Efeito do revenimento sobre a microestrutura e o desempenho mecânico de um aço multiconstituído com alto teor de Si

• Ano de defesa: 2014
• Autor(a) principal: José Alberto da Cruz Junior
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Federal de Minas Gerais; UFMG
• Palavras-chave em Português: Revenimento; Austenita retida; Propriedades mecânicas; Martensita; Bainita; Engenharia metalúrgica
• Link de acesso: http://hdl.handle.net/1843/BUOS-9N5GRK

In this work a research was carried out to study the effect of isothermal treatment temperature and tempering in a commercial steel with high content of Si. The steel used was SAE 9254 which is also used in manufacturing axles, coil springs and rods for shock absorbers. Tension specimens of this steel were austenitized at 900°C for 5 minutes and isothermally treated at different times (0.5, 2, 24, 48 h) and temperatures (200, 220, 270°C) and after being tempered (400°C/1 h). Impact specimens were treated in treatment times of 0,5 e 48 h and subjected to tempering (400°C/1 h). For means of comparison of microstructures and mechanical properties, other impact specimens were only isothermally treated for treatment times of 0.5 and 48 h. The microstructure with the major presence of tempered martensite, bainitic ferrite, retained austenite films and carbides were observed for the isothermally treated samples at 200°C and tempered._As for those samples treated at 270°C and tempered was observed a major presence of bainitic ferrite, retained austenite films, carbides and the decomposition of retained austenite blocks, due to tempering. The mechanical properties of tensile and impact specimens were similar independent from isothermal treatment times. Yield strength and ultimate tensile strength decreased with increasing isothermal treatment temperature, due to a lower volume fraction of martensite formed in the initial cooling, resulting in increased of ductility. The rise of isothermal treatment temperature provided greater impact energy absorbed due to a greater volume fraction of retained austenite. As for impact specimens, after tempered, they showed a fall in the values of energy absorbed due to decomposition of retained austenite blocks into bainite rich in carbides. In the analyses of the fracture surface, impact specimens, isothermally treated, showed mixed fracture mode with regions brittle (cleavage) and ductile (dimples). For impact specimens, isothermally treated and tempered, fracture surface was characterized as brittle (cleavage) due to the decomposition of retained austenite, which resulted in a lower impact energy absorbed.