PSI - Issue 14

Hari Krishan Yadav et al. / Procedia Structural Integrity 14 (2019) 605–611 Hari krishan Yadav/ Structural Integrity Procedia 00 (2018) 000–000

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1. Introduction Currently, for structural materials of nuclear reactor, austenitic stainless steels are becoming more preferred choice over ferritic steels as they exhibit better creep resistance, Todd and Ren (1989). In nuclear power plant, the core of the reactor is subjected to harsh neutron radiations as well as high temperatures of up to 973 K. Void swelling resistance and creep resistance are main requisite to develop the fuel clad and wrapper material of reactor, Tateishi (1989). Ti modified 14Cr–15Ni austenitic SS with additions of silicon, phosphorus, and boron is used to get better creep resistance and void swelling resistance. Void swelling induces point defects into the material due to the interaction of high energy neutron with crystal lattice. High dislocation density network produced by cold working, facilitates the interaction of point defects and dislocations, which leads to recombination of them and material exhibits more resistance to void swelling, Venkadesan et al. (1992). The presence of titanium in this alloy facilitates the precipitation of fine carbides of titanium during creep exposure of cold worked steel. These fine precipitates mostly occur near dislocation network, which reduces the motion of dislocations. So, due to reduced movement of dislocations, plastic deformation becomes difficult and creep life of material is enhanced, Kesternich and Rothaut (1981), Yadav et al. (2018). Various percentage of cold work may exhibit variation in rupture life of material and high amount of cold work levels could be resulted in recovery and recrystallization process, Jang et al. (2018). In this study, Ti modified 14Cr-15Ni austenitic SS has been subjected to various cold work followed by creep testing. Creep rupture strength of deformed material is compared with solution annealed sample and optimization of cold work is done. Microstructural examination carried out was post creep test in order to understand the damage Mill annealed Ti-modified 14Cr–15Ni austenitic stainless steel was procured in the form of rolled plate with 13 mm thickness. The chemical composition of steel is given in Table 1. Bars of 140 x 40 x 13 mm along the rolling direction of plate were made with the help of electric discharge machining. Solution annealing was done at 1333 K for 30 minutes for these bars. After the solution annealing of the alloy (CW0), 10% (CW1), 20% (CW2), 30% (CW3), and 40% (CW4) deformation was done by cold rolling process with the help of cold rolling mill (Buhler make) at room temperature. From these cold rolled bars, creep specimens were prepared as per ASTM E139 standard. Fig. 1 shows the drawing of creep specimen. Gauge diameter and gauge length were 5 mm and 50 mm, respectively. Creep tests at 973 K were performed on the annealed (CW0) as well as cold rolled samples with the help of uniaxial creep testing machine (Star Testing systems make). Creep test stress was kept at 200 MPa for all the samples. For microstructural examination, standard metallographic polishing was performed and electrochemical etching at 6 V using 10% oxalic acid solution was done. Detailed microstructural investigations were carried out with the help of (Olympus make) optical microscope and JEOL 6380 scanning electron microscope (SEM). Mitutoyo microhardness tester was used for microhardness measurement at 0.3 kgf load with 10 seconds dwell time and average value of 10 measurements is reported in this paper. ImageJ software was used to measure the grain size of the material. caused by creep exposure. 2. Experimental Work

Table 1. Chemical composition (wt%) of test material.

C

Ni

Cr

Mo

Ti

B

N

P

S

V

Nb

Si

Mn

Fe

0.041 15.52 14.09 2.21 0.23 0.005 0.0087 0.023 0.002 0.022 0.023 0.74 2.49 bal

3. Results and Discussion 3.1. Initial microstructure and hardness Initial microstructure of solution annealed Ti modified 14Cr–15Ni stainless steel is shown in Fig. 2. It is an