Abstract
In last years, the growing production and consumption of large plastic components, usually employed in the automotive industry, has notably influenced the plastic mold market. Medium carbon low alloyed steels, such as ISO 1.2738, have long been applied for these purposes. The ISO 1.2738 mold steel is usually machined from large pre-hardened blooms. Due to the dimensions, their heat treatment yields mechanical properties and mixed microstructures continuously varying from surface to core. Although these steels have been used for several years, their fatigue behavior, as well as the behavior of similar mixed microstructures steels, is not well known. The effect of these mixed microstructures on the fatigue crack growth (FCG) behavior, in the threshold and Paris regime, has been reported. FCG tests were carried out on the traditional ISO 1.2738 steel and on a quenched and hardened microalloyed steel recently proposed for plastic molds. Moreover, in order to clarify the interplay between the microstructural features and the fatigue behavior, fatigue crack growth tests were performed on ISO 1.2738 steel re-heat-treated samples with homogenous microstructures (pearlite or tempered martensite) and results were compared with those obtained from mixed microstructure samples cut from original blooms. After the tests, scanning electron microscopy (SEM) observations of fatigue fracture surfaces were carried out to understand the influence of microstructure morphologies on the FCG resistance.