## 4 Summary  
The effect of a heat treatment condition on the residual stress state of maraging 18 Ni-300 steel produced by LPBF was investigated. Cantilever-shaped specimens were fabricated to quantify the residual stress by measuring their distortions when cutting them from the supporters. The main findings of this study can be summarized as follows:  
* In the as-built state, the LPBF-processed maraging steel cantilevers showed complex distortion behavior while cutting them from the supporter. They bent downward significantly when the thickness of the cantilever was relatively thin. It was believed that the downward distortions of the thin cantilevers were due to the phase transformation of the maraging steel from martensite to austenite during the LPBF due to the IHT effect, which occurred in all regions, except on the top LPBF layer, and produced a compressive stress state on the top layer. FE simulations were carried out based on the assumption of the compressive stress state of the top layer. The deformation trends predicted by the simulations agreed fairly well with the experiments, indicating that the main reason for the downward distortion of the cantilever is the phase transformation due to the IHT effect.
* The effect of direct aging without solution heat treatment on the residual stress relief behavior was analyzed by comparing the distortions of the directly aged cantilevers without solution heat treatment with those that were aging heat treated followed by the solution heat treatment. It was found that aging heat treatment can effectively diminish the downward distortions of the cantilevers, in both cases with and without solution heat treatment. Thus, the results indicate that the solution heat treatment is not necessary for the purpose of residual stress relieving for the maraging 18 Ni-300 steel components produced by the LPBF.
* The effect of the LPBF baseplate made of medium-carbon steel on the residual stress relief behavior during the heat treatment was investigated by producing the cantilevers directly attached to the medium-carbon steel baseplate. These cantilevers were aging heat treated while they were still attached to the baseplate. Comparisons of the distortions of these cantilevers with those attached to the maraging steel artificial supporter structures showed that the carbon steel baseplate does not have any influence on stress relief behavior. Almost identical distortion behaviors were observed for the cases of the cantilevers heat treated with the maraging steel artificial supporter and with the carbon steel baseplate. This indicates that the stress relieving heat treatment of the maraging 18 Ni-300 steel components produced by the LPBF can be performed while they are attached to carbon steel baseplates without any special consideration of the thermal strain misfit effect due to the dissimilar material joint between the maraging components and the steel baseplate.  
Conceptualization, J.H.Y. and W.L.; methodology, J.H.Y. and K.H.; formal analysis, J.H.Y. and Q.-Y.J.; investigation, J.H.Y., Q.-Y.J. and K.H.; resources, W.L.; writing--original draft preparation, J.H.Y. and W.L.; writing--review and editing, Q.-Y.J. and W.L.; visualization, Q.-Y.J. All authors have read and agreed to the published version of the manuscript.  
This work was supported by a two-year research grant from the Pusan National University.