Classical controlled rolling of low C steels microalloyed with Ti and Mo

Two low-C steels, one microalloyed with Ti and the other with Ti and Mo were subjected to a classical controlled rolling schedule using torsion testing. The specific torsion schedules were a simulation of an industrial schedule adapted to the characteristic transformation temperatures of each steel. It is shown that such steels can develop a pancaked austenite, despite the fact that the alloys do not contain Nb. However, compared to Nb bearing steels, restoration between deformation passes is relatively rapid in the pancaking region. In order to reduce the loss of pancaking, the temperatures of the finishing rolling schedule were shifted to lower temperatures. This appeared to greatly reduce the restoration rates, but the transformation in the two phase region occurred towards the end of the final deformation pass. When comparing the two steels, it could be detected that Mo increases the temperature range for pancaking and reduces the restoration rates between deformation passes in the pancaking region.Peer ReviewedPostprint (published version

Characterization of strain-induced precipitation in Inconel 718 superalloy

© 2016 ASM International Inconel 718 presents excellent mechanical properties at high temperatures, as well as good corrosion resistance and weldability. These properties, oriented to satisfy the design requirements of gas turbine components, depend on microstructural features such as grain size and precipitation. In this work, precipitation-temperature-time diagrams have been derived based on a stress relaxation technique and the characterization of precipitates by scanning electron microscopy. By using this methodology, the effect of strain accumulation during processing on the precipitation kinetics can be determined. The results show that the characteristics of precipitation are significantly modified when plastic deformation is applied, and the kinetics are slightly affected by the amount of total plastic deformation.Peer ReviewedPostprint (author's final draft

Dynamic recrystallization mechanisms and twining evolution during hot deformation of Inconel 718

The hot deformation behavior of an IN718 superalloy was studied by isothermal compression tests under the deformation temperature range of 950–1100 °C and strain rate range of 0.001–1 s-1 up to true strains of 0.05, 0.2, 0.4 and 0.7. Electron backscattered diffraction (EBSD) technique was employed to investigate systematically the effects of strain, strain rate and deformation temperature on the subgrain structures, local and cumulative misorientations and twinning phenomena. The results showed that the occurrence of dynamic recrystallization (DRX) is promoted by increasing strain and deformation temperature and decreasing strain rate. The microstructural changes showed that discontinuous dynamic recrystallization (DDRX), characterized by grain boundary bulging, is the dominant nucleation mechanism in the early stages of deformation in which DRX nucleation occurs by twining behind the bulged areas. Twin boundaries of nuclei lost their ¿3 character with further deformation. However, many simple and multiple twins can be also regenerated during the growth of grains. The results showed that continuous dynamic recrystallization (CDRX) is promoted at higher strains and large strain rates, and lower temperatures, indicating that under certain conditions both DDRX and CDRX can occur simultaneously during the hot deformation of IN718.Peer ReviewedPostprint (author's final draft

Efecto de los elementos residuales e impurezas en la ductilidad y mecanismos de fragilización en caliente de un acero de construcción 0.23C-0.9Mn-0.13SI

El agrietamiento transversal en la superficie de los productos de colada continua es un problema que sigue provocando el rechazo de algunos de estos productos con las correspondientes pérdidas energéticas y económicas. A pesar de que el problema se ha conseguido minimizar para algunas calidades de acero, el reciclaje de chatarra, práctica cada vez más frecuente por sus beneficios económicos y medioambientales, está incorporando nuevos retos, especialmente en lo que se refiere a la aparición de grietas superficiales. El origen del agrietamiento para estos aceros se relaciona con el aumento en elementos residuales e impurezas que se introducen durante el reciclaje y son difíciles de eliminar.Para conocer la influencia de estos elementos residuales e impurezas en el agrietamiento transversal, se evaluó la ductilidad en caliente de un acero de construcción 0.23C-0.9Mn-0.13Si con Cu y Sn como residuales y alto S. La ductilidad en caliente fue evaluada a partir de la reducción del área (%RA) de probetas ensayadas a tracción a temperaturas entre 650ºC y 1100ºC. La velocidad de deformación elegida para los ensayos fue 5·10-3s-1. Estas condiciones están en el intervalo de las que se dan durante el desdoblado, etapa crítica para la aparición de grietas. Las curvas de %RA en función de la temperatura se completaron con el estudio fractográfico y metalográfico de las probetas.Durante los ensayos se varió la temperatura de austenización mediante recalentamientos a 1100ºC y a 1330ºC. Otra serie de ensayo consistió en la fusión y solidificación in-situ de las probetas. Además, se trabajó con el material en dos condiciones iniciales: colada (probetas extraídas de una palanquilla) y laminada (probetas extraídas de varilla corrugada). Un acero con una composición base similar pero sin elementos residuales fue evaluado con fines comparativos.Las curvas de ductilidad en caliente fueron muy parecidas independientemente de la temperatura de recalentamiento, sin embargo, cuando el acero se ensayó en su condición laminada el valle de ductilidad obtenido fue más estrecho. Por otro lado, los mecanismos de fragilización variaron con la temperatura de recalentamiento y no con la condición inicial del material. Las fracturas tras recalentamientos a 1100ºC fueron interdendríticas y se relacionaron con las microsegregaciones de solidificación. Recalentamientos a 1330ºC originaron fracturas interganulares.Las inclusiones de MnS, así como las segregaciones de S parecen tener un papel importante en este comportamiento. El efecto del S sólo puede ser evaluado adecuadamente cuando las probetas se solidifican in-situ para poner en solución el S que se encontrara formando MnS. Los resultados obtenidos mediante estas condiciones experimentales presentaron pozos de ductilidad más anchos que los obtenidos en condiciones de recalentamiento. Por otro lado, las fractografías muestran la gran tendencia al "hot shortness" que tiene el acero. Éste mecanismo de fragilización que actúa a muy altas temperaturas estaría debilitando los espacios interdendríticos a menores temperaturas. Al comparar el comportamiento del acero industrial con el del acero limpio, lo primero que se ve es que el pozo de ductilidad es significativamente más estrecho para éste último. Así, el único mecanismo de fragilización identificado consiste en la concentración de la deformación en una fina capa de ferrita que se forma rodeando la austenita a temperaturas entre Ae3 y Ar3. El cálculo de las microsegregaciones de solidificación indica que los elementos con más tendencia a enriquecer el último líquido en solidificar son el P y el S. Éste último además parece el responsable de la fragilidad del acero cuando se ensaya en condiciones de recalentamiento, tal y como se pudo determinar por espectroscopía Auger. A pesar de que no se ha podido demostrar, en Sn podría tener también un papel importante en las segregaciones intergranulares.Transverse cracking in the surface is a problem related to the continuous casting steelmaking route. Its incidence has been minimized for several steel grades, but it is very sensitive to operational or compositional variations. In particular, the production of steel from scrap, which has great economic and environmental advantages, has introduced new challenges. The high incidence of transverse cracking for these steel grades is related to their high contents on residual elements which are introduced during the recycling and are difficult to eliminate.The influence of residual elements and impurities on the transverse cracking susceptibility has been studied for a structural steel 0.23C-0.9Mn-0.13Si with high Cu and Sn as residuals, as well as high S. The evaluation was done by means of hot tensile tests. The reduction area (%RA) of the samples tested to fracture was taken as a measure of the hot ductility, and therefore, of the sensitivity of the steel to present transverse cracking. The analysis of the fracture surfaces and metallographies of the samples allowed the identification of the different embrittling mechanisms that could take place depending on the thermomechanical conditions. During the tests, samples were first reheated to 1100ºC or 1330ºC, or in- situ melted. Then, they were tested at temperatures ranging from 650ºC to 1100ºC. The strain rate was 5×10-3s-1, close to the ones that take place during the unbending operation in continuous casting. The steel was tested for two initial conditions of the material: the as-cast (samples were machined from a billet), and the as-rolled (samples were machined from a corrugated bar). Moreover, another steel residual free was evaluated for comparison purposes.The hot ductility curves (%RA vs. Temperature) were similar after different reheating treatments, but ductility troughs appeared narrower when the steel was tested in the as-rolled condition. However, fracture surfaces showed different features depending on the reheating temperature. On one side, the reheating treatment at 1100˚C promoted a mixture of intergranular and interdendritic brittle fracture. The interdendritic component of the fracture was related to microsegregations taking place during the solidification of the steel. On the other side, brittle samples tested after a reheating treatment at 1330˚C showed completely intergranular features.MnS inclusions, as well as S segregations, seemed to have a very important role in the hot ductility behaviour of the steel. Since S forms particles with high melting points, samples had to be cast in-situ in order to put all S back into solution and then evaluate its effect on the hot ductility. For samples cast in-situ, the ductility troughs are wider than the ones obtained for reheated samples. Moreover, the fractographies of in-situ melted samples showed the high tendency of the steel to embrittle through "hot shortness". This mechanism that would act at very high temperatures could also be the responsible of the brittle behaviour of the steels at testing temperatures.The ductility troughs for the clean steel were significantly narrower than the ones for the industrial steel at any testing condition. The only embrittling mechanism identified for the clean steel was the concentration of the deformations at a ferrite layer formed surrounding the austenite grains at temperatures between Ae3 and Ar3.The composition of the last solidifying liquid was calculated according to microsegregation models. The calculations showed that P and S are the elements with the highest tendency to microsegregate and thus, the last solidifying liquid is enriched in these elements. By means of Auger spectroscopy the S was proved to be the embrittling element under reheating conditions. Though it could not be verified, special attention should be paid to Sn due to its tendency to segregate intergranularly

Twin-induced plasticity of an ECAP-processed TWIP steel

The TWIP steels show high strain hardening rates with high ductility which results in high ultimate tensile strength. This makes their processing by equal channel angular pressing very difficult. Up to now, this has only been achieved at warm temperatures (above 200 °C). In this paper, a FeMnCAl TWIP steel has been processed at room temperature and the resulted microstructure and mechanical properties were investigated. For comparison, the material has also been processed at 300 °C. The TWIP steel processed at room temperature shows a large increase in yield strength (from 590 in the annealed condition to 1295 MPa) and the ultimate tensile strength (1440 MPa) as a consequence of a sharp decrease in grain size and the presence within the grains of a high density of mechanical twins and subgrains. This dense microstructure results also in a loss of strain hardening and a reduction in ductility. The material processed at 300 °C is more able to accommodate deformation and has lower reduction in grain size although there is a significant presence of mechanical twins and subgrains produced by dislocation activity. This material reaches an ultimate tensile strength of 1400 MPa with better ductility than the room temperature material.Postprint (published version

Stress-strain response and microstructural evolution of a FeMnCAl TWIP steel during tension-compression tests

© 2016. The stress-strain response of a Fe-17.5Mn-0.7C-2Al TWIP steel during cyclic loading has been investigated by means of tension-compression tests within the strain limits of ±2%, ±5% and ±10%. In addition, the microstructural evolution during the ±5% cyclic test has also been studied. The difference between the forward and reverse stress for each pre-strain has been analyzed at 0.2% offset strain and at the strains in which forward and reverse curves were parallel in order to study the Bauschinger effect (BE) and permanent softening, respectively. The evolution of the BE with pre-strain for this steel is similar to other FeMnC TWIP steels, that is, increasing values of BE are obtained as the pre-strain increases. However, its absolute values are half those reported in the literature on other FeMnC steels. This diminution of the BE is related to the lower activity of mechanical twinning in FeMnCAl TWIP steels at the pre-strains herein investigated, which promotes less polarized stresses in the matrix due to the lower dislocation storage capacity.Regarding permanent softening, the evolution is similar to that of the BE and the same analysis can be applied. During reverse compression, a slight increase of twin thickness and twin spacing with respect to the first tensile stage took place. This fact might be linked to the lower flow stress observed in the permanent softening period during reverse straining.Peer ReviewedPostprint (author's final draft

Nucleation and growth of precipitates in a V-microalloyed steel according to physical theory and experimental results

Using a theoretical model, the nucleus number and nucleation time were determined for a V-microalloyed steel. The calculated data has made it possible to plot the nucleus number vs. temperature, nucleation critical time vs. temperature, and precipitate critical radius vs. temperature. The nucleus number was calculated by integration of the nucleation rate expression. On the other hand, an experimental study was performed and the nucleation time vs. temperature was plotted (PTT diagram), thus allowing a comparison between the theoretical values and experimental results. It has been found that the growth of precipitates during precipitation obeys a quadratic growth equation and not a cubic coalescence equation. The experimentally determined growth rate coincides with the theoretically predicted growth rate. The experimental nucleation time is longer than the calculated time due to conceptual differences.Peer ReviewedPostprint (author's final draft

Theoretical and experimental nucleation and growth of precipitates in a medium carbon–vanadium steel

Using the general theory of nucleation, the nucleation period, critical radius, and growth of particles were determined for a medium carbon V-steel. Several parameters were calculated, which have allowed the plotting of nucleation critical time vs. temperature and precipitate critical radius vs. temperature. Meanwhile, an experimental study was performed and it was found that the growth of precipitates during precipitation obeys a quadratic growth equation and not a cubic coalescence equation. The experimentally determined growth rate coincides with the theoretically predicted growth ratePeer ReviewedPostprint (author's final draft

Novel method of severe plastic deformation - Continuous closed die forging: CP aluminum case study

There is a large number of methods for severe plastic deformation (SPD). Multidirectional forging (MDF) is probably one of the most easily scalable for industrial application. In general, two main conditions need to be fulfilled for successful SPD processing: constan t sample geometry and application of a quasi - hydrostatic pressure. The first condition is necessary for strain accumulation by repetitive deformation and the second one helps preventing cracking in the specimens with high accumulated strain. However, MDF i s not providing quasi - hydrostatic condition in the processed sample. This paper reports a novel method for severe plastic deformation, namely continuous close d die forging (CCDF), which fulfils both requirements for the successful deformation of samples to a very high accumulated strain. Commercially pure aluminum (1050) was processed to a total strain of 24 by CCDF. After processing, the microstructure was refined down to a mean grain size of 0. 78 µm . Tensile testing showed good mechanical properties: yiel d strength and ultimate tensile strength of the ultrafine - grained (UFG) aluminum were 180 and 226 MPa, respectively. Elongation to rupture was about 18%. The microstructure, microhardness and grain boundary statistics are discussed with regard to the high mechanical properties of the UFG aluminum processed by th is novel method.Peer ReviewedPostprint (author's final draft