EM - Escola de Minas

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Effect of thermal aging on the microstructure and mechanical properties of stainless steel UNS S31803.
(2020) Costa, Junia Maria Gândara; Lacerda, José Carlos de; Godefroid, Leonardo Barbosa; Cândido, Luiz Cláudio
Duplex stainless steel UNS S31803 exhibits high mechanical strength with high corrosion properties, due to its microstructure composed of ferrite and austenite phases, in equal proportion. When the UNS S31803 steel is submitted to high temperatures, some precipitations can occur, such as nitrites, carbides, and third phases (e.g. sigma phase -σ, and alpha prime - α'). These phases are deleterious in relation to the mechanical properties and corrosion resistance, and their effects are analyzed regarding the properties of the steel. In order to evaluate the precipitation of this deleterious phase, isothermal treatment was done at 500°C for 144 hours (α' phase) and at 850ºC for 80 minutes (σ phase). The results were obtained through the microstructural analysis and tensile tests. The presence of sigma phase was verified in the grain boundary, an increase in the mechanical resistance with a loss of toughness. There was as well as an increase in mechanical resistance with the precipitation of α', with less loss of ductility than that observed in the experiments involving the presence of sigma phase.
Effect of martensite volume fraction on the mechanical behavior of an UNS S41003 dual-phase stainless steel.
(2020) Faria, Geraldo Lúcio de; Godefroid, Leonardo Barbosa; Nunes, Isadora Pereira; Lacerda, José Carlos de
UNS S41003 (410D) steel is a relatively low chromium unstabilized ferritic stainless steel. It has superior mechanical and corrosion resistance than ordinary low carbon steels. Therefore, and for its relatively low cost among specialty steels, it is a strong candidate for replacement of low carbon common steels in many applications. In order to enable new applications for this steel, it is important to ensure good performance in relation to its mechanical strength, so the development of hardening mechanisms without significant loss of ductility is desirable. Dual-phase microstructures are an example that fits in this context, because they tend to increase mechanical strength and are favorable to the fracture toughness and fatigue resistance of steels. This research evaluated the influence of quenching heat treatments after intercritical austenitizing on the microstructure, tensile strength, hardness, fracture toughness (J x Δa curves) and fatigue crack growth resistance (da/dN x ΔK curves) of a 410D ferritic stainless steel. The used intercritical austenitization temperatures were defined based on its critical temperatures Ac1 and Ac3 measured by dilatometry. Ten different quenching heat treatments were performed, varying the austenitizing temperature and time, in order to obtain dual-phase microstructures (ferrite and martensite) with different volume fractions of constituents. The obtained results revealed that the increase of the austenitizing temperature and time favor the increase of the martensite volume fraction in the microstructure. Higher martensite volume fractions imply greater hardness and mechanical tensile and fatigue strength of the steel, but with loss of ductility and fracture toughness. The best balance among the studied mechanical properties was presented by steel treated at 825 ◦C for 15min, containing 57% of martensite.
Corrosion behavior of UNS S31803 steel with changes in the volume fraction of ferrite and the presence of chromium nitride.
(2015) Lacerda, José Carlos de; Cândido, Luiz Cláudio; Godefroid, Leonardo Barbosa
The goal of this study was to examine the changes caused in stress corrosion cracking and the critical pitting temperature behavior of a UNSS31803 duplex stain less steel due to variations in the ferrite fraction and the presence of chromium nitride in their microstructure. The steel plates were cold rolled and annealed at 1060 °C. Microstructural variations in the plates were produced by thermal annealing treatments a t1200 °C and 1300 °C. Steel annealed at 1060 °C showed greater resistance to stress corrosion cracking than steel annealed at 1300 °C. Steel annealed at 1300 °C had the lowest critical pitting temperature.
Effect of volume fraction of phases and precipitates on the mechanical behavior of UNS S31803 duplex stainless steel.
(2015) Lacerda, José Carlos de; Cândido, Luiz Cláudio; Godefroid, Leonardo Barbosa
Mechanical properties of a grade UNS S31803 duplex stainless steel were studied. Samples were annealed at three different temperatures, 1060 C, 1200 C, and 1300 C, to change the a/c proportion, to precipitate undesirable phases (for example, a0 , r and chromium nitrides), and to study the effect of the microstructure on the mechanical behavior. Specimens with a thickness of 1.8 mm were prepared according to ASTM standards and tested at room temperature. Tensile tests were conducted using displacement control at a rate of 5 mm/min. Force-controlled constant amplitude axial fatigue tests were conducted at a frequency of 30 Hz and an R stress ratio of 0.1. Microstructural analyses, performed by optical microscopy and scanning electron microscopy, showed the following modifications to the microstructure: changes in the phase volume fractions, the phase grain growths, and the morphology of the c-phase and the precipitation of chromium nitrides in the a-phase. The mechanical properties of the steel were significantly changed due to the changed microstructure. The results demonstrate the sensitivity of the steel to the annealing temperature.

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