2 s -os- 1995
VTT MANUFACTURING TECHNOLOGY
.
GUIDELINES FOR EVALUATING
IN-SERVICE CREEP DAMAGE
VALB105
Nordtest Project 1183-94
Final Report
Pertti Auerkari
Jorma Salonen
Kjeld Borggreen *)
VIT Manufacturing Technology
P.O. Box 1704, FIN-02044 VIT, Finland
*l FORCE Institute, Denmark
Espoo August 1995
GUIDELINES FOR EVALUATING
IN-SERVICE CREEP DAMAGE
VALB105
Nordtest Project 1183-94
Final Report
Pertti Auerkari
lonna Salonen
Kjeld Borggreen*)
VTI Manufacturing Technology
P.O. Box 1704, FIN-02044 VTI, Finland
*) FORCE Institute, Denmark
Espoo August 1995
2
ABSTRACT
To indicate safe life or re-evaluation periods for high-temperature components, assessment based
on measured creep cavitation damage is used worldwide. To interpret the observed damage
unambiguously requires evaluation of the extent of damage on a unique, clearly defined and
accepted scale. The presently available European damage scales in the guidelines NORDTEST NT
TR 170 and VGB-TW 507 provide reference micrographs for a range of creep damage in the most
common high-temperature steels. However, these guidelines differ in their stated and implicit
definitions of a given class (extent) of damage, although both guidelines are nominally based on
the same original Neubauer scale of creep damage.
This work provides a comparison of the guidelines NORDTEST NT TR 170 and VGB-TW 507
guidelines, as well as an attempt to unify the European creep damage scales. The proposed unified
definitions of damage are based on the quantified damage scales of NORDTEST NT TR 170 with
limited revisions and simplifications. The new definitions for damage classification also aim to
correct inconsistencies in the present guidelines, with minimum modifications to the actual
example micrographs. The main use of the definitions is foreseen in constructing or extending
compilations of reference micrographs on in-service damage, and in calibrating image analysis of
such damage.
3
FOREWORD
This is the final report of the Nordtest Project Nr 1183-94, "Riktlinjer for harmoniserad skadebedomning av kraftverkskomponenter" (Guidelines for unified damage assessment of power plant
components), performed by the VTT Manufacturing Technology, Finland, and the FORCE
Institute, Denmark. The authors gratefully acknowledge financial support from NORDTEST, as
well as technical support from the Nordic Reference Group by Jan Sturesund (Swedish Institute for
Metals Research), Knud-Erik Poulsen (Danish Technological Institute) and Liisa Muurinen
(Helsinki Energy, Finland).
4
CONTENTS
Page
ABSTRACT
2
FOREWORD
3
1 INTRODUCTION
5
2 QUANTITATIVE DEFINITIONS FOR CREEP DAMAGE
5
2.1 THE DEFINITIONS OF DAMAGE IN NT TR 170
5
2.2 THE DEFINITIONS OF DAMAGE IN VGB TW-507
8
3 COMPARISON OF THE NORDTEST AND VGB DAMAGE SCALES
9
4 UNIFIED SCALE OF DAMAGE
11
4.1 PRINCIPLES FOR DEFINITIONS
11
4.2 PROPOSED MODIFIED SCALE OF DAMAGE
12
5 SUMMARY
14
REFERENCES
14
5
1 INTRODUCTION
Creep, by definition, is time (or rate) dependent deformation. Since in real materials and structures
deformation cannot extend ad infinitum without adverse effects to integrity, creep is also finally
intervened by processes that ultimately lead to failure. The final creep failure is frequently preceded
by a sequence of accumulating creep damage, manifesting itself as emerging, growing and
coalescing cavities and cracks. It can also happen that with a suitable combination of materials,
service conditions and structural (and loading) geometry the failure occurs without such damage,
through e.g. loss of stability or fully ductile rupture. However, particularly for thick-wall
components and welded steel structures operating at high temperatures for prolonged periods,
creep cavitation is the usual sign of growing damage before creep failure. Since the sequence of
accumulating creep damage is often roughly similar for a relatively large variety of materials and
service conditions, and because the susceptible components can be important for both economy
and safety reasons, creep cavitation damage - measured non-destructively in replica inspections is widely used to indicate safe life. Consequently, metallographic assessment of creep cavitation is
one of the most important tools for life and condition assessment. This is indicated by the inherent
presence of such damage assessment in many existing and proposed procedures for life and
condition assessment (e.g. VGB R509L 1984; NORDTEST NT NDT 010 1991; Auerkari 1993).
However, before useful results can be expected from utilising the measured creep damage, the
interpretation of the observations requires that the degree or extent of damage is determined on a
well defined scale. In the early days of 1970's the damage classes were either calibrated by
comparing to example replicas or example micrographs, typically internally within one company.
This obviously had the drawback that without common reference the de facto damage scales varied
considerably between companies evaluating replica inspections. Worse still, as the original internal
references were lost even temporarily, the interpretation of damage varied in time even for a single
individual. To alleviate such problems, two European guidelines NORDTEST NT TR 170 (1992)
and VGB-TW 507 (1992) have been published to provide examples of damage by reference micrographs from most common high-temperature steel types. Since no unified international definitions
of the damage classes exist, these two guidelines are not identical but differ in the extent of damage
for a given class, although both guidelines are ultimately based on the same original though loosely
defined Neubauer damage scale (Arnswald et al 1979; Neubauer & Wedel 1984). To the authors'
knowledge, these two documents appear to be the only published guidelines of their kind.
A comparison of these guidelines is given below, and a unified creep damage scale is proposed
through rules of correspondence in the appearance of damage. Furthermore, a revised set of
damage definitions for each damage class is proposed to extend, unify and simplify the definitions
of NORDTEST NT TR 170.
2 QUANTITATIVE DEFINITIONS FOR CREEP DAMAGE
2.1 THE DEFINITIONS OF DAMAGE IN NORDTEST NT TR 170
The reference micrographs of NORDTEST NT TR 170 have been compiled for cavitation type of
creep damage of the common low alloy steels 14MoV63 (0.5Cr-0.5Mo-0.25V), 13CrMo44 (1Cr0.5Mo) and IOCrMo910 (2.25Cr-1Mo). The micrographs have been selected from a large number
6
of replicas, all taken from actual plant components after operating for more than 80 000 service
hours in the creep regime, and mostly from various regions of welds. The light optical (and mostly
also scanning electron) micrographs show the amount of damage in each damage class within the
field of view at magnifications that are commonly used in the replica inspections (200, 500 and
IOOOx). It is solely the cavitation damage which is of concern in these reference micrographs,
although microstructural changes typical for long term service can be seen in the examples. The
quantified definitions of damage given by NORDTEST NT TR 170 are shown in Table 1.
Table I. Classification of creep damage according to NORDTEST NT TR 170 ( 1992).
Damage
class
Damage type
Definition of damage
0/1
None/no cavitation
< 100 cavities/mm2
2
Cavities with no apparent directional alignment
2.3
Isolated or scattered
cavitation I)
- small amount
- medium amount
-abundant
3
Aligned/oriented cavitation 2l
Apparently aligned cavity formations, so that
3.3
- small amount
- medium amount
-abundant
DJl
> 100/mm
> 100/mm
> 100/mm
4
Microcracks7l
Cracks with a length 20 < L < I000 11m
4.1
4.2
4.3
- small amount
- medium amount
- large/abundant
Nc ~l
< 20 cracks/mm2
20 - I 00 cracks/mm 2
> 100 cracks/mm2
5
Macrocracks7l
2.1
2.2
N = 100 - 300 cavities!mm2
300 < N < I000 cavities!mm2
N > 1000 cavities/mm2
Type A
3.1
3.2
L14)
>50 11m
>50 11m
>50 11m
L2S)
< 1001-lm
100- 3001lm
> 300 11m
and
or
or
TypeB
N6l
100- 500/mm2
500 - 3000/mm2
>3000/mm2
Lmax9 l
< 10011m
100- 300 llm
> 300 llm
Cracks detectable with conventional NDT,
generally Lmax > I mm9l
NOTES to Table I:
I)
2)
3)
4)
5)
6)
7)
8)
9)
The limits of maximum cavity density N are here given for clarification; it is usually faster in practical replica evaluation
to use the appearance of actual reference micrographs.
Note that
- as a rule it is faster to evaluate the class of damage by comparison to the actual reference micrographs;
- type A damage refers to cases with relatively little damage outside the main lines along which the cavities occur;
- type B damage refers to cases with relatively widely distributed cavity formations;
- at low levels of aligned cavitation (class 3.1 lower limit) types A and B may be inseparable;
D =cavity line density along the (grain boundary) line under consideration.
L I = length of a continuous line of cavities fulfilling the line density requirement.
L2 =total summed length of continuous cavity lines fulfilling the line density requirement in a micrograph or image with
an area of 100 cm2 at 500x magnification.
N =area density of cavities (cavities!mm2 of sample surface).
In addition of defining the extent of cracking according to the appropriate classes it is recommended that
the general appearance of damage outside the cracks is indicated: e.g. 4.211 or 4.3/3.38.
Nc = maximum area density (microcracks/mm2).
Lmax = maximum length of the cracks; two cracks are counted as one, if their distance is less that the length of the
shorter crack; the total length =combined cracks and ligaments projected in the main crack direction.
7
These definitions have been defined originally for calibration purposes only, since for any practical
assessment of creep damage it is generally much faster to simply refer to the appearance of damage
in the reference micrographs. The creep damage shown in the reference micrographs is in
accordance with the cavitation damage classes defined in NORDTEST NT NOT 0 I 0, Remanent
Lifetime Assessment of High Temperature Components in Power Plants by Means of Replica
Inspection ( 1991 ). This scale is compatible with the classical discrete (0 to 5) Neubauer scale for
cavitation damage, but also includes a three-level subscale for damage classes 2 to 4 to indicate the
extent of damage within a major class. The definitions of damage have been defined with the intent
to leave intact the present limits of the major damage classes. The example micrographs have in
general been taken from various parts of welds, including the base material (BM), heat affected
zone (HAZ) and weld metal (WM), so that the designation of the micrograph location follows the
method described in NORDTEST NT NOT 010.
It is not the purpose of the NORDTEST reference micrographs in any way to fix the interpretation
of damage as consequences regarding subsequent action, when a given damage class has been
observed. Advise in such interpretation can be sought from the rules of NORDTEST NT NOT 0 I0.
The reference micrographs are only intended to show the amount of damage in each damage class
within the field of view.
To deal with damage gradients and locally variable damage, especially within welds, two
additional features have been added to the NORDTEST damage scale. Particularly for class 3, the
damage with little cavitation outside the main lines or stringers of aligned (oriented) cavity
formations is designated type 3A, and more diffuse damage is called type 3B. Secondly,
particularly for damage classes 4 and 5 it is recommended that the damage type characterizing the
damage outside the maximum damage or cracks should also be described: eg 4.l/3.3B where the
first part (4.1 or small amount of microcracks) refers to the characteristic maximum damage,
followed by a description for adjacent damage (3.3B, abundant oriented but diffuse cavitation).
Nevertheless the scale retains compatibility with the original classes of NORDTEST NT NOT OIO
for characteristic (maximum) damage and its designation.
For all damage classes a tentative numerical definition is given, to aid calibration of automatic (if
available) damage ao;sessment. Also for the purpose of the present work this is fortunate, because it
allows in principle for a direct comparison with other scales including that of VGB. These
numerical definitions for damage are shown in Table I.
The definitions of NORDTEST NT TR I70 appear fairly complicated, and need revision. Also,
they follow the classical definitions of cavitation damage, which particularly for class 3 damage
(orientated cavitation) is not defined unambiguously. This is because e.g. a equibiaxial tension or a
suitable combination of unbalanced biaxial tension and material strength will not produce clear
orientation in cavitation but escalating non-orientated damage until extensive cracking. This in
principle disrupts the original idea of damage classes growing in order of growing true damage or
loss in remanent life. The interpretation for class 3 damage has turned out to be particularly fuzzy
for welds, where additional apparent orientation in the cavity formations is induced by relatively
natTow zones of weaker material. Although NORDTEST NT TR 170 does provide example
micrographs from welded components, the coverage is not complete and the definitions of Table I
do not fully reflect the difficulties encountered in assessing cavitation in welds.
8
2.2 THE DEFINITIONS OF DAMAGE IN VGB TW-507
The German guideline VGB TW-507 does not state explicitly in detail how each class of damage is
defined but refers only to the Neubauer division of damage classes, as shown in Table 2.
Table 2. The description of Neubauer damage classes as used in VGB TW-507 ( 1992).
Assessment class
Description of damage
0
New material, no thermal exposure
I
Creep/thermal exposure, no cavities
2a
2b
Isolated cavities
Numerous cavities without preferred orientation
3a
3b
Numerous cavities with directional orientation
Chains of cavities or grain boundary separations
4
Microcracks
5
Macroscopic cracks
These types of definitions require a full set of example micrographs to define the damage classes in
practice, as has been done in VGB TW-507 for a good number of materials. However, there are
still problems in unambiguous interpretation, because
-definition for class 3 or orientated cavitation (as NT TR 170) does not recognise expected results
from e.g. equibiaxial tension in base material;
- no example micrographs have been given of welds; and
-there are questions on internal consistency between different materials (e.g. low alloy steels vs.
X20CrMoV121 and X8CrNiNb1613).
However, the VGB TW-507 guideline shows a full set of good quality micrographs on as-new
material and creep damaged material for a wider range of materials than NT TR 170. It is also
being used or referred to by a relatively large group of power plant maintenance service
professionals in Germany and neighbouring countries, and probably will have an important impact
in setting much of the internal (company-specific) interpretation on creep damage. Because of
existing problems in such interpretation and because of potential conflict between differing
practical definitions of damage, the damage scales of the guidelines VGB TW-507 and NT TR 170
are compared below.
9
3 COMPARISON OF THE NORDTEST AND VGB DAMAGE SCALES
The major damage class divisions (0 to 5) are in principle the same for both NORDTEST and
VGB classification. This is because both scales are based on the classical Neubauer (RWTUV)
damage scale, ranging from no damage (0) and no cavitation (I) through scattered cavitation (2),
orientated cavity formation (3) and microcracking (4) to macroscopic creep cracks (5). A general
comparison between the present NORDTEST and VGB damage micrograph collectives is shown
in Table 3.
Also the practice of presenting the micrographs mainly at a magnification of 200, 500 or 1000
times is the same in both NORDTEST and VGB guidelines. The most obvious differences between
the scales are related to the actual definitions of the subdivisions. In the NORDTEST scale, the
major classes have been divided into three subclasses particularly for isolated and orientated
cavitation (classes 2 and 3) as well as for microcracks (class 4). In the VGB classification, only two
subclasses are applied for classes 2 and 3.
Using the quantified damage definitions of the Table 1 for the VGB reference micrographs, more
detailed rules of correspondence can be designed. First, applying criteria of cavity density criteria
to all of the VGB example micrographs results in a comparison of Fig 1 for damage classes 2 and
3. A similar comparison for the NORDTEST micrographs is shown in Fig 2. On the other hand,
the VGB micrographs can be classified using the NORDTEST micrographs directly, and vice
versa. The results from such an exercise by the present authors are shown in Tables 4 and 5.
Table 2. A general comparison of NORDTEST and VGB reference micrographs.
Document
Materials
included
Micrographs
from
NORDTEST
TR 170
14MoV63
13CrMo44
10CrMo910
Replicas
from plant
VGB-TW
507
14MoV63
13CrMo44
10CrMo910
X20CrMoVI21
X8CrNiNbl613
Specimens
from plant &
laboratory
Dnmnge
classes
included
Welds
considered
Newlunusual microstructures
Numerical
definitions
for damage
Damage
distribution
considered
2 to 4/5
Yes
No
Ye
Yes
0 lo4/5
No
Yes
No
No
10
10000
I
I
"
/
/
/
T
1
1-
.L 3b
! .0
•
I
/"
/
/
2b
/
,-
I
I
/
~
2a
/
-
/
0
/
0
/
0
•
•
/
/
/
/
//
I
10
10
-
13CrMo44
14MoV63
10CrMo910
X20CrMoV121
XBCrNiNb1613
I
10000
100
1000
Proposed range of cavity density (1/mm2)
in the VG8 damage classes 2a, 2b and 3b
Fig 1. The estimated cavity density of the VGB micrographs in comparison with the revised
cavity density criteria for the damage classes 2 and 3.
.
,.,
~ 10000
/
(')
/
~
~
:::E--r-0---3:
(I)
/
N'Cii
~~
Ea2.3
.... ..-- 1000
..~~
-~ 2a.
/
~ e
I
~~
~ 'E
15 Cll
~~
.~
a//
/
/
2.2
I
/.
2.1
/
100
/
/
/
-g Cll
/
/
/
::J
~~
3.38
:::&Lo--.:a:
o 13CrMo44
o 14MoV63
o 10CrMo910
/
/
10~----------L-----------~--------~
10
100
1000
Proposed range of cavity density (1/mm~
in the NT damage classes 2. 1. 2.2. 2.3 and 3.38
10000
Fig 2. The estimated cavity density in the example micrographs of the NORDTEST guidelines
in comparison with the same criteria as in Fig I.
II
Tahle 4. 1'l1e damage shown in the example micrographs of the VGB guideline, as class(fied
according to the damage rules of NORDTEST NT TR 170. Note also that in some cases the VGB
guideline shows several examples, so that the expressed range implies a range shm-vn in the
original VGB example micrographs.
2a
2.1
2.1
2.1
2.1
2.2
Material
l3CrMo44
lOCrMo910
14MoV63
X20CrMo V 121
X8CrNiNb 1613
The dama~e class in the VGB guideline TW-507
2b
3a
3b
4
2.2
3.18-3.28 3.2B- 3.3B
4.3/3.3B
2.2
3.18- 3.2B
3.3B
2.2-2.3
3.2B
3.2A- 3.3B
4.3/3.3B
2.3
3.2B- 3.38 3.2A- 3.2B
4.2/2.3
3.2B- 3.3B 4.2/3.2B-3.3B
2.3
3.2B
5
5
5
5
5
Table 5. The damage shown in the example micrographs of the NORDTEST guideline, as
classified according to the micrograph examples ofVGB TW-507..
Material
l3CrMo44
10CrMo910
l4MoV63
2.1
2a
2a
-
The damage class in the NORDTEST guideline NT TR 170
2.2
2.3
3.1B
3.2B
3.3B
4.1
4.2
4.3
3b
4
2b
3a
2b
3b
3b- 4
4
2b
3a
2b
3b
4
4
2b
-
5
5
4 UNIFIED SCALE OF DAMAGE
4.1 PRINCIPLES FOR DEFINITIONS
The results from Figs l and 2 as well as from Tables 4 and 5 suggest that there are a few
inconsistencies in the micrographs of the guidelines. Nevertheless, a general picture appears to
emerge for correspondence of the guidelines, and hence it is also possible to define damage classes
quite generally from the actual micrographs of the guidelines. To revise and generalise the
definitions of damage, the following principles have been taken:
(i) There should be a minimum possible change in the present sets of example micrographs of the
guidelines NT TR 170 and VGB TW-507. This is important because the bulk of present databanks on creep damage are referable to these guidelines and therefore any changes would
induce bias in their evaluation in terms of e.g. optimal timing of inspections.
(ii) Damage is taken to extend logarithmically, as cavitation and cracking can be generally
modelled with exponential or power law growth in time, strain, loading, or characteristic
dimensions of damage. Here a factor of 4 to 5 is used to separate consequtive damage classes
where applicable.
(iii) The class 2 (isolated cavitation) damage is defined through cavity density criteria. This has in
principle the disadvantage that the equivalent life fraction depends on grain size but is assumed
to he reasonably well balanced by structural similarities. This implies e.g. that the range of
12
grain size variation is limited in typical components with material and thickness ranges of
interest, and also limited within similar locations in welds.
(iv) Inconsistent or ambiguous definitions for class 3 damage (orientated cavitation) is defined
through a revised set of criteria, applicable also in biaxial stress states.
(v) The difference between microcracks and macrocracks is defined as 2 mm in defect length, or
estimated 50% probability of detecting surface creep cracks in surface (MT) inspections;
(vi) The definitions of damage classes aim for simplicity, and therefore e.g. the crack density
criteria for microcracks have been dropped from the definitions of NT TR 170.
(vii) To help in unification of damage scales, it is proposed that the NORDTEST type of
classification is reduced to two-step subscales within a major damage class, but retaining
additional features of the NORDTEST designation system (such as those defining damage in
welds and damage distribution) that do not conflict with the VGB designation.
4.2 PROPOSED MODIFIED SCALE OF DAMAGE
The general correspondence between the NORDTEST and VGB damage scales is suggested as
Table 6. Based on the comparisons of Figs 1 and 2 as well as Table 6, a new set of cavity density
criteria is proposed for damage classes 2 and 3 in Table 7. These criteria are designed to be
compatible with both the existing NORDTEST and VGB scales with minimum changes in the
example micrographs of NORDTEST NT TR 170 and VGB-TW 507. These rules, together with
revised rules for other damage classes, are compiled as Table 8 below. Note that in the new rules
the two-step subclass system has been adopted.
Table 6. The correspondence of the creep damage classes of VGB and NORDTEST guidelines.
Note also that the NORDTEST system includes welds and an additional division according to the
damage location within a weldment, as well as for damage level outside the maximum damage (to
characterise local damage distribution as well as probable causes).
Damage scale
No damage
No caviration ,
with exposure
NTTR 170
0
I
Isolated cavitation
Oriented caviration
Microcracks
Macrocracks
2.1
3.1 13.2 13.3
4.1 14.2 14.3
5
4
5
2.2 12.3
B
VGB-1W507
0
I
2a
2b
B
3a
B
: 3b
Table 7. The proposed new cavity density criteria ( llmm2 ) for the damage classes 2 and 3.
NORDTEST present
NORDTEST proposed
VGB proposed
class 2.1:
100- 300
class 2a:
100-400
class 2a:
100-400
class 2.2:
300- 1000
cla<;s 2b:
>400
class 2b:
>400
class 2.3:
> 1000
cla<;s 3.3B:
> 3000
class 3bC:
> 1600
class 3b:
> 1600
13
Table 8. Proposed rules for unified definitions of creep damage
Damage class
Damage type
Dermition of damage
Notes
0
No damage (as-new material)
N::;; 100 cavilies/mm2 with a size~ 0.5 11m
I)
I
No cavitation (thennal exposure)
N::;; 100 cavities/mm2 with a size~ 0.5J.lm
I)
2
Isolated cavitation
Cavities without chainlike fonnation or gb separation
2a
- small amount
100::;; N::;; 400 cavities/mm2
2b
-extensive
N > 400 cavilies/mm2
Orientated cavitation
3
Cavity chains I gb separations (max 3 grains or I00 J.lm)
TypeC
TypeK
3a
- small amount
50 ::;; r.,_ ::;; 200 J.lm
400::;; N::;; 1600 cavities/mm2
3b
-extensive
4.....>200J1m
N > 1600 cavities/mm2
Microcracks
Cracks with (3 x grain size or I00 J.lm) < L.n.. ::;; 2 mm
4a
-small
Max( 3 x grain size or 100 J.lm) < L.nox::;; 400 J.lm
4b
-extensive
4
5
Macrocracks
I)
I) 2) 3)
3) 4) 5)
400 J.lm < L.n.. :s;; 2 mm
Cracks detectable in conventional NDT
4) 5)
L.n..>2mm
I)
2)
3)
•
4)
5)
N =area density of cavities on actual sample surface (cavities/mm2)
Note that
- type K damage refers to cases with little damage outside the main lines of damage;
- type C damage refers to cases with distributed cavity fonnations;
- at low levels of orientated cavitation (class 3a lower limit) types K and C may be inseparable;
- cavity chain = fonnation with several cavities on a grain boundary extending to adjacent grains
- gb = grain boundary
L.:nuJt = total summed maximum length of continuous cavity lines, each at least 50 J.lm in length and fulfilling the line
density requirement of at least I00 cavities/mm of the grain boundary line under consideration, in an image with an area
of 100 cm2 at 500x magnification.
It is recommended that also the damage outside the cracks is indicated: e.g. 4b I I; 4.b 13bC or 5 14a 13aK
L.n.. = maximum length of the cracks; two cracks are counted as one, if their distance is less that the length of the shorter
crack; the total length = combined cracks and ligaments projected in the main crack direction.
14
5 SUMMARY
•
To indicate safe life for high-temperature components of power plants, assessment based on
indicated creep cavitation damage is widely used worldwide. To interpret the observed damage
unambiguously requires evaluation of the extent of damage on a unique, clearly defined and
accepted scale. The presently available European damage scales in the guidelines NORDTEST NT
TR 170 and VGB-TW 507 provide reference micrographs of damage in the most common hightemperature steel types. However, these guidelines differ in their stated and implicit definitions of
damage for a given class (extent) of damage, although both guidelines are nominally bac;ed on the
same original Neubauer scale of creep damage. Fortunately the differences do not appear to be very
extensive, and the existing example micrographs can be used as a bac;is of more unified approach
without losing the valuable potential of present long-term inspection experience.
A comparison is made of the guidelines NORDTEST NT TR 170 and VGB-TW 507 for the
purpose of damage definitions that unify the European creep damage scales through simplifying
modifications and rules of correspondence. The correspondence is based on the quantified damage
scales of NORDTEST NT TR 170 with revisions, and on the principle of minimum required
changes in the existing guidelines. The proposed definitions of creep damage classification also
aim to correct the inconsistencies in the present guidelines. The main use of the definitions is
foreseen in constructing or extending compilations of reference micrographs on in-service damage,
and in calibrating image analysis for such damage.
REFERENCES
Amswald, W., Blum, R., Neubauer, B. & Poulsen, K.E., 1979. VGB Kraftwerkstechnik 59(7), p.
173-187.
Auerkari, P., 1993. Guidelines for inspection criteria of hot pipework. SPRINT SP 249: Technical
Report I. Technical Research Centre of Finland, Metals laboratory, Report VTT-MET C-224.
Espoo, 13 p.
Auerkari, P., Borggreen, K. & Salonen, J., 1992. Reference micrographs for evaluation of creep
damage in replica inspections. NORDTEST NT Technical Report 170. 41 p. (Available from
NORDTEST, P.O.Box 111,02101 Espoo, Finland, fax +358-0-455 4272).
Neubauer, B. & Wedel, U., 1984. NDT: Replication avoids unnecessary replacement of power
plant components. Power Engineering, May, p. 44.
•
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