Thermodynamic considerations of pore formation and hydrostatic pressure during underwater wet welding

摘要

The pore and crack formations in the weld bead during underwater wet welding are the main cause of failure to reach the required mechanical properties of the weld metal. These defects are closely associated with the decomposition of the water molecule under electric arc conditions. In this paper, the thermodynamic calculations of the complex process of the water decomposition under the conditions of high electric arc temperatures at a pressure of 1 atm of steam are discussed. The values of the partial pressures of the five main products of the vaporizations and decomposition of the water (H_2O_((g)), H_2, O_2, H and O) are calculated for temperatures between 1870 and 4000 K. Due to the fact that atomic hydrogen is mainly responsible for pore formation in the weld metal, its, partial pressure as a function of partial pressures of atomic oxygen and steam (water vapour) is expressed. Hydrogen solubility values in the liquid metal under the conditions of underwater wet welding at depths of 50 and 100m, and a comparison between the thermodynamic calculation and porosity measurement results at depths of 50 and 100 m, are presented.