Fracture toughness of the heat affected zone on Nd-YAG laser welded joints

Laser deposit welding based on modern Nd-YAG lasers is a new mould repair process with advantages relatively to the traditional methods (Micro-plasma and TIG methods). For moulds steels there are no research studies about the laser deposit welding process effects in respect to fracture toughness, hardness and residual stresses variations in the laser-deposited layer and in the heat affected zone. These variations will have an important influence in the thermal–mechanical fatigue strength of the moulds parts. This paper is concerned on the study of the fracture toughness of transient microstructure regions in Nd-YAG laser welded joints performed in mould steels. Fracture toughness tests were performed in two hot-working tool steels: AISI H13 and AISI P20. Small welded specimens were prepared with U notches and filled with laser welding deposits. The hardness profiles were obtained at the middle cross-section of specimens against distance to the surface in order to identify the microstructures present at the crack tip region. Fracture toughness was evaluated and plotted against the Vickers hardness measured at the fatigue crack tip. Depending of the crack tip depth two very different toughness levels were observed in the H13 steel, while a lower variation of toughness was observed for the P20 steel. The values of fracture toughness observed in each specimen are consistent with the correspondent crack tip microstructure hardness.