Microstructural and Mechanical Properties of Novel Co-Free Maraging Steel M789 Prepared by Additive Manufacturing

Abstract

This research aims to characterize and examine the microstructure and mechanical properties of the newly developed M789 steel, applied in additive manufacturing. The data presented herein will bring about a broader understanding of the processing–microstructure–property–performance relationships in this material based on its chemical composition and heat treatment. Samples were printed using the laser powder bed fusion (LPBF) process and then the solution was annealed at 1000 °C for 1 h, followed by aging at 500 °C for soaking times of 3, 6 and 9 h. The AM components showed a relative density of 99.1%, which arose from processing with the following parameters: laser power of 200 W, laser speed of 340 mm/s, and hatch distance of 120 µm. Optical and electron microscopy observations revealed microstructural defects, typical for LPBF processes, like voids appearing between the melted pools of different sizes with round or creviced geometries, nonmelted powder particle formation inside such cavities, and small spherical porosity that was preferentially located between the molten pools. In addition, in heat-treated conditions, AM maraging steel has combined oxide inclusions of Ti and Al (TiO2:Al2O3) that reside along the grain boundaries and secondary porosities; these may act as preferential zones for crack initiation and may increase the brittleness of the AM steel under aged conditions. Consequently, the elongation of the AM alloy was low (<3%) for both annealed and aged solution conditions. The tensile strength of AM M789 increased from 968 MPa (solution annealed) to 1500–1600 MPa after the aging process due to precipitation within the intermetallic η-phase. A tensile strength and yield point of 1607 ± 26 and 1617 ± 45 MPa were obtained, respectively, after a full heat treatment at 500 °C/6 h. The results show that 3 h aging of solution annealed AM M789 steel achieves satisfactory material properties in industrial practice. Extending the aging time of printed parts to 6 h yields slightly improved properties but may not be worth the effort, while long-term aging (9 h) was shown to even reduce quality.

Influence of in-situ process parameters, post heat treatment effects on microstructure and defects of additively manufactured maraging steel by laser powder bed fusion—A comprehensive review

“…These precipitates, when present in high concentrations, significantly diminish the material's impact strength, ductility, and toughness of the material. The pricey nature of maraging steel can be attributed to its high alloy content,and in particular to the addition of cobalt [10,[31][32][33]. The different grades maraging steels are 18 Nickel (200), 18 Nickel (250), 18 Nickel (300), 18 Nickel (350), 18 Nickel (cast) and 12-5-3(180).…”

Indentation Plastometry for Study of Anisotropy and Inhomogeneity in Maraging Steel Produced by Laser Powder Bed Fusion

“…The advantages of WAAM, as applied to maraging steels, [1][2][3] largely relate to higher production rates, but the structure and surface finish tend to be relatively coarse, and the porosity level can be high. In general, the LPBF technique has been more popular than WAAM for maraging steels, [4][5][6][7][8][9][10] offering superior scope for geometrical and microstructural control.…”

Aging Behaviour of a 12.2Cr-10Ni-1Mo-1Ti-0.6Al Precipitation-Hardening Stainless Steel Manufactured via Laser Powder Bed Fusion

“…PH-SS are easily manufacturable by LPBF due to their low C content, leading to a reduced risk of crack formation [14,[16][17][18]. The as-built (AB) microstructure of LPBF-manufactured PH-SS consists of melt pool/scan track boundaries, a martensite microstructure with oriented grains, and a cellular solidification sub-structure, and exhibits relatively low hardness and strength due to the lack of strengthening precipitates comparable to condition A for conventionally manufactured PH-SS [8,14,17,[19][20][21][22][23][24][25][26][27][28][29][30]. The AB microstructure of Cu-bearing 17-4 PH and 15-5 PH steels can possess significant amounts of metastable, retained γ-austenite, depending on the powder atomisation and LPBF inert gas atmosphere [18,[31][32][33][34][35][36][37][38][39][40].…”