Structural steels are often considered unsuitable for Laser Powder Bed Fusion (L-PBF) due to their susceptibility to cracking. These defects form due to the combined presence of brittle martensite and residual stresses, and become more prominent as the carbon content increases. This trend is partly caused by the increase in martensite brittleness with increasing carbon content, however, little is known regarding the effect of carbon on the evolution of residual stresses.

During L-PBF, residual stresses are typically generated by the contraction of the solidifying melt pool which pulls on the underlying material, leading to tensile stresses in the newly deposited layer and compressive stresses in the underlying layers. However, in structural steels there is also the martensite transformation which induces volume expansion and subsequent compressive stresses, where the magnitude of volume expansion increases with increasing carbon content. Due to this complexity, it is difficult to follow the evolution of residual stresses during L-PBF of structural steels.

In the current work, in- and ex-situ neutron diffraction measurements were conducted at the Swiss Spallation Neutron Source. These measurements were carried out on Fe-0.2C and Fe-0.45C steel using the n-SLM, which is a miniaturized L-PBF device that was specifically designed for in-situ neutron studies of bulk specimens. The in-situ measurements followed the evolution of residual strains at fixed positions along the transverse and building directions by measuring each time 5mm was printed. As for ex-situ measurements, these involved Bragg Edge Imaging along the building and transverse directions for already printed samples.

From these results, it was possible to map the evolution of residual stresses as new layers were added. Generally, the change in residual stress increased as more layers were printed, while the magnitude of compressive stresses increased as the carbon content increased. Additionally, the change in residual stress was more pronounced along the building direction than it was along the transverse direction. From Bragg Edge Imaging, strain maps were produced that outlined the local distribution of the strain along both directions for the 0.2C and 0.45C steels. These findings improve our understanding regarding how residual stresses evolve and are distributed during L-PBF of structural steels.