Low-temperature precipitation strengthening of maraging steel 1.2709 produced by powder bed fusion

Abstract

Research into precipitation hardening in additively manufactured (AM) maraging steel is mainly conducted on the material after several hours of holding at the peak hardening temperatures of 450–500 °C. Lower temperatures are expected to have little to no effect on mechanical properties even after very long holds at these temperatures. This article, however, demonstrates that new precipitates grow and cause a noticeable hardening effect in solution-annealed AM maraging steel after only a 6-hour hold at a temperature of 350 °C. High-resolution scanning and transmission electron microscopy (HRSEM, HRTEM) was used to identify the new precipitates. Very fine coherent microdomains containing mainly nickel and molybdenum layers were found. Particles of a new phase, Al0.5CNi3Ti0.5, were also detected. Additionally, various precipitates were also found in both the as-built and the AM solution-annealed maraging steel samples. Precipitates such as Ni3Ti, Al0.3Ni0.35Ti0.35 and Al2O5Ti, were also observed in the as-built condition. After subsequent solution annealing at 820 °C additional Ni3Mo particles and another type of Ti-rich particles, Ni4Ti3, were further found in the AM steel microstructure. The additional new phases which precipitated during the final hardening at 350 °C caused an 18 % (200 MPa) increase in strength.