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Laser heat treatment of galvanised steels with a martensite content superior to 80% were performed on a 1 cm wide area over an extensive temperature range [620 K-1350 K]. The material softening induced is studied through uniaxial tensile testing and SEM microstructural observation. A treatment temperature close to Ac3 yields a massive increase in the ductility of the specimens while reducing the mechanical strength. This change in mechanical properties is associated with the nucleation of new austenite islands and the vanishing of the initial martensite laths. The results presented in this paper pave the way to localised variations of the strengthductility trade-off, which could be useful for several industrial applications, particularly for enabling plastic forming or stamping of the martensitic steel sheets at low temperature.

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Carburization assisted by laser processing is a promising method to strengthen metallic materials. Direct laser beam carburization is implemented for the first time on thin AISI 430 ferritic stainless steel sheets with graphite coating under different conditions. Microstructural morphology, phase constitution, carbon content, microhardness and tensile behavior are investigated to evaluate laser carburization effect. The carburized zone presents different morphologies according to linear energy density of laser beam. The least carbon content is around 0.4 wt% in the carburized zone where austenite becomes the leading phase. Delta ferrite is found in cellular carburized area, which resembles to a duplex microstructure. Hardness of carburized zone has been at least increased by 130%. And the yield strength and ultimate tensile strength of a fully carburized sample can be increased up by respectively 90% and 85 %. This hardening effect is driven by the precipitation of carbides formed during solidification offering pinning points for dislocations and grain boundaries. These improvements could be useful to modify locally ferritic stainless steel to meet industrial needs such as wear-resistant surface.

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Laser-Power Bed Fusion (L-PBF) is continuing to grow in use among the industrial field. This process allows the manufacturing of parts with complex geometry, good dimensional accuracy, and few post-processing steps. However, deviations can still be observed on the final parts. It is known in the literature that all of these deviations can be imputed to some extent to thermal phenomena such as overheating or thermal gradient through residual stress relaxation. The objective of this study is to reach a better understanding of the influence of the thermal properties on the dimensional accuracy of parts produced by L-PBF. To do so, an infrared camera has been instrumented inside the machine, allowing the determination of the temperature of parts during the process. Thin walls with different process parameters (laser power, scanning speed…) and nominal dimensions were manufactured and measured afterwards with a coordinate measuring machine (CMM). Thermal acquisitions were performed at different moments during the fabrication and give access to the cooling rate of the observed parts. Least square fitting has been used to approximate the cooling rate function and returns characteristic times that are used to compare the different manufacturing configurations. In the end, a correlation has been established between the process parameters, the thermal parameters, and the dimensional accuracy of the parts. Form deviations, possibly due to residual stress, have only been observed on the thinnest wall, which is also the part with the highest measured thermal gradients. Other form deviations were due to roughness.