FE simulation of ductile fracture in matrix type tool steel

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, which has been compared to the strain at failure of the FE-model of the four-point bending test. A very satisfactory agreement between the experimental ductility and the simulations has been obtained.Validerat; 20101217 (root