PSI - Issue 13

Dan Eliezer et al. / Procedia Structural Integrity 13 (2018) 2233–2238 Eliezer et al/ Structural Integrity Procedia 00 (2018) 000 – 000

2234

2

metal, stress or deformation rates applied to the metal. Second phases created by the mechanism of hydrogen induced-phase transition, have a major effect on the HAC characteristics and on the trap’s activation energy for hydrogen [1], [2]. Material's Conditions affecting the HAC characteristics are due to their influence on hydrogen diffusion to the crack tip. Multiple cracks and continues crack growth with the assistance of hydrogen, created by the HAC phenomena, can be accelerated and finally lead to hydrogen fracture mechanisms. The two most applicable model for describing hydrogen embrittlement in a non-forming hydride systems are the ones determined by dislocations processes: hydrogen enhanced mechanism (HEDE) and hydrogen enhanced local plasticity (HELP) [3] – [5]. HEDE refers to a reduction in the atomic bond strength [4] and HELP refers to dislocations enhancement and acceleration by the presence of hydrogen [5]. Hydrogen ‒ induced second phases and the embrittlement mechanisms were determined by hydrogen trapping mechanisms. The dominant hydrogen phase transition mechanism will be discussed. The validity of the HELP model will be shown to be highly dependent on strain rate, due to the demand of hydrogen travel with dislocation. Microstructural and strength changes in the presence of hydrogen, and hydrogen trapping mechanism were studied by means of thermal desorption analysis (TDA), and microstructural SEM and TEM analysis. 2. Results and discussions

2.1. Microstructure changes in the presence of hydrogen

The role of hydrogen was investigated in different commercial stainless steels: duplex stainless steels (DSS), austenitic stainless steel (AUSS), and super martensitic stainless steel (SMSS). These materials were received in the form of rolled annealed sheets and were cathodicly charged with hydrogen to 72 h at STP conditions. The surface cracking of these steels after hydrogen charging can be seen in Fig. 1 a-c.

(b)

(c)

(a)

10 μm

10 μm

10 μm

(e)

(f)

(d)

ɛ -martensite

ɛ -martensite

2 μm

5 μm

30 μm

Fig. 1. SEM observations of surface cracking of 72 h cathodic charged in (a) AUSS (b) DSS, (c) SMSS. Microstructural changes after 72 h cathodic charging in (d) AUSS, (e) DSS, and (f) SMSS. The formation of needle-shaped  -martensite phase after one month at RT as a result of hydrogen-induced-phase transition can be seen in AUSS (d) and DSS (e) [2], [6], [7].