Characterization of elevated temperature properties of heat exchanger and steam generator alloys

Abstract

-The Next Generation Nuclear Plant project is considering

Effect of Strain Range on the Low Cycle Fatigue in Alloy 617 at High Temperature

“…It is notable that the yield strength (YS) and the ultimate tensile strength (UTS) values were determined from the area in which the plastic deformation occurred (non-uniform) along the engineering stress-strain relationship, as well as with the highest stress magnitude during deformation behavior, respectively. The elastic modulus was calculated according to the slope in the elastic region, which is in good agreement with that reported in the literature [5]. The tensile elongation and reduction of area (as a definition of material ductility) are also in accordance with the literature.…”

“…A thin oxide layer makes it difficult to observe the crack initiation. The cracking of surface brittle oxides at the periphery specimen was developed on the grain boundary, and this may induce a weak stress response, see [5,6]. The reduction in fatigue life under higher total strain ranges is clearly acquired partly from the activation of the slip system and an increase in plastic deformation in a cycle, and is evidenced by those non-linear cracks.…”

“…The strengthening process of this Alloy 617 is granted by the hardening precipitates of Alloy carbides, which provide excellent fatigue resistance at high temperature processes [3,4]; the high temperature oxidation resistance is derived from the high nickel and chromium content. The Alloy 617 is expected to provide good thermal stability for components of power generating plants with high temperature strength up to 950 • C [5].…”

“…Investigation into these design considerations should be carried out, because a significant degradation of mechanical strength may occur in the high temperature condition. Under this circumstance, the cyclic loadings in the low cycle fatigue (LCF) regime describe a main failure mechanism from a temperature-gradient-induced thermal strain during operation, as well as startups and shutdowns and power transients or temperature change of flowing coolant [5,6]. In the state-of-the-art, fatigue damage accumulation as a stress and strain concentration during cyclic loading is potentially causing the crack initiation site and propagation onwards [7].…”

“…Therefore, a thorough knowledge of the elastoplastic behavior is necessary in the design and life evaluation of such components that operate in the high temperature condition. Although Alloy 617 has many superior properties, many researchers have reported that the number of cycles to failure varies widely at high temperatures [5][6][7][8][9]. However, they did not provide sufficient data on the Alloy 617 in the LCF regime over a range of conditions.…”