The usual mechanical properties of martensitic steels are strongly linked to the complex microstructure obtained after heat treatment, which is generally performed to achieve good hardness and tensile strength with acceptable ductility. In the present work, X38CrMoV5-1 (AISI H11-1) steel, classed as a hot-work tool steel with many applications, has been studied from the viewpoint of phase transformations and its manufacturing process. Parts are often manufactured by die-casting followed by quenching and tempering and the microstructure obtained is highly sensitive to the process. Besides slight changes in chemical composition can have a huge impact on mechanical properties, so the amount and nature of alloying elements must be controlled because of their effect on precipitation of secondary carbides and their modification of tempering kinetics. [1-5] Previous studies on this steel have shown the optimal heat treatment to obtain an acceptable microstructure that guarantees the behaviour in service. It consists on an annealing at 780ºC/1hour, followed by a quenching at 1020ºC/1hour, and finally a double tempering at 580ºC/2 hours.

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