Processamento térmico de têmpera e partição (T&P) na obtenção de ligas metálicas fundidas de alta resistência mecânica

Abstract

The process in the casting industry is constantly looking for improvements to maximize mechanical properties in view of competition between materials such as forged steels, ductile cast irons, and aluminum alloys. One line of research, development, and innovation (RDI) for cast steels, also called nanostructured steels, is the application of austempering in steels designed for isothermal treatment at low temperatures around 200ºC. For this, the steels, besides some alloying elements, have carbon around 0.8% in mass. Thus, to obtain mechanical strength above 1500 MPa, the heat treatments are very long and may take several hours. The process of quenching and partitioning (Q&P) has been used for the improvement of high strength steels, belonging to the third generation, which results in the combination of high strength without significant deterioration of ductility. In this thesis, cast materials and heat treated by Q&P were developed to evaluate the influence of the treatment on microstructure and mechanical strength. For this purpose, a cast steel of 0.28%C-1.25%Mn-2.20%Si and a ductile cast iron of 3.27%C, 2.47%Si, 0.20%Mn were produced both with low alloying element contents. The low carbon steel aims isothermal treatments with shorter treatment times. The research lines of application of Q&P in cast steels and nodular cast iron are poorly explored. Therefore, we thought to analyze the application of this treatment in ductile cast iron as an alternative for better mechanical properties. The results show that the microstructure of the Q&P treated steel consists of martensite, bainite and retained austenite. This resulted in a better combination of tensile strength (1647 MPa) and impact toughness (27J/cm2). Compared to the Q&P treated steel, the same steel cast in the austempered condition, showed inferior mechanical properties: ultimate tensile strength of 1329 MPa, yield strength of 1311 MPa and toughness of 7 J/cm2. The Q&P heat treatment of the nodular cast iron was performed in two ways: a conventional one presented in the literature, using one cooling furnace for the quenching step and another furnace for the partitioning step. The second innovative way, using a single salt bath furnace for cooling and isothermal treatment, respectively. This form has been termed as ramp T&P treatment. The ductile cast iron obtained by Q&P processing showed promising results with about 102 graphite nodules per mm2. The microstructure of nodular cast iron after Q&P treatment consists of martensite, ferritic bainite, and retained austenite. In ramp Q&P processing, yield strength of 1659 and 1757 MPa and yield strength 1641 and 1722MPa were obtained for the partition conditions at 300 and 375°C, respectively. In the traditional two oven processing, yield strength of 1743 and 1584 MPa and tensile strength 1679 and 1486 MPa were obtained for the partition conditions at 300 and 375°C respectively. The values obtained are twice the values specified in the standard for higher strength nodular cast iron, but with elongation a little less than 2%. The increase in the strength limit in relation to ADI was around 10% with superior impact strength. The results are promising, showing that the Q&P process is a viable alternative in obtaining high strength ductile cast iron with competitive potential compared to ADI.