Abstract: When a low-core etching inspection was conducted on the crankshaft of a certain automobile engine using 40CrNiMoA steel, it was found that there were suspected cracks in the heart. The causes of defects were analyzed by low-magnitude etching comparison, chemical composition analysis, metallographic inspection, , magnetic powder detection and other methods. The results show that the core part of the material is not dense and the abnormal bainite tissue in the core part leads to its impervious corrosion resistance, and corrosion pits are generated during the acid etching process. These corrosion pits are connected together to form "false crack" defects. After optimizing the process parameter of the continuous casting and steel rolling process, its density increased, the abnormal bainite tissue disappeared, and there was no obvious corrosion pit after 30 minutes of etching of the heart.
Keywords: 40CrNiMoA steel; low-power defects; center segregation; center looseness; bainite; acid etching
Chinese picture Classification number: TG115.2 Document code: B Article number: 1001-4012 (2022)07-0056-06
40CrNiMoA steel belongs to alloy structural steel , and is often used to make tempered parts with high strength, high toughness, and large cross-sectional size, such as the transmission eccentric shaft of horizontal forging machine, forging press crankshaft, commercial vehicle engine curve shafts, blades, fasteners, gears, etc. In addition, after nitriding it, it can also be made into parts with special performance requirements. However, due to the high alloy content of this steel type and strong supercooled austenite stability, abnormal structures such as bainite and martensite are prone to appear during the rolling and cooling process. At the same time, due to its high alloy content and large material specifications, if the smelting and rolling process is unreasonable, round steel is prone to low-speed defects such as center segregation and looseness [1], which reduces the density of the material and ultimately affects the use of parts.
A certain automobile engine crankshaft parts manufacturing factory used 40CrNiMoA steel to make commercial vehicle crankshafts, and then cut the raw materials and conducted low-efficiency etching and sampling inspection in the factory. It was found that there were "cracks" defects visible to the naked eye in the center of the round steel material. The process of manufacturing and processing of the crankshaft parts is: 130mm (diameter) raw materials → sawing and cutting → raw materials entering the factory for sampling inspection → forging → tempering → machining → crankshaft magnetic powder detection, physical and chemical inspection → qualified products packaging and packaging into the warehouse. In order to analyze the causes of the "crack" defects in the heart after low-ultrasound etching of raw materials, the author studied and analyzed the defects of the materials through a series of physical and chemical inspection methods, and proposed corresponding improvement measures to avoid the recurrence of such defects.
1 Physical and chemical test
1.1 Low-magnitude test
conducted low-magnitude test on the material and found that there was a "crack" defect visible to the eye in the heart of the material, and it was suspected that there was overcorrosion. The low-magnification corrosion conditions are: an aqueous hydrochloric acid solution with a volume ratio of 1:1 is used as the corrosion solution, the heating temperature is 80 ℃, and the erosion time is 60 min. The macromorphology of material defects is shown in Figure 1, and the low-magnification defect morphology observed under a microscope is shown in Figure 2. From Figures 1 and 2, we can see that the suspected "crack" defect area observed by the naked eye is formed by multiple dot-shaped corrosion pits continuously distributed in a string, and is not a real crack defect [2].
1.2 Chemical composition analysis
was taken on low-magnitude samples, and the samples were analyzed chemically using direct reading spectrometer . The results are shown in Table 1. From Table 1, we can see that the chemical composition of the sample meets the requirements of 40CrNiMoA steel in GB/T3077-2015 " alloy structure steel ", the content of harmful segregation residual elements such as phosphorus and sulfur is less than the standard requirements, and there is no significant segregation of the core component, so such defects caused by improper chemical composition control or segregation can be ruled out.
1.3 Low-magnitude etch
Comparison In order to further understand and analyze defects, a comparison and analysis was conducted based on the low-magnitude detection defect samples. After remilling the sample with milling machine (milling amount 2~3mm), three low-magnitude etching tests were performed. The first acid etching time was 10min, the second acid etching time was 20min (total acid etching time 30min), and the third acid etching time was 20min (total acid etching time 50min). During this period, the sample was not polished again, and it was a continuous acid etching test.The defect morphology after three acid etchings As shown in Figures 3 to 5, a low-fold defect of suspected cracks was observed under a body microscope, which was actually a series of corrosion pits with multiple dot-shaped corrosion pits. As the acid etching time was extended, the suspected crack defects visible to the naked eye became more and more obvious, and the corrosion pits observed under the microscope became more and more serious. The results of low-multiple defect ratings under different acid etching conditions are shown in Table 2. It is found that as the acid etching time extends, the degree of corrosion increases, the segregation morphology will approach looseness, and the dense segregation points will be connected to segregation lines, and suspected cracks will appear, so the central segregation level has also been improved.
1.4 Metallographic inspection
After grinding the sample after the third acid etching longitudinally along the low-magnitude corrosion pit, it was found that the deepest corrosion pit was about 0.34mm, and no abnormal metallurgical defects were found in the corrosion pit (see Figure 6). After erosion of 4% (volume fraction) alcohol solution of nitrate, metallographic inspection was carried out. It was found that the bottom of the corrosion pit was mostly distributed along bainite (see Figure 7). Bainite actually consists of coarse ferrite grains and carbide particles. The interface and structure are complex. The internal stress of lattice increases, resulting in the energy of the entire system inside it, the corrosion resistance of the material becomes worse, and the corrosion resistance is uneven [3]. The corrosion pit was then grinded three times in transverse direction and observed, and found that there were fewer and fewer corrosion pits on the corrosion surface until they disappeared, and there was no obvious decarbonization at the edge of the corrosion pit (see Figure 8). The metallographic test results of
show that as the grinding progresses, the corrosion pit is gradually polished away, and the material itself does not have defects. If it is defects such as cracks and shrinkage holes in the core of the material itself, these defects are difficult to reduce or disappear. In addition, this inspection also eliminates the overheating of the material and the presence of metallurgical defects such as slag inclusions and inclusions in the steel.
1.5 Magnetic powder detection
After grinding the low-magnetic sample, magnetic powder detection was performed, the results show that there is no obvious defect in the cross-section of the sample, indicating that there is no hole defect before the low-magnetic acid etching (see Figure 9). Therefore, it can be ruled out that the material has suspected crack defects before acid etching.
2 Comprehensive analysis
By analyzing the above physical and chemical test results, it can be considered that acid etch defects are caused by the presence of bainite abnormal tissue in the center of the material, low-magnitude segregation, looseness, etc., which are not resistant to corrosion during acid etching, and are not real crack defects. Although the existence of crack defects in the core of the material was ruled out [4], it was confirmed that the core tissue was not dense enough and there was an abnormal tissue of bainite . These defects also affect the use performance of the material.
After research and analysis and review of relevant literature [5], it is believed that the main reasons for the production of bainite in the core of alloy steel are: ① The center segregation of the casting billet produced by the continuous casting process, and the segregation elements cannot diffuse during the rolling and heating process, so that the content of carbon and alloy elements in the core of the rolling material will be too high, resulting in overcooling of the core of the material. The stability of the austenite is improved, and it is easy to form abnormal martensite and bainite during the rolling and cooling process; ② After the final rolling, the core of the material is often slow to dissipate heat, which can easily cause the temperature of the core of the material to be too high, and the cooling rate of the core will be accelerated during the subsequent cooling process. When the cooling rate exceeds the lower critical cooling rate, the supercooled austenite structure will not occur during the cooling process. 2 pearlite during the cooling process. Transformation, the constant temperature of the transition temperature zone changes to bainite tissue. If the cooling rate further accelerates, the supercooled austenite tissue will undergo a phase change below Ms (the starting temperature of martensite transformation), resulting in more harmful martensite tissue .
3 Process optimization and improvement measures
After analysis, although the low-speed defect of the material can be eliminated as the core crack defect, if the central defect cannot be eliminated during the subsequent forging and processing, it will also affect the final performance of the part. In order to improve this defect, it is necessary to improve the density of the core part of the material, reduce the center looseness, and at the same time, it is necessary to reduce the center segregation and prevent the non-equilibrium structure of the core part. This requires optimization of continuous casting process parameters and rolling process process parameters.
3.1 Continuous casting process parameter optimization
The cross-sectional dimension of the casting blank produced by this material is 300 mm × 325 mm (length × width). In order to improve the looseness of the core part of the material and the center deviation analysis, the relevant parameters in the continuous casting process need to be optimized, including the first and end electromagnetic stirring parameters, continuous casting pull speed, and the second-cold water quantity. After simulation calculation and multiple field tests, the optimal continuous casting related parameters [6-7] are obtained. The comparison results before and after optimization of continuous casting process parameters are shown in Table 3.
3.2 Optimization of rolling process parameters
In order to make the material's structure denser after rolling, reduce abnormal tissues such as bainite in the center, and reduce the core defects caused by casting billets, the rolling process parameters need to be optimized as follows: increase the heating temperature and maintain a certain high-temperature diffusion and heating time, so that carbon, phosphorus, sulfur and other easy-to-segregate elements and other alloy elements can be fully diffused, and reduce the core segregation defects of the material; at the same time, appropriately increase the high-pressure descaling pressure to create cold and hot conditions outside the material during the rolling process, so that the rolling force can penetrate into the center, achieving the purpose of reducing the looseness of the core part of the material; in addition, it is also necessary to lower the final rolling temperature of the round steel to prevent the core part from exceeding the core part during the final rolling process In the subsequent cooling process, bainite or even martensite abnormal structure is prone to appear in the center [8]. The comparison results before and after optimization of rolling process parameters are shown in Table 4. After rolling, the round steel is quickly collected on the cold bed and the pit is cooled slowly. The temperature of the pit is required to be greater than 500℃, and the temperature of the pit is cooled for 48 hours is required to be discharged. The temperature of the pit is required to not exceed 200℃. The slow cooling process also prevents abnormal structures such as bainite and martensite during the cooling process after rolling [9].
3.3 Improvement effect verification
adopts the improved continuous casting process and rolling process to reorganize the continuous casting and rolling production of 40CrNiMoA steel. The process is improved and the low-magnitude etching morphology of round steel before and after is shown in Figure 10 and Table 5. From the comparison results, we can see that after improvement, the low-power structure of the round steel is dense, and there are no obvious "crack" defects that can be seen by the naked eye, and the center is loose and segregation situation has improved significantly.
The acid-etched low-magnitude sample of the improved material (corrosion time is 30 minutes). The core tissue was observed under a microscope and compared with the improvement. The results are shown in Figure 11; the samples were taken from the low-magnitude sample after 30 minutes of acid etching and longitudinal grinding were performed to analyze the longitudinal micromorphology of the front and back core. The results are shown in Figure 12. From Figures 11 and 12, we can see that the improved core sample was magnified 10 times under a microscope, and there was no obvious corrosion pit. After slightly grinding and polishing, the sample was still magnified and observed, and no corrosion pits with crack morphology were found. After improvement, there was no bainite abnormal tissue in the longitudinal tissue of the core, but pearlite + ferrite tissue, and the tissue was improved, so it can be considered that the improvement effect was obvious.
4 Conclusion
(1) The chemical composition of the low-fold sample taken by the user on site meets the standard requirements of GB/T3077-2015 for 40CrNiMoA steel, indicating that the low-fold defect is not caused by the composition.
(2) Because the bainite strip structure and loose core inside the material are not resistant to corrosion, corrosion pits (or holes) are easily formed after acid etching. As the corrosion time extends, the diameter of the corrosion pits becomes larger and larger, and finally they are connected in a string, showing a form similar to "cracks" (false cracks), which are not crack defects inside steel in the true sense.
(3) The abnormal structures such as bainite in the center of the material are caused by segregation of the core of the material and the cooling rate of the core of the material is too fast during the rolling process.
(4) reasonably optimized the continuous casting process parameters and rolling process parameters, ultimately avoiding corrosion pits in the material during the acid etching process, the density of the material is improved, and the abnormal structure of the core is controlled, thereby improving the comprehensive use performance of the material.
References:
[1] Cai Kaike. Quality control of continuous casting billets [M]. Beijing: Metallurgical Industry Press, 2010.
[2] Qin Ying, Liu Huixia, Hu Yunsheng, et al. Causes and control of low-power defects of continuous casting billets [J]. Hebei Metallurgical, 2004(4): 41-43.
[3] Li Hefei, Qin Zuoxiang. Bainite structure of 25MnCrNiMo steel Microscopic structure analysis [J]. Journal of Dalian Jiaotong University, 2015, 36 (Supplement 1): 97-101.
[4] Mao Binbin, Fan Minjie, Cao Yanfeng, et al. Analysis of causes and control measures for low-power tissue defects in continuous casting billets[J]. Jiangsu Science and Technology Information, 2018, 35 (8): 36-38.
[5] Wang Chao, Ke Jiaxiang, Zhang Hu, et al. Optimization of structural properties of 42CrMo round steel [J]. Rolled steel, 2020, 37(5): 100-102.
[6] Wang Chao, Sun Jianshe, Zhang Tongfang. Analysis of the causes of pinhole loosening of bearing steel and countermeasures [J]. Shandong Metallurgy, 2018, 40(3):36-38.
[7] Yu Zhimin. Discussion on the center segregation defects of continuous cast bearing steel [J]. Harbin Bearing, 2019, 40(4):36-37, 42.
[8] Yue Yinan, Guo Ruihua, Yang Xiong. Research on the control rolling and cold structure properties of low alloy structural steel [J]. Baosteel Technology, 2018, 44(6):42-45.
[9] Jin Lanfen, Liang Tian. The structure and properties of 16Mn steel controlled cooling of 16Mn steel [J]. Materials Science and Technology, 1995, 3(4): 52-56.
Article source Materials and Test Network Journal Paper Physical and Chemical Inspection-Physics Volume 58 Volume 7 (pp: 56-61)
