Estudio del efecto de adiciones de titanio en las propiedades mecánicas y de corrosión del acero pulvimetalúrgico AISI 316

Context: Powder metallurgy uses metallic and/or non-metallic powders that, through mixing, compacting, and sintering operations, allow obtaining large series of products. In austenitic stainless steels, Cr23C6-type carbides can precipitate at temperatures between 450 and 950 °C. When this occurs, the steel is susceptible to being attacked at its grain boundaries by a phenomenon called sensitization. Titanium is added as a ‘stabilizer’ because it has a greater affinity with carbon for the formation of species at a temperature of approximately 900 ºC, and, during cooling, it consumes the carbon forming MC-type carbides, inhibiting the precipitation of Cr23C6. Method: The composition and morphology of the powders were characterized, leading to the formulation of an alloy matrix consisting of a mixture of AISI 316 steel powders of two different particle-size distributions in a proportion that produced the highest density and the lowest porosity in the sintered material. Titanium was added at two levels (0,4 and 1,0 wt%), and sintering was carried out with nitrogen. The corrosion rate was determined by potentiodynamic polarization. Vickers hardness and pin-on-disk wear tests were performed. The stages were complemented with a microstructural analysis. Results: The addition of 0,4 wt% of titanium decreased the steel’s rate of corrosion, albeit in the absence of passivation. The microstructure consists of austenite, ferrite, and TiC precipitates. The addition of 1,0 wt%Ti showed an increase in the corrosion rate, with a microstructure containing austenite, ferrite, TiC carbides, and the Laves ɳ-Fe2Ti phase. Conclusions: The results were compared against thermodynamic simulations in the Thermo-Calc software, which were consistent with the microstructural analysis, showing the phenomena of stabilization as well as the precipitation of intermetallic phases and highlighting the importance of establishing strict controls in the formulation of powder metallurgical alloys due to the transformations that can take place due to the effect of the thermal cycles of the process.Contexto: La pulvimetalurgia utiliza polvos metálicos y/o no metálicos que, mediante operaciones de mezcla, compactación y sinterización, permiten obtener grandes series de productos. En los aceros inoxidables austeníticos, los carburos de tipo Cr23C6 pueden precipitar a temperaturas entre 450 y 950 °C. Cuando esto ocurre, el acero es susceptible de ser atacado en sus límites de grano por el fenómeno denominado sensitización. El titanio se agrega como ‘estabilizador’ porque tiene mayor afinidad con el carbono para la formación de especies a una temperatura de aproximadamente 900 ºC y, durante el enfriamiento, consume el carbono formando carburos tipo MC, inhibiendo la precipitación de Cr23C6. Método: Se caracterizó la composición y morfología de los polvos, lo que condujo a la formulación de una matriz de aleación compuesta por una mezcla de polvos de acero AISI 316 de dos granulometrías diferentes en una proporción que produjo la mayor densidad y la menor porosidad del material sinterizado. Se añadió titanio en dos niveles (0,4 y 1,0 % en peso) y se sinterizó con nitrógeno. La velocidad de corrosión se determinó por polarización potenciodinámica. Se realizaron pruebas de dureza Vickers y de desgaste pin-on-disk. Las etapas se complementaron con un análisis microestructural. Resultados: La adición de 0,4 % en peso de titanio disminuyó la velocidad de corrosión del acero, si bien en ausencia de pasivación. La microestructura consiste en precipitados de austenita, ferrita y TiC. La adición de 1,0 % en peso de Ti mostró un aumento en la velocidad de corrosión, con una microestructura que contenía austenita, ferrita, carburos de TiC y la fase Laves ɳ-Fe2Ti. Conclusiones: Los resultados fueron comparados con simulaciones termodinámicas del software Thermo-Calc, consistentes con el análisis microestructural, mostrando los fenómenos de estabilización y la precipitación de fases intermetálicas, y destacando la importancia de establecer un control estricto en la formulación de aleaciones pulvimetalúrgicas debido a las transformaciones que pueden generarse por efecto de los ciclos térmicos del proceso

Study Of the Effect of Titanium Additions on The Mechanical and Corrosion Properties of AISI 316 Powder Metallurgical Steel

Context: Powder metallurgy uses metallic and/or non-metallic powders that, through mixing, compacting, and sintering operations, allow obtaining large series of products. In austenitic stainless steels, Cr23C6-type carbides can precipitate at temperatures between 450 and 950 °C. When this occurs, the steel is susceptible to being attacked at its grain boundaries by a phenomenon called sensitization. Titanium is added as a ‘stabilizer’ because it has a greater affinity with carbon for the formation of species at a temperature of approximately 900 ºC, and, during cooling, it consumes the carbon forming MC-type carbides, inhibiting the precipitation of Cr23C6. Method: The composition and morphology of the powders were characterized, leading to the formulation of an alloy matrix consisting of a mixture of AISI 316 steel powders of two different particle-size distributions in a proportion that produced the highest density and the lowest porosity in the sintered material. Titanium was added at two levels (0,4 and 1,0 wt%), and sintering was carried out with nitrogen. The corrosion rate was determined by potentiodynamic polarization. Vickers hardness and pin-on-disk wear tests were performed. The stages were complemented with a microstructural analysis. Results: The addition of 0,4 wt% of titanium decreased the steel’s rate of corrosion, albeit in the absence of passivation. The microstructure consists of austenite, ferrite, and TiC precipitates. The addition of 1,0 wt%Ti showed an increase in the corrosion rate, with a microstructure containing austenite, ferrite, TiC carbides, and the Laves ɳ-Fe2Ti phase. Conclusions: The results were compared against thermodynamic simulations in the Thermo-Calc software, which were consistent with the microstructural analysis, showing the phenomena of stabilization as well as the precipitation of intermetallic phases and highlighting the importance of establishing strict controls in the formulation of powder metallurgical alloys due to the transformations that can take place due to the effect of the thermal cycles of the process

Effect of porosity and eutectics on the high-temperature low-cycle fatigue Performance of a nickel-base single-crystal superalloy

This work investigates the separate influence of porosity and γ/γ′-eutectics on the low-cycle fatigue life of a single-crystal Ni-base superalloy at high temperatures. A conventional vacuum furnace heat-treatment but also integrated heat-treatments in a hot isostatic press are applied to produce different material variants of the same alloy. High-resolution electron microscopy revealed that both pores and γ/γ′-eutectics act as crack starters, thus initiating early failure. Moreover, the results indicate that remaining γ/γ′-eutectics can weaken the fatigue resistance even more than pores. Furthermore, the results confirm the beneficial effect of proper integrated hot isostatic pressing heat-treatments on the fatigue Performance

Thermodynamic and Ab Initio Design of Multicomponent Alloys Based on (Fe₅₀Mn₃₀Co₁₀Cr₁₀)-xBx (x = 0, 5, 7, 10, and 15 at.%)

Multicomponent alloys have attained general interest in recent years due to their remarkable performance. Non-equiatomic alloys with boron addition as an interstitial element are being studied, exhibiting outstanding mechanical properties. In order to estimate the mechanical behavior of potential alloys, thermodynamic and ab initio calculations were utilized in this work to investigate phase stability and stacking fault energy (SFE) for (Fe50Mn30Co10Cr10)-xBx (x = 0, 5, 7, 10, and 15 at.%) systems. Thermodynamic experiments revealed two structural variations of borides, M2B(C16) with a tetragonal structure and M2B(CB) with an orthorhombic structure. Borides precipitate when boron content increases, and the FCC matrix becomes deficient in Mn and Cr. According to ab initio calculations, the presence of boron in the FCC and HCP structures primarily disrupts the surroundings of the Fe and Mn atoms, resulting in an increased distortion of the crystal lattice. This is related to the antiferromagnetic condition of the alloys. Furthermore, for alloys with a low boron concentration, the stacking fault energy was found to be near 20 mJ/m2 and greater than 50 mJ/m2 when 10 and 15 at.% boron was added. As boron concentrations increase, M2B borides are formed, generating changes in the matrix composition prone to fault-induced phase transitions that could modify and potentially impair mechanical properties

CCorrosion resistance of nitrogenated high-carbon martensitic stainless steel designed and produced at nitrogen low pressures

A new martensitic stainless steel with high nitrogen and carbon content at low pressures was designed using the CALPHAD method. The chemical composition of the steel was checked by optical emission spectrometry, obtaining 0.17 wt%-N and 1.33 wt%-C. Scanning electron microscopy (SEM) analyses were carried out for microstructural characterization. The properties of the steel were assessed by Rockwell hardness and potentiodynamic polarization tests in 0.6M NaCl. The steel showed maximum hardness values of 60 HRC, regarded as a hard material. However, the passive film formation was prevented by the high carbon content promoting excessive Cr-rich carbides

Reactivity of PVD cathodic arc coated hardmetal tools with Inconel 718: Correlation between diffusion couples and tool wear in drilling tests

The reactivity of Al0.66Ti0.33N, Al0.31Ti0.62Si0.07N, Al0.60Cr0.40N, and Al0.38Cr0.54Si0.07N cathodic arc-coated hardmetal tools with Inconel 718 has been evaluated by using diffusion couple experiments at 900 degrees C and 1100 degrees C. As expected, a strong reaction occurs at these temperatures between Inconel 718 and uncoated hard-metal samples leading to the formation of & eta; phases; mainly M12C and M6C carbides. PVD coatings act as diffusion barriers limiting the presence of such reactions as long as they maintain their structure in contact with Inconel 718 at high temperatures. Diffusion couple experiments confirm that Cr-containing coatings are significantly more unstable than Al0.66Ti0.33N or Al0.31Ti0.62Si0.07N materials, thus leading to notable differences in cutting performance. The relation between diffusion couple results and drill wear mechanisms in drilling Inconel 718 with different coating materials are also analysed. The best results are obtained either with Al0.66Ti0.33N or Al0.31Ti0.62Si0.07N coated drills, where the coating remains longer time at the cutting edge leading to more efficient protection, especially at the drill corner