Analysis of the deformation and damage development of a dissimilar weld operating in the creep regime

Nuclear Engineering and Design 119 (1990) 231-238 North-Holland

231

ANALYSIS OF THE DEFORMATION AND DAMAGE DEVELOPMENT OF A DISSIMILAR WELD OPERATING IN THE CREEP REGIME Klaus S C H N E I D E R 1, Erich T O L K S D O R F 2 a n d Giinter G N I R B 1 1 ASEA Brown Booeri AG, Mannheira, Fed Rep. Germany z VGB, Essen, Fed. Rep. Germany Received: first version 16 February 1989, revised version 3 October 1989

The analysis of the deformation and damage behaviour of stress rupture tests with specimens out of the dissimilar metal weld seam 12% Cr-steel welded with a nickel base electrode for alloy 800 exhibits two competing processes: - Crack initiation occurs along the melting line due to high thermal stresses; - Creep deformation and damage concentrates in a heat affected zone of the ferritic 12% Cr steel due to long term stresses. The velocity of stress relaxation determines the resulting damage mechanism. At high temperatures with predominant creep deformation the cracks initiated in the melting line arrest and the creep deformation is concentrated in the heat affected zone (HAZ). At lower temperature the fracture area along the melting line increases. Long term tests at 535 o C lead to lower stress rupture values compared to the seatterband of X 20 CrMoV 12 1 due to the reduced cross section after crack initiation in the melting line. The analysis of stress rupture tests leads to the conclusion that grinding of melting line cracks is a reasonable measure because of sufficient stress relaxation.

1. Introduction In power plants especially in high temperature reactors metallic components consisting of different materials are in use. The material is selected with respect to the individual maximum service temperature, fabrication procedure and for economic reasons. The maximum service temperature of ferridc steels is limited due to the stress rupture strength. 300

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At higher service temperatures austenitic Cr-Nisteels are applied or as in the case of THTR 300 the iron-nickel-chromium alloy 800 (X 10 NiCrA1Ti 32 20) which has the advantage of being applicable up to 800 *C and even higher. The welding seams of ferritic and austenitic materials are critical sections because of high stresses at service 1,0 X20 C.*~oV 12 11

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Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d )

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Fig. 3. Creep behaviour of individual materials in dissimilar welding X 20 CrMoV 12 1 - buffer.

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K. Schneider et al. / Analysis of the deformation and damage development

temperature due to differeing thermal expansion independent of service loadings like pressure (fig. 1). A possibility often used to reduce these thermal stresses is the choice of a suitable weld metal with high Ni content and a coefficient of thermal expansion between the ferritic and austenitic parent metal. Additionally a reasonable stress level can be achieved by adjusting the geometry of the weld (such as flank angles, width of the weld seam [1]). The lifetime determining zone of a dissimilar weld operating in the creep regime is located at the transition area of the ferrite to the weld metal, because the ferrite is used at the upper limit of it's application temperature. Additionally material conditions may be produced during fabrication or service in the transition zone between ferrite and weld metal and in the HAZ of the ferrite which exhibit lower creep resistance (soft zones) or which are sensitive to crack initiation (hard zones) consisting of brittle phases like carbides. Service experiences with other dissimilar weld seams are reported in [2-6]. The following contribution describes the analysis of the deformation and damage behaviour of the weld joint between X 20 CrMoV 12 1 and X 10 NiCrAITi 32 20 welded with the electrode S-NiCr 16 FeMn, DIN-No. 2.4620 (commercial name Crdni 7 (NCF9) R 800 or UTP 7015 Mo). This welding joint is applied in the water/steam circuit of the THTR 300 and is intended to be applied in the future reactors H T R 500 and H T R module. The findings are as well applicable to other high temperature components with dissimilar weld seams.

2. Materials and weld joints

233

3. Creep and stress rupture behaviour Stress rupture tests with specimens perpendicular to the weld seam exhibit deformation maxima in the transition zone of the ferrite to the buttering (fig. 2) after short times. During long term creep testing a shift of the deformation peak from the melting line or next to the melting line in the buttering to the HAZ of the ferritic steel X 20 CrMoV 121 is observed. The local creep deformation behaviour of the dissimilar metal weld joint parallel to the weld seam also shows that the maximum deformation occurs in the H A Z of the X 20 CrMoV 12 1 for long time testing (fig. 3). Stress rupture behaviour therefore is different in the two directions (figs. 4 and 5). Thus the following limits of stress rupture strength are estimated perpendicular to the weld seam: 550°C: 2 × 105 h 50 MPa (X 20 rain. 89 MPa) 600°C: 2 × 105 h 20 MPa (X 20 rain. 38 MPa). The fracture appearance of stress rupture specimens perpendicular to the weld seam is characterized by the fracture location (fig. 6). As stated when discussing the deformation behaviour the maximum deformation is shifted from the melting line into the H A Z of the X 20 CrMoV 12 1 if the external loading is comparitively low. Correspondhag to that behaviour characteristic parts of the melting line or the H A Z of the 'X 20 CrMoV 12 1 can be observed on the fracture surface (figs. 6 and 7). Even in case of 100~ H A Z fracture crack initiation in the melting line can be observed (fig. 7).

5. Discussion 5.1. Microstructural changes

Dissimilar metal weld joints with the parent metals X 20 CrMoV 12 1 and X 10 NiCrA1Ti 32 20 were fabricated by buttering the ferritic pipe with a weld metal S - N i C r 16 FeMn (materials-no. 2.4620) and a bell seam made of the same weld metal connecting the buttering with the austenitic pipe section. The buttering was welded at low preheating temperature (180-250°C) to avoid hot crack in the weld. Afterwards the buttering was annealed at 780 o C. The bell seam was welded at an intermediate layer temperature of 120°C. The flanks of the bell seam have an angle of 7 degrees. Heat treatments and creep rupture tests at 550°C and 600°C were performed to control the long term behaviour of the weld joint.

Initial state condition The (critical) transition ferrite (X 20 CrMoV 12 1) (1) to the austenitic buttering is characterized by a zone (2) located in front of the X 20 CrMoV 12 1 with equal chemical composition with the exception of a higher Ni content. In the buttering a alloying zone (3) being three layers wide = 4 mm can be determined characterized by a higher Fe content. The transition ferrite/austenite which is critical due to thermal stresses is characterized by the interface (2)/(3) (fig. 8). A fine grained austenitic zone with carbide layers, on the grain boundaries is located in front of the interface (2)/(3). Hardness values and chemical composition point towards a Ni con-

234

K. Schneider et al. / Analysis of the deformation and damage development

Fig. 6. Creep fractures of dissimilar weldings and percentage of fracture area of the melting line as a function of time to rupture at 550°C.

Fig. 7. Percentage of fracture area of the melting line as a function of time to rupture at 600 ° C, HAZ=fracture and cracks in the melting time.

K. Schneider et al. / Analysis of the deformation and damage development

235

Fig. 8. Microstructure of dissimilar welding: as-received condition.

raining soft martensite in the zone (2), which is not annealed at stress relief temperature (fig. 10). Long term loaded dissimilar metal weld seam After long term ageing at 550°C (30000 h) the soft martensite zone is annealed and exhibits carbide dispersion. Pronounced grain boundary precipitations in the

zone (3) point towards C-diffusion. In the remelting area (3) the hardness values are smaller than in the buttering. After the stress rupture test ( 5 5 0 ° C / 1 4 732 h / 1 3 0 MPa, fracture, fig. 9 and 11) the microstructure and element distribution is comparable to that after the thermal exposure.

Fig. 9. Microstructure of dissimilar welding: after creep test at 550 o C.

K. Schneider et al. / Analysis of the deformation and damage development

236

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In stress rupture tests already after 2 / 3 of the expected rupture time creep damage in the HAZ (near the melting line) of X 20 CrMoV 12 1 is observed. Broken specimens show extensive damage within the whole HAZ. Typical stress rupture development of X 20

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CrMoV 12 1 is shown in [1] and [8]. Additionally cracks along the melting line are found (fig. 12). Microcracks in the region of the melting line of dissimilar welds are reported in refs. [9-12]. They were caused by the formation of carbide layers during service. The damage development means that at the beginning of the loading in the melting line cracks may

5.2. Damage development

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237

K Schneider et al. / Analysis of the deformation and damage development Table 1 Penetration depths of important alloying elements in dissimilar weld seams Element

Grini 7 C Cr Ni Fe Nb X20 C Cr Ni Fe Nb

750 o C/2 h [/xm]

550 ° C/30000 h [pro]

600 * C/16 000 h [pro]

535 o C/3 X 105 h [/Lm]

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170

260

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0.1 240 5.5 0.5 0.27 0.1

initiate because the melting line sees the highest stresses. The further growth of these cracks is competing with stress rearrangement processes within the material transition zone. However, this crack growth does not determine the final failure. This fact shows that there is no tendency towards notch embfiddlement.

0.1 380 10 1 0.6 0.1

600 13 1 0.6 0.3

5.3. Long-term and diffusion behaoiour Metallography, hardness and stress distribution point out that the remelting zone (3) is the least creep resistant area in the beginning. Diffusion of Cr, Ni and Fe are negligible (less than 1 #m) at 550°C according to the estimation in table 1. Carbon diffusion, however, is possible (100-300/zm). At 6 0 0 ° C marked diffusion of metallic atoms is possible (Cr = 10 /~m) enabling other deformation mechanisms. At 550°C and lower temperatures thermal changes in the material up to 30000 h are controlled only by carbon diffusion and carbide precipitation. Estimations for 535 ° C show that diffusion processes for service times up to 3 × 105 h can be simulated within 50000 h at 5 5 0 ° C or 15000 h at 570°C.

6. C o n d u s i o n s

In a dissimilar weld seam between X 20 CrMoV 12 1 and X 10 NiCrAITi 32 20 loaded by long time stresses the transition zone from ferritic parent metal to austenltic weld metal reveals to be the most critical

region.

Fig. 12. Competing damase processes in dissimilar welding: crack initiation and crack propagation in melting line/damage development in HAZ.

Stresses normal to the weld cause two competing processes: - crack initiation along the melting line due to high thermal stresses and Ni-containing soft martensite, - c o n c e n t r a t i o n of creep deformation and damage within the HAZ of X 20 CrMoV 12 1 due to thermal stresses.

238

K Schneider et al. / Analysis of the deformation and damage development

Resulting damage process is controlled by the velocity of relaxation. At 600"C obviously melting line cracks will arrest and further creep deformation concentrates in the HAZ. At lower temperatures the fracture area along the melting line increases resulting in a marked reduction of the load bearing cross section and therefore also of the lifetime in a stress rupture test. At 550 ° C fracture points of the stress rupture tests may be shifted upwards to the scatter band of X 20 CrMoV 12 1 by correcting the load bearing cross section. At 600 o C, however, an additional influence on the HAZ of X 20 CrMoV 12 1 by the welding procedure has taken place leading to accelerated creep rates at high temperatures. It is expected that at dissimilar weld seam service temperatures of 535°C this influence is less pronounced. It can be concluded that grinding of melting line cracks is a reasonable measure after sufficient stress relaxation has taken place. It is only applicable to dissimilar weld seams which do not suffer from local damage in the melting line. Today this seems to be true for the dissimilar weld seam discussed in this paper, but it has to be proved by further investigations. This contribution discusses the available results of an ongoing research programme. A final evaluation of the governing damage mechanisms with respect to service related loadings is planned towards the end of the programme. The damage development in different dissimilar welds will be compared regarding metallurgical and thermo-mechanical boundary conditions.

Acknowledgement This report is part of a research programme which is funded by the Minister of Research and Technology of the Federal Republic of Germany under the funding code 03 BBC 209. The author is responsible for the content of this publication.

References [I] R. Fidler, The effect of weld width on the performance of main steam pipes welded by the narrow gap process, CEGB-Report TPRD/M/1583/R86 (August 1986). [2] H. Thier, Schwarz-Weiss-Verbindungen-Probleme und Ltstmgen, Schweiszen yon Behllltern und Rohrleitungen '82, Sondertagung des DVS, Mtinchen 1982, pp. 8-17. [3] H. Zlirn, E. Moraeh, Z. f. Werkstofftechnik 5 (1974) 146-155. [4] H. Jesper, H. Kaes, VGB-Mitteilungen 1984, pp. 668-698. [5] J. Class, VGB-Mitteilungen 1962, pp. 326-337. [6] E. Jahn, VGB-Mitteilungen 1962, pp. 86-93. [7] W. Poeppel, E. Tolksdorf, Klassifizierung der Zeitstandseh~digung von 12% CrMoV-Stahl bei 550°C, Metallographie-Tagung 1987, Osnab~ek, Sonderband 19. [8] R. Bfirgel, W. Poeppel, E. Tolksdorf, X 20 CrMoV 12 1 mad die Entwi¢ldtmg yon Zeitstandsehlidigung, VGBTagung, Essen 1987. [9] R.A. Ainswortl~ M.C. Coleman, Fatigue Fract. Eng. Mat. Struet., Vol. 10, No. 2 (1987) 129-140. [10] Int. J. Press. Ves. and Piping 11 (1983) 1-18. [11] Int. J. Press. Ves. and Piping 15 (1984) 271-290. [12] Int. J. Press. Ves. and Piping 18 (1985) 277-310.