Heat treatment and failure risk of large automotive plastic molds: a fracture mechanics approach and property assessment

Abstract

Molds for plastic automotive components such as bumpers and dashboards are usually machined
from large pre-hardened steel blocks. Due to the dimension of the blooms, the heat treatment produces
mixed microstructures, continuously varying with the distance from the quenched surface,
at which fracture toughnesses lower than those appropriate for a fully quenched and tempered
metallurgical condition are usually associated. Furthermore, final fabrication machining exposes, in part
of the mold's surface, the steel that was at heart during both ingot casting and bloom heat treatment.
The response of the mold to defects (for example, microcracks from weld bed deposition) or events
(for example, incomplete formed plastic object extraction) that can conceivably cause failure
during service (and in a few cases actually did), depends on the steel properties, that in turn depend
upon the heat treatment and the microstructure. A pointwise survey of the mechanical properties of some
commercial blooms, actually used to machine bumper molds, has shown usual and expected values
of hardness and of tensile properties, but, indeed, a low range of fracture toughness, suggesting
that the latter is a critical characteristic of this steel and that fracture mechanics verifications,
already usual in other fields of industry, should dutifully be applied to the molds' design.
The relationship between the mechanical properties, the morphology of the fracture surfaces
and the microstructure has been also investigated.