Corrosion solution and macroscopic metallographic corrosion method for low-alloy ultra-high strength steel

Abstract

The present application relates to a kind of corrosion liquid and macro etching method for low alloy ultra-high strength steel, the volume concentration of nitric acid in its corrosion liquid is (1.2+1.4*CE)% to (1.5+1.8*CE)%, the volume concentration of low carbon alcohol is 40%~50%;Acid fluorine salt mass concentration is (1.0+1.4*ASI)g / L to (1.4+1.8*ASI)g / L, the mass concentration of thiourea or its derivative is (0.5+1.5*HDI)g / L to (0.8+2.0*HDI)g / L;The balance is water;CE=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15, ASI=Nb+Ti+0.5* (V+Al), HDI=(C+Mn / 5+Cr / 3+Mo / 2+V / 2)*(1+Nb+Ti+B), above-mentioned element symbol is the mass percentage value of the element in the steel to be etched.The present application can solve the technical problems that the over-etching window of conventional etching liquid is difficult to control, weld and heat-affected zone is prone to over-etching, and corrosion duration control is seriously dependent on artificial experience, and the consistency of detection result is poor.

Description

Technical Field

[0001] This invention relates to the field of metallographic testing technology, and in particular to a corrosion solution and macroscopic metallographic corrosion method for low-alloy ultra-high strength steel. Background Technology

[0002] Low-alloy ultra-high-strength steels (such as classic S1100QL or Q1100 steels with a yield strength of not less than 1030MPa; and conventional low-alloy high-strength tempered steels such as Q890 and Q960, 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 low cost. Compared with conventional low-alloy high-strength steels (such as Q690 or steels with lower strength), these low-alloy ultra-high-strength steels tend to have a more significant tendency for microstructure degradation in the heat-affected zone during welding, and the corrosion resistance of the weld and heat-affected zone decreases more significantly, making welding relatively more difficult. To ensure welding quality, macroscopic metallographic corrosion testing is usually required on the weld joints to clearly reveal the microstructure of the weld and heat-affected zone and determine whether welding defects exist.

[0003] In existing technologies, the macroscopic metallographic corrosion of welded joints of low-alloy ultra-high-strength steel (hereinafter referred to as ultra-high-strength steel) generally follows the conventional corrosion methods for 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 welded joints of ultra-high-strength steel.

[0004] First, the corrosion resistance of the base metal and weld zone in ultra-high strength steel welded joints differs significantly, and the over-corrosion window in the weld zone is extremely narrow. Unlike conventional low-alloy high-strength steel, the base metal of ultra-high strength steel is mostly composed of fine tempered martensite or lower bainite, which is destroyed by high temperatures. It also contains more alloying elements such as Cr, Ni, and Mo to improve corrosion resistance, and has strong initial resistance to nitric acid alcohol. However, the weld metal has a significantly reduced corrosion resistance due to its cast structure and dendritic segregation. Under the welding thermal cycle, the originally uniform and dense base metal structure in the heat-affected zone may be destroyed by the high welding temperature, resulting in grain coarsening, element segregation, and residual stress concentration, which also significantly reduces corrosion resistance. Due to the coarse structure and poor chemical stability of the weld and heat-affected zone, the corrosion rate is extremely fast. The transition time window from clearly visible microstructure to a blackened, blurred over-corrosion state is extremely narrow, often only a few seconds. Secondly, and more importantly, 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 considerably depending on the manufacturer. This leads to differences in the deterioration tendency of the heat-affected zone and the corrosion resistance of the weld area between different batches of steel. The width and location of the over-corrosion window also drift significantly depending on the steel grade. This means that empirical corrosion parameters developed for a specific type of steel often fail to effectively suppress weld over-corrosion 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 results in significant discrepancies between test results from different operators and between different batches by the same operator, leading to poor repeatability and comparability. This severely restricts the efficiency and reliability of ultra-high strength steel welding quality inspection. Summary of the Invention

[0005] (a) Technical problems to be solved

[0006] In view of the above-mentioned shortcomings and deficiencies of the prior art, the present invention provides a corrosion solution and macro-metallographic corrosion method for low alloy ultra-high strength steel. It solves the technical problems of conventional corrosion solutions and corrosion methods in the prior art when used for macro-metallographic inspection of welded joints of ultra-high strength steel. The over-corrosion window is difficult to control, the weld and heat-affected zone are prone to over-corrosion, and the control of corrosion time depends heavily on manual experience, resulting in poor consistency of test results.

[0007] (II) Technical Solution

[0008] To achieve the above objectives, the main technical solutions adopted by the present invention include:

[0009] In a first aspect, the present invention provides a corrosion solution for low-alloy ultra-high-strength steel, comprising a solvent component and a solute component. The solvent component includes nitric acid, a low-carbon alcohol, and the balance being water. The solute component includes an acidic fluoride salt and a thiourea or its C1-C3 alkyl-substituted derivatives. The acidic fluoride salt, thiourea, or their C1-C3 alkyl-substituted derivatives of the solute component can all be completely dissolved in the solvent component to form a homogeneous and stable acidic corrosion solution.

[0010] The ratio of the corrosive solution must meet the following conditions:

[0011] Based on a predetermined volume of the corrosive solution, the volume percentage concentration of nitric acid is (1.2 + 1.4 × CE)% to (1.5 + 1.8 × CE)%; the volume percentage concentration of lower alcohols is 40% to 50%; the mass-volume concentration of acidic fluoride salts is (1.0 + 1.4 × ASI) g / L to (1.4 + 1.8 × ASI) g / L; the mass-volume concentration of thiourea or its C1-C3 alkyl-substituted derivatives is in the range of (0.5 + 1.5 × HDI) g / L to (0.8 + 2.0 × HDI) g / L; and water is used to make up the predetermined volume.

[0012] Wherein, carbon equivalent CE=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15, alloy strengthening index ASI=Nb+Ti+0.5×(V+Al), heat-affected zone deterioration index HDI=(C+Mn / 5+Cr / 3+Mo / 2+V / 2)×(1+Nb+Ti+B), in the above three formulas, the symbol of each element represents the mass percentage of the element in the material to be corroded, without units.

[0013] According to a preferred embodiment of the present invention, when the carbon equivalent CE of the material to be etched is ≥0.65, the solvent component further includes hydrochloric acid; the volume percentage concentration of hydrochloric acid in the etching solution is (0.01+0.05×CE)% to (0.03+0.15×CE)%.

[0014] According to a preferred embodiment of the present invention, the lower alcohol is at least one of anhydrous ethanol, methanol and isopropanol; the acidic fluoride is at least one of ammonium bifluoride and sodium bifluoride; the water is deionized water, distilled water or ultrapure water; and the C1-C3 alkyl-substituted derivative of thiourea is at least one of methylthiourea, ethylthiourea and isopropylthiourea.

[0015] 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%; the solid component consists of thiourea and ammonium bifluoride.

[0016] Secondly, the present invention also provides a method for macroscopic metallographic corrosion of low-alloy ultra-high-strength steel, comprising the following steps:

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

[0018] Wherein, CE=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15, 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 as described in any one of the first aspects based on the CE, ASI and HDI measured in S1;

[0020] S3. Calculate the corrosion time t according to the following fitting formula:

[0021] t=(A+B×CE+C×ASI-E×HDI)×exp[D×(T-20℃)];

[0022] Where A is the baseline corrosion time, in seconds;

[0023] B is the carbon equivalent influence coefficient, with units of s;

[0024] C is the influence coefficient of the alloy strengthening index, with the unit being s;

[0025] D is the temperature correction factor, in °C. -1 ;

[0026] E is the degradation index influence coefficient, with units of seconds;

[0027] T represents the ambient temperature;

[0028] A, B, C, D, and E are constants obtained by calibration in the following way: multiple groups of low-alloy ultra-high strength steel samples with different CE, ASI, and HDI were selected, and orthogonal corrosion tests were carried out using quantitative corrosion solution prepared with S2 under multi-gradient ambient temperatures. The corresponding system characteristic constants were determined by fitting and regressing the experimental data, with clear metallographic morphology, obvious microstructure boundaries, no under-corrosion, and no over-corrosion as the standard.

[0029] S4: Apply the etchant to the surface of the material to be etched and continue etching for a duration of t to complete the etching process.

[0030] 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 Ra≤1.6μm; in S2, when preparing the corrosion solution, if CE≥0.65, hydrochloric acid with a volume percentage concentration of (0.01+0.05×CE)% to (0.03+0.15×CE)% is added, and the amount of water is reduced by the same amount.

[0031] According to a preferred embodiment of the present invention, in S2, the volume percentage concentration of nitric acid in the prepared corrosive solution is (1.5 + 1.6 × CE)%, the mass volume concentration of the acidic fluoride salt is (1.2 + 1.6 × ASI) g / L, and the mass volume concentration of thiourea or its C1-C3 alkyl-substituted derivative is (0.65 + 1.75 × HDI) g / L; in S3, A = 4.0~9.0 s, B = 18.0~30.0 s, C = 6.0~12.0 s, E = 2.0~5.0 s, and D = -0.06~-0.02℃. -1 .

[0032] According to a preferred embodiment of the present invention, in S1, the material to be corroded is a welded joint including the weld and the heat-affected zone; when CE≥0.65, the corrosion solution also includes hydrochloric acid, and the volume percentage concentration of hydrochloric acid is (0.02+0.1×CE)%.

[0033] According to a preferred embodiment of the present invention, in S3, the material to be corroded is steel with a yield strength of not less than 1000 MPa, A = 6.5 s, B = 24.0 s, C = 9.0 s, E = 3.5 s, and D = -0.04℃. -1 .

[0034] 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 standard samples with known CE, ASI and HDI, and then corrosion calibration is performed. Based on the calibration results, it is determined whether the system characteristic constants in S3 should be corrected.

[0035] (III) Beneficial Effects

[0036] The present invention relates to a corrosion solution and macroscopic metallographic corrosion method for low-alloy ultra-high-strength steel. This method utilizes a quantitative formula to dynamically determine the corrosion solution ratio and corrosion duration based on carbon equivalent (CE), alloy strengthening index (ASI), and heat-affected zone degradation index (HDI). Compared to existing methods with relatively fixed formulations and reliance on experience to determine corrosion duration, this method dynamically enhances corrosion resistance and provides protection against the significant differences in corrosion resistance between the base material and weld area in ultra-high-strength steel welded joints, as well as the extremely narrow over-corrosion window of the weld. This effectively avoids over-corrosion of the weld and heat-affected zone, further improving the accuracy and consistency of macroscopic metallographic corrosion detection of ultra-high-strength steel welded joints.

[0037] Specifically, the etching solution of this invention introduces acidic fluoride salts (hereinafter referred to as fluoride salts) and thiourea or its C1-C3 alkyl-substituted derivatives (mainly thiourea is the active ingredient, referred to as thiourea below) into the nitric acid-alcohol system. The fluoride salts effectively activate grain boundaries by providing free fluoride ions, eroding the chemically stable regions formed by the segregation of alloying elements at the grain boundaries. This allows the dense grain boundaries, which are originally difficult to corrode by nitric acid alone, to become visible, thereby macroscopically improving the visibility quality of the microstructure boundaries in each region. Thiourea, through the preferential adsorption of sulfur atoms in its molecules onto high-energy unstable surfaces such as welds and heat-affected zones, forms a corrosion-inhibiting protective film, selectively slowing down the corrosion rate of these easily over-corroded areas. This homogenizes the corrosion rates of the weld, heat-affected zone, and base material, which originally differed significantly, resulting in clearer macroscopic microstructure boundaries, more significant contrast differences, and reduced likelihood of over-corrosion. The two work together to protect the weld from over-corrosion while fully revealing the base material. They can clearly show the macroscopic metallographic boundaries of the weld metal, fusion line, and heat-affected zone, providing a more reliable microstructure basis for macroscopic metallographic observation and defect identification.

[0038] Meanwhile, this invention also employs a method to calculate the corrosion time t using CE, ASI, HDI, and ambient temperature T, freeing the determination of corrosion time from reliance on manual experience. Instead, it is directly calculated from material parameters and on-site temperature through a mathematical model. CE reflects the difficulty of revealing the microstructure of ultra-high strength steel in a nitric acid-alcohol system; a higher CE indicates a denser base material microstructure, requiring a higher nitric acid concentration for effective corrosion. ASI reflects the contribution of microalloying elements such as Nb, Ti, and V to grain boundary corrosion resistance through grain refinement and precipitation strengthening; a higher ASI indicates that grain boundaries are more difficult to clearly reveal, requiring more fluoride salts for grain boundary activation. HDI characterizes the tendency of the heat-affected zone (HAZ) of steel to deteriorate under welding thermal cycling; a higher HDI indicates a more severe tendency for microstructure deterioration in the weld area and a greater susceptibility to over-corrosion. In the formula of this invention, a higher HDI results in a relatively lower calculated corrosion time, enabling the suppression of over-corrosion risk at the time level. Furthermore, the model compensates for the nonlinear effect of ambient temperature fluctuations on the corrosion rate through an exponential term, ensuring high consistency of the detection results across different seasons or regions.

[0039] Furthermore, in the etching solution of this invention, the concentrations of nitric acid and fluoride salts increase gradually with the increase of CE and ASI, ensuring that the base metal structure can be fully etched and the metallographic structure can be clearly activated; the concentration of thiourea increases significantly with the increase of HDI, thereby strengthening the corrosion inhibition protection of the weld area and further slowing down its corrosion rate. This results in an automatic increase in the concentration of protective components in the etching solution for steel with more severe heat-affected zone deterioration. This method of the present invention can effectively address the characteristics of poor corrosion resistance of welds and heat-affected zones and strong corrosion resistance of base metal in ultra-high strength steel welded joints. It allows the base metal and other areas to be fully exposed within the short corrosion time required for the weld, while the weld is protected from over-corrosion due to targeted protection. Thus, under the same corrosion operation, the microstructure of the weld, heat-affected zone, and base metal at the weld joint can be displayed in a balanced and clear manner, effectively avoiding the problem in the prior art where it is difficult to take into account the large differences in corrosion resistance of different areas.

[0040] Furthermore, since the ratio of the etching solution and the etching time are all based on the same set of three parameters—CE, ASI, and HDI—once determined, an effective quantitative control system is established, ensuring that each macroscopic metallographic etching operation using this 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 good repeatability and comparability of macroscopic metallographic detection results. It can be directly applied to fields such as large-scale testing of hydraulic support welded components, facilitating the standardization and industrial-scale batch application of macroscopic metallographic testing for ultra-high strength steel welding quality. Attached Figure Description

[0041] Figure 1 This is the macroscopic metallographic image observed after corrosion of the steel in Example 1 of the present invention. Detailed Implementation

[0042] To better explain and facilitate understanding of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0043] Firstly, this invention proposes a corrosion solution for low-alloy ultra-high-strength steel, comprising a solvent component and a solute component. The solvent component includes nitric acid, low-carbon alcohols, and water, while the solute component includes acidic fluoride salts and thiourea or their C1-C3 alkyl-substituted derivatives. The acidic fluoride salts, thiourea, or their C1-C3 alkyl-substituted derivatives of the solute component are all completely soluble in the solvent component, forming a homogeneous and stable acidic corrosion solution, i.e., the corrosion solution of this invention. Nitric acid, as the main corrosion medium, can erode grain boundaries and microstructures, forming corrosion grooves to reveal the microstructure. Low-carbon alcohols mainly reduce the surface tension of the corrosion solution, ensuring uniform spreading of the corrosion solution on the sample surface. They also act as a diluent to adjust the nitric acid concentration and aid in dehydration and drying after corrosion. Water, as a polar solvent, is used to dissolve the solute component and adjust the final volume. The acidic fluoride salts can efficiently release free fluoride ions in the acidic system, selectively eroding the chemically stable regions formed by the segregation of alloying elements at grain boundaries, thus activating the grain boundaries and making previously blurred and discontinuous grain boundaries continuous and clear. Especially for low-alloy ultra-high-strength steels containing microalloying elements such as Nb, V, and Ti, the high degree of grain refinement and strong grain boundary chemical stability make the grain boundary activation effect of acidic fluoride salts more pronounced. Simultaneously, acidic fluoride salts can 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 proportioning. Thiourea preferentially adsorbs onto grain boundary regions, guiding nitric acid to preferentially erode along the grain boundary direction, thereby enhancing the contrast and distinctiveness of the grain boundaries, making the corrosion effect more uniform and controllable, and avoiding the loss of microstructural details caused by excessive or insufficient local corrosion. Similarly, C1-C3 alkyl-substituted derivatives of thiourea also contain thiourea, and the carbon chain strength of C1-C3 can enhance the adsorption capacity of thiourea to a certain extent without causing serious steric hindrance problems. With minimal weight deviation, they can also be used.

[0044] Unless otherwise specified, the thiourea mentioned below in this invention refers to thiourea or its C1-C3 alkyl-substituted derivatives. The fluorine salts mentioned in this invention, unless otherwise specified, also refer to acidic fluorine salts.

[0045] The proportions of the etchant are determined based on the carbon equivalent (CE) and alloy strengthening index (ASI) of the material to be etched, according to the following dynamic proportioning formula:

[0046] Based on a predetermined volume of the corrosive solution, the volume percentage concentration of nitric acid ranges from (1.2 + 1.4 × CE)% to (1.5 + 1.8 × CE)%. 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 through fitting 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 fit and determine the appropriate parameters based on actual conditions to obtain parameters suitable for practical use. Corrections can be made as needed during actual application. The above range can accommodate reasonable differences in corrosion conditions among different steels and has broad applicability.

[0047] Preferably, the volume percentage concentration of nitric acid is (1.5 + 1.6 × CE)%. This preferred range is a better ratio determined based on the commonly used Q1030-1100 steel and its equivalent variant steel for hydraulic supports. It can ensure clear visualization of tissue boundaries while taking into account both the economy of reagent consumption and operational safety.

[0048] The mass-volume concentration of acidic fluoride salts ranges from (1.0 + 1.4 × ASI) g / L to (1.4 + 1.8 × ASI) g / L. The dosage increases with increasing ASI, effectively compensating for the increased difficulty of intergranular corrosion caused by microalloying element strengthening in high-ASI steels. Similar to the aforementioned range formula for nitric acid, the optimal concentration of acidic fluoride salts may fluctuate slightly for different types of low-alloy ultra-high-strength steels due to variations in specific elemental composition even with the same ASI concentration, but generally falls within this range, allowing for fine-tuning during application.

[0049] Preferably, the mass-volume concentration of the acidic fluoride salt is (1.2 + 1.6 × ASI) g / L. This preferred range is the experimentally optimal value determined for Q1030-1100 steel and its equivalent variant steel commonly used in hydraulic supports, which can effectively activate grain boundaries without causing excessive corrosion.

[0050] The mass-volume concentration of thiourea ranges from (0.5 + 1.5 × HDI) g / L to (0.8 + 2.0 × HDI) g / L. Similar to the aforementioned range formula for nitric acid, this range can cover the fluctuations in optimal thiourea dosage caused by differences in composition and process in different low-alloy ultra-high strength steels, and has wide applicability.

[0051] Preferably, the mass-volume concentration of thiourea is (0.65 + 1.75 × HDI) g / L. This preferred range is also determined for Q1030-1100 steel commonly used in hydraulic supports and its equivalent variant steels. It can effectively guide nitric acid to preferentially erode along the grain boundaries and control the direction and intensity of corrosion, thereby obtaining a clear and uniform metallographic structure.

[0052] The volume percentage concentration of low-carbon alcohol is 40%~50%. Similarly, if the amount of low-carbon alcohol is too high, it will over-dilute the nitrate substance and weaken its corrosive ability. If it is too low, the spreadability will be poor and the drying speed will be slow. The range of 40%~50% can achieve a good balance between the above effects and has a wide applicability to different steels.

[0053] The present invention uses the carbon equivalent (CE), alloy strengthening index (ASI), and heat-affected zone degradation index (HDI) of the steel to be corroded to dynamically and quantitatively determine the composition ratio of the corrosion solution, thereby forming a dynamic ratio system and achieving effective and precise matching.

[0054] Carbon equivalent CE = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, Alloy strengthening index ASI = Nb + Ti + 0.5 × (V + Al), HDI = (C + Mn / 5 + Cr / 3 + Mo / 2 + V / 2) × (1 + Nb + Ti + B). In the above three formulas, the symbol of each element represents the mass percentage of that element in the steel, and there are no units.

[0055] It should be noted that carbon equivalent (CE) is commonly used in conventional techniques to evaluate the weldability and cold cracking tendency of steel. However, in this invention, CE is not used to reflect the corrosion resistance of steel, but rather to characterize the difficulty of revealing the metallographic structure of low-alloy ultra-high-strength steel under the nitric acid-alcohol corrosion system of this invention. A higher carbon equivalent indicates a greater tendency for the steel to harden, making it more prone to forming hard phases such as martensite or bainite after welding or heat treatment. Under nitric acid-alcohol corrosion, stronger corrosion conditions (such as higher nitric acid concentrations or longer corrosion times) are required to clearly reveal its grain boundaries and microstructure. Therefore, this invention uses CE as a core parameter to measure the required corrosion conditions. Simultaneously, this invention also uses the Alloy Strengthening Index (ASI) to quantify the comprehensive contribution of microalloying elements Nb, Ti, V, and Al to grain refinement and precipitation strengthening of steel (in low-alloy high-strength steel, the main contributions are from Nb and Ti, while the contributions from V and Al are relatively weak). These elements significantly increase the difficulty of clearly revealing grain boundaries during metallographic corrosion by refining grains, increasing grain boundary area, and forming stable carbonitrides at grain boundaries. A higher ASI indicates stronger corrosion conditions required for grain boundary visibility, which also reflects its strength parameters. This invention also introduces the heat-affected zone degradation index (HDI) to characterize the tendency of the heat-affected zone (HAZ) of the base material to deteriorate under welding thermal cycling. In its formula, the (C+Mn / 5+Cr / 3+Mo / 2+V / 2) part reflects the hardening tendency of the steel itself and its sensitivity to welding heat input. The higher this value, the easier it is for the HAZ to generate coarse, unstable hard phase structures during welding, resulting in a more significant decrease in corrosion resistance. The latter part (1+Nb+Ti+B) reflects the ability of elements such as Nb, Ti, and B to pin grain boundaries and inhibit grain growth by forming fine carbonitrides. The higher the overall HDI value, the more severe the microstructure deterioration when the heat-affected zone is affected, and the greater the risk of over-corrosion of the weld. Correspondingly, a higher concentration of thiourea is needed to provide corrosion inhibition protection and shorten the corrosion time to avoid possible over-corrosion.

[0056] Preferably, when the carbon equivalent (CE) of the material to be corroded is ≥0.65, the solvent component also includes hydrochloric acid. In particular, for ultra-high-strength steel with a high CE, its microstructure is usually more dense, and nitric acid alone is not enough to fully corrode it, so hydrochloric acid needs to be added to fully reveal the microstructure of the heat-affected zone and the fusion line.

[0057] More preferably, the volume percentage concentration of hydrochloric acid is (0.01 + 0.05 × CE)% to (0.03 + 0.15 × CE)%. Similar to the aforementioned range formula for suitable nitric acid, this range can also accommodate fluctuations in the optimal amount of hydrochloric acid for different ultra-high strength steels. More preferably, the volume percentage concentration of hydrochloric acid is (0.02 + 0.1 × CE)%, this preferred range can clearly reveal the boundary of the heat-affected zone without excessively increasing the corrosion intensity.

[0058] 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.

[0059] More preferably, considering safety, volatility and cost, anhydrous ethanol can be selected as the low-carbon alcohol, and its volume percentage concentration is 43-47%, more preferably 45%.

[0060] 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.

[0061] 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.

[0062] Of course, it should be noted that in the actual operation of this invention, pure nitric acid, pure ethanol, and pure hydrochloric acid are not required as solvent components. Conventional concentrated nitric acid and concentrated hydrochloric acid aqueous solutions are also feasible. During calculation and use, the concentration of these substances (such as nitric acid) is converted to pure values ​​and substituted into the formula of this invention. The water content is calculated as the amount of water required in the formulation of the corrosion solution of this invention.

[0063] More preferably, in thiourea or its C1-C3 alkyl-substituted derivatives, thiourea is preferably used as the solute component, and in ammonium bifluoride and sodium bifluoride, ammonium bifluoride is preferably used, i.e., the solute components are thiourea and ammonium bifluoride.

[0064] In a second aspect, the present invention also provides a macroscopic metallographic etching method applicable to the etching solution as described in any one of the first aspects, comprising the following steps:

[0065] S1: Obtain the chemical composition of the material to be corroded, and calculate its carbon equivalent (CE), alloy strengthening index (ASI), and heat-affected zone degradation index (HDI) based on the obtained mass percentage of each element. Wherein, CE = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, ASI = Nb + Ti + 0.5 × (V + Al), and HDI = (C + Mn / 5 + Cr / 3 + Mo / 2 + V / 2) × (1 + Nb + Ti + B). 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 be mutually verified. The methods and equipment used in the actual measurements should ideally be consistent to ensure the uniqueness of the subsequent model input parameters and the traceability of the test results. A conventional thermometer with an accuracy of ±0.5℃ is sufficient for temperature measurement. During measurement, place the thermometer probe near the corrosion operation area to avoid deviations caused by local temperature differences.

[0066] S2: Prepare the corrosion solution based on a predetermined volume of corrosion solution, such that the volume percentage concentration of the nitric acid is (1.2 + 1.4 × CE)% to (1.5 + 1.8 × CE)%; the volume percentage concentration of the low-carbon alcohol is 40% to 50%; the mass-volume concentration of the acidic fluoride salt is (1.0 + 1.4 × ASI) g / L to (1.4 + 1.8 × ASI) g / L; the mass-volume concentration of the thiourea or its C1-C3 alkyl-substituted derivative (hereinafter referred to as thiourea or its derivative) is in the range of (0.5 + 1.5 × HDI) g / L to (0.8 + 2.0 × HDI) g / L, with the remainder being water. During operation, first measure the required volume of low-carbon alcohol and some water (water can be omitted). Add nitric acid while stirring (hydrochloric acid can be added simultaneously or later if available). After mixing thoroughly and cooling to room temperature, add acidic fluoride and thiourea or their derivatives, stirring until completely dissolved. Finally, add water to make up the remaining volume to the target predetermined volume. Since the concentrations of each component are dynamically determined according to CE, ASI, and HDI formulas, the prepared etching solution is a specific formula for the steel being tested, with the corrosion intensity precisely matched to the material properties of the steel. Prepare and use immediately to ensure consistent reagent activity for each test, avoiding concentration changes and contamination problems that may occur with pre-prepared storage.

[0067] S3. Calculate the corrosion time t according to the following fitting formula:

[0068] t=(A+B×CE+C×ASI-E×HDI)×exp[D×(T-20℃)].

[0069] Where A is the baseline corrosion time in seconds (s), B is the carbon equivalent influence coefficient in seconds, C is the alloy strengthening index influence coefficient in seconds, E is the degradation index influence coefficient in seconds, and D is the temperature correction coefficient in degrees Celsius (°C). -1 T represents the ambient temperature. (A+B×CE+C×ASI-E×HDI) represents the baseline corrosion time at the standard reference temperature of 20℃. The exponential term exp[D×(T-20℃)] is a 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, which can provide more accurate time control under conditions with large temperature differences.

[0070] Furthermore, it should be noted that the parameters A, B, C, D, and E in the corrosion time calculation formula of this invention are derived under the premise of using the dynamic proportioning method of this invention (i.e., the concentration of each component in the corrosion solution in this invention is determined according to the formula based on CE, ASI, and HDI). In a precisely controlled laboratory environment, multiple groups of ultra-high strength steels with different CE, ASI, and HDI were selected as samples. Systematic orthogonal corrosion experiments were conducted on these samples under multiple temperature gradients, and the optimal corrosion time was recorded. The criteria for determining the optimal corrosion time were clear metallographic morphology, distinct microstructure boundaries, no under-corrosion, and no over-corrosion. The data were then summarized, and the corresponding system characteristic constants were obtained by fitting the data using multiple nonlinear regression and other methods with CE, ASI, and HDI as independent variables. These parameters already include the comprehensive influence of the dynamic changes in the formulation with CE, ASI, and HDI on the corrosion rate. In practical applications, it is only necessary to obtain the CE value, ASI value and HDI value of the target material to be corroded, prepare the corrosion solution according to the ratio formula given in this invention, and calculate the corrosion time 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, and the operation is relatively simple.

[0071] S4: After calculating the required corrosion time t, apply the etching solution to the surface of the material to be etched and continue etching for the required time t to complete the etching process. The etching solution can be applied evenly to the surface using a dropper or cotton swab, ensuring complete coverage of the target area, and then start timing. During etching, the sample can be slightly shaken to ensure continuous and uniform contact between the etching solution and the surface. After the calculated time t is reached (with a maximum allowable time error of ±2s, preferably ±1s), immediately stop the etching process. Quickly rinse the sample surface with clean water to remove the etching solution, then rinse with anhydrous ethanol for dehydration, and finally dry the sample surface with cold air. This process must be rapid and continuous to avoid water residue or secondary corrosion. After etching is complete, macroscopic metallographic testing and other procedures can be performed for further analysis of the etching effect.

[0072] The corrosion method of this invention integrates the dynamic ratio of the corrosion solution and the corrosion time into a unified quantitative control process, achieving synergistic control of steel composition, corrosion solution formulation, and corrosion time. CE, ASI, and HDI, as core parameters, determine both the component concentration of the corrosion solution and participate in the calculation of corrosion time. Their interaction and combined effect make the entire testing process less reliant on the operator's personal experience and more determined by the material properties of the steel and the ambient temperature. This improves the consistency and repeatability of test results for different steels, different operators, and different ambient temperatures.

[0073] Furthermore, it should be noted that the low-alloy high-strength steel targeted in this invention has a relatively specific range of alloying elements added. Besides the elements selected in this invention, such as C, Mn, Cr, Mo, V, Ni, Cu, Nb, Ti, Al, B, and Zr, which have significant effects on hardenability, it may also contain elements such as Si, P, S, Sn, Sb, and As. Among these, Si has a negligible effect on hardening tendency within a general content range and can be ignored in the formulation of this invention. The content of P, S, Sn, Sb, and As is extremely low, and their effect on metallographic corrosion is negligible and is not considered. Meanwhile, conventional low-alloy steels typically do not contain other alloying elements such as W and rare earth elements that may have a significant impact on hardenability or microstructure. For certain special-purpose ultra-high-strength steels, even if they contain small amounts of W or rare earth elements, these substances may affect the hardenability or grain state of the steel, making it more difficult to alter the microstructure observed in a nitric acid-alcohol system. This can usually be mitigated by adjusting parameters within the range specified in this invention and correcting calibration experiments to adapt the corrosion conditions to actual needs, ensuring the applicability of the technical solution described in this invention. Furthermore, some of these effects can be offset by adjusting CE or ASI. For example, W's contribution to hardenability is similar to that of Mo, and can be incorporated to some extent through an equivalent amount of Mo, further reducing the influence of these special elements.

[0074] 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 the weld bead and its heat-affected zone. Grinding is performed using gradient sandpaper from coarse to fine until the surface is smooth and the scratches are uniform. Polishing can be done using conventional felt or diamond polishing paste or alumina suspension. Of course, this invention does not limit the grinding and polishing methods; various methods can be used to ensure a final mirror-like finish, laying the foundation for subsequent uniform corrosion and clear imaging.

[0075] Preferably, in step S2, when CE ≥ 0.65, additional hydrochloric acid with a volume percentage concentration of (0.01 + 0.05 × CE)% to (0.03 + 0.15 × CE)% is added, while the amount of water used is reduced by an equal amount, or in other words, the proportion of water is reduced. The addition of hydrochloric acid can specifically disrupt the stable carbides rich in Cr, Mo, and other elements in high-carbon equivalent ultra-high-strength steel, allowing the microstructure of the heat-affected zone and fusion line to be fully revealed, thus ensuring the quality of macroscopic metallographic testing of the ultra-high-strength steel. Appropriately reducing the proportion of water avoids dilution.

[0076] Preferably, in S3, A = 4.0~9.0s, B = 18.0~30.0s, C = 6.0~12.0s, E = 2.0~5.0s, and D = -0.06~-0.02℃. -1 The parameters are allowed to have a certain fluctuation range, allowing for reasonable differences in equipment accuracy 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 steel with a yield strength of not less than 1000 MPa, specifically not less than 1030 MPa. For these steels, the detection results are generally quite accurate. For conventional Q960, Q890, and other transitional steels, since their microstructure is often similar to that of ultra-high-strength steel, using the above scope usually yields good corrosion results for the weld and heat-affected zone as well. However, the contrast between the weld and heat-affected zone and the base material may be relatively low, but this does not affect the distinction between the weld and its heat-affected zone.

[0077] Preferably, in S1, the material to be corroded is a welded joint of low-alloy 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 is (1.5 + 1.6 × CE)%, the mass volume concentration of acidic fluoride is (1.2 + 1.6 × ASI) g / L, and the mass volume concentration of thiourea or its C1-C3 alkyl-substituted derivatives is (0.65 + 1.75 × HDI) 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 visualization of the microstructure boundaries and considering the economic efficiency of reagent consumption and operational safety.

[0078] More preferably, in S3, A = 6.5s, B = 24.0s, C = 9.0s, E = 3.5s, and D = -0.04℃. -1This set of parameters is also set in this application for welded joints of low-alloy ultra-high-strength steel with a yield strength of 1030~1100MPa, which can achieve a corrosion effect with clear microstructure boundaries and is less prone to problems such as over-corrosion or under-corrosion.

[0079] 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 (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:

[0080] (1) Select several representative grades of steel in this series (at least covering the upper, middle and lower limits of their typical carbon equivalent 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 nitric acid content of (1.5+1.6×CE)%, (1.6+1.7×CE)%, etc., and then prepare a series of corrosion solutions for systematic corrosion experiments through orthogonal experiments and / or single variable experiments, and record the corrosion time when each sample obtains better metallographic display effect at multiple gradient temperatures.

[0081] (2) Using CE, ASI, and HDI as independent variables and the optimal corrosion duration as the dependent variable, multiple nonlinear regression and other methods are used to fit time model parameters A, B, C, and D suitable for this series of steels. Simultaneously, based on the nitric acid concentrations that yielded the best results under each CE, the fluoride salt concentrations that yielded the best results under each ASI, and the thiourea concentrations that yielded the best results under each HDI in the experiments, regression is used to determine the formula coefficients for the dynamic mix proportions suitable for 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, and if the range is exceeded, it can be appropriately expanded based on new data to cover the needs of this series of steels.

[0082] 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 complete welded joint specimen including the weld metal, fusion line, and heat-affected zone. By conducting a unified corrosion experiment and effect evaluation on the entire joint, the fitted parameters enable each area of ​​the welded joint to achieve a relatively distinct and clearly defined metallographic manifestation effect, meeting the requirements of comprehensive welding quality testing for macroscopic metallographic inspection. Of course, due to the significant difference in corrosion resistance between the base material and the weld area of ​​low-alloy ultra-high-strength steel, the parameters fitted for the complete joint in this invention focus more on suppressing over-corrosion of the weld and heat-affected zone to ensure the manifestation quality of the core detection area. The corrosion effect on the base material is relatively weaker, but it is sufficient for comparison in macroscopic metallographic inspection.

[0083] Of course, independent fitting can also be performed on only the weld metal or heat-affected zone to obtain formula coefficients and time constants specific to that region. For low-alloy ultra-high-strength steel, due to the significant difference in corrosion resistance between the base metal and the weld, parameters obtained from fitting a single region may produce large deviations when applied to the complete joint. For example, parameters fitted only to the base metal or heat-affected zone may lead to over-corrosion in the weld area, while parameters fitted only to the weld may lead to under-corrosion in the base metal. Therefore, parameters obtained from independent fitting are mainly suitable for detailed observation of a specific region and are not recommended for direct use in the comprehensive inspection of complete joints. In practice, the fitting method can be flexibly selected according to the inspection purpose and accuracy requirements.

[0084] 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 with known CE, ASI, and HDI, 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 the tissue), 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 and blurred tissue 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, batches, or 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 there are changes in reagent batches or raw material storage time, the specific composition of each raw material should be determined in advance, its performance should be determined, and corresponding calculations or optimizations should be carried out to avoid fluctuations in corrosion results caused by changes in its composition.

[0085] To better understand the above technical solutions, exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present invention are shown in the drawings, 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. It should be noted in advance that, in the following description, thiourea refers only to thiourea and does not include its derivatives.

[0086] Example 1:

[0087] This embodiment provides a corrosion solution and macroscopic metallographic corrosion method for low-alloy ultra-high strength steel. The welded joint of Q1100 low-alloy ultra-high strength steel used in mining hydraulic supports is used as the material to be corroded, and the corrosion solution and macroscopic metallographic corrosion method described in this invention are used for detection.

[0088] Sample pretreatment:

[0089] 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.

[0090] S1: Obtain material parameters and measure ambient temperature:

[0091] 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.

[0092] The calculated values ​​are CE=1.059, ASI=0.145, and HDI=1.183. Simultaneously, the ambient temperature at the corrosion operation site was measured to be T=22℃.

[0093] S2. Preparation of the corrosive solution:

[0094] In this embodiment, the total volume of the etching solution is 500 mL, and its components include nitric acid, anhydrous ethanol, deionized water, ammonium bifluoride, and thiourea. The volume percentage concentration of nitric acid is (1.5 + 1.6 × CE) = 3.19%, but for ease of operation, 3.2% is used. Commercially available concentrated nitric acid with a mass fraction of 67.5% is used for conversion during preparation. The volume percentage concentration of anhydrous ethanol is 45%. The mass-volume concentration of ammonium bifluoride is (1.2 + 1.6 × ASI) g / L = 1.43 g / L. The mass-volume concentration of thiourea is (0.65 + 1.75 × HDI) g / L = 2.72 g / L. The volume percentage concentration of hydrochloric acid is (0.02 + 0.1 × CE)% = 0.126%, and 0.13% is used. Deionized water is used to bring the volume to 500 mL. During preparation, anhydrous ethanol, commercially available concentrated nitric acid, concentrated hydrochloric acid, and a portion of deionized water are mixed evenly and cooled to room temperature. Then, ammonium bifluoride and thiourea are added and stirred until completely dissolved. Finally, the volume is adjusted to 500 mL with deionized water to obtain the corrosion solution of this embodiment.

[0095] S3. Calculate corrosion duration:

[0096] The system characteristic constants are: A = 6.5s, B = 24.0s, C = 9.0s, E = 3.5s, D = -0.04℃. -1 The ambient temperature is T = 22℃. Substituting into the formula, the corrosion time is t = 29.09 × 0.923 ≈ 26.8s. For ease of operation, we take 27s.

[0097] S4. Corrosion Operation:

[0098] Using a dropper, draw up the etching solution and evenly apply it to the polished surface of the sample, covering the weld, fusion line, heat-affected zone, and base material area. Start timing simultaneously. During etching, gently shake the sample to maintain uniform contact between the etching solution and the sample surface. When the timer reaches 27 seconds, immediately rinse the sample surface with clean water to remove residual etching solution, then rinse with anhydrous ethanol to dehydrate, and finally dry with cold air.

[0099] Then, macroscopic metallographic observations were performed on the corroded samples, such as... Figure 1 As shown, the observation results indicate that the boundaries of the weld metal, fusion line, and heat-affected zone are clear and continuous, the weld layer formation is distinct, the fusion line is uninterrupted, the heat-affected zone has distinct microstructures, and there is no blackening of the microstructure caused by corrosion or blurring caused by under-corrosion. The base metal area can also be effectively distinguished from other areas.

[0100] 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.

[0101] Example 2:

[0102] This embodiment provides a corrosion solution and macroscopic metallographic corrosion method for low-alloy ultra-high strength steel. The difference from Embodiment 1 is that this embodiment uses Q960 low-alloy ultra-high strength steel welded joints for hydraulic supports as the material to be corroded. Since Q960 steel is not strictly ultra-high strength steel, its composition and microstructure may differ significantly from the standard Q1100 steel of Embodiment 1, requiring adjustment of system characteristic parameters.

[0103] In S1, the calculated values ​​are CE=0.913, ASI=0.128, and HDI=1.041. The ambient temperature is T=25℃.

[0104] In S2, based on the refitting of transitional series steels such as Q960 and Q890, the formula for the central value of nitric acid concentration in this embodiment is adjusted to (1.4 + 1.7 × CE_ultra-high)%. The total volume of the corrosion solution is 500 mL, wherein the volume percentage concentration of nitric acid is (1.4 + 1.7 × 0.913)% = 2.95%, which is taken as 3.0%; the volume percentage concentration of anhydrous ethanol is 45%; the mass-volume concentration of ammonium bifluoride is (1.2 + 1.6 × 0.128) g / L = 1.40 g / L, which is taken as 1.40 g / L; the mass-volume concentration of thiourea is (0.65 + 1.75 × 1.041) g / L = 2.47 g / L, which is taken as 2.47 g / L; and the volume percentage concentration of hydrochloric acid is (0.02 + 0.1 × 0.913)% = 0.111%, which is taken as 0.11%. The preparation operation is the same as in Example 1.

[0105] In S3, the corrosion time t = 26.94 × 0.819 ≈ 22.1s was calculated using the same system characteristic constant as in Example 1, and was taken as 22s.

[0106] Macroscopic metallographic observation was performed after 23 seconds of corrosion in S4. The results showed that the boundaries of the Q960 steel weld, fusion line and heat-affected zone were clear and continuous, the transition of the microstructure was natural and the layers were distinct, and there was no excessive corrosion in the weld and other areas. The base metal was also effectively exposed.

[0107] Example 3:

[0108] This embodiment provides a corrosion solution and macroscopic metallographic corrosion method for low-alloy ultra-high strength steel. The difference from Embodiment 1 is that the material to be corroded in this embodiment is the welded joint of HB500 grade low-alloy ultra-high strength wear-resistant steel used in coal mine scraper conveyors.

[0109] In S1, the sampling, grinding, and polishing operations for the samples were the same as in Example 1. Spectroscopic analysis revealed significant differences in the chemical composition of the HB500 steel compared to the Q1100 steel in Example 1, particularly in alloy composition. These differences were primarily reflected in a higher C content (approximately 0.26%), a significantly higher Cr content (approximately 1.20%), a lower Ni content (approximately 0.60%), and the absence of elements such as B and Zr. A small amount of W (approximately 0.30%) was also present to enhance wear resistance. The ambient temperature was T = 18℃.

[0110] In step S2, the total volume of the etching solution is 500 mL, and anhydrous ethanol is still used as the low-carbon alcohol, with a volume percentage concentration of 45%. Given the significant differences in composition between HB500 steel and Q1100 steel in Example 1, based on systematic metallographic corrosion experiments on this series of steel samples and refitting the data, the nitric acid concentration was adjusted from (1.5 + 1.6 × CE)% used in Example 1 to (1.6 + 1.4 × CE)%, i.e., the coefficients were adjusted from 1.5 and 1.6 to 1.6 and 1.4. After adjustment, the volume percentage concentration of nitric acid is 3.4%; the mass-volume concentration of sodium bifluoride is 1.31 g / L; the mass-volume concentration of thiourea is 3.00 g / L; and the volume percentage concentration of hydrochloric acid is 0.15%. The preparation procedure is the same as in Example 1, and finally, deionized water is used to bring the volume to 500 mL.

[0111] In S3, the system characteristic constants are adjusted to A=8.0s, B=20.0s, C=6.0s, and E=4.0s. The corrosion time is approximately 27s.

[0112] Steps S4 and S5 are performed in the same manner as in Example 1, with the etching time set to 27 seconds.

[0113] The observation results show that the weld zone and fusion line of HB500 steel have clear and continuous boundaries, the fusion line is continuous and uninterrupted, the transition between the sub-zones of the heat-affected zone is natural, and there is no over-corrosion or under-corrosion phenomenon, thus obtaining a high-quality macroscopic metallographic morphology.

[0114] Example 4:

[0115] This embodiment provides an etching solution and macroscopic metallographic etching method for low-alloy ultra-high-strength steel. The difference from Embodiment 1 is that the etching solution includes: nitric acid, sodium bifluoride, isopropyl thiourea, isopropanol, and water. In step S2, the total volume of the etching solution is 500 mL, the volume percentage concentration of nitric acid is 3.2%, the volume percentage concentration of isopropanol is 45%, the mass / volume concentration of sodium bifluoride is 1.43 g / L, the mass / volume concentration of isopropyl thiourea is 2.72 g / L, and the volume percentage concentration of hydrochloric acid is 0.13%. The preparation procedure is the same as in Embodiment 1, and finally, deionized water is used to bring the volume to 500 mL.

[0116] The remaining operations are the same as in Example 1. Observation results show that after replacing ammonium bifluoride with sodium bifluoride, thiourea with isopropyl thiourea, and anhydrous ethanol with isopropanol, the microstructure boundaries of the weld, fusion line, heat-affected zone, and base material are also clear and continuous, which is basically consistent with Example 1, indicating that the above-mentioned substitutes have equivalent technical effects in the corrosion liquid system of the present invention.

[0117] 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 method of the present invention can effectively achieve a corrosion effect with clear microstructure boundaries and continuous sharp grain boundaries.

[0118] Example 2 uses S1100QL as the object, which has a strength grade comparable to Q1100 but with slight differences in alloy composition. By using the dynamic formula 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.

[0119] Example 3 uses wear-resistant steel HB500 as the subject, although its composition system differs significantly from the steel in Example 1. Through systematic experiments on this series of steels, the formula coefficients and time constants were refitted, and high-quality metallographic effects were still obtained for this series, further demonstrating that the dynamically adjusted etching solution and etching time of the present invention have good adaptability.

[0120] 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.

[0121] In summary, the present invention combines the ratio of the etchant and the corrosion time with CE, 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 used for macroscopic metallographic inspection of ultra-high strength steel welded joints, such as difficulty in controlling the over-corrosion window, easy over-corrosion of the weld bead and heat-affected zone, heavy reliance on manual experience for corrosion time control, and poor consistency of test results.

[0122] 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 corrosive liquid for low-alloy ultra-high-strength steel, characterized in that, It consists of a solvent component and a solute component. The solvent component contains nitric acid, a low-carbon alcohol, and the balance water. The solute component contains an acidic fluoride salt and a thiourea or its C1-C3 alkyl-substituted derivative. The acidic fluoride salt, thiourea, or its C1-C3 alkyl-substituted derivative of the solute component can be completely dissolved in the solvent component to form a homogeneous and stable acidic corrosive liquid. The ratio of the corrosive liquid must meet the following conditions: Based on a predetermined volume of the corrosive solution, the volume percentage concentration of the nitric acid is (1.2 + 1.4 × CE)% to (1.5 + 1.8 × CE)%; the volume percentage concentration of the lower alcohol is 40% to 50%; the mass-volume concentration of the acidic fluoride is (1.0 + 1.4 × ASI) g / L to (1.4 + 1.8 × ASI) g / L; the mass-volume concentration of the thiourea or its C1-C3 alkyl-substituted derivative is in the range of (0.5 + 1.5 × HDI) g / L to (0.8 + 2.0 × HDI) g / L; and water is used to make up the predetermined volume. Wherein, carbon equivalent CE=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15, alloy strengthening index ASI=Nb+Ti+0.5×(V+Al), heat-affected zone deterioration index HDI=(C+Mn / 5+Cr / 3+Mo / 2+V / 2)×(1+Nb+Ti+B), in the above three formulas, the symbol of each element represents the mass percentage of the element in the material to be corroded, without units.

2. The corrosive liquid as described in claim 1, characterized in that, When the carbon equivalent CE of the material to be corroded is ≥0.65, the solvent component further includes hydrochloric acid; the volume percentage concentration of hydrochloric acid in the corrosive solution is (0.01+0.05×CE)% to (0.03+0.15×CE)%.

3. The corrosive liquid as described in claim 1, characterized in that, The lower alcohol is at least one of anhydrous ethanol, methanol, and isopropanol; the acidic fluoride is at least one of ammonium bifluoride and sodium bifluoride; the water is deionized water, distilled water, or ultrapure water; and the C1-C3 alkyl-substituted derivative of the thiourea is at least one of methylthiourea, ethylthiourea, and isopropylthiourea.

4. The corrosive liquid as described in claim 3, characterized in that, The low-carbon alcohol is anhydrous ethanol, and its volume percentage concentration in the corrosive solution is 43-47%; the solid component consists of thiourea and ammonium bifluoride.

5. A macroscopic metallographic corrosion method for low-alloy ultra-high strength steel, characterized in that, Includes the following steps: S1: Obtain the chemical composition of the material to be corroded, and calculate its carbon equivalent (CE), alloy strengthening index (ASI), and heat-affected zone degradation index (HDI). Wherein, CE=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15, 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 etching solution as described in any one of claims 1-4 based on the CE, ASI and HDI measured in S1; S3. Calculate the corrosion time t according to the following fitting formula: t=(A+B×CE+C×ASI-E×HDI)×exp[D×(T-20℃)]; Where A is the baseline corrosion time, in seconds; B is the carbon equivalent influence coefficient, with units of s; C is the influence coefficient of the alloy strengthening index, with the unit being s; D is the temperature correction factor, in °C. -1 ; E is the degradation index influence coefficient, with units of seconds; T represents the ambient temperature; A, B, C, D, and E are constants obtained by calibration in the following way: multiple groups of low-alloy ultra-high strength steel samples with different CE, ASI, and HDI were selected, and orthogonal corrosion tests were carried out using quantitative corrosion solution prepared with S2 under multi-gradient ambient temperatures. The corresponding system characteristic constants were determined by fitting and regressing the experimental data, with clear metallographic morphology, obvious microstructure boundaries, no under-corrosion, and no over-corrosion as the standard. S4: Apply the corrosive liquid to the surface of the material to be corroded and continue corroding for a duration of t to complete the corrosion.

6. The macroscopic metallographic etching method as described in claim 5, characterized in that, Before S1, the material to be corroded is ground and polished to make the surface roughness Ra≤1.6μm; in S2, when preparing the corrosion solution, if CE≥0.65, hydrochloric acid with a volume percentage concentration of (0.01+0.05×CE)% to (0.03+0.15×CE)% is added, and the amount of water is reduced by the same amount.

7. The macroscopic metallographic etching method as described in claim 5 or 6, characterized in that, In S2, the prepared corrosive solution contains nitric acid at a volume percentage concentration of (1.5 + 1.6 × CE)%, acidic fluoride at a mass-volume concentration of (1.2 + 1.6 × ASI) g / L, and thiourea or its C1-C3 alkyl-substituted derivatives at a mass-volume concentration of (0.65 + 1.75 × HDI) g / L; in S3, A = 4.0~9.0 s, B = 18.0~30.0 s, C = 6.0~12.0 s, E = 2.0~5.0 s, and D = -0.06~-0.02℃. -1 .

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 the weld seam and the heat-affected zone; when CE≥0.65, the corrosion solution also includes hydrochloric acid, and the volume percentage concentration of the hydrochloric acid is (0.02+0.1×CE)%.

9. The macroscopic metallographic etching method as described in claim 8, characterized in that, In S3, the material to be corroded is steel with a yield strength of not less than 1000 MPa, and A = 6.5 s, B = 24.0 s, C = 9.0 s, E = 3.5 s, 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 in S2 for standard samples with known CE, ASI, and HDI, and then corrosion calibration is performed. Based on the calibration results, it is determined whether the system characteristic constants in S3 need to be corrected.

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