Relationships between tensile and fracture mechanics properties and fatigue properties of large plastic mould steel blocks

Moulds for plastic automotive components such as bumpers and dashboards are usually machined from large pre-hardened steel blocks. Due to their dimensions, the heat treatment produces mixed microstructures, continuously varying with the distance from the quenched surface, at which fracture toughness and fatigue properties are not well known and generally lower than those corresponding to a fully quenched and tempered condition. The response of the mould to defects (for example, microcracks due to improper weld bed deposition) and stresses during service depends on steel properties, that in turn depend upon the heat treatment and the microstructure. A pointwise determination of the tensile, Charpy V-notched, fracture toughness and rotating bending fatigue properties was carried out in a large block. High cycle fatigue was investigated by the stair-case method. The samples were obtained from different depths of the blooms. The relationship between mechanical properties, fracture surfaces morphology and microstructure was also investigated.