On the relations between cyclic contraction ratio, flow stress and deformation mechanisms in bainitic CrMoV steels: Dissertation

The cyclic diametral strain and stress response of macroscopically untextured (nominally isotropic) bainitic Cr-Mo-V steels has been studied.The total axial strain amplitudes were controlled and chosen so that a range of ratios of plastic and elastic elongations were used extending from 0.04 to 5. The trend of the cyclic diametral strain was sometimes found to drastically deviate from the commonly used Poisson's ratio when the ratio of plastic and elastic elongation was around 3 for the uncycled material.The unusual initial increase in cyclic contraction ratio for these conditions was attributed to strain concentration and the decrease to strain decent ration.A condition for these unpredictable macroscopic effects seems to be that the effective strengthening structure should be sufficiently unstable during the cyclic strain applied.At room temperature fatigue slip bands of high local density and number are created in these conditions.Such conditions were found to be satisfied at ambient temperature if the carbides were spaced widely enough -or more generally- if the temperature of testing was elevated.The basic reason for the unexpected combination of softening and decreasing diametral strain range seems to be gradual latent hardening in the slip band bundles which carry the cyclic plasticity.At elevated temperature applied strains larger than ~0.3 % give rise to an increasing mechanically activated dynamic recovery which operates despite a dense carbide dispersion.The amount of recovery and simultaneous dislocation annihilation increase and act to lower the flow strength with rising strain.Their extent depend on the strain rate.An apparent maximum in dynamic recovery was observed as a minimum in cyclic yield strength at the same strain for which the pronounced unpredictable diametral strain was observed.Similar diametral strain effects in monotonic tension tests on different materials reported in the published literature indicate that the effects are most probably related to the particular dominant mode of slip at strain levels for which the ratio of plastic and elastic strain (ea~eE) is around three.Slip is then dominantly planar.Careful shape control of the specimen gauge section is necessary for reproducible diametral strain because of the unstable nature of the material in the actual conditions of the cyclic straining which produce the unexpected behaviour.