PSI - Issue 5

J. Belzunce et al. / Procedia Structural Integrity 5 (2017) 1275–1282 J.Belzunce et al./ Structural Integrity Procedia 00 (2017) 000 – 000

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Fig. 3. Influence of steel hardness on the embrittlement index (EI) calculated for strength and reduction of area for hydrogen pre-charged smooth tensile specimens. The rate displacement used in these tests was 0,04mm/min.

3.4. Tensile tests on notched specimens The results of tensile tests performed on hydrogen pre-charged notched specimens are summarized in Fig. 4. EI was calculated for the ultimate tensile strength and reduction of area and was plotted versus the steel hardness. The deleterious effect of hydrogen is clear in 42CrMo4_500 grade due to its high strength (σ ys =1086 MPa): the notched tensile strength strongly decreased due to hydrogen (σ ut =1800MPa  σ utH =1000MPa), the same occurred with the reduction of area (RA=2.5%  RA H =0%). The other grades have maintained their mechanical strength in the presence of hydrogen (EI<5%). Nevertheless, the reduction of area always showed a significant decrease.

Fig. 4. Influence of steel hardness on the EI for mechanical strength and reduction of area (notched specimens, displacement rate: 0.04 mm/min). (Right). Fracture surface of 42CrMo4_500 notched specimen pre-charged with hydrogen (0.4 mm/min).

Hydrogen distribution in the notch region of high strength steels is dominated by hydrostatic stress even when plastic strain is high [3]. Low displacement rates allow hydrogen to diffuse along the steel microstructure and move to attain the stress concentration region located in front of the notch (high triaxiality). Due to the accumulation of hydrogen in this area, a critical hydrogen concentration is reached, giving rise to a premature failure. The fracture surfaces of the tensile notched specimens were analysed. The uncharged 42CrMo4_500 grade showed the typical ductile micromechanism. On the contrary, the hydrogen pre-charged specimens, regardless of the displacement rate, always exhibited some brittle intergranular (IG) fracture. One of these areas is shown in Fig. 4. Hydrogen pre-charged specimens corresponding to the other grades did not exhibit any trace of intergranular fracture. In these cases, the localized plasticity ahead of the notch, enhanced by the presence of hydrogen, fosters an extreme growth of microvoids, which finally coalesce after a relatively low reduction of area, but maintaining a ductile fracture micromechanism [7].