Effects of thermal treatments on the ductile to brittle transition of MANET steel

The effect of the cooling rate T from austenitic field on the microstructural evolution and the mechanical stability of MANET steel for increasing times of tempering at 700 degrees C has been studied by means of Charpy tests, SEM observations with EDS microanalysis and X-ray diffraction analysis. The investigated rates T range from 150 to 3600 degrees C/min. The ductile to brittle transition temperature, the upper shelf energy and the slope a Delta E/Delta T in the transition region depend on the time of treatment with variations more pronounced in samples slowly cooled. After tempering times of about 20 h at 700 degrees C the samples cooled with T = 150 degrees C/min show a mixed mode of brittle fracture (quasi-cleavage and intercrystalline) with good correspondence between Cr-enriched zones and surfaces fractured in intercrystalline mode. Only quasi-cleavage mechanism is operative in samples quenched with T = 3600 degrees C/min, which present fracture surfaces with uniform Cr distribution. The results are discussed on the basis of C-Cr associate distributions determined by internal friction tests