To investigate the tensile deformation behavior of high martensite DP steels using nano-indentation and spherical indentation hardness testing

Abstract

Dual phase steels find extensive applications in auto-industry because of their characteristic properties. For the ferrite-martensite dual phase steels, the soft phase (ferrite) imparts ductility whereas the hard phase (martensite) imparts strength. Ferrite affects the properties mainly through its volume fraction whereas martensite affects the properties by its volume fraction as well as morphology and distribution. In the present work, the starting steel of low carbon content was processed through CAS route with slight changes resulting in two harmonic type dual phase microstructures (core and shell type). These steels containing DP microstructures had differences in martensite fraction and grain size and were designated as DP-1 and DP-2 steels. Volume fraction of martensite in DP-1 and DP-2 was 52.2% and 62.2% respectively. After tensile tests, it was found that DP-2 resulted in more ultimate tensile strength, yield strength, and ductility. Reasons for difference in the tensile properties of the two steels were investigated with SEM analysis of fractured surfaces of tensile tested specimens. The deformation behaviour/damage mechanisms shown by the two steels were also analyzed using nano-indentation and spherical indentation hardness testing techniques. For nano-indentation hardness testing, contour plots were plotted for hardness under constant low load as well as progressive load (low to high load conditions) at various locations. Spherical indentation hardness testing was also done to confirm the outcomes of nano-indentation hardness testing results. Here, plots were made for hardness as a function of depth of indentation. It was found that in DP-1 steel, hardness didn’t change significantly at various points of testing (contour plots did not show significant change in color pattern at different locations). However, in DP-2 steel, contour plots showed significant changes in colour at various points of observation. The results of hardness testing showed that for DP-2 steel showing better tensile properties, both the phases (i.e. ferrite as well as martensite) participated in the plastic deformation process. The same trend was also found in spherical indentation which showed that in DP-2 steel, both the phases accommodated plastic deformation, thus provided superior strengthductility combination.