The combination of high strength and formability of transformation induced plasticity (TRIP) steels is interesting for the automotive industry. However, the poor weldability limits its industrial application. This paper shows the results of six low-alloy TRIP steels with different chemical composition which were studied in order to correlate retained austenite (RA) and non-metallic inclusions (NMI) with their resistance spot welded zones to their joints’ final mechanical properties. RA volume fractions were quantified by X-ray microdiffraction (µSXRD) while the magnetic saturation technique was used to quantify NMI contents. Microstructural characterization and NMI of the base metals and spot welds were assessed using scanning electron microscopy (SEM). Weld nuggets macrostructures were identified using optical microscopy (OM). The lap-shear tensile test was used to determine the final mechanical properties of the welded joints. It was found that NMI content in the fusion zone (FZ) was higher than those in the base metal and heat affected zone (HAZ). Whereas, traces of RA were found in the HAZ of highly alloyed TRIP steels. Lap-shear tensile test results showed that mechanical properties of spot welds were affected by NMI contents, but in a major way by the decomposition of RA in the FZ and HAZ.
Three quaternary Al-6Si-3Cu-xMg (x= 0.59, 3.80, and 6.78 wt.%) alloys were produced by melt-spun and characterized using X-ray diffractometry (XRD), transmission electron microscopy (TEM), and microhardness techniques. Obtained second phases were for the alloy with 0.59% Mg and (Q) for the alloys with 3.80 and 6.78% Mg. These phases are present as 30–50 nm or as 5–10 nm nanoparticles. Alloying elements content in solid solution increased, mainly for Si and Mg. The high alloying elements content in solid solution and the small -Al cell size for melt-spun alloys leads to microhardness values about 2 times higher than those of ingot counterparts. The microhardness increase for melt-spun alloys with 3.80 and 6.78% Mg depends on Mg content in solid solution.
ResumenEste trabajo investiga el efecto de los ciclos térmicos sobre la microestructura de un acero inoxidable superdúplex específicamente sobre la formación de fase sigma. Los ciclos térmicos examinados son similares a los que se producen en la zona afectada térmicamente del acero inoxidable cuando se aplica el proceso de soldadura GTAW. Las temperaturas y tiempo de permanencia para el ciclo térmico se determinan usando un modelo de distribución de temperatura típico. La aplicación de los ciclos térmicos permite conocer la evolución microestructural del acero en el rango de 475°C a 1100°C con un tiempo corto de calentamiento y determinar la temperatura de formación y disolución de la fase sigma, pasando por su temperatura de máxima formación. Se examina la formación preferencial y la composición química de la fase sigma.Palabras-claves: fase sigma, ciclos térmicos, acero inoxidable superdúplex, zona afectada térmicamente.Abstract: This work investigates the effect of thermal cycling on the microstructure of a stainless steel superduplex, specifically on the sigma phase formation. The examined thermal cycles are similar to those produced in the heat affected zone of stainless steel when applying GTAW welding process. The temperatures and residence time for the cycle was determined using a typical thermal model of temperature distribution. The application of thermal cycles shows the microstructural evolution of steel in the range of 475°C to 1100°C with a short heating time and determines the temperature of formation and disolution of the sigma phase, and its maximum temperature of formation. The formation mechanism and chemical composition of the sigma phase is also examined.Key-words: sigma phase, thermal cycle, stainless steels superduplex, heat affected zone. IntroducciónLos aceros inoxidables dúplex tienen una microestructura de fase ferrita y fase austenita lo que les da ventajas sobre otros tipos de aceros inoxidables [1] como son excelente resistencia a la corrosión, alta tenacidad, buena soldabilidad y gran resistencia a la tensión [2]. Sin embargo, la resistencia a la corrosión y las propiedades mecánicas de los aceros inoxidables dúplex son sensiblemente afectadas cuando estas aleaciones son soldadas, debido a los ciclos térmicos de calentamiento y enfriamiento a que el acero es sometido [3]. Un ciclo térmico de soldadura, puede propiciar la formación de fases intermetálicas, especialmente en la zona afectada térmicamente (ZAT). La microestructura de la ZAT es determinada por los ciclos térmicos impuestos por el proceso de soldadura, los cuales dependen de las variables de soldadura [4].Una de las fases intermetálicas que se puede formar por la aplicación de un ciclo térmico es la fase sigma, la cual perjudica notablemente la resistencia a la corrosión de los aceros inoxidables superdúplex [1,5] ya que remueve el cromo que se encuentra en solución sólida en la aleación, disminuyendo la formación de la capa protectora de oxido de cromo [6].La fase sigma nuclea en la interfase ferrita-austenita en los pe...
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