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The fracture toughness of low-alloy steels significantly depends upon the microstructure, i.e. upon metallographic constituents amounts and distribution and other parameters like carbides distribution, grain size, etc. Almost all the available studies and experimental data concern single constituent microstructures, whereas specific industrial applications use steels containing variable fractions of tempered martensite and bainite, as well as ferrite-pearlite, due to the production cycle and especially to the heat treatment used. In these cases it is particularly difficult to estimate the fracture toughness on the basis of the available experimental data; thus, it isn’t possible in practice to evaluate (and minimize) the crack propagation risk. Molds for plastic automotive components are a peculiar example, being produced from pre-hardened large blocks. The fracture toughness of the core mixed microstructures, that can occupy parts of the mold face, being sought, KIc tests have been performed as a function of the sample position in a 1.2738 steel block, and results interpreted at the light of fracture morphology and local microstructure.