Influência da microestrutura na resistência à corrosão da liga ASI 444

O estudo do comportamento dos aços inoxidáveis em ambientes agressivos é um importante ramo da ciência dos materiais dado seu diálogo direto com a aplicação industrial desses materiais. Dessa forma, o conhecimento mais aprofundado da reação de ligas de aço inoxidável permite uma racionalização da seleção e tratamento desses produtos siderúrgicos para se obter os melhores desempenhos na vida em serviço de peças produzidas. Como maneira de ampliar a compreensão sobre a liga AISI 444, uma liga de aço inoxidável ferrítico, o presente trabalho foi desenvolvido para se conhecer suas propriedades de resistência à corrosão de acordo com diferentes microestruturas, estas obtidas através da realização de tratamentos térmicos em forno nos corpos de prova seguidos de resfriamento em diferentes meios (água, ao ar e ao forno). Foram analisadas as propriedades de resistência à corrosão intergranular e por pites por meio de ensaios eletroquímicos, realizadas análises metalográficas em microscópios ótico e eletrônico de varredura (MEV) e os resultados comparados com pesquisas anteriores para determinar as condições de serviço a que a liga possa estar sujeita sem comprometimento de sua performanceStainless steels are materials recognized for their corrosion resistance and good mechanical properties. The study of the behavior of these materials in harsh environments contributes to the popularization of their use and constitutes in a great branch of the materials science. In this way, the more solid knowledge of these metal alloys allows a more appropriate selection and treatment for each type of application. As a way to broaden the understanding of the AISI 444 alloy, a ferritic stainless steel alloy, the present work was developed to know its properties of resistance to corrosion according to different microstructures, obtained through the accomplishment of heat treatments in furnace in the test specimens followed by cooling in different media (water, air and furnace). The properties of resistance to intergranular and pitting corrosion were analyzed by means of electrochemical tests, metallographic analyzes in optical and electronic scanning microscopes (SEM), and the results were compared with previous researches to determine the service conditions which the alloy may be subject without loss its performanc

Influence of heat treatments on the microstructure and degree of sensitization of base metal and weld of AISI 430 stainless steel

<div><p>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.</p></div