Analysis on corrosion fatigue cracking mechanism of 17-4PH blade of low
pressure rotor of steam turbine
Yuwei Wei1,2※, 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
1. Introduction
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.