A method for preparing low-carbon steel ultra-high frequency water immersion flaw detection specimens

Abstract

This invention relates to a method for preparing a low-carbon steel ultra-high frequency water immersion flaw detection sample. The method includes: rough machining a hot-rolled low-carbon steel bar sample, cutting it to a suitable length, sawing it to the approximate size of the sample to be inspected using a saw, and then milling the surface to be inspected using a milling machine; heat-treating the rough-machined sample to obtain a microstructure that meets the usage conditions; fine machining the heat-treated sample, re-leveling it and ensuring a certain surface roughness to avoid surface unevenness affecting the tracking of interface waves and to reduce surface scattering attenuation caused by surface roughness; finally, performing ultra-high frequency water immersion flaw detection. This sample can meet the requirements of a 50MHz ultra-high frequency water immersion focusing probe and can detect the purity level of inclusions in any longitudinal section along the rolling direction.

Description

Technical Field

[0001] This invention relates to the field of ultra-high frequency water immersion testing technology for low-carbon steel, and particularly to a method for preparing ultra-high frequency water immersion testing samples of low-carbon steel. Background Technology

[0002] Low-carbon steel possesses excellent machinability and is widely used in industries such as automotive, military, power, petroleum, and manufacturing. As market demands for material performance gradually increase, the requirements for inclusion levels in steel are also rising. Not only are the size of inclusions being tested becoming smaller, but the sample area is also increasing. While metallographic inclusion testing can be used to inspect small-sized inclusions in steel, the processing of metallographic inclusion samples is time-consuming and labor-intensive, resulting in low testing efficiency. Therefore, ultra-high frequency (UHF) water immersion testing with C-scan is introduced to assess the purity level of inclusions in steel, effectively and intuitively reflecting the distribution, quantity, and macroscopic morphology of inclusions. Traditional high-frequency water immersion testing, using a 10MHz probe, can theoretically effectively detect defects with an equivalent diameter ≥ φ0.30mm. Its detection capability for smaller inclusions is limited. UHF water immersion testing utilizes an ultra-high frequency probe to inspect the purity of inclusions in steel. The use of 50MHz ultra-high frequency water immersion flaw detection focusing probes in the market is not yet mature, and there is no clear processing technology for the corresponding sample processing. Summary of the Invention

[0003] To address the shortcomings of existing technologies, this invention provides a method for preparing low-carbon steel ultra-high frequency water immersion testing samples, comprising machining and heat treatment. While meeting the requirements of a 50MHz water immersion focusing probe for ultra-high frequency water immersion testing, it also features low processing cost and short processing time. This provides a valuable reference for the preparation of ultra-high frequency water immersion testing samples.

[0004] The technical solution adopted by this invention to solve its technical problem is a method for preparing a low-carbon steel ultra-high frequency water immersion flaw detection sample, the specific steps of which are as follows: (1) Rough machining of hot-rolled low-carbon steel bars: The diameter of the cut bars is φ20mm~φ100mm. When cutting the length, cut bars of 20mm~200mm along the rolling direction. Determine the plane to be inspected. The plane to be inspected is a square longitudinal section, perpendicular to the radial direction, with a length of 20mm~200mm and a width of 20mm~100mm. Sufficient machining allowance should be left between the height of the rough-machined sample and the height of the final sample. Among them, the surface to be inspected should have a machining allowance of 3mm~5mm, and the bottom surface should have a machining allowance of 2mm~4mm. Ensure that the height of the final machined sample is 20mm~90mm. For bars with a diameter of φ20mm~φ45mm, wire cutting can be used for rough machining, and bars with a diameter of φ45mm~φ100mm can be machined by a saw. The sawn sample is then machined by a milling machine to flatten the surface to be inspected. The rough-machined specimens after milling must be free of burrs and flash to prevent stress concentration during subsequent heat treatment, which could lead to specimen cracking.

[0005] (2) Heat treatment of the rough-machined sample: The microstructure of the hot-rolled low-carbon steel bar sample is ferrite and pearlite. Its grains are coarse and the grain boundaries are well-developed, which seriously attenuates the ultrasonic waves. Therefore, heat treatment is needed to improve the microstructure of the sample. The purpose is to refine the grains, improve the attenuation of ultrasonic waves by the coarse grain boundaries, and improve the signal-to-noise ratio. Since the low-carbon steel has a low martensite carbon content after quenching, the internal stress is small, and the rough-machined sample has a regular shape, no burrs, and a small volume, making it less prone to cracking during quenching. Therefore, this invention refines the grains by directly quenching the sample as a whole, thereby improving the situation of coarse grains and well-developed grain boundaries. The sample is heated with the furnace, the quenching heating rate is 1.0℃ / min~3.0℃ / min, the quenching holding temperature is 800℃~900℃, and the holding time is 10min~60min to ensure that the sample can be completely austenitized. During the sample holding process, the holding temperature should not be too high and the holding time should not be too long to prevent overheating and grain growth. After being taken out of the furnace, it is water-quenched to obtain a fine and dense structure through a relatively fast cooling rate.

[0006] (3) Finishing the heat-treated sample: The heat-treated sample has a thick iron oxide scale, which needs to be removed from the surface to be inspected and the corresponding bottom surface to ensure the sample is level. The surface to be inspected needs to have not only the iron oxide scale generated by heat treatment removed, but also the decarburized layer with coarse grains and well-developed grain boundaries removed. Therefore, the grinding amount is 1mm to 3mm on the surface to be inspected and 1mm to 2mm on the corresponding bottom surface. Use a surface grinder to grind the upper and lower surfaces flat. The grinding speed should not be too fast and good cooling should be ensured. The finer the grinding wheel, the smaller the sample roughness and the better the flaw detection effect. The grinding wheel used should have a mesh size ≥120.

[0007] (4) The processed sample is subjected to water immersion testing: The probe used for water immersion testing is a 50MHz ultra-high frequency water immersion focusing probe, which can theoretically effectively detect defects with an equivalent diameter ≥ φ0.06mm.

[0008] This invention involves rough machining to obtain a suitable length and determine the approximate dimensions of the plane to be inspected, followed by milling. Appropriate heat treatment conditions are then applied to obtain a microstructure that meets the usage requirements. The heat-treated sample is then ground to re-level and ensure a certain surface roughness, preventing surface unevenness from affecting interface wave tracking and reducing surface scattering attenuation caused by surface roughness. Finally, ultra-high frequency (UHF) water immersion testing is performed. The sample prepared by this method meets the requirements of a 50MHz UHF water immersion focusing probe and can detect the purity level of inclusions in any longitudinal section along the rolling direction. By prioritizing rough machining of the raw material before heat treatment, this invention reduces the total amount of material requiring heat treatment, shortens the heat treatment time, and reduces the risk of bending deformation during heat treatment. While meeting the requirements of UHF water immersion testing, it reduces processing costs and improves inspection efficiency. Attached Figure Description

[0009] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0010] Figure 1 This is a schematic diagram of sample processing in an embodiment of the present invention.

[0011] Figure 2 The images show the metallographic structure of the hot-rolled low-carbon steel and the corresponding ultrasonic water immersion flaw detection C-scan images in the embodiments of the present invention. Figure 3 The images shown are metallographic photographs of the quenched low-carbon steel and the corresponding ultrasonic immersion test C-scan images from embodiments of the present invention. Detailed Implementation

[0012] The present invention will now be described in detail with reference to specific embodiments.

[0013] This embodiment relates to a processing technology for low-carbon steel ultra-high frequency water immersion flaw detection samples, characterized by the following steps: (1) Rough machining of hot-rolled low-carbon steel bars: The diameter of the cut bar is φ80mm. When cutting the length, cut a 50mm bar along the rolling direction. Determine the inspection plane at 1 / 2 radius. The inspection plane is a square longitudinal section, perpendicular to the radial direction, with a length of 69mm and a width of 50mm. Sufficient machining allowance should be left between the height of the rough-machined sample and the height of the final sample to ensure the dimensions of the final inspection plane. Leave a machining allowance of 4mm on the inspection surface and a machining allowance of 3mm on the corresponding bottom surface. Ensure that the height of the final machined sample is 20mm. Rough machining is performed using a saw. The length of the inspection surface of the rough-machined sample is 64mm, the width is 50mm, and the height is 27mm. The sawn sample is then machined using a milling machine. 1mm is milled off the top and bottom surfaces to flatten the inspection surface and the bottom surface. The rough-machined sample after milling should not have burrs or flash to prevent stress concentration during subsequent heat treatment, which could lead to sample cracking.

[0014] (2) Heat treatment of the rough-machined sample: The microstructure of the hot-rolled low-carbon steel bar sample is ferrite and pearlite, with coarse grains and well-developed grain boundaries, which severely attenuates ultrasonic waves. Therefore, heat treatment is needed to improve the microstructure of the sample, with the aim of refining the grains and improving the attenuation of ultrasonic waves by the coarse grain boundaries, thereby improving the signal-to-noise ratio. Because the rough-machined sample has a regular shape, no burrs, and a small volume, and because the low carbon content of low-carbon steel results in low carbon content and low carbon content in martensite after quenching, the internal stress is small and cracking is not easy during heat treatment. Therefore, this invention refines the grains by directly quenching the sample, thereby improving the phenomenon of coarse grains and well-developed grain boundaries. The sample is heated with the furnace, the quenching heating rate is 1.0℃ / min, the quenching holding temperature is 870℃, and the holding time is 40 min. It is necessary to ensure that the sample can be completely austenitized. During the sample holding process, the holding temperature should not be too high and the holding time should not be too long to prevent grain growth. After being taken out of the furnace, it is water-quenched to obtain a fine and dense structure through a relatively fast cooling rate.

[0015] (3) Finishing the heat-treated sample: The heat-treated sample has a thick iron oxide scale, which needs to be removed from the surface to be inspected and the corresponding bottom surface, and the sample must be level. The surface to be inspected needs to be ground to remove not only the iron oxide scale generated by heat treatment, but also the decarburized layer with coarse grains and well-developed grain boundaries. Therefore, the grinding amount of the surface to be inspected is 3 mm and the grinding amount of the bottom surface is 1 mm. The upper and lower surfaces are ground flat using a surface grinder. The grinding speed should not be too fast, and good cooling should be ensured. The finer the grinding wheel, the smaller the roughness of the sample and the better the flaw detection effect. The grinding wheel used is 120 mesh, and the final surface roughness of the sample is Ra=0.3μm.

[0016] (4) The processed sample is subjected to ultra-high frequency water immersion testing: The probe used for water immersion testing is a 50MHz ultra-high frequency water immersion focusing probe, which can theoretically effectively detect defects with an equivalent diameter ≥ φ0.06mm. The sample processed by this invention has a low noise level and good testing effect when subjected to ultra-high frequency water immersion testing.

[0017] The above describes the processing technology of a low-carbon steel ultra-high frequency water immersion test sample according to the present invention. While meeting the requirements of the 50MHz water immersion focusing probe for ultra-high frequency water immersion testing, it also takes into account the characteristics of low processing cost and high inspection efficiency.

[0018] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications to the technical solutions described in the foregoing embodiments, or equivalent substitutions for some or all of the technical features, do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions in the embodiments of the present invention, and are all covered by the claims of the present invention.

Claims

1. A method for preparing a low-carbon steel ultra-high frequency water immersion flaw detection sample, characterized in that, Includes the following steps: (1) Rough machining of hot-rolled low-carbon steel bar sample: cut the required length, determine the surface to be inspected, use a saw or wire cutting machine to process to the size of the sample to be inspected plus the machining allowance, and then perform milling machine processing to mill the surface to be inspected flat, which is the rough-machined sample; (2) Heat treatment of rough-machined specimens: The rough-machined specimens are directly quenched as a whole. The quenching heating rate is 1.0℃ / min~3.0℃ / min, the quenching holding temperature is 800℃~900℃, the quenching holding time is 10min~100min, and after taking them out of the furnace, they are water quenched or oil cooled to ensure that the specimens are completely austenitized. (3) Finish the heat-treated sample: use a surface grinder to grind away the iron oxide scale and decarburized layer produced by heat treatment to ensure that the surface of the sample to be inspected is level; the grinding wheel used has a mesh size ≥ 120. (4) The processed sample is subjected to ultra-high frequency water immersion testing.

2. The method for preparing low-carbon steel ultra-high frequency water immersion flaw detection specimens as described in claim 1, characterized in that, In step (1), the diameter of the hot-rolled low-carbon steel bar sample is φ20mm~φ100mm; a bar sample of 20mm~200mm is cut along the rolling direction.

3. The method for preparing low-carbon steel ultra-high frequency water immersion flaw detection specimens as described in claim 1, characterized in that, In step (1), the rough-machined surface to be inspected is a square longitudinal section, perpendicular to the radial direction; the length is 20mm to 200mm and the width is 20mm to 100mm.

4. The method for preparing low-carbon steel ultra-high frequency water immersion flaw detection specimens as described in claim 1, characterized in that, In step (1), a machining allowance of 3mm to 5mm is retained on the surface of the rough-machined sample to be inspected, and a machining allowance of 2mm to 4mm is retained on the bottom surface.

5. The method for preparing low-carbon steel ultra-high frequency water immersion flaw detection specimens as described in claim 1, characterized in that, The height of the rough-machined sample is 20mm to 90mm.

6. The method for preparing low-carbon steel ultra-high frequency water immersion flaw detection specimens as described in claim 1, characterized in that, In step (1), the roughing process is performed by wire cutting for bar samples with a diameter of φ20mm to φ45mm and by sawing for bar samples with a diameter of φ45mm to φ100mm.

7. The method for preparing low-carbon steel ultra-high frequency water immersion flaw detection specimens as described in claim 1, characterized in that, In step (1), the rough-machined sample after milling must not have burrs or flash, to prevent stress concentration during subsequent heat treatment that could cause the sample to crack.

8. The method for preparing low-carbon steel ultra-high frequency water immersion flaw detection specimens as described in claim 1, characterized in that, In step (3), the amount of grinding off the surface to be inspected is 1mm to 3mm, and the amount of grinding off the bottom surface is 1mm to 2mm.

9. The method for preparing a low-carbon steel ultra-high frequency water immersion flaw detection sample as described in claim 1, characterized in that, In step (4), the probe used for water immersion testing is a 50MHz ultra-high frequency water immersion focusing probe, which can theoretically effectively detect defects with an equivalent diameter ≥ φ0.06mm.

10. The method for preparing a low-carbon steel ultra-high frequency water immersion flaw detection sample as described in claim 1, characterized in that, The chemical composition (by weight percentage) of the low-carbon steel is as follows: C: 0.15–0.19%, Si: 0.17–0.37%, Mn: 1.10–1.30%, P: ≤0.025%, S: 0.020–0.035%, Cr: 0.90~1.10%, Ni: ≤0.30%, Cu: ≤0.20%, Al: 0.20~0.40%, with the remainder being Fe and unavoidable impurities.

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