Notch-fatigue Strength of Advanced TRIP-aided Martensitic Steels

The notch-fatigue limit and notch sensitivity of 0.1-0.6%C-1.5%Si-1.5%Mn transformation-induced plasticity (TRIP)-aided martensitic steels (TM steels) were investigated for use as common rails in next-generation automotive diesel engines. Also, these properties were related to the microstructural and retained austenite characteristics. When TM steels containing 0.2% to 0.4% C were subjected to heat treatment for isothermal transformation at 50 degrees C and subsequent partitioning at 250 degrees C, the steels achieved much higher notch-fatigue limits and lower notch sensitivities than those of conventional 0.2-0.4%C-1.0%Cr-0.2%Mo structural steels. This was principally associated with (i) plastic relaxation of localized stress concentration as a result of strain-induced transformation of 3-5 vol% metastable retained austenite and (ii) a large amount of finely dispersed martensite-austenite phase along prior austenitic, packet and block boundaries, as well as (Hi) a small amount of carbide only in the wide lath-martensite structure, which may contribute to making fatigue crack initiation and/or propagation difficult.