Method for preparing hardness test sample of thick steel plate large heat input welding joint

Abstract

The present application relates to the technical field of hardness testing of steel material welded joint, and particularly relates to a method for preparing a hardness testing sample of a thick steel plate welded joint with large welding energy. The whole section is prepared to solve the problem of discontinuous points of hardness values of segmented sampling, and to accurately determine the testing points of hardness of each welding area, so as to continuously determine the hardness values of the welded joint and meet the requirements of hardness testing of ship plates and other thick plates. The grinding and polishing method is used to solve the grinding and polishing problem of large-size samples which cannot be achieved by conventional metallographic welding sample preparation. A kind of alkyl benzene sulfonic acid sodium surfactant is prepared to uniformly treat the surface of the sample. The symmetry of the structure of the thick plate welded joint and the boundary lines of each area are displayed more clearly, and the boundaries of each welding area can be clearly defined, and the interval points of hardness testing can be accurately changed. The cold acid etchant is prepared to clearly present the structures of the base material, the heat affected zone and the welding zone of the welded joint.

Description

Technical Field

[0001] This invention relates to the field of hardness testing technology for welded joints of steel materials, and particularly to a method for preparing test specimens for hardness testing of welded joints of thick steel plates with high heat input. Background Technology

[0002] In the quality assessment and inspection of steel welding technology, it is usually necessary to examine the microstructure and properties of the weld seam. This inspection involves taking a metallographic sample from the weld seam of the welded steel plate, including the base metal, heat-affected zone, and weld metal. After grinding, polishing, and etching with 3% nitric acid alcohol, the entire weld area is exposed. The hardness values ​​of the three zones are measured using a Vickers hardness tester to obtain a hardness curve, which is then used to assess the quality of the welded plate.

[0003] In recent years, with the widespread application of high-strength, thick-gauge steel plates in various fields, high heat input welding technology has emerged. The quality assessment of high heat input welding performance for high-strength ship plates is more stringent. The weld pool formed by high heat input welding of thick-gauge ship plates is wider, exceeding 40 mm. Ship plate certification requires full-section weld samples to reflect symmetrical weld morphology. Hardness measurements must be taken at 2 mm intervals between the weld and base metal, and at 0.7 mm intervals in the heat-affected zone, to obtain a continuous hardness distribution curve. This serves as the technical basis for judging the weldability of the steel plate; there must be no discontinuities in hardness measurements. Therefore, the cross-sectional dimensions of the weld sample must be greater than 80 × 80 mm.

[0004] The large cross-section sample size far exceeds the size of conventional metallographic samples (20×20 mm), making it difficult to prepare hardness samples: (1) It cannot be polished on a normal polishing machine. (2) The weld surface finish cannot reach RA0.8, and when measuring Vickers hardness, the indentation cannot be focused, making it impossible to accurately determine the hardness value.

[0005] Currently, the common practice is to cut several samples into sections and prepare hardness test specimens for high heat input weld joints of thick steel plates using conventional metallographic specimen preparation methods. This method has the following drawbacks: it disrupts the overall morphology of the weld joint; hardness values ​​cannot be measured at the cut edge and approximately 3-4 mm on both sides of the cut sample; the test values ​​are discontinuous, with discontinuities in the test curve, failing to meet the certification requirements of international classification societies.

[0006] Therefore, conventional metallographic sample preparation methods cannot be used to prepare hardness test specimens for steel plates with welded joints that have undergone high heat input welding (heat input above 50 kJ / cm) and a thickness of more than 20 mm. Summary of the Invention

[0007] To overcome the shortcomings of existing technologies, this invention provides a method for preparing a test specimen for the hardness of welded joints of thick steel plates with high heat input. This method allows for accurate determination of test points for the hardness of each welded area on the entire cross-section specimen, enabling continuous measurement of the hardness value of the welded joint, thus meeting the hardness testing requirements for thick plates such as ship plates.

[0008] To achieve the above objectives, the present invention employs the following technical solution:

[0009] A method for preparing test specimens for hardness testing of welded joints of thick steel plates with high heat input, where the thickness of the steel plate is 20 mm or more and the heat input is 50 kJ / cm or more.

[0010] The preparation method specifically includes the following steps:

[0011] Step 1: Cut the sample

[0012] Take a thick steel plate sample that has been welded with high heat input (50 kJ / cm) and cut a full-section weld joint sample including the base material on both sides at the weld location.

[0013] For test surfaces with a side length of 80mm × 80mm or more, the entire cross-section of the weld joint is first rough-ground using a grinding machine. As the grinding precision increases, the feed rate is gradually controlled within a range of tens of micrometers. Finally, 2-3 passes are used to achieve spark-free grinding. The final surface roughness of the weld joint reaches RA0.6.

[0014] Step 2: Upright flat polishing + corneal polishing

[0015] 1) Place the sample surface, after being ground on a grinding machine, facing upwards, and secure it to a caliper using a clamp. Use a manual sticking wheel polisher to perform upright manual polishing of the sample surface. At the beginning of polishing, use the flat surface of the sticking wheel to make even contact with the entire sample surface for polishing. Select a polishing liquid with a particle size range of 1.5 to 2.5 W. Spray water to cool the surface while polishing to prevent localized overheating.

[0016] 2) Due to significant differences in hardness across different areas of the high-heat welded joint, when flat polishing reveals variations in gloss levels in different areas, the polishing wheel is tilted at a certain angle. The wheel's edge is then used to gradually approach the less glossy areas, employing a line-contact polishing technique to uniformly move the polishing operation. This locally polishes the softer areas, quickly improving the local surface finish. Subsequently, alternating flat polishing and angle polishing operations are used until the entire cross-section achieves a uniform gloss level of RA0.8.

[0017] Step 3: Surfactant Pretreatment

[0018] Because the surface of the upright polished sample is subjected to uneven normal pressure and grinding force from the polishing wheel, a surface stress is formed in the shallow layer of the sample, resulting in a significant difference in corrosion resistance compared to the surface prepared by the conventional inverted polishing method. Therefore, sodium alkylbenzene sulfonate surfactant was uniformly applied to the polished sample surface, rinsed with hot water after approximately 5 minutes, and then dried with high-pressure air.

[0019] Step 4: Prepare a cooling acid reagent to corrode and reveal the metallographic structure of each weld zone.

[0020] Prepare a cold acid etchant; the mixing ratio of the cold acid etchant is as follows:

[0021] Nitric acid (HNO3), concentration 20%–60%, 2–20 ml;

[0022] Picric acid C6H2OH(NO2)3, 5-50g;

[0023] Anhydrous ethanol C2H6O, 50-200 ml.

[0024] The above agents are mixed to form a cold acid etching reagent, which is then used to uniformly etch the surface of the sample treated with surfactant. After the microstructure of the base material, heat-affected zone, and weld zone of the weld joint is clearly revealed, the entire sample surface is immediately immersed in a container of anhydrous ethanol for 1-2 minutes, then removed and dried with high-pressure air. Hardness testing is then performed.

[0025] Compared with the prior art, the present invention has at least the following technical effects or advantages:

[0026] 1. Thick steel plates welded with high heat input result in wide weld pools and strong symmetry at the weld joint. This invention involves sawing a full-section weld joint sample, including both sides of the base material, at the weld location. This full-section preparation solves the problem of discontinuities in hardness values ​​caused by segmented sampling. Compared to conventional segmented sampling of steel plate weld joints, the full-section sample allows for accurate determination of hardness test points in each weld area, enabling continuous measurement of weld joint hardness values ​​and meeting the hardness testing requirements for thick plates such as ship plates.

[0027] 2. This invention employs a clamp to fix the sample, which is essential for successful grinding and polishing. Otherwise, the sample would fly away immediately upon contact with the grinding wheel. The clamp ensures effective contact between the sample surface and the grinding wheel. To achieve the desired smoothness and improve the uneven polishing effect caused by flat polishing, the polishing wheel is tilted at a certain angle, and the wheel edge is used to progressively polish the underpolished areas. A polishing method combining upright flat polishing and corner polishing is used, alternating between flat and corner polishing, ultimately achieving a uniform RA0.8 gloss across the entire cross-section. This invention utilizes special grinding and polishing methods to solve the problem of grinding and polishing large-size samples that cannot be achieved with conventional metallographic welding sample preparation.

[0028] 3. Manual polishing, due to its uneven pressure and grinding force, results in uneven surface stress distribution (a problem not present in inverted automatic polishing). This leads to highly uneven corrosion resistance of the test surface, affecting the metallographic display effect. Therefore, this invention specifically formulates a sodium alkylbenzene sulfonate surfactant to homogenize the test surface. Through the surfactant activation effect, residual stress left by shallow surface polishing of the sample can be effectively alleviated and removed, resulting in a uniform metallographic corrosion effect across the entire weld joint. This promotes the display effect of macrostructure in various regions of the high-heat weld joint, making the symmetry of the thick plate weld joint structure and the boundary lines of each region clearer. It also allows for clear definition of the boundaries of each weld zone and accurate adjustment of the hardness measurement spacing points.

[0029] 4. This invention uses a specially formulated cold acid etchant that clearly reveals the microstructure of the base material, heat-affected zone, and weld zone at the weld joint. It enables clear definition of the boundaries of each weld zone and accurate adjustment of the hardness measurement interval. Attached Figure Description

[0030] Figure 1 The macroscopic morphology of the cross-sectional hardness test specimen of the high heat input welded joint of thick steel plate prepared by the method of the present invention is shown. Detailed Implementation

[0031] This invention discloses a method for preparing a hardness test specimen for a high heat input welded joint of thick steel plate. Those skilled in the art can refer to this document and appropriately modify the process parameters to achieve the desired result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this invention. The methods and applications of this invention have been described through preferred embodiments. Those skilled in the art can clearly modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit, and scope of this invention to realize and apply the technology of this invention.

[0032] The test specimen preparation method for the hardness test of the welded joint of thick steel plate with high heat input consists of four steps: 1. flat grinding of the entire cross-section specimen of the welded joint with high heat input; 2. upright flat polishing + corneal polishing; 3. surfactant pretreatment; 4. corrosion of the welded joint structure with cold acid etching reagent.

[0033] 1. Surface grinding of the entire cross-section specimen of the high heat input welded joint.

[0034] Take a thick steel plate sample that has been welded with high heat input (50 kJ / cm) and cut a full-section weld joint sample including the base material on both sides at the weld location.

[0035] The test surface has a side length of 80mm x 80mm. First, a grinding machine is used to rough grind the entire cross-section of the welded interface. As the grinding precision increases, the feed rate is gradually controlled within a range of tens of micrometers. Finally, 2-3 passes are used to achieve spark-free grinding. The final result is RA0.6.

[0036] 2. Upright flat polishing + corneal polishing

[0037] (1) Place the sample surface, after being ground on a grinding machine, with the surface facing upwards, and fix it in a clamp. Use a manual sticking wheel polisher to perform upright manual polishing on the sample surface. At the beginning of polishing, use the flat surface of the sticking wheel to make uniform contact with the entire sample surface for polishing. Select polishing liquid with a particle size range of W1.5 to 2.5; and spray water to cool while polishing to prevent local overheating of the surface.

[0038] (2) The hardness of different areas of the high heat input welded joint varies greatly. When flat polishing shows differences in the gloss effect of different areas, the polishing wheel is tilted at a certain angle. The wheel edge is used to gradually approach the less glossy area. The polishing operation is carried out in a line contact manner to uniformly move the polishing operation and perform local polishing on the floppy drive to quickly improve the local surface finish. Afterwards, the operation of alternating flat polishing and angle polishing is used intermittently until the entire cross section uniformly achieves a gloss effect of RA0.8.

[0039] 3. Surfactant pretreatment

[0040] Because the surface of the upright polished sample is subjected to uneven normal pressure and grinding force from the polishing wheel, a surface stress is formed in the shallow layer of the sample, resulting in a significant difference in corrosion resistance compared to the surface prepared by the conventional inverted polishing method. Therefore, sodium alkylbenzene sulfonate surfactant was uniformly applied to the polished sample surface, rinsed with hot water after approximately 5 minutes, and then dried with high-pressure air.

[0041] 4. Use cold acid reagent to corrode and reveal the metallographic structure of each weld zone.

[0042] Prepare a cold acid etchant; the mixing ratio of the cold acid etchant is as follows:

[0043] Nitric acid (HNO3), concentration 20%–60%, 2–20 ml;

[0044] Picric acid C6H2OH(NO2)3, 5-50g;

[0045] Anhydrous ethanol C2H6O, 50-200 ml.

[0046] The above agents are mixed to form a cold acid etching reagent, which is then used to uniformly etch the surface of the sample treated with surfactant. After the microstructure of the base material, heat-affected zone, and weld zone of the weld joint is clearly revealed, the entire sample surface is immediately immersed in a container of anhydrous ethanol for 1-2 minutes, then removed and dried with high-pressure air. Hardness testing is then performed.

[0047] Example 1:

[0048] Preparation of weld hardness test specimens for 46mm thick low-alloy high-strength ship plates after high heat input (above 50 kJ / cm).

[0049] The chemical composition of the sample, by weight percentage, is as follows:

[0050] C: 0.14%, Si: 0.35%, Mn: 1.20%, P: 0.032%, S: 0.035%, Nb: 0.03%, V: 0.05%, Als 0.018%.

[0051] 1. Take a whole-section welded joint sample and mechanically grind it flat.

[0052] After high heat input welding, a full-section weld joint sample, including both sides of the base material, was taken from the weld seam of the steel plate. The transverse side length of the sample was 60 mm. The entire cross-section of the weld joint was rough ground using a grinding machine. During the process, as the grinding precision improved, the feed rate was gradually controlled to a range of tens of micrometers. The final 2-3 passes achieved spark-free grinding, resulting in a surface finish of RA0.6.

[0053] 2. Upright flat polishing + corneal polishing

[0054] With the ground sample surface facing upwards, secure it to the worktable with calipers. Use a manual polishing machine for upright surface polishing: first, move the polishing wheel evenly to contact the entire sample surface, using a polishing slurry with a particle size of W1.5-2.5 to assist polishing; simultaneously spray water for cooling to prevent surface overheating.

[0055] When flat polishing shows differences in gloss in different areas and areas with low substrate hardness show underpolishing, switch to corneal polishing: tilt the polishing wheel at a certain angle and use the edge of the wheel to gradually move to the underpolished area, and perform local polishing operation in a line contact manner to achieve a rapid improvement in the gloss of the local area. Afterwards, alternate between flat polishing and corner polishing to make the entire cross section uniformly achieve a gloss effect of RA0.8.

[0056] 3. Surfactant pretreatment

[0057] Take 20 ml of sodium alkylbenzene sulfonate and 100 ml of water, stir well to make a surfactant solution, apply the solution evenly to the polished sample surface, leave it for 5 minutes, rinse with hot water and cold water, and dry with high pressure air.

[0058] 4. Use cold acid reagent to corrode and reveal the metallographic structure of each weld zone.

[0059] Prepare a cold acid etchant; the mixing ratio of the cold acid etchant is as follows:

[0060] Nitric acid (HNO3) (38% concentration) 6-8 ml;

[0061] Picric acid C6H2OH(NO2)3 10g;

[0062] Anhydrous ethanol C2H6O 94~98ml.

[0063] The above agents are mixed to prepare a cold acid etching reagent, which is then evenly applied to the surface of the surfactant-treated sample. After the microstructure of the base material at the weld interface, the heat-affected zone, and the weld fusion line on the sample surface is clearly visible, the entire sample surface is immediately immersed in a container of anhydrous ethanol for 2 minutes, then removed and dried with high-pressure air. This produces a hardness test sample.

[0064] Example 2:

[0065] Preparation of test specimens for weld hardness testing of 60mm thick low-alloy high-strength steel plates after high heat input (above 50 kJ / cm).

[0066] The chemical composition of the sample, by weight percentage, is as follows:

[0067] C: 0.18%, Si: 0.33%, Mn: 1.3%, P: 0.025%, S: 0.0012%, Nb: 0.04%, V: 0.05%, Als: 0.016%.

[0068] 1. Take a whole-section welded joint sample and mechanically grind it flat:

[0069] After high heat input welding, a full-section weld joint sample, including both sides of the base material, was taken from the weld location of the low-alloy high-strength steel plate. The transverse side length of the sample was 80 mm. The entire cross-section of the weld joint was rough ground using a grinding machine. During the process, as the grinding precision improved, the feed rate was gradually controlled to a range of tens of micrometers. The final 2-3 passes achieved spark-free grinding, resulting in a surface finish of RA0.6.

[0070] 2. Upright flat polishing + corneal polishing

[0071] With the ground sample surface facing upwards, secure it to the worktable with calipers. Use a manual polishing machine for upright surface polishing: first, make the entire flat surface of the polishing wheel contact the sample surface, move it evenly, and use polishing fluid with a particle size of W1.5-2.5 to assist polishing; simultaneously spray water to cool the surface and prevent overheating.

[0072] When flat polishing shows differences in gloss in different areas and areas with low substrate hardness show underpolishing, switch to corneal polishing: tilt the polishing wheel at a certain angle and use the edge of the wheel to gradually move to the underpolished area, and perform local polishing operation in a line contact manner to achieve a rapid improvement in the gloss of the local area; afterward, alternate between flat polishing and corner polishing to make the entire cross section uniformly achieve a gloss effect of RA0.8.

[0073] 3. Surfactant pretreatment

[0074] Take 20 ml of sodium alkylbenzene sulfonate and 100 ml of water, stir well to make a surfactant solution, apply the solution evenly to the polished sample surface, leave it for 5 minutes, rinse with hot water and cold water, and dry with high pressure air.

[0075] 4. Use cold acid reagent to corrode and reveal the metallographic structure of each weld zone.

[0076] Prepare a cold acid etchant; the mixing ratio of the cold acid etchant is as follows:

[0077] Nitric acid (HNO3) (37% concentration) 6-8 ml;

[0078] Picric acid C6H2OH(NO2)3 10g;

[0079] Anhydrous ethanol C2H6O 94~98ml.

[0080] The above agents are mixed to prepare a cold acid etching reagent, which is then evenly applied to the surface of the surfactant-treated sample. After the microstructure of the base material at the weld interface, the heat-affected zone, and the weld fusion line on the sample surface is clearly visible, the entire sample surface is immediately immersed in a container of anhydrous ethanol for 2 minutes, then removed and dried with high-pressure air. This yields the hardness test sample.

[0081] Figure 1 The macroscopic morphology of the full-section hardness test specimen of the high heat input welded joint of thick steel plate prepared by the method of the present invention is shown below. Figure 1 As shown, this invention is a technique for preparing hardness test specimens for full-section welded joints of thick steel plates welded with high heat input. It utilizes special grinding and polishing methods to solve the problem of grinding and polishing large-size specimens that cannot be achieved with conventional metallographic welding specimen preparation. By introducing a surfactant activation effect during the corrosion process, the macroscopic microstructure of each region of the high heat input weld is enhanced, making the symmetry of the thick plate weld joint microstructure and the boundary lines of each region clearer. This satisfies the need for clearly defining the boundaries of each weld zone and accurately adjusting the hardness measurement interval.

[0082] Thick steel plates are welded using high heat input, resulting in a wide and highly symmetrical weld pool at the weld interface. The use of a whole-section sample solves the problem of discontinuities in hardness values ​​obtained from segmented sampling. The test points for hardness in each welded area can be accurately determined on the whole-section sample, enabling continuous measurement of the weld interface hardness value and meeting the hardness testing requirements for thick plates such as ship plates.

[0083] Pretreatment of the sample surface after upright flat grinding and polishing with surfactants can effectively alleviate and remove residual stress left by shallow grinding and polishing of the sample, making the metallographic corrosion effect of the entire welding joint uniform and the boundaries of each welding zone clearer.

[0084] Compared with conventional segmented sampling of steel plate welded joints, the method of preparing whole-section samples for high heat input welded joint hardness specimens simplifies the process, provides complete display of each welded zone, and enhances the accuracy of measurements.

[0085] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a hardness test specimen for a heavy plate large- energy- welding joint, characterized in that, The specific steps are as follows: Step 1. Cut the sample Cut a full-section weld joint sample, including the base material on both sides, at the weld location; The sample surface has a side length of 80mm×80mm or more, and is machined to make the surface roughness Ra of the welded joint < 0.6um; Step 2. Upright horizontal projectile + angled projectile With the sample surface facing up, fix it to the caliper with a clamp, and use a manual adhesive wheel polisher to perform upright manual polishing on the sample surface; At the beginning of polishing, the flat surface of the sticky wheel is used to make uniform contact with the entire sample surface for polishing; Then, tilt the polishing wheel at a certain angle and gradually approach the dull area using the edge of the wheel. Move the polishing operation evenly in a line contact manner to polish the soft area locally. Then, an alternating polishing process of flat polishing and angle polishing was adopted, and finally the entire cross section uniformly achieved a gloss effect of RA0.8; Step 3. Prepare surfactants for pretreatment. Sodium alkylbenzene sulfonate was prepared to homogenize the sample surface; Step 4. Prepare a cooling acid etching reagent and use it to etch and reveal the metallographic structure of each weld zone. The ratio of the cold acid etchant is: Nitric acid (HNO3), concentration 20%~60%, 2~20ml; Picric acid C6H2OH(NO2)3, 5~50g; Anhydrous ethanol C2H6O, 50~200ml.

2. The method for preparing a heavy plate high heat input welded joint hardness test specimen according to claim 1, characterized in that, Step 1. Cutting is sawing; machining is first rough grinding of the entire cross-section of the welded interface using a grinding machine. As the grinding precision improves, the feed rate of the grinding is gradually controlled within a range of tens of micrometers. Finally, 2-3 passes are used to achieve spark-free grinding.

3. The method of producing a heavy plate high heat input welded joint hardness test coupon according to claim 1, characterized in that, Step 3: Select sodium alkylbenzene sulfonate surfactant and apply it evenly to the polished sample surface. After 5 minutes, rinse with water at a temperature higher than 40°C and dry with high-pressure air at a pressure of 30~200KPa.

4. The method according to claim 1, wherein Step 4. Use cold acid etching reagent to uniformly etch the sample surface. After the microstructure of the base material, heat-affected zone, and weld zone of the weld joint is clearly revealed, immediately immerse the entire sample surface in the cleaning agent, then take it out and blow it dry, and perform a hardness test.

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