35CrM oA hydrogen preheater connecting bolt high temperature sulfur corrosion 1 macro analysis After 4 years of operation of a hydrogen preheater in an olefin plant, during the overhaul, 3 of the bolts connecting the tube sheets were completely broken, and the remaining bolts also had different degrees. Cracking and corrosion. The preheater tube operating pressure is 3 M Pa, inlet temperature 391 e, outlet temperature e, shell side operating pressure 4 MPa, inlet temperature 177 e, outlet temperature 375 e, tube and shell material are 0Cr18N i10T i medium Also all are hydrogen hydrocarbon compounds. Connecting bolt diameter a300 The macroscopic characteristics of the broken bolt are shown in Figure 1. The bolt has been completely disconnected. The entire fracture and the outer surface of the bolt and nut are covered with corrosion products. The corrosion product has a certain metallic luster, brittleness, and tapping is layered off. The surface color exposed after falling off is grayish black. According to the on-site technicians, the smell was thicker when disassembled.
In order to find out the cause of failure, the bolts were studied from the material, the shape and composition of the corrosion products and the metallographic structure.
2 bolt material analysis bolt material test results are shown in Table 1. It can be seen from the test results that the material is qualified.
Element name detection content ( ) standard content ( ) 3 fracture scanning electron microscopy analysis using the scanning electron microscope to analyze the fracture, the results shown in Figure 2, 3.
The outermost corrosion products with metallic luster in the fracture are closely arranged rock-like features (see Figure 2). The EDS analysis results of the corrosion products show (), these particles contain only two elements, Fe and S, and the number of atoms. The ratio is basically 1B1 (49B50), and therefore, it can be confirmed that the corrosion product is a FeS crystal.
Elemental mass content (W t ) atomic content ( At ) The outermost layer of FeS grains on the fracture surface is partially exfoliated, revealing the corrosion products of the inner layer, and the morphology and composition of the inner layer corrosion products are small. Cr, M n, S i In addition to the alloying elements in the base metal and the O element, the main component is still an atomic ratio of 1? 1 Fe and S.
Elemental mass content (W t ) atomic content ( At ) analysis results show that the corrosion products are typical high temperature sulfur (hydrogen sulfide) corrosion products, both in form and composition. Regardless of S or elemental sulfur, it will have strong corrosion to Fe at high temperatures. HS can directly react with Fe at high temperature: although elemental sulfur is stable at normal temperature, it can directly form sulfide with Fe at high temperature (35 400 e): In addition, in the presence of H, elemental sulfur is corroded. More sexual. For example, at 230 e, the sulfur vapour is less corrosive to steel, but in the rich H environment, it can react rapidly with Fe, causing the corrosion degree to be several times more serious than the HS corrosion under the same conditions. This is because the metal surface has catalytic activity, S and H are catalyzed in the metal surface shape hydrogen preheater connection bolt failure analysis and solution to high concentration of HS, resulting in extremely corrosive under the condition of isolated air, typical The metal sulfide corrosion product can be roughly divided into two layers. The outer layer structure is loose, and the brittleness is easy to fall off. The inner layer of FeS columnar crystal is fine, but the lattice defects are many. In addition to FeS, the components also contain alloying elements contained in the matrix material.
Since the structure of the sulfide layer is not dense enough and is easy to fall off, the sulfide corrosion layer of the metal does not contribute to the improvement of the corrosion resistance of the base metal. On the contrary, after the metal undergoes sulfide corrosion, the volume is expanded to the original multiple, and therefore, Large stresses are generated inside the metal, and the crack propagation is accelerated. The high-temperature sulfur corrosion generally has the characteristics of uniform corrosion. However, if the material is also subjected to a large stress, the stress due to the combination of the sulfur-containing medium and the stress may also occur. Corrosion cracking.
The hydrogen preheater has a service temperature of 177 391 e and is rich in hydrogen. Therefore, regardless of whether the working medium contains more S or S, it will cause serious sulfur corrosion to the material. In addition, the head bolts are subjected to large working tensile stresses during operation, and therefore, stress corrosion cracking caused by the combination of sulfur-containing medium and stress is likely to occur.
4 Metallographic structure analysis There are many inclusions in the bolt material, as shown in Figure 4. The inclusions are granular or strip-shaped, and are densely distributed near the outer surface, and the number of cores is small. Inclusions have an adverse effect on the strength and corrosion resistance of the material.
The bolt structure is a small amount of upper bainite in the sorbite, and the microstructure is basically normal, but the granular carbide precipitates along the grain boundary of the prior austenite (see Fig. 5), which causes the material to resist intergranular corrosion and is corroded. It is easy to crack along the crystal under the action of the medium. The shape of the secondary crack observed near the main fracture surface is shown in Fig. 6. The crack is filled with corrosion products and has obvious characteristics along the grain boundary of the prior austenite, which is related to the precipitation of carbide.
The failure reason analysis of the pressure vessel 5 and the recommended bolts under the working condition are subject to large tensile stress, and the medium environment contains more HS gas or S steam, and the stress caused by the combination of sulfur medium and working stress occurs in the high temperature environment. Corrosion cracking, the initial cracking is only some small cracks, the crack is filled with corrosion products. Since the metal sulphide corrosion, the volume expands four times. Therefore, as the corrosion progresses, the volume of the corrosion product in the crack expands, causing a large stress at the crack tip, accelerating the crack propagation until the final fracture occurs. . In addition, the bolt material itself contains certain impurities and carbides deposited along the austenite grain boundary, which reduces the mechanical properties and corrosion resistance of the material, and also provides a convenient passage for crack initiation and expansion.
Based on the above analysis, it can be determined that the fracture of the hydrogen preheater connecting bolt is a failure caused by stress corrosion cracking in a typical high temperature sulfur environment.
Hydrogen preheater is an important equipment in the whole set of equipment. The working medium is flammable and explosive hydrogen and hydrocarbon gas. Once the leakage is easy to explode, it will cause serious consequences. Therefore, the content of corrosive components in the working medium, especially S and its compounds, must be strictly controlled in production. It is recommended that the production unit must identify the source of S and control its content. In addition, the general view is that when the content of Cr in the material reaches 18 or more, the occurrence of sulfur corrosion can be suppressed, and the actual production also shows that 18 8 type stainless steel (0Cr18N i10T i) used in the same working medium, tube, Corrosion does not occur in the shell side. Therefore, when the source of S is not found or the content cannot be controlled, and it is urgent to resume production, it is recommended to use austenitic stainless steel bolts instead of the original 35CrM oA bolts under the premise of ensuring strength.
Yang Bo, Tian Songbai, Yueshanlin. High temperature damage evaluation and failure analysis of corrosion behavior of different sulfur compounds, address: High Temperature Process Equipment Research Laboratory, School of Mechanical and Power Engineering, Nanjing University of Technology, 5 Xinma Fanma Road, Nanjing, Jiangsu, China.
(3) The main reason for the nonlinearity of the compression stiffness of the multi-layer bellows is the non-linear variation of the contact area and the contact force between the layers during the compression process, because the deformation of each layer in the compression process is more complicated.
(4) For large multi-layer corrugated pipes, in order to improve the accuracy of their engineering calculations, as with the thin and thick thinning effect, important parameters such as the friction coefficient for characterizing the contact state between layers must also be accurately quantified.
Li Yongsheng 1 Wavelet Expansion Joint Practical Technology [M] 1 Beijing: Chemical Industry Press, 20001 Zhao Liansheng, Wang Yinhui, et al. U-shaped bellows were analyzed by nonlinear finite element method, Chen Yong, Li Yongsheng, Gu Boqin, et al. U-shaped ripple Yang Yijun, Wang Xinfeng with ANSYS software. Nonlinear finite element stress analysis of multi-layer bellows [6] Zhuo Jun, Cao Liya, Liao Ridong, et al. Finite element analysis of axial stiffness and critical load of multi-layer U-shaped bellows [J]. Mechanical Strength, 2004, 26 [7] Wang Yang. Strength, stiffness and buckling analysis of multi-layer bellows expansion joints [D]. Nanjing: Nanjing University of Aeronautics and Astronautics, 2005.
PhD student of the School of Power Engineering, research direction: structural vibration of aeroengine.
Failure Analysis and Solution of Hydrogen Preheater Connection Bolt
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