THE SEM IN SITU OBSERVATION OF TENSILE FRACTURE PROCESSES IN PLAIN CARBON DUAL-PHASE STEELS

The dynamic microprocesses of fracture in martensite-ferrite dual-phase steelswere observed by the in situ technique in SEM equipped with tensile platform.The mechanism of crack nucleation has been clarified.The conditions of crack prop-agation and affecting factors have been discussed.It is believed that the propaga-tion of the microcracks depends on the combined effect of both tensile stress andplastic shear strain.The dislocation pile-up model can be successfully used for ex-plaining the effect of the volume fraction of martensite,the carbon content in itand the low temperature tempering on crack nucleation.Furthmore,the factorsaffecting the ductility of dual-phase steels were analyzed according to the specificfeatures of fracture processes.