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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were applied in order to study the influence of this parameter in HE. The maximum triaxiality (calculated numerically using Abaqus CAE) in the notched specimens was also determined to analyse the effects of stress gradients in HE, being 1.66. These tests were performed onto uncharged and hydrogen-charged specimens. The extent of hydrogen embrittlement was assessed by means of the embrittlement index (EI 1 ). 2.4 Fracture toughness tests The fracture toughness determination was performed using CT specimens, with a width of 48 mm and a thickness of 10 mm, previously fatigue pre-cracked up to obtaining a crack length versus width ratio, a/W=0.5, following the ASTM E1820 standard [4]. The CT specimens were fatigued using an R ratio of 0.1 and, afterwards, lateral notches were machined on both sides to assure a plane strain state. Fracture toughness tests were carried out under nominal displacement rates of 1 mm/min, 0.1 mm/min and 0.01 mm/min in order to evaluate the influence of this variable in HE. The behaviour between uncharged and hydrogen-charged specimens was compared in this way. 2.4. Observation of microstructures and fracture surfaces The steel microstructures were observed in a scanning electron microscope (SEM JEOL-JSM5600) using an acceleration voltage of 20kV (the samples were previously ground and finally polished with diamond paste of 1µm). The fracture surfaces of the tested specimens were also carefully examined using the same technique.

3. Results

3.1. Steels

The resultant microstructures of 42CrMo4 and 2.25Cr1Mo steels after the heat treatments are shown in Fig. 1.

(b)

(a)

335HB σ ys =1086MPa σ ut =1200MPa

201HB σ ys =622MPa σ ut =710MPa e max =21% RA=65%

e max =13% RA=51%

(c)

(d)

285HB σ ys =761MPa σ ut =887MPa e max =19% RA=60%

170HB σ ys =437MPa σ ut =580MPa e max =27% RA=72%

Fig. 1. Microstructure of heat treated steels: (a) 42CrMo4_700, (b)42CrMo4_500, (c) 2.25Cr1Mo_690 and (d) 2.25Cr1Mo_600.

1 [%] = − ∙ 100 , where X and X

H are the evaluated material property without and with hydrogen.