NUMERICAL SIMULATION OF WELDMENT CREEP RESPONSE

Peter Segle

Doctoral Thesis

Report KTH / June 2002 AMT-206

Department of Materials Science and Engineering

Royal Institute of Technology (KTH) SE-100 44 STOCKHOLM, Sweden

and

Swedish Institute for Metals Research Drottning Kristinas v. 48

SE-114 28 STOCKHOLM, Sweden

ISSN 0282-9770

NUMERICAL SIMULATION OF WELDMENT CREEP RESPONSE

Peter Segle

Doctoral Thesis

Report KTH / June 2002 AMT-206

Department of Materials Science and Engineering Royal Institute of Technology (KTH) SE-100 44 STOCKHOLM, Sweden

and

Swedish Institute for Metals Research Drottning Kristinas v. 48

SE-114 28 STOCKHOLM, Sweden

ISSN 0282-9770

AKADEMISK AVHANDLING

som med tillstånd av Kungliga Tekniska Högskolan framlägges till offentlig granskning för avläggande av teknologie doktorsexamen torsdagen den 6 juni 2002 kl. 13.00 i Kollegiesalen, Administrationsbyggnaden, KTH, Valhallavägen 79, Stockholm. Fakultetsopponent är Professor Thomas Hyde, Division of Mechanical Engineering, University of Nottingham.

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NUMERICAL SIMULATION OF WELDMENT CREEP RESPONSE

Peter Segle Department of Materials Science and Engineering Royal Institute of Technology (KTH) SE-100 44 STOCKHOLM, SWEDEN

Swedish Institute for Metals Research Drottning Kristinas v. 48 SE-114 28 STOCKHOLM, SWEDEN

ABSTRACT

In-service inspections of high temperature pressure equipment show that weldments are prone to creep and fatigue damage. It is not uncommon that severely damaged weldments are found even before the design life of the component has been reached. In order to improve this situation action has been taken during the last decades, both from industry, universities and research institutes, aiming at an enhanced understanding of the weldment response.

The work presented in this thesis focuses on numerical simulation of weldment creep response. For a more profound understanding of the evolution of creep damage in mismatched low alloy weldments, simulations are performed using the continuum damage mechanics, CDM, concept. Both design and life assessment aspects are addressed. The possibility to assess seam welded pipes using results from tests of cross-weld specimens taken out from the seam is investigated. It is found that the larger the cross-weld specimen the better the correlation. The advantage to use the CDM concept prior to a regular creep analysis is also pointed out. In order to develop the CDM analysis, a modified Kachanov-Rabotnov constitutive model is implemented into ABAQUS. Using this model, a second redistribution of stresses is revealed as the tertiary creep stage is reached in the mismatched weldment.

Creep crack growth, CCG, in cross-weld compact tension, CT, specimens is investigated numerically where a fracture mechanics concept is developed in two steps. In the first one, the C* value and an averaged constraint parameter are used for characterising the fields in the process zone, while in the second step, the creep deformation rate perpendicular to the crack plane and a constraint parameter ahead of the crack tip, are used as characterising parameters. The influence of type and degree of mismatch, location of starter notch as well as size of CT specimen, is investigated. Results show that not only the material properties of the weldment constituent containing the crack, but also the deformation properties of the adjacent constituents, influence the CCG behaviour. Furthermore, the effect of size is influenced by the mismatch of the weldment constituents.

A circumferentially cracked girth weld with different mismatch is assessed numerically by use of the fracture mechanics concept developed. The results show that type and degree of mismatch have a great influence on the CCG behaviour and that C* alone cannot characterise crack tip fields. Corresponding R5 assessments are also performed. Comparison with the numerical investigation shows that the assumption of plane stress or plane strain conditions in the R5 analysis is essential for the agreement of the results. Assuming the former results in a relatively good agreement for the axial stress dominated cases while for the hoop stress dominated cases, R5 predicts higher CCG rates by an order of magnitude.

Keywords: ABAQUS, constraint effect, continuum damage mechanics, creep, creep crack

growth, design, design code, finite element method, fracture mechanics, life assessment, mismatch, numerical simulation, weldment

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Papers included in this thesis: Paper I Segle, P., Tu, S.-T., Storesund, J. and Samuelson, L.Å. (1996) Some issues in life

assessment of longitudinal seam welds based on creep tests with cross-weld specimens, Int. J. Pres. Ves. & Piping, Vol. 66, pp. 199-222.

Paper II Segle, P., Samuelson, L.Å., Andersson, P. and Moberg, F. (1996) Implementation

of constitutive equations for creep damage mechanics into the ABAQUS finite element code - Some practical cases in high temperature component design and life assessment, ”Inelasticity and Damage in Solids Subject to Microstructural Change – The Lazar M. Kachanov Symposium”, Eds. I.J. Jordaan, R. Seshadri and I.L. Meglis, September 25-27, St. John’s, Newfoundland, Canada.

Paper III Segle, P., Andersson, P. and Samuelson, L.Å. (1998) A parametric study of creep

crack growth in heterogeneous CT specimens by use of finite element simulations, Materials at High Temperature (UK), Vol. 15, No. 2, pp. 63-68.

Paper IV Segle, P., Andersson, P. and Samuelson, L.Å. (2000) Numerical investigation of

creep crack growth in cross-weld CT specimens - Part I: Influence of mismatch in creep deformation properties and notch tip location, Int. J. Fatigue & Fracture of Engineering Materials & Structures, Vol. 23, No. 6, pp. 521-531.

Paper V Andersson, P., Segle, P. and Samuelson, L.Å. (2000) Numerical investigation of

creep crack growth in cross-weld CT specimens - Part II: Influence of specimen size, Int. J. Fatigue & Fracture of Engineering Materials & Structures, Vol. 23, No. 6, pp. 533-540.

Paper VI Samuelson, L.Å, Segle, P., Andersson, P and Storesund, J. (2001) Creep crack

growth in welded components – A numerical study and comparison with the R5 procedure, Int. J. Pres. Ves. & Piping, Vol. 78, pp. 995-1002.

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TABLE OF CONTENTS Page

1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Scientific objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2 Technical objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.

THE WELDMENT COMPONENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.1 Weldment manufacturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2 Material properties of weldment constituents . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.3 Cracking of weldments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.

STRUCTURAL ANALYSIS TECHNIQUES IN HIGH TEMPERATURE APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.1 Continuum mechanics approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.2 Continuum damage mechanics approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.3 Fracture mechanics approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.4 Micro mechanics approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.5 Reference stress method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.

DEVELOPMENT IN DESIGN AND LIFE ASSESSMENT OF HIGH TEMPERATURE WELDMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4.1 High temperature design codes and life assessment procedures of today . . . . . . 94.2 Current research activities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5.

SUMMARY OF INCLUDED PAPERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

5.1 Some issues in life assessment of longitudinal seam welds based on creep tests with cross-weld specimens (Paper I) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

5.2 Implementation of constitutive equations for creep damage mechanics into the ABAQUS finite element code - Some practical cases in high temperature component design and life assessment (Paper II) . . . . . . . . . . . . . . . . . . . . . . . . . 12

5.3 A parametric study of creep crack growth in heterogeneous CT specimens by use of finite element simulations (Paper III) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5.4 Numerical investigation of creep crack growth in cross-weld CT specimens - Part I: Influence of mismatch in creep deformation properties and notch tip location (Paper IV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5.5 Numerical investigation of creep crack growth in cross-weld CT specimens - Part II: Influence of specimen size (Paper V) . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5.6 Creep crack growth in welded components – A numerical study and comparison with the R5 procedure (Paper VI) . . . . . . . . . . . . . . . . . . . . . . . . . . 13

ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 DISTRIBUTION OF WORK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

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1. INTRODUCTION

Creep in structures has been of interest to engineers for more than 200 years. Design of candles was probably one of the first areas where a continuously deforming material under constant load had to be analysed. Examples of other fields where the creep phenomenon has been of importance for the interpretation of the structural response are construction of cable suspended bridges, steam-engine design, power-, chemical-, and petrochemical-plant design, design of jet engines and, nuclear power plant design [1]. As the demand on materials with respect to creep has increased within different applications, theories and methodologies have been developed [2-4]. The individual event that probably has had the largest impact on the research and development within the area of creep is the establishment of nuclear power plants such as Liquid Metal Fast Breeder Reactors, LMFBR, Helium Cooled High Temperature Reactors, HTR, and Advanced Gas Cooled Reactors, AGR [5,6]. Here, the driving force has been the very high safety requirements.

In the context of high temperature pressure equipment the weakening effect of the weldments has been recognised for several decades. In-service inspections have shown that weldments are prone to creep and fatigue damage. It is not uncommon that severely damaged weldments are revealed even before the design life of the component has been reached. In order to improve this situation actions have been taken, both from industry, universities and research institutes, aiming at an enhanced understanding of the weldment response.

In the beginning of the eighties, the effect of stress redistribution within the weldment region due to mismatch in creep deformation properties between the weldment constituents was investigated both experimentally and numerically [7-10]. It was understood that this phenomenon was of importance for the interpretation of the behaviour of the weaker weldments. Based on numerical calculations of the stress and the strain fields, researchers predicted rupture time and rupture position. An initial attempt to define a weldment performance factor was also done. It was defined as that factor by which the pressure had to be reduced to ensure that the welded component attains its design life [10].

The influence of stress state, or degree of constraint, on creep rupture was also recognised [11,12]. Thus, not only the stress level but also the characteristics of the stress tensor is essential for the creep rupture process. For a weldment subjected to creep this fact is most essential for the weldment performance [13-15]. An enhancement in the degree of constraint also reduces the creep ductility [16]. The structural creep response of a weldment obviously becomes very complex as many parameters influence each other [17].

Another issue that has been focused on since the mid-eighties is seam welded steam pipes in fossil fired power plants [18]. Due to mismatched creep deformation properties between the weld and the parent metal in combination with non-favourable weld shapes, stress concentrations developed within the weldment eventually resulting in seams that fractured catastrophically. Extensive efforts were undertaken which led to the establishment of assessment procedures for this special component [19-21].

In order to enhance the understanding of the structural creep response of weldments, the continuum damage mechanics concept was introduced as an analyst tool [22-25]. The possibility to predict creep damage evolution, or evolution of other deteriorating mechanisms, was improved. One obstacle was still, however, the lack of material property data for the weldment constituents.

The ASME Code Case N-47 [26] was the first high temperature design code that 1987 explicitly took the weakening effect of weldments into account and weld creep and fatigue strength reduction factors were introduced [27]. The weldments were assumed to be free of

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defects and reduction factors were given for certain well defined weld systems. Since then both the French code RCC-MR [28] and the British PD6539 [29] and R5 [30] have incorporated similar concepts.

The introduction of the fracture mechanics concept in design and life assessment of high temperature weldments came during the nineties [20,29,30]. The research within this field is ongoing and further improvements of current high temperature design codes and life assessment procedures can be expected.

Despite the fact that a tremendous amount of work has been spent in understanding the high temperature weldment response, more work remains to be done in order to tackle industry-related problems. In-service inspections of high temperature pressure equipment, for example, still show that weldments are prone to creep and fatigue damage. Reasons explaining why this situation prevails are: i) deficiencies in the high temperature design code used; ii) lack of weldment constituent material property data and lack of information about loading conditions at design stage; iii) unfavourable combination of base material and weld deposit material; iv) a welding procedure resulting in unfavourable microstructures across the weldment and introduction of defects; and v) deviation in operation of plant from what was stated in the design specifications.

In order to improve the present situation, the very complex behaviour of high temperature weldments has to be further understood. Results from laboratory testing of uniaxial specimens as well as more complex component testing, acquisition of plant experiences in combination with results from numerical simulations should be the basis for this improvement work.

The present thesis focuses on numerical simulation of low alloy steel weldments subjected to creep. Both a continuum damage mechanics approach and a fracture mechanics approach are used for a better understanding of weldment performance in high temperature applications.

1.1 Scientific objective

The scientific objective with this work is to increase the physical understanding of the response of weldments subjected to creep.

1.2 Technical objective

The technical objective with this work is to improve the basis for further development of present high temperature design codes and life assessment procedures. This will contribute to an increased safety and an improved economy in high temperature plants.

2. THE WELDMENT COMPONENT

Variables of importance for the performance of a weldment are the material characteristics of the base and the weld deposit material, the weld geometry and the geometry of the component, the welding procedure used in manufacturing of the weld and, finally, the loading conditions during operation. For dissimilar metal welds where two different base materials are joined, the weldment response is further complicated [31,32]. In addition, defects may be introduced during welding which may act as initiation points for creep and/or fatigue cracks during subsequent operation.

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2.1 Weldment manufacturing

The most commonly used welding technique in high temperature applications is manual metal arc welding. A characteristic weld contains several beads and has a weld width that increases towards the outer surface of the component. The microstructure developed during welding will vary across the weldment as well as within the beads. What type of microstructure that is developed is essentially controlled by the heat cycle the material experiences and the characteristics of the material, i.e. chemical composition, microstructure before welding (base material), etc. A subsequent post weld heat treatment reduces residual stresses introduced during welding as well as influences the mechanical properties of the weldment constituents.

Depending of type of microstructure, the weldment can be divided into a number of different zones. For a low alloy steel weldment the following zones can be distinguished across the weldment, starting from the centre of the weld: weld metal, fusion line, coarse grained heat affected zone, HAZ, fine grained HAZ, intercritical HAZ and base material, see figure 1.

Fig. 1 Different microstructural zones across a low alloy steel weldment [15].

All but the weld metal and the fusion line, consist of base material experiencing different heat cycles, the closer the weld the higher the peak temperature. The fusion line and its vicinity contain both base and weld deposit material.

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2.2 Material properties of weldment constituents

Material properties such as yield strength, tensile strength, plastic hardening characteristics, fatigue strength, fracture toughness, hardness, creep deformation rate, creep rupture strength and creep ductility vary with type of microstructure [33]. Many of these properties are, in addition, influenced by the current stress state; a stress state that is a function of the loading condition, the geometry and the deformation properties of the weldment constituents, existing defects, etc. [34]. The degree of complexity of the high temperature weldment response is obviously very high.

For low alloy steel weldments, characteristic material creep properties for the different weldment constituents are shown in table 1. The different creep deformation properties will cause stress and creep strain rate redistribution to take place when the weldment is set in operation.

Table 1 Characteristic weldment constituent creep properties in low alloy steel weldments.

Weldment zone Characteristic material creep properties

weld metal similar, higher or lower creep deformation rate, creep rupture strength and creep ductility than that of the base material

coarse grained HAZ lower creep deformation rate, higher creep rupture strength and lower creep ductility than that of the base material

fine grained HAZ similar creep deformation rate, creep rupture strength and creep ductility as that of the base material

intercritical HAZ higher creep deformation rate, lower creep rupture strength and higher creep ductility than that of the base material

base material reference material

2.3. Cracking of weldments

A crack in a weldment can emanate either from a pre-existing defect, i.e. a defect introduced during welding, or from a region where service-induced damage has been accumulated [16]. In the latter case, nucleation and coalescence of creep damage and/or accumulation of plastic deformation at a microstructural level are addressed. As the crack has initiated a subsequent growth may follow. In which direction and at what rate the crack will grow are dependent on factors such as stress level and stress state, the material properties in the cracked area and the possibility to off-load the crack by stress redistribution.

Reheat cracking is a special type of cracking, in or in the vicinity of the weld, where a post weld heat treatment of the weldment or initial operation act as a trigger. The relief of residual stresses is thought of as one important factor in this context [35].

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A failure in a weldment does not always have to start from a crack. Instead, the whole section can accumulate damaged with a subsequent failure prior to crack initiation.

Depending on location in the weldment, four types of cracks are defined [36], see table 2 and figure 2. Type I and II cracks are often associated with initiation of cracks that emanate from pre-existing defects from welding while Type III cracks are a result of the relatively lower creep ductility in the coarse grained HAZ in combination with the lower creep deformation rate resulting in an enhancement in stress level due to stress redistribution during operation. Type IV cracks are those developing in the intercritical heat affected zone [37]. The material in this region is typically characterised by a relatively lower creep strength, higher minimum creep rate and higher creep ductility [25,38,39]. For a girth welded pipe subjected to an internal pressure, the weaker Type IV region is loaded in parallel with more creep resistant material which results in an off-loading of the Type IV region. The narrow width of the Type IV region introduces, however, an increased degree of constraint and thereby a reduced creep ductility. In combination with an additional axial stress from system loads, where the Type IV zone is loaded in series with the adjacent weldment constituents, cracking can occur [40].

Table 2 Definition of weldment cracks.

Designation Location of crack

Type I in weld metal

Type II in weld metal and adjacent HAZ

Type III in coarse grained HAZ

Type IV in intercritical HAZ

Fig. 2 Classification scheme for damage types in weldments [36].

3. STRUCTURAL ANALYSIS TECHNIQUES IN HIGH TEMPERATURE APPLICATIONS

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All structural analysis techniques are unified through the three variables material, geometry and loading. Without sufficient knowledge of each of these variables, design or life assessment of a component can not be done properly. The more complex the structural analysis technique is the more detailed information is needed.

Analytical [41,42], semi-analytical [43] and numerical analyses [44] are all used in design and life assessment in high temperature applications. Analytical methods are very powerful when these can be applied. The semi-analytical methods combine the best from analytical and numerical methods. Numerical methods give the possibility to account for complex non-linear material characteristics, geometries and loading conditions in detail. Results from all three analysis techniques can furthermore be combined with the reference stress method giving a powerful assessment tool in high temperature applications [45].

3.1 Continuum mechanics approach

In the continuum mechanics approach it is assumed that the material performs as a continuum. This means, for example, that local microstructural effects are not taken into account in the analysis. Instead, the material properties at the microstructural level are averaged to a continuum level. This approach is well suited for analytical solutions as well as for use together with the finite element technique. In many general purpose finite element programs, several different material models are included and complex geometries and loading conditions can be modelled [46,47].

Most high temperature design codes and life assessment procedures are based on continuum mechanics. One advantage with this approach is that it is relatively easy to use for the analyst. When a more complex situation has to be analysed, however, that approach does not give sufficient possibilities.

3.2 Continuum damage mechanics approach

The continuum damage mechanics, CDM, approach is based on continuum mechanics where a damage parameter has been introduced [3,41,48,49]. The character of this approach is phenomenological and the damage parameter can in this context represent creep damage, fatigue damage, some other deteriorative damage, or a combination of these. The reason why the material gets damaged does not necessary have to be understood in detail. Of importance is, however, that the damage is taken into account in the analysis [45].

Together with the finite element technique, CDM has proven to be a powerful tool in increasing the understanding the response of high temperature components [14,50,51]. In many general purpose finite element codes, user written material routines can be implemented taking into account one or more damage mechanisms [52].

Fracture processes can also be investigated by use of the CDM approach. Several investigations of creep crack initiation and growth in notched specimens have been performed with promising results [53-55].

3.3 Fracture mechanics approach

When creep failure of a structure is controlled by growth of a macroscopic crack, time-dependent fracture mechanics can be applied to predict the structural behaviour. Some twenty years ago, in analogy with the J integral, the C* integral was suggested as

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C W y Tux

s* * &= −

∫ s i

id d∂∂Γ

(1)

where Ws* is the deformation work rate density, Ti is the outward traction vector on ds , &ui is

the displacement rate vector at ds , x and y are co-ordinates in a rectangular co-ordinate system and finally, ds is the increment on the contour path Γ [56-58]. It has been found that this parameter characterised crack growth at global steady state creep and that the creep crack growth, CCG, could be written as

[ ]& *a D C= 0φ

(2)

where &a is the CCG rate and D0 and φ are material constants that may be temperature and stress state dependent. For non steady state creep conditions, the parameters C t( ) [59] and Ct [60] were proposed to characterise transient CCG. Once the stress redistribution is complete, both C t( ) and Ct are equal to the C* integral value.

The most precise way of calculating the C* integral is by use of some FE program. In ABAQUS [47] this can be done by a built-in procedure. If an approximate value of C* is sufficient, the reference stress method [16,30] can be used which gives

CK* &=

σ ε

σref refc I

ref

2

(3)

where

σσ

refY

Lc=

PP

. (4)

Here σ ref is the reference stress, &εrefc is the creep strain rate at reference stress, KI is the stress

intensity factor, P and PLc are the applied load and the limit load for the cracked component, respectively, and finally σY is the yield stress.

Assessment of creep cracks in weldments requires more than one characterising parameter. As the creep deformation properties in the weldment vary across the weldment, stress and creep strain rate redistributions take place. This may influence the degree of constraint in the vicinity of a creep crack why a second characterising parameter has to be introduced [61].

3.4 Micro mechanics approach

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The micro mechanics approach gives the possibility to incorporate physical phenomena into the model in a more accurate and direct way. The idea is to start from a unit cell at microstructural level for which the physics is described in detail. Based on this unit cell, a continuum mechanics model is then created by which the structural analysis is done. This technique has, for example, been used in analysing initiation and growth of creep cracks in notched specimens and in compact tension specimens [62-64].

So far the use of micro mechanics in design and life assessment of high temperature components has been very limited. The reasons for this are the high degree of complexity in using this approach and the lack of materials data needed for the analysis. What the analysts need in their daily design and life assessment work are rational methods that are easy to use. Results from analyses based on micro mechanics can, however, contribute to a deeper insight in the structural response of high temperature components and thereby contribute to the basis for improved design codes and life assessment procedures.

3.5 Reference stress method

One concept of importance in design and life assessment of structures subjected to creep is the so-called reference stress method, RSM. The development of this method started during the early forties [65] but it took several decades before it was generally accepted [45]. The RSM is an approximate method with the basic idea that creep in a component under complex variable loading can be predicted from the result of a single uniaxial creep test performed at the reference stress for that loading, and that the reference stress used in the test is virtually independent of the material properties [66]. A special interpretation of the RSM, which has been called the ”limit approach”, and which has been suggested as a suitable design procedure, leads to simple, powerful results requiring only elastic and plastic limit analysis [67]. The RSM has been developed over the last decades and is today available for analysis of phenomena such as creep deformation, cyclic loading, creep buckling, creep rupture and creep fracture [43,45,68-70]. Regarding current high temperature design codes and life assessment procedures, RSMs form basis for PD6539 [29], R5 [30] and R6 [71] which deal with the creep response of defect-free and defected structures.

4. DEVELOPMENT IN DESIGN AND LIFE ASSESSMENT OF HIGH TEMPERATURE WELDMENTS

High temperature design codes and life assessment procedures are related to each other. Basis for both of them is the failure and damage mechanisms that prevail in high temperature applications [21]. The general idea is to keep the risk for failure of a component in operation below a certain level. This level is normally set in the design code, usually not explicitly, but as a safety factor addressing the failure mechanism of concern. When performing a subsequent life assessment, the same risk level as used in the design code should be applied. There are several reasons why failure occurs in a component. Deviation in operation of plant from what was stated in the design specification may result in an unexpected premature failure. Neglecting regular inspections and life assessments may result in material deterioration, due to some damage mechanism, to such an extent that failure occurs. Only by being aware of and having a thorough understanding of both the failure and the damage mechanisms, correct actions can be taken for managing the failure risk.

4.1 High temperature design codes and life assessment procedures of today

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Only a few of today’s high temperature design codes and life assessment procedures take the weakening effect of the weldment into account explicitly [27,72-74]. In AD-Merkblatt [75] the lowest creep rupture strength value either of the parent, the weld or the HAZ material shall be used in design of fully-stressed welded joints. If the creep rupture strength of the weld metal and the HAZ is unknown, 80 % of the creep rupture strength of the parent shall be used in the weldment design.

In RCC-MR [28] and ASME Section III - NH [76], weldment reduction factors are based on results from tests of actual welded components with well-known, pre-specified weld systems. Defect-free similar and dissimilar metal welds are addressed and creep as well as fatigue loading can be taken into account. A number of stress and inelastic strain criterion limits also have to be fulfilled in the weldment region.

R5 [30] is probably the most developed procedure for assessment of high temperature weldments. The reference stress concept is used and both creep and fatigue loading are addressed in the procedure. Similar metal welds containing defects are analysed by use of the fracture mechanics concept. Redistribution of stresses within the weldment due to creep is here taken into account. Assessment of defect-free dissimilar metal welds is based on a combination of service experience, tube and pipe size vessel results, as well as stress analysis.

The BS-PD6539 [77] is a simplified version of the R5 procedure. Defect-free as well as defected similar metal welds subjected to creep and fatigue loading are addressed.

Redistribution of stresses due to creep is not considered. Furthermore, the material property variation across the weldment is accounted for in a more simplistic way.

The PODIS procedure [73] was developed for assessment of defect-free dissimilar metal welds. It is based on elastic stresses and incorporates both creep and fatigue loading. The weakening effect of the weldment is derived from in-service experiences and tests of cross-weld specimens, similar to what is done in the R5 procedures.

4.2 Current research activities

Several industries, institutes and universities are involved in research activities within the area of high temperature performance of weldments [78-81]. The research area is of great concern for industry as weldments are often found to be the weakest part of high temperature pressure equipment.

An evaluation of the influence of welding on creep resistance was performed 1993 [82]. In that work, test results from cross-weld and parent metal creep testing were compiled for ferritic, martensitic and austenitic creep resistant materials. The concept of strength reduction factors and life reduction factors are discussed, the former for design purposes and the latter for judging the lifetime of the welded components at normal design stresses. Strength reduction factors for weldments subjected to creep are suggested for a number of weld systems. For P91, in the temperature range of 600 to 650 oC, a strength reduction factor of 0.7 is suggested. The risk of determining non-conservative strength reduction factors, when performing accelerated cross-weld creep tests, is also addressed.

The importance of considering the influence of the multi-axial stress state in the weldment region when assessing weldments subjected to creep, is discussed in [83]. A semi-analytical approach for determination of weldment creep strength reduction factors is described. It is claimed that this approach gives sufficient accuracy at a low cost.

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In [84], it is suggested that the spatial distribution of constitutive parameters is determined by uniaxial testing while the creep response of components is simulated by use of numerical methods. By considering the stress multiaxiality and the corresponding stress redistribution process, weldment creep reduction factors are derived.

The use of simple weld reduction factors (0.8 with respect to creep rupture, 0.5 with respect to cyclic life) in the life prediction procedure for welds may risk being conservative [85]. A new approach to analysing the creep-fatigue life of welds by applying damage accumulation models calibrated directly from creep and fatigue tests on cross-weld specimens is suggested.

Redistribution of stresses within the weldment region influences the degree of constraint and thereby also the local creep ductility [86,87]. This will have an effect on initiation and growth of a weldment creep crack [88]. Life assessment of a weldment based on CCG data from specimen testing of homogeneous simulated HAZ, thus has do be done with great care [89].

In the EC supported LICON project [90] a technique for mapping the extent and distribution of creep damage zones in high temperature fracture mechanics type test pieces, with significant non-uniform stress fields ahead of the crack starters, has been developed. The intention is to use this technique in the development of a procedure for prediction of long-term creep behaviour in welded components using results from short term multi-axial tests [91].

In [92] the influence of mismatch in plastic and creep deformation properties between the weldment constituents on the creep and creep-fatigue crack growth behaviour in weldments is pointed out. Effects such as meandering cracks and big scatter in CCG and CFCG data are noticed. It is emphasised that developed high temperature design codes and life assessment procedures must incorporate the influence of mismatch in a more profound way.

The objectives with the EC supported project HIDA [93] were: i) to expand and validate the database of the existing high temperature defect assessment procedures by conducting feature tests; ii) to develop and validate methodologies for predicting the behaviour of high temperature components and develop a HIDA-procedure; iii) to develop a knowledge-based system based on the new HIDA-procedure; and iv) to develop a data bank for pressure vessel piping materials. The project results emphasised the very high complexity of the high temperature weldment response [94].

In the EC supported project SOTA [61] the influence of mismatch in weldment constituent creep deformation properties on the CCG behaviour of cross-weld CT specimens was investigated both experimentally and numerically with the objective to develop creep crack growth testing and data analysis for welds. The results reveal that not only the material properties of the constituent containing the crack but also the deformation properties of adjacent material are of importance. Experimental data and analytical evidence also show that C t* ( ) is not suitable for characterising creep crack growth behaviour in materials with low ductility in which the creep crack tip can advance at a rate comparable to the creep zone expansion rate. This directs attention to the crack tip parameters that correlate data in both creep ductile and creep brittle materials, such as Ct and CTOD [95].

Analytical and computational stress analyses are used for an enhanced understanding of the response of welded components subjected to creep [51,96]. Analytical methods are good for providing approximate solutions for simple geometries whereas numerical methods, the FE method in particular, are suited for analyses of most complicated cases. By use of the continuum damage mechanics concept, accurate predictions of creep rupture and initiation and growth of creep cracks in weldments can be made [50]. An obstacle is often, however, the lack of material property data for weldment constituents. Creep testing of local weldment constituent property data is suggested as remedy [38,97,98].

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5. SUMMARY OF INCLUDED PAPERS

5.1 Some issues in life assessment of longitudinal seam welds based on creep tests with cross-weld specimens (Paper I)

Several catastrophic bursts of longitudinal seam welded pipes in power generating industries, have led to an increased interest in how to perform life assessments of this component. In this paper, the possibility to predict the remaining life of a seam weld by creep testing cross-weld specimens taken out from the seam, is investigated numerically. The creep damage evolution in three seam welds with different weldment material mismatch is simulated and for each of these seam welds, the creep damage evolution in three cross-weld specimens with different radius is simulated as well. The assessment of the twelve components is based on both the steady state creep analysis and the continuum damage mechanics analysis with non-coupled equations.

MN

MX

MX

0 0.5 0.6 0.7 0.8 0.9 0.95 0.98 1.0

Fig. 3 Normalised creep damage in a creep-soft seam weld and in a corresponding cross-weld specimen with a radius of one fifth of the pipe wall thickness at 6800 and 6000 hours, respectively. The plot of the cross-weld specimen shows the HAZ with the adjacent parent (top) and weld (bottom) material. The centre line of the cross-weld specimen is given by the vertical boundary to the left.

The results show that the creep behaviour of the components is better predicted by the continuum damage mechanics approach compared to the steady state creep approach. One of the numerically investigated cross-weld specimens was also creep tested and the numerical and experimental results agreed well with respect to rupture time and rupture position.

In order to interpret the cross-weld specimen results in life assessment of the seam weld, the influence of the difference in overall loading conditions between the two components, the mismatch in creep properties between the weldment constituents and the size effect of the cross-weld specimen need to be understood. In Fig. 3, the normalised creep damage, i.e. a value of unity means fully damaged, in a creep-soft seam weld with a corresponding cross-

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weld specimen is shown just before rupture. As seen, the creep damage evolution differs between the components which is explained by the parameters mentioned above.

For all simulated cases, the creep rupture times of the cross-weld specimens are shorter than that of corresponding seam welds, independent of the cross-weld specimen radius. This means that predicting the remaining life of the present seam welds with creep testing of cross-weld specimens, is always conservative.

5.2 Implementation of constitutive equations for creep damage mechanics into the ABAQUS finite element code - Some practical cases in high temperature component design and life assessment (Paper II)

In this paper, the coupled modified Kachanov-Rabotnov constitutive equations, which account for the inhomogeneity in creep damage, are implemented into the ABAQUS finite element code. The creep damage evolution in pipes with differently mismatched V-shaped girth welds subjected to an internal pressure, is then investigated.

The results reveal that a second stress redistribution in the weldment takes place as the weakest weldment constituent approaches the tertiary phase. The most damaged zone is thus off-loaded and the surrounding material is on-loaded. A consequence of this is that the creep damage will be more evenly distributed within the weldment and that the predicted creep life will be longer compared to what is predicted by using the model in the previous paper. A growing macro crack in the most damaged region is however not considered in this discussion, and if this would be the case, the benefit of the second stress redistribution can not be gained.

5.3 A parametric study of creep crack growth in heterogeneous CT specimens by use of finite element simulations (Paper III)

A parametric study of creep crack growth in mismatched cross-weld CT specimens is performed by use of finite element simulations. The influence of different combinations of minimum creep strain rate of the weldment constituents and different locations of the starter notch is investigated. Two characterising parameters are used, i.e. the C* value and a constraint parameter, for describing the CCG process.

The results show that the mismatch in creep deformation properties between the weldment constituents is of importance for the CCG rate. Furthermore, the location of the starter notch does have a higher influence on the degree of constraint ahead of the crack tip than on the C* value.

5.4 Numerical investigation of creep crack growth in cross-weld CT specimens - Part I: Influence of mismatch in creep deformation properties and notch tip location (Paper IV)

In this paper, creep crack growth in mismatched cross-weld specimens is investigated by 2D finite element simulations. A creep-ductility based damage model is used where the creep deformation rate perpendicular to the crack plane and a constraint parameter ahead of the crack tip are used as characterising parameters.

The numerical results show that not only the material properties of the weldment constituent containing the crack, but also the deformation properties of adjacent constituents, have an

13

impact on the CCG behaviour. The location of the starter notch has a minor influence compared to that of the mismatch effect. The fracture mechanics approach developed in this paper is well suited for analyses of defected component weldments subjected to creep. By comparing numerical results from analyses of both cross-weld CT specimens and corresponding welded components, the correctness of presently used life assessment procedures can be evaluated.

5.5 Numerical investigation of creep crack growth in cross-weld CT specimens - Part II: Influence of specimen size (Paper V)

Influence of specimen size on creep crack growth behaviour of mismatched cross-weld CT specimens is investigated by 3D finite element analysis. The same creep-ductility based damage model is used as in Paper IV and the starter notch is symmetrically located in the heat affected zone for all cases analysed.

Results show that for a load level chosen such that the stress intensity factor becomes constant, the degree of constraint ahead of the crack tip is higher in the larger than in the smaller specimens. Despite this fact, the CCG rate is higher in the smaller than in the larger specimens which is explained by the higher creep strain deformation rate in the smaller specimens. If instead the CCG rate is evaluated under constant C* , the type of mismatch decides which CT specimen shows the higher or the lower CCG rate.

5.6 Creep crack growth in welded components – A numerical study and comparison with the R5 procedure (Paper VI)

In this paper, creep crack growth in a circumferentially welded low alloy steel pipe is numerically investigated for a number of different mismatch combinations. A fully circumferential creep crack located in the heat affected zone is assumed growing at a constant rate from the outer surface towards the inside. The results show that not only the properties of the HAZ containing the crack but also the deformation properties of the adjacent weldment constituents influence the CCG behaviour. It is furthermore shown that the CCG process cannot be sufficiently well characterised by the calculated C* value alone.

Corresponding R5 assessments are also performed. Comparison with the numerical investigation shows that the assumption of plane stress or plane strain conditions in the R5 analysis is essential for the agreement of the results. Assuming plane stress conditions at the crack tip results in a relatively good agreement for the axial stress dominated cases investigated. For the hoop stress dominated cases, R5 predicts higher CCG rates by an order of magnitude.

ACKNOWLEDGEMENTS

There are a number of persons without whose indefatigable encouragement, support and help this thesis might never have been completed. First of all, I would like to express my deepest acknowledgements to Professor Åke Samuelson who has been one of my supervisors and colleague since the beginning of 1990 when he introduced me into the interesting field of creep. My supervisor Professor Rolf Sandström is also gratefully acknowledged for valued technical discussions and for pushing me in finalising this thesis. Valuable technical discussions and good collaboration with Peder Andersson, Professor Shan-Tung Tu, Dr Jan

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Storesund, Dr Lars Dahlberg, Dr Iradj Sattari-Far, Henrik Andersson, Dr Rui Wu and Fredrik Moberg are also very much appreciated.

I am also grateful to the following members of the three research committees involved;

Christer Jansson, Vattenfall Energisystem AB Mikael Lauth, ABB Carbon AB Jan Stider, Birka Service AB Dr Lars Hammar, Mikael Palmgren, Tommy Larsson and Johan Strandmark, Sydkraft Värme Malmö AB Dr Lars-Erik Svensson and Dr Nils Thalberg, ESAB AB Dr Pamela Henderson, Vattenfall Energisystem AB Dr Jan Jansson, Alstom Power Sweden AB Dr Jan Storesund, Det Norske Veritas AB Dr Ahmed Shibli, ERA Technology Ltd (England) Professor Bilal Dogan, GKSS (Germany) Dr Giovanni Fedeli and Dr Stefano Concari, ENEL (Italy) Henrik Rantala, JRC Petten (Netherlands) A. Baptista, Dr A Coreia Da Cruz and Alex Levy, PROET/ISQ (Portugal) Jaime Izquierdo Gomez and Dr Samuel Perez, Iberdrola (Spain)

Funding of the first part of this research work was provided by the Board for Environmental Protection (Rådet för arbetslivsforskning), the Swedish National Board for Technical Development (NUTEK), Malmö Värme AB, Stockholm Energi AB, ABB Carbon AB, Vattenfall AB and SAQ Kontroll AB which is gratefully appreciated.

Funding of the second part of this research work was provided by the European Commission within the framework of the research and technological development programme Standards, Measurements and Testing, the Swedish Thermal Engineering Research Institute Värmeforsk and finally SAQ Kontroll AB. This support is greatly appreciated.

Last but not least, I would like to express my warmest acknowledgements to my wife Maija for her never ending support. Puss på dig!

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DISTRIBUTION OF WORK

Paper I

Segle performs numerical analyses, evaluates results and writes the paper. Tu initiates the work and participates in completing the paper. Storesund supplies creep materials data and participates in completing the paper. Samuelson participates in evaluation of results and in completing the paper.

Paper II

Segle initiates the work, evaluates results, writes the paper and act as one of two supervisors for Moberg. Samuelson evaluates results and participates in completing the paper. Andersson performs numerical analysis, evaluates results and participate in completing the paper. Moberg implements constitutive model into ABAQUS.

Paper III

Segle initiates the work, develops fracture mechanics concept, evaluates results and writes the paper. Andersson performs numerical analyses, evaluates results and participates in completing the paper. Samuelson evaluates results and participates in completing the paper.

Paper IV

Segle initiates the work, develops fracture mechanics concept, performs numerical analyses, evaluates results and writes the paper. Andersson evaluates results and participate in completing the paper. Samuelson evaluates results and participates in completing the paper.

Paper V

Anderson performs numerical analyses, evaluates results and writes the paper. Segle initiates the work, participates in numerical analysis, evaluates results and participates in completing the paper. Samuelson evaluates results and participates in completing the paper.

Paper VI

Samuelson initiates the work, performs numerical analyses, evaluates results and participates in completing the paper. Segle performs R5 analysis, evaluates results and writes the paper. Andersson evaluates results and participates in completing the paper. Storesund participates in completing the paper.

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