Microstructural deterioration assessment of ex-service ... · Mechanical representations of creep...
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Microstructural deterioration assessment of ex-service power plant steel using concentric backscatter imaging Presented By: Melody van Rooyen, Supervisor: Prof TH Becker In collaboration with: CHRTEM, NMU (Dr.J Westraadt and Dr. G Marx)
Microstructural deterioration assessment of ex-service ... · Mechanical representations of creep are often in the form of creep curves, a plot of strain versus time shown in normalised
Microstructural deterioration assessment of ex-service power plant steel using concentric
backscatter imaging
Presented By: Melody van Rooyen, Supervisor: Prof TH BeckerIn collaboration with: CHRTEM, NMU (Dr. J Westraadt and Dr. G Marx)
Presenter
Presentation Notes
Good morning ladies and gentlemen Welcome to my talk today on the assessment of the deterioration of microstructural features of ex-service power plant steels using only concentric backscatter imaging. This forms part of my PhD work conducted in Stellenbosch University with the Materials Engineering Research Group under the supervision of Prof. Thorsten Becker. This is also done in collaboration with the centre for high resolution transmission electron microscopy at NMU specifically Dr Westraadt and Dr. Marx.
MSSA Conference, Langebaan, 4 December 2019
Background – Power stations and creep
1
0
50
100
150
200
250
300
350
Life
exh
aust
ion
ratio
OL/
DL
(%)
Max. operating hoursDesign lifeLife exhaustion ratio
Exceed design life
iosrjournals.org
High temperature (> 550 °C), high steam pressures
(>15 MPa)
Creep
Presenter
Presentation Notes
Currently in SA, we are very familiar with the negative impacts of load shedding on industrial as well as private life. This graph shows the current life exhaustion relative original design life (vertical axis) of current operating coal fired power stations in the country labelled 1 to 13 on the x-axis. What we notice is that more than 75% of these stations have already exceeded their original intended lifetimes. This is because at high temperatures and pressures encountered in steam pipes, the material undergoes a damage process known as creep which can lead to pipe failure. This places a lot of strain on critical components, such as thick-walled steam piping
MSSA Conference, Langebaan, 4 December 2019
Background – Material assessment
2
Life fraction, 𝑡𝑡𝑡𝑡𝑟𝑟
Rel
ativ
e st
rain
, 𝜀𝜀 𝜀𝜀 𝑟𝑟
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
1.0
0.00.0
No damage
Isolated cavities
Oriented cavities
Microcracks
Macrocracks
12 % Cr steel:X20CrMo12-1(X20 for short)
EPPEI
Damage state Cavities/mm2
New Ideally noneLow 60-90
Medium ≈200High >220-690
Ex-service
X20
Presenter
Presentation Notes
Mechanical representations of creep are often in the form of creep curves, a plot of strain versus time shown in normalised form here. Typically during station maintenance, plant operators assess the creep damage state of the piping steel (an example shown here) using a quasi-NDT technique known as surface replication during which they paste acetate tape on the surface of the pipe to pull off a negative copy of the microstructure. These replicas are analysed under light microscopes for creep cavity densities, where a higher cavity density indicates a higher damage state or material life exhaustion shown here categorised on the right. This assists decisions in terms of component monitoring, repair or replacement to manage the continued reliable operation of the plant. In this work, we were provided with these four damage states of a common piping steel known as X20 which were classes using this method and labelled new, low, medium and high damage respectively. Each of these ex-service states show an increasing amount of cavity densities.
However, besides cavity-based deterioration assessments there are also other microstructural aspects to consider. X20 is very complicated in terms of microstructure but the main elements that contribute to creep resistance have been identified as dislocation densities, precipitates and subgrains. Now as one can see there is a range of scales to consider here when characterising these features. For instance, dislocation densities are challenging to accurately quantify using commonly available microscopy techniques due to the heterogenous nature, high resolution required and difficult specimen preparation. In this work, we rather considered looking at the subgrains and larger precipitates known as Cr23C6 carbides and the intermetallic phase that forms during creep known as Laves phase which is rich in Molybdomen. During creep these elements evolve in fairly predictable manners and it would be convenient to have a single microstructural preparation, observation and analysis stream that can indicate the level of material deterioration of ex-service X20. Such an approach will minimise assessment to maintenance decision time which can ensure more effective operation of power stations.
MSSA Conference, Langebaan, 4 December 2019
Experimental description - Materials
4
Material Type: X20 CrMoV12-1
Virgin(New)
Low Damage
Medium Damage
High Damage
Cavity density measured from BSE-SEM(cavity/mm2)
<40 70-150 150-250 >250
Operating temperature (°C) - 545 545 543
Operating steam pressure (MPa) - 17.0 19.4 18.1
Operating life (hours) - 130 000 130 000 156 000
Presenter
Presentation Notes
For this work, we looked at three different ex-service X20 materials with different levels of service exposure and previously characterised according to the surface replication technique. For instance low and medium piping sections have been run at the same temperature and length of operation but medium operated at high steam pressures. High damage was subjected to the longest time of operation which is about 18 years
MSSA Conference, Langebaan, 4 December 2019
Experimental description – Creep testing
(b)
Longitudinalcentreline
5 mm
x = 0 x = 5
Disc sampling location in grip
Disc sampling locations in gauge region(a)
Specimen extraction
Gauge region
5
Loading direction
White LED
Cameras
(d)
(c)
Gleeble vacuum chamber
Specimen
Water-cooled grips
(d)
DIC: Digital Image
Correlation
Presenter
Presentation Notes
For this analysis, specimens were machined from ex-service piping sections supplied by the local power utility. These specimens were subjected to accelerated creep testing using a technique that I developed in which the specimen is heated to a maximum temperature of 600 degrees Celsius in a Gleeble thermomechanical similar shown in (d). Three locations were selected for microstructural analysis as shown in (b) where 3 mm dia discs were wire cut, at two locations in the gauge region and one location in the grip region. Strains were measured using a photometric technique known as Digital Image Correlation or DIC for short. In brief, DIC measures strains by taking digital images of a surface during deformation and tracking the movement of unique speckle features applied to the surface using for example paint as shown in the top right of (b).
MSSA Conference, Langebaan, 4 December 2019
Experimental description – Creep testing (cont.)
6
Presenter
Presentation Notes
So this is the result of the creep testing using the Gleeble-DIC. At several locations we are able to plot the accelerated creep curves. The reason for the differences between the center and off-center creep curves is because the Gleeble applies a higher temperature at the center that parabolically decreases with distance from the center. The benefit of DIC is that we are able to spatially resolve the curves and get more data out of a single sample and correlate that with the regional microstructural analysis.
MSSA Conference, Langebaan, 4 December 2019
Experimental description – Creep testing (cont.)
7
Presenter
Presentation Notes
So this is the result of the creep testing using the Gleeble-DIC. At several locations we are able to plot the accelerated creep curves. The reason for the differences between the center and off-center creep curves is because the Gleeble applies a higher temperature at the center that parabolically decreases with distance from the center. The benefit of DIC is that we are able to spatially resolve the curves and get more data out of a single sample and correlate that with the regional microstructural analysis.
MSSA Conference, Langebaan, 4 December 2019
Experimental description – Microstructure
8
CBS-SEM: Concentric Backscatter-Scanning Electron Microscopy
FEI
CBS detector settings
Specimen preparation
• FIB-SEM• 5 kV• 0.20 nA• 4 mm WD• Total of 4
images per site (680 μm2)
• Twin-jet electropolishing
• 5 % HClO4solution
• 21-30 V• -20 °C
Presenter
Presentation Notes
Based on work of Dr Marx, we considered Concentric Backscatter analysis or CBS for the microstructural characterisation of ex-service X20. In brief, a CBS detector consists of concentric rings of backscatter electron detectors that allow simultaneous collection of compositional, grain orientation and topography contrasts. This requires a carefully electropolished surface which was obtained from twin-jet electropolishing process with the settings along with the detector settings provided in this table. In order to ensure there is sufficient data for sound statistics, four images were taken at each location.
MSSA Conference, Langebaan, 4 December 2019
Experimental description – Micro. (cont.)
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5 μm
Micro-grains
w
2.5 μm
Laves phase
Cr23C6
Precipitates
Diameter
Interparticle spacing
Numberdensity
Presenter
Presentation Notes
Once we have the image, the quantitave analysis was performed using image processing software known as MIPAR. Micro-grains were identified through a thresholding and segmentation routine. Because the micro-grains are characterised by their short widths and are normally elongated, an ellipse was fitted to the segmented outlines as shown. The particles were isolated on a threshold sequence based on the atomic contrast. For instance, the Mo rich Laves phase is heavy and so appears brighter than the Cr carbides which appear grey. From this the particle distributions were characterised by particle diameter, interparticle spacing and number density.
MSSA Conference, Langebaan, 4 December 2019
Results - Qualitative
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Low - Grip High - Grip
Medium - Grip High – Fracture (gauge)
Presenter
Presentation Notes
Here we have representative micrographs of each of the ex-service states. High damage material demonstrated a Laves phase which are the bright particles as shown. It is usually not wanted in X20 material because as shown in a SEM image of the region near fracture of a tested specimen (d), these Laves phase particles are often associated with voids that can lead to crack development and failure. Compared to high damage, we note small micrograins with dense carbide distributions in the low and medium material, although no laves phase was evident in the sample grip region of the low damage X20.
If we look qualitatively at the micro-grain measurements, we note a logarithmic distribution in terms of frequency versus width ranges and when we take the calculate the mean of this distribution we can see a trend in the level of damage state and micro-grain size, where high damage has larger grains. Interestingly, the medium damage grains were not substantially different from the low damage material. We therefore take our comparison further into the particle descriptions.
MSSA Conference, Langebaan, 4 December 2019
Results – Quantitative (Precipitates)
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Particle size
Inter-particle spacing
Volume number density
Cr23C6
Size
or i
nter
-par
ticle
spa
cing
(nm
)
Volu
me
num
ber d
ensi
ty (μ
m-3
)
:Gauge:Grip
Low Medium High
Presenter
Presentation Notes
For the Cr carbides, we have chosen to separate the particle parameters in this graph which shows for the low, medium and high X20. We include grip and gauge regions because we do not expect a measurable change in particle distributions given the short duration of the tests and gives us enhanced statistics. Given its complexity, I would like to draw your attention to the search for trends? We first notice that is considerable scatter associated with microstructural measurements.
MSSA Conference, Langebaan, 4 December 2019
Results – Quantitative (Precipitates)
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Particle size
Inter-particle spacing
Volume number density
Cr23C6
Size
or i
nter
-par
ticle
spa
cing
(nm
)
Volu
me
num
ber d
ensi
ty (μ
m-3
)
:Gauge:Grip
Dense carbide distribution
Low Medium High
Laves
Size
or i
nter
-par
ticle
spa
cing
(nm
)
Volu
me
num
ber d
ensi
ty (μ
m-3
)
Low Medium High
Smaller micro-grains Higher Laves
phase distribution
Presenter
Presentation Notes
The only element that truly stands out from a statistically significant point of view is the number density shown in white bars. We notice that the medium damage actually displays a higher number density of particles that both the low and high damage samples. This would indicate why we are finding comparably smaller grains in this material as the dense distribution pins grain boundaries and hamper growth. In terms of the Laves phase distribution, we see that although no phase was noted in the grip region, a low number density was observed in the gauge region of the low damage material. Higher Laves phase distributions were noted in the higher damage categories.
MSSA Conference, Langebaan, 4 December 2019
Results – Hardness
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LowMediumHigh
Medium: high hardness
Longitudinal distance from central gauge location
MSSA Conference, Langebaan, 4 December 2019
Mechanical response
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0
0.02
0.04
0.06
0.08
0.1
Cre
ep s
train
600 ° C
0 1 2 3 4 5Time (hours)
High
New
Low Med.
Presenter
Presentation Notes
So what does all tell us? As a final confirmation of the observed microstructure, we look at the creep curves measured using the Gleeble-DIC technique and compare how the ex-service material behaves. We have here again creep strain over time for all the material states. As expected we see a fast curve develop for the highly damage material showing a small resistance and a slow developing curve for the new X20.
MSSA Conference, Langebaan, 4 December 2019
Mechanical response
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0
0.02
0.04
0.06
0.08
0.1
Cre
ep s
train
600 ° C
0 1 2 3 4 5Time (hours)
High
New
Low Med.
0 1 2 3 4 5Time (hours)
10 -7
10 -6
10 -5
10 -4
10 -3
600 ° C
Cre
ep ra
te (s
-1)
Low Med.
Faster creep rates for low compared to
medium
Confirms CBS results: high Cr23C6 density enhances creep resistance
despite cavity density
Presenter
Presentation Notes
Interestingly, we notice the low and medium show similar curves. This is better represented by the curve gradient or a plot of creep rate versus time. Here we notice that contrary to belief, we have faster creep rates for the low when compared to the medium and this may be related to that dense carbide distribution that also translates to relatively small grains and this in spite of the higher cavity density in this material.
MSSA Conference, Langebaan, 4 December 2019
Conclusion
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• CBS is a convenient microstructural method for micro-grain and large
particle characterisation of ex-service material
• Highly damage material has large polygonal micro-grains, low density
of carbides and high fraction of Laves phase especially near voids.
• Low damage has less Laves phase but had a lower density of carbides
than medium
• Medium displays relatively smaller grains due to dense carbide
distribution. Medium and low damage display similar creep rates
despite differences in void distributions
• Using CBS allows faster damage assessment through a single
In summary, CBS offers us a convenient single process method of characterising the two most important factors in deterioration assessment of ex-service X20, namely micro-grains and particles with sufficient resolution to make comparisons We notice that on the extreme spectrum of damage, we note large almost equiaxed micro-grains bounded by sparse distribution of carbides but with a higher amount of Laves phase the negative effects of which is manifested in adjacent voids Low damage boasts less Laves but a less dense carbide distribution The result is that medium is able to stabilise its grains using this carbide distribution and actually demonstrate similar creep rates in comparison to low damage.
MSSA Conference, Langebaan, 4 December 2019
Conclusion
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“No SINGLE measurement is sufficiently comprehensive to describe the [damage state of] the steel with all requisite completeness”
~ Bhadesia et al. (1998)
Presenter
Presentation Notes
All of this work has been presented in the MPDI Materials paper with the title as shown. In this paper we tried to focus equally on the microstructural contributions as well as the resulting effect on the creep response as measured with DIC. This leads into the pull away message from my talk today. I would like to thank you for your attention and would like to end with a quote from a famous researcher in the creep realms known as Bhadesia that summarises the work quite effectively. There is no single measure whether microstructural or mechanical that can completely describe the level of creep deterioration of an ex-service steel. And so it is vital to combine several methods as CBS-SEM analysis enables us to do together with mechanical confirmation.
MSSA Conference, Langebaan, 4 December 2019
Acknowledgements and questions
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Thank you to the following:
Supervision:Prof Thorsten Becker and Materials Engineering Research Group at SUCBS-SEM imaging:NMU CHRTEM team, Dr J Westraadt and Dr G MarxGleeble testing/specimen preparation facilities:Prof Rob Knutsen and S. von Willingh at CMEStakeholder guidance and material provision:EPPEI staff and fellow students Funding:NRF (THRIP) and THUTHUKA, Frank Wilhelm Bursary Fund
