Effects of retained austenite and martensite microstructure on fatigue crack propagation in quenched and tempered high carbon steels

• High-carbon steel microstructures with higher initial retained austenite content show improved fatigue crack growth resistance. • Stress assisted retained austenite to martensite transformation observed in the vicinity of a fatigue crack in high-carbon martensitic microstructures. • Presence of retained austenite suppresses brittle intergranular fracture and promotes tortuous transgranular fracture during stable mode I fatigue crack propagation due to transformation toughening and transformation induced micro-crack coalescence. Commercial high-carbon 52,100 steel was subjected to varying quenching and tempering heat treatments to develop plate martensite and retained austenite (RA). The mode I fatigue crack growth (FCG) behavior of quenched and tempered microstructures with varying amount of RA and degree of martensite tempering was investigated. The interaction between the fatigue cracks and the surrounding microstructure was characterized using electron microscopy/backscatter diffraction and quantitative fractography. Higher initial amounts of RA and greater degree of martensite tempering improved FCG resistance of high-carbon plate-martensite austenite microstructures. Stress-assisted RA to martensite transformation was observed in the vicinity of the fatigue cracks in the microstructure heat treated to have an elevated amount of RA. The fatigue fracture surfaces of the different microstructures exhibited varying combinations of intergranular (IG) fracture, mixed-ductile brittle (MDB) and transgranular (TG: cleavage, quasi-cleavage, and ductile striations) fracture as a function of stress intensity range at the crack tip. Microstructure conditions showing greater fractions of MDB and TG fatigue fracture had better FCG resistance. The presence of a higher initial RA content suppressed brittle IG fracture and promoted TG fracture, potentially due to transformation–toughening associated with RA to martensite transformation or transformation induced micro-crack coalescence near the fatigue crack.