Ni Maraging Steel Research Articles

Effect of Scanning Strategy in the L-PBF Process of 18Ni300 Maraging Steel

Maraging steels are good candidates for the laser powder bed fusion process (L-PBF), also known as Selective Laser Melting, due to excellent weldability and resistance to quench cracking. Powders physical and chemical characteristics dominate the final microstructure and properties of the printed parts, that are also heavily influenced by the process parameters. In this study, the effects of the scanning strategies on dimensions, average surface roughness, density and material hardness were evaluated, keeping the powder type and the volumetric energy density (Andrew number) constant. The effects of the scanning strategy on these properties are far less understood than on other important ones, like residual stresses and distortion, strongly affected by the scanning strategy. In this study, parallel stripes, chessboard and hexagonal pattern strategies were studied, keeping the Andrew number constant but varying the interlayer rotation. In general, the hexagonal strategy underperformed compared to the chessboard and the stripes ones.

Effects of TiC addition on microstructure, microhardness and wear resistance of 18Ni300 maraging steel by direct laser deposition

18Ni300 maraging steel with different TiC contents (0−10 wt.%), for the first time, have been fabricated by a direct laser deposition (DLD) technique. The microstructure and properties of the as-deposited and heat-treated samples have been carefully investigated. The obtained results show that the original 18Ni300 maraging steel exhibits extremely fine cellular microstructure with an average diameter of 3.7 μm, composed of martensite (M) and Ti2N. In contrast, the microstructure of the TiC-reinforced composites is significantly refined, in which the volume fraction of TiC formed with different morphologies varies with the TiC addition content. However, some cracks exist in the composite sample with the TiC addition up to 10 wt.%. The average microhardness of the as-deposited samples increases initially and then decreases with increasing TiC content, and the maximum microhardness (444 HV0.2) is obtained after adding 5 wt.% TiC. In addition, the sample with 5 wt.% TiC has been experienced a solution treatment at 850 ℃ for 1 h and then a subsequent aging treatment at 500 ℃ for 3 h. A number of nano-precipitates Ni3(Mo,Ti) and a few austenite are formed, which are dispersed in the martensite matrix after the heat treatment. This results in a high microhardness of 576 HV0.2 and a low wear rate of 1.37 × 10-6 mm3/(N·m), compared with those of the TiC-free sample (368 HV0.2 and 5.61 × 10-5 mm3/(N·m)), respectively. The possible microstructural mechanisms accounting for the property changes of the TiC-reinforced 18Ni300 maraging steel have been discussed in detail.

Aluminum-alloyed lightweight stainless steels strengthened by B2-(Ni,Fe)Al precipitates

The age hardenability of Al-alloyed lightweight stainless steels with the base chemical composition Fe–10.5Cr–3Al (wt.%) and Ni concentrations in the range 3–15 wt.% was studied. Alloys containing 3% and 6%Ni exhibited almost fully ferritic matrix microstructures and a weak age hardening response. Alloys containing 9%, 12%, and 15%Ni, on the other hand, developed primarily martensitic microstructures. Differential scanning calorimetry measurements indicated the occurrence of an exothermic reaction in the approximate temperature range 375–625 °C. Dilatometry measurements indicated that the exothermic reaction was accompanied by a net contraction. Hardness measurements after aging for 5 min indicated significant hardening of alloys already at 350 °C due to the formation of B2-(Ni,Fe)Al intermetallic precipitates. The age hardening response was significantly superior to that of conventional precipitation-hardenable martensitic stainless steels. Tensile elongation in the aged condition was negatively influenced by the presence of soft ferrite regions. Processing conditions associated with a fully martensitic microstructure prior to aging are required to render a uniform age hardening response. Guidelines for the development of a new family of lightweight precipitation-hardenable steels with lower raw material costs and a higher corrosion resistance compared to the standard 18Ni maraging steels are provided.

Structure and Mechanical Properties of the 18Ni300 Maraging Steel Produced by Spark Plasma Sintering

In this work, a new approach for compaction of the gas-atomized 18Ni300 maraging steel at two different temperatures of 1050 °C and 1150 °C using a progressive SPS technology is studied. Moreover, the influence of two heat treatments combining solution annealing and aging (SAT) and simply aging treatment (AT) on the microstructure and mechanical properties is investigated. It is found that samples compacted at 1050 °C had higher porosity compared to the almost non-porous material produced at 1150 °C. In addition, the difference of 100 °C for the compaction temperature successfully reduces the porosity from 0.86% down to 0.08%. Additionally, we discovered that the higher the compaction temperature, the higher the amount of retained γ-Fe which positively affects the ductility of the samples. The subsequential heat treatment results in precipitation strengthening via the Ni3Mo precipitates. Microhardness of the SPS1050 and SPS1150 samples increase from 303 ± 13 HV0.1 and 360 ± 5 HV0.1 to 563 ± 31 HV0.1 and 606 ± 17 HV0.1, respectively. The sample compacted at 1150 °C shows the highest ultimate tensile strengths reaching up to 1940 ± 6 MPa, while also showing 4% ductility.

An investigation of the anisotropic properties of heat-treated maraging steel grade 300 processed by laser powder bed fusion

In order to explore the possibilities enabled by laser beam powder bed fusion of metals (PBF-LB/M), reliable material models are necessary to optimize designs with respect to weight and stiffness. Due to the unique processing conditions in PBF-LB/M, materials often develop a dominating microstructure that leads to anisotropic mechanical properties, and thus isotropic material models fail to account for the orientation-dependent mechanical properties. To investigate the anisotropy of 18Ni300 maraging steel, tensile specimens were built in seven different orientations. The specimens were heat treated at two different conditions and tested for their tensile properties using digital image correlation (DIC) technique. The microstructure and fracture surfaces are investigated with scanning electron microscope and electron backscatter diffraction. The tensile properties are typical for the material, with a yield strength in the range of 1850 MPa to 1950 MPa, and ultimate tensile strength in the range of 1900 MPa to 2000 MPa. The elastic modulus is 180 GPa, and the elongation at fracture is in the range of 2–6% for all specimens. The strain fields analysed with DIC reveals anisotropic straining in both the elastic and plastic parts of the flow curve for both direct ageing and solution treatment plus ageing specimens. In the former condition, the elastic anisotropy is dictated by the fraction of melt pool boundaries on the transverse surfaces of the specimens. When the material is solution treated prior to ageing, the melt pool boundary effect was supressed.

A relationship between the build and texture orientation in tensile loading of the additively manufactured maraging steels

This research reveals the effect of building direction on the microstructure, mechanical properties and the deformation behaviour of additively manufactured maraging steels. 18Ni-300 maraging steel samples were manufactured throughout the laser powder bed fusion (LPBF) process in horizontal and vertical directions. Uniaxial tensile tests were then conducted for as-built and heat-treated specimens to study the mechanical properties before and after the heat treatment process. The horizontal samples showed higher strength and ductility than the vertical ones, while the heat-treatment procedure reduced the building direction effect on the mechanical strength. Ductility was slightly different amongst the four sets of specimens. The heat-treatment increased the mechanical strength significantly at the expense of decreasing the ductility. Fractography through scanning electron microscopy showed that the aged fracture surfaces contain cleavage along with dimples. Transmission electron microscopy revealed the austenite phase enclosed by the martensite laths and segregated alloying elements and precipitations, resulting in a hierarchical microstructure. Further studies on the deformation behaviour in the four cases of as-built/aged, horizontal/vertical combinations were conducted. X-ray diffraction technique was adopted to study the phase changes through the deformation process. The as-built specimens showed a fully transformed austenite phase, while the aged samples reduced the austenite fraction to half at the fracture. Finally, the EBSD technique was conducted to show the microstructure evolution through the manufacturing process. The material showed an elongated grain structure towards the building direction. Heat-treatment did not eliminate the texture along the building direction, while the deformation process led to a texture evolution from < 100 > and < 111 > into < 110 > directions.

Fatigue Behavior of 18% Ni Maraging Steels: A Review

18% Ni maraging steels are based on the Fe-Ni-Co-Mo system with low-carbon content. They display an excellent combination of high strength and high toughness. However, they suffer from a low fatigue ratio, the ratio decreasing monotonically with increasing strength. Considering prospective applications for these steels involving fatigue loading, attempts were made by researchers to improve their fatigue life. The studies suggest that fatigue strengths higher than those realized in peak aged condition can be obtained through controlled overaging with a small amount of reversed austenite playing a critical role. Corrosion fatigue in different environments is a serious problem with these steels. Double aging seems to reduce the susceptibility to corrosion fatigue under high humidity conditions. Inclusion content has a strong influence on the fatigue life, inclusion size and type playing an important role. Surface treatments such as shot peening, laser peening, and nitriding were found to improve the fatigue life; however, it is important to optimize the process parameters. This paper attempts a critical review of studies reported in the published literature aiming to improve the fatigue life of 18% Ni maraging steels.

Short fatigue crack growth behavior in 18Ni marageing steel

In recent decades, it has been found that the fatigue crack growth rate could be slowed down by the zigzag crack growth path in some materials, but the mechanism was not well clarified. In this study, the effect of crack growth path on the fatigue crack growth rate of 18Ni maraging steel under three different heat treatment states was studied, and two factors influencing the fatigue crack growth rate were outlined. It was found that, firstly, the main factor affecting the fatigue crack growth rate was the strength of the material; secondly, the zigzag crack growth path did not slow down the crack growth rate but increased the crack growth rate. Therefore, a two-phase model was established to understand the influence of crack growth path on the growth rate. In different phases, their resistances to the crack growth are different, and the difference in the strength between the two phases determines whether the zigzag cracking path can depress the fatigue crack growth or not. Specifically, when the difference in the strength between the second phase and the matrix is greater than a certain value, the zigzag cracking path leads to the acceleration of fatigue crack growth. The consequences show that not all the zigzag crack propagation paths are beneficial for the higher fatigue cracking resistance, and the specific analysis should be combined with the material microstructure in detail.

Effects of aging time on the microstructure and mechanical properties of laser-cladded 18Ni300 maraging steel

In this work, 18Ni300 maraging steel was successfully fabricated, for the first time, by a laser cladding technique under atmospheric condition. The effects of different aging times (1, 3, 6, and 9 h) at 500 °C on the microstructure and mechanical properties of the as-cladded 18Ni300 maraging steel were carefully characterized and analyzed. The microhardness and tensile strength increase with increasing aging time up to 6 h, and subsequently, decrease with the time extending to 9 h. On the contrary, the elongation is shown a reverse trend. The original 18Ni300 maraging steel exhibits cellular microstructure with an average grain size of 2 µm, composed of martensite and nano-Ti2N particles. After aging treatment, Ni-rich nano-precipitates (Ni3(Mo,Ti) and Ni(Mo,Ti)) together with reverted austenite were formed and promoted with the extension of aging time. The optimal comprehensive performance of the 18Ni300 maraging steel can be obtained by aging 3 h at 500°C, with microhardness of 509 HV0.2, ultimate tensile strength of 1686 MPa, and elongation of 11.5%, respectively. The microstructural mechanisms accounting for the property changes are discussed in detail.

Use of plasma nitriding to improve the wear and corrosion resistance of 18Ni-300 maraging steel manufactured by selective laser melting

18Ni-300 maraging steel manufactured by selective laser melting was plasma nitrided to improve its wear and corrosion resistance. The effects of a prior solution treatment, aging and the combination of both on the microstructure and the properties after nitriding were investigated. The results were compared with conventionally produced 18Ni-300 counterparts subjected to the same heat- and thermo-chemical treatments. The plasma nitriding was performed under the same conditions (temperature of 520 °C and time of 6 h) as the aging in order to investigate whether the nitriding and the aging could be carried out simultaneously in a single step. The aim of this work was to provide a better understanding of the morphology and chemical composition of the nitrided layer in the additive-manufactured maraging steel as a function of the prior heat treatments and to compare the wear and corrosion resistance with those of conventional maraging steel. The results show that nitriding without any prior aging leads to cracks in the compound layer, while nitriding of the prior-heat-treated additive-manufactured maraging steel leads to benefits from the thermochemical treatment in terms of wear and corrosion resistance. Some explanations for the origins of the cracks and pores in the nitride layers are provided.

Surface hardness determination of 3D printed parts using laser-induced breakdown spectroscopy.

Laser-induced breakdown spectroscopy was used to determine the relationship between the spectral line intensity and surface hardness of 3D printed 18Ni300 maraging steel. Research found that there is a linear relationship between the spectral intensity ratio of ion line to atomic line and the surface hardness of the samples. This linear relationship is closely related to the selected elements and spectral lines. The weak self-absorption spectrum of minor elements can obtain a better linear relationship. We study the effect of the number of laser pulses on the linear relationship. The results show that the ideal results can be obtained by using 100 pulses, which can minimize the damage to the sample.

Mechanical Behaviour of Maraging Steel Produced by SLM

Metallic additive manufacturing has been developed as a manufacturing process for complex functional parts that may be potentially applied in various engineering fields, with very distinct mechanical performance requisites. Therefore, it is of primary importance to characterize additive manufacturing materials under various loading conditions in other to understand possible distinctive behaviours as regards the conventional material counterparts. This paper aims at characterizing the mechanical behaviour of a very common material for additive manufacturing applications – the 18Ni300 maraging steel. A set of physical and mechanical tests were performed on conventional and additive maraging steel. Regarding the physical properties, the heat capacity and conductivity were measured with temperature; regarding mechanical characterization, compression tests were performed at various (high) strain rates. Also, tensile tests were performed covering distinct stress triaxialities. Finally, multiaxial tests were performed to evaluate the quasi-static mechanical response of the materials under combined tension/compression and shear loadings with the assistance of Digital Image Correlation. Inverse numerical analyses were performed to evaluate the best constitutive modelling strategies for the materials. Tests revealed the very complex behaviour of the maraging steels and the additive material showed higher strength than the conventional material.

Effect of remaining ligament on fatigue crack growth

Fatigue crack growth (FCG) has been studied assuming that ∆Keff is the crack driving force. However, the effect of crack ligament on FCG rate is not clear. Experimental-numerical work was developed in the 18Ni300 maraging steel considering different load blocks with the same initial ∆K. No effect of crack ligament was observed experimentally, which was explained by the inexistence of crack closure effects. In the numerical work transient effects were observed for the plane stress state, which disappear for the plane strain state and some effect of crack ligament was found. In both cases this was attributed to crack closure phenomenon.

Improving surface mechanical properties of the selective laser melted 18Ni300 maraging steel via plasma nitriding

Enhancing the hardness and wear resistance of selective laser melted (SLM) maraging steel is challenging. In this work, plasma nitriding was developed to improve the surface mechanical properties of SLM 18Ni300 maraging steel based on the effect of simultaneous aging and nitriding. The nitriding behaviors of the as-fabricated sample and heat-treated (i.e., solution, aging and combined solution-aging treatments) samples were systematically analyzed using microstructure and property characterizations as well as first principles calculations. The results show that the hardness and wear resistance of all samples were notably enhanced by a nitrided layer consisting of an outermost N-rich bright white layer and an inner Mo-rich diffusion layer. Among all nitrided samples, the directly nitrided sample without heat treatment possesses the best mechanical properties, which are closely related to the strongest Mo segregation and a novel nitriding structure induced by additive manufacturing. The novel nitriding structure is an interlaced lamellar structure consisting of coarse γN lamellae along the prior SLM subgrain interfaces and parallel fine γN lamellae in the αN matrix, which obeys the typical Kurdjumov-Sachs orientation relationship between αN and γN. Compared to the traditional nitriding structure originated from the homogenized maraging steel, the novel nitriding structure benefits strengthening and toughening and encourages the formation of an Fe-O-Si self-lubricating film. The abovementioned results suggest that direct nitriding of the as-fabricated sample without heat treatment is the best method to obtain the best surface mechanical properties.

Plastic deformation throughout strain-induced phase transformation in additively manufactured maraging steels

A comprehensive study was intended to show the microstructural features of additively manufactured (AM) 18Ni-300 maraging steel. Uniaxial tensile tests were conducted on specimens built using laser powder bed fusion (LPBF) technique for two different powder layer thicknesses. The specimens were built to have the lowest possible porosity, and tensile tests showed two stages of strain hardening. In stage I, the dislocation density increased, leading to a positive strain hardening rate. A negative strain hardening rate due to the necking effect was then followed in stage II. X-ray diffraction (XRD) analyses revealed a phase transformation through the deformation. Various analyses via electron backscattered diffraction (EBSD) technique was then conducted with large scans over three different zones representing undeformed, deformed, and severely deformed close to the fracture area. The pole figures and orientation distribution functions (ODF) revealed a texture evolving through the deformation process in agreement with the kernel average misorientation (KAM) and grain boundary maps. Transmission electron microscopy (TEM) was used to detect the inclusions and segregated alloying elements adjacent to the fractured surfaces. Results indicated that the deformation led to diminishing the austenite (γ) phase, while the transformed austenite sourced the high dislocation density area at cell boundaries.

Selective Laser Melting of 18NI-300 Maraging Steel.

In the present study, 18% Ni 300 maraging steel powder was processed using a selective laser melting (SLM) technique to study porosity variations, microstructure, and hardness using various process conditions, while maintaining a constant level of energy density. Nowadays, there is wide range of utilization of metal technologies and its products can obtain high relative density. A dilatometry study revealed that, through heating cycles, two solid-state effects took place, i.e., precipitation of intermetallic compounds and the reversion of martensite to austenite. During the cooling process, one reaction took place (i.e., martensitic transformation), which was confirmed by microstructure observation. The improvements in the Rockwell hardness of the analyzed material from 42 ± 2 to 52 ± 0.5 HRC was improved as a result of aging treatment at 480 °C for 5 h. The results revealed that the relative density increased using laser speed (340 mm/s), layer thickness (30 µm), and hatch distance (120 µm). Relative density was found approximately 99.3%. Knowledge about the influence of individual parameters in the SLM process on porosity will enable potential manufacturers to produce high quality components with desired properties.

Influence of Selective Laser Melting Process Parameters on Densification Behavior, Surface Quality and Hardness of 18Ni300 Steel

This study investigated the effect of laser power (P), scan speed (v) and hatch space (h) on densification behavior, surface quality and hardness of 18Ni300 maraging steel fabricated by selective laser melting (SLM). The results indicated that the relative density of the SLMed samples has a shape increase from 73% to 97% with the laser energy density increasing from 0.5 to 2.2 J/mm2. The relative density ≥ 99% was achieved at the energy density in the range of 2.2~5.9 J/mm2. The optimum process parameters were found to be laser power of 150~200 W, scan speed of 600mm/s and hatch space of 0.105mm. In addition, it was found that the hardness increased initially with the increasing relative density up to relative density of 90% and then little relationship, but finally increase again significantly. This work provides reference for determining process parameters for SLMed maraging steel and the development of 3D printing of die steels.

Post‐treatment selection for tailored fatigue performance of 18Ni300 maraging steel manufactured by laser powder bed fusion

AbstractThis study investigates the fatigue behaviour of additively manufactured 18Ni300 maraging steel. Specifically, the surface and material parameters impacting fatigue performance are analysed through various post‐treatment combinations. Vertically built miniaturised test samples produced by laser powder bed fusion are tested in as‐built and age‐hardening heat‐treated conditions. To utilise the potential of using additive manufacturing for complex‐shaped parts in which conventional machining tools could have limited access, vibratory finishing and sand blasting are employed. The fatigue results show that in as‐built microstructural condition, both the surface treatments significantly enhanced the fatigue performance, with vibratory finishing outperforming sand blasting owing to better surface finish. After heat treatment, sand‐blasted samples performed better than vibratory‐finished ones because of higher residual stresses. This competing interaction between post‐treatments sheds light on identifying the relative influence of various factors. With systematic postfracture and microstructural analyses highlighting the fatigue influencing factors, recommendations are drawn to select post‐treatments to achieve the desired fatigue performance.

High-Throughput Exploration of the Process Space in 18% Ni (350) Maraging Steels via Spherical Indentation Stress–Strain Protocols and Gaussian Process Models

Several challenges are encountered in the development of maraging steels with desired combinations of mechanical properties. These include the need to explore a large process design space, the time- and effort-consuming standardized testing protocols for property evaluations, and the lack of a formal design of experiments strategy that guides the selection of the process conditions for the next set of experiments based on a thorough analyses of the previously accumulated data. In this work, we explored the effect of the different combinations of the aging temperature and the aging time on the yield strength of 350-grade maraging steels using high-throughput protocols. For this purpose, a total of 21 small volumes of differently aged samples were produced and studied using spherical nanoindentation stress–strain protocols. Furthermore, the results of these tests were modeled using Gaussian process regression (GPR) to establish data-driven linkages between the yield strengths and the aging parameters. The predicted yield strengths were found to be in reasonable agreement with the experimentally measured ones. It is also demonstrated that the GPR model can provide objective guidance in the selection of the next set of experiments.

Kinetics of Reverted Austenite in 18 wt.% Ni Grade 300 Maraging Steel: An In-Situ Synchrotron X-Ray Diffraction and Texture Study

In this study, we investigated the transformation kinetics of martensite → reverted austenite in 18 wt.% grade 300 Ni maraging steel. The kinetics was evaluated based on the in situ synchrotron x-ray diffraction data collected during isothermal heat treatment at 570°C. The onset of transformation martensite → reverted austenite was detected after ~ 5 min of aging. The austenite fraction increased as a function of annealing time and reached approximately 30 vol.% after 3 h of heat treatment. The electron backscatter diffraction technique revealed that reverted austenite is formed preferentially on both the martensitic lath boundaries and sub-grain boundaries inside the laths, in particular in those with high Taylor factor values. The reverted austenite maintains an orientation relationship with the prior austenite; however, variant selection can take place.