The thermal fatigue behavior of the combustor alloys In 617 and HAYNES 230 before and after welding |
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Authors: | F Meyer-Olbersleben N Kasik B Ilschner F Rézaï-Aria |
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Affiliation: | (1) Present address: the Swiss Federal Institute of Technology, is with Siemens AG, Power Generation (KWU), Mülheim, Germany;(2) ABB Power Generation Ltd., Baden, Switzerland;(3) the Swiss Federal Institute of Technology, Lausanne, Switzerland;(4) the Swiss Federal Institute of Technology, is with Ecole des Mines d’Albi-Carmaux, 81013 Albi Cedex 9, France |
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Abstract: | The nickel-base alloys IN 617 and HAYNES 230 for welded high-temperature components have been subjected to thermal fatigue
(TF) loading. In a series of TF tests in air, single wedge specimens were induction heated and compressed-air cooled at the
leading edge for various temperature cycles between 200 °C and either 850 °C, 950 °C, or 1050 °C. The test rigs permitted
simultaneous measurements of temperature and total strain along the edge of specimen during TF cycling. Both materials have
been tested in conditions relevant for hot path components in the gas turbines, e.g., “as delivered,” “welded,” and “welded + notched”. Under identical temperature cycles and thermal gradients, HAYNES 230 showed
a higher TF strength than IN 617 in the as-delivered condition. It is suggested that this advantage of HAYNES 230 is primarily
related to its lower value of the relevant combination of properties of this alloy: coefficient of thermal expansion, thermal
conductivity, elastic modulus, ultimate tensile strength, taken at maximal operating temperature. In addition, the advantage
of the HAYNES 230 is described by a lower plastic strain, which is induced at the wedge region during TF loading. Moreover,
microstructural details of crack initiation, crack propagation, and reactions with the gaseous environment play an important
role. Both alloys investigated in the present work showed plastic deformation with a maximum in the central zone of the wedge
tip. In this zone, slip bands and grain distortion occurred, whereas both ends of the wedge tip free of visible plastic deformation.
The TF cycles led to multiple transgranular crack initiation and propagation. In welded specimens of IN 617 and HAYNES 230,
cracks appeared first in the center of the weld. The susceptibility of welds to TF cracking depends considerably on the weld
filler and the surface quality. It was shown for HAYNES 230 that a mismatched weld could reduce the TF life to less than 50
pct of non-welded specimens. The lower TF-fatigue strength of the welded specimens can be explained by the difficulty of the
cast alloy in the welded zone to accommodate the repeated thermal shocks by plastic deformation. Notches introduced in the
heat-affected zone (depth about 0.1 mm) reduced the TF life of both alloys by a factor as high as 4. The thermal fatigue strength
of the welded material can almost reach the values of the base alloy provided the use of matching electrodes, post-weld heat
treatment, and grinding off the weld beads is carefully executed. |
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