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

10 25 50
 
Description The effect of martensite start temperature (Ms) on martensite microstructure has been systematically investigated for the first time. This has shown the importance of Ms on the size of the microstructural subunits (block size, packet size, lath size etc), which strongly affects the mechanical properties. The role of carbon was believed to be of principal importance, but we have shown that it is in fact secondary. The transformation process has been observed mid-way giving a unique insight into the mechanisms. The role of each alloy element addition has been investigated, with the role of cobalt and chromium being particularly interesting. Not only has the work developed completely new understanding of the martensite transformation mechanism, it also opens up routes to design new martensitic steels in a resource efficient manner. It also opens the possibility of using martensitic steels in their as-quenched condition.
Exploitation Route The research has laid the route to design resource efficient martensite steels with high strength and ductility, which will be of interest to industry.
The research has opened the possibility of martensitic steels being used in their as-quenched state.
Sectors Aerospace

Defence and Marine

Construction

Energy

Manufacturing

including Industrial Biotechology

Transport

URL https://www.martensite.org/
 
Description The understanding of martensite structure has allowed the design of a new Reduced Activation Ferrite/Martensite Steel (RAFM) that will allow higher temperature operation in the STEP reactor. This is being upscaled with UKAEA. The design paradigm will allow further developments in all high temperature steels. The understanding of the formation of martensite is being used in the design of new ultra-high strength steels for wire applications, for example, for high tensile cables for bridges and deepwater oil rig moorings that are some of the most demanding applications for any material.
First Year Of Impact 2022
Sector Aerospace, Defence and Marine,Construction,Energy,Transport
 
Description British Steel 
Organisation British Steel
Country United Kingdom 
Sector Private 
PI Contribution Design of a new steel for high strength wire applications Determination of fracture modes in rail steels
Collaborator Contribution Sponsoreship of 3 PhD students Supply of materials for the project Intellectual input into the projects
Impact Design of a new steel composition and process route for high strength wire Recommendations for microstructure of rail steels
Start Year 2006
 
Description Rolls-Royce Submarines 
Organisation Rolls Royce Group Plc
Department Rolls Royce Submarines
Country United Kingdom 
Sector Private 
PI Contribution Design of new iron based hard facing alloys for pump and valve applications in the nuclear industry
Collaborator Contribution Provision of raw materials (powders etc) and processing (laser deposition) Intellectual input.
Impact Design of a new iron based hard facing alloy to replace existing cobalt based alloys for nuclear applications
Start Year 2017
 
Description The Hatfield Memorial Lecture 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact The Hatfield Memorial Lecture is an annual event hosted by the Department of Materials Science and Engineering at Royce Partner the University of Sheffield. It featured presentations from noted academics and industrialists on topics as diverse as space exploration and biomaterials.
Year(s) Of Engagement Activity 2021
URL https://www.sheffield.ac.uk/materials/department/community/events/hatfield-memorial-lecture