3. Analysis and discussion
According to the test results, the mechanical properties and chemical
composition of the blade meet the requirements of the standard. The
metallographic structure is low carbon martensite + dispersed
intermetallic strengthening phase, and the metallographic structure is
normal. From the metallographic structure of the crack section, it can
be seen that most of the cracks originate from the corrosion pits on the
blade surface (as shown in the Fig.8,9). The main cracks originate from
the inner arc and propagate to the center, while the other secondary
cracks originate from the corrosion pits on the outer arc and propagate
to the center. Most of the cracks in the origin region propagate along
the grain. The macro morphology of blade fracture can be clearly divided
into fracture source region, crack growth region and instantaneous
fracture region. There are shell like patterns in the crack growth area
and shear lip shape in the 45° direction in the instantaneous fracture
area, which is a typical fatigue fracture morphology. According to XRF
analysis, there are a lot of corrosion products, many secondary cracks,
and mud like patterns in the fracture area, and there are corrosion
sensitive elements such as S, Cl which indicates that the turbine blade
fracture belongs to a typical corrosion fatigue fracture.
During the operation of the last stage blade, when the impeller rotates,
there are two forces acting on the blade: one is the centrifugal force
generated by the mass of the blade itself, the shroud and the drawing
mass; the other is the steam flow force generated by the high-speed
steam flow through the blade channel. Generally, the centrifugal force
not only produces tensile stress but also bending stress in the moving
blade because the penetration line of the centrifugal force acting point
does not pass through the center of gravity of the cross section for
calculation through the blade. The steam force acting on the blade is
composed of two parts: one does not change with time, and the other
changes with time. Among them, the component that changes with time will
cause the exciting force of the blade. Centrifugal and steam forces may
also cause torsional stresses in the blades. The exciting force causes
forced vibration and even resonance, which makes the alternating stress
of the turbine blade increase sharply [9]. Under
the action of periodic alternating stress caused by the above-mentioned
multiple stresses, high cycle fatigue crack on blade is created,then
gradually expands, and finally breaks due to insufficient strength. The
vibration intensified sharply and the impact of the broken blade
accelerate the fracture of the remaining blades while the rotor dynamic
balance is damaged after one blade breaks.
In addition, the content of chloride ion, calcium and silicon in the
steam exceed the standard due to the unstable quality control of steam
water. The saturated steam or even wet steam is finally transformed with
the gradual expansion of steam whose pressure and temperature decrease.
The various ions which corrode nearby parts finally gradually
precipitates on the last several stages of blades while the partial
pressure of various ions in the steam changes. In particular,chloride
ions do great harm to stainless steel. The surface damage (corrosion
pit) of the blade caused by corrosion not only destroys the hardened
layer, but also forms the crack source, which provides conditions for
the generation of cracks. However, the alternating stress is the stress
state that the blade must bear. Under the joint action of the
alternating stress and the corrosive environment, the cracks are formed
and continue to expand, which eventually leads to the blade fracture.
The detailed mechanism of crack formation is analyzed as follows.
Blade cracking originates from corrosion pits on blade surface or water
erosion on blade edge. As a result of water erosion, a sharp groove
which changes the stress state of the blade surface is formed at the
edge of the blade, and makes the crack appear and expand more easily at
this position. Pitting is considered to be the precursor of corrosion
fatigue damage again and again, which has been proved by many blade
failure analysis [10-15]. Pitting not only reduces
the actual strength of blade, but also leads to stress concentration,
which can become the core of crack in the range of low cycle stress. The
formation of corrosion pit begins with pitting caused by chloride ion.
Chloride ion pitting can be divided into two stages: pitting nucleation
stage and pitting growth stage. There are two theories of pitting
nucleation, that is, the theory of passive film destruction and the
theory of adsorption [16]. According to the
failure theory of passivation film, it is most likely to penetrate the
tiny pores in the oxide film and reach the metal surface, and interact
with the metal to form a soluble compound due to the small radius and
strong penetration ability of chloride ion so that the structure of the
oxide film changes and the metal produces corrosion. According to the
adsorption theory, the fundamental reason why chloride ion destroys the
oxide film is that chloride ion has a strong ability to be adsorbed by
metal, and they are preferentially adsorbed by metal, and oxygen is
discharged from the metal surface. Because oxygen determines the
passivation state of the metal, chloride ion and oxygen compete for the
adsorption point on the metal surface, and even can replace the
passivation ion in the adsorption to form chloride with the metal. The
adsorption of chloride on the metal surface is not stable, forming
soluble substances, which leads to the acceleration of corrosion.
Once pore erosion is formed, it develops rapidly, and there are many
models of pore erosion development. It is generally recognized that
acidification autocatalytic process occurs in the pores. Once the
pitting is formed, the external and internal reactions of the pitting
are different. The metal in the hole is in the active state (the
potential is negative) and becomes the anode. The metal outside the hole
is in the passive state, and the potential is positive and becomes the
cathode. An activation passivation micro galvanic corrosion cell is
formed inside and outside the hole.
Due to oxygen enrichment outside the pore, cathode reaction occurs:
O2+H2O+4e→4OH- (1)
Anode reaction in the hole:
M→Mn++ne (2)
Due to the increase of pH at the orifice, the secondary reaction of
metal ions takes place, taking Fe2 + as an example,
Fe2++2OH-→Fe(OH)2 (3)
Fe(OH)2+2H2O+O2→4Fe(OH)3(4)
The iron hydroxide formed by the reaction is deposited on the orifice.
As the corrosion proceeds, the pH outside the orifice increases
gradually, and the rust layer and the scale layer together accumulate in
the crater to form an occluded battery, which hinders the ion migration,
finally form oxygen concentration cell. Because the metal ions in the
pore are not easy to diffuse outwards, resulting in the increase of
cations. In order to maintain the electrical neutrality, the
Cl- ions outside the pore migrate to the pore, and the
Cl- concentration inside the pore increases.
Hydrolysis of metal ions in the pores results in serious acidification
in the pores, which in turn causes more Cl- migration
and acidification. The autocatalytic process of block cell is formed.
Mn++nH2O→M(OH)n+nH+(5)
It can be said that there is a linear relationship between the pitting
potential and the concentration of the aggressive ion, as it is also
reported by other authors. Eq. (6) is
given based on their works and describes this linear
relationship[17]:
E pit = A − B log Cx (6)
where A and B are constants, Cx is
the concentration of the aggressive ion
and E pit is the pitting potential.
The increase of pitting depth mainly depends on the positive dissolution
of metal in the pit. The higher the current density of anodic
dissolution is, the faster the anodic dissolution speed is, the larger
the corrosion pit is. In the above analysis, the blade is subjected to
tension, bending, torsion, exciting force and other complex stresses.
The influence of complex stress on the current density of anodic
dissolution can be expressed as the sum of the change of internal energy
caused by the anisotropic stress and the change of enthalpy caused by
the plastic deformation. Under the action of complex load, the current
density of activation dissolution of metal can be expressed as a
function of complex stress state [18]:
\begin{equation}
i=i_{0}\exp\left(\frac{G}{\text{RT}}\right)\nonumber \\
\end{equation}\(\ \ \ =i_{0}\exp\left[\frac{1}{\text{RT}}\left(\frac{M\left(1-2\upsilon\right)\left(\sigma_{1}+\sigma_{2}+\sigma_{3}\right)}{3E\rho}+\eta\frac{\text{Mn}}{\rho}\frac{\sigma_{1}^{n+1}+\sigma_{2}^{n+1}+\sigma_{3}^{n+1}}{P^{n}\left(1+n\right)}\right)\right]\)(7)
i is the current density of activation dissolution ,E is
the modulus of elasticity,P is the
pressure,σ1 ,σ2 ,σ3is the main stress,ν is the Poisson’s ratio,ρ is the
density of metal electrode,η is the percentage of plastic strain
energy density stored in metal.
From the above formula, it can be seen that the larger the complex load
is, the greater the current density of anodic dissolution at the bottom
of the pit is, the faster the anodic dissolution rate of the pit is, and
the larger the pit is. According to fracture mechanics, the size of
corrosion pit directly affects the threshold strength factor and
critical cracking stress.
The relationship between crack stress field intensity factor,stress and
crack half length:
\(K=Y\sigma\left(\text{πa}\right)^{2}\) (8)
Threshold nominal stress range for crack
elongation[19]:
\({\sigma}_{th}=\frac{1}{\text{Y.}\sqrt{\pi}}\ \frac{{K}_{th,lc}}{\sqrt{l+l_{0}}}\)(9)
\({K}_{th,lc}\) is the threshold stress intensity factor range for long
cracks, Y is a geometry factor, l is the crack length and
l0 is a hypothetical “intrinsic” crack length given
by:
l0= \(\frac{1}{\pi}\left(\frac{{K}_{th,lc}}{{\sigma}_{0}Y}\right)^{2}\)(10)
It was found [20,21] that using conventional
solutions for the geometry factor according to the pit shape (e.g. the
solution for a semi-elliptical crack provided by Newman and Raju[22,23]) leads to highly conservative results
(overestimation of up to 40%). Therefore, Eqs. (11) and (12) were
evaluated according to extensive fatigue results. Following equations
were suggested for the threshold nominal stress range\({\sigma}_{\text{th}}\):
\({\sigma}_{\text{th}}=\frac{1}{0.42.\sqrt{\pi}}\ \frac{{K}_{th,lc}}{\sqrt{a+a_{0}}}\)(11)
\({\sigma}_{\text{th}}=\frac{1}{0.65.\sqrt{\pi}}\ \frac{{K}_{th,lc}}{\sqrt{c+c_{0}}}\)(12)
\({K}_{\text{th},\text{lc}}\): threshold stress intensity factor range
for long cracks,
\({\sigma}_{\text{th}}\ \): threshold nominal stress range for crack
elongation
Y: geometry factor
l: crack length
l0: hypothetical “intrinsic” crack length
a,c: a is the corrosion pit depth and c is the half corrosion pit width
(a0 and c0 are calculated according to
Eq. (12)).
It can be seen from the above formula that crack growth is closely
related to stress, depth and width of corrosion pit.The critical
cracking stress decreases when the width a and depth c increase. As the
width and depth of the corrosion pit increase, the critical stress
decreases and the blade cracks more easily.To sum up, on the one hand,
the corrosion pit which becomes the crack source under the external
conditions of complex alternating stress formed by tensile force,
bending force, torsion force and exciting force forms, widens and
deepens under the action of Cl -; on the other hand,
the complex alternating stress directly promotes the crack growth until
the fracture failure.
It has been proposed by some authors [24]that
laser cladding of alloy elements which were a mixture of Cr, Ni, Co and
W powder on the surface can improve the corrosion cracking resistance of
blade surface. After laser alloying, the surface layer was denser and
the grain refined, while the microhardness of the surface(average
610HV0.2) was about one times higher than that of the substrate material
(330HV0.2). The friction coefficient of the laser-alloyed 17-4PH layer
was much lower than that of the substrate.