Preliminary Characterization and Mechanical Performance of Additively Manufactured HT9


Laser-blown powder deposition of HT9 was performed to evaluate the feasibility of using advanced manufacturing to fabricate creep strength enhanced ferritic steels. The as-printed materials were tensile tested at room temperature, 330�C, and 550�C to provide a baseline value for the strength levels. The as-fabricated strengths are then compared with wrought materials in the published literature. The strength parameters for the as-fabricated HT9 was found to be significantly higher than that of wrought HT9 without sacrifice in ductility. Two different heat treatment cycles, deemed ACO3 and FCRD, involving two different austentizing temperatures (1065C and 1040C) and tempering temperatures (750C and 760C) were then evaluated. The heat-treated samples were also tensile tested and were found to show strength and ductility parameters which closely mimicked that of the wrought HT9 literature data. Detailed multi-scale characterization was performed at various length scales. The as-fabricated structure showed a refined martensitic structure with a significant fraction of -ferrite present. Closer examination using TEM showed a refined distribution of carbides (predominantly M23C6 and MX or M2X), which would have precipitated during the deposition process. Upon heat treatment, it was seen that the precipitates were more homogenous and refined in the case of the specimens heat treated according to the ACO3 cycle although there was no significant difference in the strength or the hardness results. However, the specimens heat-treated per the FCRD cycle showed significantly smaller prior austenite grains (measure and give quantitative numbers), which could result in better Charpy toughness. However, the toughness was not evaluated and will be performed in the future. In addition, possible thermos-mechanical treatments (TMT) to refine the grain structure and improve the toughness without compromising the ductility will also be evaluated in the future.