Ni Maraging Steel Research Articles

Microstructural heterogeneity and mechanical anisotropy of 18Ni-330 maraging steel fabricated by selective laser melting: The effect of build orientation and height

Abstract

A fast evaluation method for fatigue strength of maraging steel: The minimum strength principle

The relationship between fatigue strength and tensile properties has drawn widespread concern in recent decades. It is gradually recognized that the fatigue strength is dependent on the strength of the weakest zone in materials, while the regular tensile test could only obtain the global strength. The present study, for the first time, constructed a fast evaluation method, in which extremely slender specimens were employed. It is found that, when the specimen is slender enough, i.e., the soft oriented grain could occupy the whole section of the specimen, the strength of the soft domain could be related to the fatigue strength. In the 18Ni maraging steels, the fatigue crack would nucleate in the precipitate free zone (PFZ), which is the soft domain. Furthermore, the fatigue strength of fully reversed loading test nearly equals to the yield strength of the PFZ measured by the slender tensile test. This founding may shed light on the fast evaluation of fatigue strength, which could bring great economic benefits and save invaluable time.

Fracture mechanism and model of 18Ni maraging steel

ABSTRACT18Ni maraging steel is normally used to make key components by machining processes. It is necessary to understand the fracture mechanism and behaviour of 18Ni maraging steel for the study and further optimisation of machining processes. In this paper, the Split Hopkinson Tensile Bar tests accompanied by quasi-static mechanical tests are carried out to relate the stress triaxiality, deformation temperature and strain rate with the fracture strain. During the building up of the fracture model, it is noticed that the change of fracture mechanism from 600°C to 900°C brings about a non-linear relationship of fracture strain with the deformation temperature. This results in a large error in fitting the Johnson–Cook (J-C) fracture model, so a modified fracture model is put forward.

Structural State and Mechanical Behavior of Fe–Cr–Ni Maraging Steels

The structure and the mechanical properties of low-carbon Fe–Cr–Ni maraging steels with molybdenum and titanium additives that were measured during static and cyclic deformation are studied in comparison with the structure and the mechanical properties of an austenitic–martensitic steel of the same alloying system. The mechanisms of fatigue crack nucleation and propagation are investigated by fractographic analysis.

Friction and Wear Characteristics of 18Ni(300) Maraging Steel under High-Speed Dry Sliding Conditions.

18Ni(300) maraging steel, which has exceptional strength and toughness, is used in the field of aviation and aerospace. In this paper, using a high-speed tribo-tester, tribological behaviors of 18Ni(300) maraging steel were investigated under high-speed dry sliding conditions. Morphology of the worn surfaces and the debris was analyzed by scanning electron microscope, and the oxides of worn surfaces caused by friction heat were detected by X-ray diffraction. The experiment results reveal that the friction coefficient of frictional pairs declines with increasing load and speed. With the speed and load increasing, oxides of the worn surfaces of 18Ni(300) maraging steel change from FeO to Fe3O4 and the wear mechanism converts from adhesive wear into severe oxidative or extrusion wear.

Effect of Heat Treatment Condition on Microstructural and Mechanical Anisotropies of Selective Laser Melted Maraging 18Ni-300 Steel

18Ni-300 maraging steel produced by the selective laser melting (SLM) process has a unique microstructure that is different from that of the same alloy processed by conventional methods. In this paper, maraging steels were fabricated by the selective laser melting process and their microstructures and mechanical properties were investigated in terms of post heat treatment conditions. Moreover, the effect of different heat treatments on the mechanical anisotropy was studied in detail. The micro Vickers hardness in the as-built state was around 340 Hv and could be increased to approximately 600 Hv by aging heat treatments. It was found that the solution heat treatment was not necessary to obtain a fully hardened state. From tensile tests of the maraging steels heat treated with different conditions, it was found that the highest strength was achieved by aging and solution treatment (ST) temperatures lower than the commonly used temperatures. In the direction parallel to the laser scanning, the highest ultimate tensile strength was obtained when 450 °C aging was done without solution heat treatment. In the other two directions tested, i.e., directions normal to the building and 45 degrees to the laser scanning direction, the highest tensile strength was obtained when aging was done at 450 °C after 750 °C solution treatment.

Hybrid parts produced by deposition of 18Ni300 maraging steel via selective laser melting on forged and heat treated advanced high strength steel

Long production times, the associated high costs of the products and product size limitations belong among current issues of selective laser melting (SLM) technology. Hybrid products containing small and complex-shaped parts deposited by SLM on the forged, rolled or hot stamped semi-products could offer a practical solution to these limitations. Cylindrical hybrid parts were additively manufactured by depositing 18Ni300 maraging steel on the cylindrical semi-products of CMnAlNb low-alloy advanced high strength steel (AHSS). The AHSS was used either in forged and air cooled condition or after heat treatments typically used for inducing the TRIP (transformation induced plasticity) effect. Various post-build heat treatments of the hybrid parts were performed. The mechanical properties of the hybrid parts were determined by hardness measurement across the interface and by a tensile test of the dissimilar joints. All tensile samples fractured in the high-strength steel side, several millimetres from the interface. Microstructure analysis of both materials and the interface region was carried out using light and scanning electron microscopes. The hybrid parts had the ultimate tensile strengths of 840−940 MPa, with total elongations of 12–19 %. The best combination of tensile strength and elongation was obtained with two-step heat treatment of the TRIP steel prior to additive manufacturing with no post-build heat treatment of the hybrid part.

A trade-off between powder layer thickness and mechanical properties in additively manufactured maraging steels

In this paper, a comprehensive study on the microstructure and mechanical properties of an additively manufactured 18Ni-300 maraging steel (with the brand name MS1), fabricated through the laser-powder bed fusion (LPBF) technique is presented. The influence of powder layer thickness and the characteristics of feedstock powder as the input in the LPBF process is investigated on the microstructure and mechanical properties of solid cubes and cylindrical rods. Relative density and hardness are measured through the depth of the manufactured cubes. The study of porosity and hardness through the depth of LPBF-MS1 cubes proves homogeneous properties in the core of the material in comparison with more heterogeneous properties closer to the subsurface layers. X-ray diffraction techniques both on the powder and the as-built samples are then performed to identify phases in the fabricated samples. A correlation between lower austenite content and higher strength is observed for the tensile samples manufactured with lower powder layer thickness. Texture analysis shows a directional grain growth along the building direction resulting in a weak texture, while the material induces a stronger texture with an increased amount of austenite after the deformation. Studying the effects of powder layer thickness shows slightly lower strength and ductility for the samples manufactured with higher powder layer thickness, while the energy consumption, as well as the manufacturing time, are reduced.

The phase transitions in selective laser-melted 18-NI (300-grade) maraging steel

Dilatometric studies in 18-Ni steel components fabricated by selective laser melting technique were carried out to determine the influence of heating rate on transitions occurring during the heating cycle. SLM components have been examined in controlled heating and cooling cycles. For analysis, heating of the analysed materials was carried out at heating rates of 10, 15, 20, 30 and 60 °C min−1. During the heating process, two solid-state reactions were identified—i.e. precipitation of intermetallic phases and the reversion of martensite to austenite. A simplified procedure based on the Kissinger equation was used to determine the activation energy of individual reactions. For precipitation of intermetallic phases, the activation energy was estimated 301 kJ mol−1, while the martensite to austenite reversion was determined at the activation energy 478 kJ mol−1.

Anisotropy of additively manufactured 18Ni-300 maraging steel: Threads and surface characteristics

The purpose was to evaluate part properties of additively manufactured 18Ni-300 maraging steel in a unified set-up when the printing orientation and the heat treatment were independently changed. Though the hardness, shrinkage, and drilling thrust force showed isotropic behavior to a certain extent, anisotropy was observed in the flatness, surface roughness, dross, and thread profiles. Solution treatment increases the surface roughness and reduces the flatness, surface hardness, drilling thrust force, and thread quality to some extent. Aging treatment significantly increases the surface hardness and yet still allows good-quality threads to be cut. The study aids designers in establishing and corroborating design for additive manufacturing.

Determination of the minimum chip thickness and the effect of the plowing depth on the residual stress field in micro-cutting of 18 Ni maraging steel

Due to the cutting-edge radius effect in micro-cutting, there exists some material sticking in front of the cutting-edge named stagnation zone, above which the material flows as chips. The location of the stagnation zone is reported to be associated with the minimum chip thickness in micro-cutting, which is a significant value influencing the cutting mechanics and the machined surface integrity in micro-cutting. The determination of the minimum chip thickness is of great importance in micro-cutting process. In this paper, the cutting force analysis on the shearing plane was carried out considering the ductile fracture and cutting-edge radius. Based on the cutting force analysis, this paper proposed a new method to determine the minimum chip thickness in micro-cutting of 18Ni maraging steel through finite element simulation. The minimum chip thickness was calculated to be 0.25 times the cutting-edge radius, which is in a good agreement with the velocity distribution analysis of the material around the cutting-edge. In addition, the effect of the plowing depth which was equivalent to the height of the stagnation zone on the residual stress filed was investigated using energy criterion. The energy stored in the machined surface increased with the plowing depth, resulting from the increasing thermal-mechanical load due to plowing.

A critical review on susceptibility of stress corrosion cracking in maraging steel weldments

Stress corrosion cracking is the decay of mechanical characteristics of sensitive material under the mixed effects of stress and corrosive atmosphere. Because of their high strength and extremely good fracture toughness 18 Ni maraging steel (250 grade) is suitable for aerospace application such as rocket engines as these steel materials can be welded without much hassles over a broad range of thickness. The maraging steel weldment causes catastrophic failure during service owing to stress corrosion cracking. It is a challenge to comprehend the SCC behaviour for the weldment of this material. The attempts of this review paper are to analyse the mechanical and microstructural changes related to SCC due to some post weld heat treatment or surface modifications. Further, review was made to intuit the effect of different welding processes on SCC behaviour of this material. Apart from a brief review of 18 Ni maraging steel, this paper presents the impact of different corrosive environments on SCC activity. The effect of different filler materials and composition of the filler material were also investigated.

Investigation on the cracking resistances of different ageing treated 18Ni maraging steels

The cracking resistance is an important indicator concerned by the industry. In this study, the cracking resistances (impact toughness, fracture toughness and fatigue crack growth resistance) of different ageing treated 18Ni maraging steels were investigated. Meanwhile, the differences in microstructures and fractographies were compared. Finally, the key factors affecting the cracking resistances were discussed and confirmed. It is found that, with the increment in ageing temperature, the size and spacing of precipitated phases would augment, as well as, the impact and fracture toughness, and fatigue crack growth resistance would improve. The fracture morphologies of impact and fracture toughness specimens display that, with the improvement of toughness, the size of dimple would gradually upgrade, and the characteristic of cleavage fracture would gradually disappear. The key factor influencing the cracking resistances of maraging steel is the interparticle spacing. Augmenting the interparticle spacing would cause the improvement of cracking resistances for maraging steel.

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.

Effect of heat treatment on microstructure and mechanical behaviours of 18Ni-300 maraging steel manufactured by selective laser melting

Selective laser melting (SLM) of 18Ni-300 maraging steel is an important research area in view of its numerous applications in the automotive domain. Heat treatment plays a significant role in the microstructure and mechanical behaviour of maraging steels and is a major area of interest. This paper investigated the effect of heat treatment on microstructure and mechanical behaviour of SLM-built 18Ni-300 maraging steel. The experimental results showed that the densest parts with the smallest number of defects were fabricated at optimum laser energy density of 70 J/mm3 and laser power of 275 W. When the laser power was fixed at 275 W, lower laser energy density resulted in the formation of balling and irregular pores, while higher laser energy density induced spherical pores and microcracks. The as-built samples consisted of cellular and columnar microstructures due to the fast cooling and solidification rates during SLM. However, solution treatment led to changes in the typical microstructure and massive lath martensite phase. The tensile strength and microhardness decreased slightly due to grain growth and residual stress relief upon solution treatment; an opposite effect was observed when the samples were subjected to solution treatment followed by aging at 490 °C for 2 h. With regard to the tensile anisotropy, yield strength and ultimate tensile strength of the horizontally-built samples slightly exceeded those vertically-built. These findings are significant as they allow an informed prediction about the effect of various heat treatments on the microstructure and mechanical behaviour of components manufactured from 18Ni-300 maraging steel using the SLM process.

Austenite reversion kinetics and stability during tempering of an additively manufactured maraging 300 steel

Reverted austenite is a metastable phase that can be used in maraging steels to increase ductility via transformation-induced plasticity or TRIP effect. In the present study, 18Ni maraging steel samples were built by selective laser melting, homogenized at 820 °C and then subjected to different isothermal tempering cycles aiming for martensite-to-austenite reversion. Thermodynamic simulations were used to estimate the inter-critical austenite + ferrite field and to interpret the results obtained after tempering. In-situ synchrotron X-ray diffraction was performed during the heating, soaking and cooling of the samples to characterize the martensite-to-austenite reversion kinetics and the reverted austenite stability upon cooling to room temperature. The reverted austenite size and distribution were measured by Electron Backscattered Diffraction. Results showed that the selected soaking temperatures of 610 °C and 650 °C promoted significant and gradual martensite-to-austenite reversion with high thermal stability. Tempering at 690 °C caused massive and complete austenitization, resulting in low austenite stability upon cooling due to compositional homogenization.

Influence of cycling frequency and testing volume on the VHCF properties of 18Ni maraging steel

Fatigue properties of nitrided 18Ni maraging steel sheets under cyclic tension loading were investigated at cycling frequency 95 Hz by using a resonance tester and at 20 kHz using ultrasonic fatigue testing equipment. High and very high cycle fatigue (VHCF) tests have been performed with two specimen shapes with distinctly different stressed volumes. Fatigue cracks started at internal aluminate or Zr(N,C) inclusions. Inclusions are considered as initial cracks, and fatigue lifetimes as the numbers of cycles necessary to propagate the cracks to fracture. Specimen size effect is explained with volume influence on statistical distribution of inclusion sizes. Comparable mean lifetimes for the same specimen shape and similar dependence of lifetimes on stress intensity factor range were found at 95 Hz and 20 kHz, which indicates the absence of frequency effects in maraging steel.

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.

Mechanical Properties of 18Ni-300 maraging steel manufactured by LPBF

The purpose of this document is to investigate the influence of heat treatment on the mechanical properties of parts from MAR 300 (EN 1.2709) produced by LPBF. In order to compare the influence of the heat treatment on the parts properties, some parts were in the state solution annealed and others age hardened tested. For the quality in the LPBF the feedstock powder as starting material is of great importance. Therefore, powder characterization has been performed in order to work out ageing effects after multiple use. As a major ageing effect an increasing oxygen content could be identified which might affect process and parts quality. For industrial acceptance, AM parts need to be produced to the best available tolerances and with well-understood mechanical properties. For this purpose, the mechanical properties of the LPBF samples have been analysed and are compared to those properties for conventional samples. In this investigation, the mechanical tests (hardness test, tensile test and notch impact resistance) have been performed in horizontal and vertical directions of manufacturing by LPBF. The hardness increased from 333~341 HV10 to 640~656 HV10 after heat treatment. Simultaneously, the ultimate tensile strength (Rm) increases from 1056~1096 MPa to 1964~2102 MPa, while the break elongation (εtot) is reduced from 11.3~16.0% to 2.0~4.5%. Aged samples show a significant decrease in toughness in the Notch Impact Test. A specifically designed ball joint test setup has shown that the frame manufactured by LPBF provides comparable results as a conventionally manufactured frame and is robust and suitable for the prototype production and test of car headlamps.

Effect of Scan Direction on Tensile properties and Fractography of Laser Additive Manufactured Maraging Steel

Laser additive manufacturing (LAM) is the process used to fabricate near net shape from metallic powders. A major challenge with LAM fabricated parts is the materials performance to achieve required mechanical properties. This work presents the fabrication of 18% Ni Maraging steel 300 (MS) using a LAM process by adopting two scan direction (bi-directional). The scan directions are selected in two perpendicular directions, one is parallel to the loading axis of the tensile specimen (0°) and another is in perpendicular direction to the loading axis of the tensile specimen (90°). Tensile properties and its fractography are investigated to analyse the scan direction effect due to the variation in thermal history. Further, to analyse the defect and pores in the fractured surface, scanning electron microscopy is used. The tensile properties of 90° scan direction are found to be higher owing to the nucleation and micro-void coalescence. This work facilitates to provide a better understanding of the selection of scan direction patterns for fabrication of better quality MS parts using the DMLS AM process.