PSI - Issue 2_A

G Sudhakar Rao et al. / Procedia Structural Integrity 2 (2016) 3399–3406 G.S. Rao et al. / Structural Integrity Procedia 00 (2016) 000–000

3400

Peer-review under responsibility of the Scientific Committee of ECF21.

Keywords: Reactor pressure vessel steels; hydrogen embrittlement; dynamic strain aging; intergrannular cracking; fracture

1. Introduction Structural integrity of the reactor pressure vessel (RPV) of light water reactors (LWR) is of utmost importance with regard to safety of operation and service lifetime and may be affected by different degradation processes like strain-induced corrosion cracking and corrosion fatigue or irradiation embrittlement, the latter being recognized as life limiting factor, Seifert (2008a and 2008b), Roychowdhury (2016). There is growing concern that hydrogen, absorbed from the high temperature water environment and corrosion reactions, may potentially reduce the toughness of the RPV steel in synergy (or competition) with other embrittlement mechanism like irradiation embrittlement, thermal ageing or dynamic strain aging (DSA), Roychowdhury (2016). Strain rate, temperature, grain size, strength and occurrence of DSA may affect the severity of the hydrogen effects in these steels. There are several investigations on the hydrogen effects on fracture of different materials including various RPV steels, but mostly at room temperature. Essentially the coarse grain weld heat affected zones (HAZ) with high hardness were found to be detrimentally affected by the hydrogen. The literature at elevated temperatures in the range of 250-288 °C is very limited, see references in Roychowdhury (2016). In this investigation, effects of hydrogen on tensile and fracture behavior of the base material of RPV steels were studied over a wide range of strain rates in synergy with the effect of DSA. For this purpose a low-alloy RPV steel (20 MnMoNi 5 5) with a high susceptibility to DSA was investigated and compared with previous results with a RPV steel with a moderate susceptibility to DSA, Roychowdhury (2016), Ritter (2002). The hydrogen contents in this study (~ 5 wppm) are significantly higher than the hydrogen activity in the RPV from corrosion and dissolved hydrogen in the coolant in case of an intact cladding and under typical steady-state LWR power operation conditions (< 0.1 wppm) and thus may appear as unrealistic at a first glance. However, similar hydrogen levels might be reached in the process zone in case of cracks in the RPV with stressed and plastically strained bare crack-tips with an aggressive occluded crack crevice chemistry and a high hydrostatic triaxial stress state, e.g., in fracture mechanics tests in high-temperature water or in a loss of coolant accident under suitable strain rate conditions, Roychowdhury (2016). 2. Materials and Methods Two base materials of RPV steels 20 MnMoNi 5 5 (similar to SA 508 Cl. 3 grade) and 22 NiMoCr 3 7 (similar to SA 508 Cl. 2) with different DSA susceptibilities and free nitrogen contents of 30 and 3 wppm, respectively, were used in this investigation. Both steels are austenitized and tempered having a bainitic microstructure with a mean prior austenite grain size of 10-20 µm. The 20 MnMoNi 55 steel was austenitized at 910-920 °C for 6h then water quenched; tempering treatment was carried out at 640-650 °C for 9.5h following furnace cooling. The 22 NiMoCr 3 7 steel was austenitized at 870-905 °C for 6.9h then water quenched; tempering treatment was given at 635-655 °C for 11.3h then air cooled. The chemical composition of the steels is given in Table 1. The 20 MnMoNi 55 and 22 NiMoCr 3 7 steels have an average yield stress of 420 and 400 MPa respectively at 288 °C and strain rate of 10 -3 s -1 .

Table 1. Chemical composition of the steels (wt%).

Elements

C

Si

Mn

P

S

Ni

Cr

Mo

V

Cu

Co

Al

N total N free

O total

Materials

20 MnMoNi 5 5 22 NiMoCr 3 7

0.21 0.25 0.22 0.20

1.26 0.91

0.004 0.004 0.77 0.008 0.007 0.88

0.15 0.50 0.42 0.53

0.008 0.06 0.007 0.04

-

0.013 0.07 0.03

0.14

0.1

0.018 0.08 0.003 -

The DSA behaviour in air was characterized by tensile tests between 25 and 400 °C and at strain rates between 10 -1 and 10 -6 s -1 . Cylindrical tensile specimens of gauge length and gauge diameter of 36 mm and 6 mm were prepared from the forged steel blocks along the transverse direction. Tensile tests with hydrogen pre-charged