Numerical simulation of similar and dissimilar welds of martensitic steels and Nickel alloys

D. Hüggenberg, P. Buhl, T. Klein, A. Klenk

Materialprüfungsanstalt Universität Stuttgart (MPA), Pfaffenwaldring 32, 70569 Stuttgart, Germany

39th MPA-Seminar October 8th and 9th, 2013 in Stuttgart

Abstract

In components of power or process plants operating at high temperatures weldments often

become the weakest links due to the specific characteristics of the weld or heat affected

zones. In design and life assessment this is considered by weld strength factors reducing the

allowable stresses. For common materials material and temperature specific factors are

available. These factors are usually obtained by evaluation of crossweld creep tests. Thus,

they are applicable for creep loading and describe the weld strength of a fully loaded and at

least in the long term range approximately homogeneous stress distribution. However, this

often does not reflect the real situation in components.

Numerical simulations considering the specific properties of heat affected zones in ferritic

and martensitic steels are able to predict the stress and strain situation and the stress

redistribution more accurate. In this paper assessments of martensitic welds in piping

systems using numerical simulation are described.

The demand for efficiency increase in fossil fired plants which lead to the development of

700°C-technology, causes the necessity of the application of Nickel alloys in at least parts of

boiler and turbine components. This implies also the necessity of dissimilar welds in such

components. In the paper similar assessments for dissimilar welds are shown allowing

conclusions regarding stress redistributions and explanation of failure mechanisms.

1 Introduction / Motivation

Welded joints in piping systems are subjected to complex loading conditions. On the one

hand the development of the damage depends on the primary stresses, which act permanent

like the internal pressure and weight of the system. On the other hand there are secondary

stresses, which influence the redistribution of the stresses. These stresses occur due to

restraint of the thermal expansion in components with inhomogeneous temperature

distributions. A welded joint consists of the heat affected zone, the base and weld material.

Due to the different material properties within the weld and its specific loading conditions it is

very difficult to evaluate the most critical location and to describe stress redistribution during

operation.

The design of components in the current regulations DIN EN 12952-3 and DIN EN 13445-3

is carried out with equations for linear elastic material behavior. For components, where

irreversible deformations occur due to plastic deformation or creep strain accumulation the

use of these regulations lead to conservative results because the redistribution of stresses

and the reduction of local stress peaks are usually not considered. Even the treatment of

additional forces, bending and torsion moments of the piping system which influence the size

and direction of maximum principal stresses is not clearly handled by these regulations.

Therefore nonlinear viscoplastic simulations can be carried out to calculate the redistribution

of the stresses and the accumulation of inelastic strains in components with welded joints

with regard to its different material properties.

2 Comparison of technical rules for lifetime assessement of components

for elevated temperatures

Table 1 shows an overview of the procedures of the standards and regulations which are

used for design or lifetime evaluation: DIN EN 13480-3 [1], FDBR [2], AD2000 [3] and TRD

[4], with regard to the calculation of welded joints in pressurized components.

Regulation DIN EN13480-3 FDBR AD2000 TRD

σeff

(

( )

) (

( )

)

σallow

Additional

reduction of

σallow

-20% -20% - -

Table 1: Overview of the procedures of different regulations

The effective and the allowable stress are treated identical in the regulations DIN EN

13480-3 and FDBR. The allowable stress in AD2000 is equal to DIN EN 13480-3 and FDBR.

TRD includes differences in the determination of the allowable stresses for 200.000 h of

creep. There is used a denominator of 1 and in the other regulations it has a value of 1,25.

The calculation of the effective stress is different in the TRD and AD2000.

All these procedures make use of equivalent stress formulations but do not consider effects

of multiaxial stress states on the material or component behavior. The influence of primary

and secondary stresses in welded joints is not handled in a consistent manner. Due to this

reason a comparative assessment using these procedures and a detailed inelastic

calculation is of interest.

3 Numerical Investigations for welds in pipework

3.1 Welded joint with a high bending moment

For comparion with the results obtained using standard regulations, an inelastic finite

element simulation of a pipe section with a detailed modeled welded joint was performed.

The pipe section has the material properties of the martensitic steel P92 and is subjected to

internal pressure and bending moment. The level of the bending moment was defined

according to the FDBR regulation with the aim to get an ineligible load factor in the welded

joint. The critical value of the bending moment amounts to MB=6,1*108 Nmm. Furthermore

the internal pressure and the temperature were assumed to be 85 bar respectively 618°C for

the whole simulation for a service period of 200.000 h. At the determined temperature the

time dependent material behavior of the P92 base material was modeled by a modification of

the incremental Graham Walles creep law which is given by equation (1) and (2).

2

2

21

1

1

110

110 m

n

Am

n

A

DDdt

d

(1)

2

2

21

1

1 310

310 mD

nD

ADmD

nD

AD

qqdt

dD

(2)

Equation (1) is the modified Graham-Walles creep law whereby it is possible to describe the

primary, secondary and with the damage parameter D the tertiary creep effects. The damage

parameter D is calculated with equation (2) and depends on the multiaxiality factor q. The

parameter AD1, nD1, mD1, AD2, nD2, mD2, A1, n1, m1, A2, n2 and m2 are material

dependent parameter that are fitted with uniaxial creep tests.

For modeling purposes the welded joint was divided into four areas representing the

intercritical, the fine and the coarse grained zone and the weld metal as shown in figure 1.

The different creep behavior of these areas was determined with uniaxial creep tests of

specimens with simulated microstructure (weld simulation using Gleeble test). The resulting

creep curves were the basis of the approximation of the Graham-Walles parameter for each

area. The same material properties as the base material were assigned to the welde metal.

The creep curves of the different areas are shown in figure 2.

Using an user defined material (UMAT) the implementation of the time dependent material

behavior of material P92 in a finite element calculation was performed.

Figure 1: finite element model of a pipe section with a detailed modeled heat affected zones (HAZ)

Figure 2: Creep curves of the three areas of the HAZ and WM at a stress of 80 MPa

For geometrical modeling two reference points in cylindrical coordinates were defined. The

first reference point was used for the application of the bending moment and the second to

fix the model. Both reference points were coupled with „kinematic couplings“ and one degree

of freedom in radial direction was coupled to the free surfaces of the pipe section. By

choosing these boundary conditions expansion of the pipe as a result of the internal pressure

is possible.

With the described model a stress and a strain controlled simulation were performed. In the

stress controlled simulation all loads were treated like primary loads and in the strain

controlled simulation all loads with the exception of the internal pressure were treated like

secondary loads.

In figure 3 the stress and creep strain distribution in the weld region is shown in detail. The

top of the contour plot represents the inner side of the pipe wall, the bottom of the picture

shows the outer side of the pipe wall. In the upper row of the figure the equivalent Mises

stress after loading and after 200.000 h of creep is depicted, in the lower row the equivalent

creep strain after 200.000 h of creep is shown. For the 200.000 h state a distinction has been

made between the stress controlled and the strain controlled case.

After the first loading step the equivalent Mises stress on the inner side of the pipe is slightly

higher than on the outer side. Since both weld metal and heat affected zones have been

modeled using identical elastic material parameters, they cannot yet be distinguished in the

contour plot in this state. After 200.000 h the influence of the weld on the stress distribution

becomes apparent in both stress and strain controlled case, the maximum equivalent

stresses are encountered in HAZ 1, the minimal equivalent stresses can be found in HAZ 3.

Again in both cases (stress and strain controlled) the highest creep strains appear in HAZ3

on the inner side of wall.

Figure 3: Contourplots of the equivalent stress and creep strain in the weld for the stress and strain controlled loadcase

Figure 4 shows evaluations of the equivalent Mises stress and creep strain along a path from

inner to outer pipe surface in HAZ3 for stress and strain controlled loading case.

Stress controlled :

Strain controlled:

Figure 4: Path evaluation for equivalent Mises stress and creep strain from inner to outer surface along the HAZ 3

The finite-element-simulations have been evaluated with respect to time and the equivalent

stresses and creep strains are given in figure 5.

Figure 5: Temporal courses of the equivalent Mises stresses and creep strains evaluated at the respective maximum for the stress and strain controlled simulation

The development over time clearly shows that the stress controlled case yields significantly

higher stresses and strains than the strain controlled case. In addition, the Mises equivalent

stress obtained by analytical formula valid for linear elastic thickwalled pipes is plotted in the

diagram. The path evaluation of the HAZ3 shows a maximum accumulated creep strain close

to inner surface.

In order to assess the results of the FE-simulations, values taken from different locations

(base metal: BM, heat affected zone: HAZ1 and HAZ3) at several time points are compared

to the 100,000-creep strength of P92 base metal at 618 °C.

Table 2: Comparison of the finite element results with 100,000 h-creep strength acc. to VdTÜV und ECCC

In the regulations AD2000 and TRD the bending moment is not considered in the effective

stress calculation. The comparison of the evaluation of local values of effective and axial

(perpendicular to the weld) stress shows that after short relaxation periods the stresses in the

critical zones especially in the intercritical heat affected zone are small. It is also visible that

the axial stress which is the critical one since it is perpendicular to the weld is significantly

smaller than the effective stress. This is in coincidence with the estimated strains (see Fig.

4).

Normalized

stress

0 h 5.000 h 200.000 h

HAZ1 HAZ3 BM HAZ1 HAZ3 BM HAZ1 HAZ3 BM

Mises 0.93 0.93 0.93 0.70 0.47 0.58 0.70 0.45 0.58

Axial 0.23 0.23 0.23 0.30 0.29 0.30 0.30 0.31 0.31

3.2 Dissimilar welded joints

The 700°C power plants currently under development will utilize Ni-base alloys such as

Alloy 617. Due to technical and economic reasons CrMoV-steels will be used for components

or parts of components which are subjected to temperatures < 650 °C. Therefore dissimilar

joining of Ni-base alloys and Cr-steels is necessary in these plants.

The following investigation focuses on welds between Alloy 617 and a 2Cr-steel. Figure 6

shows the creep-rupture strength at 550 °C. In creep tests two competing failure

mechanisms are observed:

Failure in the fusion line (FL) between ferritic/martensitic steel and nickel based weld metal. The crack usually occurs on the outer surface.

Failure in the heat affected zone (HAZ) of ferritic/martensitic steel. The crack usually occurs in the specimen center.

Figure 6: Creep-rupture strength of 2Cr/A617-welds at 550°C [5]

Several research projects, [6]-[10], have identified the intercritical heat affected zone (HAZ3)

as the “weakest link” in welded components under service conditions. The heat input during

the welding procedure affects the microstructure of this zone [6].

Inelastic analysis is able to obtain detailed information on the deformation and failure

behavior of welds. Figure 6 shows contour plots of an axisymmetric finite-element analysis at

different time points. The increase in stress in the Ni-base weld metal near the fusion line

and (due to the lower creep resistance) the concentration of creep strain in the intercritical

HAZ3 can be seen.

In Figure 7 the development of stress and creep strain over time is shown for two different

loads. The time to rupture in the experiment is marked respectively. In case of the lower load

(92 MPa) the creep strain rate in the HAZ is so small that the axial stress in the weld metal

becomes predominant and as a consequence crack growth completely takes place in the

fusion line. The higher load (125 MPa) leads to higher stresses in the weld metal and higher

creep rates in the heat affected zone. Thus, the two competing damage mechanisms in weld

metal and HAZ become nearly equally important. On the one hand the higher creep rate

promotes failure in the HAZ. On the other hand the maximum principal stress in the fusion

line increases, even compared to the equivalent stress, which still leads to cracks in the

fusion line.

Figure 7: Regions of highest stress and strain in a creep-rupture specimen (FEM-simulation)

Figure 8: Development of stresses in fusion line and strain in HAZ over time of 2Cr/A617-welds at 550°C (FEM-simulation)

The different stress distributions in weld metal and HAZ are illustrated in figure 8. The

highest values of maximum principal stress in the weld metal are found at the outer surface,

while the mostly stressed region in the HAZ is near the center, which corresponds to the

crack positions found in the specimens.

0 2000 4000 60000,0

0,5

1,0

1,5

2,0

HAZ3: intercritical zone

eq

uiv

ale

nt

cre

ep

str

ain

/ %

time / h

SV1Z19, 550°C, 125 MPa

0 2000 4000 6000

0

50

100

150

200

250

Mis

es s

tre

ss / M

Pa

time / h

Mises (below surface)

Mises (center)

axial stress (below surface)

axial stress (center)

SV1Z19, 550°C, 125 MPa

0 2000 4000 6000 8000 10000 12000

0

50

100

150

200

Mis

es s

tre

ss / M

Pa

time / h

Mises (below surface)

Mises (center)

axial stress (below surface)

axial stress (center)

SV1Z20, 550°C, 92 MPa

0 2000 4000 6000 8000 10000 120000,0

0,5

1,0

1,5

2,0

HAZ3: intercritical zone

eq

uiv

ale

nt

cre

ep

str

ain

/ %

time / h

SV1Z20, 550°C, 92 MPa

Figure 9: Stress distributions in weld metal and HAZ at time to rupture (FEM-simulation)

To summarise, the following findings have been presented:

• Creep tests of dissimilar welds have shown significant creep strength reduction • Two competing damage mechanisms have been identified • Numerical simulation of crossweld specimens gave evidence to the development of

different damage mechanisms by correlation to the development of stresses and strains

Since the numerical calculations can explain the damage occurring in specimens similar

calculations for components can be used to make predictions on the failure behavior of

components.

LITERATURE

1. Metallic industrial piping – Part 3: Design and calculation; German version EN 13480-3:2012, Beuth Verlag, Berlin

2. FDBR-Taschenbuch Rohrleitungstechnik 1: Planung und Berechnung, Günter Wossog, Vulkan, 2005

3. AD2000 Regelwerk Taschenbuch – Ausgabe 2009, Verband der TÜV e.V., Berlin 4. TRD - Technische Regeln für Dampfkessel mit den Vereinbarungen der Verbände:

Taschenbuch-Ausgabe 1998/2, Heymann 5. T. Klein, C. Feuillette and A. Klenk: „Verhalten von optimierten Werkstoffen und

Werkstoffverbunden für 700-720°C-Dampfturbinenbauteile“, Teilvorhaben MPA Stuttgart zu Verbundvorhaben COORETEC DT3, 2010

6. K. Maile et al.: „Zeitstandverhalten von P91-Schweißverbindungen“, 19. Vortragsveranstaltung AGW/AGHT, Düsseldorf, 1996

7. R. U. Husemann, et al.: „Langzeiteigenschaften von Schweißverbindungen moderner Stähle für Dampferzeuger“, 23. Vortragsveranstaltung AGW/AGHT, Düsseldorf, 2000

8. K. Maile et al.: “Behavior of similar welds in T24 Tubes and E911 pipes”, 3rd HIDA and Integrity Conference, Integrity of High Temperature Repair Welds, Oeiras-Lisabon, Portugal, 16-18 September, 2002, pp. 69/80.

9. M. Bauer et al.: “Evaluation of cross weld creep strength of 9-11% Cr steels”, Proceedings Int. Conf. on Plant Life Extension, Cambridge, 16-18 April 2004

10. M. Bauer, A. Klenk, K. Maile and E. Roos: “Numerical Investigations On Optimisation Of Weld Creep Performance In Martensitic Steel”, 8 th Seminar of Mathematical Modelling of Weld Phenomena, Seggau, 2008

0 2 4

20

40

60

80

100

120

140

160

180

200

220

240

260

280

300

SV1Z20, 550°C, 92 MPa

str

ess / M

Pa

radius [mm]

weld metal_Mises

weld metal_SMax

HAZ3_Mises

HAZ3_SMax

0 2 4

20

40

60

80

100

120

140

160

180

200

220

240

260

280

300

SV1Z19, 550°C, 125 MPa

str

ess / M

Pa

radius [mm]

weld metal_Mises

weld metal_SMax

HAZ3_Mises

HAZ3_SMax