INFLUENCE OF SEA-WATER ON THE FATIGUE-STRENGTH AND NOTCH SENSITIVITY OF A LASER-HARDENED B-MN STEEL

Following earlier publications, the effects of sea water and laser har dening on fatigue strength and notch sensitivity of smooth and notched quench and tempered (Q&T) and laser hardened (LH) specimens were inve stigated. Constant-amplitude plane-reversed bending corrosion fatigue tests (R=-1) at 47 Hz in sea water were conducted using cylindrical LH and Q&T steel specimens made of a B-Mn SS2131(similar to AlSl 15B21H) steel with K-t=1.05 and 1.7. The results are compared with those obta ined for plasma-nitrided (PN) specimens. LH specimens show a general c orrosion attack by sea water with a much higher corrosion rate than th at for PN specimens. The S-N curves show that sea water suppresses the fatigue limit and reduces fatigue strength (especially at long fatigu e lives) of smooth and notched Q&T and LH specimens. At long fatigue l ives, laser hardening produces a significant improvement of the corros ion fatigue resistance of smooth Q&T specimens, but no improvement is observed for the notched specimens. Cracks arising from pits produce c orrosion fatigue failure at intermediate and long fatigue lives. The m echanisms for corrosion fatigue crack initiation, suggested in this wo rk, permit a clear explanation of the experimental features, based on the formation of a pit around a damaged zone and subsequent crack nucl eation. Application of an equation derived by the authors in a previou s work, gives good prediction of the corrosion fatigue strength of not ched specimens. (C) Munksgaard, 1998.