PSI - Issue 60

Gopal Sanyal et al. / Procedia Structural Integrity 60 (2024) 311–323 Author name / Structural Integrity Procedia 00 (2019) 000–000

321 11

initiation and growth from second phase particle (the carbide particles within dimples are visible in the SEM fractograph, figure 9(a)) to brittle intergranular failure is the severest manifestation of HE in the RPV steel. In order to quantify the degree of embrittlement the conditional fracture toughness (K, converted from calculated J), K J , has been evaluated following the procedure in ASTM E1820-01 (2001). The calculation procedure for K J has been briefly enumerated below and the value of all interim parameters has been shown in Table 1. The J-value at the peak load point, is divided into elastic and plastic components as per the following:

   2 i K 1



2



J J

J

J

  



(2)

 

 

  pl i

i

el i

pl i

E

W a i

  

  

W a i

BB W P N i

  

  

f

f

K



; For compact tension specimen the

is given by the following

where

  i

expression:

   

   

2

3

4

i W a

i W a

i W a

i W a

i W a

  

  

  

   

  

   

  

  

2

0.866 4.64 

13.32

14.72

5.6





i W a

  

   

f

(3)

3/ 2

i W a

  

  

1



  pl i J , the plastic part at the maximum load has been computed from the following equation �� �� � �� � �� � � � � (4) where:A pl = area under force verses load line displacement record ;  pl = 1.9 for load line displacement. ; B N = net specimen thickness.; B o = W – a o ; The K J was calculated using the following relationship: � ��� � �� � � � (5) The temperature dependence of Young Modulus (E) was derived from the relationship: E = 204 – T/16 GPa[18], where, T = temperature in  C. 2 ) has been determined to be ~165 and ~105 MPa  m respectively. Thus, the HE in the Cr-Mo-V-Ni RPV steel decreases the fracture toughness by about ~ 40% from that in air. The presence of intergranular fracture at the crack tip resulting in sharp decrease of initiation fracture toughness and the reduction of crack tip opening displacement as result of hydrogen exposure point to a decohesion related mechanisms to cause HE in RPV steel. This work shows that presence of cathodically generated hydrogen ingressed under continuously strained sample is able to reduce ductility and fracture toughness of RPV steel (summarized in Table 1) to a great extent, caused by a change in failure mode from ductile to brittle. The mechanical behavior in a tensile or fracture toughness test that induces stress gradient in the sample (for example necking in tensile test and crack initiation in fracture toughness test) are severely affected. The fractographs showing brittle intergranular failure suggests that decohesion related mechanismsare having important role for causing the degradation of tensile ductility and fracture toughness in steel The calculated K J for specimens tested in air and in-situ (20 mA/cm