Electric-furnace steel for electrical applications
Fine-grain stable case-hardening steel for hybrid and electric gear systems
Dr.-Ing. T. Wurm, T. Duda
Legislation and efforts to reduce CO2 emissions are set to change the drivetrains of cars and commercial vehicles drastically. The development and optimisation of classical internal combustion engines (ICE) runs up against limitations with respect to a reduction of fuel consumption sufficient to achieve already enacted and future CO2 limits. Solutions currently practicable involve only the electrification of drivetrains, either using hybrid, electric-battery or fuel-cell technology. New power-train concepts are being developed employing higher-speed electric motors compared to the internal combustion engines. These electric motors require greater gearing reduction and fewer gear speeds. The gearwheels and gear shafts used as standard in drivetrains may, however, be subjected to stresses different to those familiar up to now, either in the form of higher motor speeds and load cycles or due to additional loads resulting during recuperation when the vehicle decelerates.
These new applications and systems give rise to new demands on the materials used and on component design. The market and political pressure exerted is immense – both time and money for Research & Development are a rarity during this transformation phase. For this reason, many gear-system manufacturers currently make use of tried and proven, low-cost materials in order to take their first steps in these unfamiliar applications. In this context, some crucial questions arise for all producers throughout the steel-processing value chain:
- Will the case-hardening steels used up to now for gearing components be able to withstand the stresses anticipated in electric drivetrains?
- Can established grades be optimised or will it be necessary to develop, test and make ready for series production new, higher-strength steels?
OEMs, gear manufacturers and Georgsmarienhütte GmbH (GMH), one of Europe’s leading producers of steel bar, perceive potentials for further rises in the load- and stress-bearing capacity of steels in the enhancement of the so-called fine-grain stability of case-hardening steels, i.e. the stability of the internal “character” (microstructure) of steels even under exposure to high carburisation temperatures. Grain sizes and their distribution in the structure of the steel could be capable of significantly influencing the mechanical and technological properties of these materials and the components produced from them.
Influence of grain size on dynamic strength properties of components
The mechanical-technological properties of steels can be influenced by a wide range of different mechanisms. These include such measures and treatments as the addition of specific alloying elements to strengthen the solid solution, precipitation hardening, phase transformation, plastic forming and grain refinement. The yield strength and dynamic strength properties of steel, for example, can be enhanced by treatments that cause the formation of a finer-grained microstructure while preventing the formation of mixed/coarse grains. Fig. 1 [1].
![Fig. 1: Influence of coarse grain formation on fatigue cracking properties [1]](https://www.e-motec.net/wp-content/uploads/2024/12/GMH-EMOTEC-FIG-1.jpg)
Today, gear components are primarily made using Mn-, Cr-, Mo- and Ni-alloyedcase-hardening steels with relatively low carbon contents of around 0.2 wt%. GMH supplies steel bar in these grades as the primary material to companies that produce semi-finished gear components by means of hot, warm and cold-forming processes. To achieve extra-high mechanical strength and wear resistance, particularly in the tooth face and root areas of the future gear components, the semi-finished parts are usually subjected to a case-hardening treatment, i.e. carburising of the component surface followed by quenching and tempering. As a result, the finished component will possess different properties internally and at its surface: it will have a relatively tough centre and a surface characterised by high strength and good wear resistance.
Carburising is usually performed at temperatures of up to about 950°C. Temperatures above 1,000°C (up to about 1,050°C) may accelerate the process, as they strongly promote the diffusion of carbon into the steel matrix. However, special attention must be devoted to the fact that the higher the carburising temperature the higher the risk of grain coarsening becomes [2, 3], resulting in poorer dynamic strength properties. It is therefore vital that the treatment is performed within precisely the correct temperature range to assure fine-grain stability in the respective steel grades. A coarse-grain austenite after carburising results in a significant deterioration in the material’s fatigue strength (see Fig. 1).
The challenge throughout the production process is to achieve the perfect balance between treatment temperature and treatment time in order to avoid the formation of a coarse-grained microstructure.
The process chain from steel bar to e-gear
Fine-grain stability in case-hardened steels and the risk of grain coarsening in components depend not only on the carburising temperature and time, but also on a number of other process parameters, such as the chemical composition and precipitation characteristics of the steel.
Fine-grain stability should therefore be considered a critical process parameter – not a material property – because the influencing effects, such as precipitation, for example, may vary along the entire process chain due to the influence of temperatures and the forming processes. Only with the combination of in-depth knowledge of the entire process chain – from steel production down to the finish-machining of the gear components – and judicious consideration of all material-related aspects will it be possible to further enhance the performance properties of critical gear parts. The starting point for adequate fine-grain stability – in other words, stable, homogeneously distributed precipitations of microscopic sizes between 10 and 100 nm – is the steelmaking process itself. This starts with the production of the steel heat based on a sophisticated alloying concept and the correct ratio of the chemical elements involved in the precipitation process. Aluminium and niobium in combination with nitrogen, in particular, have proved to achieve the desired effect as micro-alloying elements, thanks to their capacity to form stable nitrides and carbonitrides, which are extremely effective in countering grain growth. There has also been growing discussion of titanium-based concepts. These are associated with the risk of coarse titanium nitrides forming in the steel and causing poorer fatigue strength performance [4, 5]. During continuous casting, coarse precipitations will form in the billet as a result of relatively low cooling rates. Due to their size and distribution, however, these precipitations have no effect on the fine-grain stability of the ultimate product.
![Fig. 2: Time saved by increasing the carburising temperature versus the risk of grain growth at higher carburising temperatures [2].](https://www.e-motec.net/wp-content/uploads/2024/12/GHM-EMOTEC-FIG-21-1.png)
Casting of the heat is followed at the steelmaker’s by hot rolling of the untreated strand into steel bar, the primary material for further heat treatment and/or shaping of component blanks. The hot-rolling process has the function not only of shaping the steel into bars, but also of ensuring effective precipitation of (carbo-) nitrides. In this process, an important part is played by correct selection and optimisation of process parameters for the dissolution of precipitates in the rolling-mill furnace. For process optimisation, GMH employs the know-how of its own materials-science engineers, backed up by such software solutions as, for example, JMatPro®, performs calculations of the thermodynamic equilibrium of, and for example, selected aluminium and nitrogen contents and calculates aluminium nitrides content as a function of various rolling temperatures. The aim here is to assure an optimum aluminium nitrides content in the steel bar by optimising the chemical composition of the steel and the rolling parameters. At the same time, the aluminium nitrides must, however, be solubilised at the hot-forming temperature of the gear-system components. And, at carburising temperature, they must be present as stable precipitates of the effective size and distribution, since these will then counteract any shift in grain boundaries, thus preventing grain coarsening [4].
GMH Gruppe is your development partner for tailor-made steel
Based on its many years of experience and close strategic cooperation with customers and steel users, GMH Gruppe develops and supplies from its Georgsmarienhütte location specifically optimised fine-grain stabilised steel bar as a primary material – including the feed material for gearing components of vehicles featuring partially or all-electric propulsion.
“Even today, we are the strategic partner for new e-mobility applications and we intend to continuously transform our company and our processes ever more to serve this and other future-orientated markets. We aim to be the supplier of choice of primary material for our customers for a technologically demanding and, at the same time, sustainable supply chain. We – an electric-furnace steel plant using 100% scrap as our feed material – are ideally positioned for the future. Our steel production for further processing in international supplier chains even now saves more than 1,000 kg of CO2 per tonne of liquid steel compared to the industry average.” (Oliver Santelli, Chief Sales Officer BU Mobility, GMH Gruppe)
Fig. 3 shows two different microstructure variants of 16MnCr5 mod. as an example of an optimised fine-grain stable case-hardening steel: Fig. 3a shows the “standard” and Fig. 3b the optimised grain structure. Adjustment of the Al and N contents in the steel, together with optimised process parameters during hot rolling of the steel bar, made it possible to reduce or even eliminate coarse-grain contents following a reference heat treatment which simulated subsequent forging and case-hardening processes.
Fig. 3b: right column: Grain-size distribution in optimised 16MnCr5 mod. with grain sizes G ≥ 5
In addition to the use of fine-grain optimised steel bar as primary material, the downstream processes, such as forging, subsequent heat treatments, if required, and the concluding case-hardening process, determine the grain-size distribution in the microstructure of the final component and its in-service resistance to dynamic stresses. Fig. 4 illustrates how the downstream processing stages of steel bar influence fine-grain stability and grain-size distribution in a finished gear component produced in the bainitic steel 16MnCrV7-7, for example. This steel grade, developed by GMH in cooperation with a globally active operator of massive forming facilities, possesses ideal case-hardening properties and has been designed specifically to assure very good fatigue resistance to high dynamic stresses.
Ultimately, the influence of the downstream processing operations on the occurrence of microscopic precipitations and their effectiveness must be considered throughout the production chain and for each individual case.
The following questions arise:
- Will the precipitations present in the bar steel completely dissolve during hot forming in order to be able to subsequently reprecipitate in finely dispersed form and provide the desired fine-grain stabilising effect?
- How will the precipitations react to warm-forming and to subsequent heat treatment(s)?
- At which temperatures and for what times are the components formed to be case-hardened?
The process chains examined demonstrate that, thanks to the optimised microalloying concept, fine-grain stability in 16MnCrV7-7 can be achieved at warm-forming temperatures of up to 1,000°C. In warm- and cold-forged components fine-grain stabilities can be attained at carburising temperatures and times of up to of 1,050°C and two hours, respectively, depending on the heat treatment which follows the forging process.
Fig. 4: Possible influence of forging process, heat treatment and case-hardening process on fine-grain stability / grain size of components produced in 16MnCrV7-7 [6].
GMH is currently engaged in various development projects with customers, manufacturers of semi-finished parts and gear systems, and even OEMs. These projects are aimed at investigating the influence of forming processes on fine-grain stability and developing further possibilities for optimisation of other case-hardening steels, process chains and gear components, including materials for customised applications. These projects, and additional dynamic tests with components, have achieved highly promising results – both for established and approved case-hardening steels and for other, newly optimised grades.
Close cooperation between the steel producer and all steel users, including OEMs, has proved increasingly essential in order to succeed on the world market at all stages of the process chain.
Summary
The progressive electrification of drivetrains gives rise to new demands on the materials used and on component design. Grain sizes and grain-size distribution within the microstructure of gear components produced using case-hardening steels are known to have a significant influence on the resistance of these components to dynamic stresses. The steels and the entire process chain up to the finished part, therefore, provide great potential for enhancement of the fine-grain stability and, ultimately, in-service properties of the component. The materials development departments within GMH are cooperating closely on specific projects with the materials, process and component design experts at the forging shops, their direct customers. This cooperation may be expanded to include other stakeholders along the process chain, up to and including OEMs. Various successful projects have meanwhile shown that optimised, fine-grain stabilised steels will play a key role in assuring that even future needs generated by advanced hybrid and electric gear systems can be met.
Dr.-Ing. T. Wurm, Head of Technical Customer Support and Application Development, Georgsmarienhütte GmbH
T. Duda, Senior Sales Manager / New Business Development / Business Unit Mobility / GMH Gruppe
References
[1] S. Hock, J. Kleff, M. Schulz, A. Sollich, D. Wiedmann: HTM 54 (1999) No.1, S. 45
[4] J.C. Florian, H. Dickert, B. Kontiokari, O. Rösch, J. Gervelmeyer: Optimization of micro-alloying concepts and the influence of the process chain on fine grain stability of case hardening steels, 24th IFHTSE Congress and European Conference on Heat Treatment 2017, Nice, France
[5] F. Hippenstiel: Mikrolegierte Einsatzstähle als maßgeschneiderte Werkstofflösung zur Hochtemperaturaufkohlung von Getriebekomponenten, Dissertation, Aachen 2001
[6] S. Konovalov, J. Gervelmeyer, J.C. Florian, H.-W. Raedt: Innovative steels for lightweight transmission, 7th FVA GETPRO International Conference on Gear and Drivetrain Production, 2019, Würzburg, Germany
