Design of novel bainitic steels

Mixed microstructures consisting of fine plates of upper bainitic ferrite separated by thin films os stable retained austenite have seen many applications in recent years because of their impressive combination of strength and toughness. There may also be some martensite present in the microstructure, but its formation can be controlled increasing the stability of the retained austenite. On the other hand. carbides are avoided by the judicious use of silicon as an alloying element. The aim of the present work was to see how far these concepts can be extended in order to achieve the highest ever combination of strength and toughness in bulk-samples subjected to continuous cooling transformation. Three alloys were proposed and manufactured, and the results of metallographic characterisation and mechanical tests have shown that the designed steels have the highest ever combination of strength and toughness for bainitic microstructures, matching even the maraging steels which are at least thirty times more expensive. The experimental results confirm the alloy design procedures. (Author) 29 refs