Directed energy deposition (DED) is one of the most promising additive manufacturing technologies, particularly for partial coating, repairing, and dissimilar metal depositing. However, determining the optimal deposition parameters to achieve desired shapes and mechanical properties is challenging because of the complex process of repeated melting and solidification driven by thermal energy. While some studies have focused on the cooling rate and its impact on mechanical properties, they have not evaluated the range within which the cooling rates vary. In this study, we investigated a range of cooling rates and their influence on hardness in DED under practical conditions using austenitic stainless steel SUS316L. The primary objective was to control the hardness of the deposited part by adjusting the cooling rate. Among the factors influencing hardness, grain size is particularly affected by cooling rates. Therefore, this study focused on grain size to evaluate the effects of DED technology. We proposed a method to calculate the cooling rate based on temperature distribution in the melt pool, captured by a camera coaxial with the laser beam. We varied the interpass dwell time, the resting interval between deposition layers, and investigated its effects on cooling rate and hardness. Additionally, the surface temperature of the workpiece during deposition was measured with a thermal camera. Results showed that while the dwell time did not significantly affect the calculated micro cooling rate, the deposition height had a notable impact. Conversely, the macroscopic temperature change was influenced by the dwell time. The cooling rate ranged from 3×10³°C/s to 10×10³°C/s, with hardness varying between 160 HV and 220 HV, and a linear correlation was observed between them.

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