Abstract
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°C and subsequent partitioning at 250°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 (iii) a small amount of carbide only in the wide lath-martensite structure, which may contribute to making fatigue crack initiation and/or propagation difficult.
