Showing papers on "Maraging steel published in 2021"

Effect of solution annealing and precipitation hardening at 250 °C–550 °C on microstructure and mechanical properties of additively manufactured 1.2709 maraging steel

Abstract: Post-processing heat treatment of additively manufactured (AM) maraging steel is an important issue in the tailoring of the final mechanical properties of a product. Up to now, mainly precipitation hardening at temperatures of 450 °C–500 °C has been studied either in as-built or solution-annealed samples. This work, however, presents an overview of the effect of much broader tempering temperatures of 250 °C–550 °C on the microstructure and mechanical properties of as-built maraging steel. Furthermore, the effect of previous solution annealing of AM steel at a lower temperature of 820 °C and at a higher temperature of 940 °C on subsequent precipitation hardening is also described. The results obtained for the precipitation behaviour of AM maraging steel are compared with those of conventionally produced maraging steel. The microstructure and mechanical properties of AM samples pre-annealed at 940 °C and precipitation hardened were found to be comparable to the conventionally processed reference sample at all hardening temperatures. On the other hand, the microstructures and properties of AM samples pre-annealed at 820 °C and precipitation hardened strongly resembled the results of as-built samples. However, even after a 6-h hold at the highest tempering temperature of 550 °C, distinct differences could still be found in the samples prepared with various initial conditions.

On the role of the powder stream on the heat and fluid flow conditions during Directed Energy Deposition of maraging steel—Multiphysics modeling and experimental validation

Abstract: The Directed Energy Deposition (DED) process of metals, has a broad range of applications in several industrial sectors. Surface modification, component repairing, production of functionally graded materials and more importantly, manufacturing of complex geometries are major DED’s applications. In this work, a multi-physics numerical model of the DED process of maraging steel is developed to study the influence of the powder stream specifications on the melt pool’s thermal and fluid dynamics conditions. The model is developed based on the Finite Volume Method (FVM) framework using the commercial software package Flow-3D. Different physical phenomena e.g. solidification, evaporation, the Marangoni effect and the recoil pressure are included in the model. As a new feature, the powder particles’ dynamics are modeled using a Lagrangian framework and their impact on the melt pool conditions is taken into account as well. In-situ and ex-situ experiments are carried out using a thermal camera and optical microscopy. The predicted track morphology is in good agreement with the experimental measurements. Besides, the predicted melt pool evolution follows the same trend as observed with the online thermal camera. Furthermore, a parametric study is carried out to investigate the effect of the powder particles incoming velocity on the track morphology. It is shown that the height-to-width ratio of tracks increases while using higher powder velocities. Moreover, it is shown that by tripling the powder particles velocity, the height-to-width ratio increases by 104% and the wettability of the track decreases by 24%.

Concurrent improvement of strength and ductility in heat-treated C300 maraging steels produced by laser powder bed fusion technique

Abstract: In this work, C300 maraging steel powder feedstocks at different titanium contents (0.72 and 1.17 wt%) were additively manufactured using the laser powder bed fusion (LPBF) technique to systematically study the mechanical behavior of the material in both as-built and heat-treated conditions. X-ray diffraction techniques along with electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were employed to investigate the microstructural characteristics and phase formation in the as-built and heat-treated samples. After heat-treating at 490 °C for 6 h, the HighTi maraging steel showed higher strength and ductility (2057.74 MPa and 4.05%). The microstructural characterization proved that this alloy contains a higher amount of reverted austenite (17.89 wt%), which resulted at higher strength as results of the transformation induced plasticity (TRIP) effect. In terms of ductility improvement, the fiber/copper texture developed in the HighTi sample provided a higher driving force to transform the reverted austenite to martensite. As the TEM analyses revealed, needle-shaped Ni3Ti and spherical Ni3Mo precipitates were found in both LowTi and HighTi alloys, where the HighTi one showed higher volume fraction of Ni3Ti precipitates, and consequently, higher values of tensile strength, which can be explained, based on the Orowan mechanism.

Microstructure and mechanical properties of additively manufactured multi-material component with maraging steel on CrMn steel

Abstract: High manufacturing freedom of additive manufacturing (AM) technology, such as selective laser melting (SLM), innovates an effective way for the fabrication of multi-material components which is highly challenging or even impossible for the conventional manufacturing processes. The bonding performance between dissimilar materials in the AMed multi-material component needs further research prior to application. In this study, maraging steel (MS1) part was manufactured on top of the cast CrMn steel using SLM to obtain a multi-material component. The microstructure of CrMn steel substrate and MS1 steel, as well as the interfacial morphology of the hybrid CrMn-MS1 component, was characterized to study the metallurgical property. The microhardness and tensile property tests were conducted to investigate the mechanical behaviour of the hybrid cast/SLM multi-material component. An interface with a width of about 130 μm was formed between the two dissimilar materials. Marangoni convection caused complex phenomena at the interface, leading to the cross-regional distribution of alloy elements and a variation in the microstructure of the first several layers of SLMed MS1. Good metallurgical bonding of the dissimilar materials manufactured by SLM is achieved without pores, inclusions or cracks in the interfacial region. The microhardness value of the interface region is 309 ± 9 HV0.05 . This mitigates the difference in performance mismatch by smoothening the mechanical-property transition between CrMn steel (277 ± 11 HV0.05) and SLMed MS1 (360 ± 9 HV0.05). The hybrid CrMn-MS1 steel presents a higher ultimate tensile strength of 986 ± 30 MPa than cast CrMn steel and higher elongation of 24.5 ± 1.0% than SLMed MS1. This study proves feasibility to manufacture a reliable multi-martial component with a cast substrate and SLMed part of potentially higher complexity.

Dependence of the wear resistance of additive-manufactured maraging steel on the build direction and heat treatment

Abstract: The aim of this research work was to study the effect of the build direction and heat treatment on the anti-wear properties of maraging steel produced by laser powder bed fusion (LPBF) and how these properties correlate with the resulting mechanical properties. The investigation involved a commercial EOS laser power bed fusion system which included horizontal, vertical and inclined (45°) build directions. After the additive manufacturing the specimens were subjected to aging or solution treatment and aging, after which the wear and mechanical properties were evaluated in different build directions. The results show that even after aging build direction of AM maraging steel parts plays an important role in terms of mechanical and wear properties. After aging the interfacial features between the laser tracks tend to disappear, with the cellular sub-structures being broken into particles. However, although broken and diminished, the cellular sub-structure, defects, nano-segregations and a weak texture in the build direction are still present after aging. This results in lower strength and elastic properties as well as reduced resistance to crack initiation for vertical printing direction. On the other hand, it still provides superior resistance to crack propagation. Best combination of abrasive and adhesive wear is obtained for horizontal direction and in-plane sliding, while sliding across the layers is the most detrimental. Inclined printing bypasses anisotropy and provides good combination of high strength and toughness. However, it is inferior in terms of wear resistance. Properties anisotropy can also be eliminated by a solution treatment, which homogenizes the microstructure and improves toughness, but may also result in reduced strength and wear resistance.

Selective Laser Melting of Maraging Steels Using Recycled Powders: A Comprehensive Microstructural and Mechanical Investigation

Abstract: Selective laser melting (SLM) is an additive manufacturing (AM) technique designed to use a high energy density laser to fuse metallic powders for producing three-dimensional parts. So far, most studies of SLM have been focused on using virgin metal powders. There are few comprehensive studies on the microstructure and mechanical properties of SLM-produced parts using recycled powders, especially for maraging steels. In this study, we employ recycled steel powder (reused after 113 building cycles) in the SLM process to print multiple shaped components and systematically characterize the microstructure and mechanical properties (indentation, tensile, and Charpy testing). Our results show that maraging steel produced with recycled powder exhibit the nearly identical microstructure and mechanical properties (940 MPa yield strength, 1127 MPa ultimate tensile strength, 11 pct elongation, and 47.5 J room temperature impact fracture energy) to those produced using virgin powders. This study provides a useful generic guide towards using recycled metal powders in the SLM processing, promoting an economic solution to industrial productions.

Effect of aging temperature on the austenite reversion and mechanical properties of a Fe–10Cr–10Ni cryogenic maraging steel

Abstract: Fe–10Cr–10Ni cryogenic maraging steel is one of the key candidate materials applied in harsh cryogenic condition due to its high strength and good cryogenic toughness. In this study, the effect of aging temperature on its microstructure and mechanical properties was systematically studied based on austenite reversion and nano-precipitation. It shows that a desirable combination of high strength (834 MPa, 25 °C) and excellent cryogenic impact toughness (164 J, −196 °C) can be obtained by aging treatment at 500 °C. Multi-scale characterizations were conducted to reveal the microstructure characteristics of the steel. It was found that obvious film-like reversed austenite nucleate and grow at the high-angle grain boundaries of martensite matrix in the steel aged at 500 °C, whereas the higher aging temperature resulted in a larger content of blocky reversed austenite in martensite blocks. Austenite reversion mechanism was proposed based on the double-spherical-cap model and diffusion kinetics of Ni element. Besides, it was found that the precipitation sites of Ti-rich particles are not only distributed in matrix but also located at the dislocations, and they were identified as the clusters of Ni3Ti precipitates. Finally, the origin of the above cryogenic toughness includes the transformation-induced plasticity (TRIP) effect from the soft film-like austenite, higher density of high angle grain boundaries of martensite and fine nanoscale precipitates. Moreover, the precipitation strengthening from the clusters of Ni3Ti precipitates contributes to the high strength of the steel aged at 500 °C.

Investigation of Building Orientation and Aging on Strength–Stiffness Performance of Additively Manufactured Maraging Steel

Abstract: The interaction between building orientation and heat treatment may change the strength–stiffness behavior of maraging steel manufactured by powder bed fusion (PBF). Further investigations about the correlation of the fracture characteristics and microstructures with the measured properties are needed to improve the process findings. Thus, this study investigated the way an interaction between building orientation (0°, 45°, and 90°) and specimen condition (as-built and aged) may change the defects interaction mechanism and the mechanical properties of the maraging 300 steel manufactured by PBF. Aging treatment, microstructure characterization, density measuring, tensile and microhardness tests, and fractography were performed on the specimens. The aging treatment strengthened the material, which also reduced the probability of defect coalescence. The interaction of building orientation and aging affected the stiffness, rising this in ≈ 70.5 GPa for the 0° aged specimens. Furthermore, this interaction changed the melt pool stretching direction, and the behavior of the defect coalescence rises the elongation at break in ≈ 16.5% for 0° as-built specimens. The investigated mechanisms show the importance of considering the interaction between the building orientation and condition (as-built and aged) for the mechanical performance of additively manufactured maraging steel 300.

Microstructural evolution and high strain rate compressive behavior of as-built and heat-treated additively manufactured maraging steels

Abstract: Maraging steel is ultra-high-strength steel with low carbon content hardened by secondary precipitation during aging treatments. Additive manufacturing is an advanced technique for fabricating near net-shaped components from a powder or wire feedstock. In the present study, maraging 300 samples were additively manufactured using laser powder bed fusion (LPBF) and were subjected to dynamic impacts. Using a Split Hopkinson Pressure Bar apparatus, compressive loads were applied at strain rates ranging between 1500 s−1 and 4000 s−1 for the as-built and 150 s−1 to 1930 s−1 for the heat-treated LPBF-maraging steel samples. Scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) were used to analyze the microstructure and texture evolution during the dynamic impact loadings. While as-built LPBF-maraging steel samples fractured when impacted at a strain rate of 3500 s−1, aged LPBF-maraging steel samples fractured when loaded at a strain rate of 1930 s−1. The microstructural analysis of the as-built samples revealed the formation of adiabatic shear bands as a result of heat accumulation in strain rates of 1500 s−1, 2000 s−1, and 3200 s−1. In addition, adiabatic shear bands were detected at the strain rate of 890 s−1 in the aged maraging steel samples as well. Finally, a constitutive model was developed to understand better the high strain rate behavior of LPBF-maraging steel samples. For both as-built and heat treat conditions, the Chang-Asaro hardening model showed good agreement with experimental results.

Tuning process parameters to optimize microstructure and mechanical properties of novel maraging steel fabricated by selective laser melting

Abstract: Maraging steel is a promising material for additive manufacturing due to its ultrahigh yield strength, reasonable ductility and good weldability. However, the ductility of the fabricated part will be degraded after aging treatment. In this regard, we firstly designed a new composition of maraging steel Fe-18.3Ni-9Co-4.84Mo-0.92Ti-0.27Al-0.13Cr-0.01C (wt.%), whose strength and ductility can be simultaneously improved by selective laser melting. The relationship between laser process parameters and forming defects has been studied. Using single-track and single-layer experiments, fully dense parts were fabricated with a certain range of process parameters, corresponding to the 30–50% lap rate in the X–Y plane and 70% remelted rate along Z-axis. Besides, we found film-like reverted austenite along the martensite lath boundaries in the as-fabricated part. The effects of heat treatment processes on the reverted austenite and mechanical properties of the fabricated parts were also studied; the strength-ductility trade-off of maraging steel after heat treatment can be alleviated. The tensile strength and elongation of printed samples after direct aging treatment can respectively reach 2037 MPa and 6.4%, and 2182 MPa, 4.8% after solution and aging treatment.

Effect of Cryogenic Grinding on Fatigue Life of Additively Manufactured Maraging Steel

Abstract: Additive manufacturing (AM) is replacing conventional manufacturing techniques due to its ability to manufacture complex structures with near-net shape and reduced material wastage. However, the poor surface integrity of the AM parts deteriorates the service life of the components. The AM parts should be subjected to post-processing treatment for improving surface integrity and fatigue life. In this research, maraging steel is printed using direct metal laser sintering (DMLS) process and the influence of grinding on the fatigue life of this additively manufactured material was investigated. For this purpose, the grinding experiments were performed under two different grinding environments such as dry and cryogenic conditions using a cubic boron nitride (CBN) grinding wheel. The results revealed that surface roughness could be reduced by about 87% under cryogenic condition over dry grinding. The fatigue tests carried out on the additive manufactured materials exposed a substantial increase of about 170% in their fatigue life when subjected to cryogenic grinding.

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

Abstract: 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.

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

Abstract: 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.

Evaluation of Maraging Steel Produced Using Hybrid Additive/Subtractive Manufacturing

Abstract: Hybrid manufacturing is often used to describe a combination of additive and subtractive processes in the same build envelope. In this research study, hybrid manufacturing of 18Ni-300 maraging steel was investigated using a Matsuura LUMEX Avance-25 system that integrates metal additive manufacturing using laser powder bed fusion (LPBF) processing with high-speed machining. A series of benchmarking coupons were additively printed at four different power levels (160 W, 240 W, 320 W, 380 W) and with the integration of sequential machining passes after every 10 deposited layers, as well as final finishing of selected surfaces. Using non-contact three-dimensional laser scanning, inspection of the final geometry of the 18Ni-300 maraging steel coupons against the computer-aided design (CAD) model indicated the good capability of the Matsuura LUMEX Avance-25 system for net-shape manufacturing. Linear and areal roughness measurements of the surfaces showed average Ra/Sa values of 8.02–14.64 µm for the as-printed walls versus 0.32–0.80 µm for the machined walls/faces. Using Archimedes and helium (He) gas pycnometry methods, the part density was measured to be lowest for coupons produced at 160 W (relative density of 93.3–98.5%) relative to those at high power levels of 240 W to 380 W (relative density of 99.0–99.8%). This finding agreed well with the results of the porosity size distribution determined through X-ray micro-computed tomography (µCT). Evaluation of the static tensile properties indicated that the coupons manufactured at the lowest power of 160 W were ~30% lower in strength, 24% lower in stiffness, and more than 80% lower in ductility relative to higher power conditions (240 W to 380 W) due to the lower density at 160 W.

Effect of the as-built microstructure on the martensite to austenite transformation in a 18Ni maraging steel after laser-based powder bed fusion

Abstract: During laser-based powder bed fusion, the non-equilibrium solidification conditions promote local elemental segregation, leading to a characteristic microstructure composed of cellular walls. These walls can display either low carbon BCC martensite or FCC retained austenite crystal structures, thus affecting the subsequent isochronal or isothermal martensite to austenite phase transformation mechanisms. In the present study, the effect of the non-homogeneous as-built microstructure on the martensite-to-austenite reversion phenomena was studied for a 18Ni maraging steel fabricated by laser-based powder bed fusion. In-situ synchrotron X-ray diffraction was used to retrieve the austenite volume fraction and lattice parameter evolution during the physical simulation of continuous heating cycles to the austenitic field; and during isothermal tempering cycles throughout the inter-critical tempered martensite + austenite (α’ + γ) field. The as-built microstructure resulted in the expansion of the inter-critical α’ + γ field during very slow heating rates. This was associated to the synergic effects of compositional segregations (anticipating reversion) and pre-existing retained austenite (delaying solubilization). During conventional inter-critical tempering, the as-built microstructure did not fundamentally alter the austenite reversion kinetics, resulting in similar high temperature microstructures at the end of the isothermal stage relative to the solution treated state.

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

Abstract: 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 and into directions.

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

Abstract: 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 HV02, ultimate tensile strength of 1686 MPa, and elongation of 115%, respectively The microstructural mechanisms accounting for the property changes are discussed in detail

Optimization of direct aging temperature of Ti free grade 300 maraging steel manufactured using laser powder bed fusion (LPBF)

Abstract: Maraging steels are of interest to the tool and die industry owing to their high strength, toughness and machinability. Additive manufacturing technologies like laser powder bed fusion (LPBF) can result in a paradigm shift in the design of maraging steel tools. The lack of precipitates, in combination with solute segregation and non-equilibrium microstructure in Grade 300 maraging steel fabricated via LPBF makes the steel amenable to strengthening via direct aging post fabrication instead of the conventional solution treatment and aging. In this study we have focused on optimizing the direct aging temperature for a Ti-free Grade 300 maraging steel fabricated via LPBF for two different aging times. Through strain hardening analysis and detailed microstructural characterization, we show that direct aging at a temperature of 440 ∘C for 6 h resulted in the best strength-ductility combination. Aging samples at a lower temperature or shorter time resulted in no strain hardening prior to necking as a result of lower fraction of reverted austenite, whereas aging samples at a higher temperature resulted in extensive recrystallization of martensite, coarsening of precipitates, and extensive austenite reversion, resulting in softening of the fabricated parts.

High pressure torsion processing of maraging steel 250: Microstructure and mechanical behaviour evolution

Abstract: Maraging steels are precipitation strengthened martensitic steels with an unusual combination of strength and ductility. High Pressure Torsion (HPT) has been used in this study to produce maraging steel 250 grade (AMS 6512) with finer laths and higher dislocation density, both of which act as nucleation sites for precipitation, and reverted austenite formation. This study focusses on the effect of such a processing on the evolution of the microstructure, including kinetics of precipitation, recrystallisation and austenite reversion as well as the stability of the precipitates thus formed. It was found that the aging kinetics accelerated substantially in the HPT processed samples, by achieving peak aging conditions at considerably shorter temperature/time durations and also a peak hardness higher than the as-received sample by 41%. Detailed microstructural characterisation revealed a change in the precipitate morphology from spherical to plate like form in the overaged conditions. The impact of this on mechanical response of these steels was quantified using tensile tests. A 70% increase in ultimate tensile strength was achieved in HPT processed samples after peak aging. Changes in strength and ductility were correlated to the changes in the microstructure and their impending contributions to different strengthening mechanisms at play to enable better design of maraging steels.