PSI - Issue 60

P.A. Jadhav et al. / Procedia Structural Integrity 60 (2024) 631–654 P. A. Jadhav et.al. / Structural Integrity Procedia 00 (2019) 000 – 000

640 10

The presence of a volumetric flaw does not always result in the formation of DHC crack. The crack formation occurs only when a set of conditions are met. The model for the crack initiation ahead of the tip of a volumetric flaws is recommended by the CSA Standard (2016). This methodology is based on process zone method and is developed using the reported work of Scarth (2002), Scarth and Smith (2001), (2002). The formation of the crack is dependent of the geometry of pressure tube, geometry of the flaw (length, depth and root radius), applied loading, hydrogen concentration in the material and the material tensile and fracture properties. The process zone method is used to assess if the sufficient stress condition exists for the initiation of DHC. This method is quite involved and requires an iterative solution. The CSA Standard has also given an engineering estimate to this method by fitting an equation to the procedure. The primary condition which needs to be checked for DHC initiation is the presence of sufficient hydrogen ahead of flaw tip such that the precipitation occurs. The hydrogen migrates to the flaw tip due to stress concentration. The hydrogen concentration at the flaw tip as a function of bulk concentration and the stress levels is estimated by equation. The hydrostatic stress 1 hyd  remote from flaw tip is calculated using eq. 4. In case of the flaw is in the rolled joint region, 1 0 hyd   .

( 4 )

 

3 





h

a

1

hyd

The hydrostatic stress ℎ 2 at the flaw tip is calculated using eq. 5.   2 1 3 p hyd     

(5)

The terminal solid solubility for hydrogen dissolution (TSSD) is given by eq. 1. The threshold bulk hydrogen equivalent concentration, ℎ , at which the equilibrium concentration at the flaw tip is equal to TSSD (eq. 11) at sustained hot conditions is estimated using eq. 6.     2 1 exp H hy o d hyd o p th p R V H TSSD T T            (6) When the bulk hydrogen equivalent concentration exceeds ℎ , the DHC inititiation condition exists. The bulk hydrogen equivalent concentration is estimated from the eq. 7.

(7)

D

H H  

eq

i

2

The peak principal flaw-tip stress is estimated by eq. 8. p t k   

(8)

The stress based condition necessary for the DHC initiation when hydrogen has precipitated ahead of flaw tip is given by eq. 9.

(9)

p th   