A metallographic etching liquid and etching method for hydraulic support welding heat-affected zone

Abstract

This invention relates to a metallographic etching solution and method for the heat-affected zone of hydraulic support welds, comprising a first-step base etching solution and a second-step selective coloring solution; the first-step base etching solution comprises nitric acid, a low-carbon alcohol, and water; the second-step selective coloring solution comprises nitric acid, a low-carbon alcohol, water, an acidic fluoride salt, and a selective inhibitor; the volume concentration of nitric acid is (0.3 + 0.3 × HGI)% to (0.5 + 0.6 × HGI)%, the mass-volume concentration of the acidic fluoride salt is (0.5 + 1.0 × ASI) g / L to (1.0 + 1.5 × ASI) g / L, and the selective coloring solution comprises nitric acid, a low-carbon alcohol, water, an acidic fluoride salt, and a selective inhibitor; the volume concentration of nitric acid is (0.3 + 0.3 × HGI)% to (0.5 + 0.6 × HGI)%, the mass-volume concentration of the acidic fluoride salt is (0.5 + 1.0 ... Corrosion inhibitors include molybdate corrosion inhibitors and benzotriazole corrosion inhibitors or their derivatives, with a total mass-volume concentration ranging from (0.2 + 0.8 × HGI) g / L to (0.5 + 1.2 × HGI) g / L. The heat-affected zone microstructure gradient index HGI = (C + Mn / 6 + Cr / 5 + Mo / 4 + V / 3) × (1 + Ni + Cu) / (1 + Nb + Ti + Al), and the alloy strengthening index ASI = Nb + Ti + 0.5 × (V + Al). In the above two formulas, the symbols of each element represent the mass percentage of that element in the steel. This invention can solve the technical problems of the difficulty in clearly distinguishing the sub-regions within the heat-affected zone formed by conventional corrosion solutions, and the reliance on manual experience for corrosion parameter control, resulting in poor consistency of test results.

Description

Technical Field

[0001] This invention relates to the field of metallographic testing technology, and in particular to a metallographic etching solution and etching method for the heat-affected zone of hydraulic support welding. Background Technology

[0002] Low-alloy ultra-high-strength steels (such as classic Q1100, S1100QL, and 35CrMnSiA, with yield strengths not less than 1030MPa; and conventional low-alloy high-strength quenched and tempered steels such as Q890, Q960, and 30CrMnSi, whose strength grades are in the transition range, are sometimes considered atypical ultra-high-strength steels) are widely used in key structural components of heavy-duty equipment such as hydraulic supports due to their excellent strength and relatively low cost. To ensure welding quality, macroscopic metallographic corrosion testing is usually required on the welded joints to clearly reveal the microstructure of the heat-affected zone and determine whether welding defects such as coarse grains, weld hardening, and tempering softening exist.

[0003] In existing technologies, the macroscopic metallographic corrosion of such ultra-high strength steel welded joints generally follows the corrosion methods used for conventional low-alloy high-strength steel. This involves using a fixed-ratio nitric acid-alcohol solution as the etchant, with operators controlling the corrosion time based on experience, and visually observing the degree of discoloration on the sample surface to determine the corrosion endpoint. However, these conventional methods present significant technical problems when applied to ultra-high strength steel welded joints, especially when analyzing the fine structure of the heat-affected zone subregion (which is a macroscopic structure obtained through low-magnification observation).

[0004] Specifically, the base metal structure of ultra-high strength steel is mostly fine-grained tempered martensite or lower bainite, and contains a relatively high amount of alloying elements such as Cr, Ni, and Mo to improve corrosion resistance, exhibiting strong initial resistance to nitric acid alcohol. However, under the thermal cycling of welding, the originally uniform and dense base metal structure in the heat-affected zone (HAZ) is destroyed by the high welding temperature, forming different sub-regions from the fusion line outwards: a coarse-grained zone, a fine-grained zone, an incomplete recrystallization zone, and a tempered softening zone. Due to different peak heating temperatures, each sub-region exhibits a gradient distribution of grain size and microstructure deterioration, resulting in a corresponding gradient in corrosion resistance, all significantly lower than that of the base metal. If the corrosion time is set based on the full exposure of the base metal, the sub-regions of the HAZ may already be over-corroded, with subtle contrast differences between sub-regions masked by overall blackening. Conversely, if the HAZ is considered to be completely uncorroded, the sub-regions within the HAZ may be difficult to distinguish due to insufficient corrosion depth, and areas such as the base metal may still be in an under-corroded state, unable to be effectively distinguished from the HAZ. Because the difference in corrosion resistance between the sub-regions of the heat-affected zone is much smaller than the overall difference between the heat-affected zone and the base metal, even if a compromise corrosion time is found in the traditional one-step corrosion method, it can often only show the basic outline of the weld, the heat-affected zone and the base metal. The coarse-grained zone, fine-grained zone, incomplete recrystallization zone and tempering softening zone of the heat-affected zone usually appear as a blurry gray transition zone under low magnification, and it is impossible to clearly and accurately distinguish the boundaries and transition states of each sub-region.

[0005] Secondly, the alloy composition systems of different grades of ultra-high strength steel vary significantly. Even for steel of the same grade, the content of elements such as Cr, Ni, Mo, and V can vary significantly depending on the manufacturer or batch. This results in different microstructure gradients and corrosion resistance distributions in the sub-zones of the heat-affected zone (HAZ) of different batches of steel. This means that empirical corrosion parameters developed for a specific type of steel often cannot effectively distinguish the sub-zones of the HAZ when applied to another batch or grade of steel. Consequently, even experienced operators struggle to establish stable and reliable judgment criteria when dealing with new steel, often resorting to trial and error. This leads to significant differences in test results between different operators and between different batches by the same operator, resulting in poor repeatability and comparability. This severely restricts the efficiency and reliability of ultra-high strength steel welding quality inspection. Summary of the Invention

[0006] (a) Technical problems to be solved

[0007] In view of the above-mentioned shortcomings and deficiencies of the prior art, the present invention provides a metallographic etching solution and etching method for the heat-affected zone of hydraulic support welding. It solves the technical problems of conventional etching solutions and etching methods in the prior art when used for ultra-high strength steel welded joints, where it is difficult to clearly distinguish the various sub-zones inside the heat-affected zone, and the corrosion parameter control depends on manual experience and the test results are inconsistent.

[0008] (II) Technical Solution

[0009] To achieve the above objectives, the main technical solutions adopted by the present invention include:

[0010] In a first aspect, the present invention provides a metallographic etching solution for the heat-affected zone of a hydraulic support weld, comprising a first-step base etching solution and a second-step selective coloring solution; the first-step base etching solution comprises nitric acid, a low-carbon alcohol and water; based on a predetermined volume of the etching solution, the volume percentage concentration of nitric acid in the etching solution is in the range of (0.6 + 1.0 × HGI)% to (1.0 + 1.5 × HGI)%, the volume percentage concentration of the low-carbon alcohol is 40% to 50%, and water is used to make up the predetermined volume;

[0011] The second step involves selecting a coloring solution containing nitric acid, low-carbon alcohols, water, acidic fluoride salts, and selective inhibitors. Based on a predetermined volume of the etching solution, the volume percentage concentration of nitric acid in the etching solution is in the range of (0.3 + 0.3 × HGI)% to (0.5 + 0.6 × HGI)%, the volume percentage concentration of low-carbon alcohols is 40% to 50%, the mass-volume concentration of acidic fluoride salts is in the range of (0.5 + 1.0 × ASI) g / L to (1.0 + 1.5 × ASI) g / L, and the selective inhibitors include molybdate corrosion inhibitors and benzotriazole corrosion inhibitors or their derivatives, with a mass ratio of 1:1 to 2:1 and a total mass-volume concentration in the range of (0.2 + 0.8 × HGI) g / L to (0.5 + 1.2 × HGI) g / L, with the remainder being water.

[0012] The heat-affected zone microstructure gradient index HGI = (C + Mn / 6 + Cr / 5 + Mo / 4 + V / 3) × (1 + Ni + Cu) / (1 + Nb + Ti + Al), and the alloy strengthening index ASI = Nb + Ti + 0.5 × (V + Al). In the above two formulas, the symbols of each element represent the mass percentage of that element in the steel, and there are no units.

[0013] According to a preferred embodiment of the present invention, the molybdate corrosion inhibitor is at least one of sodium molybdate and potassium molybdate; the benzotriazole or its derivative corrosion inhibitor is at least one of benzotriazole or methylbenzotriazole; the lower alcohol is at least one of anhydrous ethanol, methanol and isopropanol; the acid fluoride is at least one of ammonium hydrogen fluoride and sodium hydrogen fluoride; and the water is deionized water, distilled water or ultrapure water.

[0014] According to a preferred embodiment of the present invention, the lower alcohol is anhydrous ethanol, and its volume percentage concentration in the corrosive solution is 43-47%.

[0015] According to a preferred embodiment of the present invention, the selective inhibitor is composed of sodium molybdate and benzotriazole in a mass ratio of 1.5:1.

[0016] Secondly, the present invention also provides a corrosion method for the heat-affected zone of a hydraulic support weld, comprising the following steps:

[0017] S1: Obtain the chemical composition of the material to be corroded, and calculate its alloy strengthening index (ASI) and heat-affected zone degradation index (HDI).

[0018] Wherein, ASI = Nb + Ti + 0.5 × (V + Al), HDI = (C + Mn / 5 + Cr / 3 + Mo / 2 + V / 2) × (1 + Nb + Ti + B); and the ambient temperature T at the corrosion operation site is measured;

[0019] S2: Prepare the etching solution according to any one of claims 1-4 based on the CE, ASI and HDI measured in S1;

[0020] S3: Apply the first step of the basic etching solution to the surface of the material to be etched and continue etching for the first etching time t1; after etching, rinse and dry the surface to be etched.

[0021] S4. Apply the selected coloring solution from step two to the surface to be tested after treatment in step S3, and continue etching for the second etching time t2. After etching, rinse and dry to complete the etching process.

[0022] Wherein, t1 and t2 are determined by the following formulas:

[0023] t1=(A1+B1×HGI)×exp[D×(T-20℃)];

[0024] t2=(A2-B2×HGI)×exp[D×(T-20℃)];

[0025] In the formula, A1 and A2 are the reference corrosion times for the first and second steps, respectively, in seconds; B1 and B2 are the influence coefficients of the microstructure gradient index in the heat-affected zone for the first and second steps, respectively, in seconds; and D is the temperature correction coefficient, in degrees Celsius. -1 ; Parameters A1, B1, A2, B2 and D

[0026] The constants were determined by the following method: multiple groups of low-alloy ultra-high-strength steel samples with different ASI and HDI were selected, and orthogonal corrosion tests were carried out using quantitative etching solution prepared with S2 under multi-gradient ambient temperatures. The standard was clear metallographic morphology, obvious microstructure boundary, no under-corrosion, and no over-corrosion. The corresponding system characteristic constants were determined by fitting and regressing the experimental data.

[0027] According to a preferred embodiment of the present invention, before performing S1, the material to be corroded is first ground and polished, and the surface roughness Ra of the material to be corroded is ≤1.6μm.

[0028] According to a preferred embodiment of the present invention, in S2, the volume percentage concentration of nitric acid in the first step base etching solution is (0.8 + 1.2 × HGI)%; in the second step selective coloring solution, the volume percentage concentration of nitric acid is (0.4 + 0.4 × HGI)%, the mass volume concentration of acidic fluoride is (0.8 + 1.2 × ASI) g / L, and the total mass volume concentration of selective inhibitors is (0.3 + 1.0 × HGI) g / L.

[0029] According to a preferred embodiment of the present invention, in S1, the material to be corroded is a welded joint including a heat-affected zone, and the material to be corroded is low-alloy steel with a yield strength of not less than 1000 MPa; in S3, A1 = 12.0~25.0 s, B1 = 6.0~15.0 s, A2 = 12.0~28.0 s, B2 = 5.0~12.0 s, and D = -0.06~-0.02℃. -1 .

[0030] According to a preferred embodiment of the present invention, in S3, A1 = 18.0s, B1 = 10.0s, A2 = 20.0s, B2 = 8.0s, and D = -0.04℃. -1 .

[0031] According to a preferred embodiment of the present invention, a calibration experiment is required before performing S3: in the same environment, according to the method of S2, a corrosion solution is prepared for the standard samples known for ASI and HDI, and then corrosion calibration is performed. Based on the calibration results, it is determined whether to correct A1, B1, A2, B2 and D.

[0032] (III) Beneficial Effects

[0033] The metallographic etching solution and etching method for the heat-affected zone of hydraulic support welding of the present invention adopts a scheme based on the Alloy Strengthening Index (ASI) and the Heat-affected Zone Degradation Index (HDI), and dynamically determines the ratio of the two-step etching solution and the step-by-step etching time through a quantitative formula. Compared with the existing technology with fixed steps and formulas, it can achieve clear differentiation of coarse-grained zone, fine-grained zone, incomplete recrystallization zone and tempering softening zone inside the heat-affected zone by step-by-step selective coloring. It can effectively improve the precision and consistency of macroscopic metallographic inspection of ultra-high strength steel welded joints, and improve the low-magnification detection effect of macroscopic metallographic inspection.

[0034] Specifically, this invention employs a two-step method: a first step of basic etching and a second step of selective coloring. The first step, the basic etching solution, primarily uses a low-concentration nitric acid-alcohol system to initially erode the weld joint profile, revealing the basic outlines of the weld, heat-affected zone, and base material, thus laying the foundation for subsequent sub-zone differentiation. The second step, the selective coloring solution, introduces acidic fluoride salts and selective corrosion inhibitors (sodium molybdate and benzotriazole or their derivatives) into the nitric acid-alcohol system. Among them, acidic fluoride salts can effectively activate grain boundaries by providing free fluoride ions, assisting nitric acid in the differentiated etching of various sub-regions within the heat-affected zone. Sodium molybdate, as a passivating agent that is relatively sensitive to iron, can quickly combine to form a basic passivation film on the surface that has already been etched in the first step, thus delaying the overall etching rate of nitric acid and providing time for benzotriazole or its derivatives to take effect. Benzotriazole or its derivatives, as corrosion inhibitors, can preferentially adsorb onto the surface of the base material and fine-grained regions with higher chemical stability by utilizing the coordination adsorption of nitrogen atoms and aromatic rings in its molecules with the metal surface, forming selective protection. However, the coarse-grained regions and incompletely recrystallized regions have insufficient adsorption due to severe structural deterioration and poor chemical stability, resulting in weaker protective effects. Nitric acid will continue to etch in the weakly protected coarse-grained regions and incompletely recrystallized regions, further increasing their corrosion depth and exhibiting a darker contrast under low magnification, while the base material and fine-grained regions, being better protected, show relatively shallow contrast. The rapid passivation effect of sodium molybdate and the selective passivation effect of benzotriazole work synergistically to effectively amplify the contrast differences between the sub-regions of the heat-affected zone, making the originally blurred sub-region boundaries clearer and more distinguishable.

[0035] Meanwhile, this invention also employs a method to calculate the first-step corrosion time t1 and the second-step corrosion time t2 using HGI, ASI, and ambient temperature T, respectively. This eliminates reliance on manual experience in determining corrosion time, instead relying on calculations based on material parameters and on-site temperature. HGI comprehensively reflects the magnitude of the microstructure gradient within the heat-affected zone of the steel under welding thermal cycling; a higher HGI indicates more significant microstructure differences between sub-zones. In the first-step basic corrosion, a higher HGI requires a longer basic corrosion time to fully establish the overall microstructure profile of the welded joint. In the second-step selective coloring, a higher HGI indicates a more pronounced contrast in corrosion resistance between sub-zones, correspondingly shortening the required selective coloring time and preventing issues such as the smoothing out of contrast differences due to excessive corrosion. Furthermore, the introduction of an exponential term in the calculation process compensates for the impact of ambient temperature fluctuations on the corrosion rate, ensuring high consistency in test results across different seasons or regions.

[0036] Furthermore, since the ratio of the two-step etching solution and the step-by-step etching time are both determined based on the same set of HGI and ASI parameters, they constitute an effective step-by-step quantitative control system, ensuring that each macroscopic metallographic etching operation using the present invention is completed under a unified standard. The experimental procedure of this invention is standardized, the overall operation is simple and easy to perform, and the experimental equipment and reagents used are all conventional consumables, eliminating the need to purchase additional specialized equipment. This results in low testing costs and ensures that the macroscopic metallographic detection results have good repeatability and comparability. It can be directly applied to fields such as large-scale testing of hydraulic support welded components, efficiently completing the heat-affected zone detection of hydraulic support welds, and providing scientific and reliable experimental data support for the quality control of hydraulic support welded components. Detailed Implementation

[0037] To better explain and facilitate understanding of the present invention, the present invention will be described in detail below through specific embodiments.

[0038] In a first aspect, the present invention proposes a step-by-step etching solution for fine display of the sub-regions of the heat-affected zone in low-alloy high-strength steel welding, comprising a first-step base etching solution and a second-step selective coloring solution.

[0039] The first step involves a basic etching solution comprising nitric acid, low-carbon alcohol, and water. Nitric acid serves as the primary corrosive medium, initially and uniformly eroding the weld joint profile. Utilizing the overall difference in corrosion resistance between the base metal and the heat-affected zone (HAZ), the basic outlines of the weld, HAZ, and base metal are revealed. The low-carbon alcohol primarily reduces the surface tension of the etching solution, ensuring its uniform spread across the sample surface. It also acts as a diluent to adjust the nitric acid concentration and aids in dehydration and drying after etching. Water, as a polar solvent, is used to dissolve the components and adjust the final volume.

[0040] The second step involves selecting a coloring solution comprising nitric acid, low-carbon alcohols, water, acidic fluoride salts, and a selective inhibitor. The effects of nitric acid, low-carbon alcohols, and water remain unchanged. The addition of acidic fluoride salts allows for the efficient release of free fluoride ions in an acidic system, selectively eroding chemically stable regions formed by alloy element segregation at grain boundaries. This activates the grain boundaries and assists nitric acid in effectively eroding the subregions within the heat-affected zone, making previously blurred and discontinuous grain boundaries appear continuous and clear. This is particularly effective for low-alloy ultra-high-strength steels containing microalloying elements such as Nb, V, and Ti, which exhibit high grain refinement and strong grain boundary chemical stability, where the grain boundary activation effect of acidic fluoride salts is even more pronounced. Simultaneously, acidic fluoride salts stabilize the hydrogen fluoride released during hydrolysis and do not significantly affect the acidity or alkalinity of the corrosion solution after hydrolysis. The high fluoride ion supply efficiency and precise stoichiometry also facilitate accurate formulation. The selective inhibitors include molybdate corrosion inhibitors and benzotriazole or its derivatives, with a mass ratio of 1:1 to 2:1. Molybdate corrosion inhibitors, as passivating agents relatively sensitive to iron, can first form a basic passivation film on the surface already eroded in the first step, thus slowing down the overall corrosion rate of nitric acid and creating conditions for subsequent selective adsorption. Benzotriazole or its derivative corrosion inhibitors utilize the coordination adsorption of nitrogen atoms and aromatic rings in their molecules with the metal surface, preferentially adsorbing onto the surface of chemically stable substrates and fine-grained regions, forming selective protection. However, coarse-grained regions and incompletely recrystallized regions, due to severe microstructural degradation and poor chemical stability, have lower adsorption amounts (and the adsorption effects also differ between these two groups), resulting in weaker protection. Nitric acid therefore continues to erode in the weakly protected coarse-grained and incompletely recrystallized regions, further increasing the corrosion depth and exhibiting darker contrast under low magnification, while the substrate and fine-grained regions, being better protected, show relatively shallower contrast. At the ratio selected in this invention, the rapid passivation effect of molybdates synergistically with the selective adsorption of benzotriazole or its derivative corrosion inhibitors, effectively amplifying the contrast differences between the various sub-regions of the heat-affected zone. In benzotriazole-derived corrosion inhibitors, benzotriazole is the main active ingredient, and its derivatives generally have the same selective adsorption effect.

[0041] Unless otherwise specified, the fluoride salts used in this invention refer to acidic fluoride salts.

[0042] The ratio of the two-step etching solution is determined according to the heat-affected zone microstructure gradient index (HGI) and alloy strengthening index (ASI) of the material to be etched, using the following dynamic ratio formula.

[0043] In the first step of the basic etching solution, based on a predetermined volume of the etching solution, the volume percentage concentration of nitric acid is within the range of (0.6 + 1.0 × HGI)% to (1.0 + 1.5 × HGI)%. It should be noted that for different types of ultra-high strength steel, due to differences in the specific proportions of alloying elements and metallurgical processes, the appropriate nitric acid concentration obtained may fluctuate to some extent. However, the optimal values ​​generally fall within the range defined by the above formula. Those skilled in the art can determine the appropriate parameters based on actual conditions to obtain parameters suitable for actual use. Corrections can be made as needed during actual use. The above range can accommodate reasonable differences in corrosion conditions for different steels and has wide applicability. Preferably, the volume percentage concentration of nitric acid in the first step is (0.8 + 1.2 × HGI)%. This preferred range is a better proportion determined based on the commonly used Q1030-1100 steel and its equivalent variants for hydraulic supports. It can balance reagent consumption economy and operational safety while fully establishing the overall microstructure of the welded joint. Furthermore, this range is generally compatible with non-standard ultra-high strength steels such as Q980 and Q890, which fall within the transition range of strength grades. The specific range can be determined based on actual conditions. Other ranges provided by this invention, unless otherwise specified, are typically optimized for standard ultra-high strength steels, but they can also generally accommodate non-standard ultra-high strength steels such as Q980 and Q890, which fall within the transition range of strength grades. Adjustments can be made based on actual conditions, and will not be elaborated further.

[0044] In the second step, the volume percentage concentration of nitric acid in the coloring solution is selected based on a predetermined volume of the etching solution, ranging from (0.3 + 0.3 × HGI)% to (0.5 + 0.6 × HGI)%, preferably (0.4 + 0.4 × HGI)%. The overall nitric acid concentration in the second step is lower than that in the first step, so as to slow down the corrosion rate and provide a sufficient time window for the selective inhibitor to exert its differentiated protective effect.

[0045] The mass-volume concentration of the acidic fluoride salt is in the range of (0.5 + 1.0 × ASI) g / L to (1.0 + 1.5 × ASI) g / L, preferably (0.8 + 1.2 × ASI) g / L. The amount of acidic fluoride salt increases with increasing ASI, effectively compensating for the increased difficulty of grain boundary corrosion caused by microalloying element strengthening in high-ASI steels, and assisting nitric acid in the differentiated etching of different sub-regions of the heat-affected zone. Similar to the aforementioned range formula for nitric acid, the optimal concentration of acidic fluoride salt may fluctuate to some extent for different types of ultra-high-strength steels due to differences in specific elemental composition even with the same ASI, but it generally falls within this range.

[0046] The total mass-volume concentration of the selective inhibitor is in the range of (0.2 + 0.8 × HGI) g / L to (0.5 + 1.2 × HGI) g / L. The molybdate corrosion inhibitor provides basic passivation protection, while benzotriazole or its derivatives provide selective adsorption fine-tuning. The two work synergistically; for steels with a larger microstructure gradient in the heat-affected zone, the total concentration of the selective inhibitor automatically increases to enhance protection of the base metal and fine-grained regions, thus widening the contrast difference between sub-regions. Preferably, the selective inhibitor consists of sodium molybdate and benzotriazole in a mass ratio of 1.5:1, with a total mass-volume concentration of (0.3 + 1.0 × HGI) g / L.

[0047] In both steps of the etching solution, the volume percentage concentration of low-carbon alcohol is 40% to 50%. Similarly, if the amount of low-carbon alcohol is too high, it will over-dilute substances such as nitric acid and weaken the etching ability; if it is too low, the spreading properties will be poor and the drying speed will be slow. The range of 40% to 50% can achieve a good balance between the above effects and has a wide applicability to different types of steel.

[0048] The present invention uses the heat-affected zone microstructure gradient index (HGI) and alloy strengthening index (ASI) of the steel to be corroded to dynamically and quantitatively determine the composition ratio of the two-step corrosion solution, thereby forming a stepwise dynamic ratio system and achieving precise and detailed display of the sub-regions of the heat-affected zone.

[0049] The heat-affected zone microstructure gradient index HGI = (C + Mn / 6 + Cr / 5 + Mo / 4 + V / 3) × (1 + Ni + Cu) / (1 + Nb + Ti + Al), and the alloy strengthening index ASI = Nb + Ti + 0.5 × (V + Al). In the above two formulas, the symbol of each element represents the mass percentage of that element in the steel, and there are no units.

[0050] It should be noted that HGI is a parameter specifically set in this invention to comprehensively characterize the microstructure gradient within the heat-affected zone (HAZ) of steel under welding thermal cycling. In its formula, the numerator (C+Mn / 6+Cr / 5+Mo / 4+V / 3) reflects the steel's inherent hardening tendency and sensitivity to welding heat input; the higher this value, the more significant the microstructure differences between the various sub-regions of the HAZ during welding. The (1+Ni+Cu) part reflects the influence of corrosion-resistant elements such as Ni and Cu; the higher the content of these elements, the greater the difference in corrosion resistance between the base metal and the HAZ. The denominator (1+Nb+Ti+Al) reflects the ability of microalloying elements such as Nb, Ti, and Al to pin grain boundaries and inhibit grain growth during welding by forming fine carbonitrides; the higher the content of these elements, the smaller the microstructure gradient within the HAZ. A higher overall HGI value indicates more significant differences in microstructure between the subregions of the heat-affected zone, necessitating a more precise differentiation using a step-by-step etching strategy. Correspondingly, the first step of basic etching requires a longer time to fully establish the overall outline, while the second step of selective coloring needs a shorter time to avoid over-etching and smoothing out contrast differences. Simultaneously, this invention employs the Alloy Strengthening Index (ASI) to quantify the combined contribution of microalloying elements Nb, Ti, V, and Al to grain refinement and precipitation strengthening in steel (in low-alloy high-strength steel, Nb and Ti are the primary contributors, while V and Al contribute less). These elements significantly increase the difficulty of clearly revealing grain boundaries during metallographic etching by refining grains, increasing grain boundary area, and forming stable carbonitrides at grain boundaries. A higher ASI value requires stronger etching conditions for grain boundary visualization, necessitating more fluoride salts in the second step of selective coloring to assist grain boundary activation, in conjunction with selective inhibitors to achieve differentiated display of each subregion.

[0051] Preferably, the low-carbon alcohol can be an alcohol with fewer than three carbon atoms in the main chain, which usually has good volatility and solubility. For example, at least one of the common anhydrous ethanol, methanol and isopropanol can be selected.

[0052] More preferably, considering safety, volatility and cost, anhydrous ethanol can be selected as the lower alcohol, with a volume percentage concentration of 43-47%, more preferably 45%.

[0053] Preferably, the acidic fluoride salt includes at least one of ammonium bifluoride and sodium bifluoride. These two types of substances have similar properties and can both stably dissociate in water, thus providing a stable supply of free fluoride.

[0054] The water used should be deionized, distilled, or ultrapure, ensuring its purity and avoiding the introduction of large amounts of other ions that could affect the detection results. C1-C3 alkyl-substituted derivatives of thiourea include at least one of methylthiourea, ethylthiourea, and isopropylthiourea.

[0055] Of course, it should be noted that in the actual operation of this invention, pure nitric acid and pure ethanol are not required as solvent components. Conventional concentrated nitric acid (nitric acid aqueous solution) is also feasible. When calculating and using it, the concentration of nitric acid and other substances in these substances is converted to pure substances and substituted into the formula of this invention. The water contained therein can be calculated as the water required in the formulation of the corrosion solution of this invention.

[0056] In a second aspect, the present invention also provides a macroscopic metallographic etching method applicable to the stepwise etching solution as described in any of the first aspects, comprising the following steps:

[0057] S1: Obtain the chemical composition of the material to be corroded, and calculate its Alloy Strengthening Index (ASI) and Heat Affected Zone Gradient Index (HGI) based on the obtained mass percentage of each element. Wherein, ASI = Nb + Ti + 0.5 × (V + Al), HGI = (C + Mn / 6 + Cr / 5 + Mo / 4 + V / 3) × (1 + Ni + Cu) / (1 + Nb + Ti + Al). Measure the ambient temperature T at the corrosion operation site. The chemical composition can be obtained from the steel smelting composition ratio provided by the manufacturer, or from on-site measurements using methods such as spark spectroscopy; both methods can corroborate each other. The methods and equipment used in the actual measurements should be kept consistent to ensure the uniqueness of the subsequent model input parameters and the traceability of the test results. A conventional thermometer can be used for temperature measurement; an accuracy of ±0.5℃ is sufficient to meet basic requirements. During measurement, the thermometer probe should be placed near the corrosion operation area to avoid deviations caused by local temperature differences.

[0058] S2: Based on the predetermined volume of the etching solution, prepare the first-step base etching solution and the second-step selective coloring solution. In the first-step base etching solution, the volume percentage concentration of nitric acid is in the range of (0.6 + 1.0 × HGI)% to (1.0 + 1.5 × HGI)%, the volume percentage concentration of lower alcohols is 40% to 50%, and the balance is water. In the second-step selective coloring solution, the volume percentage concentration of nitric acid is in the range of (0.3 + 0.3 × HGI)% to (0.5 + 0.6 × HGI)%, the volume percentage concentration of lower alcohols is 40% to 50%, the mass-volume concentration of acidic fluoride salts is in the range of (0.5 + 1.0 × ASI) g / L to (1.0 + 1.5 × ASI) g / L, the total mass-volume concentration of selective inhibitors is in the range of (0.2 + 0.8 × HGI) g / L to (0.5 + 1.2 × HGI) g / L, and the balance is water. During the operation, first measure the required volumes of low-carbon alcohol and a portion of water for each step. Add nitric acid while stirring, and after thorough mixing and cooling to room temperature, add the solid components such as acidic fluoride salts and selective inhibitors. Stir until completely dissolved, and finally add water to bring the remaining volume to the target predetermined volume. Prepare and use immediately to ensure consistent reagent activity for each test, avoiding potential concentration variations and contamination issues that may result from pre-prepared storage. Since the concentrations of each component are dynamically determined using formulas based on ASI and HGI, the prepared two-step etching solution is a specific formulation for this test material (steel), with the corrosion intensity precisely matched to the material properties of the steel.

[0059] S3: Apply the base etching solution from step one to the surface of the material to be etched, and continue etching for a first etching time t1. After etching, rinse and dry the surface. Then, apply the selective staining solution from step two to the surface treated in S3, and continue etching for a second etching time t2. After etching, rinse and dry to complete the etching process. t1 and t2 are determined by the following fitted formulas:

[0060] t1=(A1+B1×HGI)×exp[D×(T-20℃)];

[0061] t2=(A2-B2×HGI)×exp[D×(T-20℃)];

[0062] In the formula, A1 and A2 are the reference corrosion times for the first and second steps, respectively, in seconds. B1 and B2 are the influence coefficients of the microstructure gradient index in the heat-affected zone for the first and second steps, respectively, in seconds. D is the temperature correction coefficient, in degrees Celsius. -1 The exponential term exp[D×(T-20℃)] represents temperature compensation based on the change in corrosion reaction rate with temperature: when the ambient temperature T is higher than 20℃, the corrosion time is shortened. Conversely, when T is lower than 20℃, the corrosion time is extended, providing more precise time control under conditions with large temperature differences.

[0063] It should be noted that the formula for t1 uses A1 + B1 × HGI, while the formula for t2 uses a subtraction term (A2 - B2 × HGI). This is because the first step aims to establish the overall profile through corrosion. A larger HGI indicates a more significant difference in microstructure between the sub-regions of the heat-affected zone, requiring a longer base corrosion time to fully reveal the overall microstructure profile of the welded joint. The second step, selective coloring, aims to increase the contrast difference between sub-regions. A larger HGI indicates a more obvious difference in corrosion resistance between the sub-regions, requiring a correspondingly shorter selective coloring time to avoid excessive corrosion that would smooth out the contrast differences.

[0064] Furthermore, it should be noted that the parameters A1, B1, A2, B2, and D in the corrosion duration calculation formula of this invention are derived under the premise of adopting the stepwise dynamic proportioning method of this invention. This is achieved by selecting multiple groups of ultra-high strength steel welded joints with different ASI and HGI values ​​as samples in a precisely controlled laboratory environment. Systematic orthogonal corrosion experiments are conducted on these samples under multi-gradient temperatures, and the optimal corrosion duration is recorded. The criteria for determining the optimal corrosion duration are clear microstructure morphology of each subzone of the heat-affected zone, distinct subzone boundaries, no under-corrosion, and no over-corrosion. The data are then summarized, and the corresponding system characteristic constants are obtained by fitting the data using multiple nonlinear regression and other methods with HGI as the independent variable. These parameters already include the comprehensive influence of the stepwise proportioning dynamic change with HGI on the corrosion rate. In practical applications, it is only necessary to obtain the ASI and HGI values ​​of the target material to be corroded, prepare the two-step corrosion solution according to the proportioning formula given in this invention, and calculate the corrosion duration of each step according to the time formula to obtain a stable corrosion effect. There is no need to consider the interaction between formula changes and time changes separately, making the operation relatively simple.

[0065] S4: After etching, quickly rinse the sample surface with clean water to remove the etching solution, then rinse with anhydrous ethanol to dehydrate, and finally dry the sample surface with cold air. This process must be quick and continuous to avoid water residue or secondary etching. After etching, macroscopic metallographic testing can be performed to observe the microstructure of each sub-region of the heat-affected zone under low magnification, and to further analyze the etching effect.

[0066] The corrosion method of this invention integrates the dynamic ratio of the step-by-step corrosion solution with the two-step corrosion time into a unified quantitative control process, achieving synergistic control of steel composition, corrosion solution formulation, and corrosion time. ASI and HGI, as core parameters, determine the component concentrations of the two-step corrosion solution and participate in the calculation of corrosion time. Their mutual cooperation and combined effect ensure that the entire detection process does not rely entirely on the operator's personal experience, but is determined by the material properties of the steel itself and the ambient temperature. This improves the consistency and repeatability of the detection results for different steels, different operators, and different ambient temperatures.

[0067] Preferably, before performing S1, the material to be corroded is ground and polished to achieve a surface roughness Ra ≤ 1.6 μm. The material to be corroded should at least include weld beads and their heat-affected zones. Grinding is performed using gradient sandpaper, progressing from coarse to fine, until the surface is smooth and the scratches are uniform. Polishing can be done using conventional felt or other polishing pastes such as diamond polishing compound or alumina suspension. Of course, this invention does not limit the grinding and polishing methods; various methods can be used to ensure a mirror-like finish, laying the foundation for subsequent uniform corrosion and clear imaging.

[0068] Preferably, in S2, the volume percentage concentration of nitric acid in the first-step base etching solution is (0.8 + 1.2 × HGI)%. In the second-step selective coloring solution, the volume percentage concentration of nitric acid is (0.4 + 0.4 × HGI)%, the mass volume concentration of acidic fluoride salt is (0.8 + 1.2 × ASI) g / L, and the total mass volume concentration of selective inhibitor is (0.3 + 1.0 × HGI) g / L.

[0069] Preferably, in S3, A1 = 12.0~25.0s, B1 = 6.0~15.0s, A2 = 12.0~28.0s, B2 = 5.0~12.0s, and D = -0.06~-0.02℃. -1The parameters are allowed to have a certain fluctuation range, allowing for reasonable differences in equipment precision and environmental control between different laboratories. Furthermore, these parameters can be corrected and optimized based on data from actual use to adapt to different operating environments or system errors. It should be noted that the scope of this invention primarily targets standard low-alloy ultra-high-strength steels with a yield strength of not less than 1000 MPa, specifically not less than 1030 MPa, such as Q1100, S1100QL, and 35CrMnSiA. For these steels, relatively accurate differentiation of heat-affected zone sub-regions can usually be achieved. For conventional steels in the transitional stage, such as Q960, Q890, and 30CrMnSi, since their microstructure is generally similar to that of ultra-high-strength steels, the above scope can usually also achieve good differentiation of their heat-affected zone sub-regions.

[0070] Preferably, in S1, the material to be corroded is a welded joint of low-alloy ultra-high-strength steel with a yield strength of 1030-1100 MPa, commonly used in large steel structures such as hydraulic supports. More preferably, in S2, the volume percentage concentration of nitric acid in the first-step base etching solution is (0.8 + 1.2 × HGI)%, the volume percentage concentration of nitric acid in the second-step coloring solution is (0.4 + 0.4 × HGI)%, the mass volume concentration of acidic fluoride salt is (0.8 + 1.2 × ASI) g / L, and the total mass volume concentration of selective inhibitor is (0.3 + 1.0 × HGI) g / L. The above range represents a preferred proportion of low-alloy ultra-high-strength steel with a relative yield strength of 1030-1100 MPa, determined by this application while ensuring clear differentiation of the heat-affected zone sub-regions and considering the economy of reagent consumption and operational safety.

[0071] More preferably, in S3, A1 = 18.0 s, B1 = 10.0 s, A2 = 20.0 s, B2 = 8.0 s, and D = -0.04℃. -1 This set of parameters is also set by this application for welded joints of low-alloy ultra-high-strength steel with a yield strength of 1030~1100MPa. It can effectively increase the contrast difference between sub-regions by selective coloring in the second step after fully establishing the overall outline in the first step, so as to obtain a corrosion effect with distinct sub-region boundaries and less prone to problems such as over-corrosion or under-corrosion.

[0072] When determining the coefficients of the dynamic proportioning formula, depending on the applicable environment, steel from the same series or with relatively small compositional differences can be selected as the target to obtain more accurate and specific system characteristic parameters for that series of steels. For other types or grades of low-alloy ultra-high-strength steel, if their alloy composition may differ significantly (such as the difference between Chinese and EU grades of the same strength), the formula coefficients in the dynamic formulation and the system characteristic constants used in calculating corrosion time can be determined using the following methods:

[0073] (1) Select several representative grades of steel in this series (at least covering the upper, middle and lower limits of their typical HGI range), and according to the overall framework of the method of this invention, based on experience and the possible effects of different components, pre-set several initial coefficients, such as the nitric acid content of the first step basic corrosion solution being (0.8+1.2×HGI)%, (0.9+1.3×HGI)%, etc., and the nitric acid content of the second step coloring solution being (0.4+0.4×HGI)%, (0.4+0.5×HGI)%, etc., and then prepare a series of step corrosion solutions for systematic corrosion experiments through orthogonal experiments and / or single variable experiments, and record the two-step corrosion time when each sample obtains a better heat-affected zone sub-region manifestation effect under multi-gradient temperatures.

[0074] (2) Using ASI and HGI as independent variables and the optimal two-step corrosion duration as the dependent variable, multiple nonlinear regression and other methods were used to fit the time model parameters A1, B1, A2, B2, and D applicable to this series of steels. Simultaneously, based on the optimal nitric acid concentrations for the first and second steps under each HGI, the optimal fluoride salt concentrations under each ASI, and the optimal selective inhibitor concentrations under each HGI, regression was used to determine the formula coefficients for the stepwise dynamic proportions applicable to this series of steels. Generally, for low-alloy high-strength steels, the regression coefficients fall within the range of the formula in this invention. Of course, if necessary, if the range exceeds this limit, it can be appropriately expanded based on new data to cover the needs of this series of steels.

[0075] Furthermore, it should be noted that when determining the coefficients and system characteristic constants of the dynamic proportioning formula, the preferred experimental object in this invention is a welded joint sample including the fusion line and the heat-affected zone. By conducting a unified stepwise corrosion experiment and effect evaluation on the entire heat-affected zone, the fitted parameters enable the coarse-grained zone, fine-grained zone, incomplete recrystallization zone, and tempering softening zone within the heat-affected zone to achieve a distinct and clearly defined appearance, meeting the requirements of macroscopic metallographic testing for fine detection of the heat-affected zone. Of course, it is also possible to independently fit only a specific sub-region of the heat-affected zone to obtain the formula coefficients and time constants specific to that sub-region for fine observation of that specific sub-region. In practice, the fitting method can be flexibly selected according to the testing purpose and accuracy requirements.

[0076] Preferably, if necessary, a calibration experiment is required before performing S3: In the same environment, following the method in S2, prepare the etching solution for standard samples known in ASI and HGI, and then perform etching calibration. Based on the calibration results, determine whether to correct the system characteristic constants in S3. Specifically, if the standard sample shows slight over-etching (e.g., localized blackening of a subregion), it may be necessary to repeat several sets of experiments and adjust the relevant parameters downwards proportionally. If slight under-etching occurs (e.g., discontinuous blurring of subregion boundaries), it may be necessary to repeat several sets of experiments and adjust the corresponding parameters upwards. The specific correction method can be determined according to the actual situation, and will not be elaborated upon in this invention. "If necessary" refers to situations such as changing reagent manufacturers or batches, significant changes in ambient temperature, or excessively long storage time of raw materials, which may have uncertain effects on the composition of the etching solution or its etching effect. This method ensures that the test results remain stable, guaranteeing the accuracy and consistency of the etching results. Of course, when changing reagent batches or the storage time of raw materials, the specific composition of each raw material should be determined in advance, its performance should be determined, and corresponding calculations or optimizations should be performed to avoid fluctuations in the etching results due to changes in composition.

[0077] To better understand the above technical solutions, exemplary embodiments of the present invention will be described in more detail below. However, it should be understood that the present invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a clearer and more thorough understanding of the present invention and to fully convey the scope of the invention to those skilled in the art.

[0078] Example 1:

[0079] This embodiment provides a metallographic etching solution and etching method for the heat-affected zone of hydraulic support welding. The Q1100 low-alloy ultra-high-strength steel welded joint for mining hydraulic supports is used as the material to be etched, and the etching solution and macroscopic metallographic etching method of this invention are used for detection.

[0080] Sample pretreatment:

[0081] The target steel was a long strip of steel obtained through conventional welding. Samples were taken perpendicular to the weld joint of the target steel to obtain specimens, which were then used to obtain the material to be corroded. The specimens included complete weld metal, fusion lines, heat-affected zones, and the base metal matrix. The specimens were then subjected to gradient wet grinding using 180-grit, 400-grit, 800-grit, and 1200-grit water-resistant sandpaper. Each time a different grit was used, the specimen was rotated 90° until the scratches from the previous grit were completely eliminated. Deionized water was continuously used as both coolant and cleaning agent during the wet grinding process. After grinding, diamond polishing paste was applied to a polishing cloth for polishing. Following polishing, the surface was rinsed with anhydrous ethanol and dried with cold air to ensure that the surface roughness Ra of the material to be corroded was ≤0.8 μm.

[0082] S1: Obtain material parameters and measure ambient temperature:

[0083] Chemical composition analysis of the sample parent material region was performed using a spark direct-reading spectrometer, and the mass percentage of each element was as follows: C=0.20%, Mn=1.70%, Cr=0.70%, Mo=0.60%, V=0.06%, Ti=0.02%, Nb=0.05%, Ni=2.00%, Cu=0.30%, Al=0.05%, B=0.0020%, Zr=0.005%, Si=0.28%, P=0.010%, S=0.004%, with the balance being Fe.

[0084] The calculated HGI values ​​were 1.183 and ASI values ​​were 0.145. Simultaneously, the ambient temperature at the corrosion operation site was measured to be T = 22℃.

[0085] S2. Preparation of the corrosive solution:

[0086] In this embodiment, the total amount of the two-step corrosion solution is 500 mL.

[0087] The first step involves preparing the basic etching solution, which consists of nitric acid, anhydrous ethanol, and deionized water. The nitric acid has a volume percentage concentration of (0.8 + 1.2 × 1.183)% = 2.22%, which is taken as 2.2%. The anhydrous ethanol has a volume percentage concentration of 45%. The remaining volume is brought to 500 mL with deionized water. To prepare the solution, mix the anhydrous ethanol, commercially available concentrated nitric acid, and a portion of the deionized water thoroughly and cool to ambient temperature. Finally, bring the volume to 500 mL with deionized water.

[0088] The second step involves selecting a coloring solution comprising nitric acid, anhydrous ethanol, deionized water, ammonium bifluoride, sodium molybdate, and benzotriazole. The nitric acid has a volume percentage concentration of (0.3 + 0.4 × 1.183)% = 0.77%, which is set to 0.8%. The anhydrous ethanol has a volume percentage concentration of 45%. The ammonium bifluoride has a mass-volume concentration of (0.8 + 1.2 × 0.145) g / L = 0.97 g / L. The mass ratio of sodium molybdate to benzotriazole is 1.5:1, with a total mass-volume concentration of (0.3 + 1.0 × 1.183) g / L = 1.48 g / L, of which approximately 0.89 g / L is sodium molybdate and approximately 0.59 g / L is benzotriazole. The remaining volume is brought to 500 mL with deionized water. When preparing the solution, mix anhydrous ethanol, commercially available concentrated nitric acid and some deionized water evenly and cool to room temperature. Then add ammonium fluoride, sodium molybdate and benzotriazole and stir until completely dissolved. Finally, dilute to 500 mL with deionized water.

[0089] S3. Calculate corrosion duration:

[0090] System characteristic constants used: A1=18.0s, B1=10.0s, A2=20.0s, B2=8.0s, D=-0.04℃ -1 The ambient temperature T = 22℃. Substituting into the formula, the corrosion time t1 = 29.83 × 0.923 ≈ 27.5s, which is taken as 28s. t2 = 10.54 × 0.923 ≈ 9.7s, which is taken as 10s.

[0091] S4. Corrosion Operation:

[0092] The first step, applying the base etching solution evenly with a dropper to the polished surface of the sample, covering the weld, fusion line, heat-affected zone, and base material area, begins timing simultaneously. During etching, the sample is gently agitated to maintain uniform contact between the etching solution and the sample surface. After 28 seconds, the sample surface is immediately rinsed with clean water to remove residual etching solution, followed by rinsing with anhydrous ethanol to dehydrate, and then dried with cold air. Next, the second step, applying the selected coloring solution evenly with droppers, is completed after 10 seconds. The sample is then immediately rinsed with clean water, dehydrated with anhydrous ethanol, and dried with cold air.

[0093] Then, macroscopic metallographic observation was performed on the corroded sample. The observation results showed that the weld, fusion line, heat-affected zone and the outline of the base material of the welded joint were basically clearly visible. The coarse grain zone, fine grain zone, incomplete recrystallization zone and tempering softening zone inside the heat-affected zone showed different contrasts. The boundaries between each sub-zone were clearly distinguishable. There was no blackening of the structure caused by corrosion or blurring caused by under-corrosion.

[0094] According to GB / T 26955-2011 "Destructive Testing of Welded Joints in Metallic Materials - Macroscopic and Microscopic Inspection", the welded joints are evaluated. If the weld fusion is good and there are no welding defects such as incomplete penetration, cracks, porosity, or slag inclusions, it is judged as qualified.

[0095] Example 2:

[0096] This embodiment provides a metallographic etching solution and etching method for the heat-affected zone of hydraulic support welding. The difference from Embodiment 1 is that this embodiment uses 35CrMnSiA low-alloy ultra-high-strength tempered steel welded joints as the material to be etched. 35CrMnSiA is a medium-carbon tempered ultra-high-strength steel, which differs significantly from the Q1100 steel in Embodiment 1 in its composition and welding characteristics. It has a higher carbon content and does not contain elements such as Ni and Cu that improve corrosion resistance, resulting in a more pronounced tendency for microstructural degradation in the weld heat-affected zone.

[0097] In S1, compared with the Q1100 steel in Example 1, the carbon content of 35CrMnSiA is significantly higher, but the Ni and Cu contents are extremely low and it contains no B. Although the Cr content is relatively high, the Mo content is relatively low. The microstructure differences between the sub-regions of its heat-affected zone are relatively small, and the HGI is slightly lower than that of Q1100. The calculated HGI is 0.981 and ASI is 0.085. The ambient temperature is T = 22℃.

[0098] In S2, the volume percentage concentration of nitric acid in the first-step basic etching solution is (0.8 + 1.2 × 0.981)% = 1.98%, which is taken as 2.0%. The volume percentage concentration of anhydrous ethanol is 45%. In the second-step selected coloring solution, the volume percentage concentration of nitric acid is (0.4 + 0.4 × 0.981)% = 0.79%, which is taken as 0.8%. The volume percentage concentration of anhydrous ethanol is 45%. The mass-volume concentration of ammonium bifluoride is (0.8 + 1.2 × 0.085) g / L = 0.90 g / L. The total mass-volume concentration of sodium molybdate and benzotriazole is (0.3 + 1.0 × 0.981) g / L = 1.28 g / L, with a mass ratio of 1.5:1. The preparation procedure is the same as in Example 1.

[0099] In S3, the corrosion time was calculated using the same system characteristic constants as in Example 1: t1 = 27.81 × 0.923 ≈ 25.7 s, which was taken as 26 s. t2 = 12.15 × 0.923 ≈ 11.2 s, which was taken as 11 s.

[0100] Macroscopic metallographic observation was performed after 23 seconds of corrosion in S4. The results showed that the weld, fusion line and heat-affected zone of S1100QL steel had clear and continuous microstructure boundaries, natural microstructure transition and distinct layers, no excessive corrosion in the weld and other areas, and the base metal was also effectively visible.

[0101] Example 3:

[0102] This embodiment provides a metallographic etching solution and etching method for the heat-affected zone of hydraulic support welding. The difference from Embodiment 1 is that, in this embodiment, the material to be etched is the welded joint of Q960 low-alloy high-strength quenched and tempered steel used in hydraulic supports. Q960 is a steel with a strength grade in the transition range, with a yield strength of 960 MPa. The heat-affected zone also presents a problem of difficulty in distinguishing between its various sub-zones; the method of this invention is used for detection.

[0103] In S1, the sampling, grinding, and polishing operations for the samples were the same as in Example 1. Spectroscopic analysis revealed certain differences in the chemical composition of the Q960 steel compared to the Q1100 steel in Example 1, primarily in slightly lower C content (0.18%), slightly lower Mn content (1.60%), lower Ni content (1.50%), and the absence of Zr (not detected). The calculated HGI was 1.041, and the ASI was 0.128. The ambient temperature was T = 20℃.

[0104] In step S2, the volume percentage concentration of nitric acid in the first-step base etching solution is (0.8 + 1.2 × 1.041)% = 2.05%, which is taken as 2.0%. The volume percentage concentration of anhydrous ethanol is 45%. In the second-step selected coloring solution, the volume percentage concentration of nitric acid is (0.3 + 0.4 × 1.041)% = 0.72%, which is taken as 0.7%. The volume percentage concentration of anhydrous ethanol is 45%. The mass-volume concentration of ammonium bifluoride is (0.8 + 1.2 × 0.128) g / L = 0.95 g / L. The total mass-volume concentration of sodium molybdate and benzotriazole is (0.3 + 1.0 × 1.041) g / L = 1.34 g / L, with a mass ratio of 1.5:1. The preparation operation is the same as in Example 1.

[0105] In S3, the same system characteristic constants as in Example 1 are used for calculation: t1 = 18.0 + 10.0 × 1.041 = 28.4s, which is taken as 28s. t2 = 20.0 - 8.0 × 1.041 = 11.7s, which is taken as 12s.

[0106] The operations of steps S4 and S5 are the same as in Example 1. In S4, macroscopic metallographic observation is performed after step-by-step corrosion at 28s and 12s.

[0107] The observation results show that the sub-regions within the heat-affected zone also exhibit discernible contrast differences. The boundaries of the coarse-grained zone, fine-grained zone, incomplete recrystallization zone, and tempering softening zone are basically clear. The fusion lines are continuous and uninterrupted, and there is no over-corrosion or under-corrosion phenomenon, resulting in a better macroscopic metallographic morphology.

[0108] Example 4:

[0109] This embodiment provides a metallographic etching solution and etching method for the heat-affected zone of hydraulic support welding. The difference from Embodiment 1 is that in the first step (basic etching solution) and the second step (selective coloring solution), the etching solution includes: nitric acid, sodium bifluoride, methylbenzotriazole, isopropanol, and water. In step S2, the volume percentage concentration of nitric acid in the first step basic etching solution is 2.2%, and the volume percentage concentration of isopropanol is 45%. In the second step selective coloring solution, the volume percentage concentration of nitric acid is 0.8%, the volume percentage concentration of isopropanol is 45%, the mass-volume concentration of sodium bifluoride is 0.97 g / L, and the total mass-volume concentration of sodium molybdate and methylbenzotriazole is 1.48 g / L, with a mass ratio of 1.5:1. The remaining operations are the same as in Embodiment 1.

[0110] The observation results show that after replacing ammonium fluoride with sodium fluoride, benzotriazole with methylbenzotriazole, and anhydrous ethanol with isopropanol, the different sub-regions inside the heat-affected zone of the welded joint also exhibit different contrasts and have clearly distinguishable boundaries, which is basically consistent with Example 1. This indicates that the above-mentioned substitutes have basically the same technical effects in the stepwise corrosion liquid system of the present invention.

[0111] Based on the above, it can be seen that Example 1 uses Q1100 steel, a conventional low-alloy ultra-high-strength steel for hydraulic supports, as the object. The step-by-step corrosion method of the present invention can effectively distinguish the coarse-grained zone, fine-grained zone, incomplete recrystallization zone and tempering softening zone inside the heat-affected zone. The boundaries of each sub-zone are clear and the contrast difference is obvious.

[0112] Example 2 uses 35CrMnSiA as the object, which has a strength grade comparable to Q1100 but has a significant difference in alloy composition. By using the dynamic formulation of the present invention and adjusting the time, a relatively stable corrosion effect can also be obtained, indicating that the method of the present invention has a certain adaptability and adjustment capability for steels of different standard systems.

[0113] Example 3 uses Q960 as the object, which belongs to the steel in the strength grade transition range. Its microstructure gradient is slightly smaller than that of typical ultra-high strength steel. The second step of coloring time is relatively extended to compensate for the small contrast difference. After processing with the method of the present invention, the sub-regions of the heat-affected zone can still be effectively distinguished under low magnification observation conditions, which can further demonstrate the compatibility of the method of the present invention with steel in the transition range.

[0114] Example 4 shows that a relatively stable corrosion effect can be achieved even after changing the material, indicating that the solution of the present invention can allow for certain material fluctuations and errors, and has good reproducibility stability.

[0115] In summary, the present invention combines the ratio of the etchant and the corrosion time with ASI and HDI to form a dynamic model, which can effectively match the corrosion conditions with the characteristics of the steel material. It can effectively overcome the technical problems of conventional etchants and corrosion methods when used for ultra-high strength steel welded joints, such as the difficulty in clearly distinguishing the sub-zones within the heat-affected zone, the reliance on manual experience for corrosion parameter control, and the poor consistency of test results.

[0116] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions 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 etching solution for the heat-affected zone of hydraulic support welding, characterized in that, The process includes a first-step base etching solution and a second-step selective coloring solution. The first-step base etching solution contains nitric acid, low-carbon alcohol, and water. Based on a predetermined volume of etching solution, the volume percentage concentration of nitric acid in the etching solution is in the range of (0.6 + 1.0 × HGI)% to (1.0 + 1.5 × HGI)%, the volume percentage concentration of low-carbon alcohol is 40% to 50%, and water is used to make up the predetermined volume. The second step involves selecting a coloring solution containing nitric acid, low-carbon alcohol, water, acidic fluoride, and a selective inhibitor. Based on a predetermined volume of the etching solution, the volume percentage concentration of nitric acid in the etching solution is in the range of (0.3 + 0.3 × HGI)% to (0.5 + 0.6 × HGI)%, the volume percentage concentration of low-carbon alcohol is 40% to 50%, the mass-volume concentration of acidic fluoride is in the range of (0.5 + 1.0 × ASI) g / L to (1.0 + 1.5 × ASI) g / L, and the selective inhibitor includes molybdate corrosion inhibitor and benzotriazole corrosion inhibitor or their derivatives, with a mass ratio of 1:1 to 2:1 and a total mass-volume concentration in the range of (0.2 + 0.8 × HGI) g / L to (0.5 + 1.2 × HGI) g / L, with the remainder being water. The heat-affected zone microstructure gradient index HGI = (C + Mn / 6 + Cr / 5 + Mo / 4 + V / 3) × (1 + Ni + Cu) / (1 + Nb + Ti + Al), and the alloy strengthening index ASI = Nb + Ti + 0.5 × (V + Al). In the above two formulas, the symbol of each element represents the mass percentage of that element in the steel, and there are no units.

2. The corrosive liquid as described in claim 1, characterized in that, The molybdate corrosion inhibitor is at least one of sodium molybdate and potassium molybdate; the benzotriazole or its derivative corrosion inhibitor is at least one of benzotriazole or methylbenzotriazole; the low alcohol is at least one of anhydrous ethanol, methanol and isopropanol; the acidic fluoride is at least one of ammonium hydrogen fluoride and sodium hydrogen fluoride; and the water is deionized water, distilled water or ultrapure water.

3. The corrosive liquid as described in claim 2, characterized in that, The low-carbon alcohol is anhydrous ethanol, and its volume percentage concentration in the corrosive solution is 43-47%.

4. The corrosive liquid as described in claim 2, characterized in that, The selective inhibitor is composed of sodium molybdate and benzotriazole in a mass ratio of 1.5:

1.

5. A corrosion method for the heat-affected zone of hydraulic support welds, characterized in that, Includes the following steps: S1: Obtain the chemical composition of the material to be corroded, and calculate its alloy strengthening index (ASI) and heat-affected zone degradation index (HDI). Wherein, ASI = Nb + Ti + 0.5 × (V + Al), HDI = (C + Mn / 5 + Cr / 3 + Mo / 2 + V / 2) × (1 + Nb + Ti + B); and the ambient temperature T at the corrosion operation site is measured; S2: Prepare the corrosion solution as described in any one of claims 1-4 based on the CE, ASI and HDI measured in S1; S3: Apply the first step of the basic etching solution to the surface of the material to be etched and continue etching for the first etching time t1; after etching, rinse and dry the surface to be etched. S4. Apply the selected coloring solution from step two to the surface to be tested after treatment in step S3, and continue etching for the second etching time t2. After etching, rinse and dry to complete the etching process. Wherein, t1 and t2 are determined by the following formulas: t1=(A1+B1×HGI)×exp[D×(T-20℃)]; t2=(A2-B2×HGI)×exp[D×(T-20℃)]; In the formula, A1 and A2 are the reference corrosion times for the first and second steps, respectively, in seconds; B1 and B2 are the influence coefficients of the microstructure gradient index in the heat-affected zone for the first and second steps, respectively, in seconds; and D is the temperature correction coefficient, in degrees Celsius. -1 The parameters A1, B1, A2, B2 and D The constants were determined by the following method: multiple groups of low-alloy ultra-high-strength steel samples with different ASI and HDI were selected, and orthogonal corrosion tests were carried out using quantitative etching solution prepared with S2 under multi-gradient ambient temperatures. The standard was clear metallographic morphology, obvious microstructure boundary, no under-corrosion, and no over-corrosion. The corresponding system characteristic constants were determined by fitting and regressing the experimental data.

6. The macroscopic metallographic etching method as described in claim 5, characterized in that, Before performing S1, the material to be corroded is ground and polished to make the surface roughness Ra ≤ 1.6 μm.

7. The macroscopic metallographic etching method as described in claim 5, characterized in that, In S2, the prepared etching solution has the following characteristics: the volume percentage concentration of nitric acid in the first step basic etching solution is (0.8 + 1.2 × HGI)%; the volume percentage concentration of nitric acid in the second step selective coloring solution is (0.4 + 0.4 × HGI)%, the mass volume concentration of acidic fluoride salt is (0.8 + 1.2 × ASI) g / L, and the total mass volume concentration of selective inhibitor is (0.3 + 1.0 × HGI) g / L.

8. The macroscopic metallographic etching method as described in claim 7, characterized in that, In S1, the material to be corroded is a welded joint including a heat-affected zone, and the material to be corroded is low-alloy steel with a yield strength of not less than 1000 MPa; in S3, A1 = 12.0~25.0 s, B1 = 6.0~15.0 s, A2 = 12.0~28.0 s, B2 = 5.0~12.0 s, and D = -0.06~-0.02℃. -1 .

9. The macroscopic metallographic etching method as described in claim 8, characterized in that, In S3, A1 = 18.0s, B1 = 10.0s, A2 = 20.0s, B2 = 8.0s, and D = -0.04℃. -1 .

10. The macroscopic metallographic etching method as described in claim 5, characterized in that, Before proceeding with S3, a calibration experiment is required: In the same environment, the etching solution is prepared according to the method of S2 for standard samples known to ASI and HDI, and then the etching is calibrated. Based on the calibration results, it is determined whether to correct A1, B1, A2, B2 and D.