Use of Replication and Portable Hardness Testing for High Temperature Plant Integrity and Life Assessment

Seminar Power & Process Plant Issues
Lahore, Pakistan: May 2011

Use of Replication and Portable
Hardness Testing for High
Temperature Plant Integrity and Life
Assessment
M Hussain, ETD Pakistan
1
mhussain@etd1.co.uk, +92 345 812 4575

Overview
 Material degradation assessment is one of the
most important fields of life assessment
 It allows to identify, localize and quantify the
damage mechanism impact in a certain
component
 It is used worldwide specially in the power
plants (conventional and CCGTs),
petrochemical and process industries

European Technology Development Ltd.

2

Introduction
 Life assessment based on calculational
procedures is conservative (use minimum
parent material properties, etc)
 Difficult to account for welds
 Metallographic methods assess the actual
material condition of the component/weld
 Metallographic replication
 Hardness assessment


Life Limiting Areas
 Welds are frequently the locations for high
temperature plant failures
 HAZ with differing properties
 Residual stresses
 Fabrication defects
 Welds are often located at stress concentrations
 Welds need detailed evaluation of defects and
properties
 NDE and possibly materials testing carried out


Microstructure & Damage
 Microstructure degrades
& creep cavitation
damage develops as the
consumed life fraction
increases
 Can see these changes if
polish and etch at site
 Hard to use microscope
in power plant
 Need flat surface - Not
normal in plant
 Hence replication


Creep Damage Accumulation
Isolated and oriented cavities,
linking to form micro-cracks, then
macro-cracks


Precipitate Coarsening
Progressive coarsening of
precipitates in a low alloy steel



8

Metallographic Replication
 Must be targeted at the areas most likely to show
creep damage
 Normally welds and bends in pipework
 High temperatures; end-loads on pipework; known
problems
 Apply across Weld, HAZ and Parent material
 Examine replicas at site optically before re-
lagging
 Further examination in laboratory


Replication Procedure
1. Area is grinded to high metallographic standard
2. Then polished and etched several times to remove all
traces of cold work from polishing
3. Finally surface wetted with acetone and acetate foil
applied
4. Acetone softens foil and capillary action forces the
foil to conform to the etched structure
5. After drying, the foil is removed with image of the
etched surface impressed on foil surface


1) Rough component surface

2) Component surface polished

3) Component etched to reveal microstructure
(Steps 2 and 3 repeated ~4 times)

Film 4) Film applied to surface

Film 5) Film conforms to surface as solvent dries

Film
6) Film stripped from surface with record of
microstructure



Polishing of a P91 reducer Replication of etched cross
welded to a CrMoV HPHT weld surface
steam turbine valve (the arrows show the acetate foil)


12

Replica Positions
 Pipework
 4 points round welds
 Intrados, extrados of bends
 Intersections - saddles and crotch
positions
 Tubing
 Hottest tubes - superheater, reheater
outlets
 Swollen tubes to quantify remaining life
 Headers
 Antler/stub tubes – minimum grinding
 Plain barrel if any signs of distress
 Nozzles
 Turbines
 Rotor bores


Portable grinding and polishing
equipment
Grinding and
polishing head

Control
unit


14

Sentencing of Replicas –
Quantifying the Damage
 Several means of quantifying
damage:
 ‘A’ parameter depends on
counting the number of
cavitated grain boundaries
 Gives numerical answer but
time-consuming
 Normal use life based on
damage classification schemes
 Action may be advised but
depends on
 History
 Future outage schedules
 Operating practice
 Best assessed individually


Cavitation Damage Classification
A: Clear

B: Isolated cavitation

C: Orientated cavitation

D: Microcracking <2mm
NDE non-detectable

E: Macrocrack >2mm
NDE detectable
After Neubauer


17

Introduction to Hardness
 Hardness is used in power,
petrochemical and process industry for:
 quality control
 life assessment
 It is defined as the ability of a material
to resist permanent indentation or
deformation when in contact with an
indenter under load

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Introduction to Hardness
 Basically a hardness test consists of
pressing an indenter of known geometry
and mechanical properties into the test
material


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Hardness – Life relationship
 Hardness indentation technique is one
of the oldest testing methods applied to
analyse the materials properties
 It gives the hardenability of a certain
component
 Conversion charts can convert hardness
values to tensile strain and consequent
probability to type IV damage.

20

Hardness changes and testing
 Several investigators have developed
hardness models to calculate the
remaining life of piping and tubing
components.
 Models based on hardness have been
developed for low alloy steels and
modern steels based on creep data.


21

Portable hardness testing equipment

MIC 10
control unit

Vickers
5kgf Probe


22

Hardness test for superheater header

Shape of indent of on-site hardness measurement
Robertson D. et al; ETD Lifing Procedure


23


24

Main steam line

Material Properties

Material of pipe A 335 P11
Material of fittings A 234 WP11
Pipe Geometry
Outer diameter, mm 450
Wall thickness, mm 40
Operating Conditions
Pressure, Bar 95
Temperature, C 530
Case Study from Recent ETD Work in Europe
Service time, hours* 180,071


25

Main steam line (contd.)

 Base Metal microstructures consisted of
ferrite and bainite (or ferrite and pearlite).
 HAZ microstructure was bainitic and/or
martensitic, the Weld Metal microstructure
was martensitic.
 Bainite and pearlite microstructure showed
some degradation due to long-term exposure
at elevated temperature (i.e. spheroidisation)

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BM HAZ WM

Pipe thermally Moderately Martensite; 166 HB
degraded, partially degraded bainite, (400x)
spheroidized ferrite partly spheroidized;
/ bainite and 130HB 151 HB (400x)
(400x)

27


 Using optical microscopy, one of the
valves, showed intergranular crack of
~2mm length in the coarse-grained
region of the HAZ (CGHAZ) on the
forging side of the joint (see next slide)
 The morphology and the crack location
were indicative of stress relief damage


28


Stress relief cracking at the forging side (100X)


29


 Hardness testing was carried out at-site for
each of the replica locations.
 The hardness values of Base Metal, Weld
Metal and HAZ were within the expected
ranges for 1¼Cr-½Mo steel after long-term,
high-temperature exposure.
 Some welds exhibited hardness differentials
between the weld metal/HAZ and the base
metal of ~60-80HB.

30


Hardness (HB)
Position Weld HAZ BM
12 O’Clock 203 204 126
4 O’Clock 202 195 128
Valve Pipe
8 O’Clock 204 198 132
Avg. Hardness 203 199 129
12 O’Clock 208 198 135
4 O’Clock 204 201 136
Valve Forging Side 8 O’Clock 206 195 140
Avg. Hardness 206 198 137

The hardness test results for the base materials, HAZ and
weld metals were within the expected range for P11 steel
after long-term operation at elevated temperature.

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 The creep life fraction consumed for the components
exhibiting isolated creep cavities is estimated to be
50% - as the worst case scenario. This means that
the remaining creep life of these components is at
least 190,000 hours.
 The components examined showed limited
microstructural degradation which was consistent with
the plant operating conditions and service time, and
the hardness levels at the examined locations were
within the expected range for P11 steel after long-
term operation at elevated temperature.

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 Due to the presence of the micro crack,
it was recommended that the valve
should be re-inspected using
metallographic replication and
appropriate NDE (MT and UT flaw
detection) after a further 10,000 hours
service.


33

Advantages of replication & hardness

 The technique can be used easily on-site and it is non-
destructive.
 Good resolution of microstructural constituents if surface is
well prepared. This technique has a good adaptation on flat
and curved surfaces
 Can be used to monitor the evolution of microstructural
changes and it is useful for assessing creep, fatigue,
corrosion damage in elevated-temperature components.
 Can be applied to conventional materials and also to the
steel alloys used in turbines and boilers.
 Can be used to complement other non-destructive
techniques such as ultrasonic testing.

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Limitations of replication & hardness
 The technique only analyzes the microstructure of the
outer surface of a material/component.
 In many cases the surface microstructure can be
different from the microstructure found in the interior
of the component
 The replica only reveals the topographic features at
the surface; therefore it is impossible to analyse the
chemical composition of the elements
 Contamination may be a problem in harsh or dusty
environments
 Precision is required to operate the hardness probe


35

Conclusion
 It has been proved that replication and
hardness can be efficiently used to
perform life assessment
 Sampling removal and analysis are
relatively simply and not time
consumable
 Cost of performing these tests is lower
in comparison with other non-
destructive techniques

36

Thank You very much for your
attention.

Questions?


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Use of Replication and Portable Hardness Testing for High Temperature Plant Integrity and Life Assessment

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