The Role of Microstructure on Damage Tolerance in Nano-Bainitic Steels

Abstract

Three nanostructured bainitic steels have been produced by austempering at 250, 300 and 350°C respectively after a similar austenitization treatment at 950°C. It has been found that a decrease in austempering temperature causes increase in content of bainitic ferrite (BF) as well as refinement of BF and retained austenite (RA) resulting in strengthening of the steel. However, results of sub-critical fatigue crack growth rate tests on these steels revealed that the steel transformed at higher austempering temperature which showed coarser morphology of BF and RA and also higher content of RA are more damage tolerant. It appeared that the thickness of BF plates and its volume fraction in the microstructure plays a major role in enhancing the strength whereas improved damage tolerance has been attributed to higher content of ductile phase RA. Scanning electron microscopy, nano-indentation and surface profilometry techniques revealed that the larger plastic zone size, higher amount of strain energy consumed during austenite to martensite transformation (TRIP effect) and tortuous crack path are the main factors for slower fatigue crack propagation rates in steel with the coarser morphology and higher retained austenite content.