Analysis on corrosion fatigue cracking mechanism of 17-4PH blade of low pressure rotor of steam turbine

Yuwei Wei^1,2※, Yongjun Li²※, Jiafeng Lai³※, Qinxin Zhao¹,Lili Yang⁴,Qingyu Lin², Xiaolin Wang²

1.Key laboratory of Thermo-Fluid Science and Engineering of MOE ,Xi’an Jiaotong University, Xi’an,710049, China;

2.Guangxi Special Equipment Supervision and Inspection Institute, Nanning, 530219,China

3. College of Petroleum and Chemical Engineering,Beibu Gulf University,Qinzhou, 535000,China

4. School of Electronic and Information Engineering,South China University of Technology，Guangzhou, 510006,China

※These authors contributed equally to this work.

Abstract: The causes of many blade fractures of a steam turbine in a power plant were analyzed by means of macro analysis, mechanical examination, metallographic examination, SEM and X-ray fluorescence spectrum analysis(XRF). The results show that the blade cracks due to corrosion fatigue. Cl ^-, K ⁺, etc. react with the turbine blades in the steam environment in physical, chemical and electrochemical ways, causing local spot corrosion on the blades, forming corrosion pits.In addition, the steam condensate has an erosion effect on the blades, both of which form a corrosion fatigue source. The autocatalytic process of block cell is formed when Cl^-, K ⁺, etc. react with the turbine blades. Under the action of complex load, the current density \(i\) of activation dissolution of metal can be expressed as a function of complex stress state ^.Crack growth has an important relationship with stress, depth and width of corrosion pits. The larger the width of corrosion pit is, the deeper the depth is, the smaller threshold nominal stress range for crack elongation\(\ {\sigma}_{\text{th}}\) is, the easier the blade cracks. Under the external conditions of complex alternating stress formed by tensile force, bending force, torsion force and exciting force, the complex alternating stress directly promotes the crack propagation until the fracture failure.

Keywords:XRF;Corrosion fatigue;Corrosion pit;Threshold nominal stress range for crack elongation\(\ {\sigma}_{\text{th}}\);Complex alternating stress

Among all failure accidents in the power plant, turbine blade failure accounts for the largest proportion. Fatigue fracture is the most common type of turbine blade failure. The working condition and environment of steam turbine blades are very bad, mainly in stress state, working temperature, environmental medium and so on. When the blade breaks, the fracture often occurs in the middle and root of the blade^[1]. In the working process, the turbine blades bear the tensile stress caused by the centrifugal force when the rotor rotates, and the bending stress and torsion caused by the steam flow pressure; the excited vibration force of the blades will produce forced vibration, even cause resonance, and the complex alternating stress will eventually lead to the fatigue fracture of the blades^[2-3]. The working temperature of each stage blade of steam turbine is different. The temperature of the first stage blade is the highest (above 500℃), and then the temperature will be reduced gradually due to the steam working step by step, and the temperature will be reduced to below 100℃ until the last stage blade. During the operation of steam turbine, the steam is easy to condense into small water drops at the last stage blade. If there are corrosive elements in the steam, it will form electrolyte with water. It adsorbs on the surface of blade and forms micro cell, which causes electrochemical corrosion. These local corrosion points will become the weak points of blades, and the failure often originates from them. Xie Jianfeng^[4] analyzed the fracture failure of steam turbine blades in a petrochemical thermal power plant. The fracture of the blades was caused by fatigue. The fatigue source was located in the exhaust side of the blades, which suffered from high stress and strain, and formed a sharp groove at the edge of the blades due to cavitation, which changed the stress state of the blade surface, making the cracks easier appear and be growth. S. Qu ^[5] thinks that the reason of blade fracture is the stress concentration. With the gradual expansion of the crack, the blade finally breaks. Anson^[6] analyzed the vibration characteristics, frequency data and macro characteristics of imported 300MW and 600MW steam turbines, and summarized the failure forms of blades, roots and rims. B.M. Schönbauer ^[7] found that the cracking of the blade of low pressure steam turbine was caused by stress corrosion through conventional detection technology and accident site exploration. Zhu Baotian et al. ^[8] analyzed the fatigue life of the last stage 905mm blade of the steam turbine, and thought that the factors affecting the life of the blade were the alternating stress, stable stress, surface corrosion pit and so on, such as exciting force, dynamic stress, bending stress, centrifugal force, low cycle and high cycle fatigue load.

Accident unit was put into operation in September 2016. The steam turbine (Model:CLN350-24.2/566/566 ) is supercritical, primary intermediate reheat, single shaft, single back pressure, wet condensing steam turbine produced by Harbin steam turbine works Co., Ltd., which has the operation capacity of steam extraction with heat supply. The unit has started 12 times since it was put into operation, and entered into overhaul in late June 2018.

The rated speed of the steam turbine is 3000r/min.The rated pressure in front of the main steam valve is 24.2MPa, and the rated temperature is 566℃. The pumping stage is 8.The length of the last stage blade of the low pressure cylinder is 550mm.

The model of steam turbine supporting boiler is DG-1110/25.4-Ⅱ1, manufactured by Dongfang Boiler Factory Co., Ltd. in April 2015, with design pressure 25.4MPa, design temperature 571℃, and cumulative operation 18000 hours.