Comparison between Traditional and Innovative Steels for Large Plastic Moulds

Moulds for plastic automotive components such as bumpers and dashboards are usually machined from large pre-hardened steel blooms. Due to the blooms size, the heat treatment of the standard 1.2738 steel produces mixed microstructures (continuously varying from surface to core) and a very low fracture toughness, that makes this steel sensible to the defects that may occur in the moulds. In the present work, two alternative steels are investigated: a quenched and tempered microalloyed steel, and a precipitation hardenable steel. Whereas the former can be subjected to the same mould production cycle currently employed for the 1.2738 steel, the latter can be hardened by a subcritical treatment after machining. The microstructures and the ensuing mechanical behavior of steels samples, representative of different positions inside large bloom, have been examined by metallographic techniques, by fracture toughness and tensile tests, and by ensuing fractographic examinations.