Translated Abstract
The working environment of last stage blades of steam turbine is very poor. They suffer harsh conditions of corrosion, vibration, water erosion and so on. The last stage blades of low pressure area are more prone to stress corrosion and corrosion fatigue damage than the blades in medium pressure and high pressure zone. Surface high-frequency induction hardening is a common technology to prevent the turbine last stage blades from water erosion. In addition, the technology can also improve stress corrosion and corrosion fatigue resistance to a certain extent. Therefore, comprehensive evaluation of the stress corrosion and corrosion fatigue performance of last stage blades’ steel of steam turbine is extremely important to its safe use and the improvement of performance. The analysis of microstructure and fracture mechanism of the steels are significant to improving the overall performance of materials.The stress corrosion performance of 1Cr12Ni3Mo2VN and X12CrNiMoV12-2 steels in 22% NaCl solution (pH=5, 80℃) are investigated. The fatigue behavior of 1Cr12Ni3Mo2VN and X12CrNiMoV12-2 steels after high-frequency induction hardening is investigated by corrosion fatigue tests in atmosphere at room temperature, 22% NaCl solution (pH = 5, 80℃) and steam(95℃), respectively. The paper also studies the fatigue crack initiation and propagation behavior of the two steels in atmosphere at room temperature, analyzes the distributions of residual stress and microhardness of the steels after high-frequency induction hardening, measures the polarization curve of different steels and determines the effects of high-frequency induction hardening on pitting corrosion inclination. Besides, the microstructure and fracture are analyzed.The experimental results show that high-frequency induction hardening significantly improve the stress corrosion and fatigue properties of 1Cr12Ni3Mo2VN and X12CrNiMoV12 steels. Their high-frequency induction hardening zones contain a small amount of (Fe,Cr)3C, (Fe,Cr)7C3 carbides respectively. But there are more coarse (Fe,Cr)7C3, (Fe ,Cr)23C6 carbides in quenched area and tempered zone, and the segregation of carbides is common, which leads to local depleted of Cr in the base, then the corrosion resistance of steels decreased. The test results of polarization curve also show that the pitting tendency of high-frequency induction hardening area is significantly smaller than the tempered zone. In addition, the size and quantity of carbides in the quenched zone and tempered area also affect the fatigue performance of the two steels, the bigger carbides in tempered zone are always the initiations of fatigue cracks. High-frequency induction hardening produces a compressive residual stress on the surface of 1Cr12Ni3Mo2VN and X12CrNiMoV12-2 steel and lowers the fatigue crack growth rate of specimen with analog pits. The stress corrosion fracture of 1Cr12Ni3Mo2VN steel has the characteristics of intergranular fracture, while the corrosion pit of X12CrNiMoV12-2 steel has the characteristics of quasi-cleavage. The initiation of fatigue crack is in the processing traces of specimen surface. Only in 22% NaCl aqueous solution, the crack initiates in surface pits. The microstructure of high-frequency induction hardening zone and tempered zone were tempered martensite and tempering sorbite, respectively. The microstructure of the former finer than the latter.
Translated Keyword
[High-frequency induction hardening Stress corrosion Corrosion fatigue Pitting Last-stage blade steel]
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