Effect of local variations in thermal history on the microstructures and hardness of a high-strength, low-alloy martensitic steel made by directed energy deposition

This work presents the effects of spatially varying thermal history on local microstructure and properties in directed energy deposition additively manufactured AF9628 high-strength low-alloy steel walls. Hardness was shown to decrease with increasing height followed by an abrupt increase in hardness at the top of the wall. Austenite content was characterized with x-ray diffraction, martensite structures were characterized with electron backscatter diffraction, and carbide character was observed via electron microscopy. Carbide precipitation and the evolution of matrix composition during building were modeled using Thermo-Calc TC-PRISMA with ABAQUS temperature history simulations calibrated with thermocouple data during fabrication. A hardness model was used to describe the contributions of each microstructural feature measured or simulated to the local hardness. Toward the top of the martensitic structure, all subsequent layers reheated the area only above the austenitization temperature, resulting in an untempered microstructure. This study demonstrates how variations in thermal history result in differing extents of intrinsic tempering and hardness throughout the build.