The objective of our research programme is to study the microstructure and stress state evolution in the heat affected zone (HAZ) on welds of P91 and other 9Cr steels. The work to be undertaken in this proposal aims to quantify the residual stress in a welded specimen to provide validation for the numerical tools. It is expected to implement a strong link between ND measurements and modelling simulations in order to quantify the residual stress development during the welding process of P91 steel.