Design of novel high strength bainitic steels: Part 2

Abstract

Mixed microstructures consisting of ®ne plates of upper bainitic ferrite separated by thin ®lms of stable retained austenite have seen many applications in recent years. There may also be some martensite present, although carbides are avoided by the judicious use of silicon as an alloying element. The essential principles governing the optimisation of such microstructures are well established, particularly that large regions of unstable high carbon retained austenite must be avoided. With careful design, impressive combinations of strength and toughness have been reported for high silicon bainitic steels. The aim of the present work was to ascertain how far these concepts could be extended to achieve unprecedented combinations of strength and toughness in bulk samples subjected to continuous cooling transformation, consistent with certain hardenability and processing requirements. Thus, this paper (part 1 of a two part study) deals with the design, using phase transformation theory, of a series of bainitic alloys, given a set of industrial constraints. Part 2 of the study concerns the experimental veri®cation of the design process.MST/4644

<i>In Situ</i> High Temperature X-Ray Studies on Bainitic Transformation of Austempered Silicon Alloyed Steels

“…The forged steel A has been investigated earlier by F. G. Caballero [15]. The test samples were prepared following the experimental procedures of reference 15.…”

Tensile Ductility of Nanostructured Bainitic Steels: Influence of Retained Austenite Stability

“…They consist of ultrafine bainitic laths (typical width under 50 nm) surrounded by retained austenite [3,11]. Interestingly, the initial mechanical properties [2] were at best on par with those of quenched and tempered high strength spring steels [16], but were later improved to reach an unprecedented 21% elongation for over 2.1 GPa in tensile strength [10,11].…”

“…Bainitic microstructures formed at low temperatures (350 • C or less) have received a considerable amount of attention in the recent years [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. These microstructures are obtained in relatively high carbon steels (0.6-1.2 wt %, although the concept can be extended to lower carbon contents) through isothermal transformation over durations ranging from 10 to over 100 h [5,11,15].…”

Nanostructured steel industrialisation: Plausible reality

“…The reported mechanical properties [3,4] and the impressive improvement in transformation kinetics achieved through different generations of NANOBAIN steel grades [5]were highly appreciated by potential final users, but still, there were some issues to be addressed before further steps towards the industrialization of such microstructure could be taken.…”

“…Thus, during three years a consortium of steel makers, end users and researchers work together on the design of two tailored grades to manufacture two very different components with a nanostructured bainite microstructure, -small component, 20x2x3 cm 3 , heat-treated using dry gas technologies, DryBain TM , [6] with emphasis on its fatigue resistance and strength UTS > 2000 MPa -larg component, 70x40x20 cm 3 , heat-treated in salt-bath, with emphasis on wear resistance and UTS > 1600 MPa.…”