PSI - Issue 60

K. Mariappan et al. / Procedia Structural Integrity 60 (2024) 444–455 Author name / StructuralIntegrity Procedia 00 (2019) 000 – 000

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1. Introduction AISI 316L(N) stainless steel (SS) is the material of choice for the Indian fast breeder reactor components in view of its good high temperature mechanical properties, compatibility with liquid sodium coolant, weldability, availability of design data and a fairly vast and satisfactory international plant experiences [Baldev Raj etal (2002)]. A detailed understanding of the constitutive flow behaviour of structural material such as 316L(N) SS is essential to predict the performance of the components in service conditions. During the reactor operation, temperature transients due to the start-ups, shut-downs introduce low cycle fatigue loading conditions, specifically at bends in the thick components. Hence, an understanding of the effects of the extent of prior fatigue loading cycles on the remnant flow behaviour of the structural materials becomes essential to monitor the health and remaining life of the operating plants. Such studies are relatively rare and the present work aims to examine in detail the role of prior fatigue loading on tensile flow behaviour of 316L(N) SS. Towards this, well known constitutive equations, namely Holloman [Hollomon (1945)], Swift [Swift (1952)], Ludwigson [Ludwigson (1971)], and Voce [Voce (1948)] equations have been used for describing the tensile flow behaviour of 316L(N) SS. The plastic flow behaviour in many metals and alloys within the uniform elongation regime has been described by the Hollomon power law relation, as given below [Hollomon (1945)]: = . (1) where σ is the true stress, ε p is the true plastic strain, K and n are the strength coefficient and strain-hardening exponent, respectively. Swift proposed a relationship with an additional strain term, ε 0 that accounts for the pre-strain left in the material, as mentioned below [Swift (1952)]: = . ( 0 + ) (2) Low stacking fault energy materials such as austenitic stainless steels exhibit an initial non-linear transient behaviour followed by the linear variation on a double logarithmic plot of true stress-true plastic strain. An additional term, exp ( K 2 + n 2 ε p ), to the Hollomon equation (eq.1) accounts for the non-linear transient as [Ludwigson (1971)]: = 1 . ( 0 + ) 1 + ⁡( 2 + 2 . ) (3) However, the saturation in stress values exhibited at large strains in the σ - ε p plots at elevated temperatures cannot be described by the Hollomon, Swift and Ludwigson equation. In describing such flow curve, Voce equation can be employed which is given as [Voce (1948)]: = +( − ). [1 − ⁡ ( −( − ) )] (4) where σ i represents the initial true stress at the commencement of the plastic deformation ε i , and the σ s represents saturation stress, and ε c is a strain constant. Of these constants, ε i is generally set as zero. Thus, eq.(4) reduces to = +( − ). ⁡( . ) (5) where n v is fit constant. 2. Experimental Nuclear grade AISI 316L(N) SS of chemical composition: 0.02C-17.93Cr-12.09Ni-2.43Mo-1.76Mn-0.44Cu 0.3Si-0.013P-0.06N-0.01S (wt-%) and 20 ppm of B was used in the present study. The specimen blanks were solution annealed at 1373 K for 30 mins. followed by water quenching. The typical initial microstructure of 316L(N) SS in the solution annealed condition is shown in Fig. 1. The average grain size of the alloy computed based on the linear