Fig. 1. Metallurgical defects in L-PBF built C300 parts: (a) pores; (b) lack-of-fusion defects [77].

Fig. 2. Optimized processing parameters in L-PBF for fabricating C300 maraging steels (see detailed data in Appendix A).

Fig. 3. Metallurgical defects present in WAAM produced C250 parts: (a) spatters and (b) lack-of-fusion defects [91].

Fig. 4. Macrostructure features in the vertical cross-section of additive manufactured 18% nickel maraging steels: (a) L-PBF produced C300 part [36]; (b) L-DED fabricated C300 part [99]; and (c) WAAM fabricated C250 part [91]. ↑ Shows build direction.

Fig. 5. Solidification structures formed in the maraging steels produced by different AM techniques: (a) L-PBF produced C300 part [35]; (b) L-DED produced Fe-19Ni-xAl part [48]; and (c) WAAM built C250 part [91].

Fig. 6. Transmission electron microscopy (TEM) images of martensite formed in as-built L-PBF C300 part: (a) cellular structure; (b) high-density dislocations in cells in (a); and (c) selected area electron diffraction showing lath martensite [31].

Fig. 7. Radial distribution function of Ti atom pairs of as-built L-DED and L-PBF C300 maraging steels as a function of interatomic distance [34].

Fig. 8. EBSD phase map indicating the intermetallic precipitates formed at the top and bottom of WAAM built C250 maraging steel [6].

Fig. 9. SEM image of austenite formed in as-built WAAM C250 maraging steel [6].

Fig. 10. Pole figures of C250 single-pass multilayer maraging steel produced by WAAM technique: (a) as-built condition; (b) after rolling with 50 kN [114]. TD: transverse direction and ND: normal direction. The build direction is along ND.

Fig. 11. Crystallographic orientation maps (a-d) for different scanning strategies and corresponding {001} pole figures (e-h) of the top and side planes [108]. The build direction is along the z axis.

Fig. 12. Structure features of C300 steels produced by L-PBF technique: (a) as-built; (b) solution-annealed (900 °C/45 min) [102].

Fig. 13. Microstructure of L-PBF produced C300 parts after different heat treatments: (a) as-fabricated specimen; (b) specimen subjected to DAT (direct aged at 480 °C for 6 h); (c) specimen subjected to ST at 900 °C for 1 h; and (d) specimen subjected to SAT (solution annealed at 900 °C for 1 h+ aged at 480 °C for 6 h)) [41].

Fig. 14. TEM analysis of nanoprecipitate characteristics in L-PBF produced samples after being subjected to DAT: (a) massive nanoprecipitates embedded in martensite matrix, and (b) and (c) morphology of nanoprecipitates in (a) [31].

Fig. 15. Volume fractions of austenite in L-PBF built parts under different aging conditions.

Fig. 16. Phase transformation in L-PBF built C300 maraging steels during the continuous heating with different heating rates: (a) phase transformation in the as-built and solution-treated parts during the continuous heating at heating rate of 0.167 °C/s; (b) phase transformation in the as-built parts during the continuous heating processes at heating rates of 0.167 and 5 °C/s; and (c) volume fraction of austenite in the as-built and solution-treated parts during continuous heating, which was observed through in-situ synchrotron XRD observations [122].

Fig. 17. Volume fraction of reverted austenite in L-PBF built C300 maraging steels in the as-built and solution-treated conditions during isothermal tempering at different intercritical temperatures: (a) 530 °C; (b) 570 °C; (c) 650 °C; (d) 670 °C [122]. Heating rate of 500 °C/s was used in the isothermal intercritical tempering processes.

Fig. 18. Tensile properties of C300 parts printed in different L-PBF build directions: (a) UTS, and (b) EL (data derived from [43]).

Fig. 19. Tensile performance of L-PBF produced C300 parts tested at different orientations (data derived from [85]).

Fig. 20. UTS and EL of L-PBF and WAAM built 18% nickel maraging steels after post-manufacturing heat treatments and those of conventionally produced alloys after SAT (The data shown in Fig. 20 are extracted from Appendix C).

Fig. 21. Hardness of C300 parts printed with different L-PBF build directions: (a) data derived from [31]; (b) data derived from [43]; (c) data derived from [89]; and (d) data derived from [81]. 0°, 45°, and 90° are the angles between the build direction and horizontal direction.

Fig. 22. Hardness distribution along the build direction of additive manufactured parts: (a) WAAM produced C250 part [6], and (b) L-DED built C300 part [46].

Fig. 23. Influence of AT on the hardness of L-PBF built C300 parts: (a) effect of aging time on the hardness of L-PBF built parts aged at various aging temperatures [39]; (b) time required for the peak hardness of L-PBF built parts aged at different aging temperatures (Data derived from [39] and [43]).

Fig. 24. Charpy impact toughness of L-PBF built C300 maraging steels in the as-fabricated and heat-treated conditions: (a) influence of solution temperature on the impact toughness with the duration of 1 h; (b) influence of solution time on the impact toughness at 900 °C; (c) effects of aging temperatures in DAT and SAT routines on the impact toughness with a holding time of 6 h; (d) effect of the aging duration in DAT and SAT on the impact toughness at 520 °C [41].

Fig. 25. Comparison in the fatigue resistance between L-PBF built and wrought C300 maraging steels: (a) fatigue crack growth rate as a function of the applied stress intensity factor range, which were obtained in the compact tension fatigue tests; (b) stress amplitude vs number of cycles to failure (S-N), which were acquired from fully-reversed, load controlled axial tension-compression fatigue tests. P means the applied load direction is parallel to build direction, and V represents the direction of the load is vertical to build direction in (a). 90° and 0° are the angles between the build direction and horizontal direction in (b).