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Creep Life of Pipe Girth Weld Repairs OMMI (Vol. 1, Issue 3) Dec. 2002 .

CREEP LIFE ASSESSMENT OF PIPE GIRTH WELD REPAIRS WITH RECOMMENDATIONS Jan Storesund* and Lars Å. Samuelson** Det Norske Veritas, P. O. Box 30234, SE-104 25 Stockholm, Sweden

PhD, EWE Jan Storesund Consultant Working areas, experience: - Power plant condition and life management - High temperature materials - Welding in high temperature materials - Damage and failure analyses

Prof., PhD Lars Å. Samuelson Consultant Working areas, experience: - Structural analysis in general - Structural stability - High temperature damage and fracture mechanics

Abstract It is an established fact that creep damage in high temperature piping and pressure vessels predominantly occurs in, or in the vicinity of, weldments. Several investigations into the problem have been published but due to the complex nature, �accurate� solutions are not possible. The present investigation includes a literature search for weld repairs world wide and an assemblage of plant weld repair experience in Denmark and Sweden. In addition, metallography and hardness measurements were carried out in retired components containing repair weldments in order to improve the knowledge of the material properties in the different material zones. Based on earlier experience regarding the material properties, in particular in the HAZ, finite element simulations were carried out for a repaired weldment based on a creep damage model. Results show that low creep resistance of the HAZ (creep soft HAZ) leads to premature creep damage and initial cracking. Furthermore, influences of the repair weld filler material creep properties and the geometry of the repair weld preparation are indicated. The investigation has resulted in a number of recommendations for weld repair work in high temperature/high pressure components. * [email protected] ** [email protected]

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Creep Life of Pipe Girth Weld Repairs OMMI (Vol. 1, Issue 3) Dec. 2002

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Introduction The behaviour of weld repairs in creep exposed components depends on a complex of factors and is therefore not always fully understood. The present work aims to increase the understanding of weld repair behaviour and weld repair assessment. For this purpose different kinds of investigations can be undertaken such as: - Search of plant experience. A number of weld repairs has been performed over the

years. - Literature study. Amounts of research have been carried out on this subject and a lot of

useful information can be found in the literature. - Metallographical work, where the microstructures and damage development in weld

repairs can be studied. - Creep testing of specimens, where the weld repair creep properties can be studied. - Finite element simulations, where the creep behaviour of the weld repair can be studied.

This can also be studied in creep testing of full-scale components. All kinds of studies described above, except creep testing, have been carried out in the present work. From the findings some recommendations have been formulated. In the present paper the work on finite element simulations is described in detail whereas the search of plant experience and the metallographical investigations are described more briefly. The literature survey has been submitted for publication elsewhere. Plant experience A number of 44 cases of weld repairs from Swedish and Danish power plant have been collected and compiled by help of a questionnaire. The collected information is mainly from repairs of creep exposed steam piping. Creep cracks caused the major part of the repairs and they were almost in all cases found in welds. A number, but far from all of the cracks seems to be due to system stresses. Several repairs have been performed in all of the occurring low alloy heat resistant steels. The highest number was found in the material 14MoV6 3 (0,5%Cr-0,5%Mo-0,25%V). Repairs related to service induced damage was also found in dissimilar welds between low alloy and 12%Cr steels. No repairs were found in pipework of 9-12%Cr steels. Most repairs were performed after 100 000 � 200 000 hours service time (the design life is generally 100 000 hours) but there is also a number of repairs performed after less than 50 000 hours. The repairs after less than 50 000 hours were all in the 14MoV6 3 material. Some of the repairs were repaired repeatedly. The repairs show large variations in the groove geometry, from full repairs of the damaged weld to quite local ones were not much more than the crack was excavated. Different types of repairs are shown in Figure 1. The repair welding is commonly performed according to existing codes without particular control of the welding parameters except that temper beads (on the top of the weld) have been applied in a couple of cases.


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Figure 1. Different types of repair geometries

The service time of the repairs varies from 0 to 106 000 hours. A number of the repairs have so far been in service only for quite a short time. There are several cases where re-cracking have occurred after less than 10.000 hours of operation. Some cases of grinding of cracks without weld repair were also studied. These cases show high frequency of re-cracking after relatively short service times. Metallographical investigations Repairs of 14MoV 63 Two full repairs, due to smaller creep cracks, were carried out on branch welds on a thick walled (50 mm) HP steam line after 100 000 hours at 193 bars and 538°C. The repairs were MMA welded with relatively low heat inputs. The service time after the repairs was only 5000 hours, the steam line was then taken out of operation. Results of hardness measures are shown in Figure 2. The first repair profile (triangles) is taken across the HAZ at a position with no refining effects and scarcely no annealing effects from adjacent weld beads. The hardness is high in the coarse grained part of the HAZ although a post weld heat treatment has been performed (700°C/1h). The amount of coarse grained HAZ along the fusion boundary of the repair weld was measured to be about 45%. The second repair profile is measured across the whole weld. The microstructure is fully refined at both the fusion boundaries in the position of this profile. Thus, all the HAZ and the weld metal close to the fusion line consist of a homogenous microstructure with regard to the fine grain size. However, there is a peak in the hardness that may be related to the

a) b)

c) d)

e) f)


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precipitation of carbides, which was observed to be inhomogeneous in the area. Still the hardness is significantly lower than in the microstructure without refinement.

Figure 2. Repair profile 1 (triangles) and 2 (filled points) and measures on service-exposed material of an adjacent weld in the same steam line. The repair is post weld heat treated as well as the adjacent weld The profile of the service-exposed weld is measured on a section of a girth-weld from the same steam line which was cut out after approximately 90 000 hours service time (creep cracks were found). The hardness is about 40% lower in the HAZ area and about 30% lower in the weld metal compared to the repair without refined microstructures (5000 h service exposure). Repairs of 10 CrMo 9 10 The repairs of 10 CrMo 9 10 were performed on welds in an IP steam line and in a smaller branch of a HP steam line. The wall thickness at these repairs was rather small. (10-15 mm). The welds were repaired after 118 000 hours service time and were then in operation for 23 000 hours before the steam lines were taken out of operation. The repairs were due to hot cracks and porosity that were fond in an inspection of the welds. The repairs penetrated about half the wall thickness and were carried out with a few weld beads and weaving of the electrode. Examination of the microstructure indicate high heat inputs. The repair weld metal hardness (up to 250 HV) was higher than in the old weld metal and parent metal (145-210 HV) in these repairs. The hardness was not elevated in the HAZs. Discussion of metallographical examinations The repairs show a significantly higher hardness in the repair weld metal than in the service-exposed weld metal and parent metal. The differences between the service-exposed

0

50

100

150

200

250

300

350

400

450

-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30

Distance from weld centre (mm)

Har

dnes

s H

V 0,

5 Repair

Service exposed


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materials in the repair are relatively small. This is quite typical for repairs after long service time. Differences in hardness between different parts of a repair weld as well as in an ordinary weld indicate that also the creep strain resistance may be significantly different. Such differences are known to reduce the lifetime of the weld. The creep strain resistance in the HAZ is also very important in this context. It is recognised that the creep strain resistance in the fine grained and the intercritical parts of the HAZ is much lower than the parent metal whereas it is much higher in coarse grained HAZ. The magnitude of these differences as well as the size of the HAZ influences the creep behaviour of the weld. A model of a weld that can take the properties in all parts of the HAZ into account more exactly is quite complicated and it will be hard to make finite element simulations by use of such a model. A model of the weld that is described only by one property in the HAZ can be suggested in two types: 1. In case of refinement of the HAZ all the HAZ and possibly also the weld metal close

to the HAZ is fine grained and the creep properties correspond to the ones of a fine grained HAZ in all that area.

2. In case of an existing coarse-grained part of the HAZ, this part will be included in the

weld metal and the creep properties will thus be approximated to the ones of the weld metal. The rest of the HAZ have properties of a fine grained HAZ.

In many welds and repairs the amount of refined microstructure is high. For example, it is advisable to butter a repair by use a 50% overlap technique. This is practical and will typically give about 80% refinement. However the amount of refinement can be rather low, as seen in the studied repairs of 14MoV 6 3, when the welding is performed without particular focus on HAZ refinement. Although the grain size is quite homogeneous in a refined HAZ it was seen that the hardness could vary significantly in the 14MoV 6 3 material. An assumption of an equal creep strain resistance in the fine grained and the refined fine grained HAZ is therefore questionable under such circumstances. Finite element simulations Objectives The creep life of a weldment is a function of several parameters as described above. In most cases, the dependence on specific parameters can not be separated out and assessment of the remaining life becomes extremely complex. FE simulation provides a powerful tool in investigations regarding the effects of structural geometry, material properties and loading. The present study focuses on the creep life of a repaired weldment and its dependence on material mis-matching and the geometry of the weld repair preparation. The choice of material properties was made with reference to data given in the literature, [1-6], in particular relating to parts of the HAZ showing markedly �creep soft� properties. It should be pointed out that the material properties vary more or less continuously through the weldment and in addition �exact� material property values of the different zones are not known. Obviously the result of the simulation will be approximate but, the indications they provide do improve the understanding of the processes involved.


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An important aspect, however, is the possibility to choose weld geometry, filler material and weld procedure such that critical creep damage will develop at the outside of the pipe. Hence, the weld can be monitored by NDT methods which investigates the surface. Theory Design of pressure equipment subjected to creep requires application of the design stresses and (in some cases) mis-matching of the weld filler material is considered in a simplistic way. It would be preferable to apply the �true� creep properties in the design as indicated in Fig. 3 in order to predict more accurately the creep life of a component. However, since the number of parameters involved is high and some are not adequately investigated, the cases analysed here were chosen to demonstrate the effect of some of the important ones in order to improve the understanding of the creep processes as described by the damage theory.

ε,D

1.0 Fracture

Damage rupturet

fε

Strain

t

Fig. 3. Creep strain and damage as interpreted from a regular creep test The 3-D theory, given by Kachanov-Rabotnov, as used in this paper was described in detail in [4]. The equations and material properties assumed in the analyses are summarised in the following:

dtd

dtd

dtd cr

ijelij

totij εεε

+= , where:

+

−

+= ijkkij

elij

dtd

dtd

Edtd

δσν

νσνε1

1

( ) ( )[ ]υδδσε n

ijne

crij DsB

dtd −− −+−= 11

23 1

[ ]φ

υσαασφ )1(

)1(1 D

AgdtdD eI

−−+

+= , and:

( ) )1/(1cr 11 +−−= φgD


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The parameters are: totijε = total strain, el

ijε = elastic strain, crijε = creep strain, ijσ = stress

tensor and sij = the stress deviator. Iσ = max principal stress eσ = the von Mises stress. E = Young�s modulus, ν = Poisson�s ratio, D = the damage variable and Dcr the critical value. The life of the component is assumed exhausted when the ratio cr/ DD reaches the value of 1.0. α is a material constant relating to the multi-axial rupture criterion which ranges from zero to unity. B, n, A and υ are material constants relating to the minimum creep strain rate and the rupture time, g, φ and ρ the constants accounting for the in-homogeneity of the damage where ρ represents the volumetric ratio of the damaged phase. The theory was coded in the UMAT subroutine, [7], operative with the ABAQUS finite element system, [8].

The damage is a scalar parameter and its value, at a particular point in the structure, is assumed to represent the creep life utilised. In the analyses the damage throughout the model was first mapped at the time when the repair was carried out, rept according to Fig. 4. When resuming the analysis, the damage in the aged material is to be preserved while the virgin filler material starts out with zero damage. Naturally it would be preferable to carry out the analyses �exactly� by use of the finite element code. However, by the time the analyses were done, restart of the analysis considering creep damage was not operative and thus an approximation was applied as follows:

In order to be able to define the material data for the aged material zones after a weld repair, a uni-axial solution of the Kachanov-Rabotnov set of equations, [4] was utilised. 1. At the time of the repair, damage and strain distributions in the (aged) material zones

were plotted and an average damage value in the material close to the boundary on the new weld material was chosen. The �point in time p � in Fig. 4 is taken to represent the damage D and the strain ε .

2. Then, the damage and strain level of the material component was compared with the damage calculated for the uni-directional stress level and a �starting point� 0p for the continued analysis of the aged material was defined, Fig. 4.

3. Two additional �points�, 1p and 2p , along the strain and damage curves were defined to be used in the estimation of a new set of material constants valid for the material component after restart of the analysis. These values were calculated through integration of the set of uni-directional strain-damage equations.

4. All points were transposed to a new strain-time and damage-time co-ordinate system where the � '

0p points� were defined as starting points for the �new� material curves, see Fig. 4. Thus 3 strain and 3 damage values were available for estimation of the material properties from the time of repair.

5. By aid of a MathCad procedure, 6 new material constants B’, n’, A’, υ ’, Φ ’ and ρ � were thus defined. Some of the properties evaluated are included in Table 1.


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D Repaired pipe 2p 2p′ Parent pipe 1p 0p 1p′ repD 0p′ t = 0 reptt = t Fig. 4. Method used to evaluate new material properties valid after repair, t > rept (When inspecting the damage calculated at itt = , rep)( DtDD i +′= is to be observed)

Table 1a. Material properties of non-aged material zones Creep hard Matched Creep soft

Constants PM HAZ1 WM1 WM3 WM4

B 1.940e-15 1.540e-12 5.907e-15 1.771e-14 8.860e-14

n 4.354 3.925 3.870 3.870 3.870

A 8.325e-13 4.365e-11 3.304e-13 3.800e-13 4.749e-13

υ 3.955 3.750 4.110 4.110 4.110

g 0.9755 0.9180 0.9680 0.9720 0.8930

Φ 1.423 2.017 0.6517 0.6517 0.6517

ρ 0.393 0.280 0.0985 0.0985 0.0985

α 0.43 0.43 0.43 0.43 0.43

E 160000 160000 160000 160000 160000

ν 0.3 0.3 0.3 0.3 0.3

εf 0.15 0.15/0.25 0.05/0.05 0.05/0.15 0.15/0.25


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Table 1b. Material properties of aged material zones

Constants PM� HAZ1� WM3�

B� 1.590e-15 4.347e-14 1.004e-14

n� 4.403 5.026 3.991

A� 9.756e-13 4.365e-12 3.800e-13

ν ' 3.990 3.857 4.156

g� 0.9755 0.9180 0.9720

Φ� 1.732 2.377 0.8283

ρ� 0.393 0.315 0.0630

Material properties The degree of mis-matching is indicated by the factor f which is defined by: PMZONE /εε &&=f at time = zero '

PMZONE /εε &&=′f at the time of the repair FE modelling, strategy The pipe was originally assumed to contain a V-weld according to Fig. 5.a. If a weld repair becomes required, several types of repair excavations are possible, compare [5,9]. However, the present study was limited to a symmetric preparation according to Fig. 5b. Several geometries were studied according to Fig. 5c,d. a) Original V-weldment b) Partial weld repair BM HAZ WM HAZ BM Aged New WM Aged

L

d t Aged

c)

⇒ Shallow Wide �Base� preparation Narrow Deep

d) Model Depth D Length L

�Base� 0.5*t 2*t

Shallow 0.25*t 2*t

Deep 0.625*t 2*t

Wide 0.5*t 2.5*t

Narrow 0.5*t 1.75*t

Fig. 5a-d. Definition of the repair weld preparation geometry


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Basic cases Original V-weldment The FE-model of the original V- weldment, Fig. 5a, is shown in Fig. 6. It was subjected to an internal pressure giving the stresses 64=ϕσ , and 4,28x =σ MPa. The material properties of the three welding zones are defined in Table 1. The weld filler material is nearly matching the parent material (PM), the factor f = 1.22 , while the HAZ zones were given properties significantly �softer�, that is the HAZ creeps faster than the PM when subjected to the same hoop stress, f = 133 (The factor was accidentally selected somewhat too high but since the level is not unrealistic, the results of the simulations are judged to be of value within the scope given). The relative damage caused by creep was calculated throughout the model and the analysis was continued until the damage level reached the value cr/ DD = 1.0 somewhere in the structure, thus indicating severe local degradation and initiation of a creep crack.

The results are given in Fig. 6 indicating a life of 23000 h till development of a crack in the HAZ at the outside of the pipe. The life is thus approximately ¼ of the life of a pipe consisting of PM only. The damage distribution in the weldment is fairly homogeneous in the different zones apart from the local HAZ high damage area. In addition, an analysis was carried out for the V-weld under the same initial pressure but subjected to an additional axial load giving a 50 per cent increase in axial stress. The result showed that the damage level cr/ DD = 1.0 was reached after 6800 h thus indicating the extreme sensitivity to system loads. This sensitivity will be smaller with a lower value of the factor f in the HAZ.

When the welded pipe reached 23000 h, damaged material was removed and new filler material was added. The aged material had reached a damage level of cr/ DD = 0.15 to 0.20 in the areas adjacent to material zones to be removed which was considered in the later evaluation of damage in the repaired weldments.

Figure 6. Relative damage at 23 000 h in the base model before the weld repair


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Repaired weldment Load case: Internal pressure only, f = 1.25 The weld filler material of the �base� model was given properties closely matching the (aged) parent metal. The new HAZ was given the same material properties as in the original V-weldment. The material properties of the aged zones were modified as described above so as to correspond to the properties at 23000 h, at the start of continued simulation. In the base case the mis-match factor chosen was f = 1.25. The result of the repair simulation is given in Fig. 7 where the damage is plotted at the time when the critical level cr/ DD reached the value 1.0. This occurred at 36000 h after the repair and again the maximum damage was found in the HAZ at the outside of the pipe. Since the aged material zones had accumulated a damage level cr/ DD of 0.15 to 0.20, that damage must be added to the values plotted in Fig. 7. Maximum damage in the original aged material was estimated as cr/ DD = 0.55 + 0.17 = 0.72.

Figure 7. Relative damage in the repaired weldment, weld material matching parent metal

Load case: Internal pressure including 50 % increased axial stress, f = 1.25 A fairly moderate increase of axial stresses, for instance caused by system loading, leads to a substantial reduction of the creep life after reparation from 36000 to 13200 h. As in the


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reference case, max damage developed in the HAZ at the outside of the pipe. The damage in the aged, original weld was estimated at cr/ DD = 0.4.

Comment: In most of the cases analysed, the damage in the embedded aged material was found to be of the magnitude 0.6 to 0.8 and the maximum damage in the HAZ at the outside of the pipe. However, in the case of a creep hard weld repair, the damage development in the filler material may accelerate as indicated in Fig. 8. A Similar behaviour was noted in the case of a matched repair where the HAZ properties were slightly creep hard. The noted behaviour is a consequence of the fact that the creep rate in the aged material is higher and thus forces the new material to take a higher part of the load.

Figure 8. Relative damage in the repaired weldment, weld material creep hard in relation to the parent metal Results A brief summary of the results of the simulations is given below. The intention is to point to the indications found through varying the material properties and the weld preparation geometry.

A. Influence of the material properties of the weld repair material:

Load case: Internal pressure only: Creep hard weld rep. f =0.41 L = 41500 h after repair

Matched f =1.25 L = 36000 h (�Base case�)

Creep soft f =6.1 L = 30670 h


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Load case: Internal pressure including 50 % increased axial stress:

Creep hard weld rep. f =0.41 L = 12040 h after repair

Matched f =1.25 L = 13200 h

Creep soft f =6.1 L = 12590 h

B. Influence of the weld preparation geometry on the life of the weldment: In all cases f =1.25

Load case: Internal pressure only: Creep life after repair Repair geometries studied

37500 h

38259 h 36000 h (�Base*) 32700 h

34200 h

The results indicate that the weld repair excavation should not be narrower and/or deeper than that of the �base case�.

Load case: Internal pressure including 50 % increased axial stress:

Creep life after repair Repair geometries studied

12820 h

15060 h 13200 h 11070 h

12150 h

Again, the cases analysed show that a wider weld preparation should give a longer life after the repair than a narrower one. However, results for shallow and deep preparations are not conclusive, and the effect depends on the presence / amount of system stresses.

Recommendations

1 Avoid welding procedures that may cause a strongly creep soft HAZ The HAZ will always have a creep soft part (the fine grained and intercritical parts of the HAZ). That part as well as the whole HAZ will be wider with increasing heat input. A wider HAZ decreases the creep strength of the weld. In addition, the coarsening of carbides may be more pronounced during the weld thermal cycle with higher heat input, particularly in the intercritical HAZ. The coarsening of carbides results in lower creep strength as well. Therefore, as low heat input as practically possible is recommended. In order to avoid wide HAZs it is also recommended to use inter pass temperatures of max. 300°C.

2 Select design solutions that minimise system stresses Even small system stresses reduce the creep strength significantly in welds. If the repair was due to system stresses it is very important to eliminate these.


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3 Select material for weld repair somewhat overmatched (creep hard) in relation to the remaining service exposed material Hardness testing, replica testing or boat samples, service data and chemical analysis give understanding of the creep strain resistance in the aged material. The new weld metal is recommended to be tested corresponding to an ordinary weld test. These investigations will be helpful at the selection of weld repair material and procedure. Typically a new weld metal of the same material as the service-exposed ones will give a strongly overmatched repair-A repair weld material, which then can be another material than the original, that matches or somewhat overmatches the creep strain resistance in the service exposed ones is recommended.

4 Wide and medium deep geometry of the excavation is optimal for the life time of a weld repair Deeper or “full repairs” are however necessary if cracks, micro-cracks or creep cavities are present deep in the material. Repair of at least the whole width of the original weld, including the HAZs is always recommended, even if the damage is local such as cracks in the HAZ only at one side of the weld. References 1. Storesund, J.: �Life assessment from weld microstructures at high temperatures�,

Doctoral Thesis, Royal Institute of Technology, Stockholm, Report No AMT-179, May 1998.

2. Hayhurst, D. R. : �The Use of Continuum Damage Mechanics in Creep Analysis for Design�. Journal of Strain Analysis, Vol. 29, No. 3, 1994 pp 233-241.

3. Hyde, T. H., Sun W. and Becker A. A.: �creep crack growth in welds: a damage mechanics approach to predicting initiation and growth of circumferential cracks. Advances in Defect Assessment in High Temperature Plant�, MPA Stuttgart, Germany, Oct. 2000.

4. Storesund, J., Andersson, P., Samuelson. L. Å. and Segle, P.: �Prediction of creep cracks in low alloy steel pipe welds by use of the continuum damage mechanics approach�, in CAPE �97, Penny (ed.), Balkema, Rotterdam, 1997.

5. Storesund, J., Repair welding of creep exposed high temperature plant components including recommendations, The Thermal Engineering Research Institute in Sweden, Report No.657, 1999 (in Swedish).

6. Eggeler, G. et al., �Analysis of creep in a welded �P91� pressure vessel�. Int. J. Pres. Ves. & Piping, Vol. 60 (1994) pp 237 � 257.

7. Moberg, F.: �Implementation of constitutive equations for creep damage mechanics into the ABAQUS finite element code � Subroutine UMAT�, SAQ/R&D Report No 95/05, Stockholm 1995.

8. ABAQUS User�s Manual, Version 5.8.

9. Hyde, T. H. et. al., A review of the finite element analysis of repaired welds under creep conditions. 3rd Int. Conf. Integrity of High Temperature Welds, ISQ Oeiras-Lisbon, Portugal, !&-18 September 2002, pp29-44.

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