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