近年来，18Ni300马氏体时效钢在增材制造随形冷却模具中得到广泛应用，而热处理工艺是决定打印件组织和性能否满足服役要求的重要因素。本工作研究了直接时效和固溶时效2种典型热处理工艺对选区激光熔化成型18Ni300马氏体时效钢微观组织和力学性能的影响。结果表明，打印态、直接时效态和固溶时效态试样中均存在奥氏体，且与马氏体基体存在经典Nishiyama-Wassermann取向关系。打印态试样元素分布均匀，存在明显的熔池结构和胞状组织，胞状组织由位错缠结形成，在晶界处存在少量长条状的奥氏体。直接时效处理后胞状组织和熔池结构发生部分溶解，Ni在部分晶界和胞壁处富集，具有较高含量的奥氏体。固溶时效处理后获得近全马氏体组织，元素分布均匀，胞状组织和熔池结构基本消失，Ni在部分晶界处富集，存在微量奥氏体。打印态试样的奥氏体无明显化学成分偏析，为残余奥氏体；而直接时效态和固溶时效态试样的奥氏体存在Ni富集，为逆转奥氏体。直接时效和固溶时效处理使Ni在部分晶界和胞壁处富集，Ni富集促进逆转奥氏体生成并使其稳定存在。打印态试样的屈服强度为(1090 ± 1.5) MPa，直接时效和固溶时效处理后基体析出大量棒状η-Ni₃Ti金属间化合物，强度提升，其中，直接时效处理后屈服强度可达(1854 ± 13.2) MPa，固溶时效处理后屈服强度可达(2059 ± 9.9) MPa。打印态试样的强度主要由马氏体相变强化和固溶强化贡献，而直接时效态和固溶时效态试样的强度主要由马氏体相变强化、固溶强化和析出强化贡献，且固溶时效态试样具有更强的析出强化效果，这主要是由于固溶时效态试样的析出相具有更高的密度和长径比。

Recently, 18Ni300 maraging steel has been widely used for preparing conformal cooling molds via additive manufacturing. The requirements pertaining to the service life of these molds have become more stringent, but whether the microstructures and properties of these molds can meet the service requirements largely depends on the applied heat treatment. This paper studies the effects of two typical heat treatment processes—direct aging and solution aging—on the microstructure and tensile properties of 18Ni300 maraging steel fabricated via selective laser melting. In all prepared specimens, austenite was present and the classical Nishiyama-Wassermann orientation relationship was observed between austenite and the martensitic matrix. Elements in the as-prepared samples were evenly distributed, with obvious molten-pool and cell structures composed mainly of dislocation entanglements. In addition, a small number of long austenite strips appeared at the grain boundaries. Direct aging partially dissolved the cell and molten-pool structures and enriched Ni at some grain boundaries. The direct-aging sample exhibited relatively high austenite content. Meanwhile, the solution-aging sample exhibited a nearly complete martensite structure with evenly distributed elements. In addition, cell and molten-pool structures were almost completely removed and Ni was enriched at some grain boundaries. Further, trace amounts of austenite remained. Austenite retained in the as-prepared samples showed no obvious chemical composition segregation. Austenite present in the direct-aging and solution-aging samples was Ni enriched and confirmed to be of the reverted type. Ni at certain grain boundaries and cell walls was enriched due to cell-wall dissolution during the direct- and solution-aging treatments. Ni enrichment promoted the formation and stability of reverted austenite. Numerous round rod-shaped Ni₃Ti intermetallic compounds precipitated from the matrix after both the treatments, greatly increasing the yield strength from (1090 ± 1.5) MPa of the untreated sample to (1854 ± 13.2) MPa and (2059 ± 9.9) MPa of the direct-aging and solution-aging samples, respectively. The strength of the as-prepared samples was mainly contributed by austenite-to-martensitic phase transformation and solid-solution strengthening, while those of the direct- and solution-aging samples were mainly contributed by austenite-to-martensitic phase transformation, solid-solution strengthening, and precipitation strengthening. Moreover, the solution-aging samples exhibited greater precipitation strengthening than the direct-aging samples, mainly owing to the high density and large length-diameter ratio of their precipitates.