Metallographic detection method for ultra-low carbon steel resistance spot welding nugget zone

Abstract

The invention discloses a metallographic detection method for an ultra-low carbon steel resistance spot welding nugget zone, and belongs to the technical field of steel and iron material detection. The method comprises the following steps: firstly, cutting a metallographic specimen containing a complete welding part, cutting the metallographic specimen at the diameter position of a welding spot to obtain a specimen detection surface, grinding the specimen detection surface by using 150-850 # abrasive paper with different granularities, polishing, putting into a corrosive liquid consisting of hydrogen peroxide, organic acid, soluble ferric salt and water for corrosion, cleaning the specimen detection surface, drying, and then grinding the specimen detection surface by using the abrasive paper with different granularities. And placing under an optical microscope to collect microscopic images, and measuring the diameter of the nugget and the perimeter of the nugget area. The detection method provided by the invention is simple and feasible, has high efficiency, can clearly display the fusion area of the ultra-low carbon steel, and has a very good application prospect.

Description

Technical Field

[0001] The invention belongs to the technical field of steel material detection, and in particular relates to a metallographic detection method for a nugget zone of ultra-low carbon steel resistance spot welding. Background Art

[0002] Resistance spot welding involves clamping steel plates with electrodes and then applying electricity. This generates a large amount of heat between the plates, liquefying the joints that subsequently cool to form welds. This method of connecting the plates by melting the base metal using resistance heat and then cooling the molten zone to form welds is called resistance spot welding. As an economical and stable welding technology, resistance spot welding has developed rapidly. Its high welding efficiency, stable quality, and ease of automation have led to its widespread application in the automotive, aerospace, and various industrial production industries. Resistance spot welding is particularly popular for connecting sheet metal stampings with a thickness of less than 3mm, which are mostly used.

[0003] Due to the varying chemical compositions and mechanical properties of different steel grades, achieving good joint performance requires a suitable welding process. Before new steel plates are applied to large-scale industrial production, the product's welding process must be explored and certified. This involves testing and testing the corresponding weld nugget size under different welding process parameters. Measuring the weld nugget diameter falls within the scope of metallographic testing and is a critical parameter closely related to the welding process.

[0004] The nugget length is primarily measured at the interface between the two steel plates and the fusion line. The measurement location is the distance between the two points where the weld surface meets the fusion line. The weld to be tested is first sectioned, then the cut surface is ground and polished, then etched. The cross-section is then observed and measured using a microscope. After etching, a white circle appears in the center of the weld, known as the fusion line. The area enclosed by the fusion line is the nugget, and the measurement area is the area enclosed by the fusion line. Therefore, clearly identifying the location of the white fusion line is crucial. Currently, the commonly used etching method for measuring nugget length is 4% nital. While this method is effective for some low-carbon steels, it cannot distinguish between the heat-affected zone and the nugget, making nugget diameter measurement difficult for ultra-low-carbon steels (carbon content approximately 0.02%). Therefore, there is an urgent need for a detection method that clearly displays the fusion zone, facilitates measurement, and ensures accuracy. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a metallographic detection method for the molten zone of ultra-low carbon steel resistance spot welding. The detection method of the present invention can simply and conveniently detect parameters such as the diameter and circumference of the molten zone of ultra-low carbon steel resistance spot welding, and has good application prospects.

[0006] The object of the present invention is to achieve the following goals:

[0007] The present invention provides a metallographic detection method for the nugget zone of ultra-low carbon steel resistance spot welding, comprising the following steps:

[0008] (1) Cut the metallographic specimen containing the complete welded part from the steel plate after resistance point welding, cut the metallographic specimen through the diameter of the weld, and the longitudinal cut surface of the weld is the specimen inspection surface;

[0009] (2) Clamp or mount the metallographic sample of step (1), polish the sample detection surface using sandpaper of different grit sizes from 150# to 850#, and polish the polished sample detection surface using a polishing agent or polishing cloth until the sample detection surface is smooth and free of scratches;

[0010] (3) placing the test surface of the sample in step (2) into a corrosive solution and corroding it under ultrasonic vibration conditions, wherein the corrosive solution consists of 6-10 mL of 25-35 wt% hydrogen peroxide, 4-8 g of an organic acid, 3-6 g of a soluble iron salt, and 180-220 ml of water, the temperature of the corrosive solution is controlled at 20-40° C., and the corrosion time is 0.5-3 min;

[0011] (4) cleaning and drying the corroded test surface of the sample obtained in step (3);

[0012] (5) Place the test surface of the sample obtained in step (4) under an optical microscope, adjust the microscope until the image is clear, collect the microscopic image, and measure the diameter of the weld core and the perimeter of the weld core area.

[0013] Based on the above technical solution, further, the ultra-low carbon steel includes DC series cold-rolled steel sheets and cold-rolled galvanized steel sheets.

[0014] Based on the above technical solution, further, in step (1), the solder joints are cut by wire cutting, and overheating of the sample is avoided to minimize the damage to the solder joints caused by cutting.

[0015] Based on the above technical solution, further, in step (2), the sample test surface is polished with 180#, 320#, 500#, and 800# sandpaper respectively. Each time the sandpaper is changed, the new polishing direction should be perpendicular to the scratches formed by the previous polishing, and the newly generated scratches completely cover the scratches of the previous polishing.

[0016] Based on the above technical solution, further, the preparation process of the etching solution in step (3) is: first adding hydrogen peroxide to water, then adding organic acid and soluble iron salt, stirring evenly to obtain the etching solution.

[0017] Based on the above technical solution, further, the organic acid described in step (3) includes citric acid or its hydrate, malic acid, and ascorbic acid; and the soluble iron salt includes ferric chloride, ferric nitrate, and ferric sulfate.

[0018] Based on the above technical solution, further, the etching solution in step (3) is composed of 6-10 mL of 30 wt% hydrogen peroxide, 4-8 g of citric acid monohydrate, 3-6 g of ferric chloride and 180-220 ml of water.

[0019] Based on the above technical solution, further, in step (4), distilled water is used to clean the sample detection surface, and then the sample detection surface is blown dry.

[0020] Based on the above technical solution, further, the microscopic image in step (5) shows a clear boundary between the weld nugget zone and the surrounding heat-affected zone.

[0021] The present invention has the following beneficial effects compared to the prior art:

[0022] 1. The detection method of the present application is simple, feasible, and highly efficient, and can clearly display the fusion area of ​​ultra-low carbon steel.

[0023] 2. The components of the corrosive liquid used in the detection method of the present application are simple and easily available, and the corrosive method is environmentally friendly and harmless to the human body. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the embodiments of the present invention, the drawings related to the embodiments are briefly introduced below.

[0025] Figure 1 This is a morphology of the weld nugget area after corrosion shown in Comparative Example 1.

[0026] Figure 2 This is a morphology diagram of the weld nugget area after corrosion shown in Example 1.

[0027] Figure 3 The nugget morphology and total perimeter diagram are outlined for the nugget area of ​​Example 1.

[0028] Figure 4 This is a morphology of the weld nugget area after corrosion shown in Comparative Example 2.

[0029] Figure 5 This is a morphology of the weld nugget area after corrosion shown in Example 2. DETAILED DESCRIPTION

[0030] The present invention is described in detail below with reference to the embodiments, but the embodiments of the present invention are not limited thereto. Obviously, the embodiments described below are only some embodiments of the present invention. For those skilled in the art, other similar embodiments obtained without creative work all fall within the scope of protection of the present invention.

[0031] Example 1

[0032] This embodiment provides a metallographic detection method for the nugget zone of ultra-low carbon steel resistance spot welding, taking the corrosion of DC51D+Z hot-dip galvanized steel plate spot welding specimen as an example, including the following steps:

[0033] 1) Sample preparation

[0034] Cut the resistance spot welded specimen into the size of a metallographic specimen and cut the weld at the point where the weld diameter is the largest. The longitudinal cut surface of the weld is the test surface of the specimen. During the cutting process, pay attention to prevent the specimen from overheating to ensure that the size of the specimen test surface is the largest part of the weld diameter. Hot mount the specimen and polish the test surface with 180#, 320#, 500#, and 800# sandpaper respectively. Each time the sandpaper is changed, the new polishing direction should be perpendicular to the scratches formed by the previous polishing. The newly generated scratches completely cover the scratches of the previous polishing until uniform and fine scratches are formed. Use a polishing cloth and a polishing agent with a particle size of 3.0μm to polish the test surface until the surface is smooth and scratch-free.

[0035] 2) Corrosion of the sample

[0036] The reagents selected are 30% hydrogen peroxide, citric acid monohydrate, ferric chloride, and distilled water; 200 ml of distilled water is added to a beaker, 6 ml of 30% hydrogen peroxide is taken out with a measuring cylinder and poured into the beaker, 5 g of citric acid monohydrate and 3 g of ferric chloride are respectively weighed and poured into the beaker, and the mixture is mixed and stirred to obtain a corrosion solution.

[0037] Place the beaker containing the etching solution into an ultrasonic cleaning machine, turn on the ultrasonic wave at a frequency of 50kHz, immerse the sample in the etching solution for etching (25°C) for 2 minutes, and the sample surface changes from a mirror surface to light gray. Take out the sample, rinse it with distilled water, and then blow dry it with a hair dryer.

[0038] 3) Sample testing

[0039] 3.1 Place the prepared sample under an optical microscope and observe the test surface;

[0040] 3.2 Adjust the magnification of the microscope to 25X;

[0041] 3.3 Collect images to obtain a clear microstructure of the weld area. The weld area is black and elliptical after corrosion. The outer circle of the black area is the heat-affected zone, the inner white circle is the fusion line, and the area inside the fusion line is the nugget area.

[0042] 3.4 Select the corresponding scale and measure the diameter of the weld nugget. The measurement result is 6358μm, the perimeter of the weld nugget area is 12833μm, and the total area is about 2207393μm 2 .

[0043] Comparative Example 1

[0044] The experimental process of this comparative example differs from that of Example 1 only in that 4% nitric acid alcohol solution is used to corrode the sample.

[0045] The boundary between the weld nugget area and the surrounding heat-affected zone in the collected microscopic images is unclear and cannot be measured.

[0046] Example 2

[0047] This embodiment provides a metallographic detection method for the nugget zone of ultra-low carbon steel resistance spot welding, taking the corrosion of DC53D+Z hot-dip galvanized steel plate spot welding specimens as an example, including the following steps:

[0048] 1) Sample preparation

[0049] Cut the resistance spot welded specimen into the size of a metallographic specimen and cut the weld at the point where the weld diameter is the largest. The longitudinal cut surface of the weld is the test surface of the specimen. During the cutting process, pay attention to prevent the specimen from overheating to ensure that the size of the specimen test surface is the largest part of the weld diameter. Hot mount the specimen and polish the test surface with 180#, 320#, 500#, and 800# sandpaper respectively. Each time the sandpaper is changed, the new polishing direction should be perpendicular to the scratches formed by the previous polishing. The newly generated scratches completely cover the scratches of the previous polishing until uniform and fine scratches are formed. Use a polishing cloth and a polishing agent with a particle size of 3.0μm to polish the test surface until the surface is smooth and scratch-free.

[0050] 2) Corrosion of the sample

[0051] The reagents selected are 30% hydrogen peroxide, citric acid monohydrate, ferric chloride, and distilled water; 200 ml of distilled water is added to a beaker, 10 ml of 30% hydrogen peroxide is taken out with a measuring cylinder and poured into the beaker, 8 g of citric acid monohydrate and 6 g of ferric chloride are respectively weighed and poured into the beaker, and the mixture is mixed and stirred to obtain a corrosion solution.

[0052] Place the beaker containing the etching solution into an ultrasonic cleaning machine with a frequency of 20kHz. Turn on the ultrasonic wave and immerse the sample in the etching solution for etching (30°C). The etching time is 1.5 minutes. When the sample surface changes from a mirror surface to a light gray, take out the sample, rinse it with distilled water, and then blow dry it with a hair dryer.

[0053] 3) Sample testing

[0054] 3.1 Place the prepared sample under an optical microscope and observe the test surface;

[0055] 3.2 Adjust the magnification of the microscope to 25X;

[0056] 3.3 Collect images to obtain a clear microstructure of the weld area. The weld area is black and elliptical after corrosion. The outer circle of the black area is the heat-affected zone, the inner white circle is the fusion line, and the area inside the fusion line is the nugget area.

[0057] 3.4 Select the corresponding ruler and measure the diameter of the weld nugget. The measurement result is 1064μm.

[0058] Comparative Example 2

[0059] The experimental process of this comparative example differs from that of Example 2 only in that 4% nitric acid alcohol solution is used to corrode the sample.

[0060] The boundary between the weld nugget area and the surrounding heat-affected zone in the collected microscopic images is unclear and cannot be measured.

[0061] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A metallographic detection method for the nugget zone of ultra-low carbon steel resistance spot welding, characterized in that: The steps include: (1) Cut the metallographic specimen containing the complete welded part from the steel plate after resistance point welding, cut the metallographic specimen through the diameter of the weld, and the longitudinal cut surface of the weld is the specimen inspection surface; (2) Clamp or mount the metallographic sample of step (1), polish the sample detection surface using sandpaper of different grit sizes from 150# to 850#, and polish the polished sample detection surface using a polishing agent or polishing cloth until the sample detection surface is smooth and free of scratches; (3) placing the test surface of the sample in step (2) into a corrosive solution and performing oscillation corrosion at an ultrasonic frequency of 20 to 50 kHz, wherein the corrosive solution is composed of 6 to 10 mL of 25 to 35 wt% hydrogen peroxide, 4 to 8 g of an organic acid, 3 to 6 g of a soluble iron salt, and 180 to 220 ml of water, the temperature of the corrosive solution is controlled at 20 to 40° C., and the corrosion time is 0.5 to 3 min; (4) cleaning and drying the corroded test surface of the sample obtained in step (3); (5) Place the test surface of the sample obtained in step (4) under an optical microscope, adjust the microscope until the image is clear, collect the microscopic image, and measure the diameter of the weld core and the perimeter of the weld core area.

2. The metallographic detection method according to claim 1, wherein The ultra-low carbon steel includes DC series cold-rolled steel sheets and cold-rolled galvanized steel sheets.

3. The metallographic detection method according to claim 1, wherein In step (1), the solder joints are cut by wire cutting and overheating of the sample is avoided to minimize damage to the solder joints caused by cutting.

4. The metallographic detection method according to claim 1, wherein: In step (2), the test surface of the sample is polished with 180#, 320#, 500#, and 800# sandpaper respectively. Each time the sandpaper is changed, the new polishing direction should be perpendicular to the scratches formed by the previous polishing, and the new scratches should completely cover the scratches of the previous polishing.

5. The metallographic detection method according to claim 1, wherein: The preparation process of the etching solution in step (3) is as follows: first, hydrogen peroxide is added to water, then an organic acid and a soluble iron salt are added, and the mixture is stirred evenly to obtain the etching solution.

6. The metallographic detection method according to claim 1, wherein: The organic acid described in step (3) includes citric acid or its hydrate, malic acid, and ascorbic acid; the soluble iron salt includes ferric chloride, ferric nitrate, and ferric sulfate.

7. The metallographic detection method according to claim 1, wherein: The etching solution in step (3) consists of 6-10 mL of 30 wt% hydrogen peroxide, 4-8 g of citric acid monohydrate, 3-6 g of ferric chloride and 180-220 ml of water.

8. The metallographic detection method according to claim 1, wherein: In step (4), the sample detection surface is cleaned with distilled water and then blown dry.

9. The metallographic detection method according to claim 1, wherein: In the microscopic image described in step (5), the boundary between the weld nugget area and the surrounding heat-affected zone is clear.

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