Showing papers on "Maraging steel published in 2022"

Effect of heat treatment on the strength and fracture resistance of a laser powder bed fusion-processed 18Ni-300 maraging steel

Abstract: Engineering materials processed using additive manufacturing (AM) techniques such as laser powder bed fusion (LPBF) often exhibit unique microstructures and defects that must be controlled to obtain peak performance in mechanical properties. Towards this end, we here investigate how three heat-treatments (direct aging, solution treatment + aging, and thermal cycling + aging) impact microstructure evolution and corresponding mechanical performance of LPBF-processed 18Ni-300 maraging steel. Specifically, we focus on how strength and fracture toughness measured in two orthogonal directions correlate with phase transformation kinetics as well as micro- and meso-structural features and compare findings with results from as-built and conventionally processed material. Aging results in the formation of Ni–Ti and Mo-rich nano-precipitates that enhance strength but reduce both ductility and fracture resistance when compared to as-built material. With the solution treatment, however, AM-characteristics such as cellular sub-structures and melt-pool boundaries dissolve almost entirely and as a result the damage-tolerance capacity of the material degrades and becomes comparable to cast material. Thermal cycling, on the other hand, results in the formation of reverted austenite thereby enabling the transformation-induced plasticity (TRIP) effect. This leads to extensive gains in ductility that are accompanied by only moderate reductions in strength while fracture toughness is improved significantly. While anisotropy in the obtained data from both the tensile tests and the crack resistance curve (R-curve) measurements is noticeable, it is low relative to many LPBF-processed alloys due to the strong bonding characteristics between the individual layers.

Precipitation behavior of a Co-free Fe-Ni-Cr-Mo-Ti-Al maraging steel after severe plastic deformation

Abstract: Maraging steels are martensitic steels that are strengthened by the precipitation of nano-sized intermetallic phases. Severe plastic deformation is known to influence diffusion, stability and properties of metallic materials. The goal of this study is to reveal the influence of severe plastic deformation on the hardness and the precipitation behavior of a Co-free maraging steel. For this study the as-quenched material was deformed with high-pressure torsion. Afterwards, a series of heat treatments were conducted. The hardness increase was measured and correlated with micro-to-nanoscale microstructural characteristics, focused on the precipitate characterization with atom probe tomography. Severe plastic deformation transformed the microstructure to a globular microstructure, delayed and minimized austenite reversion as well as accelerated and influenced the precipitation reaction, affecting the size, morphology and type of intermetallic phases.

Synergistic enhancement of strength and ductility of cobalt-free maraging steel via nanometer-scaled microstructures

Abstract: Maraging steel with ultra-high strength and good ductility is usually achieved via semi-coherent nanometer-sized precipitates and a dual-phase structure. In this work, we studied the effect of solution treatment parameters on the microstructural evolution and the mechanical properties of cobalt-free maraging steel, with high strength (1852 MPa) and satisfactory tensile elongation (11.5%) at room temperature was obtained. The nano-scaled precipitates in the martensite matrix were analyzed, in which the η-Ni3(Ti, Mo) precipitates were distributed in width of 7.1 nm and length of 19 nm. It was found that the needle-like Ni3Ti phase plays a predominant role in enhancing the strength of the maraging steel, while the ductility can be attributed to the nano laminated austenite structure, which could effectively prevent the propagation of microcracks in the martensite matrix. A heterogeneous microstructure of precipitates was formed in the martensite matrix. Besides, the size and number density of precipitates could be regulated by tuning the solution treatment conditions, in which a finer size and a larger number of precipitates could be gained by annealing at a high temperature for a short time.

Processing optimization, microstructure, mechanical properties and nanoprecipitation behavior of 18Ni300 maraging steel in selective laser melting

Abstract: Thanks to the application needs of conformal cooling channels in mold field, selective laser melting (SLM) has become a key technique for fabricating novel mold with high cooling efficiency. This work systematically investigates the action mechanism of SLM processing parameters on densification of 18Ni300 maraging steel, and corresponding phase identification, microstructural evolution, nanoprecipitation behaviors and mechanical properties of as-built samples with optimal relative density (RD) before and after solution-aged treatment. Response surface method and analysis of variance are employed to analyze the significance levels of effects for SLM processing parameters on RD. The optimal processing parameter set and resultant RD of 99.45% as well as corresponding processing windows for SLMed 18Ni300 sample can be obtained. It is found that as-built sample consists of major martensite α phase and scarce austenite γ phase. Solution-aged treatment is in favor of reversion of martensite to austenite and precipitation of intermetallic compounds. Both as-built and solution-aged samples exhibit no obvious texture fiber, but there are different grain sizes and orientations. It is worthy noted that the mechanical properties (ultimate tensile strength of ∼1245 MPa) of SLMed 18Ni300 sample are comparable to that of corresponding wrought sample.

Influences of manganese content and heat treatment on mechanical properties of precipitation-strengthened steels

Abstract: In order to reduce the cost and improve the ductility of maraging steels without sacrificing strength, the high content of costly Ni element is partially substituted by inexpensive Mn to achieve metastable austenite. Thermodynamic simulation, composition screening and critical experiments are integrated to design ultrastrong dual-phase steels with the compositions of Fe–10Ni–3Al–4Mo–xMn (x = 0–10, wt%) by tailoring austenite fraction and heat treatment. The results show that for ageing following solid solution and quenching (soft ageing for commercial maraging steels, SA), the phase fraction of fcc should be less than that of bcc to achieve B2 precipitation in the bcc matrix. For the 9Mn alloy with a volume fraction of fcc more than 50%, ageing following cold rolling (hard ageing, HA) recovers the B2 precipitation that is hindered by fcc reversion during SA, resulting in an ultrahigh ultimate strength (∼1600 MPa) with an improved tensile ductility (∼7%) compared with the SA alloys (∼1450 MPa and ∼5% at best). The improved strength-ductility synergy at ultrahigh strength is attributed to transformation-induced plasticity (TRIP) and ultrafine grain size, both of which turn the fracture mode from transgranular cleavages to ductile dimples. This work demonstrates a promising strategy by addition of austenite-stabilized element and ageing without solid solution for achieving low cost and high strength-ductility synergy, which paves the way for the development of precipitation-strengthened dual-phase steels.

Processing optimization, microstructure, mechanical properties and nanoprecipitation behavior of 18Ni300 maraging steel in selective laser melting

Abstract: Thanks to the application needs of conformal cooling channels in mold field, selective laser melting (SLM) has become a key technique for fabricating novel mold with high cooling efficiency. This work systematically investigates the action mechanism of SLM processing parameters on densification of 18Ni300 maraging steel, and corresponding phase identification, microstructural evolution, nanoprecipitation behaviors and mechanical properties of as-built samples with optimal relative density (RD) before and after solution-aged treatment. Response surface method and analysis of variance are employed to analyze the significance levels of effects for SLM processing parameters on RD. The optimal processing parameter set and resultant RD of 99.45% as well as corresponding processing windows for SLMed 18Ni300 sample can be obtained. It is found that as-built sample consists of major martensite α phase and scarce austenite γ phase. Solution-aged treatment is in favor of reversion of martensite to austenite and precipitation of intermetallic compounds. Both as-built and solution-aged samples exhibit no obvious texture fiber, but there are different grain sizes and orientations. It is worthy noted that the mechanical properties (ultimate tensile strength of ∼1245 MPa) of SLMed 18Ni300 sample are comparable to that of corresponding wrought sample.

Additive manufacturing SiC-reinforced maraging steel: Parameter optimisation, microstructure and properties

Abstract: The unique deposition manner of additive manufacturing (AM) allows the near-net-shaping of components with multiple materials configurations and complex geometries, which sheds light on the process of high-performance metal matrix composites (MMCs). This work explores laser powder bed fusion (LPBF) AM of SiC-reinforced maraging steel MMCs to consolidate the merits of both ceramics and metal matrix for improving overall properties. The laser processing parameters were systematically optimised based on the density, roughness and hardness of the deposited samples. The effects of SiC content on the microstructures, mechanical properties, tribological performance, and wear resistance are elucidated. SiC particles are refined with uniform distribution in the metal matrix after laser processing. The highest tensile strength reaches 1611 ​MPa together with an elongation of about 10.1% with 3 ​vol% SiC addition. The tribological performance of MMCs is investigated by studying the coefficient of friction (COF), wear rate, and worn morphology. The COF has been slightly reduced with the SiC addition, and the wear rate of MS reduced from 3.25 ​× ​10−5 to 1.72 ​× ​10−5 mm3/Nm with the 12 ​vol% SiC addition. The underlying wear mechanisms are also investigated. Besides, the corrosion behaviour of MMCs is also investigated; the addition of SiC (≥6 ​vol%) has improved the corrosion properties of the matrix.

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.

Effect of heat treatment on the microstructure and mechanical properties of 2.4 GPa grade maraging steel fabricated by laser powder bed fusion

Abstract: The effects of three different heat treatments (direct aging, austenitizing–aging, and solutionizing–austenitizing–aging) on the meso-/micro-structures and the mechanical properties of the 18 wt% Ni-350 maraging steel (M350), specifically designed for ultra-high strength, additively manufactured using the laser powder bed fusion (LPBF) are studied. In both the as printed and directly aged conditions, the dendritic growth induced cellular structures constitute the unit microstructural features of the martensite; the martensitic transformation in these conditions occurs within the cell matrix and the cell boundaries act as the preferred sites for the austenite retention/reversion. The cell structures partially dissolve upon the austenitizing–aging heat treatment, while solutionizing fully erases them; both these heat treatments can facilitate the formation of the conventional packet–block–lath hierarchy, which is absent in the as printed and directed aged samples. Uniaxial tensile and mode I fracture toughness measurements reveal that the presence of the austenite in the directly aged M350 maraging steel lowers its strength but increases the fracture toughness. The absence of austenite in the other two heat treatment conditions results in high strength, but lower toughness. These findings shed light on understanding fundamental relationship between the unique features of the LPBF process and the martensitic transformation in maraging steels.

Texture evolution during processing and post-processing of maraging steel fabricated by laser powder bed fusion

Abstract: In this study, the evolution of solidification texture during LPBF of Ti-free grade 300 maraging steel, and its effect on texture development during subsequent post-fabrication heat treatments was characterized using Electron Backscatter Diffraction (EBSD). It was found that in the as-fabricated state, no texture was observed in the room temperature martensitic phase. However, the reconstructed parent austenite phase displayed a Cube texture with a minor fraction of Rotated Goss texture. During subsequent aging treatments involving two different routes, namely direct aging of the as-fabricated samples, and conventional solution treatment + aging of the as-fabricated samples, significant changes in the texture components of parent austenite were observed, whereas no changes in texture were observed in the room temperature martensitic phase. During direct aging, it was found that with an increase in the aging temperature up to 520 °C, the texture components of the parent austenite changed from Cube/Rotated Goss to Brass, whereas during the conventional solution treatment and aging cycle, interestingly a change in texture component to rotated copper was observed. The transitions in texture components have been discussed using the concepts of recrystallization and twinning in austenite during annealing and/or aging, and strain energy release maximization (SERM) theory. Furthermore, the importance of these preferred orientations on the mechanical properties was quantified using transformation potential diagrams.

Numerical investigation into laser-based powder bed fusion of cantilevers produced in 300-grade maraging steel

Abstract: Laser-based powder bed fusion of 300-grade maraging steel allows the production of parts with a high hardness, which improves the service life and wear resistance of tooling or mould insert produced from this material. The material typically consists of a martensitic matrix material, with retained austenite and nano-precipitation. The transformation from austenite to martensite has been linked to compressive stresses at the surface of parts produced in 300-grade maraging steel. In a cantilever beam-type part, this means that after cutting from the base-plate, the part will bend downwards, which is the opposite direction from the deformation found in most other materials after additive manufacturing. One way to gain insight into processing 300-grade maraging steel, while limiting the number of test samples that need to be printed, is by means of a numerical model. Using previously established models, additive manufacturing of a cantilever part in 300-grade maraging steel is simulated. Inclusion of the transformation from austenite to martensite into a numerical simulation of the laser-based powder bed fusion revealed the origin of the compressive stress at the surface of a simple cantilever beam-type sample. Additionally, changing the effective laser power through the laser absorptivity shows that the behaviour of the post-cutting deformation flips as compared to more conventional materials. Information about the laser absorption coefficient is rare, while it can greatly affect the results of a simulation. It is included in the presented result through the effective laser power, which is the product of the input laser power and laser absorption coefficient. When the effective laser power is changed from 95 W to 47.5 W, the cantilever bends upwards rather than downwards after release from the base plate. The results demonstrate the major influence played by the laser absorption coefficient on the simulation, an aspect to which little attention is paid in literature, but is proven to be one of the main factors to determine the component distortions after the laser-based powder bed fusion process.

Influence of delta ferrite on the impact toughness of a PH 13-8 Mo maraging steel

Abstract: Due to the fact that maraging steels are widely utilized as structural parts for the aerospace industry, high and consistent impact toughness is crucial to ensure reliability under extreme mechanical loads. The toughness of maraging steels is heavily influenced by the martensitic structure and reverted austenite. Another microstructural constituent is residual delta ferrite that originates from non-equilibrium solidification. This work focuses on the effect of delta ferrite on the impact toughness of a PH 13-8 Mo maraging steel, while keeping other toughness-influencing factors as constant as possible. Three-step heat treatments were applied to samples for adjusting different phase fractions of delta ferrite. Charpy impact testing revealed that the impact toughness decreases with an increasing phase fraction of delta ferrite. However, no significant influence on the lower energy shelf, i.e. the impact energies below the ductile-to-brittle transition temperature range, was found. In addition, no decrease in hardness at room temperature was measured when delta ferrite is present in the microstructure. Particle analysis by APT measurements revealed that delta ferrite contains Ni- and Al-rich precipitates. It is assumed that those precipitates in combination with effective solid solution hardening by Cr, Mo and Al significantly contribute to the hardness of delta ferrite, which is in the range of martensite. Furthermore, EDS analysis showed a depletion in Ni in delta ferrite, presumably resulting in a lower cleavage fracture resistance compared to martensite, and, therefore, causing embrittlement. Moreover, the interface between delta ferrite and martensite possibly plays an additional role for crack initiation due to amplified local stresses.

Characterization of a Novel Maraging Steel for Laser-Based Powder Bed Fusion: Optimization of Process Parameters and Post Heat Treatments

Abstract: A novel maraging steel with vanadium supplement called Specialis® has been developed for additive manufacturing by powder bed fusion using a laser beam. This study characterized this material after processing and post-processing. An intensive process optimization was carried out by means of a single track melt pool analysis to investigate optimal parameter sets for manufacturing of dense parts. Furthermore, the development of post heat treatment strategies and their influence on mechanical and microstructural characteristics of the material was evaluated. Two main concepts of direct aging treatment (AT) and solution treatment followed by aging treatment (ST+AT) were tested by dilatometry, to analyse the material behaviour with different initial microstructures: as-built and recrystallized. Both heat treatments resulted in a considerable improvement of hardness after only 2 h of aging, increasing to approximately 700 HV and 760 HV respectively, which exceeded the peak hardness of commonly known maraging steel 18Ni300 (660 HV after 6 h). These results were confirmed by tensile tests, where a tensile strength of more than 2300 MPa was achieved. Alongside the precipitation hardening known for maraging steels, the increased hardness was a result of grain refinement due to the addition of vanadium.

Texture evolution during processing and post-processing of maraging steel fabricated by laser powder bed fusion

Abstract: In this study, the evolution of solidification texture during LPBF of Ti-free grade 300 maraging steel, and its effect on texture development during subsequent post-fabrication heat treatments was characterized using Electron Backscatter Diffraction (EBSD). It was found that in the as-fabricated state, no texture was observed in the room temperature martensitic phase. However, the reconstructed parent austenite phase displayed a Cube texture with a minor fraction of Rotated Goss texture. During subsequent aging treatments involving two different routes, namely direct aging of the as-fabricated samples, and conventional solution treatment + aging of the as-fabricated samples, significant changes in the texture components of parent austenite were observed, whereas no changes in texture were observed in the room temperature martensitic phase. During direct aging, it was found that with an increase in the aging temperature up to 520 °C, the texture components of the parent austenite changed from Cube/Rotated Goss to Brass, whereas during the conventional solution treatment and aging cycle, interestingly a change in texture component to rotated copper was observed. The transitions in texture components have been discussed using the concepts of recrystallization and twinning in austenite during annealing and/or aging, and strain energy release maximization (SERM) theory. Furthermore, the importance of these preferred orientations on the mechanical properties was quantified using transformation potential diagrams.

Influence of shielding gas nitrogen content on the microstructure and mechanical properties of Cu-reinforced maraging steel fabricated by wire arc additive manufacturing

Abstract: This study investigated the effects of using shielding gases with different nitrogen concentrations on the microstructures and mechanical properties of SHER-120G maraging steel fabricated by wire arc additive manufacturing (WAAM). With increases in the nitrogen concentration, the contents of nitrogen and austenite in the bulk specimens increased significantly, while δ-ferrite formation was inhibited. In addition, nitrogen was absorbed by the molten pool and decomposed into N atoms during WAAM. Some N atoms formed a solid solution with the matrix and the rest reacted with Cr to form Cr2N, which plays an important role in the process of martensite lath refinement. The optimal shielding gas composition was 94% Ar + 2% O2 + 4% N2, which resulted in a tensile strength and micro-hardness of 1324 MPa and 441 HV, respectively. These values are 12.3% and 12.5% higher, respectively, than those of samples fabricated without nitrogen in the shielding gas.

Unveiling the precipitate evolutions and relationships between the nano-precipitates and mechanical properties in PH13–8Mo stainless steel

Abstract: Precipitation hardening maraging steels possess of great importance with attractive properties in various industrial applications. However, a major confusion about the fine precipitate evolutions and the relationships between the strengthened particles and mechanical properties of PH13–8Mo steels under various heat treatment conditions remains to be further investigated. To shed light on the direct relationships between the fine nano-phase and mechanical properties, a combination of thermodynamics predictions and experimental measurements were carried out to model the issues. The investigation results reveal that the coherent NiAl nano-precipitates distributed on the lath-like martensite matrix firstly presented a steady growth and then a significant coarsening of approximately 9 nm at 593 °C for 5 h. The acicular or block-like diffusion-controlled reverted austenite enriched in Ni element with a tendency of growth was clarified, which maintains the K–S orientation relationship with martensite matrix after diverse aging treatments. The current studied PH13–8Mo steel displays comparable mechanical properties despite over-aging conditions, and the sharp drops in hardness and the steady increment in impact energy were systematically examined. The excellent work hardening behaviors of the present steel were modeled, which indicates that the modified Ludwik model displays significantly improved agreement with the experimental data than the widely accepted Hollomon model. The five strengthening mechanism models are discussed and the C–O-M model exhibits high consistency with the experimental data with a value of 652 MPa. The growth of nano-precipitates maybe prohibits the dislocations from cutting the particles, which promotes the hardening behaviors. This work offers a valuable reference for further quantitively experimental illustrates.

Microstructural evolution of a FeCo15Cr14Ni4Mo3 maraging steel with high ductility prepared by selective laser melting

Abstract: In this study, FeCo15Cr14Ni4Mo3, a new precipitation hardening martensitic stainless steel with ultra-high ductility, was used for selective laser melting (SLM). SLM technology is gradually applied in mold design and manufacturing, but there are still problems including materials. Maraging stainless steel with high ductility for additive manufacturing can greatly increase the life of mold and solve problems such as mold fracture. However, there are few maraging steels whose ductility after heat treatment exceeds 15%. From the examination of the microstructure characteristics, this study observed a large amount of martensite in the as-built samples and up to 50% of austenite (FCC) remained after aging. In addition, the heat-treated SLM FeCo15Cr14Ni4Mo3 sample shows a finer grain structure compared with the as-built samples. These microstructural features are favorable for enhancing the mechanical properties and ductility. Under optimized aging conditions, the SLM fabricated steel exhibits a tensile strength of 1484 ± 6 MPa, a yield strength of 1376 ± 4 MPa and a large elongation at break of 25 ± 0.6%. These results are promising for the application of the SLM fabricated FeCo15Cr14Ni4Mo3 parts in different fields such as molds.

Geometry, Structure and Surface Quality of a Maraging Steel Milling Cutter Printed by Direct Metal Laser Melting

Abstract: This article considers the use of additive manufacturing to produce cutting tools for various machining operations, especially turning, milling, and drilling. The right geometry and material of the tool as well as coatings applied on cutting edges are crucial as they improve the life and performance of the tool. The study described here focused on a four-flute end mill made of maraging steel 1.2709 using a Concept Laser M2 Cusing Direct Metal Laser Melting (DMLM) machine. Before the printed tool was first used, it was examined to determine its dimensional and geometric accuracy, surface roughness, and surface structure. The measurement data showed that the tool required machining, e.g., grinding, to improve its geometry because the total runout of the shank and the cutting edge radius were too high, amounting to 120 μm and 217 μm, respectively. The cutting edges were sharpened to obtain a fully functional cutting tool ready to perform milling operations. The study aimed to check the dimensional and geometric accuracy of the 3D printed milling cutter and determine the optimal machining allowance for its finishing.

Additive manufacturing of corrosion-resistant maraging steel M789 by directed energy deposition

Abstract: Conventional maraging steels feature a combination of high strength and toughness, but they often suffer from low corrosion resistance. Hence, maraging steel M789 was developed to alleviate this issue while maintaining its strength. Most studies of maraging steels processed by additive manufacturing (AM) focus on utilizing laser powder bed fusion (LPBF). However, the research in laser-directed energy deposition (L-DED) fabrication of corrosion-resistant maraging steels is limited. The different cooling rates experienced by materials during L-DED and LPBF processing give rise to differing microstructures and mechanical properties. In this study, the L-DED process was adopted to manufacture nearly-fully dense M789 parts, which were subsequently subjected to direct aging and solutionizing + aging heat treatments. Electron backscatter diffraction (EBSD) analysis reveals a martensitic structure in both as-fabricated and heat-treated samples with the presence of austenite in the as-fabricated and directly aged samples. Scanning transmission electron microscopy (STEM) and transmission Kikuchi diffraction (TKD) reveal the presence of Ti- and Al-rich precipitates within the martensites after the solution and aging treatment, suggesting that Orowan looping around precipitates, grain boundary strengthening, and solid solution strengthening are responsible for the high yield strength of L-DED M789. Besides, the as-fabricated alloy shows higher pitting potential than solutioned and aged sample. This work serves as a guidance for the fabrication of corrosion-resistant maraging steels by L-DED and accelerate the implementation of maraging steels for marine and offshore applications.