Effect of solution annealing and precipitation hardening at 250 °C–550 °C on microstructure and mechanical properties of additively manufactured 1.2709 maraging steel

Production of Hybrid Joints by Selective Laser Melting of Maraging Tool Steel 1.2709 on Conventionally Produced Parts of the Same Steel

“…No austenite was found in the conventional VACO 180 steel in an initial condition. This is in agreement with other research, as solution annealed conventional maraging steel does not typically possess any austenite in the microstructure, while several percent of retained austenite is commonly present in AM steels in the as-built condition, as reported by [ 15 , 22 , 28 , 29 , 30 ]. The austenite phase fraction detected by those authors in the AM as-built maraging steel 1.2709 varied from approximately 3% to 6%.…”

“…This effect is also clearly seen in the microstructure and distribution of the alloying elements in the initial powder ( Figure 5 and Figure 6 ). Particles of various sizes with increased Ti content were observed in the AM microstructure ( Figure 8 a,b), and they are expected to correspond mainly to Ni 3 Ti particles which have been reported by other authors to be dominant precipitates in AM maraging steel [ 21 , 22 ] and Ti oxides [ 28 ]. Precipitation and epitaxial growth of cells across the boundaries of laser tracks ( Figure 7 d) are caused by intrinsic heat treatment occurring during additive manufacturing.…”

“…A fine dispersion of numerous Ni 3 Ti—type particles appears at those temperatures and only a negligible fraction of reversed austenite is formed at the same time. The total amount of austenite in the AM steel would be for peak annealing temperatures slightly higher than that in the conventional steel [ 28 ], due to the original 3–6% of retained austenite which remains in the microstructure of AM steel during the precipitation annealing. This combination of microstructure features maximises the strengthening of the steel, while coarsening of the precipitates and increase of reversed austenite fraction at higher annealing temperatures (above 500 °C) would result in a decline of strength and hardness after annealing at higher temperatures [ 28 ].…”

“…The fine, more homogeneous microstructure of AM steel without residual stresses achieved better total elongation of 15%, which could be the comparable value to the 16% elongation of conventional steel. As the solution annealing did not decrease the number of printing defects in the steel [ 28 ], the improvement of the elongation has to be attributed to the changes in the microstructure and residual stresses. After solution annealing, the yield tensile strength of the AM steel was observed to decrease more sharply than the ultimate tensile strength, increasing the work hardening rate of the AM steel.…”

“…This could be caused by the change in the initial dislocation arrangement and density during the solution annealing of the AM steel. The high local dislocation density was already in the AM part before the mechanical testing [ 28 ], while annealing would result in recovery and recrystallisation.…”