Finite element (FE) models that simulates the ductile fracture process in a
tool steel has been developed in order to determine the influence of the
carbides on the ductility. The studied steel is an uphill ingot cast,
Electro Slag Refined (ESR) and hot worked tool steel with a low fraction of
carbides, so called matrix type tool steel. The microstructure consists of
a steel matrix with high toughness and brittle carbides, which are
approximately half a ìm in size with an almost spherical shape.
An experimental four-point bending test has been carried out in order to
establish the ductility properties of the steel and a FE-model of the four-
point bending test has been designed for the calculations of the
macroscopic deformations. A subsequent FE-model of the microstructure of
the material has been designed based on Scanning Electron Microscopy (SEM)
pictures. The matrix has been modelled as elastic-plastic and the carbides
have been considered to be elastic. A failure criterion has been applied to
the matrix, which eliminates an element from the mesh when the plastic
strain exceeds a critical level. The FE-model of the microstructure has
been loaded so that cracks have been generated and the load has been
applied until total failure.
SEM pictures of the fracture surface show that carbides above a critical
size crack during the fracture process while smaller carbides remain
intact. Different models of describing the failure at the carbides have
been evaluated: i) intact carbides fixed to the matrix, ii) intact carbides
with no cohesion to the matrix and iii) carbides with internal cracks.
Several combinations of the failure models have been simulated. Failure
curves from the simulations of the microstructure model have been evaluated
in order to establish the strain at failure, ...
