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

1276

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Although the deleterious effects of hydrogen on quenched and tempered steels similar to 42CrMo4 and 2.25Cr1Mo have been studied for some time, a full understanding of the problem is still lacking [3]. Additionally, the quantitative relationship between the steel microstructure, the hydrogen content in the steel and the decrease of tensile properties and fracture toughness has seldom been reported. In this context, the main objective of this study is to analyse the influence of strength level and microstructure in the hydrogen embrittlement susceptibility of both aforementioned steels. The susceptibility of high strength ferritic steels to hydrogen-assistant fracture in hydrogen gas is usually evaluated by means of mechanical testing in high-pressure hydrogen gas or testing in air after pre-charging the specimens with. We have used this second methodology, conventionally known as internal hydrogen, charging the specimens with gaseous hydrogen at high temperature. Tensile behaviour of smooth and circumferentially-notched round-bar and compact tension (CT) specimens pre charged with gaseous hydrogen was compared in this work, and the influence of strain rate was also checked. Moreover, the hydrogen content of the specimens was measured and also their desorption behaviour.

2. Experimental procedure

2.1. Materials and heat treatments Two different chromium-molybdenum steels, 42CrMo4 and 2.25Cr1Mo, were used in the present study. Their chemical composition in weight % is shown in Table 1.

Table 1. Chemical composition of 42CrMo4 and 2.25Cr1Mo steels (weight %). Steel C Mn Si P S

Cr

Mo

42CrMo4

0.42

0.62

0.18

0.008

0.002

0.98

0.22

2.25CrMo1

0.14

0.56

0.16

0.005

0.002

2.23

1.00

Hot rolled plates (250x250x12 mm 3 ) of 42CrMo4 steel were austenitized at 845ºC for 40min, quenched in water, and then tempered at 500 (42CrMo4_500) and at 700ºC (42CrMo4_700) for 2h. Regarding 2.25Cr1Mo steel, the as received condition was studied (2.25Cr1Mo_690), and additionally the steel was austenitized at 940ºC for 30 min, quenched in water and tempered at 600ºC for 2h (2.25Cr1Mo_600). Two grades were thus produced with both steels, in order to analyse the influence of microstructure and strength level in the susceptibility to hydrogen embrittlement. 2.2. Hydrogen charging The specimens were pre-charged with gaseous hydrogen in a high-pressure reactor. In order to make sure that the specimens were saturated with hydrogen, the following conditions were applied: 21h at 450ºC under a pressure of 19.5 MPa of pure hydrogen. Afterwards, the hydrogen-charged specimens were removed from the reactor and rapidly immersed in liquid nitrogen, where they were kept until the moment of testing. The hydrogen desorption curves at room temperature of the different steel grades were determined. With this purpose, cylindrical samples with a diameter of 10mm and a length of 30mm were used (≈20g) . The hydrogen content was measured in a LECO DH603 analyser. The procedure to obtain the desorption curves was the following: all the samples were removed from the liquid nitrogen at the same time, and left in air at room temperature. Then, the hydrogen concentration of the samples was measured at different time intervals. Before starting the measure, each sample was cleaned in an ultrasonic bath with acetone for 5 minutes, and then carefully dried using cold air. The analysis to determine hydrogen concentration consisted on keeping the sample at 1100ºC during approximately 400s. 2.3. Tensile tests on smooth and notched specimens Tensile tests on smooth specimens of 5mm in diameter and 28mm in nominal length and circumferentially-notched (K t =4,25) round-bar specimens with a notched section diameter of 5mm and notch root radius (ρ) of 0.15mm , were performed on an Instron 5582 tensile testing machine. Different displacement rates (0.4, 0.04 and 0.004 mm/min)