Effects of thermal history and microstructure on phosphorus and manganese segregation at grain boundaries in C–Mn welds

This work describes a model development to predict the microstructural behaviour of submerged arc C–Mn weld materials used in pressure vessels. Element segregation can be predicted according to the typical operating conditions and microstructure of the materials. Phosphorus, of great interest because it causes embrittlement, is enhanced at the grain boundary and increases with operating temperature and time. Its interaction with carbon and manganese is described. Site competition has to be applied; its prediction results fit best to the experimental data. Sufficient experimental data have been taken by application of a high-resolution analytical transmission electron microscope. ermodynamic software package MTDATA to determine the solubility of second phases, such as cementite and alloy carbides, is applied. MTDATA is a software/data package for the calculation of phase equilibria in multicomponent multiphase systems. It uses, as a basis, critically assessed thermodynamic data. By using these data, reduced free amount of solute in the alloy, needed as an input to segregation modelling, is predictable. The final grain boundary segregation consists of various segregation types during the thermal history and is the sum of each single step with a final check on the maximum equilibrium segregation, which cannot be exceeded. Weld materials have a very complex microstructure, resulting in coarse and fine grains. For each grain boundary type, prior austenite and ferrite grain boundaries in coarse grain and ferrite grain boundaries in fine grain, the final segregation level has been predicted and good agreement with practical results has been shown.