The galling behaviour of 316L stainless steel was investigated in both the unoxidised and oxidised states, after exposure in simulated PWR water for 850 h. Galling testing was performed according to ASTM G196 in ambient conditions. 316L was found to gall by the wedge growth and flow mechanism in both conditions. This resulted in folds ahead of the prow and adhesive junction, forming a heavily sheared multilayered prow. The galling trough was seen to have failed through successive shear failure during wedge flow. Immediately beneath the surface a highly sheared nanocrystalline layer was seen, termed the tribologically affected zone (TAZ). It was observed that strain-induced martensite formed within the TAZ. Galling damage was quantified using R t (maximum height -maximum depth) and galling area (the proportion of the sample which is considered galled), and it was shown that both damage measures decreased significantly on the oxidised samples. At an applied normal stress of 4.2 MPa the galled area was 14 % vs. 1.2 % and the R t was 780 µm vs. 26 µm for the unoxidised and oxidised sample respectively. This trend was present at higher applied normal stresses, although less prominent. This difference in galling behaviour is likely to be a result of a reduction in adhesion in the case of the oxidised surface.
Multi-axial forging (MAF) at room temperature is employed to investigate the effects of deformation processing path and the amount of imposed strain on the deformation mechanisms of a twinning-induced plasticity (TWIP) steel.The results indicate that the twin frequency is decreased by applying the 2nd compression ( P ɛ ¼ 0.8), however, an unexpected increase is realized at the end of first MAF pass ( P ɛ ¼ 1.2). This is attributed to the change in strain path and orientation dependence of deformation twinning. The latter is phenomenal considering the previous researches reporting the suppression of twinning at the early stage of deformation (ɛ < 0.4). The same sequence is followed during the second pass of MAF process. Interestingly, the progressive and continues substructure development along with dynamic Hall-Petch effect results from deformation twinning leads to an appreciable grain refinement. The latter is accompanied by the sharp drop of hardening rate in corresponding flow curves. The microtexture analysis indicates the strengthened texture of the 1 pass-processed specimens which is weaken at the end of 2 pass due to the recrystallization and increasing the number of texture component. The current work also explores the room temperature mechanical properties of the multi axial forged workpiece through elaborating the miniaturized tensile testing method.
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