Method for determining the cause of cracking after quenching of high-strength steel forgings and use thereof

Abstract

The application discloses a method for determining the cause of cracking of high-strength steel forgings after quenching and application, and the method comprises the following steps: observing the crack direction and the fracture morphology, streamlines and microstructure after dissection of the forgings, and simulating the distribution of thermal stress in the quenching process of the forgings by means of ANSYS WORKBENCH software, and finally determining the cause of cracking of the forgings. The method of the patent provides a basis for optimizing the heat processing process parameters of the forgings, and provides a reference for solving the cracking problem of similar forgings.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of metal hot plastic forming and heat treatment, and in particular to a method for determining the cause of cracking of high-strength steel forgings after quenching and application thereof. BACKGROUND

[0002] After forging and quenching of some automobile high-strength steel forgings, some parts cracked, which not only affected the performance of the parts, but also increased the production cost. It is urgent to trace back the cracking cause in the production process and develop a solution. The inventors found through consulting domestic and foreign literatures that scholars have carried out a lot of researches on the cracking cause of forgings.

[0003] Xie Yunqian et al. [1] proposed that microcracks in the raw materials of forgings would induce macroscopic cracks of 42CrMo shaft forgings under the action of phase transformation stress and thermal stress; the research results of Liu Zhenwei et al. [2] showed that uneven stress and strain distribution and abnormal grain growth would also lead to cracking of forgings.

[0004] Takefumi Arikawa et al. [3] proposed that local microstructure defects and microstructure segregation would induce cracks. Similarly, Zhang Quanxin et al. [4] also proposed that low-melting-point copper-rich phases and segregation distributed along the grain would cause cracking of 4Cr5MoSiV1 steel forgings.

[0005] A.A. Kuznetsov et al. [5] studied the causes of quenching cracking of steel and cast iron parts, and proposed that surface heat treatment and chemical treatment were beneficial to reducing the tensile stress on the surface of the parts, thereby reducing cracking.

[0006] The research results of Sanja Šolić et al. [6] showed that the higher the austenitizing temperature, the more likely the quenching cracks would occur.

[0007] Eun Hye Hwang et al. [7] studied the influencing factors of quenching crack sensitivity of super-strength martensitic steel, and the results showed that the sample with higher carbon content was more likely to have intergranular cracks, and reducing the quenching cooling rate could reduce cracking.

[0008] Wang Gaoyuan et al. [8] studied the quenching cracking phenomenon of workpieces based on computer simulation, and proposed that increasing the transition zone allowance of forgings and reducing the quenching temperature could reduce the risk of cracking.

[0009] Nikolai Kobasko et al. [9] studied the quenching cooling process of steel in polyalkylene glycol aqueous solution, and found that step quenching could prevent the formation of cracks and reduce quenching deformation.

[0010] Vasundhara Singh et al.

[10] studied the influence of non-metallic inclusions on the cracking of forged steel, and found that alumina inclusions were more likely to cause cracks than MnS inclusions.

[0011] Zhang Kun et al.

[11] studied the reasons for quenching cracking of 45 steel at different temperatures, and the results showed that the reason for quenching cracking of 45 steel at 800-830 ℃ is that the cooling rate of supercooled austenite is too large, and it is proposed to avoid cracking by slow cooling for 3-5 s before quenching. Due to space limitations, similar research at home and abroad will not be repeated here.

[0012] Summarizing the existing technology at home and abroad, it can be found that many factors during forging and heat treatment of forgings can lead to cracking. The composition of raw materials, microstructure of raw materials, shape of forgings, forging process parameters, heat treatment medium, quenching temperature, etc. may induce cracks. Especially, only a small part of forgings in production has cracking problem, and production management and raw material consistency may eventually cause the forging to crack. Because there are too many possibilities for the causes of cracking, it is difficult to accurately determine which step is unreasonable after the forging cracks, making it difficult to solve the problem of forging cracking. Therefore, it is urgent to determine the main factors leading to cracking in order to solve the cracking problem.

[0013] Search literature:

[0014] [1] Xie Yunqian, He Xiyang, Kong Mengze, Wang Meng, Kong Shuping. Analysis and prevention of heat treatment cracking of shaft workpiece [J]. Metalworking (hot working), 2022, 854(11): 82-84

[0015] [2] Liu Zhenwei et al. Microstructure evolution and process optimization of molybdenum rods during loose tooling forging [J]. International Journal of Refractory Metals and Hard Materials, 2022, 108

[0016] [3] Takefumi Arikawa, Daisuke Yamabe, Hideki Kakimoto. Influence of Anvil Shape of Surface Crack Generation in Large Hot Forging Process [J]. Procedia Engineering, 2014, 81 : 480-485

[0017] [4] Zhang Quanxin, Jiang Haijun, Wang Chunyi. Cracking Reason of 4Cr5MoSiV1 Steel Die Forging Module[J]. Physical Edition of Physics and Chemistry Examination, 2020, 56(07): 66-68+72

[0018] [5] A. A. Kuznetsov and V.I. Rudnev. Causes of Cracking in Quenching of the Parts Made of Steels and Cast Iron and Recommendations for Their Removal: A Review[J]. Russian Metallurgy (Metally), 2017, 13: 1125-1130

[0019] [6] Sanja Šolić, Bojan Podgornik, Vojteh Leskovšek. The occurrence of quenching cracks in high-carbon tool steel depending on the austenitizing temperature[J]. Engineering Failure Analysis, 2018, 92: 140-148

[0020] [7] Eun Hye-Hwang, Jin Sung-Park, Si On-Kim, et al. Study on the controlling factors for the quenching crack sensitivity of ultra-strong automotive steel[J]. Journal of Materials Science, 2020, 55(636)

[0021] [8] Wang Gaoyuan, Wang Kai, Zhang Zhimei. Analysis of Quenching Cracks and Optimization of Heat Treatment Process for 2Cr13 Steel Special-shaped Die Forgings[J]. Metal Heat Treatment, 2022, 47(11): 45-53

[0022] [9] Nikolai Kobasko. Cooling process optimization during hardening steel in water polyalkylene glycol solutions[J]. Technology audit and production reserves, 2021, 6(1) : 27-35

[0023]

[10] Vasundhara Singh, Rashulkhan, Bharath Bandi, et al. Effect of non-metallic inclusions (NMI) on crack formation in forged steel[J]. Materials Today: Proceedings, 2020 (prepublish)

[0024]

[11] Zhang Kun, Luo Wen-feng, Guo Xiao-tong, et al. Effect of Quenching Temperature on Quenching Cracking of 45 Steel[J]. Materials for Mechanical Engineering, 2022, 46(03):63-67. SUMMARY

[0025] In view of the above problems, the present patent proposes a complete set of methods for determining the cause of cracking of high-strength steel forgings after quenching. These methods can determine the main cause of forging cracking. The present method provides a basis for optimizing the hot working process parameters of the forgings and provides a reference for solving similar forging cracking problems.

[0026] To this end, the technical scheme of the present application is a method for determining the cause of cracking of high-strength steel forgings after quenching and its application, and the specific steps are as follows:

[0027] S1. Dissect the forging along the direction of the vertical section, observe the crack depth and direction, and obtain the macroscopic distribution information of the forging;

[0028] S2. A sample with a thickness of 20 mm is taken from the sectioning surface of the forging, and is ground using 400-mesh, 800-mesh, 1200-mesh and 2000-mesh metallographic sandpaper and polished on a cloth using diamond polishing paste; the sample is immersed in etchant one at about 70 DEG C, and after etching for about 3 minutes, the sample is washed with water and dried, and the metal flow lines are observed on the surface of the sample by the naked eye, so as to determine whether the forging cracks due to unreasonable forging process;

[0029] S3. A sample is taken in a direction perpendicular to the fracture surface near the fracture surface, and after being embedded, ground using 400-mesh, 800-mesh, 1200-mesh and 2000-mesh metallographic sandpaper and polished on a cloth using diamond polishing paste, the sample is dried, and etchant two is prepared; the sample surface is wiped with cotton dipped in the etchant at a temperature of 40 DEG C, and the change in the sample surface is observed, and after etching for about 5 minutes, the sample is washed with water and dried, and the grain morphology is observed under a metallographic microscope, so as to determine whether the forging cracks due to increased brittleness caused by coarse grains;

[0030] S4. A sample is taken in a direction perpendicular to the fracture surface near the fracture surface, and after being embedded, ground using a standard process, and polished, the sample is etched in etchant three, and after etching for about 20 seconds, the microstructure morphology at room temperature is observed under a metallographic microscope and photographed, so as to determine whether the forging cracks due to insufficient plasticity of ferrite formed after surface decarburization;

[0031] S5. A sample is taken along the fracture surface, and after being cleaned with alcohol, the sample is directly observed under a field emission scanning electron microscope, the test voltage is 15 keV, the chemical element distribution is tested using an energy spectrometer, the test voltage is 0-20 keV, and the fracture surface is observed after being photographed, and according to the element content, it is determined whether the oxide scale at the fracture surface of the forging is magnetite or magnetite, so as to determine whether the forging cracks at high temperature or at room temperature;

[0032] S6. The element content of the forging material is input into Jmatpro software, and the expansion rate, thermal conductivity, Young's modulus, Poisson's ratio and specific heat of the steel material at different temperatures can be obtained; the material parameters are input into a finite element software for coupling analysis of the temperature field and the structure field of the forging quenching process, and the temperature distribution and stress distribution during quenching and cooling are observed;

[0033] S7. If the streamline is folded at the cracking position or the material flow direction at the cracking position is observed to be very unreasonable, it can be determined that the main reason for the cracking is that the forging process is unreasonable, and the forging process and the die design scheme need to be improved; if the streamline is normal, the grain at the fracture is fine, and no obvious decarburized ferrite is found, it can be determined that the forging is cracked due to too fast cooling speed and too large local stress, and the cooling speed needs to be slowed down; if there is no obvious oxide skin at the fracture, it can also be determined that the forging is cracked due to too fast quenching cooling speed. If the cracking position is consistent with the maximum stress obtained by finite element simulation, it can also be determined that the forging is cracked due to too fast quenching cooling speed; however, if the cracking position is obviously not at the maximum stress obtained by simulation, it can be determined that the cracking of the forging is not caused by quenching, but by unreasonable forging process.

[0034] As preferred, the corrosion agent one in S2 comprises: 15 mL of hydrochloric acid, 15 mL of water, and 3 g of copper sulfate; the corrosion agent with the above ratio can better corrode the streamline of the steel.

[0035] As preferred, the corrosion agent two in S3 comprises: 20 mL of a saturated aqueous solution of picric acid, 8 mL of carbon tetrachloride, one drop of hydrochloric acid, and 2 mL of dishwashing liquid; the corrosion agent with the above ratio can better corrode the grain morphology of the steel.

[0036] As preferred, the corrosion agent three in S4 is a 4% nitric acid alcohol solution by volume; the corrosion agent with the above ratio can better corrode the martensite, bainite and pearlite structure of the steel.

[0037] Beneficial effects: the present application has the following beneficial effects: 1. various analysis and calculation means are reasonably applied to accurately determine the reason for the cracking of the forging after quenching; 2. according to the test and simulation results, the hot working process of the forging can be more effectively optimized, thereby solving the problem of the forging. BRIEF DESCRIPTION OF DRAWINGS

[0038] Fig. 1 is a fracture depth and streamline diagram in the embodiment of the present application.

[0039] Figure 2 Fig. 2 is a fracture microstructure diagram in the embodiment of the present application, wherein: (a) is a grain morphology near the fracture A; (b) is a room temperature structure near the fracture A; (c) is a grain morphology near the fracture B; (d) is a room temperature structure near the fracture B.

[0040] Figure 3 Fig. 3 is a fracture A morphology diagram near the surface in the embodiment of the present application, wherein: (a) is a fracture A photo; (b) is an electronic image of the fracture A.

[0041] Figure 4are the element distribution maps of the fracture A near surface in the specific embodiment of the present application, wherein: (a) is an electron image; (b) is a total spectrum map of the distribution map; (c) is C; (d) is Cr; (e) is Fe; (f) is O; (g) is Si; (h) is Ni.

[0042] Figure 5 are the SAE5137H material parameter curve diagrams obtained by Jmatpro calculation in the specific embodiment of the present application, wherein: (a) is the expansion rate; (b) is the thermal conductivity; (c) is the Young's modulus; (d) is the Poisson's ratio; (e) is the specific heat.

[0043] Figure 6 are the equivalent stress distributions at room temperature at the cracking positions in the specific embodiment of the present application, wherein: (a) is the equivalent stress distribution of the section A; (b) is the equivalent stress distribution of the section B. DETAILED DESCRIPTION

[0044] The technical solutions of the present application will be described clearly and completely in combination with the drawings, but the embodiments should not be understood as limitations to the present application.

[0045] The present application is as shown in Figures 1 to 6 :

[0046] The specific application of the present application will be described in combination with the example of determining the cracking reason of a certain high-strength steel forging after quenching, and the specific implementation steps are as follows:

[0047] S1. Cracks are found at positions A and B of the forging, the forging is dissected along the direction perpendicular to the section, the crack depth and direction are observed, and the macroscopic distribution information of the forging is obtained;

[0048] S2. A sample with a thickness of 20 mm is taken from the section of the forging, is ground using 400 mesh, 800 mesh, 1200 mesh and 2000 mesh metallographic sandpaper and is polished on a cloth using diamond polishing paste, and the sample is immersed in an etchant (containing 15 mL of hydrochloric acid, 15 mL of water and 3 g of copper sulfate) at about 70 DEG C for etching. After etching for about 3 minutes, the sample is washed with water and dried, and metal flow lines are observed on the surface of the sample by naked eye, as shown in Figure 1 : In Figure 1 , slip bands caused by uneven material flow are observed near the two fractures, especially the cracking direction at position B is along the direction of the slip band, which proves that during the forging process, the material at the center of the forging produces severe shearing flow, thereby generating high stress near the slip band, reducing the strength of the local forging and possibly leading to quenching cracking, and in Figure 1 , a bright white area different from the core is observed near the surface of position A, and both fractures are near the parting surface, which may be the result of the combined action of material folding and oxidative decarburization during the forging process;

[0049] S3. Sampling in the direction perpendicular to the fracture near the fracture, after inlaying, grinding with 400 mesh, 800 mesh, 1200 mesh, 2000 mesh metallographic sandpaper and polishing with diamond polishing paste on a cloth, after blowing dry, preparing the etchant (containing 20 mL saturated aqueous solution of picric acid, 8 mL carbon tetrachloride, one drop of hydrochloric acid, 2 mL detergent), at a temperature of 40 ℃, using cotton to dip the etchant to wipe the sample surface and observe the changes of the sample surface, after about 5 minutes of etching, rinsing with water, blowing dry, observing the grain morphology under a metallographic microscope, as shown in Figure 2 , from Figure 2 a and Figure 2 c can be seen that the grain size near the fracture A and fracture B is relatively uniform, the average grain size is about 14 μm, without grain coarsening or mixed grain defects; from Figure 2 b and Figure 2 d can be seen that the structure near the fracture is fine martensite structure without obvious ferrite decarburization layer, so the forging has no risk of overburning in quenching; from Figure 2 d can be seen that the forging surface has an average thickness of about 5 μm of oxide skin, and the cross section is observed to be severely oxidized with red-brown color, so it can be judged that the crack is formed at high temperature and oxidized, the oxidation of the outer skin is more serious, and the oxidation of the central part is weaker, proving that the crack is formed on the outer surface and expands to the center;

[0050] S4. Sampling in the direction perpendicular to the fracture near the fracture, after inlaying, grinding and polishing according to the standard process, etching in 4% nitric acid alcohol solution, after about 20 seconds of etching, observing the microstructure morphology at room temperature under a metallographic microscope and taking a photo, as shown in Figure 3 and Figure 4 , from Figure 3 can be seen that there is a red-brown oxide skin on the outside of the fracture A, and the fracture is smooth, it is preliminarily determined that the fracture is formed at high temperature;

[0051] S5. Sampling along the fracture, the sample is directly observed under a field emission scanning electron microscope after alcohol cleaning, the test voltage is 15 keV, the chemical element distribution is tested using an energy spectrometer, the test voltage is 0-20 keV, a photo is taken for observing the fracture morphology, according to the element content, the oxide skin at the fracture of the forging is ferric oxide or magnetite, as shown in Figure 4 , from Figure 4It can be seen that the element distribution is uneven along the fracture depth direction, with the oxygen content being higher near the surface of the forging than inside. In the tested area, the mass ratio of oxygen to iron (47.6%) is very close to that of ferric oxide (42.8%), proving that ferric oxide was formed on the outer surface of fracture A of the forging. Comparative analysis of elements at different locations on the fracture surface reveals that, except for oxygen and carbon, the contents of other elements are not significantly different, proving that there is no obvious segregation of alloying elements at the fracture crack.

[0052] S6. Inputting the elemental composition of the forging material into Jmatpro software will yield parameters such as the expansion coefficient, thermal conductivity, Young's modulus, Poisson's ratio, and specific heat of the steel at different temperatures (e.g., ...). Figure 5 As shown); input these material parameters into finite element software (such as ANSYS) to perform coupled analysis of the temperature field and structural field during the quenching process of the forging, and observe the temperature distribution and stress distribution during the quenching and cooling process, such as... Figure 6 As shown; from Figure 6 It can be seen that the maximum stresses both occurred on the surface of the forging, at 285 MPa and 367 MPa respectively. Based on the stress distribution results of various regions of the forging, it can be seen that the maximum stresses did not occur at positions A and B during the entire cooling process, and the stresses at positions A and B did not reach the yield strength of the forging. This indicates that the main cause of cracking at these two locations was not quenching stress.

[0053] S7. Determining the cause of cracking: Magnetic particle testing of a high-strength steel forging for automobiles before quenching revealed no cracks. However, cracks were found at locations A and B on the forging after quenching. The cracks extended from the surface towards the center of the forging, and a thickness of approximately 5 mm was observed on the crack surface. The reddish-brown oxide scale at μm was observed. Scanning electron microscopy (SEM) results showed that the fracture surface was oxidized to form ferric oxide, indicating that the crack appeared during the high-temperature quenching process. The grain size was uniform and fine, indicating no risk of overheating during quenching, and no obvious alloy element segregation was observed at the fracture surface. Folding marks were observed at location A near the surface, while the crack at location B followed the direction of the slip band. Finite element simulation of the quenching process revealed that the maximum stress at the two quenching crack locations was far below the tensile strength of the material, indicating that the main cause of the crack was not quenching stress. This further validated the fracture analysis results. Therefore, the uneven material flow during forging and the resulting severe slippage reduced the material's plasticity. Although no direct cracking occurred after forging, insufficient plasticity under thermal stress during quenching led to cracking.

[0054] Further analysis of the uneven flow of the forging material found that when the forging at position B was bent and deformed, the compression deformation on the inner side of the bend angle and the tensile deformation on the outer side of the bend angle were both insufficient due to the lower temperature of the skin and the higher temperature of the core, the deformation was concentrated in the core, a slip band was formed in the core, and the strength of the forging was reduced; the cause of the defect at position A was that the metal was folded when extruded in the closed cavity; the study determined that the main cause of cracking of the certain automobile high-strength steel forging was from the forging process, and after making corresponding adjustments to the forging die, the proportion of quenching cracking of the forging was significantly reduced, and the cracking problem was solved.

[0055] The details not described in the specification are known in the art.

[0056] Through the description of the above processing method, those skilled in the art should understand that the present application is not limited to the specific embodiments described above, and improvements and substitutions of known techniques in the art based on the present application all fall within the protection scope of the present application, which should be defined by the claims.

Claims

1. A method for determining the cause of cracking of a high-strength steel forging after quenching, comprising the following steps: S1. dissecting the forging along the direction of the vertical section, observing the crack depth and direction, and obtaining the macroscopic distribution information of the forging; S2. taking a sample with a thickness of 20 mm from the sectioned surface of the forging, grinding using 400 mesh, 800 mesh, 1200 mesh, and 2000 mesh metallographic sandpaper and polishing on a cloth using diamond polishing paste; immersing the sample in etchant one at about 70°C, etching for about 3 minutes, rinsing with water and blowing dry, and observing the metal flow lines on the surface of the sample by the naked eye to determine whether the forging cracks due to unreasonable forging process; S3. taking a sample along the direction perpendicular to the fracture near the fracture, embedding, grinding using 400 mesh, 800 mesh, 1200 mesh, and 2000 mesh metallographic sandpaper and polishing on a cloth using diamond polishing paste, and blowing dry, and preparing etchant two; wiping the surface of the sample with cotton dipped in etchant two at a temperature of 40°C and observing the changes on the surface of the sample, etching for about 5 minutes, rinsing with water and blowing dry, and observing the grain morphology under a metallographic microscope to determine whether the forging cracks due to increased brittleness caused by coarse grains; S4. taking a sample along the direction perpendicular to the fracture near the fracture, embedding the sample, grinding according to the standard process, and polishing, and etching in etchant three, observing the microstructure morphology at room temperature under a metallographic microscope and taking a photo after etching for about 20 seconds to determine whether the forging cracks due to insufficient plasticity of ferrite formed after surface decarburization; S5. taking a sample along the fracture, cleaning the sample with alcohol, and directly observing under a field emission scanning electron microscope, testing the chemical element distribution using an energy spectrometer at a test voltage of 0-20 keV, and taking a photo to observe the fracture morphology, calculating the scale at the fracture of the forging as being either ferroferric oxide or ferriferrous oxide according to the element content, and determining whether the forging cracks at high temperature or at room temperature; S6. inputting the element content of the forging material into Jmatpro software to obtain the expansion rate, thermal conductivity, Young's modulus, Poisson's ratio, and specific heat of the material at different temperatures; inputting the material parameters into a finite element software to perform coupling analysis of the temperature field and structure field during the quenching process of the forging, and observing the temperature distribution and stress distribution during the quenching and cooling process; S7. if the flow lines are folded or a very unreasonable material flow direction is observed at the cracking position, it can be determined that the main cause of cracking is unreasonable forging process, and the forging process and die design scheme need to be improved; if the flow lines are normal, the grains at the fracture are fine, and no obvious decarburized ferrite is found, it can be determined that the forging cracks due to excessive cooling speed and excessive local stress, and the cooling speed needs to be slowed down; if there is no obvious scale at the fracture, it can also be determined that the forging cracks due to excessive quenching and cooling speed; if the cracking position is consistent with the maximum stress obtained by finite element simulation, it can also be determined that the forging cracks due to excessive quenching and cooling speed; however, if the cracking position is obviously not at the maximum stress obtained by simulation, it can be determined that the cracking of the forging is not caused by quenching, but by unreasonable forging process. ​ ​ ​ ​ ​ ​ ​ S2 in the corrosion agent by hydrochloric acid 15 mL, 15 mL of water, copper sulfate 3 g prepared; S3 in the second corrosion agent by 20 mL of picric acid saturated aqueous solution, 8 mL of carbon tetrachloride, a drop of hydrochloric acid, 2 mL of detergent prepared; S4 in the third corrosion agent is a volume percentage of 4% nitric acid alcohol solution.

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