The influence of duplex aging (DA) heat treatment and consequent microstructural changes in the stress corrosion cracking (SCC) resistance of Ti-6Al-4V α+β titanium alloy was examined through constant load SCC test and metallographic examination. The time for fracture (TFF) of duplex aged samples in the SCC test was compared with single aged samples TFF. The results of the constant load SCC test showed an improvement in corrosion resistance with the second stage of aging in the duplex aging heat treatment. Microstructural examination revealed a preferential path for SCC crack propagation through prior β grains (lamellar structure alternating α and β phases) which indicated that the bigger SCC resistance observed in DA samples is a consequence of greater β phase decomposition due to the second stage of aging. It wasn’t observed a bigger grain growth in the DA heat treatment, in comparison with single aged (SA) samples. The observed fracture aspect was mixed (ductile-fragile) in peripheral regions and ductile in the interior of the body proofs, what indicate the major influence of SCC in the fracture mechanism.

Sigma phase leads to deterioration of corrosion resistance of super duplex stainless steels (SDSS) and its volume fraction increases as the time which the materials is exposed in determined temperature increases. In the present study, the effect of the short aging time on the formation of the sigma phase and on the corrosion behavior of UNS S32520 SDSS at 3.5% sodium chloride solution was evaluated using microstructural characterization, linear potentiodynamic polarization curve and electrochemical impedance spectroscopy. The results indicated that the SDSS sample solution-treated at 1150ºC and aged at 850ºC for 5 min may improve the corrosion resistance in comparison with the as-received and solution-treated conditions, due to the better partition of the elements Cr, Mo and dissolution of certain amount of sigma phase in the microstructure. However, results also showed that when SDSS solution-treated at 1150ºC is aged at 850ºC with the aging time slightly superior (10 min), the precipitation of sigma phase increases leading to a rapid deterioration of the corrosion resistance of the material.

Os aços inoxidáveis duplex são aqueles que possuem uma microestrutura constituída por duas fases distintas, ferrita (α) ferromagnética e austenita (γ) paramagnética, presentes em quantidades aproximadamente iguais. Neste trabalho, a microestrutura do aço duplex UNS S31803 foi modificada através de tratamento térmico (TT) de solubilização à temperatura de 1050°C por 30 minutos, seguido de resfriamento em água. Em seguida, as amostras foram envelhecidas por 5, 15, 30, 60 e 180 minutos, à temperatura de 850°C. Este tratamento promoveu a formação de uma fase deletéria, a fase sigma, que se forma através da decomposição da ferrita, rica nos elementos Cr e Mo, que são formadores desta fase. A formação desta fase altera as propriedades mecânicas e químicas (resistência à corrosão) dos aços duplex e superduplex. As propriedades magnéticas e mecânicas foram avaliadas em cada uma das condições estudadas e correlacionadas com a microestrutura. As propriedades magnéticas das amostras foram obtidas em um Magnetômetro de Amostra Vibrante (MAV), no qual se fez o levantamento da curva de histerese deste material. As propriedades mecânicas das amostras foram caracterizadas por microdureza Vickers empregando-se uma carga de 0,2 kgf. A caracterização microestrutural foi realizada utilizando um microscópio óptico. Com o aumento do tempo de tratamento térmico de envelhecimento, ocorreu o aumento do percentual de fase sigma formada, e diminuição da fase ferrita. Devido a essa transformação, houve alteração das propriedades magnéticas do aço duplex. A fase ferrítica foi calculada através de dois métodos: ferritoscópia e medição da magnetização de saturação; em ambos os casos foi observada a diminuição da fase ferromagnética.

Duplex stainless steels are those that have a microstructure constituted by two phases, ferromagnetic ferrite (α) and paramagnetic austenite (γ), which are present in approximately equal volumetric fraction. In this work, the microstructure of duplex steel UNS S31803 was modified by a thermal solubility treatment at temperature of 1050 °C for 30 minutes, followed by water cooling. Next, specimens were aged for 5, 15, 30, 60 and 180 minutes at temperature of 850°C. This treatment promoted the formation of a deleterious phase, the sigma phase, which is formed by the decomposition of ferrite, rich in the elements Cr and Mo. The formation of this phase changes the mechanical and chemical properties of the duplex and super duplex steels. The magnetic and mechanical properties were evaluated in each of the conditions mentioned and a correlation with the microstructure was made. The magnetic properties of the specimens were obtained in a Vibrating Sample Magnetometer (VSM), where the material hysteresis curves were obtained. The mechanical properties of the samples were characterized by Vickers micro-hardness measurements performed with a load of 0.2 kgf. Microstructural characterization was carried out by using an optical microscope. VSM tests results showed that as the aging time increased, the volume fraction of sigma phase also increased, causing a diminution of the steel duplex magnetic properties. The diminution of the magnetic properties of the duplex steel with the aging time is attributed to the lower fraction of ferrite as more sigma phase is formed by the decomposition of ferrite. The hardness results showed an increasing of the hardness of the material as the aging temperature increased, which confirms the higher brittleness of the steel due to the higher amount of sigma phase.

A thermodynamic model was used to investigate the solidification of a hardfacing material in Fe-Cr-C-Nb system designated CNO, and developed for cladding components subjected to severe abrasive wear by welding. Microstructural characterization of the alloy showed that the theoretical simulations carried out to predict the amount and volume fraction of the phases using the module Scheil of MT-DATA Software gave very realistic results in the case of the rapid solidification of the hardfacing alloy CNO. The results obtained in this study indicate that the utilization of this tool may provide a good understanding of alloys solidification. As a consequence, it is possible to enable the refinement of alloy composition during the early stages of hardfacing materials development, leading to a decreasing of overall cost and time of manufacturing of hardfacing electrodes.