PSI - Issue 59

Jesús Toribio et al. / Procedia Structural Integrity 59 (2024) 131–136 Jesús Toribio / Procedia Structural Integrity 00 ( 2024) 000 – 000

135

5

6. HELP versus HEDE in progressively cold drawn pearlitic steels In this section the role is discussed of the two classical hydrogen-assisted deterioration micro-mechanisms, namely, hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE), together with their role in the embrittlement process as a function of the degree of cold-drawing undergone by each steel (associated with a different level of microstructural orientation of pearlitic colonies and lamellae). To elucidate which of these two degradation modes, HELP or HEDE is predominant in drawn pearlitic steels, a micromechanical modeling of the fracture/damage process in hydrogen is needed. Fig. 6 shows the micromechanical model of HD/HE in hot rolled and slightly cold-drawn pearlitic steels in the form of TTS. Such a tearing mode can be associated with a HELP mechanism and a certain degree of plasticity.

Fig. 6. Micromechanical model of HD/HE in hot rolled and slightly cold-drawn steels by HELP in the form of TTS.

Fig. 7 shows the micromechanical model of HD/HE in heavily cold-drawn pearlitic steels. Firstly, hydrogen diffuses through preferential paths parallel to the ferrite/cementite lamellae ( anisotropic diffusion of hydrogen ). Later, hydrogen-assisted fracture takes place in the form of HEDE, with a probable meaning of hydrogen-enhanced delamination between ferrite and cementite lamellae and an appearance of EOTTS.

D

//

HEDE

D

^

D

//

Fig. 7. Micromechanical model of HD/HE in heavily cold-drawn pearlitic steels: (a) hydrogen diffusion in longitudinal and transverse directions; (b) fracture by HEDE. 7. Closure While in hot rolled and slightly drawn pearlitic steels the predominant HD/HE mechanism is the more ductile HELP in the form of TTS resembling MSST by Michelangelo , in the case of heavily drawn steels the predominant HD/HE mechanism is the more brittle HEDE in the form of EOTTS resembling DWST by Donatello . References Costa, J.E., Thompson, A.W., 1982. Hydrogen Cracking in Nominally Pearlitic 1045 Steel. Metallurgical Transactions 13A, 1315-1318. Embury, JD., Fisher, RM., 1966. The Structure and Properties of Drawn Pearlite. Acta Metallurgica 14, 147-159. Gil-Sevillano, J., 1986. Cleavage-Limited Maximum Strength of Work-Hardened B.C.C. Polycrystals. Acta Metallurgical 34, 1473-1485.