The aim of the present paper is to investigate the metallurgical behaviour of AISI P20 tool steel frequently used in injection moulds, after welding. For this purpose Tig and Laser welding tests were conducted to determine the influence of the welding parameters, materials and preheating. The welding quality was assessed through the microstructural properties and microhardness evolution. The results achieved with the different processes were compared. This work allows to obtain a deeper knowledge concerning tool steels behaviour after welding.