Modelamento computacional das transformações de fase durante os ciclos térmicos de processamento de um aço inoxidável superdúplex

This project aims to perform the computer simulation of the transformation’s kinetics and phase evolution during the thermal processing cycles of a superduplex stainless steel, considering the stages of heating, hot working and cooling, using DICTRA® software. The input data for the simulations were chemical composition and phase size, desired simulation temperature, and heating and cooling rates when necessary to describe the thermal cycle. TCFE9 thermodynamic database and MOBFE4 atomic mobility database were used in order to obtain results for different models, determining the one that best describes the phase transformation kinetics. Different rates were simulated during the heating of the material from 950 ° C, considering the initial microstructural condition of 50% of ferrite [alpha] and 50% austenite [gamma], up to 1250 ° C, typical forming temperature. In heating, a maximum fraction of 66.6% [alpha] was obtained at a rate of 0.30 ° C/s, [alpha] value close to the 70% expected by the equilibrium simulation in Thermocalc®. After 1000 s of plateau at 1250 ° C and cooling to the solubilization temperature, 1090 ° C, at the rate of 0.30 ° C/s, the fraction of [alpha] reduced to values of 58.7%. In the sequence, different cooling rates were also simulated with or without the presence of solubilizations plateau. Considering 3600 s of plateau at 1090 ° C, it was possible to recover the desired duplex condition, reaching 55.5% [alpha], but not reaching the 50% expected by the equilibrium balance, since there is still a composition gradient in [alpha] and [gamma] by DICTRA® simulations. Seeking the maintenance of the duplex microstructure, a cooling was performed from 1090 °C to 790 ° C at a critical rate of 3.0 ° C/s, obtaining volumetric fractions of 56% [alpha], 43% [gamma] and sigma fractions equal to or less than 1%. If the plateau at 1090 ° C was not considered, that is, promoting cooling from 1250 °C, where the condition of 58.7% [alpha] was reached, to 790 ° C at the rate of 3.0 ° C/s volumetric fractions of 59.6% alpha], 39.4% [gamma] and 0.9% [sigma] were obtained. DICTRA® was unable to simulate the precipitation of chromium nitrides (Cr2N) during cooling, either because there was no nitrogen supersaturation (N) in [alpha] or because it was unable to predict this supersaturation. From the results of kinetics and evolution of the phases’ volumetric fraction obtained in the thermal cycle of steel processing UNS S32750, it was possible to obtain the computational model that best describes the real behavior of the studied steel

Study on the Piezomagnetic Response to Low-Cycle Fatigue of X80 Pipeline Steel

The objective of this work is to explore the relationship between the progressive material degradation process of fatigue and the evolution of the piezomagnetic field surrounding a ferromagnetic sample. The continuous examination of changes in the magnetomechanical behavior during a series of strain-controlled fatigue tests was performed on X80 pipeline steel samples. Analysis of the data obtained allowed the division of the fatigue life of the investigated steel into three stages: incipient stage, steady stage and terminal stage. Furthermore, the piezomagnetic field evolution demonstrates conspicuous changes in the initial stage of fatigue loading, then reverts to a relatively stable phase, and finally, drastic variations appear again before terminal failure. The progressive degradation of the steel under cyclic loading can therefore be tracked by following the evolution of the piezomagnetic field. The characteristics of the evolution of the piezomagnetic responses are also discussed in terms of the mechanical and microstructural condition of the steel during the fatigue process.

Comparison of the Annealing Behavior of Three Cold Rolled (Ti-bearing, Nb-bearing and V-bearing) HSLA Steels

ABSTRACTThe annealing behavior of three HSLA steels is studied using the combined techniques of EBSD-KAM and Sub-grain Method. These techniques have been successfully used to assess the annealing behavior of AK, IF and other high strength steels. Stored energy maps in the hot band, cold rolled and after annealing are constructed and analyzed. The combined usage of the Sub-grain Method and EBSD-KAM techniques are employed to calculate and compare the evolution of the stored energy and recrystallization behaviour during the annealing of Ti-bearing, Nb-bearing, and V-bearing HSLA steels. Orientation dependent stored energy distribution maps at different annealing stages are constructed and analysed. The results show that the stored energy distribution through the thickness of the samples is not uniform and is independent of the steel composition. Similarly the recrystallization behaviour is strongly related to the initial microstructural condition and particularly to the grain boundary character distribution of the steels.

Application of Metallographic Evaluation for Repair of FSX 414: 1st Stage Nozzle Segments

The 1st stage nozzle segments experience damage such as creep, fatigue, high temperature corrosion and foreign object damage in service. Amongst the other gas turbine components the 1st stage nozzle segments needs to be refurbished or repaired after stipulated number of running hours. The decision on the extent of repairs for the 1st stage nozzle segments are based on incoming inspection observations. There are various stages of incoming inspection which includes visual, dimensional and non-destructive testing methods. Metallographic evaluation is one of the most important incoming inspection method based on which the extent or class of repairs can be decided. The 1st stage nozzle segment material is FSX 414, which is a cobalt based alloy. It is critical to estimate the microstructural condition of the nozzle segments in the as received condition. As a part of incoming inspection sections at various locations are taken on the nozzle segments for metallographic evaluation. The microstructures are examined in the as received condition and after solution annealing heat treatment. Microstructural transformations and damages in terms of overheating, oxidation and hot corrosion can be identified at ease. The microstructural condition of the nozzle segments can thus be assessed after stipulated running hours. It is concluded that metallographic evaluation is effective in assessing the microstructural damage and proves to be vital in deciding the category of repairs. Typical microstructural evaluation case studies of 1st stage nozzle segments from Frame 5, 6001B and 7001EA gas turbine units are presented and discussed in this paper.

Self-Affine Analysis of Fractures Surfaces of an Aluminum Alloy Using Fractographic Techniques

We report the self-affine analysis on fracture surfaces of an A319-type aluminum alloy with different modification and refinement treatments, broken both in Charpy impact and cyclic impulse tests. The in-plane and out-of-plane Hurst exponent as well as the correlation length are obtained using quantitative fractographic techniques. It is found that the Hurst exponent, ζ, has a value of about 0.8 and is not influenced neither by the crack propagation modes nor by the microstructural condition. The self-affine correlation length is found to be related to the grain size resulting from the refining and modifying treatments applied to the alloy.

Microstructure and Self-Affine Fracture Surface Parameters in Steel

The fracture surfaces of SAE-1018 steel tension and impact test specimens with different grain sizes are analyzed in order to explore the possible relations between the microstructure and the self-affine fracture surface parameters such as the roughness exponent, ζ, and the correlation length, ξ. The topography of the fracture surfaces was observed and quantified by means of scanning electron microscopy, atomic force microscopy, optical microscopy and optical digitizer. It is confirmed that the fracture surfaces exhibit a self-affine behavior extending over six decades of length scale, from nanometers up to a few millimeters. The roughness exponent exhibits a value of ζ∼0.82 for all the cases regardless of the microstructural condition.