Desenvolvimento do aço alto C-Si e Cr com matriz nanoestruturada de martensita e bainita

Abstract

In this work was carried out the study and development of a commercial steel with high content of C, Si, Cr and with a nanostructured matrix of martensite and bainite. The steel used in this work was the SAE 9254 which is also used in the manufacture of axles, coil springs and rods for shock absorbers. Samples 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). Different mechanical behavior was observed for samples treated under various conditions due to the microstructural evolution. A majority microstructure of martensite with the presence of bainitic ferrite and retained austenite blocks was observed for samples treated at 200°C. However for samples treated at 270°C was observed a predominant presence of bainitic ferrite sheaves interspersed with films of retained austenite and the presence of some islands of martensite and retained austenite blocks. Sample treated for 2 h at 200°C showed lower yield strength (979 MPa), however a higher tensile strength (2248 MPa). While the sample treated at 270°C showed higher yield strength (1363 MPa) for the same treatment time. In the analysis of fracture, the mechanism of brittle fracture was predominant for the samples treated at 200°C. While the mechanism of ductile fracture was predominant for the samples treated at 270°C. In this study was also observed the appearance of cracks in the microstructure of the treated samples from 220°C