High-strength Maraging Steel Research Articles

Nanosized austenite and coherent nanoprecipitates making maraging steel strong and ductile

Here we report a microstructural design in maraging steel, which renders superior strength-ductility synergy by combining the advantages of transformation induced plasticity and dislocation cutting mechanisms, via an economic thermomechanical route suitable for industrial scale. It aims at innovating ductile maraging steel without conceding too much strength. Specifically, a high-strength maraging steel is first treated by cold-rolling and intercritical annealing, which leads to partial retransformation and precipitation of supersaturated needlelike/near-spherical B2-structured nanoparticles occurring simultaneously in dislocated α′-martensite matrix. The material is further optimized by peak-aging, yielding a desired microstructure, with nanosized γ-austenite (d‾γ ∼150 nm) dispersed in lath α′-martensite (d‾α′ ∼130 nm) with dense spherical B2-nanoprecipitates (d‾B2 ∼6.5 nm), i.e., an austenite-maraging structure with hierarchical heterogeneity at nanoscale. The yield strength is measured to be ∼1.65 GPa, which is largely retained as it combines boundary strengthening, dislocation strengthening and ordering strengthening of nanoprecipitates. Surprisingly, significant work hardening renders an ultimate strength even high up to ∼1.9 GPa and a decent ductility of ∼6–7%, twice that of conventional maraging steels. Interrupted microstructure examination unravels that it is the hierarchical microstructure heterogeneity-induced sequential activation of plasticity-enhancing mechanisms that confers such excellent deformability. At the stage soon after yielding (ε∼1.5–3%), deformation-induced transformation from metastable γ-austenite to α′-martensite imparts a significant strain hardening rate (Θ) up-turning. At the high strain stage (ε>3 %), multiple dislocations cutting through B2-nanoprecipitates promotes the interaction and accumulation of dislocations, thereby postponing Θ attenuation and prolonging ductility.

Laser powder bed fusion of crack-susceptible stainless maraging steel undergoing solid-state phase transformations

For alloys that experience solid-state phase transformation upon cooling, the absence of as-solidified chemical segregation information makes the elimination of hot cracking challenging. One such material is the hot-crack-susceptible high-strength maraging steel C465. Here, we resolve its hot cracking problem under the laser powder bed fusion process through the addition of titanium nitride (TiN) particles. During solidification, TiN promotes grain refinement and reduces the formation of liquid films with low solidus temperatures. Under cooling, the partial dissolution of TiN lowers the martensite start temperature and yields more retained austenite. Upon subsequent annealing, the dissolved titanium slows down the austenite reversion kinetics, while the dissolved nitrogen improves the yield strength. Materials under tensile deformation follow a three-stage work-hardening behavior, indicative of strain-induced martensitic transformations. This work highlights that besides the nucleant's grain refinement potency, the effect of its partial dissolution during processing needs to be critically examined, when addressing the hot cracking problem for alloys prone to phase transformations.

Efficient post-processing of additive manufactured maraging steel enhanced by the mechanochemical effect

Additive manufacturing technologies are beginning to shift toward hybridization with subtractive processes and it is vital to identify techniques that can enhance the machinability of the difficult-to-cut additively manufactured metals and offer easy integration. The mechanochemical effect, which can be induced by surfactant, is a feasible solution for hybrid integration due to the beneficial enhancements to the cutting performance, online integrability, and negligible impact on the AM process as compared to cutting fluids, cryogenic cutting, etc. To realize the successful integration of the mechanochemical effect and hybrid additive/subtractive manufacturing, micro-cutting of AMed high-strength maraging steel was performed to study the relationship between microstructural features, mechanical properties, cutting performance and effectiveness of the mechanochemical effect. The results show that the mechanochemical effect was successfully induced in the as-built and solution-treated steels by inhibiting dislocation movement to induce the embrittlement of chip surface and strain localization within the chip, thereby leading to substantial reductions in cutting forces of up to 35.24 % and 53.09 %, respectively, with significant improvement in the machined surface quality. However, the presence of 7.7 nm nanoparticles in the age-treated steels renders the mechanochemical effect ineffective in improving machinability. The nanoparticles sharply increased the strength, hardness, and brittleness of the AMed maraging steel where the brittleness replaced the role of surfactant that suppressed plasticity in the chip free surface. The notion was affirmed by the similarities between the cutting chips of the brittle aged steel without surfactant and the as-built steel with surfactant. This study systematically revealed the underlying mechanism of inducing the mechanochemical effect during the micro-cutting of AMed high-strength materials with different microstructures and mechanical properties. More importantly, it is evident that the mechanochemical effect is a highly feasible solution for enhanced hybrid manufacturing, especially for robot-based fabrication works that involve high degrees of freedom and large working ranges but are limited by low mechanical stiffness.

Influence of Laser Additive Manufacturing and Laser Polishing on Microstructures and Mechanical Properties of High-Strength Maraging Steel Metal Materials

To increase the surface quality of the high-strength maraging steel metal materials, a new method of executing the additive manufacturing process and subtraction polishing process of maraging steel metal materials was studied. The mechanical properties of maraging steel metal materials before and after laser powder bed fusion (LPBF) polishing were compared and analyzed. The influence of laser parameters on the formability of high-strength MS metal materials was studied, with MS additive parts successfully prepared. The initial surfaces had roughness values of 6.198–7.92 μm. The metal additive manufacturing parts were polished with double laser beams. Confocal microscopy, scanning electron microscopy, and X-ray diffraction were used to obtain the microstructure and phase composition of the microstructures. The microhardness of high-strength maraging forming parts by using a microhardness tester and the mechanical properties were analyzed. The results showed that the surface roughness was considerably reduced to lengthen the service life of the high-strength MS metal materials from an initial roughness of Sa = 6.3 μm to Sa = 0.98 μm, with the surface hardness increased and the martensite content decreased after using double-laser-beam polishing.

Laser powder bed fusion of high-strength maraging steel with concurrently enhanced strength and ductility after heat treatments

Laser powder bed fusion (LPBF) has been demonstrated as a metal additive manufacturing technique that can manufacture high-performance metal components with complex shapes directly from raw powder materials. Although 18Ni-300 maraging steel has been extensively employed in this process, a significant trade-off has always existed between strength and ductility when using traditional solution and ageing heat treatments. This study addresses this problem by introducing a heat treatment combination that includes hot isostatic pressing (HIP) before the conventional ageing treatment. The experimental results demonstrated an enhancement of the relative density from 99.6% in the as-built conditions to 99.96% after HIP, although a small number of residual gas pores remained. The subsequent ageing treatment after HIP had a negligible effect on the reopening of these closed defects. Microstructure observations revealed that a fully martensitic structure can be obtained after HIP, and austenite reversion was less sensitive with the HIP + ageing treatment (HAT) compared to direct ageing treatment (DAT). Grain growth and alloying homogenisation also occurred during HIP. Massive nanoscale precipitations were formed both after DAT and HAT; quantitative analysis based on TEM observation indicated that the size of these precipitates was larger after subjecting to HAT. The room-temperature ultimate tensile strength and elongation to failure after HAT were approximately 6% and 100% higher than DAT, confirming the simultaneous enhancement of the strength and ductility after HAT. The findings of the present study offer insights into heat treatment approaches that can overcome the strength-ductility trade-off of LPBF-manufactured 18Ni-300 maraging steel, while also providing an alternative strategy for obtaining the simultaneous improvement of strength and ductility through heat treatments.

Mechanical Properties and Fracture of High-Strength Maraging Steel Fabricated by Selective Laser Melting

Mechanical Properties and Fracture of High-Strength Maraging Steel Fabricated by Selective Laser Melting

Toxicity evaluation of particles formed during 3D-printing: Cytotoxic, genotoxic, and inflammatory response in lung and macrophage models

Additive manufacturing (AM) or “3D-printing” is a ground-breaking technology that enables the production of complex 3D parts. Its rapid growth calls for immediate toxicological investigations of possible human exposures in order to estimate occupational health risks. Several laser-based powder bed fusion AM techniques are available of which many use metal powder in the micrometer range as feedstock. Large energy input from the laser on metal powders generates several by-products, like spatter and condensate particles. Due to often altered physicochemical properties and composition, spatter and condensate particles can result in different toxicological responses compared to the original powder particles. The toxicity of such particles has, however, not yet been investigated. The aim of the present study was to investigate the toxicity of condensate/spatter particles formed and collected upon selective laser melting (SLM) printing of metal alloy powders, including a nickel-chromium-based superalloy (IN939), a nickel-based alloy (Hastelloy X, HX), a high-strength maraging steel (18Ni300), a stainless steel (316L), and a titanium alloy (Ti6Al4V). Toxicological endpoints investigated included cytotoxicity, generation of reactive oxygen species (ROS), genotoxicity (comet and micronucleus formation), and inflammatory response (cytokine/chemokine profiling) following exposure of human bronchial epithelial cells (HBEC) or monocytes/macrophages (THP-1). The results showed no or minor cytotoxicity in the doses tested (10−100 μg/mL). Furthermore, no ROS generation or formation of micronucleus was observed in the HBEC cells. However, an increase in DNA strand breaks (detected by comet assay) was noted in cells exposed to HX, IN939, and Ti6Al4V, whereas no evident release of pro-inflammatory cytokine was observed from macrophages. Particle and surface characterization showed agglomeration in solution and different surface oxide compositions compared to the nominal bulk content. The extent of released nickel was small and related to the nickel content of the surface oxides, which was largely different from the bulk content. This may explain the limited toxicity found despite the high Ni bulk content of several powders. Taken together, this study suggests relatively low acute toxicity of condensates/spatter particles formed during SLM-printing using IN939, HX, 18Ni300, 316L, and Ti6Al4V as original metal powders.

Ballistic impact resistance of additive manufactured high-strength maraging steel: An experimental study

Maraging steel is a low carbon steel known for its ultra high-strength after heat treatment. In combination with Additive Manufacturing (AM), the properties of maraging steel indicate potential to enable complex geometries and improved performance-to-weight ratios for ballistic protection. This study investigates the ballistic performance of AM maraging steel monolithic plates and profile panels fabricated by powder bed fusion. The mechanical properties of the maraging steel, both in the as-built state and after heat treatment, were revealed through quasi-static and dynamic tests in three different directions with respect to the build direction. Metallurgical studies were also conducted to investigate the microstructure of the material both before and after testing. The ballistic perforation resistance of the maraging steel samples was disclosed in a ballistic range by firing 7.62 mm APM2 bullets towards the different target configurations. Ballistic limit curves and velocities were obtained, demonstrating that the thickest heat-treated AM maraging steel plate has a particularly good potential for ballistic protection. The hard core of the armour piercing bullet broke in all tests and occasionally shattered during tests with heat-treated targets. However, due to the severe brittleness of the material, the targets showed significant fragmentation in some cases and most significantly for the profile panels.

Acoustic emission examination of high strength steel M250 pressure vessels for aerospace applications

This paper deals at experiences with the Acoustic Emission (AE) monitoring of critical pressure vessels during qualification proof pressure test. Conventional Non Destructive Evaluation (NDE) methods have limitations in evaluating the vessel. There is a high probability that they may not provide information required to evaluate the quality of vessel. In contrary, Acoustic emission technique has a high probability of detecting the likely defects in pressure vessels prior to the failure. This technique has to be applied under pressure load. These pressure loads create stresses in the vessel. During increasing pressure loads AE signals are obtained from discontinuities and other locations of stress concentrations. AE is very useful in estimating the soundness of critical pressure vessels used by aerospace industry that is made up of high strength Maraging steels. AE Testing and result analysis was carried out according to standard procedures. During the tests, AE signals were recorded and post analysis was done to understand the response from the material. The experimentation revealed that the amplitude of AE signals were less than the corresponding AE signals for critical crack opening, during pressurization cycle. Also, at proof pressure, a 2 minutes hold time was observed with no increase in amplitude of signals. AE activity can detect any defect growth during proof pressure test. Inspection was carried-out in accordance with a set of acceptance criteria based on available literature and specimen level tests.

Microstructure and machinability of selective laser melted high-strength maraging steel with heat treatment

Additive manufacturing (AM) provides a higher degree-of-freedom manufacturing process for high-mix low-volume manufacturing. However, the as-built surface quality by AM processes is generally inferior to that fabricated by conventional subtractive manufacturing and the machinability investigation for post-processing of additively manufactured (AMed) parts is insufficient. In this paper, milling experiments are undertaken to improve the surface finish of the AMed high-strength maraging steel (18Ni300) manufactured by additive manufacturing with and without heat treatment. The influence of microstructure on the machinability is explored, including microhardness, cutting force, surface roughness, tool wear, and chip formation. Significant variation in machinability is identified between the AMed samples that feature distinct microstructures. Surface microhardness of the as-built and heat-treated samples both increased after milling. Cutting forces and tool wear increased sharply after ageing treatment but only a minor change was observed in the as-built samples and those subjected to solution treatment. Roughness value of the as-built samples was reduced from ∼10 to <0.4 μm after milling. Ageing treatment induced chip adhesion on the tool surface and high degree of chip curling. According to the chip morphology analysis, machining of both as-built and solution-treated samples will produce smaller chip serrations and continuous chip formation comparing to the large serrations and fracture morphology in ageing-treated chips. This paper elucidates the relationship between material microstructure and machinability of AMed maraging steel.

Failure Analysis on a Collapsed Flat Cover of an Adjustable Ballast Tank Used in Deep-Sea Submersibles

A flat cover of an adjustable ballast tank made of high-strength maraging steel used in deep-sea submersibles collapsed during the loading process of external pressure in the high-pressure chamber. The pressure was high, which was the trigger of the collapse, but still considerably below the design limit of the adjustable ballast tank. The failure may have been caused by material properties that may be defective, the possible stress concentration resulting from design/processing, or inappropriate installation method. The present paper focuses on the visual inspections of the material inhomogeneity, ultimate cause of the collapse of the flat cover in pressure testing, and finite element analysis. Special attention is paid to the toughness characteristics of the material. The present study demonstrates the importance of material selection for engineering components based on the comprehensive properties of the materials.

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.

Design and evaluation of a distally actuated powered finger prosthesis with self-contained transmission for individuals with partial hand loss

Partial hand loss accounts for the overwhelming majority of upper limb deficiencies. Despite this, individuals with partial hand loss have a limited number of prosthetic options at their disposal. Existing externally powered devices typically house both motor and gear transmission in the proximal phalanx, which provides function at the expense of anthropomorphism. We present a novel design for an externally powered finger prosthesis with a custom gear transmission that is capable of higher intermittent torques compared to commercial gearheads of equivalent volume. We manufacture a fully functional transmission using a high-strength maraging steel alloy and direct laser metal sintering. The transmission consists of multiple planetary and spur gear stages arranged in a stackable (or laminar) configuration and accommodates joint movement at the proximal interphalangeal joint. The powered finger is equivalent in size to a 50th percentile female index finger and is capable of generating pinch forces comparable to those of commercial powered fingers at flexion speeds that exceed those of existing devices. While there are several practical and functional improvements for future iterations, our design represents a viable option for a powered finger capable of accommodating a wide range of individuals with partial hand loss.

On Reliability of Stainless Maraging Steel

The investigation results on the problem of the reliability of high-strength low-carbon maraging steel products have been generalized. The influence of a method for remelting on the reliability behavior is shown and the ways for the reliability behavior improvement are suggested. The study of the reasons for decreasing KCU during heat treatment shows that in addition to the precipitation of phases causing brittleness at cooling, chromium zones at heating, and formation of chemical and structure inhomogeneity in the two-phase region, the main reason is the remelting method with the parameters which predetermine the variation in grain size in the structure, a small number of interstitial elements (IE), retained austenite in the structure, and lower level of KCU of the steel prepared by VAR both after quenching and after TST. Shows influence of the quenching temperature on the amount of retained austenite and level of impact strength (KCU), of the time of aging on the work of the crack development (KCV) at the temperature of maximal development of brittleness in steel 08Cr15Ni5Cu2Ti and on the position of brittleness transition temperature prepared by ESR and VAR. After cooling down to the liquid nitrogen temperature, the VAR-steel is less liable to brittle fracture after maximal strengthening aging and more reliable after 1.5h-aging (KCV is twice as much as that in ESR-steel despite the low KCU level). The science-based regimes are developed for stamped semi-finished items from steel 08Cr15Ni5Cu2Ti allowing guaranteeing the proper quality and reliability of functioning of the items made from them.

Grain Size Effect on Low Cycle Fatigue Behavior of High Strength Structural Materials

This paper presents grain size effect on low cycle fatigue behavior of high strength maraging steel with gain size of 20,60 and 100μm and Ti-6Al-4V alloy with grain size of 0.5,1.4 and 5.1μm. Low cycle fatigue strength of the maraging steel depends on grain size in number of cycles up to 103.The smaller the grain size, the higher the low cycle fatigue strength was. Quasci-cleavage fracture surfaces were predominant for material with grain size of 20μm,while intergranular fracture surfaces were predominant for materials with larger grain size in number of cycles lower than 60. Striation was predominant for all tested materials in number of cycles higher than 60.Low cycle fatigue strength of Ti-6Al-4V alloy also depends on grain size in number of cycles up to 104. Grain size dependent transgranular fracture surfaces were predominant for materials with ultra-fine grain size of 0.5μm and fine grain size of 1.4μm.

Corrosion inhibition of high strength maraging steel by cerium(III) ions in sulfuric acid solution

The influence of rare earth Ce(III) ions on the corrosion behaviour of high strength maraging steel in 0.5 M sulfuric acid solution was studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. Ce3+ can significantly reduce corrosion deterioration up to 5.0 mM. Beyond that level the corrosion rate increases till a steady value. High corrosion protection of steel surface after exposure to the acidic solution containing the optimal Ce(III) concentration is revealed by SEM investigations.

Исследование сопротивления деформации мартенситно-стареющей стали ЭП679

The resistance of high-strength maraging steel EP679 to plastic deformation σs is studied as dependent on the amount of strain, strain rate and deformation temperature, as well as on heattreatment conditions. Quantitative evaluation is obtained and regularities are found in the change of σs values depending on the studied factors, with the structural evolution of steel under heating and cooling taken into account. A positive effect of preheating up to 920 °C and subsequent cooling to the test temperature on the decrease in the value of σs during subsequent deformation at temperatures ranging between 20 and 900 °C is shown.

МЕТАЛЛУРГИЧЕСКИЕ ОСОБЕННОСТИ ВЫПЛАВКИ ВЫСОКОПРОЧНОЙ МАРТЕНСИТОСТАРЕЮЩЕЙ СТАЛИ ВКС-180-ИД

МЕТАЛЛУРГИЧЕСКИЕ ОСОБЕННОСТИ ВЫПЛАВКИ ВЫСОКОПРОЧНОЙ МАРТЕНСИТОСТАРЕЮЩЕЙ СТАЛИ ВКС-180-ИД

РАЗРАБОТКА ТЕХНОЛОГИИ ВЫПЛАВКИ ВЫСОКОПРОЧНОЙ МАРТЕНСИТОСТАРЕЮЩЕЙ СТАЛИ ВКС-180-ИД (01Н18К9М5Т), МИКРОЛЕГИРОВАННОЙ РЗМ

Сталь ВКС-180-ИД рекомендуется для производства высоконагруженных силовых деталей шасси, планера, двигателя. Исследованы металлургические особенности выплавки высокопрочной мартенситостареющей стали ВКС-180-ИД в вакуумной индукционной (ВИ) печи с переплавом в вакуумной дуговой (ВД) печи с применением выбранной системы микролегирования на основе РЗМ и введением оксида никеля. Технология позволяет получать стали с низким содержанием газов и примесей (углерод ≤0,005%; кислород ≤0,004%; азот ≤0,003%), что обеспечивает высокий уровень механических свойств как в продольном, так и поперечном направлении (предел прочности, ударная вязкость, пластичность).

Precipitation of aluminum nitride in a high strength maraging steel with low nitrogen content

In the present work, aluminum nitride (AlN) precipitation was investigated in a X23NiCoCrMoAl13-6-3 maraging steel with low nitrogen content (wt.% N=5.5ppm). A reliable and robust automatic method by scanning electron microscopy observations coupled with energy dispersive X-ray spectroscopy was developed for the quantification of AlN precipitates. The first stage was to identify the solvus temperature and to develop a heat treatment able to dissolve the AlN precipitates. The experimental determination of equilibrium conditions and solvus temperature show good agreement with ThermoCalc® simulation. Then, from this AlN-free state, the cooling rate, isothermal holding time and temperature were the subject of an intensive investigation in the austenite region of this maraging steel. In spite of the high temperatures used during heat treatments, the growth kinetic of the largest AlN precipitates (>1μm) is slow. The cooling rate has a major effect on the size and the number density of AlN due to a higher driving force for nucleation at low temperatures. At last, quenching prior to isothermal annealing at high temperatures leads to fine and dense AlN precipitation, resulting from the martensite to austenite transformation. Experimental results will be discussed and compared with kinetic data obtained with the mobility database MobFe2 implemented in Dictra® software.