Abstract :
[en] Over recent years, manufacturing of tool steels by Laser Powder Bed Fusion (LPBF) has been the object of increasing attention for the possibility of producing parts, with complex shape difficult to obtain through conventional methods. Moreover, the ultra-fast heating and cooling rates pertaining to the LPBF process are responsible for the formation of strongly out-of-equilibrium microstructures, involving supersaturated solid solution and new metastable phases, thus offering new possibilities in terms of usage properties. Therefore, research is now focusing on the development of tool steels with complex chemical composition and higher carbon contents. Under the conditions achieved during the LPBF process, such tool steels composition may lead to the presence of residual stresses within the part promoting cracks nucleation and propagation, thus making the part unusable.
In this work, a low alloy tool steel AISI S2 with a medium carbon content (0.5 wt. %) was successfully processed by LPBF. As-built microstructure revealed fresh martensite, bainite and tempered martensite in a supersaturated condition, with the presence of residual stresses. In addition, phase transformations can occur in service at high temperature, affecting the mechanical properties of the part. Therefore, Differential Scanning Calorimetry (DSC) was performed on as-built samples to investigate the thermal behaviour of the material within the range 200 – 400 °C, with the aim to design a post-thermal treatment to stabilize the microstructure. Microstructural characterization was carried out on as-built and heat treated samples, in combination with macro- and nano-hardness analysis. Tribological tests were also carried out on as-built and heat treated samples. Wear properties were further assessed through observations of the worn track and counter-body surfaces and of transversal sections underneath the worn track.
