Development of a Periodic Laminate Structure in H13 Steel Using Laser Powder Bed Fusion: Effects of Tempering on Hardness Evolution

H13 steel with a sub-millimeter-order periodic laminate structure is fabricated using the laser powder bed fusion process by changing the input energy periodically, and the effect of heat treatment on the hardness distribution of the alloy is examined. When the as-built specimen is directly tempered at 550 degrees C to induce secondary hardening, the hardness values at the matrix area of the specimen, which average 638 HV under the as-built condition, increase to an average of 685 HV, whereas those of intermediate areas, which initially average 658 HV, reach an average hardness of 722 HV, which is higher than that of the matrix area even under the tempered condition. Consequently, the hardness distribution maintains the initial trend, wherein the hardness varies periodically depending on the area, exhibiting a shift toward higher hardness values under the tempered condition. The factors affecting the hardness and tempering behavior are investigated by analyzing the phase constituents and microstructural evolution. The tempered specimen comprises alpha '-martensite, indicating that retained austenite present in the as-built microstructure is almost completely decomposed. At the nanolevel, only V segregates at the lath boundaries under the as-built condition, while various metallic carbides are formed at the lath boundaries after tempering.