Manuel Carsí

Characterization of a δ/γ duplex stainless steel

A duplex stainless steel was investigated in both as-received sheet and after annealing at temperatures ranging from 850 to 1100°C. The sheet presents a deformation texture in both phases, austenite and ferrite, induced by cold rolling. Microstructure in the as-received material consists of island-like austenitic grains in a ferrite matrix. These austenitic grains are elongated with an average size of 6, 20 and 40 μm along the normal (ND), transversal (TD) and rolling direction (RD). Quantitative texture measurements demonstrated that texture components are distributed mainly along the θ-fiber (ND ∥ ‹100›) and α-fiber (RD ∥ ‹110›) for the ferrite and the ζ-fiber (ND ∥ ‹110›) for the austenite. After recrystallization, a decrease in the intensity of the mean fibers and an increase in the minor components was observed in both, ferrite and austenite. Therefore, a similar texture was reached in both phases after annealing at 1050°C. Microstructural characterization after annealing at temperatures above 850°C showed that the elongated austenitic grains transform in colonies of equiaxic grains of about 10–15 μm in size. These colonies are surrounded by a ferritic matrix at annealing temperatures above 1000°C or by a laminar microstructure at temperatures below 950°C. This laminar microstructure includes sigma phase and austenite formed from delta ferrite, and untransformed delta ferrite.

Austenitic grain size evolution and continuous cooling transformation diagrams in vanadium and titanium microalloyed steels

The evolution of the austenitic grain size in medium carbon steels microalloyed with vanadium and titanium was studied as a function of reheating temperature, heating rate, and titanium content. High resolution dilatometric techniques were used to determine the continuous cooling transformation (CCT) diagrams for two different austenitization temperatures. The microstructure and hardness were determined for different cooling rates. The results revealed a significant effect of titanium concentration on the austenitic grain growth control. The smallest grain size was found in the steel with a Ti concentration = 0.019 wt%. Low heating rates produced smaller grain sizes than high heating rates although an abnormal grain growth took place. In these steels, at temperatures above 1050 °C the influence of the reheating temperature on their hardness for cooling rates around 2 °C · s−1 was negligible. The higher reheating temperatures caused a slight increase in their hardenability. Finally, it was found that the greater the titanium content, the greater the hardness of these steels, but only when the titanium percentages were higher than 0.020 wt%.

Creep behavior of Fe–C alloys at high temperatures and high strain rates

Abstract The creep behavior of Fe–C alloys (1–1.8%C) has been studied at high temperatures (0.7–0.9Tm) and high strain rates (1–100 s−1). The dominant deformation resistance has been found to be climb-controlled dislocation creep and thus the creep rates are a function of elastic modulus, lattice diffusivity and stacking fault energy. The self-diffusion coefficient of iron in austenite was found to be solely a function of Tm/T and to vary as D=6.8×10−6 exp(−17Tm/T) m2 s−1. The Fe–C alloys were observed to have a high stacking fault energy which was unaffected by carbon and manganese. The stacking fault energy was observed to decrease with increasing concentrations of silicon, aluminum and chromium. At high stresses, deviation from power law behavior was accounted for by considering the contributions to diffusivity by dislocation pipe diffusion. The results have been used to develop a rate equation for these steels of varying composition that depends on only three material characteristics – alloy melting temperature, elastic modulus and stacking fault energy.

New numerical method for the fit of Garofalo equation and its application for predicting hot workability of a (V–N) microalloyed steel

Abstract A new method, Rieiro, Carsí, Ruano (RCR), for solving the Garofalo equation is developed. This method is based on an integrated algorithm that allows determination of the equation parameters for any given material and does not need initial values. The RCR method is used to analyse the Garofalo equation best fit applied to torsion data at various temperatures and strain rates from a (V–N) microalloyed steel. The predictive capability of the RCR method on experimental results is ∼6% in stress and the magnitude of the predicted errors is of the same order as the interpolation errors. On the other hand, the n value is slightly lower and the Q value slightly higher than those expected for a slip creep mechanism controlled by lattice diffusion. However, the obtained values agree with those found in the literature for microalloyed steels. These differences can be attributed to microstructure changes during deformation.

Superplastic properties of a δ/γ stainless steel

Abstract The high temperature deformation behavior of a duplex stainless steel has been studied by tensile tests at temperatures ranging from 850 to 1100°C. The microstructure of the as-received material consists of elongated islands of austenitic grains in a more or less continuous ferrite matrix. Metallographic studies after tensile tests showed that the material recrystallizes during heating at testing temperature and the austenite transforms into fine colonies of grains of 10–15 μm in size. The results of the mechanical tests in the temperature range 850–1100°C show a high elongation to failure (up to 300%) and a low value of n , 2–3, for strain rates up to 10 −3 s −1 . This result suggests that the controlling deformation mechanism is grain boundary sliding, GBS. Microstructure and texture evolution studies of deformed samples confirmed the importance of GBS. The colonies of austenitic grains decrease in size during superplastic deformation and, at the same time, only minor changes in the texture was observed in the deformed region. Finally, at high strain rates an increase of stress exponent to 5 is observed, which suggests a transition to a slip-creep controlling mechanism.

Integral method from initial values to obtain the best fit of the Garofalo's creep equation

Abstract An integral computational method has been developed to provide initial values to a subsequent fitting of creep data based on non-linear and iterative methods. The fitting of the Garofalos equation, which describes creep data, utilizes a strong non-linear objective function. Various algorithms are developed which provide the solution for the fitting. An analysis is made of the significance of this solution in relation to the problem studied. The dispersion of the solution values is related to the final values provided by other non-linear and iterative methods. The method is applied to three types of steels and screens the experimental creep data with a statistical analysis that improves the fitting results.

The strain rate as a factor influencing the hot forming simulation of medium carbon microalloyed steels

Abstract Two medium carbon microalloyed steels, one with vanadium and the other with vanadium and titanium, have been studied by means of high-temperature torsion tests. The evolution of the austenitic grain size with deformation has been determined together with the rheological characteristics. On the basis of these data, the apparent activation energy for deformation and the Zener-Hollomon parameter have been calculated. The dependence of the final austenitic grain size on the initial one and on the thermomechanical treatment conditions (true strain, true strain rate and temperature) have been analyzed. Two mathematical expressions are proposed to described the evolution of the austenitic grain size due to static or dynamic recrystallizaton. It was found that the strain rate can be as important as the strain to influence the final microstructure and therefore the mechanical properties of the final product.

Influence of interfaces on the mechanical properties of ultrahigh carbon steel multilayer laminates

Abstract To improve impact and flexure properties of ultrahigh carbon steel (UHCS-1.3 wt.% C), a multilayer laminate consisting of 20 layers of the same UHCS has been processed by hot rolling. Two different rolling conditions gave rise to “soft” and “severe” laminates. The toughness of the “soft” laminate, measured by impact test absorbed energy, was more than forty times higher than that of the monolithic UHCS and three times higher than that of mild steels. In contrast, the “severe” laminate showed no increase in toughness with respect to the monolithic UHCS. This large difference in behaviour was studied by means of shear and bend tests, which gave valuable information on bond quality and delamination.

Processing, microstructure, strength, and ductility relationships in ultrahigh carbon steel assessed by high strain rate torsion testing

AbstractThe torsional ductility and strength of an unalloyed ultrahigh carbon steel containing 1·3%C (UHCS–1·3C) has been studied at high strain rates (0·2–26 s-1 ) and high temperatures (750–1200°C). The strength–strain rate relationships are in agreement with a diffusion controlled dislocation creep model, where power law creep is observed with a stress exponent n of ∼5. The results were compared with the high temperature ductility and strength of a medium carbon (0·3%C) high strength, low alloyed (HSLA) steel, 304 stainless steel, and an alloyed ultrahigh carbon steel (UHCS–1·8C–1·6Al–1·5Cr). It is shown that the UHCS–1·3C material is the most ductile of the four materials, and has the lowest stress for plastic flow. The results are explained by the high rate of iron lattice diffusion and by the high stacking fault energy in the UHCS–1·3C material. It is proposed that contemporary processing and manufacturing equipment can be used to make pearlitic structure ultrahigh carbon steels for high strength ro...

Influence of data conversion methods from torsion tests on the Garofalo equation parameters for a high nitrogen steel

Abstract Torsion tests in the temperature range 900 to 1 200 °C and strain rates varying from 0.005 to 5s−1 for a high nitrogen steel were analyzed. The influence of various conversion methods from torsion tests on the stress – strain curves and on the Garofalo equation parameters as a function of strain was studied for this steel. In addition, the creep behavior of the high nitrogen steel was analyzed using the Garofalo equation. The high values of the activation energy and stress exponent using the peak stress are attributed to the microstructure that changes strongly during the first stages of deformation. The analysis of the Garofalo parameters allowed determination of a steady state at strains of about 2.5. At this strain, the values of the activation energy and the stress exponent correlated well with slip creep mechanisms operating in other reinforced steels at high strain rates.