Rodrigo Magnabosco

Kinetics of sigma phase formation in a Duplex Stainless Steel

This work determines the kinetics of sigma phase formation in UNS S31803 Duplex Stainless Steel (DSS), describing the phase transformations that occur in isothermal aging between 700 and 900 oC for time periods up to 1032 hours, allowing the determination of the Time-Temperature-Precipitation (TTP) diagram for sigma phase and proposing a model to predict the kinetics of sigma phase formation using a Johnson-Mehl-Avrami (JMA) type expression. The higher kinetics of sigma phase formation occurs at 850 oC. However, isothermal aging between 700 and 900 oC for time periods up to 1032 hours are not sufficient to the establishment of thermodynamic equilibrium. Activation energy for both nucleation and growth of sigma phase is determined (185 kJ.mol-1) and its value is equivalent to the activation energy for Cr diffusion in ferrite, indicating that diffusion of Cr is probably the major thermally activated process involved in sigma phase formation. The determined JMA type expression presents good fit with experimental data between 700 and 850 oC.

Pit Morphology and Its Relation to Microstructure of 850°C Aged Duplex Stainless Steel

Abstract The relation between pit morphology and microstructure formed during 850°C isothermal aging of UNS S31803 duplex stainless steel (DSS) could be reached through the explanation of pit nucle...

Influence of Sigma Phase Formation on Pitting Corrosion of an Aged UNS S31803 Duplex Stainless Steel

The main purpose of this contribution was the study of the relation between sigma formation and pitting corrosion resistance of a UNS S31803 duplex stainless steel aged at 850°C. Solution-treated s...

Influence of the microstructure on the degree of sensitization of a duplex stainless steel UNS S31803 aged at 650ºC

This work evaluates the phase transformations during aging of an UNS S31803 DSS at 650oC and its influence on the DOS. The material was solution treated at 1175oC and then aged at 650oC for times up to 360 h. SEM-BSC images indicate the formation of Cr2N, σ and χ phases in the samples aged at 650oC. The analysis of DL-EPR curves, obtained in a 1 M H2SO4 + 0.25 M NaCl + 0.01 M KSCN solution, shows an increase in DOS values for samples aged at 650oC. Probably, this increase observed in DOS values is mainly related to the presence of Cr- and/or Mo-depleted α, as a result of σ phase, χ phase and/or Cr2N formation at 650oC. Moreover, a possible healing up of Cr- and/ or Mo-depleted areas is observed between 1 h and 4 h of aging at 650oC.

Sigma Phase Formation and Polarization Response of UNS S31803 in Sulfuric Acid

Abstract For a better understanding of the relationship between the microstructure of UNS S31803 duplex stainless steel (DSS) and the shape of the polarization curves, this study evaluated the infl...

Corrosion Versus Mechanical Tests for Indirect Detection of Alpha Prime Phase in UNS S32520 Super Duplex Stainless Steel

Abstract Alpha prime formation leads to material embrittlement and deterioration of corrosion resistance. In the present study, the mechanical and corrosion behavior of super duplex stainless steel UNS S32520 aged at 475°C from 0.5 h to 1,032 h was evaluated using microhardness measurements, Charpy impact tests, electrochemical impedance spectroscopy, and cyclic polarization curves. The sensibility of these tests to the effects of alpha prime phase was investigated. The microhardness test showed a gradual increase in hardness with aging time, whereas the impact tests revealed losses of about 80% in the energy absorption capacity for the material aged for 12 h in comparison with the solution-annealed samples. The most responsive analysis was the impact test, which indirectly revealed the presence of this deleterious phase in samples aged for 0.5 h. The electrochemical impedance spectroscopy and polarization tests were not highly sensitive to the alpha prime phase unless these are present in large amounts i...

Toughness of two HSLA steel sheets

This paper presents a study concerning the toughness anisotropic behavior of two high-strength low-alloy steel sheets having ferrite-martensite (dual-phase) and ferrite-perlite grain-refined structures. The anisotropic behavior quantification was possible through tension and Charpy impact tests. It has been concluded that mechanical resistance has higher values in the longitudinal direction for both steels, and the inverse occurs for ductility. This could be explained with the microstructure preferential orientation with rolling direction. The higher tensile strength found in dual-phase steel is related to the presence of martensite islands, and the higher yield strength found in ferrite-perlite grain-refined steel is related to the fine structure observed. The higher toughness is a ferrite-perlite grain-refined steel characteristic, and this could be explained with the presence of only globular-oxide type inclusions, which reduces fracture nucleation sites, and does not promote fracture easy-propagation. Sulfide type inclusions found in dual-phase steel, associated with globular-oxide ones, give to this material a more brittle behavior. Toughness anisotropy is clearly noted, and in both cases the highest values are found in the longitudinal direction, related to the preferential inclusion alignment achieved in rolling. The low ductile-to-brittle transition temperature found in both steels, however, could be related to the very fine structures found.

Selective Corrosion in Sodium Chloride Aqueous Solution of Cupronickel Alloys with Aluminum and Iron Additions

Abstract Cupronickel pipes are widely used in cooling systems. Under this application, localized corrosion is observed, particularly associated with selective corrosion of nickel (denickelification). This work studied the denickelification process in solution-treated cupronickel alloys 90-10 (Cu-10wt%Ni), with aluminum and iron additions, in 0.01 M, 0.1 M, or 0.6 M sodium chloride (NaCl) aqueous solutions. Polished surfaces of the studied materials were submitted to potentiodynamic polarization, and after that, the polarized surfaces were studied using optical microscopy (OM) and scanning electron microscopy (SEM) assisted by energy-dispersive spectroscopy (EDS). A passivity break potential (Epb) was determined in all solutions, related to the initiation of selective corrosion of nickel. When submitted to cyclic potentiodynamic polarization, alloys with higher Epb showed the higher nickel losses in localized spots. However, potentiodynamic polarization tests conducted in 0.01 M NaCl until current densitie...

Modeling and Characterization of Residual Stresses in Material Processing

Residual stresses can seriously alter (detrimentally or favorably) the mechanical behavior of materials and a component’s fatigue life and crack growth behavior. They can arise from different sources and can be simulated and experimentally quantified using several different techniques. As a step in the direction of better understanding residual stresses and the different methods for its prediction, simulation, and quantification, this work reviews the different types of stresses, their physical basis, their sources, and critically presents the current most relevant approaches and limitations for residual stress modeling and experimental quantification. At the end, a brief exploratory comparison between experimental results from X-ray diffraction and hole-drilling methods using finite element (FE) models as benchmarks is conducted. Results reveal the limitations of those methods, mainly regarding plasticity effects.

Effects of Weld Strength Mismatch on Estimation Procedures for J Fracture Parameter Using Clamped SE(T) Specimens

This work presents an exploratory development of J estimation procedures for deep and shallow cracked tension SE(T) specimens based upon plastic eta factors which incorporate the effects of weld strength mismatch. The considered technique includes estimation of J from plastic work. The primary objective is to derive estimation procedures which are applicable to determine J fracture parameters for a wide range of a/W-ratios, material flow properties and different levels of weld strength mismatch using clamped SE(T) specimens of varying geometries. Very detailed non-linear finite element analyses for plane-strain models provide the evolution of load with increased load-line displacement and crack mouth opening displacement, which are needed to determine the estimation procedures. Laboratory testing of a welded structural steel using SE(T) specimens provide the data needed to evaluate J evolution for the welded joints based upon the proposed methodology. The crack driving force evolution is compared against the results obtained using plastic eta factors for homogeneous specimens and demonstrates the importance of considering mismatch effects on crack-tip stress fields and fracture parameters. The present analyses, when taken together with previous studies, extend the body of results which serve to determine J integral using tension SE(T) specimens with varying geometries and mismatch levels.Copyright