Microstructural response on the room temperature low cycle fatigue behaviour of two high strength duplex ferrite–martensite steels and a normalised ferrite–pearlite steel

Strain controlled low cycle fatigue (LCF) behaviour of two high strength duplex ferrite–martensite (DFM) steels with continuous martensite developed from a commercial grade vanadium bearing microalloyed steel through two different intercritical annealing routes has been studied at room temperature. The results have been compared with that of normalised ferrite–pearlite structure of the same steel. A mixed cyclic stress-strain behaviour and much higher cyclic life were observed in steel containing ∼50% martensite as compared to that containing ∼80% martensite exhibiting complete cyclic softening behaviour. Much higher cyclic life of the steel containing ∼50% martensite, particularly at low strain amplitude, has been discussed in terms of crack growth retardation phenomena. Bilinear Coffin–Manson (C-M) behaviour has been observed in case of the steel with 80% martensite. Under total strain control mode, better LCF resistance has been observed for DFM steels as compared to the normalised ferrite–pearlite structure.