non destructive testing review for detection of creep damage in power plant steel
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non destructive testing review for detection of creep damage in power plant steelTRANSCRIPT
Non destructive techniques for the detection of creep damage in power plant steels
Presented by Pintu Kumar(13MT60R30)
Creep
the time-dependent deformation of materials subject to high stress at elevated temperature
Significance• To design components used in the power generation
industry, in fossil fuel plants and nuclear reactors• in the assessment of their remaining life
A creep curve
• Creep mechanism• Volumetric/localised creep• Critical locations of creep dailure• Premature failure due to localised creep in
weldments
Classification of creep cracks in weldments
non-destructive evaluation (NDE) techniques
• continuous monitoring or periodic inspection• Cr–Mo steels-A power plant steel
Replication(Replica metallography)
• the most commonly used method for creep damage detection
polishing and etching
Application of a film
Allow to solidify
Softening with a solvent
Removal of film
Observation of film under microscope
Limitations
• It is commonly assumed that voids can be detected at a relatively early stage of creep
• A parameter -the number of cavities per unit area may be misleading
• The development of apparent voids during surface preparation
• individual judgement • cannot detect subsurface defects. • the lengthy procedure required for the testing
Ultrasonics • Volumetric creep evaluation- Back-wall echo based
Techniques• Localised (e.g. Type IV) creep- ultrasonic back- scatter
techniques • inspection at room temperatures only
Ultrasonics
a. Wave velocityb. Birefringencec. Wave attenuationd. Backscatter
Wave velocity
• based on the variations in physical properties
• longitudinal wave velocity decreases monotonically with increasing porosity
• microstructural changes play a greater role than void formation as in 9Cr-1Mo steel
Birefringence
• based on the velocity difference of two shear waves polarised in perpendicular directions
• Underlying principlecavities and microcracks tend to be preferentially
oriented along the direction of main stress• Limitationcannot be applied at early stages of creep
Backscatter
• Due to partial reflection of the ultrasonic wave • Spatial or material noise- Frequencies received
between the excitation pulse and the arrival of the first back-wall echo
• Source of error -Material noise due toa. Inhomogeneities at early stages b. Coarsening of precipitates due to thermal ageing
welded 9Cr-1Mo steel specimen
A greater backscatter -In the final stages of creep life.
Lower frequency part of the signal become more sensitive to the scattering as creep damage proceeds
Fig. below shows• Backscatter at various stages of creep life at 650 C
and 66 MPa. • The parameter plotted in colour scale is the integral
of the frequency spectrum of the backscattered signal between 4 and 20 MHz
Most damaged area-A, B
Positron annihilation
The positron -the antiparticle counterpart of the electron
Trap for positrons-vacancies, dislocations and microvoids
Parameters to indicate the presence of open-volume defects
• increase in life time• decrease in doppler shift• decrease in angular deviation
limitations
• Can inspect only a very thin subsurface layer. • In fatigue damage, parameters respond upto 10% of
fatigue life mainly. • decrease in positron lifetime in some steels
Magnetic methods
ferromagnetic materials mainly. • Barkhausen Emission/noise• Magneto-acoustic emission
magnetic properties change largely due to• coldwork,• localised differences in the composition of the alloy,
and• the presence of residual stresses
13-CrMoV-44 steel
• Maximum value of Barkhausen emission (noise)as a function of creep life increased initially and then decreased
Reason• Initial coarsening of grains and precipitates helps
domain wall movement• Formation of cavities restrict the movement of
magnetic domain walls.
Eddy current
• Eddy current depends on microstructure, surface roughness, hardness, local permeability variations or differences in chemical composition
• Difficult to distinguish between changes in the signal due to creep damage and those deriving from other sources.
Austenitic chrome steel boiler tubes
• Magnetic oxide scale- on the outer surface
• A magnetic ferrite phase- in the underlying base metal
• Ferrite phase concentrations- proportional to creep damage
Potential drop technique
• In creep tests, the potential drop decreases first, then stabilises and finally increases as the specimen approaches rupture
• Potential drop ratio (b/w the gauge area and an undamaged area)
• Volumetric damage type
Hardness • In Cr–Mo steelsA linear decrease in hardness with creep life, and a
further drop at rupture• LimitationsSensitive to local microstructural variations
X-ray diffraction
• To measure local strains • peak diffraction direction- determines the absolute
elastic strain• the widening of the diffraction peak- the lack of
periodicity in the lattice.• limitationPenetration depth (10 µm) in most metals
Discussion and conclusions
• ultrasonic measurements are affected by material noise in the early stages, but can give satisfactory results once microcracks start developing.
• Barkhausen or magneto-acoustic emission, have more potential for localised creep detection in ferromagnetic materials
• Potential drop techniques may be able to detect creep damage at earlier stages than other conventional methods.
References
• G. Sposito, C. Ward, P.Cawley, P.B.Nagy, C.Scruby, A review of non-destructive techniques for the detection of creep damage in power plant steels, NDT&E International vol. 43, pp. 555–567 (2010)
• A. Aghajani, Ch. Somsen, G. Eggeler; On the effect of long-term creep on the microstructure of a 12% chromium tempered martensite ferritic steel Acta Materialia 57 (2009) 5093–5106
Thank you