PSI - Issue 60

P.K. Sharma et al. / Procedia Structural Integrity 60 (2024) 335–344 P.K. Sharma/ StructuralIntegrity Procedia 00 (2023) 000 – 000

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1. Introduction Alloy 690 is a nickel-based alloy that has a nominal composition of Ni-30Cr-10Fe (UNS N06690). Due to its high chromium content, this material is resistant to various forms of intergranular stress corrosion cracking (Kuang et al. 2018 and Stiller et al. 1996) making it ideal candidate for its application in steam generator tubes of pressurized water reactor (Allen et al. 1996 and Harrod et al. 2001). Nickel base in Alloy 690 has a face-centred-cubic matrix, better phase stability and reduced diffusivity of secondary alloying elements at high temperatures providing greater resistance to creep assisted crack growth (Hu et al. 2012). Alloy 690 is also used in fabrication of vitrification melter components (Kaushik et al. 2006) where the operating temperature may reach upto 1100 ˚ C. During operation of vitrification melters, the material is exposed to very high temperatures, thermal stresses, and dead weight with aggressive corrosive and chemical environment (Kaushik et al. 2013) that may develop high stresses at the concentration zones. This high temperature corrosive environment may lead to formation of small micro-crack that might grow under different operating conditions (Li et al. 2013). Hence, it is important to understand the fracture behavior of Alloy 690 at high temperatures in order to prevent any catastrophic failure of components during its normal operation. The microstructure and mechanical properties of Alloy 690 are influenced by its chemical composition, production route, and thermo-mechanical heat treatment techniques. Comparison of fracture behavior of Inconel 690 material manufactured using hot isostatically pressed and forging was carried out by Cooper et al. 2018 over the temperature range of 300˚C to - 196˚C using charpy impact technique. Creep behavior of Alloy 690 was obtained in the temperature range of 800˚C to 1000˚C (Kumar et al. 2022) to understand the dominating creep deformation mechanism and prediction of creep life. The stress-corrosion cracking tests were conducted by Moss et al. 2017 by using standard compact tension (CT) specimens of Alloy 690 material in super critical water (SCW) and inert gas environments. Higher crack growth rates was observed at temperatures between 450°C and 550°C due to extensive cavity formation ahead of the crack tip that weakens the grain boundary and accelerate inter-granular crack growth. The effect of intergranular precipitation of carbide on the hydrogen embrittlement of alloy 690 was studied in Symons 1998. As the material ages, the brittleness caused by hydrogen increases, resulting in the precipitation of carbides at grain boundaries, and the increase in brittleness reduces ductility and fracture toughness of the material. This acts as a preferable site for nucleation of the crack that might grow under different loading conditions. The number of cavities at or near grain boundary in Alloy 690 increases with increasing cold work reduction ratio and with increase in temperature (Yonezawa et al. 2021). Inter-granular stress-corrosion cracking resistance of Inconel 690 Alloy can be improved through thermo-mechanical processing (Boehlert 2008) following strain annealing sequence, i.e., cold rolling to 25% deformation followed by a solution treatment at 1000°C for 1 hour followed by air cooling. The effect of heat treatment on the chromium depletion and precipitate evolution near grain boundaries of Alloy 690 was studied by Kai et al. 1988 concluding that the lowest chromium content at the grain boundary was found to be a function of heating time and at temperatures of 538°C, 600°C, and 700°C.

Nomenclature a

Crack length

W

Specimen width

a 0 B e

Initial crack length including fatigue pre-crack net thickness after side-grooving of the specimen

J i,0.2

Crack initiation toughness Stress Intensity factor Hardening exponent Load applied on the specimen Coefficients of factor G(a/W)

K

C ∗ E c o -c 5

elastic compliance

n P

Unloading compliance constants

Blunting coefficient Young’s modulus

t

A lot of work has been carried out on the stress corrosion cracking, heat treatments and carbide precipitation of Alloy 690 material, however, the study of fracture behavior of Alloy 690 material in the temperature range of 600 - 1000°C is limited. After development of micro-cracks due to stress-corrosion cracking or any other mechanisms in Alloy 690 material, the crack propagation needs to be examined in order to maintain the components under safe operating condition. Hence, the present work aims to evaluate the ductile crack initiation and propagation toughness