Additive manufacturing of an Fe–Cr–Ni–Al maraging stainless steel: Microstructure evolution, heat treatment, and strengthening mechanisms
Introduction
The emergence of additive manufacturing (AM) techniques in the marine and shipbuilding industries as revolutionary manufacturing techniques [1] make it essential to establish the knowledge on the fabrication of a variety of materials, including maraging stainless steels. One major advantage of additively manufactured maraging stainless steels is their low carbon content, which reduces the tendency of cracking and pore formation during or after manufacturing [2]. In addition, additively manufactured maraging stainless steels can offer a significant enhancement in strength, corrosion, ductility, and fracture toughness, which ultimately improve the performance of the next generation of vessels for naval applications [3]. Maraging stainless steels are among the top candidate materials for applications under severe environmental conditions and currently used in advanced industries such as aerospace, marine, and nuclear [4]. These steels possess high strength, good ductility and toughness, and resistance to both general corrosion and stress-corrosion cracking [5]. Maraging stainless steels are usually low in carbon content and their microstructure consist of a ductile martensitic matrix, strengthened by precipitation of nanometric intermetallic phases [6].
Maraging stainless steels are mainly developed by the improvement of the corrosion resistance of maraging steels by the addition of high Cr content (10–17 Wt.%) [7,8]. Conventional maraging steels are high in Ni-content (generally about 18 wt% [9]) and contain other alloying elements, including Co and Mo with the amount of 8–13 wt% and 3–5 wt%, respectively [10]. The high content of Ni ensures that decomposition of austenite results in the formation of only martensite, even at slow cooling rates. However, a drawback of high Ni is the instability of martensite at elevated temperatures and the formation of reverted austenite [11]. To resolve this issue, the chemical composition is developed with a lower Ni content (about 8–10 wt%), which then results in strengthening through the evolution of the β-NiAl intermetallic phase rather than the conventional Ni3Ti precipitates [4]. Among the available maraging stainless steels, PH 13-8 Mo possesses superior mechanical properties compared to other types (such as PH 17–4 and PH 15–5) [12]. PH 13-8 Mo, is hardened by the evolution of β-NiAl intermetallic phase, possessing an ordered CsCl structure [13,14]. NiAl precipitates are amongst the most effective intermetallic phases to strengthen these steels [4,10,15]. This is mainly due to their lattice parameters, which are close to those of α-Fe (bcc) such that NiAl precipitates meet the particle-lattice coherency requirement [16].
Additive manufacturing of different types of maraging steels and maraging stainless steels has received significant interest recently and several alloy systems including but not limited to PH 17–4 [[17], [18], [19]] and PH 15–5 [20,21] have been processed and promising results were reported. In addition, other classes of martensitic and ferritic stainless steels with superior mechanical properties were processed through selective laser meting as well, see e.g. AISI 420 [22], S136 mold steel [23], duplex [24], HY100 [25], Fe–14Cr [26], and 1Cr18Ni9Ti [27].
Additively manufactured PH 17–4 can be hardened to 41HRC and ~1350 MPa ultimate tensile strength in heat-treated conditions (EOS data sheet 17–4 [28]). Thus, this alloy is unable to offer competitive properties, where there is a need for both superior wear and corrosion resistance. EOS GmbH has recently developed a new grade of low carbon Fe–Cr–Ni–Al maraging stainless steel (EOS Stainless Steel CX) in the family of PH 13-8 Mo [28], compatible with the laser-powder bed fusion (LPBF) process. It was previously shown that defect-free CX could be fabricated through the LPBF process [[3], [29]]. The superior mechanical and corrosion properties of laser processed stainless steel CX in the heat-treated condition can place this alloy in a stellar position for marine, shipbuilding, nuclear submarines, and energy applications.
The current study aims to analyze the microstructure and strength of LPBF-CX in the as-built and heat-treated conditions and identify the strengthening mechanisms in this alloy. Since there is no study available on the hierarchical microstructure evolution in the LPBF-CX in the as-built and heat-treated conditions, this study is mainly focused on the multi-scale characterization of the microstructure of as-built and the heat-treated LPBF-CX under the proposed heat treatment cycle of the powder vendor. While the heat treatment procedure used in the current study is not necessarily an optimum one, it sheds light on the microstructural evolution at different length scales and the effect of hierarchical microstructure in the LPBF-CX on the strength of the material. The microstructure of as-built LPBF-CX is studied using advanced electron microscopy. The material is then heat treated under two different cycles and the microstructure is analyzed and correlated to the hardness using fundamentals of strengthening in metallic materials.
Section snippets
Powder and AM process
Cuboid samples of Fe–Cr–Ni–Al maraging stainless steel material (called CX hereafter) with dimensions of 15 mm × 15 mm × 15 mm were fabricated through the LPBF process, as shown schematically in Fig. 1. The samples were fabricated using an EOS M290 additive machine with a 400 W Yb-fiber laser. The process parameters were set based on the proposed ones by EOS GmbH to achieve the least porosity. These parameters include laser power (258.7 W), scan speed (1066.7 mm/s), hatch distance
Characteristics of the as-built LPBF-CX
The OM microstructure of the as-built LPBF-CX from the side and top views is shown in Fig. 2. The structure of LPBF-CX from the side view consists of overlapped melt pools, due to the successive solidification of melt pools on top of each other [1]. Similar to other LPBF processed materials, the melt pools are almost half-cylindrical [30] with the common fish-scale shapes. The top view OM microstructure reveals the scan tracks, with non-uniform arrangement due to the stripe scanning strategy [3
Discussion
A significant advantage of the metal AM processes is the resulting hierarchical microstructure, where microstructural features at different length scales contribute to the strength of the material [35,57]. The microstructure of the aged LPBF-CX consists of micron and submicron characteristics including the martensite laths, pre-existing dislocation networks, and nanoscale β-NiAl precipitates. This hierarchy of micron to nano components enhance the yield strength of LPBF-CX through sub-boundary
Conclusions
In summary, in the current study, a martensitic stainless steel, CX, was successfully manufactured using the LPBF process. The material was then heat-treated through a standard procedure consisted of austenitization at 900 °C for 1 h (in argon) followed by rapid air cooling and aging at 530 °C for 3 h. Moreover, a direct aging treatment at 530 °C for 3 h on an as-built LPBF-CX was also conducted. The as-built and heat-treated samples were studied using different multi-scale microscopy
Data availability statement
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
CRediT authorship contribution statement
Amir Hadadzadeh: Conceptualization, Formal analysis, Methodology, Writing - original draft, Writing - review & editing. Ayda Shahriari: Methodology, Writing - review & editing. Babak Shalchi Amirkhiz: Formal analysis, Methodology, Supervision, Writing - review & editing. Jian Li: Methodology, Writing - review & editing. Mohsen Mohammadi: Conceptualization, Supervision, Funding acquisition, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The funding from the New Brunswick Innovation Foundation (NBIF-RIF2017-071) and Natural Sciences and Engineering Research Council of Canada (NSERC-RGPIN-2016-04221) is highly acknowledged. The authors would like to thank Catherine Bibby for TEM sample preparations, Dr. Mark Kozdras for facilitating the research, Raul Santos for the heat treatments, and Pei Liu for hardness measurements.
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