Mechanical homogeneity induced by bainitic transformation in laser powder bed Fused H13 steel at Ultra-High substrate preheating temperatures

• Nearly dense LPBF H13 steel was produced under extreme high preheating substrate conditions. • The mechanical homogeneity along the build direction was validated through multi-scale compression tests. • The bainite transition triggered by the extreme high preheating substrate improved the mechanical homogeneity. Improving mechanical homogeneity is essential for laser powder bed fusion (LPBF) components. This study introduces an in-situ modification technique tailored for LPBF H13 steel that uses ultra-high preheating temperatures (up to 500 ℃) to achieve superior mechanical homogeneity. Macrocompression, microhardness, nanoindentation tests, and residual stress analyses validated the effectiveness of this approach cross multiple scales along building height. The results show that samples preheated at 500 ℃ exhibited fluctuations in compressive performance from the macro- to nanoscales of 10.6%, 17.6%, and 17.8%, respectively, which are lower than those of samples preheated at 200 ℃, which showed fluctuations of 54.1%, 32.8%, and 24.3%, respectively. Additionally, compared with the 200℃-preheated samples, the 500℃-preheated samples demonstrated the opposite trend, with improved compressive strength in the middle and lower sections, outperforming that in the top section. These phenomena are attributed mainly to the bainitic transformation triggered at elevated preheating temperatures, in addition to the overall reduction in residual stress. This transformation stabilizes the mechanical properties by reducing the material’s sensitivity to tempering. Furthermore, the precipitation of carbides at high preheating temperatures helps maintain high strength in the as-built state. These insights provide valuable strategies for optimizing LPBF processes for manufacturing large-scale, high-performance molds.