13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- The tensile properties of NiCrMo1 steel under conditions of hydrogen charging studied using the linearly increasing stress test Qian Liu1, Bartolomeus Irwanto2, Andrej Atrens1* 1 The University of Queensland, Materials Engineering, St Lucia, Qld 4072, Australia 2 Alstom (Switzerland) Ltd, CH-5242 Birr, Switzerland * Corresponding author: Andrejs.Atrens@uq.edu.au Abstract The tensile properties of NiCrMo1 steel were investigated using the linearly increasing stress test (LIST) in air and with hydrogen charging. Hydrogen charging was carried out by applying an increasingly negative applied potential to -1550 mVAg/AgCl in acidified 0.1 M Na2SO4, pH 2 solution. The LIST results showed that the yield stress of the steel was similar in air or with hydrogen charging. SEM examination showed that the specimens tested in air had no surface cracks, and the fracture surfaces comprised dimples. The failure in air was due to ductile overload. The specimens tested with hydrogen charging had obvious surface cracks in the necked region, and their length increased to about 550 µm with a more negative potential. However, the fracture surfaces were nevertheless dominated by ductile feature. These results imply that the influence of hydrogen was only associated with the final ductile fracture after the onset of necking. Keywords: Steel 1, LIST 2, hydrogen embrittlement 3, SEM 4. 1. Introduction Operation of metallic components is required in hydrogen (H) in the H economy being developed in response for the need for clean energy. The H economy requires pressure vessels resistant to hydrogen embrittlement (HE) for the production, distribution, storage and use of gaseous H2. Though laboratory scale vessels can be made from expensive exotic materials, there are significant cost imperatives to use less expensive materials like steels in a commercial H economy. Some steels are susceptible to hydrogen, particularly high strength steels [1-11]. Some medium strength steels are resistant to hydrogen even under severe hydrogen charging conditions [12-15]. Using the linearly increasing stress test (LIST) [16], our previous investigation of 3.5NiCrMoV steel [15] showed that there was negligible influence of hydrogen on the yield stress and fracture stress, though the ductility was decreased for some tests. Detailed examination of the surface appearance and the fracture surfaces indicated that (i) there was no influence of hydrogen up to the yield stress of the steel; and (ii) hydrogen caused some small brittle fracture events in small localised areas associated with the final ductile fracture after the onset of necking. The LIST [16] is a stress-controlled version of the constant extension rate test (CERT) [17]. A LIST is identical to a CERT up to the onset of yielding, or the onset on subcritical crack growth. It is expect that the same apparent threshold stress would be measured using CERT as measured using LIST, an apparent threshold that related to the onset of plastic deformation. The aim of this research was to evaluate using LIST the behavior of medium strength NiCrMo1 steel under hydrogen conditions. The results could be considered as a reference to the suitability of this steel for the H economy. 2. Experimental procedure The material was the nickel chromium molybdenum steel, NiCrMo1. It is a quenched and tempered martensitic steel; the microstructure is tempered martensite as shown in Fig. 1. Table 1 presents the chemical composition. The steel was machined into specimens with 10 mm gauge length, 3 mm diameter and an overall length of 110 mm. The specimen was evaluated by LIST. The threshold stresses were measured with the potential drop (PD) measurement. LISTs were carried out in air, and in solution, (i) at the free corrosion potential (Ecorr) and (ii) under hydrogen charging conditions.
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