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Recently, it has been reported that the wear resistance of different stainless steel grades can be substantially improved by addition of boron as alloying element, however, with a trade-off in the corrosion resistance. In this work, CALPHAD method was employed to design a ferritic and an austenitic stainless steel modified with 1 wt pct B with high corrosion resistance. The designed alloys were produced by conventional casting in graphite mold and characterized by X-ray diffraction (XRD), optical microscopy and scanning electron microscopy (SEM). Structural characterization revealed that the target microstructures composed of ferrite and austenite matrixes with hard borides were successfully achieved. Their corrosion resistance was evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests in synthetic seawater and compared to conventional 316 L stainless steel. It was shown that designing the chemical composition of the alloys’ matrixes is the key to obtain corrosion resistant boron-modified stainless steels.

Cast Al(5.5 to 6.3 wt pct)Si-(3.1 to 3.7 wt pct)Cu alloys containing 0.6 and 1.2 wt pct Fe were processed by spray forming, rotary swaging, and homogenization/solution heat treatment, which led to a tensile ductility of 16 pct. Such outstanding ductility is ascribed to the microstructure formed: globular silicon particles, refined iron-rich intermetallics, and a supersaturated solid solution of Cu in the α-Al matrix that promoted dynamic strain aging and homogenization of the deformation process.

Spray forming is an advanced casting process that produces refined and homogenous microstructure directly from the liquid metal regardless of the alloy system. However, the microstructure evolution during spray forming is complex because the process comprises two sequential steps with very different cooling rates, i.e., atomization and deposition. It is well known that the microstructure of cast irons is highly dependent on the chemical composition and the cooling rate imposed to the liquid. In order to better understand the microstructural evolution during solidification by spray forming, this study investigated the solidification of two cast irons with different stable–metastable eutectic temperature interval (ΔTES−M), 30 K and 17 K. The microstructures of both overspray powders presented a dendritic array of austenite with cementite in the inter-dendritic spacing. Despite the high cooling rates imposed to the alloys during the atomization step, the final microstructure was defined by the cooling conditions prevailing during the final step of deposit solidification and stable eutectic was formed. This was ascribed to the dynamic process involving heating and remelting of the low melting temperature phases present in the droplets that arrives completely or partially solid in the deposition zone.

The wear resistance of boron-modified supermartensitic stainless steel coatings produced by high-velocity oxygen fuel (HVOF) was investigated through pin-on-disk measurements. It was shown that addition of boron leads to the formation of an interconnected and rigid boride network delimiting the grain boundaries of the martensitic matrix. A very refined structure was formed as result of the high cooling rates imposed to the molten alloy during the HVOF process. The specific wear rates of the HVOF coatings were about tenfold lower than the boron-free supermartensitic stainless steel, lying in the order of 10−5 mm3/N m. The refined boride skeleton along the HVOF coating was found to be effective to reduce the materials’ removal from the exposed softer martensitic matrix. While the supermartensitic stainless steel master alloy and the mild steel substrate displayed severe adhesive wear, the HVOF coatings exhibited mild delamination wear at low sliding velocities (10 and 20 cm/s) and abrasive wear at the highest tested velocity (40 cm/s). The studied boron-modified supermartensitic HVOF coatings are an interesting approach to protect the surface of inexpensive steel substrates against wear.

In this work, boron-modified duplex stainless steels were prepared by spray forming using design guidelines provided by thermodynamic calculations. Firstly, an investigation of stable phases and phase formation sequence in duplex steels containing high levels of boron was conducted. The calculation indicated that there was an eutectic point at around 1 wt% boron with different primary phase formations upon equilibrium solidification. For hypoeutectic compositions, the primary phase was δ-Ferrite, whereas for hypereutectic a metallic boride (M2B) should form. Additionally, eutectic reactions for both compositions should lead to the formation of borides M2B and M3B2. Secondly, spray forming experiments were conducted based on the thermodynamic calculations. Sample preparation was carried out using a conventional superduplex steel (2507) as starting material. Two different compositions were selected: one hypoeutectic (0.8 wt% B) and one hypereutectic (2.5 wt% B). The microstructural investigation revealed the formation of different types of borides embedded in an austenitic-ferritic matrix. Finally, the wear resistance was evaluated with the dry sand/rubber wheel test and a significant improvement was observed for boron-containing steels in comparison with the same steel without boron. This improvement was attributed to the presence of fine and well-distributed boride particles that protected the austenitic-ferritic matrix from material removal.

Thermodynamic calculations in equilibrium condition were used to investigate the phase formation sequence and microstructure formation in a boron-modified ferritic stainless steel. The results were also compared with spray forming experimental heats. The thermodynamic calculations indicated that there is an eutectic point at around 1.1 wt.% B and therefore two different compositions (hypo and hypereutectic) were experimentally investigated. For the hypoeutectic composition, the primary phase to form from the melt was δ-ferrite with the boride phase M2B originating eutectically. Conversely, for the hypereutectic composition, the boride M2B was formed as the primary phase. The prediction of phase formation and stable phases at room temperature was accurate. The calculated melting temperatures showed considerable deviation from values measured by Differential Scanning Calorimetry. The final spray formed microstructure and the phase fractions were to some extent in accordance with the thermodynamic calculations.

Phase formation was investigated via thermodynamic calculations of the Al-Si-Fe system within the range of commercial compositions with additions of Mn as a quaternary element. The calculations were performed using a commercial aluminum thermodynamic database and were compared with experimental data on sand casting from the literature. Both equilibrium and Scheil–Gulliver models were analyzed. The results showed that by adding Mn to the alloy it was possible to favor the formation of the Chinese script α-Al15(Fe,Mn)3Si2 intermetallic instead of the harmful platelet-shaped β-Al5SiFe. Furthermore, the calculations indicated that with higher amounts of Mn, the β-Al5SiFe intermetallic can be formed via a solid-state reaction, while α-Al15(Fe,Mn)3Si2 becomes the primary phase to form from the melt (also known as “sludge” in the aluminum foundry industry). A relationship between Fe and Mn contents was identified that can prevent the formation of the β-Al5SiFe intermetallic.

Spray-formed boron-modified supermartensitic stainless steel (SMSS) grades are alloys developed to withstand severe wear conditions. The addition of boron to the conventional chemical composition of SMSS, combined with the solidification features promoted by the spray forming process, leads to a microstructure composed of low carbon martensitic matrix reinforced by an eutectic network of M2B-type borides, which considerably increases the wear resistance of the stainless steel. Although the presence of borides in the microstructure has a very beneficial effect on the wear properties of the alloy, their effect on the corrosion resistance of the stainless steel was not comprehensively evaluated. The present work presents a study of the effect of boron addition on the corrosion resistance of the spray-formed boron-modified SMSS grades by means of electrochemical techniques. The borides fraction seems to have some influence on the repassivation kinetics of the spray-formed boron-modified SMSS. It was shown that the Cr content of the martensitic matrix is the microstructural feature deciding the corrosion resistance of this sort of alloys. Therefore, if the Cr content in the alloy is increased to around 14 wt pct to compensate for the boron consumed by the borides formation, the corrosion resistance of the alloy is kept at the same level of the alloy without boron addition.

The microstructure formation and wear resistance of a superduplex stainless steel modified with the addition of 3 wt% boron produced by spray forming were investigated. Thermodynamic simulations were used as comparison basis and to explain the experimentally observed microstructure, which was composed by primary M2B-type borides, an austenitic-ferritic matrix, and eutectic M3B2-type borides. The predicted solidification sequence started with the precipitation of primary M2B boride, followed by ferrite/austenite formation and a final eutectic reaction resulting in M3B2 borides. A good correlation with the simulations and final microstructure was found. The abrasive wear resistance was investigated with the dry sand/rubber wheel test and the results indicated an outstanding performance, similar to the cobalt-based Stellite 1016 alloy. The excellent wear resistance resulted from the presence of a significant amount (about 35 vol%) of hard borides homogeneously dispersed in the microstructure, which was effective at increasing hardness and protecting the duplex matrix against abrasion.

Solidification in spray-forming is still an open discussion in the atomization and deposition area. This paper proposes a solidification model based on the equilibrium solidification path of alloys. The main assumptions of the model are that the deposition zone temperature must be above the alloy’s solidus temperature and that the equilibrium liquid fraction at this temperature is reached, which involves partial remelting and/or redissolution of completely solidified droplets. When the deposition zone is cooled, solidification of the remaining liquid takes place under near equilibrium conditions. Scanning electron microscopy (SEM) and optical microscopy (OM) were used to analyze the microstructures of two different spray-formed steel grades: (1) boron modified supermartensitic stainless steel (SMSS) and (2) D2 tool steel. The microstructures were analyzed to determine the sequence of phase formation during solidification. In both cases, the solidification model proposed was validated.