STUDIES ON PLASTIC INSTABILITY, NOTCH DEFORMATION AND HYDROGEN EFFECTS IN SPHEROIDIZED STEEL

Detailed studies on the mechanics of deformation and crack initiation at a U-notch in spheroidized AlSl 1090 steel were conducted both with and without cathodically charged hydrogen. For large scale plasticity at a U-notch root, the principal plastic strain distribution does not follow a hyperbolic law, but may be described in the form (epsilon)(,2) = P(,1)exp('P(,2)x/(rho)) where P(,1) and P(,2) are functions of plastic bend angle. The region of maximum strain gradient may be approximated by (epsilon)(,2) = 0.08(1+x*/(rho)) where x*/(rho) represents the process zone size for each plastic bend angle.;In-situ H-charging and bend tests on spheroidized 1090 steel show that hydrogen promotes early onset of surface instabilities independent of void formation. Early and multiple surface cracking are also promoted by hydrogen as a consequence of hydrogen-induced early onset of plastic instability. Localized void profusion along characteristic slip lines is enhanced by hydrogen, thereby accentuating the growth of surface microcracks and early fracture. Studies on the effect of notch root strain rate, notch root prestrains, different charging and testing modes, as well as the effect of hydrogen on microhardness, together, show that hydrogen degradation is greatest when atomic hydrogen is localized to the near surface region where it promotes early onset of plastic instability. The higher embrittlement in dynamic hydrogen charging and bend testing as compared to precharge experiments is associated with contact of the electrolyte with plastically deforming notch surface.;Analysis of notch surface deformation patterns shows that surface instabilities form at a critical strain that is in fair agreement with predictions based on bifurcation analysis. Plastic instability initiates first at the notch surface in the form of instability bands independent of the presence of voids. Surface microcracks initiate within the instability bands. Crack initiation in the notch root process zone is characterized by the opening and extension of surface microcracks along characteristic slip lines accentuated by the extent of localized void profusion beneath the notch root.