Austenitic-ferritic stainless steels are alloys with controlled additions of Cr, Ni, Mo and N which, after a suitable thermomechanical treatment, results in a balanced microstructure with similar proportions of ferrite (δ) and austenite (γ). Thus, it is possible to obtain good combinations of high mechanical properties and corrosion resistance through microstructural refinement, which also leads to a relatively good Pitting Resistance Equivalent value (PRE) at both phases. However, inadequate heat treatment and/or welding processes might result in the precipitation of deleterious phases, leading to poor mechanical properties and/or corrosion resistance. In this sense, the use of non-destructive microstructural characterization techniques becomes a valuable resource to access such alterations. Therefore, this work evaluates the precipitation of deleterious phase’s in welded thermal aged joints by portable Double Loop Electrochemical Polarization Reactivation (DL-EPR) taking into account a preliminary metallographic replica characterization. The results show that the proposed portable method can detect even a small percentage of deleterious phases, in addition to having a strong potential to be a non-destructive microstructural characterization technique.

Duplex stainless steels (DSS) are corrosion resistant alloys largely used in chemical and petrochemical industries. Some commercial DSS contain 0.5-1.0% copper addition to improve the corrosion resistance by reducing the corrosion rate in non-oxidizing environments. Higher copper addition (≥ 2%) can also hard by precipitation, especially when fine copper precipitates (ε phase) are produced. In this work, a cast copper alloyed DSS type ASTM A890 grade 1B with 3.01%Cu was investigated. Different levels of hardness were produced by solution treatment and aging at 450, 500, 550 and 600 oC for periods of time up to 1 hour. The corrosion resistance of aged DSS was evaluated by electrochemical tests in three media: 0.6 mol/L NaCl, 0.3 mol/L H2SO4 and 0.6 mol/L NaCl + 0.3 mol/L H2SO4 solutions. The results indicate that the effect of Cu addition depends on the media studied. Polarization studies in 0.6 mol/L NaCl showed a small anodic current peak occurred at around 400 mV vs. Ag/AgCl, with a strong influence on the passive film stability. Additionally, chronoamperometric measurements at 400 mV vs. Ag/AgCl showed a high electrochemical activity for the samples in 0.6 mol/L NaCl.

This work studies the application of a Non-Destructive Inspection Technique (NDIT), based on the Magnetic Barkhausen Noise (MBN), for the detection of non-homogeneous regions in carbon steel plates. It presents the advances in the development of a variant of a new non-destructive magnetic test technique, called Continuous Scanning Magnetic Barkhausen Noise (CSMBN). The non-homogeneous or damaged regions were produced by plastic deformations in rolled and annealed sheets of SAE 1060 and 1070 steels. MBN is generated by abrupt changes in the magnetization of materials when subjected to alteration. The sensitivity of the MBN signals in the detection of plastic deformations was analyzed varying parameters such as excitation magnetic field frequency and the speed of movement of the probe. The behavior of the root mean square of magnetic Barkhausen Noise signal (RMSMBN) parameter was correlated with the position of the nonhomogeneous regions detected in the samples. An evaluation of the technique applied to the measurement was made, comparing the results obtained with the Static Magnetic Barkhausen Noise (SMBN) technique, which is a method already established in the literature. The results showed that in all cases studied it was possible to detect the position of the damage, due to variation of the magnetic field. These changes are affected by the presence and distribution of elastic stresses (compression and traction), plastic deformation and microstructure modification. The results of CSMBN were similar to the SBMN with the advantage of being a continuous and more productive procedure. This new technique increases the spectrum of NDIT solutions for problems not contemplated by existing techniques.

Resumo O presente trabalho avalia a influência de adições entre 10 e 25% de CO2 como gases de proteção a base de argônio (Ar) nas propriedades mecânicas e resistência à corrosão de juntas soldadas pelo processo FCAW no aço inoxidável duplex (AID) UNS S31803 fazendo uso de arame eletrodo E2209T1-4. Sob estas condições o efeito da participação de 2% de oxigênio (O2) na mistura foi também levado em consideração. Ensaios mecânicos de tração e impacto Charpy, assim como de microdureza nas fases austenita e ferrita foram realizados em diversos locais das juntas assim produzidas. Paralelamente foram efetuados ensaios de corrosão para determinação da temperatura critica de pites (CPT) e perda de massa por unidade de área baseando-se nas especificações das normas ASTM G150 e G48, respectivamente. O estudo foi complementado com análises metalográficas mediante o emprego de diversos reagentes quantificando-se as fases presentes, assim como revelando características microestruturais nas condições supracitadas. Os valores das propriedades mecânicas obtidos nas juntas analisadas foram satisfatórios quando comparadas com valores normativos assim como de trabalhos publicados recentemente. Contudo, constatou-se ainda um aumento da CPT nas juntas soldadas com o emprego de gases com teores crescentes de CO 2. Finalmente observou-se, que dentre as juntas soldadas estudadas, aquelas obtidas utilizando-se a mistura com composição de 73%Ar+25%CO2 +2%O2, apresentou o melhor desempenho, tanto no que tange às propriedades mecânicas, quanto na resistência à corrosão. Os resultados obtidos permitem considerar a utilização de outras misturas gasosas na soldagem destes aços inoxidáveis com o processo FCAW, atém das indicadas normativamente, visando a grande demanda na construção e montagem de diversos componentes desta família de aço inoxidáveis na indústria offshore.

Abstract The present work evaluates the influence of additions between 10 and 25% of CO2 in argon-based shielded gases on the mechanical properties and corrosion resistance of welded joints by the FCAW process in duplex stainless steel (DSS) UNS S31803 using electrode wire E2209T1-4. Under these conditions the effect of the 2% O2 addition in the mixture was also taken into account. Mechanical tensile test, Charpy toughness and microhardness in the austenite and ferrite phases were carried out in several places of the joints thus produced. Besides, corrosion tests were carried out to determine the critical pitting temperature (CPT) and mass loss per unit area based on the specifications of ASTM G150 and G48, respectively. The study was complemented with metallographic analyzes using several reagents, quantifying the present phases, as well as revealing microstructural characteristics in the studied conditions. The mechanical properties values obtained in the analyzed joints were satisfactory when compared with normative values as well in recently published works. However, it was also observed a CPT increase value in the joints welded with the use of gases with higher levels of CO2. Finally, it was observed that, among the welded joints studied, those obtained using the mixture with composition of 73% Ar + 25% CO2 + 2% O2, presented the best performance, both with regard to mechanical properties and the corrosion resistance. The results allow determinates the use of other gaseous mixtures in the welding of DSS with the FCAW process, in addition to the normative one, in view of the great demand in the construction and assembly of several components of this stainless steel family in the offshore industry.

The supermartensitic stainless steels (SMSS) are a relatively new class of corrosion resistant alloys developed to obtain a better combination of weldability, strength, toughness and corrosion resistance than conventional martensitic stainless steels. The final properties of SMSS are strongly influenced by quenching and tempering heat treatments. In this work, different routes of heat treatments were tested in a Ti-stabilized 12%Cr supermartensitic stainless steel with the objective to improve mechanical properties, specially the low temperature (-46ºC) toughness. Double and triple quenching were tested and compared to single quenching heat treatments. Two tempering temperatures (500ºC and 650ºC) were tested. The results obtained with instrumented Charpy impact tests showed that a triple quenching treatment was able to increase the density of fine TiC particles and improve the mechanical properties of specimens heat treated by quenching and tempering at 650ºC.

The hydrogen interaction in palladium alloys with addition of Ce, Zr and Y was studied. These alloys are produced by arc-melting and then cold rolled into thin foils. Heat treatment at 800ºC for 24h in air atmosphere is applied to induce internal oxidation. Hydrogen permeation tests and TDS (Thermal Desorption Spectroscopy) are carried out to evaluate hydrogen interaction with three different microstructures which can be characterized by dislocations, substitutional atoms and nanoxides. For alloys argon annealed, Ce is the element that delays the most hydrogen diffusion, followed by Zr and Y. However alloys with yttrium nanoxides exhibit the highest solubility and the smallest diffusivity due to their small size, spherical shape and dispersion throughout the matrix. Analyzing TDS curves, it is shown that nanoxides are irreversible traps that improve hydrogen stored on palladium alloys. Dislocations and impurity atom on solid solution have little binding energy that detraps hydrogen at smaller temperatures.

Maraging steels with 18wt%Ni and 10wt%Co are precipitation hardenable steels selected for special applications. These steels are quenched and aged in the 480 - 600ºC range. Ti and Mo are added to precipitate during aging as fine Ni3(Ti,Mo) and Fe2Mo particles. Aging at high temperatures causes overaging due to coarsening of particles and austenite formation. SAE 4340 is a typical low alloy medium carbon steel for quenching and tempering. The best combination of mechanical properties is attained by quenching and tempering in the 650 - 670ºC range. These two steels are selected for services where an optimum combination of mechanical strength, toughness and fatigue resistance is required. In this work, the austenitizing temperature in the quenching treatment was varied in order to evaluate the effects on microstructure and mechanical properties of both steels. The results showed that the effect of previous austenite grain size on the toughness was different in the two steels analyzed.

Duplex stainless steels (DSS) are Fe-Cr-Ni-Mo alloys, with low carbon content, which have a favorable combination of mechanical properties and corrosion resistance. These characteristics are mainly attributed to a dual phase and refined microstructure, typical of these alloys, consisting of similar amounts of ferrite and austenite. For this reason, DSS are widely used in various segments of industry, especially in offshore applications. However, DSS components may have a decrease of their properties when they are exposed high temperatures, often due to incorrect manufacturing processes. Intermetallic phases may precipitate causing embrittlement and corrosion resistance decrease. This work presents the effects of short time aging at 475 °C on the mechanical properties and localized corrosion resistance of a cast DSS. The results show that mechanical properties, such as toughness and hardness, are more susceptible to aging, having already been affected during the first 4 hours of heat treatment, while the corrosion was affected in 12 h of aging.

The AISI 317L stainless steel is an austenitic grade with at least 3%Mo. Recently, this steel has replaced AISI 316L in many applications in chemical and petrochemical industries, due to its higher pitting corrosion resistance. The microstructure of the hot rolled and annealed material studied in this work consists of austenitic grains and 4.0% of delta ferrite (δ) with elongated islands morphology. This microstructure was modified by three processes: cold rolling with 87% of reduction, aging at 450ºC for 400 h, and welding by gas tungsten arc welding (GTAW) process. The corrosion resistance was evaluated by anodic polarization tests (ASTM G-61) and critical pitting temperature tests (ASTM G-150). Cold rolling produced a microstructure consisting of elongated grains of austenite and martensite α', high dislocation density and the elongated islands of delta ferrite present in the annealed material. Welding produced a dendritic microstructure with 7.0% of delta ferrite and some σ precipitated in the ferrite. Finally, the aging at 450ºC for 400 h provoked the decomposition of ferrite. The results show that these microstructural changes affected the pitting corrosion resistance, as detected by ASTM G-61 and ASTM G-150 tests.

Superduplex stainless steels (SDSS) are frequently employed in the petrochemical industries where is required high mechanical strength, toughness, and corrosion resistance. However, these properties can be affected by deleterious phases formation due thermomechanical processes applied in the field during pipes and vessels construction. This work propose the nondestructive microstructural characterization of deleterious phases precipitated in SDSS isothermally treated in 800 and 850ºC using portable double loop electrochemical polarization reactivation tests (DL-EPR). The results obtained in this nondestructive test are quite close to those obtained by conventional test, and can be correlated with the amount of deleterious phases precipitated. It can be concluded that the microstructural degradation of superduplex stainless steel can be evaluated by portable DL-EPR test with slow sweep rates, using a special cell and a proper electrolyte at room temperature.

ABSTRACT AISI 430 is a non-stabilized ferritic stainless steel grade with carbon content lower than 0.12%.After hot and cold rolling this material is annealed. The slow cooling after soaking at temperatures between 900oC and 1000oC promotes the formation of a high quantity of carbides and nitrides, while the rapid cooling partially suppresses the formation of these precipitates, but introduces martensite in the microstructure. Intergranular martensite can also be produced in the weld metal and in the heat affected zone (HAZ) of welds of non-stabilized ferritic stainless steels. In this work, several heat treatments between 900oC and 1000oC, with different cooling rates, were performed in a commercial sheet of AISI 430 grade. Also, an autogenous welding was produced with GTAW process, and post weld heat treatment at 700°C was carried out. The different microstructures produced were analyzed by optical and scanning electron microscopy (SEM). The degree of sensitization was measured by double loop electrochemical potentiodynamic tests (DL-EPR). The pitting corrosion resistance was evaluated by cyclic polarization tests in 3.5%NaCl solution. Hardness and toughness tests were also performed in selected heat treatment conditions. The results indicate that the slow cooling results in a higher degree of sensitization than observed in the material rapid cooled from the annealing temperature. The ferritic martensitic structure produced by water cooling has higher pitting potential and lower degree of sensitization, but is brittle at room temperature. A subsequent tempering treatment between 600 and 800oC can increase the toughness, but the corrosion resistance may decrease due to carbides precipitation.The heat affected zone of AISI 430 welds contains intergranular martensite, which is brittle and susceptible to corrosion attack. Post weld heat treatment at 700oC decomposed the martensite into ferrite and carbides and improved the corrosion resistance.

Austenitic stainless steels are high corrosion resistant alloys widely used in many industrial fields. Among this family of steels, AISI 317L stands out due to its higher localized corrosion resistance when compared to the traditional grades AISI 304L and AISI 316L. In some applications in oil refineries, the AISI 317L is being specified for services at moderately high temperatures. At the same time as it is sought to use new stainless steels, is also desirable to apply and develop emerging welding processes, replacing conventional ones, in order to achieve better behavior in service. In this respect, this work studied the effects of thermal aging on the toughness and resistance to pitting corrosion of AISI 317L steel weld metals produced by the Gas Tungsten Arc Welding (GTAW) and Friction Stir Welding (FSW) joining processes. After prolonged exposures at 450°C, for 200h, 300h and 400h, the microstructural characterization by scanning electron microscopy (SEM), toughness evaluation and anodic polarization tests in 3.5% NaCl solution were performed. The results showed that the increase of the exposure time in both weld metals caused a toughness decrease. The pitting potentials measured in the polarization tests also decreased with the aging at 450ºC.

The AISI 317L grade is an austenitic stainless steel with high Mo content (3.0 wt% min.). Due to the higher pitting resistance, this grade has replaced AISI 316L steel in many applications where the corrosion resistance is a critical property. However, the high Mo can induce phase transformations in high temperature services. In modern oil refinaries and petrochemical industries AISI 317L has been selected for temperatures as high as 550 °C. The goal of this work was to analyze the microstructural evolution and corrosion resistance of base and weld metal of AISI 317L stainless steel. The welded joints were produced by friction stir welding (FSW). The effect of prolonged exposure at 550 °C was investigated in specimens aged for 200h, 300h and 400h. After each aging treatment microstructural characterization was performed by scanning electron microscopy (SEM). Double loop electrochemical polarization reactivation tests (DL-EPR) were performed to evaluate the degree of sensitization of the samples. The results indicated that the increase of the exposure time at 550 °C promotes the formation of intermetallic phases, which causes corrosion decay of the weld metal.

This work investigates the influence of deleterious phases precipitation on the corrosion resistance of two wrought superduplex stainless steels UNS S32750 with similar composition, but different grain sizes. Isothermal treatments were carried out to introduce different amounts of deleterious phases, such as chi (χ), sigma (σ) and secondary austenite (γ2). The specimens were tested by double loop electrochemical potentiodynamic reactivation test (DL-EPR). The kinetics of precipitation in the two steels was different due to the difference of the grain sizes. The results show a correlation between the sensitization degrees, measured by DL-EPR, with the amount of deleterious phases precipitated.

Os aços inoxidáveis superduplex (AISD) são ligas extensamente utilizadas nas atividades de exploração e produção de petróleo naval e offshore em componentes tais como, trocadores de calor, cabos umbilicais, linhas de injeção de água salgada e em diversos equipamentos que requeiram uma elevada resistência à corrosão com altos valores de resistência mecânica. Estas características são devidas à sua fina microestrutura bifásica composta por proporções semelhantes de austenita e ferrita e aos elementos de liga Cr, Ni, Mo, N e W. Entretanto, durante a soldagem, tal como nas fases de fabricação e montagem de tubulações, podem ocorrer a formação de defeitos, sendo necessários procedimentos de reparos das juntas soldadas. Deste modo, o presente trabalho pretendeu caracterizar a microestrutura, propriedades mecânicas, teor de nitrogênio e a temperatura crítica de pites (CPT) em determinadas regiões de uma junta soldada de AISD UNS S32760 efetuada através do processo GTAW com dois reparos sequenciais. Os resultados exibem um decréscimo do valor da CPT na raiz da solda, apesar de não se manifestar variações tão acentuadas nas propriedades mecânicas analisadas.

Superduplex stainless steels (SDSS) are alloys widely used in the exploration and production of oil. They are used in marine and offshore components such as heat exchangers, umbilicals, seawater injection lines and various other equipment that require high corrosion resistance with elevated mechanical strength values. These characteristics are due to its fine two-phase microstructure composed of similar proportions of austenite and ferrite and the alloying elements of Cr, Ni, Mo, N and W. However, during welding, and also in the stages of fabrication and pipe assembly, the presence of defects may occur that requires the need for repair procedures to be carried out on the welded joint. This study aims to characterize the microstructure, mechanical properties, nitrogen content and critical pitting temperature (CPT) of a SDSS pipe, UNS S32760, in certain regions of a welded joint, where the GTAW welding process was performed, with the completion of two sequential repair procedures. The results show that there is a decrease in the value of the critical pitting temperature at the root of the weld, though there aren't substantial variations in the mechanical properties analyzed.