Practice and theory in the design of martensitic steels

Lead Research Organisation: University of Sheffield
Department Name: Materials Science and Engineering

Abstract

Written records of the quenching of steel exist as early as the first century of the European Iron Age.

In Homer we find "... As when the smith an hatchet or a large axe ... plunges the hissing blade deep in cold water: whence the strength of steel ..."

Archeological evidence for quenched and tempered steel exists from several centuries earlier. Tempering has been regarded as essential in order to mitigate the extreme hardness and brittleness of as-quenched steel. On the other hand the strength limit has been reached in low cost hardened martensitic and hard-drawn pearlitic steels.

We propose to push the envelope making the radical move of dispensing with the tempering step and designing new multiphase, as quenched, tough, lean (low cost, resource efficient) steel (MATLeS). The key is to exploit recently acquired understanding of the plasticity of body centered cubic metals; work hardening; and interplay between dissolved carbon, dislocations and metal carbonitrides. This will be put together with novel state-of-the-art experimental techniques: in particular precession electron microscopy and tensile stress relaxation. In powerful combination these will furnish us with the means to manipulate and exploit the hierarchical lath martensite microstructure (HiLaMM). The new steels we design will have excellent green credentials: resource efficiency, recyclability, high strength-to-weight ratio. Our vision is toward the electric vehicle economy, light-weighting of structural offshore wind farm components and super-strong cables for undersea and civil engineering projects. Making full-circle, our outcomes will inform modern theories in materials science, advancing solutions to one of the world's outstanding scientific questions: what is the nature of work hardening? (Why can I not straighten the poker you have just bent?)

Planned Impact

We have three industry project partners: two end users and a steel maker.

Impact is to be delivered through four structured industry focused Case Studies. These are presented in detail in the Pathways to Impact attachment to this proposal. Our thinking and research are directed towards the prospects of e-mobility, urgently required if we are to meet zero carbon targets by 2020 as envisaged by current UK Government policy. Interestingly in the case of electric vehicle (EV) technology, as emphasised by our Project Partner thyssenkrupp Steel Europe (in their letter of support), impact resistance takes greater importance over strength. The danger of battery fire has been highlighted recently and Tesla has a much publicised three-layer firewall to protect the driver. In order to bring EVs into low cost mass production the materials selection process for battery housing needs to be decided upon in order to design against intrusion of the battery room during a high impact incident. In their letter of support to us, thyssenkrupp write, "The content of the proposed project promises fundamental insights into the development of new high-strength martensitic steels, which we believe will deliver a decisive contribution towards making e-mobility accessible for the masses."

e-mobility is an issue also in the aerospace sector. In a press release on Dec 19th, 2019, Rolls-Royce announced, "Rolls-Royce's ambitions to build the world's fastest all-electric aircraft have taken an important step forward with the unveiling of the plane at Gloucestershire Airport. Work will now begin on integrating the ground-breaking electrical propulsion system to enable the zero-emissions plane to make a run for the record books with a target speed of 300+ MPH in late Spring 2020." While all-electric aircraft are still in the design stage, Rolls-Royce are developing the geared turbofan as a means of reducing emissions in next generation, high by-pass, turbofan engines. The materials selection exercise for steels for shafts, sprockets and teeth is not complete and as Rolls-Royce express in their letter of support to us, "The development of disruptive technology in the form of multiphase martensitic steels which avoid the need for tempering and will produce as quenched tough lean low cost resource efficient steel will be a game changer for next generation of steels for future gear steels and components for electric engine technology."

In these ways, we expect impact primarily to be funnelled through our industrial project partners. As evidenced in our previous track record, we expect to design new alloys with the ever-present view to these being manufactured at the appropriate tonnage level. While we will be preparing laboratory scale melts for initial characterisation and for carrying out fundamental science, because we have the end goal in mind there is a high likelihood of a technology readiness level that is appropriate to industrial manufacture. Therefore the design process will involve our industry partners at all stages. Our further experience with industry indicates a very open mind when it comes to computer simulation and modelling. Rolls-Royce write in their letter of support to us, "The inclusion in the proposal of advanced physics based modelling methods is also consistent with Rolls-Royce's strategy for applying such methods to improve the development of advanced materials through cost effective and faster introduction to market." Therefore we anticipate that the theory side of the work will generate its own impact in setting some of the agenda in the modelling efforts of our project partners.

Additionally we intend to act as advocates in the mantra that "Steel is a New Material." We will expect our PDRA and PhD students associated with the project to take an ambassadorial role in speaking to Government and to the Media, through activities such as STEM for Britain and though the manipulation of social media.

Publications

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