Abstract: The hardnesses of ferrite and martensite were measured using nanoindentation of each phase. It was found that the sheared hole expansion ratio (HER) is directly related to the ratio of the hardnesses of the hard (martensite) and soft (ferrite) phases. The increase in hardness of both phases occurred during testing, i.e. hole punching and expansion. In the case of the ferrite this increase is due to second stage work hardening attributed to the co-deformation of ferrite and martensite. A strong positive linear correlation exists between the hardness difference between ferrite and martensite of the undeformed plate and UTS where larger differences lead to higher UTS but low HER. Fractal analysis has also been investigated to characterize the ferrite/martensite interface where higher fractal dimensions (rougher boundaries) of more complex microstructures lead to a reduction in HER.
Authors: Jason Andring, Cyrus Eason, and Anthony DeArdo
Keywords:Dual phase steel, fractal analysis, nanohardness of individual phases, hole expansion ratio