Microstructure evolution and strengthening behavior of maraging steel fabricated by wire arc additive manufacturing at different heat treatment processes

Defect-free (single-pass multi-layer) maraging steel components were prepared by wire arc additive manufacturing (WAAM). The study on microstructure evolution and strengthening behavior of the components at different conditions was deeply investigated. It was found that the as-printed components consisted of lath martensite, residual austenite, and a few in-situ strengthening phases. There was significant element segregation among intragranular and intergranular, and the ultimate tensile strength (UTS) of the components reached 1184 MPa with an elongation of 16.8 %. After direct aging treatment at 480 °C for 4 h, unevenly distributed nanoprecipitates formed within the lath martensite, resulting in an increase of UTS about 1484 MPa. The presence of over 10 % reverted austenite in the microstructure led to elongation reached 14.8 %. 840 °C for 4 h solution treatment was applied to eliminate element segregation and reduce the content of retained austenite to below 3 %. Subsequent aging treatment at 480 °C for 4 h resulted in the dispersed precipitation of strengthening phases such as Ni3Ti and Ni3Mo, which lead to UTS increased to 1686 MPa while the elongation decreased to 9.2 %. Finally, the Orowan-modified model was used to calculate the theoretical yield strength (YS) of the aged components. The calculated values were found to be in good agreement with the actual results.