Fatigue behavior of homogeneous-microstructure and mixed-microstructure steels

Molds for plastic automotive components such as bumpers and dashboards are usually machined from large pre-hardened steel blocks. Due to the dimensions, the heat treatment of these blocks produces mixed microstructures, continuously varying with the distance from the quenched surface. The fatigue behavior of these mixed microstructures is not well known, yet peculiar results stemmed from our previous tests. Such a theme has been re-examined through experimental tests on two different plastic mold steels, the traditionally used ISO 1.2738 steel and a quenched and tempered microalloyed steel, recently proposed for the purpose. The fatigue crack growth behavior has been investigated. Moreover, in order to clarify the correlations between the microstructural features and the fatigue behavior, fatigue crack growth tests have been performed on re-heat-treated samples with homogenous microstructures (pearlite or tempered martensite), and have been compared with the results obtained while testing original blooms mixed microstructure sample