The influence of minor additions of boron and the as-built (AB) microstructure on stress-rupture behavior of a modified crack-free Hastelloy X fabricated by laser powder bed fusion (L-PBF) was investigated. Isothermal stress rupture tests were performed at 816 degrees C under a static tensile load of 103 MPa. Micro-void formation in the vicinity of carbide precipitates and their coalescence was only observed at chevron-like high-angle grain boundaries, characteristic of L-PBF process. These grain boundaries, laying on the planes with maximum resolved shear stress with respect to the loading direction, directly governed the intergranular crack propagation. In view of the fracture mechanism and the time to rupture, increasing boron content significantly improves timeto-rupture through a diffusion-controlled mechanism by hindering the carbon diffusion to the grain boundaries. Adequate additions of boron (>10 ppm) guarantee the stress-rupture properties (strength) of the AB components without the need for additional post-thermal treatments. Further increase in boron content (i.e., 30 ppm), led to about five times increase in time to rupture (500 h vs. 110 h), and significantly improved creep elongation (30% vs. 9%) compared with the low boron alloy.

One generic challenge in powder bed fusion - laser beam (PBF-LB) is the formation of epitaxially grown columnar grains, which lead to the undesirable anisotropy of mechanical properties. This anisotropy could be rectified by ex-situ or in-situ inoculation in some particular alloy systems. Understanding the grain refinement mechanism caused by in-situ inoculation is, however, complicated by remelting caused by the overlapping between neighboring scan tracks, when printing bulk samples using multiple tracks. Here in this work, a series of single tracks using ferritic stainless steels feedstock powder with and without pre-alloyed inoculant-forming elements, were printed at different scanning speeds to gain refreshed understanding on the mechanism of the observed grain refinement. Interestingly, the grain refinement in single tracks and bulk samples printed from the powder with and without inoculant-forming elements showed an opposite tendency. When using the powder without inoculant-forming elements, the single tracks showed large columnar grains, while the bulk samples showed even larger grain sizes; when using the powder with pre-alloyed inoculant-forming elements, fine equiaxed grains are found at the centers of the melt pools, surrounded by slightly coarser columnar grains at melt pool boundaries, in both single tracks and bulk samples. Noticeably, the mean grain sizes in the bulk samples are however smaller compared to those for single tracks because of remelting. Our work provides new insights on the grain refinement via in-situ inoculation during the PBF-LB process and highlights the importance of studying single tracks to better understand the melting and solidification behavior.

Solidification during fusion-based additive manufacturing (AM) is characterized by high solidification velocities and large thermal gradients, two factors that control the solidification mode of metals and alloys. Using two synchrotron-based, in situ setups, we perform high-speed X-ray diffraction measurements to investigate the impact of the solidification velocities and thermal gradients on the solidification mode of a hot-work tool steel over a wide range of thermal conditions of relevance to AM of metals. The solidification mode of primary delta-ferrite is observed at a cooling rate of 2.12 x 104 K/s, and at a higher cooling rate of 1.5 x 106 K/s, delta-ferrite is sup-pressed, and primary austenite is observed. The experimental thermal conditions are evaluated and linked to a Kurz-Giovanola-Trivedi (KGT) based solidification model. The modelling results show that the predictions from the multicomponent KGT model agree with the experimental observations. This work highlights the role of in situ XRD measurements for a fundamental understanding of the microstructure evolution during AM and for vali-dation of computational thermodynamics and kinetics models, facilitating parameter and alloy development for AM processes.

The microstructure response of laser-powder bed fusion (L-PBF)-processed ferritic stainless steel (AISI 441) during post-heat treatments is studied in detail. Focus is on the precipitation kinetics of the Nb-rich phases: Laves (Fe2Nb) and the cubic carbo-nitride (NbC), as well as the grain structure evolution. The evolution of the precipitates is characterized using scanning and transmission electron microscopy (SEM and TEM) and the experimental results are used to calibrate precipitation kinetics simulations using the precipitation module (TC-PRISMA) within the Thermo-Calc Software package. The calculations reproduce the main trend for both the mean radii for the Laves phase and the NbC, and the amount of Laves phase, as a function of temperature. The calibrated model can be used to optimize the post-heat treatment of additively manufactured ferritic stainless steel components and offer a creator tool for process and structure linkages in an integrated computational materials engineering (ICME) framework for alloy and process development of additively manufactured ferritic steels.

Duplex stainless steels are a group of widely used stainless steels, because of their attractive combination of strength and corrosion resistance. However, these steels embrittle because of a phase separation phenomenon in the ferrite phase when exposed to temperatures within the miscibility gap. This manuscript investigates the phase separation in two commercial stainless steels, the duplex stainless steel (DSS) 22Cr-5Ni (2205 or UNS S32205), and the super-duplex stainless steel (SDSS) 25Cr-7Ni (2507 or UNS S32750), and its subsequent effect on mechanical property evolution. Long-term isothermal aging heat treatments were carried out at industrially relevant temperatures between 250 °C and 350 °C for up to 48,000 hours, and quantitative measurements of the amplitude and wavelength of the phase separated nanostructure were obtained using Small-Angle Neutron Scattering (SANS). These quantifications were used as input parameters in hardness models to predict the hardness evolution. It is concluded that the quantitative information from SANS combined with these hardness models enables the prediction of hardness evolution in DSS at low temperatures, which in turn correlates with the embrittlement of the DSS. 

For ferritic stainless steels, TiN has effectively been used as an inoculant to produce equiaxed grain structures in casting and welding. However, it is not established whether TiN would be an effective inoculant in additive manufacturing. In this study, the effectiveness of TiN as an inoculant in a ferritic stainless steel processed by laser powder-bed fusion is studied. An alloy without Ti is fabricated and compared to an alloy designed to form a high amount of TiN early during solidification. The work shows that the presence of TiN provides general grain refinement and that TiN-covered oxide particles are effective in enabling columnar-to-equiaxed transition in certain regions of the meltpool. The applied approach of pre-alloying powders with inoculant-forming elements offers a straightforward route to achieving fine, equiaxed grain structures in additively manufactured metallic materials. It also shows how oxygen present during the process can be utilized to nucleate effective inoculating phases.

We report on the fabrication of a Co-free ultra-high strength maraging steel designed for the laser-powder bed fusion (L-PBF) process using computational approaches. The aim was to obtain an essentially martensitic microstructure after the L-PBF process, and to achieve an ultimate tensile strength (UTS) of minimum 1700 MPa after direct ageing of the as-built microstructure. Microstructural characterization, dilatometry, and tensile and impact toughness tests were used to evaluate the employed approach.

The microstructure of a hot-work tool steel additively manufactured using laser powder-bed fusion (L-PBF), and its response to post heat treatment, is studied in detail by microstructure characterization and computational thermodynamics and kinetics. The high solidification and cooling rates during the L-PBF process lead to suppression of delta-ferrite and instead solidification of an austenite phase directly containing a cellular substructure where the alloying elements have segregated to the inter-cellular regions and where solidification carbides have formed in the cell junctions. The austenite is then partly decomposed into martensite at lower temperatures. The micro-segregation can be predicted by reducing the complex solidification behavior to a diffusion problem in one dimension enabling detailed comparisons with the measured segregation profiles quantified at a nanometer scale. Martensite start temperature (M-s) calculations along the spatially varying composition show that the M-s temperature decreases in the inter-cellular regions where austenite is observed. The network of austenite in the as-built microstructure can be understood from the combined influence of the composition dependence of the M-s temperature in relation to the build plate temperature and the mechanical stabilization of the small-sized austenite regions. This work demonstrates the power of computational tools based on computational thermodynamics and kinetics for designing tool steels for additive manufacturing by predictions of the steel's response to the L-PBF process and post heat treatments.

High-performance stainless steels are alloyed with nitrogen for several different reasons. For the austenitic grades, the kinetics of deleterious phase transformations are delayed, the strength is increased, and the corrosion resistance is improved. For duplex steels, the weldability and corrosion resistance depend directly on their nitrogen contents. Reliable databases are crucial for the development of new stainless steels and their processing. In order to calibrate the existing thermodynamic data, a series of experiments was undertaken. Several laboratory alloys in Fe–Cr–X systems, where X = Ni, Mn, Cu, were reacted with nitrogen gas at nitrogen activities 0.5 and 1 in the temperature range from 1050 °C to 1350 °C and subsequently analyzed for nitrogen solubility and phase constitution. A novel, simplified experimental technique was used, providing a dew point that is low enough to allow for fast nitrogen transport through the specimen surface, ensuring equilibrium conditions. The results are compared to thermodynamic equilibrium calculations. 

In this study, the ferritic stainless steel SS441 was produced with excellent mechanical properties using laser powder bed fusion (L-PBF) compared to samples produced by conventional casting and hot-rolling. In addition, thermodynamic calculations were utilized to study the phase stability at elevated temperatures and to understand the solidification behavior. The hot-rolled sample showed a grain size up to several hundred mu m with additional precipitates of TiN and Nb(C,N). In contrast, the as-built L-PBF samples displayed a grain size in the mu m range. Spherical precipitates with a size of around 50 nm could be observed and were attributed to a corundum phase from the thermodynamic calculations. The printed material shows superior mechanical properties, with more than 30 times higher impact energy compared to the hot-rolled alloy (217 +/- 5 J vs. 7 +/- 0.5 J). Furthermore, the properties are anisotropic for the L-PBF produced alloy, with the highest tensile strength vertical to the build direction. The superior mechanical properties of the L-PBF produced sample can be attributed to a smaller grain size, giving a higher strength according to the Hall-Petch relationship. The anisotropy of the material can be eliminated by heat treatments at 900 degrees C followed by water quenching, but the absolute strength decreases slightly due to formation of intermetallic phases such as Nb(C,N) and the Fe2Nb Laves phase. The results clearly illustrates that L-PBF provides a promising manufacturing mute for enhanced strength of ferritic stainless steels.