| Centrifugal compressor is a type of machinery with a high-speed rotating impeller working through gas medium. The gas pressure and speed are improved by the centrifugal force while an impeller rotates rapidly. The centrifugal compressor impeller is a key equipment that is extensively used in the petroleum, chemical, natural gas transportation, metallurgy, and mining industries. As the core component of a centrifugal compressor, an impeller operates at a high speed in poor working conditions with high temperature, high pressure, and different types of working medium. In the long-term service process, a centrifugal compressor is subjected to rotary centrifugal force, aerodynamic load, vibration force, corrosive medium, and impurity particles impact wear; these factors may cause various types of damage, such as cracking, breakage, thinning, and deformation. Stress corrosion cracking (SCC) often occurs in metal components that have been used for long periods and leads to unexpected and sudden failure. SCC occurring in the centrifugal compressor impeller could cause significant loss. Hence, research on SCC of an impeller and analysis of the stress corrosion mechanism of the impeller material in the actual working environment is significant for the design and manufacture of such impeller. This type of research can also ensure safety in production and improve the efficiency of enterprises. This work is supported by the National Basic Research Program of China (973 Program)-"Behavior and Mechanism of Reman-parts under complex environments, critical threshold of Reman-life (Grant No.2011CB013401)". By simulating the corrosion environments of centrifugal compressor for natural gas pipeline, SCC of the FV520B steel and weld joint was studied using the electrochemical, slow strain rate testing(SSRT), and prefabricated crack stress corrosion test methods. The stress and corrosive medium distribution models of the pipeline’s centrifugal compressor impeller were established using the numerical simulation method. The main conclusions are as follows:(1)For the numerical simulation method, this study analyzes the influence of the internal flow field of the centrifugal compressor impeller on the stress, temperature, and velocity fields, as well as the corrosive medium distribution law of the blade. The results show that the tendencies of the equivalent stress distributions, which were caused by the centrifugal load, coupling effect of the centrifugal load, and aerodynamic load, are all the same. The largest equivalent stresses are on the suction surface of the blade near the junction of the leading edge and wheel cover. The concentration of H2S is high in the impeller blade surface near the front wheel cover, and its mass fraction increased gradually from the impeller inlet to the outlet. CO2 is mainly distributed in the suction surface of the blade near the rear cover and the front part of the pressure surface because the molecular mass of CO2 is larger than that of H2S; the mass fraction of CO2 is reduced gradually from the impeller inlet to the outlet. The equivalent stress at the corrosion pit is larger than all the others. The fluid velocity decreases considerably in the pitting corrosion pits, thereby enriching both H2S and CO2 in the corrosion pits. Thereafter, the pits can develop SCC.(2)By analyzing the electrochemical polarization curve of the FV520B impeller steel and its weld joint in the H2S/CO2 environment with high temperature and high pressure, the effects of H2S and CO2 concentrations, temperature, and pressure on the self-corrosion potential and self-corrosion current density are studied. The results show that H2S is dominant in the electrochemical corrosion process of FV520B. H2S promotes the active dissolution of the FV520B steel and plays a significant role in promoting cathodic reaction. As the H2S concentration increases, the self-corrosion potential of FV520B reduces, the self-corrosion current density increases, and the corrosion rate increases. When the H2S molar percentage is above 12%, the self-corrosion potential of FV520B becomes stable. Compared with the parent metal, the self-corrosion potential of the weld joint in the same environment is more negative, the self-corrosion current density is higher, and the corrosion rate is faster.(3)The FV520B steel has an evident tendency toward stress corrosion in the H2S/CO2 environment with high temperature and pressure. SCC originates in the material surface and propagates inside the material. The propagation direction of SCC is perpendicular to the direction of the stress, and is characterized by transgranular cracking. Several corrosion holes and small cracks exist in the side faces, and a few small cracks develop into big cracks that are vertical to the loading direction. The tiny crack fractures on the fracture surface significantly increase compared with the secondary cracks with step-form, thereby showing that the fracture type of FV520B in the H2S/CO2 environment is the brittle fracture of the transgranular cleavage fracture form. H2S is dominant in the SCC process of FV520B; with the increase in H2S concentration, the material mechanical properties decrease and the stress corrosion sensitivity index increases. The increase in H2S concentration can accelerate the anodic dissolution reaction of FV520B. Numerous hydrogen molecules diffuse and permeate inside the metal and accumulate in the internal defects, thereby eventually making the steel fragile and leading to hydrogen-induced cracking. Therefore, the stress corrosion mechanism of FV520B in the test H2S/CO2 environment is anodic dissolution, and the hydrogen-induced cracking mixing mechanisms and the crack extension model is transgranular SCC.(4)The stress corrosion resistance of the welded joints in the H2S/CO2 environment is poor. Even in environments with low H2S concentration, the stress corrosion tendency is still considerably serious. The defects resulting from the welding process, such as pores and inclusion, have a significant effect on the SCC of the weld joint. The surface defects will lead to the corrosion pits and develop into transverse cracks. The internal defects will be the hydrogen accumulation place, which develops into holes with the action of hydrogen pressure and stress. These holes cause the hydrogen-induced cracking. The weld joint fracture is uneven because of the weld defects, and is characterized by cleavage with a few secondary step-form cracks. The SCC mechanism is hydrogen-induced cracking. Temperature has a substantial influence on the stress corrosion of the weld joint in the H2S/CO2 environment. The changing law of corrosion potential in the stretching process and stress corrosion sensitivity of the FV520B weld joint with temperature are the same. The increase in temperature results in an increase in corrosion potential and a reduction in stress corrosion sensitivity.(5)The mathematical models between the environmental parameters and the stress corrosion sensitivity of FV520B and weld joint were established using the regression analysis method. The models show that the influence of H2S concentration and temperature on stress corrosion sensitivity is significant, and temperature interacts with the stress corrosion sensitivity and the other three parameters. For the weld joint, the stress corrosion sensitivity is affected by the temperature and the interaction of the CO2 concentration and pressure.(6)The results of the constant displacement method with preexisting crack show that the SCC growth rate of FV520B gradually increases with an increase of the initial loading stress intensity factor in a linear manner. With the increase of the H2S concentration, the critical stress intensity factor Kiscc reduces, and the SCC growth rate increases. When the initial loading stress intensity factor is low, Kiscc initially decreases but subsequently increases with an increase in temperature. In high initial loading stress intensity factor condition, the stress intensity factor Kiscc decreases with an increase in temperature. When the temperature is below 82 ℃, the SCC growth rate decreases with an increase in temperature. However, the SCC growth rate increases when the temperature is above 82 ℃,which conforms with the stress corrosion sensitivity results. |
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