The enormous magnitude of 2 billion tons of alloys produced per year demands a change in design philosophy to make materials environmentally, economically, and socially more sustainable. This disqualifies the use of critical elements that are rare or have questionable origin. Amongst the major alloy strengthening mechanisms, a high-dispersion of second-phase precipitates with sizes in the nanometre range is particularly effective for achieving ultra-high strength. Here, we propose an alternative segregation-based strategy for sustainable steels, free of critical elements, which are rendered ultrastrong by second-phase nano-precipitation. We increase the Mn-content in a supersaturated, metastable Fe-Mn solid solution to trigger compositional fluctuations and nano-segregation in the bulk. These fluctuations act as precursors for the nucleation of an unexpected α-Mn phase, which impedes dislocation motion, thus enabling precipitation strengthening. Our steel outperforms most common commercial alloys, yet it is free of critical elements, making it a new platform for sustainable alloy design.

An investigation was carried out to examine the influence of as-cast heterogeneities on the resulting microstructure and mechanical properties of an austempered ductile cast iron (ADI). A commercial hypereutectic ductile iron alloy with 3.59% C, 2.68% Si, 0.24% Mn and 0.46% Cu (in wt. %) was produced in a coreless induction furnace. The specimens were obtained by casting the alloy into Y-block molds. The austempering heat treatments consisted of pre-heating at 500°C, followed by austenitizing step at 900°C and 930°C during 60 minutes. Austempering was carried out in molten metal baths at temperatures of 300°C and 370°C for 30 minutes. Microstructural characterization was carried out by light optical microscopy (LOM) with image analysis, scanning electron microscopy (SEM-FEG) and X-ray diffraction with Rietveld refinement. The mechanical properties were evaluated by tensile and Vickers hardness tests. The results of this investigation indicate that as-cast heterogeneities have a detrimental effect on the transformed microstructure and mechanical properties of ADI. During austempering, non uniform distribution of graphite nodules and segregation of alloying elements at cell boundaries resulted in transformation gradients throughout the metallic matrix. Intercellular regions displayed inhomogeneous microstructures containing eutectic carbides and martensite. Yield strength, tensile strength and hardness decrease for increasing austempering temperature. Fracture surfaces showed a rapid transition from ductile to cleavage mode at intercellular regions, containing solidification carbides.

Resumo O desgaste por cavitação representa um dos principais problemas encontrados em turbinas hidráulicas, sendo as áreas danificadas recuperadas com a deposição de revestimentos resistentes a cavitação pelo processo MIG/MAG. Este trabalho buscou caracterizar amostras obtidas após deposição de um revestimento inoxidável austenítico a base de cobalto sobre substrato de ASTM A-743 CA6NM, realizada por meio do processo MIG mecanizado, avaliando as microestruturas obtidas com auxílio das técnicas de microscopia ótica, MEV/EDS, EBSD, DRX e Microdureza. Pode-se identificar a presença de microestrutura austenítica na superfície depositada, com tamanho de grão médio de 11,27 μm e traços de ferrita delta próximos a linha de fusão, com substrato majoritariamente martensítico. Acima da ZF a morfologia da microestrutura se deu em forma colunar na direção da extração de calor. O aporte térmico não afetou a superfície do revestimento, mantendo suas características químicas e de dureza, sem que houvesse uma diluição elevada do metal base no revestimento, se apresentando como uma excelente alternativa para utilização na recuperação de componentes cavitados, devido a sua baixa EFE e microestrutura que favorecem a absorção da energia proveniente do processo de cavitação.

Fish-eye defects are usually observed in the surface of ductile iron casting. Their formation mechanism is not fully understood and very little has been published concerning the subject. This paper investigates the sleeve material effect on fish-eye defect formation in ductile cast irons. Unburnt exothermic and insulating sleeves with different fluorine concentrations have been crushed and mixed in with green sand. These mixtures have been used as facing sands for the casting molds. The morphological and chemical characterization of the fish-eye defects have been performed by light optical microscopy (LOM) and scanning electron microscopy with energy-dispersive spectroscopy (SEM/EDS). Experimental results suggest that surface fish-eye defects on ductile cast irons are caused by the buildup of fluorine-containing sleeve materials in the molding sand.

This work aims to evaluate the effect of the number of welding passes and the thickness of ASTM A743 CA6NM martensitic stainless-steel plates on joints welded by the TIG process. Plates with 5 mm and 15 mm thickness were welded in a single pass and in eight passes, respectively. The filling material used was the ER410NiMo alloy, in the form of rods with a 2.4 mm diameter. Before the welding passes, the samples were preheated up to 150 °C. The characterization of the welded samples was performed with the aid of optical microscopy, scanning electron microscopy (SEM-FEG), electron backscatter diffraction (EBSD), and Vickers hardness. The experiments revealed that the sample welded in a single pass presented a higher amount of delta ferrite, with a network morphology, preferably located inside the weld bead and in the high-temperature (HT) heat-affected zone (HAZ). In the sample welded in eight passes, the delta ferrite presented different morphologies, preferably observed in the inter-pass regions, inside the weld beads, and in the high-temperature HAZ. Overheating in the inter-pass region resulted in HAZ width variation, which increased towards the bottom of the welded joint.

A indústria metalúrgica tem buscado empregar processos de fusão mais eficientes. Os fornos rotativos a gás natural e óleo de xisto surgem como uma alternativa interessante para a produção de ligas ferrosas. Durante a fusão no forno rotativo acontece uma maior queima de elementos químicos, exigindo a utilização de técnicas de carburação adequadas. No presente trabalho foram avaliadas diferentes formas de adição de carburantes durante a fusão de ferros fundidos cinzentos e nodulares, utilizando fornos rotativos a óleo de xisto e gás natural. Dois tipos de carburantes, com diferentes granulometrias, foram adicionados em diferentes estágios do processo de fusão. Os resultados mostram que o rendimento de carburação é fortemente afetado pelo estado da carga metálica. O melhor rendimento foi obtido pelo carburante de coque de petróleo calcinado com granulometria de 1-3 mm, por meio da adição à carga no estado pastoso.