Welded steel in marine environment-Experimental and numerical study of mechanical properties degradation

In this study, mechanical properties of the butt-welded joint of AH36 shipbuilding steel are investigated experimentally and numerically. Welded specimens were exposed for 6, 12, 24 and 36 months to corrosive environment (free corrosion in water, sea water, sea splash). The tensile test was carried out to evaluate the effect of the environment on the ultimate tensile and yield strength. Charpy impact and Vickers hardness tests were performed in order to estimate mechanical properties of the base metal, heat affected zone, weld metal (BM, HAZ, WM) of the butt-welded joint. The corroded surface was examined using optical and scanning electron microscope (SEM) and a pit gauge. Experimental results showed that the greatest influence on the values of tensile and yield strength has the exposure to the sea splash followed by the sea water and water. The most intensive changes to the surface of samples occurred during the first year of exposure. As for the numerical analysis, the corrosion pits on the BM, HAZ, WM were modelled stochastically using Poisson's process for the pit distribution over surface and normal distribution for the pit depth progress. The finite element model was developed for all exposed samples in order to simulate tensile tests using stress modified critical strain (SMCS) ductile fracture criterion. It is shown that with the increase of the stress triaxiality, which depends of the pit size and distribution, critical plastic strain decreases. This leads to the earlier ductile fracture initiation and degra-dation of the mechanical properties.