The thermal stability of bulk nanocrystalline steels

Nanocrystalline bainite, commonly known as “superbainite”, is a novel class of steel that utilises careful alloy design to reduce the bainite transformation temperature to below 300℃. This results in grains that are tens of nanometres in width, which make steel strong and tough. The structure can be produced in large volumes without the need for rapid cooling or severe deformation. The presence of austenite in nanocrystalline bainite is largely responsible for the toughness. Unfortunately, the austenite is metastable and previous work has shown that it decomposes into cementite and ferrite upon heating. This decomposition makes the material weak and brittle. The present work aims to develop new alloys that form nanocrystalline bainite, but which are able to survive heating. Previous work has shown that cementite precipitation is the first stage in the decomposition process and so the first alloys developed aimed to suppress cementite precipitation. This resulted in a noticeable improvement in thermal stability, although the austenite does eventually decompose at higher temperatures. Subsequent work led to an alloy which was designed not to resist cementite precipitation, but to tolerate it without the associated loss of austenite. This was achieved by the addition of large quantities of nickel to stabilise the austenite even if its carbon content is depleted. This alloy is able to survive exposure to elevated temperatures with the majority of austenite being retained. The thermal stability of the alloys was assessed using time-resolved synchrotron X-ray diffractometry at both the Deutsches Elektronen Synchrotron (DESY) and Diamond Light Source. The high-quality data that were collected allowed the symmetry of the ferrite lattice to be investigated and it was found that the lattice was best-described using a body-centred tetragonal crystal structure. This is the first evidence of its kind. Mechanical properties were investigated in the as-transformed condition at ambient temperature and at a temperature representative of aeronautic applications. The alloys developed in this project have comparable strength, toughness and fatigue performance to existing nanocrystalline bainitic steels. Mechanical properties were also measured after heating at 480℃ for 8 d and this was found to reduce strength and toughness, consistent with the measured loss of austenite.