Influence of sea water on the fatigue strength and notch sensitivity of a plasma nitrided B–Mn steel

Notched and smooth cylindrical plasma nitrided (PN) and quench and tempered (Q&T) steel specimens made of a B–Mn SS2131(≈AISI 15B21H) steel have been exposed to constant amplitude plane reversed bending corrosion fatigue tests (R=−1) at 47 Hz in sea water. S–N curves show that sea water suppresses the fatigue limit and reduces fatigue strength (especially at long lives) of smooth and notched Q&T and PN specimens. Plasma nitriding improves the corrosion fatigue resistance of Q&T specimens; this is associated with the good corrosion resistance of ε and γ′-phases, the enhancement of corrosion and fatigue by compressive residual stresses, and the consumption of H+ ions during reduction of nitrogen. This improvement is more significant for smooth specimens and for long lives. Notch sensitivity of Q&T and PN specimens decreases with fatigue life. Pitting corrosion, cyclic applied stress and residual stresses due to plasma nitriding are the main topics of this work. An equation to predict corrosion fatigue strength from air fatigue strength of smooth specimens, has been established.