Elevated substrate preheating tailors microstructural evolution and enhances fatigue resistance in laser powder bed fusion H13 tool steel

Fatigue performance remains a key limitation for laser powder bed fusion (LPBF) tool steels due to the strong coupling between process-induced defects and microstructural heterogeneity. Elevated substrate preheating has been proposed as an effective strategy to improve the processability of hard-to-print alloys; however, its combined influence on defect characteristics, microstructure, and fatigue cracking behavior remains insufficiently understood. In this study, the fatigue behavior of LPBF-fabricated H13 tool steel was investigated by increasing the substrate preheating temperature from 200 °C (PT200) to 500 °C (PT500). μ-CT analysis reveals that defects in PT500 samples are predominantly spherical with high sphericity (>0.8), whereas PT200 samples contain both spherical and irregular defects with sphericity ranging from 0.4 to 1. Microstructural characterization shows that PT200 exhibits a dual-phase cellular structure composed of lath martensite and retained austenite films, while PT500 develops a relatively coarser multiphase microstructure dominated by bainite. These changes lead to a transition in crack initiation behavior, shifting from irregular-defect-controlled cracking in PT200 to a competitive mechanism involving spherical-defect-induced and non-defect-induced initiation in PT500. As a result, PT500 samples exhibit a 134–482% improvement in fatigue life within the stress amplitude range of 500–1200 MPa, attributed to reduced defect sensitivity and improved deformation compatibility.