Grade Maraging Steel Research Articles

Directional fatigue behaviour of maraging steel grade 300 produced by laser powder bed fusion

Surfaces of metals produced by additive manufacturing (AM) are known to be rough and populated with defects, this is, in particular, true for downward-facing (down-skin) surfaces. When dealing with the fatigue of as-built surfaces produced by AM, fatigue is typically initiating from surface defects. In this work, the fatigue behaviour of maraging steel grade 300 (18Ni300) produced by laser beam powder bed fusion (PBF-LB) is investigated. Fatigue initiation from surfaces built both up- and down-skin are investigated. This is done by using specimens where all surfaces are machined, except the one at interest. Specimens were built in 10 orientation ranging from 0° (horizontal, up-skin) to 135° (down-skin). The surface roughness was measured for all orientations; high surface roughness was found for down-skin surfaces while wavy surfaces were found for up-skin surfaces. The fatigue behaviour was found to be correlated to the build orientation and the surface roughness.

A characterization method for mechanical properties of metal powder bed fusion parts

The main objective of this research is to develop a characterization method for parts fabricated in metal powder bed fusion (PBF) processes, a type of additive manufacturing (AM) process. To characterize conventionally manufactured parts, many methods and standards are available, while no comprehensive standards or methods are available for AM parts. In this paper, a mechanical property characterization method is proposed that involves statistical analysis to select sample sizes and test coupon design considerations for metal printing, printing parameters, and applicability of existing testing standards for metal PBF, in addition to several mechanical properties. As proof of concept for the proposed characterization method, material and mechanical property characterization of the metal printed (EOS M280) EOS maraging steel grade 300 material is presented along with selected sample lots and relative errors. The characterization method presented in this paper is applicable to any metal AM process.

Hot Workability and Microstructure Control through the Analysis of Stress–Strain Curves during Hot Deformation of M350 Grade Maraging Steel

Abstract The ultrahigh strength (2,400 MPa) 18Ni maraging steel (M350 grade) is widely used for critical structural applications, such as aircraft landing gears, in which the strength–toughness balance is the essential criterion for material selection. Microstructure control during thermomechanical processing is the key to obtain the desired mechanical properties on a repeatable basis in a manufacturing environment. This involves thorough understanding of the hot deformation behavior under a wide range of temperatures and strain rates to map the microstructural evolution as a function of process parameters to obtain defect-free products. Towards achieving this goal of optimization of hot workability with a view to control microstructure for M350 grade maraging steel, hot deformation processing maps have been developed and correlated to the microstructure evolved. Further, analysis of stress–strain curves was carried out to obtain fine prior austenite grain (PAG) size via discontinuous dynamic recrystallization, and the same was verified experimentally by the microstructures evolved through hot isothermal compression tests on cylindrical specimens subjected to different strain levels. A single peak DRX type σ − ε curve was selected for analyses. The theoretically determined critical strain value was verified experimentally for initiation of DRX (DRXI) and transition from DRX dominant region to grain growth dominant region (DRXT). Hot isothermal compression tests have been conducted at T = 950°C and ε˙=0.01 s−1 and obtained PAG size of 3.14 µm in the specimen deformed to theoretically determined optimum strain of 0.74, thereby validating the used models.

Precipitate evolution and strengthening behavior during aging process in a 2.5 GPa grade maraging steel

Development of precipitation strengthening steels with ultrahigh strength and high ductility requires thorough understanding of nanoscale precipitation mechanisms. In this study, atom probe tomography (APT), HRTEM and first-principles calculations were used to reveal an interesting co-precipitation mechanism of Ni3Ti and Mo-rich nanoparticles in a 2.5 GPa grade maraging steel. The Ni–Ti rich clusters preferentially nucleate from the supersaturated solid solution and grow into Ni3Ti with extension of aging time, meanwhile the rejection of Mo atoms leads to heterogeneous precipitation of Mo-rich nanoparticles adjacent to the Ni3Ti particles and finally forms a core-shell structure along with Ni3Ti phase. Calculations of interaction energy between alloying elements in different aging process exhibit that the preferential formation of Ni–Ti rich cluster is due to the low interaction energy between Ni and Ti atoms, however, the Ni–Ti cluster is only a transitional phase, and when stable Ni3Ti is formed, Mo atoms are rejected from Ni3Ti to form a core-shell structure along with Ni3Ti precipitates. Finally, four modified theoretical prediction models are introduced to describe the yield strength as a function of microstructure and precipitates characteristics of the experimental steel.

Effect of deformation on precipitation hardening behavior of a maraging steel in the aging process

The effect of deformation on precipitation hardening behavior of a 2.8 GPa grade maraging steel was studied. The hardness test showed, plastic deformation promoted an earlier peak aging time and a higher peak hardness. By employing Transmission electron microscopy (TEM) and atom probe tomography (APT) analysis, nano-size precipitates formed during the aging process were identified as Ni3(Ti,Mo). The increased density of Ni3(Ti,Mo) precipitates was attributed as the main reason for the hardness enhancement after plastic deformation.

EBSD study of early fractured phenomena in a 350 grade Maraging steel elbows exposed to hydrofluoric acid

The effect of grain orientations, boundary characteristics, and crystal effects on the crack formation of a 350 high strength Maraging steel elbows exposed to hydrofluoric acid was investigated in the present research. Tempered low tetragonal martensite and fresh martensite were characterized by scanning electron microcopy. Moreover, no segregation zone and precipitates were found near the crack path. Detail analyzing the spatial distribution of crystal orientations by electron backscattered diffraction technique revealed the development of two soft and hard grains. Soft grains oriented approximately along {110} planes provide adequate slip systems to promote dislocation mobility, indicating as low Taylor factor grains distributed well the strain gradient. However, hard grains associated with low compact {001} planes decrease dislocation mobility and increasing the strain accumulation. Micro-cracks initiation and propagation along these high internal energy grains are expected consequently.

Dissimilar Welding of Maraging Steel (250) and 13-8 Mo Stainless Steel by GTCAW, LBW and EBW Processes

The frequently used aerospace materials, i.e., ultra-high-strength maraging steel (250) and corrosion-resistant 13-8 Mo stainless steel in the solution-annealed and cold-worked condition, have been joined by three fusion welding processes, namely interpulse TIG welding, and high energy density fusion processes like electron beam welding (EBW) and laser beam welding (LBW). The interpulse TIG welding process was carried out by using W2 grade maraging steel filler wire. The dissimilar joints were welded by EBW and LBW processes without any filler wire. All the dissimilar welded joints were characterized by microstructural observations and validated by mechanical properties in the as-welded as well as precipitation-hardened conditions after welding. The weld microstructures and microhardness profiles were correlated to the tensile strength of weld. Electron beam welded joint with precipitation hardening after welding, i.e., soaking at 510 °C and subsequent air cooling, demonstrated the superior mechanical properties among all the welds.

Precipitate Evolution and Strengthening Behavior During Aging Process in a 2.5 GPA Grade Maraging Steel

Possible nucleation mechanism as well as the following evolution of precipitation, microstructure and mechanical properties have been studied in a 2.5 GPa grade maraging steel during aging at 480 ℃. An initial hardening reaction within 15 min is reported, and this strengthening is caused by the formation of complex multi-component atomic co-clusters containing primarily Ni and Ti. First principle calculations showed that Ni and Ti were preferred to segregate together for their lowest interaction energy. With the extension of aging time, Ni-Ti clusters grew to Ni3Ti, and regardless of Co or Fe occupied position the formation energy of Ni-Ti clusters was higher than pure Ni3Ti. Therefore, Co and other atoms were eventually expelled from Ni3Ti. Meantime, owing to difficult Mo diffusion, Mo-rich phase adjacent to Ni3Tiphase grew up and formed a core-shell structure with Ni3Ti phase together. Finally, three modified theoretical prediction models were introduced to describe the yield strength as a function of microstructure and precipitates characteristics of the experimental steel. When both the volume fraction and dimension of precipitates were considered, the modified equation of Orowan relationship showed more reasonable prediction compared with experimental results of the yield strength.

Effect of thermal cycling and aging stages on the microstructure and bending strength of a selective laser melted 300-grade maraging steel

Additive manufacturing techniques allow the creation of complex parts in a layer by layer fashion, bringing new opportunities in terms of applications and properties when compared to conventional manufacturing processes. Among other ultra-high-strength steels, the 18 Ni maraging 300 steel offers a good toughness/strength ratio. However, when fabricated by additive manufacturing, this steel presents lower ductility and strain-hardening than its forging counterparts. One way to enhance ductility and toughness is to promote martensite-to-austenite reversion. Therefore, in the present study, 18 Ni maraging steel powder was processed by selective laser melting and different heat treatments were applied to the built parts, aiming for homogenization, microstructural refinement and martensite-to-austenite reversion. Thermodynamic simulations were used to assess a range of temperatures for the reversion heat treatments. Microstructural characterization was performed by scanning electron microscopy, electron backscattered diffraction and x-ray diffraction.

Precipitation in 300 grade maraging steel built by selective laser melting: Aging at 510 °C for 2 h

This paper focuses on the precipitates found in the 300 grade maraging steel built by Selective Laser Melting (SLM), which is globally one of the fastest-growing Additive Manufacturing (AM) technology. The test samples were built layer by layer via full melting and rapid solidification of the deposited powder bed of 300 grade maraging steel, using an M2 Concept Laser machine. The samples were aged at 510 °C for 2 h, and then air-cooled. Optical microscopy, scanning electron microscopy, transmission electron microscopy, atom probe tomography, and hardness testing were used. No precipitates were found in the structure of the as-built samples. However, for the heat-treated samples, two distinct phases with three different morphologies were detected. These phases were as follow: spherical and plate-like precipitates enriched in Ti and Ni with considerable amounts of Mo, and spherical precipitates enriched in Mo of Fe7Mo6 stoichiometry.

Environment-Induced Degradation in Maraging Steel Grade 18Ni1700

Samples extracted from flow formed tubes made of 18% nickel maraging steel grade C18Ni1750 were subjected to tensile testing at room temperature in laboratory environment at two different strain rates. Testing was carried out in as flow formed as well as flow formed and aged conditions. Aging was carried out adopting four different cycles. Distinct loss of ductility was observed at the lower strain rate in all tested conditions. The embrittlement occurring during low strain rate testing is explained in terms of hydrogen induced damage, hydrogen coming from the moisture in the environment. It is also concluded that the heavy cold work imparted to the material during flow-forming is importantly responsible for the ductility loss observed at low strain rate.

Optimization of Hot Workability and Control of Microstructure in 18Ni (M250 Grade) Maraging Steel Using Processing Maps

This article examines the hot working characteristics of M250 grade maraging steel by performing isothermal compression tests varying the temperatures and strain rates. A processing map has been developed, indicating microstructurally “stable” and “unstable” regions during hot working. At strain rates (ϵ, ˙, , , , , ) above 0.1 s−1 and temperatures (T) below 1,000°C, stress–strain curves show strain hardening behavior, whereas steady-state behavior is shown at strain rates below 0.01 s−1 and temperatures above 1,050°C. Above 1,150°C, flow softening is observed at 0.001 and 0.01 s−1. In the processing map, two distinct microstructurally stable domains are observed: one is at T = 1,150–1,200°C and ϵ, ˙, , , , , = 0.001–0.01 s−1, and another is at T = 1,000–1,150°C and ϵ, ˙, , , , , = 0.001–0.1 s−1. Based on microstructural observations, electron backscattered diffraction results, and the high efficiency of power dissipation, dynamic recrystallization is the softening mechanism. A constitutive relation useful in computer modeling is developed, using the generated flow stress data.

On the corrosion resistance of some selective laser melted alloys

The electrochemical corrosion resistances of selective laser melted (SLM) 316 L austenitic stainless steel (SS), 18(Ni) 300-grade maraging steel (MS), and Al-12 wt.% Si (AS) alloy in a 0.1 M NaCl solution at room temperature were evaluated. The effects of laser scanning strategy (single melt vs. checker board styles), post-SLM heat treatment, and corroding surface orientation (with respect to the scan and build directions) on the corrosion behavior were examined. In all cases, results were compared with those obtained on samples with the same compositions, but manufactured using conventional means. The experimental results show that, for the particular set of experimental conditions employed in this study, SLM in general improves the corrosion resistances of Al-12 wt.% Si and stainless steel alloys and degrades the corrosion resistance of the maraging steel, in comparison to the respective corrosion resistances of their conventionally manufactured counterparts. These results are discussed in terms of microstructural refinement and porosity that are common to the SLM alloys.

Corrosion investigation of the 18Ni 300 grade maraging steel in aqueous chloride medium containing H2S and CO2

The corrosion behavior of the 18Ni 300-grade maraging steel in 0.6 mol L−1 NaCl solution, containing 1 mmol L−1 H2S, and saturated with CO2, was investigated by measurement of the open circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy techniques. The maraging steel was solution-treated at 840 °C for 1 h and aged at 650 °C for 3 h. All corrosion electrochemical tests proved that the austenitic phase formed during the aging thermal treatment was detrimental to corrosion resistance of the 18Ni 300-grade maraging steel. The analysis of the XPS spectra suggested that FeCO3, Fe2O3, FeS2, FeSO4, NiO, Ni(OH)2, NiS, NiSO4, MoO2, MoO3 and MoS2 were the corrosion products deposited on the surface of both maraging steel samples after 29 days of immersion in the testing solution. Lastly, for industrial application requiring the corrosion resistance property of the 18Ni 300 grade maraging steel, it is recommended that it be thermal solution-treated before its use.

Effect of 1,2,3benzotriazole on the corrosion of aged 18Ni250 grade Maraging steel in Phosphoric acid solution

The 18 Ni 250 grade maraging steel is a potential high strength steel for advanced technologies such as aerospace, nuclear, and sporting goods. Phosphoric acid and Nitric acid solutions are used in pickling of delicate and precision items where re-rusting after pickling has to be avoided. The present work addresses the study of corrosion behaviour and inhibition using 1,2,3 benzotriazole of aged 18 Ni 250 grade maraging steel in phosphoric acid medium at higher concentration by Potentiodynamic Polarization Technique. The corrosion rates were determined in 1M, 1.5M and 2M by Tafel extrapolation technique in the temperature range 300C-500C with different concentrations of inhibitor. The results indicate that the corrosion rate increases with increase in acid concentration and temperature.Inhibition efficiency of 1,2,3benzotriazolewas found to increase with the increase in 1,2,3 benzotriazole concentration and decrease with the increase in temperature.The activation energy Ea and other thermodynamic parameters (ΔG˚,ΔH# andΔS#) have been evaluated and discussed. The standard free energy of adsorptionΔG˚ads values indicates that the adsorption is of mixed type. Scanning electron microscopy (SEM) study confirmed the formation of an adsorbed protective film on the metal surface.

Microstructure, properties and hot workability of M300 grade maraging steel

Abstract This paper presents isothermal uniaxial compression test results of M300 grade maraging steel over a wide range of temperatures (900–1200 °C) and strain rates (0.001–100s −1 ) to examine hot deformability and concurrent microstructural evolution. Processing map is generated and indicated the optimum processing parameters in the temperature range of 1125°-1200 °C and strain rate range of 0.001–0.1 s −1 . High values of the efficiency of power dissipation, microstructural observations and EBSD results indicate softening mechanism to be the occurrence of dynamic recrystallisation. Material constants in a constitutive relation are evaluated from the flow stress data useful in computer modelling.

EFFECT OF HEAT INPUT AND POST-WELD HEAT TREATMENT ON THE MECHANICAL AND METALLURGICAL CHARACTERISTICS OF LASER-WELDED MARAGING STEEL JOINTS

This paper investigates the impact of heat input and post-weld aging behavior at different temperatures on the laser paper welded maraging steel grade 250. Three different levels of heat inputs were chosen and CO2 laser welding was performed. Aging was done at six different temperatures: 360[Formula: see text]C, 400[Formula: see text]C, 440[Formula: see text]C, 480[Formula: see text]C, 520[Formula: see text]C and 560[Formula: see text]C. The macrostructure and microstructure of the fusion zone were obtained using optical microscope. The microhardness test was performed on the weld zone. Tensile tests and impact tests were carried out for the weld samples and different age-treated weld samples. Fracture surfaces were investigated by scanning electron microscopy (SEM). Microhardness values of the fusion zone increased with increasing aging temperature, while the base metal microhardness value decreased. Tensile properties increased with aging temperature up to 480[Formula: see text]C and reduced for 520[Formula: see text]C and 560[Formula: see text]C. This was mainly due to the formation of reverted austenite beyond 500[Formula: see text]C. XRD analysis confirmed the formation of reverted austenite.

Tensile, fracture, and fatigue crack growth properties of a 3D printed maraging steel through selective laser melting

The microstructure and room temperature mechanical properties of a18Ni (300) grade maraging steel (MS) fabricated using the selective laser melting (SLM) technique were studied, in both before- and after-aging conditions. Microstructural analysis reveals fine cellular structure in the as-SLM MS. Upon aging, nanoscale precipitation of intermetallic compounds occurs within the cells, which in turn, result in marked improvements in yield and ultimate tensile strengths, substantial reductions in ductility and fracture toughness, and a transition from dimple to quasi-cleavage fracture morphology. Overall, the mechanical performance, including the fatigue crack growth characteristics, of the SLM MS after aging is found to be similar to that of conventionally manufactured MS of the same grade. Importantly, the reduced ductility does not lead to a reduction in toughness, attributed to the stress induced martensitic transformation as a result of austenite presence in aged SLM MS. Although the SLM alloy possesses a mesostructure, which is a result of line-by-line laser scanning and layer-by-layer building of the components, no significant anisotropy in the mechanical behavior is observed, which is a result of strong metallurgical bonding between adjoining lines and layers. These results are discussed in terms of the meso- and micro-structural features.

Short-term Creep Properties and Fracture Surface of 18 Ni (300) Maraging Steel Plasma Nitrided

Plasma nitriding of a solution annealed and aged 300 grade maraging steel was studied aiming to increase the creep resistance. The surface microhardness reached 1,140 HV, producing 50 µm layer composed of e-Fe3N and γ'-Fe4N nitrides at the uppermost sample layer. The inner core remained unaltered presenting typical plate-like martensite microstructure of maraging steels with average microhardness of 604 HV. Surface RMS roughness in the nanometric scale increase from 52 nm to 71 nm. The continuous layer of iron nitrides seems to behave as a barrier for oxidation and for inward oxygen diffusion improving the creep resistance by reducing the steady-state creep rate (es) in 52-65% when compared with the literature results. Dominant creep mechanism is controlled by dislocations climb. Fracture surfaces of specimens presented ductile failure consisting of equiaxed and bi-modal dimples in the fibrous zone surrounded by 45o shear lip. Nitrided sample presented a reduced ductility, associated to the hard surface layer.

Simultaneous ageing and plasma nitriding of grade 300 maraging steel: How working pressure determines the effective nitrogen diffusion into narrow cavities

Maraging steels combine excellent mechanical properties at medium-high temperatures with good fracture toughness, but they usually require the aid of thermochemical treatments to enhance superficial properties such as resistance to abrasive wear and corrosion resistance. In particular, plasma nitriding, due to its duration and temperature, which are compatible with the age-hardening cycles of maraging steels, permits the realization of both processes simultaneously.In this work, simultaneous ageing and arc-activated plasma nitriding of maraging steel grade 300 were studied as a function of the working pressure of the gases (N2 and Ar) present in the vacuum reactor chamber. In addition, the influence of the operating pressure was established by analysing the capacity of the plasma to penetrate in 2.5mm in diameter blind holes.In-depth chemical composition, hardness and micro-hardness profiles, X-Ray diffraction, roughness, tribological characterization and corrosion tests were carried out to compare maraging steel samples treated in one simultaneous plasma nitriding and ageing process and in two-step processes. Metallographic tests and nanoindentation measurements were performed to study how the plasma penetrates and diffuses into the blind holes and how the nitride case layer thickness formed at the inner surfaces of the holes changes as a function of the distance to the surface for various working pressures.