Design of Novel Bainitic Steels: Moving from UltraFine to Nanoscale Structures

Abstract

The concepts of phase transformation theory can be exploited to design nanostructured steels that transform to bainite at temperatures as low as 150 ºC. The microstructure obtained is so refined that it is possible to achieve a strength in excess of 2.5 GPa in a material which has considerable toughness (40 MPam 1/2). Such combination of properties has never been achieved before with bainite. A description of the characteristics and significance of this remarkable microstructure in the context of the mechanism of transformation is provided. Advanced bainitic steels ready for the Nano Era There is a growing current awareness of the potential benefits of nanoengineering in the modern steel industry, and leading research and development institutes and companies are pursuing research in the area of nanostructured steels. In this industry, the term ultra-fine grained is generally used to describe steels with average grain sizes between 1 and 2 μm and the term submicron (submicrometre) structure to refer to grain sizes between 100 and 1000 nm. Until recently, effective processing techniques to reduce the grain size of these materials to less than 100 nm did not exist. There are major difficulties in creating novel nanostructures that have a combination of properties appropriate for large scale applications. An important requirement is to be able to manufacture large 3-dimensional nanostructured components. In addition, the material concerned must be cost effective to produce, if it is not to be limited to niche applications. Severe deformation by methods, such as mechanical milling, equal channel angular processing and high pressure torsional straining, has not succeeded in this respect. Although mechanical milling and alloying can produce powders containing nanosized grains, grain growth cannot effectively be suppressed during consolidation processes such as sintering and hot pressing. Therefore, processing bulk

Transmission Kikuchi Diffraction study of texture and orientation development in nanostructured hard turning layers

“…The retained austenite grains are extremely fine ($50 nm) and equiaxed. Recent studies have found inverse correlation between size of the austenite grain and amount of carbon dissolved in the solution [7]. The higher carbon in the retained austenite could occur from dissolution of carbides and movement of carbon out of martensite due to dislocation movement.…”

“…The higher carbon in the retained austenite could occur from dissolution of carbides and movement of carbon out of martensite due to dislocation movement. It is also known that smaller grain size and higher amount of dissolved carbon significantly enhances mechanical stability of retained austenite [7]. In the application point of view, the retained austenite in the thermally transformed layer will have higher mechanical stability to resist the strain induced transformation (TRIP effect); on the other hand, it also means dramatic improvement in the dimensional stability.…”

Martensite and bainite in nanocrystalline steels: understanding, design and applications

“…In general, low transformation temperatures are associated with fine microstructures which in turn generally possess both strength and toughness. Following this simple concept, a new generation of steels has been designed in which transformation at low temperature leads to a nanoscale microstructure consisting of extremely fine, 20-40-nmthick, plates of ferrite and retained austenite [7]. These microstructures are achieved through isothermal transformation to bainite of high carbon high silicon steels with low martensite start temperature (approx.…”

Thermally Stable Nanocrystalline Steel

“…At temperatures where the atomic mobility of carbon atoms is sufficient, there will then be a tendency for the austenite to decompose into a mixture of ferrite and cementite. Many studies have observed a carbon supersaturation with respect to cementite in both austenite [5,6,[10][11][12][13][14][15] and ferrite. [12,13,[15][16][17][18][19] There is therefore a large driving force for the formation of cementite in both phases.…”