Issue 42

A. Strafella et alii, Frattura ed Integrità Strutturale, 42 (2017) 352-365; DOI: 10.3221/IGF-ESIS.42.36 361 Although they weren’t obtained at the same stress level, creep curve at 575MPa in lead are plotted in Fig.11. It can be noticed that rupture time of specimen tested at 575MPa in lead is lower than that tested in air at 560MPa, as expected. Up to 850h, creep strain of specimen in lead is higher than creep strain in air and this supports the hypothesis that lead corrosion appears after a long time of steel/lead contact. For time greater than 850h, it can be observed a trend inversion. It is due to LME, which produces a loss of ductility and competes with creep strain at higher stress and long time. Indeed, it can be noticed that the specimen tested at 575MPa in lead shows a lower percentage of final creep strain than specimen tested at 560MPa in air, although subjected to higher stress level, but it shows a lower time of rupture too; this means that it is less ductile than specimen tested in air and it is in accordance with LME effect. It is important to underline that these tests in stagnant liquid lead were performed to verify the steel sensitivity to LME. The Handbook on Lead-bismuth Eutectic Alloy and Lead Properties, Materials Compatibility, Thermalhydraulics and Technologies [8] proposes the following definition of LME : LME is the loss of ductility of a normally ductile metal or metallic alloy when stressed in contact with a liquid metal that can result in brittle fracture. Typically the phenomenon is accompanied with a change from ductile to brittle fracture modes, intergranular or transgranular cleavage-like fracture modes. According to this definition, the LME justifies the decrease of rupture time, the reduction of creep strain and then the loss of specimen ductility tested in lead. The effect of lead is more evident at the lower stress (500MPa, Fig. 12). The comparison between curves in lead and air at 500 MPa can better explain this phenomenon: a relevant increment of deformation growths with the time in the whole curve, unlike tests at 560MPa. It means that lead influences the sscr, then the shape of creep curves. In addition, the lead presence anticipates the tertiary stage which occurs at about 700h, unlike test in air that remains in the secondary stage. All these effects are ascribable to LME. Figure 12 : Comparison between creep curves 500MPa in air and in lead. Then, the analysis on tests in lead can be summarized as follows: - For high stresses (near yield stress) the presence of liquid lead does not significantly affect the deformation of the specimens and its deformation rate sscr. This means that the creep effect dominates the deformation in primary and secondary stages. The effect of Pb becomes prevalent only in the tertiary stage and it occurs with a reduction of the final time and final deformation percentage and then with a change in the rupture mode. - For low stresses (significantly below the yield), the effect of Pb is more evident. It was noticed that lead influences sscr and increases the deformation, with the time. Then, LME tends to prevail on creep effect: the shape of curve changes, the sscr is higher than the test in air and the tertiary stage occurs at lower time. 0 200 400 600 800 1000 1200 0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40 0,45 0,50 0,55 Creep strain % Creep Strain % - [500 MPa] Creep Strain % - [500 MPa]- Pb Time [h]