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Research Papers: Materials and Fabrication

High Temperature Fatigue of Welded Joints—Experimental Investigation and Local Analysis of Butt Welded Flat and Cruciform Specimens

[+] Author and Article Information
Kay Langschwager

Institute for Material Technology,
TU Darmstadt,
Darmstadt 64283, Germany
e-mail: langschwager@mpa-ifw.tu-darmstadt.de

Jürgen Rudolph

AREVA GmbH,
Erlangen 91052, Germany
e-mail: rudolph.juergen@areva.com

Alfred Scholz

Institute for Material Technology,
TU Darmstadt,
Darmstadt 64283, Germany
e-mail: Scholz@mpa-ifw.tu-darmstadt.de

Matthias Oechsner

Institute for Material Technology,
TU Darmstadt,
Darmstadt 64283, Germany
e-mail: oechsner@mpa-ifw.tu-darmstadt.de

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received June 27, 2016; final manuscript received February 23, 2017; published online April 24, 2017. Assoc. Editor: Xian-Kui Zhu.

J. Pressure Vessel Technol 139(4), 041408 (Apr 24, 2017) (9 pages) Paper No: PVT-16-1099; doi: 10.1115/1.4036140 History: Received June 27, 2016; Revised February 23, 2017

Austenitic stainless steel of type X6CrNiNb18-10 exhibits advantageous mechanical and chemical properties and is a common material for numerous applications in the nuclear power plant and chemical industries. Besides the mechanical strain induced by high pressure, the fatigue life in welded pipelines is affected by additional thermomechanical strains due to thermal loading. The welding process mainly determines the geometry and metallurgical constitution of the welded joint. Therefore, the butt welds additionally influence the strain gradient along the component and reduce its lifetime. While the base and weld material are similar, they show different softening and hardening behavior, especially at ambient temperature. Cyclic hardening occurs in the base material, whereas cyclic softening can be observed in the weld material. The hardness distribution along the welded joint reveals no clear differentiation of the base material, the heat affected zone, and the weld material. The attributes of the individual materials cannot be transferred to the welded joint automatically. Thus, the analysis of the interaction between the materials along the welded joint is a main topic of this research. To this end, digital image correlation (DIC) is used for different kinds of specimens and load conditions. The position along the testing area at which fatigue failure occurs depends on the specimen type and the load condition but not on the temperature. Further, isothermal and anisothermal fatigue tests on welded cruciform specimens are presented. The common practice of the effective strain is discussed for the analyzed conditions.

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References

Figures

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Fig. 1

Geometry of the uniaxial LCF and flat specimens, schematic representation of the weld above the transverse section in the case of the as-welded joint and the weld ground in accordance with normal practice at this application

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Fig. 2

Measurement and sectioning of the welded semifinished pipes into various specimen types, standardized LCF specimens out of the base material and along the weld, as well as flat specimens with typical weld [4]

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Fig. 3

Schematic representation of the types of load along the transverse section of pipe and representation of a microhardness profile (HV 0.5) along an as-welded joint in transverse section [5]

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Fig. 4

Representation of the hardening and softening behavior of selected specimens of the austenitic base and filler materials in their original state. Test temperature: 350 °C.

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Fig. 5

Representation of the hardening and softening behavior of selected specimens of the austenitic base and filler materials in their original state. Test temperature: 25 °C.

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Fig. 6

Test setup for uniaxial testing with a system for attendant DIC and a schematic representation of the displacement field within the test zone, ARAMIS system[4]

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Fig. 7

Strain field in longitudinal direction as the result of the DIC for the welded round specimen under compression loading, virtual extensometer for validation of the measurements and for determination of the local strain at the incipient crack, εa = ±0.36%, Ni = 5037 cycles

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Fig. 8

Comparison between mechanically controlled stress–strain hysteresis and global optically measured comparative hysteresis, as well as representation of the local stress–strain hysteresis at the crack, Ref. [4]

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Fig. 9

Results of the uniaxial LCF experiments on standardized round specimens for the base material, the weld filler and the welded state, KTA mean and design curves, and comparison between global control and actual local strain at ambient temperature [4]

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Fig. 10

Results of the uniaxial LCF experiments on standardized round specimens for the base material, the weld filler and the welded state, KTA mean, and design curves at higher temperatures [4]

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Fig. 11

Showing the different failure locations on the welded LCF round specimens dependent on the controlled strain amplitude at ambient temperature [4]

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Fig. 12

Results of the uniaxial LCF experiments on finished and unfinished flat specimens, KTA mean, and design curves at ambient temperature [4]

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Fig. 13

Results of the uniaxial LCF experiments on finished flat specimens, KTA mean, and design curves at high temperatures [4]

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Fig. 14

Longitudinal strain trajectory along a selected average path of a welded finished flat specimen under compression loading, failure location within the weld, originating at the face of the weld

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Fig. 15

Results of the uniaxial LCF experiments on unfinished flat specimens, KTA mean, and design curves at high temperatures [4]

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Fig. 16

Illustration of a welded cruciform specimen with electropolished surface, prepared from a welded austenitic plate, geometry of the cruciform specimen

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Fig. 17

Test setup for biaxial testing under thermomechanical loads

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Fig. 18

Calculated effective strain along the cruciform specimen during tensile loading, schematic representation of the weld and the test zone and the virtual area of analysis Fa = ±35 kN, Ni = 3720 cycles

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Fig. 19

Results of the biaxial cruciform specimen tests for the welded state, KTA mean and design curve, as well as the comparison between global and actual locally occurring effective strain under TMF loading [4]

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