Mechanical properties of nanostructured bainitic steels

Abstract Nanostructured bainitic steels possess ultra-high strength and have been the focus of extensive research due to their suitability in many demanding engineering applications, especially in rail-wheel system, gears, bearings and automobile industry. The composite microstructure of nano-scaled bainitic ferrite (BF) and retained austenite (RA) balances the strength-ductility combination to reach 21% elongation for over 2.1 GPa of tensile strength. The strength mainly arises from large numbers of bainitic interfaces and adequate ductility is gained due to the softness as well as transformation-induced plasticity (TRIP) effect of RA. Moreover, recent works devoted to characterize the toughness of nanostructured bainitic steels show that a plane strain fracture toughness of 30 MPam0.5 is routinely obtained for a tensile strength of 1.5 GPa. The toughness values can be increased to as much as 128 MPam0.5 for a tensile strength of 1.6 GPa by appropriately tailoring the microstructure through specially designed thermo-mechanical heat-treatment procedures. However, excellent quasi-static properties alone do not suffice and consequences of microstructural refinement on fatigue properties should also be known for designing a damage tolerant material. Above all, a fundamental understanding of process-microstructure-property correlation in nanostructured bainitic steels is of prime importance. In this review article on the mechanical properties of nanostructured bainitic steels, we have first presented the background on development of these steels through appropriate alloy design and specific heat-treatment procedures. Moreover, the mechanism of phase-transformation and microstructure evolution has been discussed briefly. Subsequently, we have reviewed a range of original works focused on various mechanical properties including tensile properties, torsion, fracture, fatigue and wear behavior of nanostructured bainitic steels. The deformation mechanisms and mechanical properties have been shown to be primarily determined by the volume fraction, morphology and size of BF and RA as well as the alloying content of the steel.