pulse frequency 0.6-6kHz, pulse width 0.2-6ms, current density 20-200A / mm 2 , processing time 5 to 120 minutes.

5. The method of using pulse current to quickly homogenize the microstructure of the heat affected zone of low temperature high manganese steel welding according to claim 1, characterized in that: In the step (4), the sample after the pulse current treatment is polished by using 200-400#, 800-1000#, and 1500-2000# water-abrasive sandpaper to polish the surface of the sample until only scratches in a single direction exist, and using a diamond grinding paste with a particle size of 0.5-2.5μm to polish the surface of the sample to a scratch-free mirror surface.

6. The method of using pulse current to quickly homogenize the microstructure of the heat affected zone of low temperature high manganese steel welding according to claim 1, characterized in that: In the step (4), the sample that has been polished and ground after the pulse current treatment is chemically etched at room temperature, the etching solution is composed of 2-4g picric acid, 4-6mL hydrochloric acid and 90-100mL anhydrous ethanol, and the etching time is 2-4min.

7. The method for rapidly homogenizing the microstructure of the heat-affected zone of low-temperature high-manganese steel welding by using pulse current according to claim 1, characterized in that: In the step (4), the sample polished after the pulse current treatment is subjected to vibration polishing at room temperature, the vibration polishing frequency is 5400-7200 r / min, the vibration polishing liquid is 0.02-0.05 μm non-crystalline silicon dioxide polishing liquid, and the vibration polishing time is 60-120 min.

8. The method of using pulse current to quickly homogenize the microstructure of the heat affected zone of low temperature high manganese steel welding according to claim 1, characterized in that: The characterization of the element distribution state in step (5) requires electron probe X-ray micro-area characterization analysis of the element distribution characteristics within the crystal and at the grain boundaries at no less than 8 locations of the same sample to evaluate the uniformity of the element distribution of the sample after the pulse current treatment.

9. The method for rapidly homogenizing the microstructure of the heat-affected zone of low-temperature high-manganese steel welding by using pulse current according to claim 1, characterized in that: In the step (5), the quantification of the grain boundary structure characteristics requires the use of electron backscatter diffraction technology to quantify the number and distribution characteristics of large and small angle grain boundaries at no less than 6 locations of the same sample, so as to evaluate the uniformity of the grain boundary structure distribution of the sample after pulse current treatment.

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