Obtenção e caracterização de aço fundido bainítico com elevada resistência á fadiga mecânica de alto ciclo

Abstract

The purpose of the present work was to characterize and further develop a novel kind of nanostructured bainitic steel. This steel casting not required thermomechanical processing, can be produced in large pieces, and present GPa strength. The special structure consisting of ferrite laths surrounded by retained austenite with highconcentration of carbon can be formed in steels containing 1.5% of silicon or more. In this high carbon steel, including molybdenium, chromium, nickel, manganese and cobalt, the very fine bainite can be obtained by isothermal transformation at temperature lower than 200°C. The time taken for nucleation at this temperature can be many days,resulting in a nanostructured steel casting due to the growth of extremely thin small plates of bainite. The present work reports the first experiments on this system developed in Brazil. The steel was produced using an air melting technique. The heat treatments were conducted including austenitisation at 875°C for 30 min prior to isothermal transformation at 200°C and 300°C for 240 h and 18 h, respectively. Thedilatometric technique was used in thermal cycle heat treatment definitions. It was obtained a combination of properties with ultimate strength and ductility like as 2,1 GPa and 1%; 1,7 GPa and 15%, for austempering at 200°C or 300°C respectively. It is reported the first results of fatigue tests by high-cycle rotating bending which permittedthe determination of the fatigue life, as defined to be the maximum cycles stress for the samples must survive over 107 cycles. The fatigue life stress was found to be 593 MPa for austempering at 200°C, and 516 MPa for austempering at 300°C. These fatigue life results are higher than those presented by important engineering materials such asforged steel and austempered ductile cast iron, in spite of the fact that was observed fish eye fracture patterns, characteristic of fatigue fracture initiation at shrinkages defects. The results suggest further investigations in order to eliminate these defects and enhance the novel steel for future engineering applications.