Strain Induced Martensitic Transformation and Low Cycle Fatigue in Metastable Austenitic Steels

The effect of strain induced martensite on the fatigue life was investigated in the air and brine by using metastable austenitic steels. A center hole, double edge key-hole or groove notches were introduced in tensile and fatigue specimens as elastic and plastic strain concentrators. Smooth specimens tested in tension exhibit the transformation product more in brine than in the air both in SUS 301 and 302. The induced martensite distribution around a center hole in tension shows a butterflywing type extent in the alloy D. In the air and brine a fatigue crack propagates in three stages both in SUS 301 and 302. The fatigue life in brine is always shorter than that in the air. The volume of induced martensite in brine is usually more than that in the air and it is confirmed that the transformation strats at the earlier cycle in brine than in the air in SUS 301. The crack growth rate per unit cycle in the second stage increases with the designed stress concentration factor on the key-hole and groove notch specimens and the complete reversed relationship is observed on the center hole specimen in which the stress gradient is seemed to be another important factor. The fringes around freshly propagated fatigue cracks, corroded by brine are often observed both in SUS 301 and 302.